Compaction of 

Sediments Underlying 

Areas of Land Subsidence 

in Central California

GEOLOGICAL SURVEY PROFESSIONAL PAPER 497-D
Compaction of 

Sediments Underlying 

Areas of Land Subsidence 

in Central California

By ROBERT H. MEADE

MECHANICS OF AQUIFER SYSTEMS

GEOLOGICAL SURVEY PROFESSIONAL PAPER 497-D

A study) partly statistical^ of the factors 

that influence the pore volume and fabric 

of water-bearing sediments compacted by 

effective overburden loads ranging from 

J to JO kilograms per square centimeter

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1968
UNITED STATES DEPARTMENT OF THE INTERIOR 

STEWART L. UDALL, Secretary

GEOLOGICAL SURVEY 

William T. Pecora, Director

For sale by the Superintendent of Documents, U.S. Government Printing Office 

Washington, D.C. 20402 - Price 35 cents (paper cover)
CONTENTS

Abstract_ _---------_-____________________________

Introduction- -_-------_____________________________

Purpose of report_______________________________

Acknowledgments__ __________________________

Review of factors influencing compaction of clays and 

sands__________________________________________

Removal of water from clays.____________________

Rearrangement of clay-mineral particles. __________

Development of preferred orientation_______

Development of turbostratic orientation. ______

Reduction of pore volume in sands at pressures less 

than 100 kilograms per square centimeter..______

Influence of particle size.____________________

Influence of particle sorting._________________

Influence of particle roundness.______________

Influence of mica particles.__________________

Influence of interstitial water.________________

Pore volume of sandstones at pressures greater than 

100 kilograms per square centimeter__________

Summary of petrology of sediments in areas of land 

subsidence _______________________________________

Analysis of factors influencing compaction of the sediments. __________________________________________

Use of multiple-regression statistics for analysis.____

Effect of overburden load________________________

Effect of particle size__________________________

Page 

Dl

1

2

2

3

3

5

9

11

12

12

12

12

14

17

17

18

22

Page

Combined effects of overburden load and particle 

size______________________---____ D23

Effect of particle sorting________________________ 24

Combined effects of selected physical and chemical 

factors.______--_--____--______-_-_------_--- 25

Fabric of the sediments and its relation to overburden

load and other factors.____________________________ 28

Orientation of clay-mineral particles_____________ 28

Observation and measurement______________ 28

Relation to depth of burial and type of sedimentary deposit._________________________ 29

Relation to particle size and chemical factors 31 

Domainlike aggregates._____________-_--_--- 31

Distribution of montmorillonite orientation 

within the sediments,_____________________ 32

Fabric of sands___________-.________-__----_---_ 32

Preparation of thin sections._________________ 32

Distortion of compressible grains._______--__- 32

Orientation of mica particles___________---- 34

Conclusions.___________________________-_--_-----_- 34

Glossary of statistical terms_________________-___-_--- 36

References.________________________________________ 37

Appendix A. Samples used in statistical studies.-_______ 39

Appendix B. Effective overburden loads at different 

depths in the cored sections at time of coring._____ 39

ILLUSTRATIONS

Page 

FIGURE 1. Graphs showing influence of different factors on the relations between void ratio and pressure in clayey

materials_________________________________________________________________________________ D4

2. Graph showing influence of pH on relations between void ratio and pressure in <4-micron fractions of kaolinite mixed with 10~3 M sodium chloride solution____-____-_--_-_--------------------_--------- 5

3. Sketches showing idealized clay-mineral particle arrangements that may be formed during compaction _ __ 6 

4-8. Graphs showing:

4. Influence of particle size on void ratio of silts, sands, and sandstones______________-_----_---_ 11

5. Influence of particle sorting on void ratio of sands_______--_------------------------_____---_- H

6. Influence of degree of roundness of pure quartz, 420 to 840 microns in size, on relation between void

ratio and pressure______--___________________________________-___-_---__----------- 12

7. Influence of proportion and size of mica particles on relations between void ratio and pressure in sands

and silts___________________________________________________________________________ 13

8. Influence of interstitial water on relation between void ratio and pressure in well-sorted clean quartz

sands_____________________________________________________________________________ 14

9. Map showing locations of core holes in central California._________________-_-_-__-_--___---------- 15

10.. Composite logs of petrologic characteristics of sediments cored in areas of land subsidence in central

California. _________________________________________________________________________________ 16

11. Graphs and diagrams showing simple relations between void ratio and effective overburden load in freshwater-bearing alluvial sediments._____________________________________-_-_-_-___----_--------- 20
IV CONTENTS

FIGURES 12-17. Graphs showing: Page

12. Relations between median diameter and the residuals of the void ratio-load regressions.________ D22

13. Simple relations between void ratio and median particle diameter in fresh-water-bearing alluvial

sediments____ ____________________________________________________________________ 23

14. Influence of particle size on relations between void ratio and effective overburden load.__________ 23

15. Relations between quartile deviation and the residuals of void ratio-load-Md^ and void ratioMd<t> regressions___________________________________________________________________ 25

16. Simple relation between void ratio and quartile deviation in silty sands from Los Banos-Kettleman

City area___________________________-____-______--__-----------------------______ 25

17. Relations of void ratio of fine sediments in Richgrove core to depth of burial, type of sedimentary

deposit, and effective overburden load_______________________-___-_________________^__ 26

18-20. Diagrams showing:

18. Relations of montmorillonite-particle orientation to depth of burial and type of deposit represented

by fine sediments cored in Los Banos-Kettleman City area_____________________________ 30

19. Relations of montmorillonite-particle orientation to depth of burial and type of deposit represented

by fine sediments in Richgrove core _________________________________________________ 31

20. Orientation of montmorillonite in sections cut at different angles to the bedding of sediments from

Huron and Richgrove cores_________________________________________________________ 33

21. Sketches showing distortion of compressible sand grains____________________________________________ 34

22. Sketches showing sections of micaceous sands from Mendota core-____-------------_----_-------______ 35

TABLES

Page 

TABLE 1. Summary of experimental data on compaction of unconsolidated sands_______________________________ DIO

2. Details of significant regressions of void ratio on effective overburden load, median particle diameter, and

quartile deviation of particle-size distribution____-_______-______-_-----------------_-_------__ 21

3. Selected properties of fine sediments in Richgrove core____________________________________________ 27

4. Selected details of multiple regressions of void ratio on different combinations of variables, representing

properties of selected fine sediments in Richgrove core_________________________________________ 27

5. Selected details of multiple regressions of orientation ratio on different combinations of variables representing

properties of marine siltstone in Richgrove core______________________________________________ 31
MECHANICS OF AQUIFER SYSTEMS

COMPACTION OF SEDIMENTS UNDERLYING AREAS OF LAND SUBSIDENCE IN CENTRAL

CALIFORNIA

By ROBERT H. MEADE

ABSTRACT

An increase in effective overburden load from 3 to 70 kilograms per square centimeter, partly natural and partly manmade, has caused an average reduction of 10 to 15 percent in 

the volume of alluvial sediments in the San Joaquin and Santa 

Clara Valleys of California. The effects of load, however, are 

complicated by the effects of other factors on the pore volume. 

The most easily discerned of these factors in the alluvial sediments are particle size and particle sorting. The variation in 

pore volume that is related to differences in average particle 

size is of the same order as the variation related to differences 

in load. The effects of particle sorting are probably subsidiary to 

the effects of load and average particle size.

A multiple-regression analysis of the pore volume, overburden 

load, and selected petrologic characteristics of a group of 20 

fine alluvial and shallow-marine sediments from the east side of 

the San Joaquin Valley shows the pore volume to be most closely 

related to particle size, diatom content, and the proportion of 

sodium (relative to other exchangeable cations) adsorbed by the 

clay minerals. Other factors that may have direct or indirect influence on the pore volume are the large proportions of montmorillonite in the sediments, and the variable pH and concentration of the interstitial electrolytes. The effects of overburden 

load on the pore volume of this group of sediments is completely 

obscured by the influence of the petrologic factors.

The degree of preferred orientation of the montmorillonite 

particles in the fine sediments in the San Joaquin Valley shows 

no regular relation to the depth of burial. The degree of orientation is related most consistently to the types of deposits represented by the sediments and decreases in the order: lacustrine, 

shallow marine, flood plain, and alluvial fan. The identity of 

the specific sediment properties or environmental characteristics 

that control the orientation, however, is uncertain.

The only effects of compaction observed in the sands are distorted and broken fragments of mica, shale, and metamorphic 

rock.

INTRODUCTION

The compaction of clastic sediments is influenced by 

several factors in addition to the load exerted by overlying sediments. During the early stages of compaction, 

these factors include such textural properties as particle 

size and sorting, such compositional characteristics as 

the proportions of mica or the different clay minerals,

and such chemical properties as the composition and 

concentration of the material dissolved in the interstitial 

waters. Our understanding of the process of compaction, 

therefore, involves understanding how these factors 

interact with each other and with increasing overburden 

loads to inhibit or to enhance the removal of fluids and 

the reduction of pore volume in sediments.

The compaction of the water-bearing sediments in 

three areas of land-surface subsidence in the San 

Joaquin and Santa Clara Valleys of California is the 

subject of this report. The historic compaction and land 

subsidence in these areas are clearly related to'the 

depletion of artesian pressure (increase of grain-tograin load) in the' sediments, which is a result of confined ground water being pumped from the sediments 

faster than it is being replenished. Because the rates of 

compaction and subsidence are rapid and can be measured, these areas were selected as field laboratories for 

a program of study of the mechanics of aquifer systems. Because the compaction represents an acceleration of the natural processes that would have taken 

place with the further accumulation of overlying sediments, these are also opportune areas for general studies 

of the compaction of sediments.

Two factors complicate these studies of compaction 

which are based on void ratios (pore volumes) determined in the laboratory from core samples obtained between 1957 and 1960. First, the void ratios reflect the 

combined effect of (1) the natural compaction due to 

the slow long-term increase in effective stress (effective 

overburden load) accompanying gradual burial and 

(2) the manmade compaction due to the rapid shortterm increase in effective stress resulting from decline 

in artesian head. At the time the cores were taken, the 

increment of void ratio resulting from the natural compaction was in equilibrium with the effective overburden 

load because time, an essential ingredient, had been adequate. The deeper cores, 1,000 to 2,000 feet beneath the

Dl
D2 MEOHANIOS OF AQUIFER SYSTEM'S

land surface, have been compacting under increasing 

sediment burial for one to possibly 5 million years. On 

the other hand, the application of the additional manmade stress began only 30 to 50 years ago and this stress 

had increased irregularly but rapidly to the coring date. 

The thicker fine-grained beds of low permeability were 

still experiencing pore-pressure decay and had not had 

time to attain hydraulic equilibrium with the head decline in the beds of coarser texture and higher permeability the aquifers. Hence, in the fine-grained beds, the 

void ratio increment had not reached equilibrium because of man's change of the hydrologic regimen.

Second, data are not available for differentiating the 

increments of void ratio due to the natural and manmade increases in effective stress partly because, at the 

time of coring, the manmade increment was transient. 

The reader should keep this in mind, especially in the 

chapter on analysis of factors influencing compaction 

of the sediments.

At the time the samples were cored, the ratio of manmade to natural stresses varied with depth and hydrologic regimen and ranged from 0 to about 22 percent. 

For example, in the Santa Clara Valley at San Jose, the 

natural effective overburden load in 1960 (time of coring) at a depth of 500 feet was about 16 kg per cm2 

(kilograms per square centimeter) or 230 psi (pounds 

per square inch), and the manmade increase was about 

3.5 kg per cm2 based on 120 feet of artesian-head decline. 

Thus, the manmade increase in effective stress at the 

500-foot depth in San Jose as of 1960 was 22 percent. 

On the other hand, in the San Joaquin Valley at Cantua 

Creek, the natural effective stress in 1958 at a depth of 

300 feet was 13 kg per cm2 (185 psi) and the manmade 

increase was zero (based on a constant water table) ; at 

a depth of 2,000 feet, however, the natural effective 

stress was about 65 kg per cm2 (930 psi), and the manmade increase was about 13 kg per cm2 based on 400 feet 

of artesian-head decline. Thus the manmade increase 

in effective stress at the 2,000-foot depth at Cantua 

Creek as of 1958 was 20 percent.

A discussion of the problems of land subsidence and 

aquifer compaction and the ways in which the problems 

have been approached are given by J. F. Poland in his 

foreword to the first chapter of this series (Johnson 

and others, 1967).

PURPOSE OF REPORT

This report is one of a series of chapters of Geological 

Survey Professional Paper 497, on the mechanics of 

aquifer systems in California and elsewhere. It is also 

the last of three chapters by the same author that concern the petrologic aspects of compaction. The first of 

these three chapters (Meade, 1964) is a comprehensive

review of previous work on the factors that influence 

pore volume and fabric of clayey sediments under increasing overburden loads. The second of the chapters 

(Meade, 1967) is a description of the variation and distribution, in the sediments of central California, of 

some of the petrologic factors that the review of previous work showed to be significant influences on compaction. The third chapter this report relates, partly by 

statistical analysis, the variations in overburden load 

and petrologic factors to variations in the pore volume 

and fabric of the sediments. Although these reports are 

part of a comprehensive series, their organization and 

content are such that they can be read separately.

The first section of this report is a review of previous 

work that identifies and evaluates factors that influence 

the responses of clastic sediments to loads between 0 and 

100 kg per cm2 or 1,422 psi (15 psi?»l atmosphere^ 

1 ton per sq ft«l kg per cm2 ). This section consists 

of (1) a brief summary of the first chapter (Meade, 

1964) plus a discussion of new evidence, mostly on the 

development of preferred orientation in clays, that has 

become available since the earlier chapter was completed, and (2) a review of the factors that influence the 

compaction of sands. Then follows a brief summary of 

the second chapter (Meade, 1967), describing the distributions of relevant petrologic factors in the sediments 

of the San Joaquin and Santa Clara Valleys in enough 

detail to set the stage for what follows.

The final sections are detailed studies, largely statistical, that explore the following questions: (1) What 

factors influence the water content and fabric of these 

sediments under effective overburden loads between 3 

and 70 kg per cm2 ? (2) What is the relative importance 

of these factors? Some preliminary results of these 

studies have been published in three earlier papers 

(Meade, 1961b, 1963a, 1963b), which are now superseded 

by the present report.

ACKNOWLEDGMENTS

I would like to thank the following people, all of 

whom are members or former members of the Geological 

Survey: J. B. Corliss, for assistance in measuring the 

fabric of the clayey sediments; D. R. Dawdy, for instructive discussions in the uses and misuses of statistics; 

H. C. Starkey, Claude Huffman, and A. J. Bartell, for 

determining selected chemical characteristics of the 

clayey sediments; and P. H. Held and Ruperto Laniz, 

for careful attention to the difficulties of making thin 

sections of unconsolidated clays and sands.

I also appreciate the critical discussions and reviews 

of the manuscript by my supervisor, J. F. Poland, and 

my colleagues, W. B. Bull, D. R. Dawdy, A. I. Johnson, 

B. E. Lofgren, L. K. Lustig, and R. P. Moston.
COMPACTION OF SEDIMENTlS UNDERLYING AREAS OF LAND SUBSIDENCE IN CENTRAL CALIFORNIA D3

REVIEW OF FACTORS INFLUENCING COMPACTION OF 

CLAYS AND SANDS

Because the pore volume of clastic sediments decreases with increasing overburden loads, it is a useful and convenient measure of compaction. In watersaturated sediments, it is also a measure of water 

content. Factors other than overburden load, however, influence the pore volume and water content 

of sediments and must be considered in any comprehensive study of compaction. This section of the report summarizes the pertinent previous studies that 

identify these factors and indicate the nature and 

degree of their influence on compaction processes.

This review is restricted largely to compaction in 

the range of pressures between 0 and 100 kg per cm2 , 

which includes the range that applies to the waterbearing sediments whose compaction accounts for the 

observed land subsidence in central California. It is 

further restricted mainly to consolidation processes 

that involve direct responses to load and mostly ignores the reduction of pore volume by the formation 

of interstitial cement.

The measure of pore volume used most extensively 

in this report is the void ratio, which is denned as 

the ratio of the volume of pore space to the volume 

of solids. Percent porosity is also included in most 

of the figures and in some of the text discussions.

REMOVAL OF WATER FROM CLAYS

Because of the intimacy of the relations between 

water and clay, the reduction of pore volume in clays 

under increasing overburden loads is best considered 

in terms of the removal of water. The factors that 

are known to influence the water content of clayey 

sediments under applied loads are particle size, clay 

minerals, adsorbed cations, interstitial electrolyte solutions, acidity, and temperature. The effects of all but 

the last two of these factors are shown in figure 1. 

With the exception of particle size, the influence of 

these factors is inferred mainly from laboratory 

studies of monomineralic clays mixed with simple 

electrolytes. Very little of the present knowledge is 

based on the study of sediments in nature or on 

laboratory mixtures of clays and electrolytes that 

approach the complexity found in nature.

This section of the review is mainly a summary 

of material that was treated earlier in much 'greater 

detail (Meade, 1964, p. B2-B13). Some newer evidence on the influence of acidity and temperature, 

taken from the work of Warner (1964), is included 

here to bring the earlier paper up to date (1965). 

Void ratio is used as a volumetric measure of water 

content.

The influence of particle size on the pore volume 

and water content of fine sediments is represented 

in figures IA and IB. The five lines in figure IA 

represent groups of sediments from different areas 

river reservoirs, an estuary, and the deep sea. None 

of these sediments were buried more than 20 feet 

below the water-sediment interface. Figure IB is a 

summary of the effects of particle size on the water 

content of sediments under pressure that was prepared by Skempton (1953, p. 55) from studies of 

sediments in the United States and Great Britain. 

The two graphs show the inverse relations between 

particle size and pore volume and how these relations persist during compaction under pressures 

approaching 100 kg per cm2.

The influence of different clay minerals on water 

content is discernible over a wide range of pressures, 

as shown in figure W. The observed differences are 

related to differences in the surface areas per unit 

weight (specific surfaces) of the clay minerals. The 

larger the specific surface, the larger the amount of 

water retained by the clay under pressure. Montmorillonites have larger specific surfaces than illites, 

which in turn have larger specific surfaces than 

kaolinites.

The influence of different exchangeable cations on 

the water content of montmorillonite is shown in 

figure 1Z>. Sodium-saturated montmorillonites retain 

more water than montmorillonites whose exchange 

positions are saturated with other common cations. 

These effects, however, have only been observed at 

pressures less than 50 kg per cm2 ; at greater pressures, differences in the exchangeable cations do not 

seem to be related to differences in water content. 

Because of the larger specific surface of montmorillonite and its associated larger capacity for adsorbing cations, the effects of different exchangeable cations on water content are much more pronounced 

in montmorillonite than in the other clay minerals. 

But in addition to showing effects of a lesser degree, 

other clay minerals may be affected in different ways 

by the exchangeable cations. An experiment by Samuels (1950; see Meade, 1964, fig. 12) on kaolinite 

saturated with different cations showed the largest 

water content in kaolinite saturated with aluminum 

and the smallest in kaolinite saturated with sodium 

an effect opposite to the one shown in figure 1Z>. 

The relative differences in water content, however, 

were substantially less than those shown in figure 

W.

The graphs in figures \E and IF show two opposite 

effects of electrolyte concentration on the water content 

of clays under pressure. The relation shown in figure
D4 MECHANICS OF AQUIFER SYSTEMS

10

80

50

10 100

0

1000 

MEDIAN DIAMETER, IN MICRONS

0.1

B

S//ts

1 10

UJ

o

70 gj 

Q.

Ld

o

>

50 - 

>

v>

o

o:

O

n O.

100

EFFECTIVE OVERBURDEN PRESSURE, IN KILOGRAMS PER 

SQUARE CENTIMETER

15 r

70

ZUJ

 o

O LL"

50 O=3 

O

100 10,000

PRESSURE, IN KILOGRAMS 

PER SQUARE CENTIMETER

PRESSURE, IN KILOGRAMS 

PER SQUARE CENTIMETER

2 ,- E

o

0.1

6 r

60

50

1 10

PRESSURE, IN KILOGRAMS 

PER SQUARE CENTIMETER

O

a:

Ld

0.

80 ,

50 g

Q: 

O Q. 

0

0.1 1 10

PRESSURE, IN KILOGRAMS 

PER SQUARE CENTIMETER

FIGURE 1. Influence of different factors on the relations between void ratio and pressure in clayey materials (summarized from Meade, 1964, p. B5-B12). Note different void-ratio and pressure scales. A, Relation of void ratio to 

median particle diameter at overburden pressures less than 1 kg per cm2 (Meade, 1964, p. B6). B, Generalized influence 

of particle size (modified from Skempton, 1953, p. 55)'. 0, Influence of clay-mineral species (modified from Chilingar 

and Knight, 1960, p. 104). D, Influence of cations adsorbed by montmorillonite (modified from Samuels, 1950), E, 

Influence of sodium chloride concentrations in uafractionated illite about 60 percent coarser than 2 microns (modified 

from Mltchell, 1960, flg. M-3). F, Influence of sodium chloride concentration in illite finer thau 0.2 micron (modified 

from Bolt, 19S6, p. 92>).
COMPACTION OF SEDIMENTS UNDERLYING AREAS OF LAND SUBSIDENCE UN CENTRAL CALIFORNIA D5

IE greater water contents associated with more concentrated electrolyte seems to be characteristic of most 

clay-water-electrolyte systems. The effect is related to 

the tendency of the clays to flocculate more readily in 

more concentrated electrolyte and to form openwork 

aggregate structures that resist compaction. The opposite relation shown in figure IF is characteristic of a 

restricted clay-water-electrolyte system, namely fine 

clays (montmorillonite or fine illite) associated with 

sodium electrolytes in concentrations less than about 

0.3 M (molar solution). In this system, the effect of the 

electrolyte concentration may be its control (as predicted by double-layer theory) of the tendency of the 

adsorbed sodium to diffuse and its consequent control 

of the thickness of the layer of adsorbed water that 

surrounds each particle. The different electrolyte concentrations do not seem to affect the water content of 

clays at pressures greater than 50 kg per cm2 . They do, 

however, seem to affect the fabric of clays at greater 

pressures, as discussed in the next section.

Recent experiments by Warner (1964, p. 51-62) describe some of the effects of acidity on the water content 

of clays. In figure 2, showing some results of Warner's 

pressure experiments on kaolinite (<4 microns) mixed 

with 10"3 M sodium chloride at different pH values, the 

greater void ratios are associated with the lower pH. 

This effect is related to the tendency of kaolinite and illite flakes to form more open flocculated structures in acid 

than in alkaline solutions as shown by the studies reviewed earlier (Meade, 1964, p. B15), and supported 

by Warner's (1964, p. 38-46) own sedimentation experiments. Warner found, however, that kaolinite mixed 

with a more concentrated electrolyte (0.55 M sodium 

chloride) showed void ratio-pressure relations that did

100 1000

PRESSURE, IN KILOGRAMS 

PER SQUARE CENTIMETER

FIQDEB 2. Influence of pH on relations between void 

ratio and pressure in <4-micron fractions of kaoJinite mixed with 10-3 M sodium chloride solution. 

Modified from Warner, 1964, p. 52.

£80-804 O--6I7 2

not vary with changes in pH (in the range 2.0 to 8.5). 

Furthermore, he found no variations in the void ratiopressure relations in illite or montmorillonite (mixed 

with different concentrations of sodium chloride solution) that could be related to variations in pH. The sum 

of his results seems to indicate that (1) the degree of 

acidity influences the water content of kaolinite under 

pressure and in contact with dilute electrolyte solutions 

and (2) that it has no influence on the pressure response 

of kaolinite mixed with more concentrated electrolyte, 

of illite, or of montmorillonite.

The influence of temperature on the water content of 

clayey sediments under pressure is not well defined. It 

is well known, however, that temperatures between 100° 

and 150° Celsius (centigrade) will remove the interstitial water from most clays under otherwise atmospheric conditions. One would expect the natural increase 

in temperature that accompanies increasing depth of 

burial to enhance the tendency of the increasing overburden load to remove water from clays. Little evidence 

is available, however, to support this expectation. At 

low pressures, furthermore, the temperature may have 

little effect. Warner's experiments (1964, p. 51-55, 59- 

62, 70-75) showed that variations in temperature in the 

range between 25° and 80 °C caused no differences in 

the equilibrium water content of kaolinite or montmorillonite at pressures between 1 and 8 kg per cm2. 

The rates of compaction under these pressures, however, 

did increase with increasing temperature. The experimental studies of van Olphen (1963, p. 183-186; reviewed by Meade, 1964, p. B13), suggest that increasing 

temperatures may assist in the removal of water at pressures greater than several hundred kilograms per square 

centimeter. The influence of temperature on the compaction of clays is a promising subject for future 

study especially enlightening would be experiments in 

which pressure and temperature were increased simultaneously by appropriate increments that corresponded 

to geostatic and geothermal gradients.

REARRANGEMENT OF CLAY-MINERAL PARTICLES

As a corollary to the reduction of pore volume under 

increasing loads, clay particles must move closer together in space and into more efficient packing arrangements. Many different arrangements are possible. The 

most widely considered arrangement, taking the characteristically platy shape of clay-mineral particles into 

account, is a preferred orientation such as is represented 

in figure 3.4.. This arrangement has been assumed to be 

the most likely result of the compression of clays often 

to the exclusion of other possibilities. Another arrangement which some evidence suggests might be formed 

during compaction is shown in figure 32?, and consists
D6 ME'CHANTOS OF AQUIFER SYSTEMS

of packets or domains of clay particles within which 

the planar orientation is perfect and between which the 

orientation is random. This fabric has been called turbostratic by Aylmore and Quirk (1960, p. 1046). Other 

regular fabrics as yet undescribed may be formed during 

compaction. Some clays may be compacted without developing any regular arrangement of particles.

DEVELOPMENT OP PREFERRED ORIENTATION

Because preferred orientation has received most of 

the attention in studies of the changes in clay fabric 

under increasing loads, many of the factors that influence its development have been identified. In the 

studies published through 1962, as reviewed earlier 

(Meade, 1964), much of the evidence on which the 

identification was based was indirect and included few 

actual measurements of the degree of orientation. In the 

years 1963 through 1965, several new studies have been 

completed that provide more direct evidence in the form 

of X-ray-diffraction measurements of clay fabric. 

Mainly on the basis of the newer studies, we may make 

a systematic assessment of the nature and degree of the 

influence of the following factors on the relations between pressure and preferred orientation in clayey 

sediments: initial water content, particle size, clay 

minerals, exchangeable cations, electrolyte concentration, acidity, and organic matter.

FIGURE 3. Idealized arrangements of 

clay-mineral particles that may be 

formed during compaction. A., Preferred orientation. B, Turbostratic 

orientation.

Experiments by Martin (1965) on kaolinite slurries 

and by O'Brien (1963, p. 20-40) on illite and kaolinite 

pastes indicate that, given sufficient initial water content, 

most of the reorientation of clay particles that occurs 

at pressures less than about 100 kg per cm3 takes place 

during the very early stages of compaction. Martin 

stated that a pressure of 1 kg per cm2 produced "a very 

marked preferred orientation which does not change 

upon further consolidation up to 32 kg/cm2." O'Brien's 

experimental results tabulated in the next paragraph 

also show well-developed preferred orientation at low 

pressures 0.02 to 0.2 kg per cm2.

The influence of the initial water content on the 

development of preferred orientation is shown in the 

following results obtained by O'Brien (1963, p. 36-37; 

1964-, p. 827-828) during pressure experiments on pastes 

of kaolinite and illite mixed with a prepared "marine" 

water (27 grams sodium chloride plus 1 gram calcium 

chloride per liter).

Initial water content 

(percent of wet weight)

Pressure 

(kg per cm2)

Orientation value

Kaolinite (unfractionated)

Not specified

23

32

Uncompressed

48

48

1.2

2.4

7.6

Dike «2 microns)

Not specified 

44

Uncompressed 

0.08

1.5 

1.5

Dlite-kaolinite (<2 microns)

Not specified 

50 

55 

67

Uncompressed

0.-17

.08

.02

3.0

20.0

38.0

28.0

The orientation value here is similar to the orientation 

ratio described on page D29: values near 1.0 denote 

random orientation, higher values denote greater degrees of preferred orientation. These data show fairly 

clearly that the initial water content is a critical factor 

in determining the degree of preferred orientation that 

develops during the early stages of compaction. A certain amount of interstitial water is required to allow 

the particles to move easily past one another into positions of preferred orientation. The studies cited in this 

and the previous paragraphs substantiate Hedberg's 

(1936, p. 272,281) inference that most of the mechanical 

rearrangement of particles during compaction takes 

place while porosities of the sediment are on the order 

of 75 to 90 percent and perhaps within 5 to 10 centimeters of the surface of deposition.
COMPACTION .OF SEDIMENTS UNDERLYING AREAS OF LAND SUBSIDENCE IN CENTRAL CALIFORNIA D7

The relation of particle size to the preferred orientation of clays is not clear. Grim, Bradley, and White 

(1957) in studying Paleozoic shales in Illinois suggested 

that preferred orientation was better formed in finegrained shales (in which most of the particles were finer 

than 2 microns) than in coarser grained shales that 

contained appreciable amounts of nonclay mineral 

grains. A similar observation was reported by Folk 

(1962, p. 541) in Silurian black shales in West Virginia: 

the most extreme examples of preferred orientation of 

clay-mineral particles were seen in shales having the 

least calcite and the least silt. On the other hand, Beall 

(1964, p. 79) said that he could find no relation between 

the degree of orientation and the grain size of the 

Cretaceous mudstones that he studied in Texas. No experimental evidence is available to shed light on these 

studies of natural claystones. They may only indicate 

that some fine-grained sedimentary rocks have welldeveloped preferred orientation and some have not.

Studies reviewed earlier (Meade, 1964, p. B17-B19), 

supported by the experiments of Martin and O'Brien 

cited above, give the impression that low pressures on 

the order oi a few kilograms per square centimeter can 

produce marked preferred orientation in kaolinite and 

illite but do not seem to produce much orientation in 

montmorillonite. Greater pressures on the order of 100 

kg per cm2 or more, as applied in laboratory studies, 

can produce preferred orientation in any clay minerals 

of platy habit including montmorillonite. At lower 

pressures, however, the larger clay particles of illite and 

kaolinite seem to be more susceptible to preferred orientation than the smaller particles typical of montmorillonite. This impression is substantiated by Beall's 

(1964, p. 42) studies of the fabric of Cretaceous mudstones of Texas in which kaolinite and illite always 

showed a greater degree of preferred orientation than 

montmorillonite, within the same rocks. Odom (1963, 

1964), on the other hand, noted a lack of correlation 

between the variable clay-mineral composition and the 

preferred orientation of different Paleozoic argillaceous 

rocks in Illinois.

The only previous direct evidence of the influences of 

the different exchangeable cations on fabric was presented by Beall (1964, p. 82-84). He noted that the degree of preferred orientation of the clay minerals was 

inversely related to the amount of exchangeable sodium, 

relative to exchangeable calcium and magnesium, in 

seven samples of the Cretaceous mudstones that he 

studied. However, the opposite relation a direct relation between the degree of montmorillonite orientation 

and the proportion of adsorbed sodium was found in 

11 samples of shallow-marine sediment in the San 

Joaquin Valley (described later in this report).

The influence of electrolyte concentration on the fabric 

of clays under pressure is indicated by the results of 

experiments made by Engelhardt and Gaida (1963, p. 

926-927), and by Warner (1964, p. 84-91). In montmorillonite compacted under a pressure of 800 kg per 

cm2 , Engelhardt and Gaida found an increase in the 

degree of preferred orientation with decreasing concentrations of sodium chloride solutions in the range 1.1 to 

0.16 M. A similar increase in orientation with decreasing sodium chloride concentration (range 2.4 to 0 M} 

was shown in experiments with kaolinite compacted 

under a pressure of 160 kg per cm2 , although these results were not as clearly expressed >a$ the results of the 

experiments with montmorillonite. From experiments 

with kaolinite sedimented at a pH of 8.5 in sodium 

chloride solutions of two concentrations (0.55 and 0.001 

M) and compacted under a pressure of 4,200 kg per 

cm2 , Warner reported that the better developed preferred orientation was associated with the lower electrolyte concentration. These results, however, pertain to 

pressures greater than 100 kg per cm2 .

No clearly expressed relations between electrolyte concentration and preferred orientation resulted from the 

experiments conducted at low overburden pressures by 

O'Brien (1963, p. 21-28). He suspended two different 

concentrations of kaolinite in three different electrolyte 

solutions and allowed the clay to settle out. Then, over 

a period of 3 weeks, he sedimented 3 feet of silt and sand 

on top of the clay. The volume of the clay was reduced 

by about half during the 3-week period. The degrees of 

orientation produced are illustrated in the following table. The number in the last column is the final orientation value after the overburden load was applied.

Solution

Electrolytes 

(grams per liter)

NaCl CaCh

Kaolinite 

in original 

suspension 

(grams per 

liter)

Final orientation 

' value

"Salt lake" water.

Do_____.___. 

"Marine" water__

Do_________.

" Fresh" water_ _

85

85

27

27

0. 05

. 05

0.55

. 55

1

1

0. 1

. 1

5

50

5

50

5

50

48

11.2

64

10

4. 5

34

Whereas the data do not show a simple relation between 

electrolyte concentration and fabric, they may indicate 

a complex, and as yet undefined, interrelation between 

fabric, concentration of clay suspensions, and concentration of electrolytes.

The effects of variations in acidity on the orientation 

of clay particles under pressure can only be inferred 

indirectly from Warner's (1964, p. 51-62) experiments 

that were discussed earlier (fig. 2). In kaolinite mixed
D8 MiECSHANTOS OF AQUIFER SYSTEM'S

with electrolyte solutions of low concentration, the degree of preferred orientation may be better developed 

in alkaline than in acid solutions. On the other hand, 

acidity may have little or no effect on the orientation of 

kaolinite associated with more concentrated electrolyte 

solutions, of illite, or of montmorillonite.

The presence of organic matter seems to enhance the 

degree of preferred orientation in argillaceous rocks. 

Odom (1963, 1964) found, in claystones overlying coal 

beds in Illinois, that the degree of preferred orientation 

correlated directly with the content of organic carbon 

(as C). This is substantiated by O'Brien's (1963, p. 13) 

summary of his measurements of the preferred orientation of illite and kaolinite in black and gray shales of 

Pennsylvanian age in Illinois the black shales, having 

an approximate range of orientation values from 10 to 

20, contain more organic matter than the gray shales, 

having a range of orientation values from 6 to 10. Beall 

(1964, p. 86-87) also reported a greater degree of preferred orientation in the darker colored of some of the 

Cretaceous mudstones that he studied in Texas. These 

newer studies confirm the earlier work of Ingram (1953, 

p. 873-875) on the orientation of clay-mineral particles 

in clayey rocks, Ordovician to Eocene in age, from 

Colorado, Iowa, and Wisconsin. Ingram noted, in thin 

sections of the rocks, that organic stain was associated 

with clays that showed preferred orientation and was 

absent from clays in which the orientation was random. 

In one rock sample, this relation was expressed in alternate organic and nonorganic clay layers. Folk (1962, p. 

541) has suggested that a large proportion of organic 

matter in a sediment may aid the development of preferred orientation by its initial deposition as a stagnant 

clay-organic "soup" which is compacted markedly and 

allows the clay flakes to become well oriented.

Spatial variations of the preferred orientation of 

clay minerals with respect to depth of burial and 

the inferred environments of deposition of Cretaceous 

mudstones in Texas have been described by Beall 

(1964, p. 46-54, 104-107). He noted no variation in 

preferred orientation that could be related to overburden load or degree of compaction that is, no 

relation between fabric and bulk density. He did note, 

however, lateral changes in preferred orientation that 

were related to changes in the inferred depositional 

environment: in one horizon that graded laterally 

from shallow-marine to near-shore sediments, the better preferred orientation was associated with deposits 

of a tidal mud flat or shallow lagoon. In an overlying horizon, the whole pattern of depositional types 

and associated preferred orientation was shifted laterally, indicating a westward transgression of the 

Cretaceous sea. Beall concluded that preferred orientation was best developed in a low-energy brackishwater environment. The specific characteristics of the 

environment that are most responsible for the development of preferred orientation are not clearly 

shown, but he did note (as cited in the preceding 

paragraphs) relations with clay minerals, exchangeable cations, and the dark color of the rocks.

These laboratory and field observations of preferred 

orientation can be summarized as follows. During 

the compaction of clayey sediments under pressures 

between 0 and 100 kg per cm2 , most of the rearrangement of clay particles takes place and most of the 

preferred orientation is developed very early at pressures of only a few kilograms per square centimeter. 

During this very early stage, the amount of water 

held in the clayey sediment may be the most critical factor. If enough water is present, the particles 

may slip past one another easily into efficient packing arrangements; if not, the preferred orientation 

may be poorly developed or absent. A large concentration of carbonaceous organic matter seems to 

assist the development of preferred orientation by its 

ability to retain large amounts of water during the 

early stages. The influence of other factors is less 

certain, but one might expect the development of 

preferred orientation to be favored by larger proportions of kaolinite and illite relative to montmorillonite, by lower concentrations of interstitial electrolytes, and perhaps by greater alkalinity of interstitial 

solutions.

DEVELOPMENT OF TURBOSTRATIC ORIENTATION

Whether a turbostratic fabric of the kind shown in 

figure 3.Z? can be formed during the compaction of 

clayey sediments is not certain. This fabric consists of 

domains of clay particles within which the planar orientation is nearly perfect and between which the orientation is random. If the domains were also oriented 

with respect to each other, this fabric would be nearly 

indistinguishable from the preferred orientation of 

individual particles shown in figure 3A Note that the 

permeability of a clay that has a turbostratic fabric 

is greater than the permeability of a clay of the same 

water content that has preferred orientation the two 

sketches in figure 3 are drawn with approximately 

equal void ratios. Several indirect lines of evidence, 

summarized from an earlier chapter (Meade, 1964, B9- 

B10, B18), suggest that a turbostratic fabric may 

develop under pressure in some montmorillonites.

Montmorillonite that is mixed with electrolyte solutions containing calcium or magnesium and that has 

exchange positions saturated with these cations will 

not swell to interparticle distances greater than 9 A
COMPACTION OF SEDIMENTS UNDERLYING AREAS OF LAND SUBSIDENCE IN CENTRAL CALIFORNIA D9

(angstrom), regardless of the concentration of the interstitial electrolyte (Norrish, 1954). This is in contrast to mixtures of montmorillonite and sodium electrolytes in which the interparticle distance is a direct 

function of water content (greater than about 50 percent) and an inverse function of electrolyte concentration (less than about 0.3 M). Nitrogen-sorption measurements of calcium montmorillonite by Aylmore and 

Quirk (1962, p. 109-112) showed the existence of two 

different sizes of interparticle spaces, the smaller of 

which was on the order of 10 A, which suggested a 

particle arrangement such as the one shown in figure 

3#.

Other experiments suggest that pressure may assist 

in the development of domains in calcium montmorillonite or in sodium montmorillonite associated with a 

concentrated electrolyte. From measurements of the 

sharpness of X-ray reflections from compressed calcium 

montmorillonite, Blackmore and Miller (1961, p. 171) 

inferred that the number of unit 10-A sheets per domain increased progressively with pressure and ranged 

from about five unit sheets at 0.5 kg per cm2 to nearly 

eight unit sheets at 90 kg per cm2 . The process seemed 

to be irreversible; that is, the sheets in the domains did 

not seem to dissociate when the pressure was released. 

In sodium montmorillonite compressed under 800 kg 

per cm2 , Engelhardt and Gaida (1963, p. 926) found 

that increasing concentrations of interstitial sodium 

chloride solution (from 0.16 to 1.1 M) were related to 

decreasing degrees of preferred orientation and increasing permeabilities. The equilibrium water content, however, was the same at all sodium chloride concentrations. 

These results may be explained by a lessening development of preferred orientation and a greater degree of 

turbostratic orientation at the higher concentrations. 

Because of the small size of the domains and their random orientation, however, the. existence of a turbostratic 

fabric is difficult to prove, either optically or by X-ray 

diffraction.

REDUCTION OF PORE VOLUME IN SANDS AT PRESSURES LESS THAN 100 KILOGRAMS PER SQUARE 

CENTIMETER

The compaction of sands and the factors that influence the pore volume of sands and sandstones under 

pressure have received less attention than the corresponding processes and factors in clays and shales. One 

reason is that the decrease in pore volume with increasing depth of burial is not as easily observed in sands 

and sandstones as it is in clays and shales because cementation and other effects obscure the relation. Another 

reason is that unconsolidated sands are difficult to core 

and nearly impossible to bring to the laboratory without 

disturbing their natural pore volume. Furthermore,

sands have not received the same attention from soil 

engineers that clays have because sands cause fewer 

difficulties in foundation engineering.

Information on the response of sands to increasing 

pressures between 0 and 100 kg per cm2 consists of 

laboratory compression tests summarized in table 1. One 

cannot extrapolate indiscriminately from these results 

to most natural sands because the comprehensive testing 

to date has all been done on pure quartz sands in a 

restricted range of sizes. Although the experiments were 

made under different conditions (sample dimensions, 

rate of pressure application), they seem to show that 

the void ratio of a well-sorted and rounded quartz sand 

will decrease by about 0.03 in pressures from 0 to 70 kg 

per cm2 . If the sand consists of angular grains, its void 

ratio will decrease by 0.15 to 0.30 in the same pressure 

range.

Under pressures between 0 and 100 kg per cm2 , the 

experimental quartz sands compacted principally by 

the shifting of particles into more dense packing arrangements and, to a lesser degree, by elastic compression of the grains 'themselves. At pressures in excess of 

about 100 kg per cm2 (especially in the experiments of 

Koberts and de Souza), the rate of compaction increased 

as the grains cracked and shattered under pressure. I 

doubt, however, that quartz grains would shatter extensively at these pressures in naturally compacting 

sands. The slow rate of pressure application in nature 

allows the grains to accommodate themselves in other 

ways, perhaps mainly by solution and reprecipitation 

at points of contact. Furthermore, polymineralic sands 

should respond differently to pressures, the softer grains 

deforming more readily and bearing much of the load.

The factors that influence the pore volume of sands 

at pressures between 0 and 100 kg per cm2 are mainly 

the textural characteristics of the constituent particles: 

size, sorting, roundness, shape, and flexibility. The 

influence of these factors on the pore volume of uncompaoted sands has been reviewed and discussed 

extensively by Engelhardt (1960, p. 3-16), Fraser 

(1935), Gaither (1953), and Hamilton and Menard 

(1956). They will be reviewed here rather briefly, with 

some emphasis on their effects under pressure.

INFLUENCE OF PARTICLE SIZE

Although ideally the pore volume of well-sorted sands 

is independent of particle size, most natural sands show 

a slight inverse relation between pore volume and average size (fig. 4). Notice, however, that the relation is 

much more pronounced in silts than in sands.

In sandstones or in sands that are buried under several 

hundred feet or more of overburden, the relation between pore volume and particle size is both similar to
D10 ME'GHATSTIOS OF AQUIFE.R SYSTEMS

TABLE 1. Summary of experimental data on compaction of unconsolidated sands

[ Void ratio and porosity at 70 kg per cm2 are interpolated from data as presented; 70 kg per cm2 represents approximate maximum pressure to which sands from cores in San

Joaqum and Santa Clara Valleys have been subjected by natural overburden loads]

Properties of sand Pore volume

Reference

Size (mm)

Mineral 

composition Initial packing Roundness

Interstitial 

fluid

Void ratio at indicated Porosity in percent volpressure ume at indicated pressure

Initial 70kg Maxi- Initial 70kg Maxiper cm2 mum per cm2 mum

Terzaghi (1925, p. 987- 0.25-1.0 Quartz.___ Loose-____ Angular.__ Dry

DoBotset and Reed 

(1935).

Do.

Urul (1945 data 

reported by Weller, 

1959, p. 295).

25-1.0 _-do__.___ Compacted.

42- .6 _--_______ Compacted by 

tamping.

42- .6 __--__-___ Compacted six 

times at 

210kg 

per cm2 .

.do.

._ 0.97 0.66 0.56 49.2 39.8 35.9

.67 .53 .45 40. 1 34.7 31.0

Kerosene.. .70 .67 .60 41.2 40.1 37.5

__do .58 .56 .56 36.8 36.0 35.9

79 .74 .33 44.0 42.5 25.0

Roberts and de Souza

(1958). 

Do 

Do... ___________

Do ___________

Do ___________

.42- .84

.42- .84

. 42- . 84

. 42- . 84

. 1 - . 84

Quartz

(Ottawa) . 

-_do______

__ do__ _

(Ottawa) .

__do--_-.

__ do .

__do.___.

Rounded

._ __do____-_

._ _-do-__.__

Rounded _

Dry______

__do--____

Dry______

.65

.60

.55

.97

.49

.62

.58

.53

.71

.47

49

.31

.44

30

32

39.4

37.5

35.5

49.2

32.9

38.3

36.7

34.7

41.5

32.0

32.9

23.7

30.6

23. 1

24.2

TABLE 1. Summary of experimental data on compaction of unconsolidated sands Continued

Applied pressure 

(kg per cm2)

Initial Maximum

Elastic recovery 

(percent difference 

in porosity at initial Grains crushed at maximum 

and maximum pressure (weight percent) 

pressures)

Pressure application

Minimum interval 

Increment (kg per cm2) between applications (hours)

Dimensions of 

sample (cm)

Initial 

height

Diameter

Terzaghi (1925, p. 987-

988). 

Do.. ._--_____-__

Botset and Reed

(1935). 

Do __________

0

0

0

0

110 __-.

115 __-_

240

210

___-_-__ _ do _

27 _____----_-____-_-_

80 8 ("after 7 load1

. !_____________-

- 5-12. __________

5-12.. _________

1/60

1/60

2

2

4.0

4.0

6.7

15

15

7

7

unload cycles).

Urul (1945 data

reported by Weller, 

1959, p. 295). 

Roberts and de Souza

(1958). 

Do__ _____ __

Do _ ___________

Do __-____.. .

Do -.---_____-

1.3

1.3

1.3

1.3

1.4

i Q7n

410 ______

985 ______

575 ______

705 ______

915 ______

Extensive

______ About 50 _ _

Two times previous increment.

.-___ dodo

24

24

24

24

24

1-2

1-2

1-2

1-2

1-2

3-7

3-7

3-7

3-7

3-7

and different from the relation in unconsolidated sands. 

It is similar in that the pore space, if it is not filled by 

cement, increases with decreasing particle size: curve V 

in figure 4 represents Cretaceous sandstones buried 

under 3,580 to 3,680 feet of overburden. The relation 

may be different, however, in that the void ratio may

be expressed as a linear, rather than curvilinear, function of the logarithm of the median particle size. Although this is only suggested in the narrow range of 

particle sizes represented by curve V, it is supported by 

other evidence that will be presented in the final sections 

of this paper.
COMPACTION OF SEDIMENTS UNDERLYING AREAS OF LAND SUBSIDENCE IN CENTRAL CALIFORNIA Dll

INFLUENCE OF PARTICLE SORTING

Sorting of constituent particles also affects the pore 

volume of a sand. "Generally the porosity decreases as 

grains vary from a uniform size because (a) the finer 

grains tend to occupy voids between larger grains and 

(b) the coarsest grains reduce porosity by occupying a

2 

UJ 

O 

(£. 

UJ 

Q_

70 UJ

50 o:

10 100 1000 

MEDIAN DIAMETER, IN MICRONS

FIGURE 4. Influence of particle size on void ratio of 

silts, sands, and sandstones. Curves I through IV 

represent unconsolidated sediments. I, North Sea 

bottom off Wilhelmshaven (modified from Fiichtbauer 

and Reineck, as reported by Engelhardt, 1960, p. 15) ; 

II and III, shallow sea bottom off San Diego (II, 

data from Shumway, 1960, p. 454 457; III, data from 

Hamilton and others, 1956, table 1) ; IV, Colorado 

River delta in Lake Mead (modified from Sherman, 

1953, p. 399). Curve V represents Cretaceous sandstone (Germany) buried under 3,580 to 3,680 feet of 

overburden (modified from Engelhardt, 1960, p. 21).

1.0 r

0.5

-] 50

30

0.5 

QUARTILE DEVIATION

volume that would otherwise be occupied by finer grains 

and voids" (Gaither, 1953, p. 184). This statement is 

supported by the experiments of Kogers and Head 

(1961) on lognormally distributed artificial mixtures of 

different sand sizes (fig. 5A). The void ratio is related 

inversely to quartile deviation (QD^} a measure of 

sorting which was introduced and explained by Krumbein (1936, p. 102-103), and which is the logarithm to 

the base 2 of the Trask sorting coefficient, So (Trask, 

1930, p. 594). Although the relations in figure 5A seem 

to plot as straight lines, they probably apply only to 

the short range of QD$ values indicated. If they were 

extrapolated, they would imply that sands whose quartile deviations amounted to 2.5 or more would have no 

porosity at all.

The results plotted in figure 5A also show how particle sorting influences the relation between the pore 

volume and particle size of sands. In the well-sorted 

sands, the void ratio is virtually independent of the 

average particle size. As the sorting becomes poorer 

(QDj, increases), the void ratios of the different-sized 

sands diverge the finer sands having the greater pore 

volumes.

The influence of particle sorting on the relation between void ratio and pressure is indicated in figure 5Z?, 

which is adapted from the experimental data of Roberts, and de Souza (1958) on well-rounded quartz grains. 

The void ratio of the less well sorted sand (greater 

QD$ ) remained below that of the two better sorted 

sands at pressures up to a few hundred kilograms per 

square centimeter. At the greater experimental pressures, the well sorted sand was fractured more extensively (see details in table 1), and its void ratio-pressure curve converged with the curve for the less well

B

1.0 r

0.5

-I 50

30

10 100 1000

PRESSURE, IN KILOGRAMS 

PER SQUARE CENTIMETER

FIGURE 5. Influence of particle sorting on void ratio of sands. A, In artificial mixtures of fractions of two natural sands from Texas 

(modified from Rogers and Head, 1961, p. 469). B, In well-rounded pure-quartz sand under pressure (modified from Roberts and 

de Souza, 1958) ; median diameter (Md) of the two better sorted sands, 600 micro-ns; median diameter of the poorer sorted sand, 

480 microns; details of pressure experiments listed in table 1.
Dl,2 MECHANICS OF AQUIFER SYSTEM'S

sorted sand. As pointed out, however, this type and degree of fracturing is probably not common at these pressures in nature.

INFLUENCE OP PAiRTICLE ROUNDNESS

The angularity of sand particles also affects the pore 

volume, as shown in figure 6 by the experiments of Roberts and de Souza (1958). Whereas the initial void ratio 

of angular, ground quartz was nearly 1.0, the initial 

void ratio of rounded quartz (Ottawa sand) was considerably lower on the order of 0.6. The initial void 

ratio of the loose angular quartz used by Terzaghi 

(1925) in his experiments was also large (0.97), as 

shown in table 1. These void ratios reflect the instability 

of the initial packing of angular grains. Furthermore, 

angular sands are more compressible than rounded ones. 

This relation also has been observed in consolidated 

sandstone by Fatt (1958, p. 1940-1941) who noted that 

sandstones consisting of poorly sorted, angular grains 

are more compressible than sandstones whose grains are 

well sorted and rounded.

INFLUENCE OP MICA PARTICLES

If a sand contains a few percent of mica flakes, the 

flexibility and elasticity of the micas contribute to the 

compressibility of the whole. The pronounced effect of 

increasing mica content on the compressibility of prepared sand-mica mixtures has been demonstrated by 

Gilboy (1928). Figure 7A shows the results of some of 

his experiments on mixtures of rounded quartz grains 

and "white mica" (presumably muscovite) flakes. The 

larger initial pore volume of the more micaceous sands 

is apparently caused by bridging of open spaces by the 

mica flakes. Because the flakes respond to pressure by 

bending around spherical grains, the more micaceous

1.0 r Angu/

0.5

0 ' 

1

50

30

10 100 1000

PRESSURE, IN KILOGRAMS 

PER SQUARE CENTIMETER

FIGURE 6. Influence of degree of roundness of pure 

quartz, 420 to 840 microns in size, on relation between 

void ratio and pressure. Modified from Roberts and 

de Souza (1958). Details of experiments listed in 

table 1.

sands are more compressible and their greatest compaction takes place at lower pressures than in less micaceous sands. Note also the greater elastic rebound 

upon release of pressure on the more micaceous sands. 

Results of experiments made by McCarthy and 

Leonard (1963) on the compressibility of mixtures of 

different proportions of two different sizes of fine muscovite and a natural sandy silt from Pennsylvania are 

shown in figures 7B and 7G. Because the samples were 

compacted dynamically by tamping rather than being 

compressed under a static load, the results are not strictly 

comparable to those in figure 7A, nor are they directly 

applicable to equivalent sediments under natural loads. 

The results do show, however, that the addition of mica 

increases the void ratio of the silt and that, the finer 

the mica, the greater the increase in void ratio per unit 

increase in mica content.

INFLUENCE OP INTERiSTITTA'L WATER

The degree to which the presence or absence of interstitial water influences the compressibility of a sand is 

related to the amount of finer material that the sand 

contains. In clean sands, the compressibility is apparently independent of the water content. Terzaghi (1925, 

p. 987) noted this in his early experiments, and the 

results obtained by Koberts and de Souza (1958) substantiate his observations. In figure 8, the difference in 

void ratio between wet and dry sands is the same as the 

difference between duplicate tests on the dry sand.

When clay is present in sufficient quantity to act as a 

binder or as a coating on sand grains, however, the 

difference between the behavior of the wet and dry 

aggregates is substantial. Bull (1964, p. A58-A59), for 

example, made compression tests on a sand that contained 5 percent clay, most of which was montmorillonite. The sand was markedly more compressible when 

wet than when dry. In the compression tests made by 

McCarthy and Leonard (1963) on sands and silts 

mixed with different proportions of mica, the water 

content had a small but markedly consistent effect on 

the compaction. In the range of water contents roughly 

between 39 and 84 percent of saturation and in the range 

of mica contents between 0 and 100 percent, the void 

ratio at a given pressure was nearly always smaller (by 

0.01 to 0.04) in the more saturated sediments. This 

indicated a small but consistent influence of water 

content on the efficiency of the compaction.

PORE VOLUME OF SANDSTONES AT PRESSURES 

GREATER THAN 100 KILOGRAMS PER SQUARE 

CENTIMETER

Although studies of the porosity of sandstones and 

the factors that influence it at pressures greater than 

100 kg per cm2 (depth of burial about 3,000 feet) are
COMPACTION OF SEDIMENTS UNDERLYING AREAS OF LAN'D SUBSIDENCE IN CENTRAL CALIFORNIA D13

not directly applicable to the studies that follow this 

review, they indicate effects that may be incipient at 

lower pressures and they provide useful insights. Although the work reviewed below relates to sandstones 

rather than sands, its chief asset is that most of it is 

based on natural sedimentary rocks rather than on sands 

prepared in the laboratory.

The porosity of sandstones seems to decrease regularly 

with increasing depth of burial. In well-sorted quartzose 

sandstones, the relation between depth of burial and 

percent porosity (rather than void ratio) may be linear 

(Maxwell, 1964; Philipp and others, 1963a, p. 461;

1963b, p. 465; or Fiichtbauer and Reineck, 1963, p. 304). 

On the basis of the available studies, however, one 

cannot be certain of the depth range in which such a 

linear relation might apply (Walker and Maxwell, 

1964), or what form the relation might have in poorly 

sorted mixtures of sand, silt, and clay.

One of the most informative studies of the processes 

by which sandstones respond to pressure was made by 

Taylor (1950). In thin sections of Mesozoic sandstones 

from Wyoming, she studied the number and types of 

contacts between sand grains. The number of contacts 

increased with increasing depth of burial, and the type

10 0.1 1 10 0.1 1 10 0.1 1 

PRESSURE, IN KILOGRAMS PER SQUARE CENTIMETER

10 0.1

60

50 o

30 g 

o: 

O

Q.

0

10

1.0

0.5

B

100%

50

30

0.1 10

PRESSURE, IN KILOGRAMS 

PER SQUARE CENTIMETER

1.0

0.5

50

a

_l

o

30 Z

0.1 10

PRESSURE, IN KILOGRAMS 

PER SQUARE CENTIMETER

FIGURE 7. Influence of proportion and size of mica particles on relations between void ratio and pressure in sands and silts. Percentage of mica by weight indica-tedi on each diagram. A, Artificial mixtures of mica and quartz ; particles of both constituents 420 to 

590 microns in size (modified from Gilboy, 1928, p. 560). B, Sandy silt (Md=60 microns, QD<j> =1.7, material finer than 4 

microns in size=12 percent) mixed with silt-sized mica (Md = 2A microns, QD<t> =2.5)<; water saturation 80 to 82 percent (modifled from McCarthy and Leonard, 1963, p. 33). G, Sandy silt as in B, mixed with finer grained mica (Md = 7 microns, QD$ = 

0.7) ; water saturation 78 to 80 percent (modified after McCarthy and Leonard, 1963, p. 34).

280-804 O 67 5
D14 MECHANICS OF AQUIFER SYSTEMS

changed progressively from tangential through long 

and concavo-convex to sutured. She believed that the 

progressive change of contact type was due to (1) solid 

flow of material under pressure and (2) solution and 

redeposition of dissolved material, both of which were 

caused by the pressure that accompanied deep burial. 

Other pressure effects that she observed were "the crushing and yielding of micas, feldspar and rock fragments, 

and the development of tension and pressure cracks in 

the grains" (p. 715). Unfortunately, she did not report 

the depths at which these phenomena became important.

Factors other than pressure that have been clearly 

shown to affect the porosity of sandstones are the interstitial cement (carbonate or silica), the size of the sand 

particles, and temperature.

The effect of interparticle cement on porosity is intuitively obvious, and it is documented by recent detailed studies. Clear inverse relations between porosity 

and calcite content, for example, are described by 

Fiichtbauer (1964, p. 243-244) in Tertiary sandstones 

buried at depths between 3,600 and 11,500 feet in Switzerland, Austria, and Bavaria. Similar inverse relations 

between porosity and silica overgrowths on quartz 

grains are shown by Fiichtbauer (1961, p. 172-173) in 

Mesozoic sandstones buried at depths between 4,700 and 

5,300 feet in northwest Germany. Inverse relations between porosity and the amount of cementing material 

(both carbonate and silica) are also revealed by the 

statistical studies of Paleozoic sandstones in the Appalachian region by Griffiths (1964, p. 653-664) and his 

students. (See also Griffiths, 1958, p. 27-29.)

In sandstones that do not contain large proportions 

of cement, the relations of porosity to particle size and, 

to a lesser extent, particle sorting are often clearly visible (curve V in fig. 4; Griffiths, 1958, p. 27-29; 1964,

1.0 r

0.5

-I 50

30

10 100 1000

PRESSURE, IN KILOGRAMS 

PER SQUARE CENTIMETER

FIGURE 8. Influence of Interstitial water on relation 

between void ratio and pressure In well-sorted clean 

quartz sands. Modified from Roberts and de Souza 

1958). Details of experiments In table 1.

p. 652- 664). Whereas the relation between porosity and 

particle size in sandstones is often inverse, it occasionally is found to be direct that is, in some sandstones 

the porosity increases with increasing particle size 

(Fiichtbauer, 1964, p. 243-244; McCulloh, 1964; Modaressi and Griffiths, 1963, p. 258). This reversal in the 

relation between porosity and particle size is at least 

partly a function of the depth of burial. Although finer 

sediments have greater initial porosities than coarser 

sediments, they are compressed more rapidly during the 

early stages of compaction. As compaction continues 

with progressive depth of burial, a critical depth or 

depth range must be reached at which the porosity of 

the finer sediments equals that of the coarser and below 

which the porosity of the finer sediments is less than 

that of the coarser. This extrapolation is supported by 

the well-known fact that, among older or deeply buried 

sediments, sandstones are usually more porous than 

closely associated claystones see, for example, the data 

of Proshlyakov cited by Maxwell (1964, p. 698).

The effects of temperature on the porosity of quartzose sandstones are best shown in the laboratory experiments and, to a lesser extent, in the analysis of field 

data by Maxwell (1960, 1964). Two main effects were 

observed in the laboratory experiments. Increasing temperatures from 20° to 235° C and from 270° to 345° C 

increased the compaction of quartz sands under a fixed 

pressure of about 2,000 kg per cm2 roughly equivalent 

 to a depth of burial of 26,500 feet. The greater temperatures apparently decreased the strength of the quartz 

grains so that they failed mechanically under pressure. 

Furthermore, temperatures greater than 270° C seem 

to enhance the solution of silica and its reprecipitation 

as interparticle cement.

The reduction of porosity in sandstones seems to be 

a time-dependent process. In Maxwell's laboratory studies (1960), the longer the experimental conditions of 

temperature and pressure were maintained (up to 100 

days), the greater the observed decrease in porosity. 

Observations of progressively smaller porosities in successively older natural sediments by Maxwell (1964) 

and McCulloh (1964) supported the laboratory results 

and led Maxwell to suggest (1964, p. 708) "that compaction will continue so long as porosity exists; that is, 

there is no suggestion of an equilibrium porosity at a 

given depth which would persist throughout geologic 

time."

SUMMARY OF PETROLOGY OF SEDIMENTS IN AREAS 

OF LAND SUBSIDENCE

The petrologic features of the sediments whose compaction accounts for the observed land subsidence in 

central California were described in some detail in the
COMPACTION OF SEDIMENTS UNDERLYING AREAS OF LAND SUBSIDENCE IN CENTRAL CALIFORNIA D15

preceding chapter in this series (Meade, 1967). Special 

emphasis was given to the features that influence the 

compaction behavior of the sediments. A short summary 

is given here as general 'background for the studies that 

follow. Selected petrologic features are plotted against 

depth in figure 10.

The sediments included in the studies that follow 

were collected from eight cored sections whose locations 

are shown in figure 9. Four of the cores Oro Loma, 

Mendota, Cantua Creek, and Huron were taken in the 

Los Banos-Kettleman City area on the west side of the 

San Joaquin Valley. Two cores Pixley and Richgrove were taken in the Tulare-Wasco area on the east 

side of the San Joaquin Valley. Two cores Sunnyvale 

and San Jose were taken in the Santa Clara Valley. 

Details of the coring procedure, core recovery, and sampling are given in an earlier chapter (Meade, 1967, 

p. 2-3,48).

The source terranes of the sediments, identified by 

the distinctive assemblages of minerals and rock fragments that comprise the sand and gravel fractions, are 

represented by letter symbols in the composite logs in 

figure 10. The sediments in the Sunnyvale and San Jose 

cores in the Santa Clara Valley were derived entirely 

from the Coast Ranges specifically, the Diablo and 

Santa Cruz ranges that flank the valley on the northeast 

and southwest. The sediments in the Pixley and Richgrove cores in the Tulare-Wasco area were derived entirely from the Sierra Nevada. The sediments in the

FIGURE 9. Locations of core holes In central California.

Oro Loma, Mendota, Cantua Creek, and Huron cores 

in the Los Banos-Kettleman City area were derived 

from both the Sierra Nevada and the Coast Ranges.

The sediments are mainly alluvial, having been deposited either on alluvial fans or on the flood plains of 

perennial streams. This is shown in the second column 

of each composite log in figure 10. Other types of deposits, subsidiary in their abundance, are the lacustrine 

sediments cored in five of the six holes shown in the 

San Joaquin Valley, the shallow-marine sediments 

below about 760 feet in the Richgrove core, and the 

deltaic sediments below 1,800 feet in the Huron core.

Particle sizes are represented indirectly by the electrical resistivity of the sediments and numerically by 

the median diameter. Resistivity, in the third column 

of each composite in figure 10, increases to the right; 

the greater resistivities are characteristic of the coarser 

sediments. Median diameter, in microns on a logarithmic scale, is plotted in the fourth column. The particle 

sizes are diverse, ranging from fine clay to gravel. The 

geometric mean size is probably in the coarse-silt range, 

or between 30 and 60 microns. The general degree of 

sorting is fair to poor; the average quartile deviation 

( QD $) is about 2.0. The general degree of skewness of 

the particle-size distributions indicates (1) that the sizes 

are not distributed lognormally and (2) that the sediments contain disproportionately large admixtures of 

finer particles.

Variations in the degree of rounding of the sand-sized 

particles are not extreme, ranging from subangular in

<4U.ai.iiZi gia.ins IAJ auuiuu.iiu.eu. 111 tut; »uj.i*;r IUU.B.

fragments. 

The proportions of mica flakes in the sediments reflect 

the source terranes.

Source of sediments Percent of mica

Relative size 

of mica Hakes 

to associated 

nonmica 

particles

Los Banos-Kettleman City area

[Cores: Oro Loma, Mendota, Cantua Creek, Huron]

Sierra Nevada.______ ___ 2to5,max 10 __ . _____ __ Same or 

larger. 

Same or 

smaller.

Tulare-Wasco area

[Cores: Pixley, Richgrove]

__________ 2 to 5, max 10 ____ ____ Same.

Santa Clara Valley

[Cores: Sunnyvale, San Jose]
D16 MECHANICS OF AQUIFER SYSTEMS

EXPLANATION

500'

1000'

500'

1000' -

1500'

500'

1000'

1500'

2000'

-L500' -

1000' -

1500' -

2000' - i

Other 

clays

NO

PARTICLE 

SIZE 

DATA

ORO LOMA 

1 8 62 500

Ca 

& 

Mg

C

-

»: 

s

IM:

c

A

-F-

 

F

T~

.

"*.. ' 

 '"V':'

J,:

 .V**"'

i * i P.* . i

1 8 62 500

CANTUA CREEK 

1 8 62500

C

-MC

-

JL

s

A

T

A

S

o

i \

., *

V-v.

 ** s

* ' V

,*r 

N %

i i

HURON

NO 

VOID 

RATIO 

DATA

0.5 1

0.5 1

|Source terrane Type of deposit

1 te.

diameter, Median 

in microns CO

>> S a 5

o.E

E

Exchangeable 

cations

.2 

IS

 c

I

1 8 62500

500'

PIXLEY

1 8 62 4000

500'

1000'

1500'

2000'

RICHGROVE 

1 8 62 500

500'

1000'

Ca 

& 

Mg

Na

SUNNYVALE 

1 8 62 4000

500'

1000'

Ca 

& 

Mg

SAN JOSE

0.5 1 1.5

0.5 1 1.5

0.5

1 i.V

0.5 1

FIGURE 10. Composite logs of petrologic characteristics of sediments cored in areas of land subsidence in central California. Key to 

composites in upper right corner. Source terranes indicated by C(Coast Ranges), S (Sierra Nevada) and M (mixed Coast Range and 

Sierra Nevada). Types of deposits indicated by A (alluvial fan), P (flood plain), and L (lacustrine). Locations of core holes shown in 

figure 9.
COMPACTION (OF SEDIMENTS UNDERLYING AREAS OF LAND SUBSIDENCE IN CENTRAL CALIFORNIA D17

More mica in larger flakes is present in the sediments 

derived from the Sierra Nevada.

Montmorillonite is consistently the dominant clay 

mineral, as shown by the shaded area of the central 

column of each composite in figure 10. It comprises 60 

to 80 percent of the clay-mineral assemblages and 5 to 

25 percent of the different sections of sediments represented in the several cores. Vermiculite is included with 

montmorillonite in the log for the Pixley core. Other 

common but subsidiary clay minerals in the sediments, 

included in "other clays" in figure 10, are illite, chlorite, 

a kaolinite-type mineral, and mixed-layer illitemontmorillonite.

Exchangeable cations, adsorbed largely by the montmorillonite, are represented in figure 10 in terms of the 

proportion of sodium (and hydrogen, in the Richgrove 

core) relative to calcium and magnesium. Calcium is the 

most abundant of the exchangeable cations. The proportion of adsorbed sodium increases downward in most 

of the cores, probably reflecting the corresponding 

downward increase in the proportion of sodium dissolved in the associated interstitial waters.

Although pore volume was not treated in detail in the 

preceding chapter in this series (Meade, 1967), it is included in figure 10 to show its spatial variations relative to variations in the petrologic characteristics. Expressed as void ratio, it is plotted in the right column 

of each composite. The largest void ratios are found 

in some of the lacustrine sediments of the Mendota and 

Pixley cores and in the shallow-marine sediments of the 

Richgrove core. No decrease in void ratio with increasing depth of burial is obvious. The most apparent relation is with particle size: the graphs of void ratio for 

several cores especially Mendota, Richgrove, and Sunnyvale are distorted mirror images of the graphs of 

median diameter.

ANALYSIS OF FACTORS INFLUENCING 

COMPACTION OF THE SEDIMENTS

This section of the report is an assessment of the 

influences of selected factors on the pore volume of 

the sediments whose compaction accounts for the land 

subsidence observed in central California. It consists 

mainly of two progressive multiple-regression analyses of successive factors or groups of factors. The first 

analysis explores the influence of overburden load, 

particle size, and particle sorting in a large number of 

samples in the following general steps: (1) evaluation 

of effects of overburden load, (2) relation of particle 

size to the variation in void ratio that is "unexplained" 

by load, (3) evaluation of combined effects of load and 

size, and (4) relation of particle sorting to the variation in void ratio that is "unexplained" by the combination of load and size. The other multiple-regression analysis is an attempt to sort out the relative 

influences of overburden load, particle size, clay minerals, exchangeable cations, electrolyte concentration, 

pH, fabric, and diatom content in a small group of 

fine-grained samples by successive eliminations of the 

less significant factors.

In addition, the influence of montmorillonite is assessed qualitatively without resort to statistical techniques. Judging from experimental results such as the 

ones shown in figure W (others are shown in Meade, 

1964, fig. 4) the large proportions of montmorillonite 

must exert a strong influence on the compaction behavior of these sediments. This influence cannot be 

demonstrated by statistical analysis because the montmorillonite content within the sediments does not vary 

sufficiently to cause any discernible variations in void 

ratio. The abundance of montmorillonite, however, 

probably contributes to the compaction behavior in 

several ways. The void ratios are probably larger and 

the sediments are probably more sensitive to changes in 

load than they would be if less montmorillonite were 

present. And the large exchange capacity and surface 

area of montmorillonite certainly contribute to the discernible effects of the different exchangeable cations on 

the void ratio shown in this report.

Three progress reports of this work have been published (Meade, 1961b, 1963a, 1963b). The approach used 

in the earliest of these involving computations of the 

amounts of adsorbed water and free pore water in 

clayey sediments in the Cantua Creek core may not 

be valid. Two of the assumptions that were made, concerning the thickness of the layers of adsorbed water 

and the uniformity of interparticle distances, are questionable enough to throw serious doubts on that approach to the removal of water. In any event, all three 

earlier papers are superseded by this report.

USE OF MULTIPLE-REGRESSION STATISTICS 

FOB ANALYSIS

The statistical techniques of multiple-regression analysis are the principal tools used in this section of the 

study. These techniques have been used previously and 

successfully by Griffiths (1964) and his students to sort 

out the relative influences of different petrologic factors 

on the porosity of Paleozoic sandstones. Although this 

is a promising approach, especially because the necessary arduous computations can be done quickly on a 

digital computer, it involves some difficulties. In the 

first place, one must be able to give numerical values 

to each factor. For some factors, such as effective overburden load, one can use numerical measures that describe the factor fairly completely. For others, one
D18 MECHANICS OF AQUIFER (SYSTEMS

must use generalized index measures that do not describe the factor completely: the average size and sorting, for instance, do not characterize entirely the nonnormal distribution of particle sizes in many natural 

sediments.

Another difficulty in using the techniques of multiple 

regression is that one must either find out or assume 

the nature of the numerical relations between the measure of pore volume and the measures of the influencing factors. That is, one needs to know whether the 

relations are linear or nonlinear. As these relations are 

virtually unknown, much of this chapter is concerned 

with attempts to discern their nature. This is a first 

step in applying numerical statistical techniques to the 

study of pore volume.

For selected groups of samples, the following statistics were computed on the U.S. Geological Survey's 

Burroughs 220 computer:

1. Correlation coefficients simple, multiple, and partial using void ratio as the dependent variable.

2. Probability of significance (Student's t test) of correlation coefficients.

3. Regression equation of the straight line that best 

represents the observed relation between void ratio 

and the other variables.

4. Residuals (arithmetic differences) between the 

measured values of the void ratio and the values 

predicted from the regression equation.

5. Standard error of estimate of the void ratio.

The simple correlation coefficient is a measure of the 

degree of linear correlation between any two variables; 

the multiple correlation coefficient measures the linear 

correlation between the dependent variable (void ratio 

in this study) and a combination of independent variables. When correlation is perfect, the coefficient (r) 

equals ±1 (the sign is a convention to indicate whether 

correlation is direct or inverse); when correlation is 

poor or absent, r values fall near zero.

The significance test yields a probability (P) that the 

observed correlation is not due to chance alone. For 

example, when P equals 0.95, chances are 19 to 1 that 

the observed correlation is real rather than merely 

fortuitous.

The standard error of estimate, expressed in units of 

the void ratio, is a measure of the deviation of the observed void ratios from the regression line. Approximately two-thirds of the observed void ratios should 

fall within one standard error of the values predicted 

from the regression equation. Approximately 19 observed values in 20 should fall within 2 standard errors 

of the predicted values.

Several assumptions, in addition to the assumption 

that the numerical relations between void ratio and the

independent variables are linear, are involved when 

probability statements, such as those in the preceding 

paragraphs, are made on the basis of the computed statistics. None of the assumptions are entirely satisfied 

by the data used in this study. The correlation coefficient 

is computed with the assumption that the sample errors 

are random and normally distributed. Significance tests 

concerning the equation of the regression line are based 

on the assumption that the distribution of the values of 

the void ratio that correspond to any single value of the 

independent variable follows the normal distribution 

law. The use of the standard error of estimate as in the 

preceding paragraph adds the additional assumption 

that the distribution of void ratio with respect to the independent variable is constant for all values of the independent variable. Although their underlying assumptions are not fulfilled entirely, these statistical measures 

and methods are used with the understanding that their 

interpretation involves more risk than is indicated by 

the statistics themselves.

The statistical terms used in this paper are defined 

in the glossary. For simple and nontheoretical discussions of correlation and regression, see Ezekiel and Fox 

(1959), Fisher (1950), and Williams (1959).

EFFECT OF OVERBURDEN LOAD

The relations between pore volume and overburden 

load are examined in selected groups of alluvial sediments, including 135 samples from the Mendota, Cantua 

Creek, and Huron cores of the Los Banos-Kettleman 

City area and 23 samples from the Sunnyvale and San 

Jose cores of the Santa Clara Valley. The alluvial sediments are singled out for study because they are more 

abundant than the other types. No distinction is made 

between alluvial-fan and flood-plain deposits. The clays 

and clayey silts included among the analyzed samples 

are only those whose textures suggest that they were 

deposited by moving water (Meade, 1967, p. 7). 

Lacustrine sediments and other sediments whose finely 

laminated textures suggest that they were deposited in 

standing water (that is, some of the fine flood-plain 

deposits) are not included in the statistical study, nor 

are samples included that represent heterogeneous or 

cemented alluvial sediments. Clean, well-sorted sands 

are also excluded because they are the most likely to have 

been disrupted during the coring process; only those 

sands containing 25 percent or more of silt and clay are 

included. All sediments included in the analysis were 

collected from below the water table. No sediments from 

the Tulare-Wasco area are included in the analysis 

the range of effective load between the water table and 

the bottom of the alluvial-fan sediments in the Pixley
COMPACTION OF SEDIMENTS UNDERLYING AREAS OF LAND SUBSIDENCE IN CENTRAL CALIFORNIA D19

and Eichgrove cores (roughly 16 to 27 kg per cm2 ) is 

too small for the effects of load to be clearly visible.

Void ratio and effective overburden load are the dependent and independent variables in this analysis. 

Depth of burial is not used as a variable because the 

presence of artesian pressures in the sediments causes 

the relation between depth and load to be irregular. I 

computed the void ratios from the porosity measurements that were made on these sediments by Johnson, 

Moston, and Morris (1967). Effective overburden loads 

were computed by subtracting the fluid pressures in the 

sands and gravels, as measured at the time that the sediments were cored, from the total load (bulk weight) of 

the overlying sediments. R. E. Miller computed the effective loads in the Los Banos-Kettleman City area; J. 

H. Green computed those in the Santa Clara Valley. 

Computed effective overburden loads are listed in "Appendix B." The tabulated data represent loads that were 

computed at discrete points in the sections, based on the 

physical properties of the sediments and water-level 

conditions. The effective overburden loads at intervening 

depths, which were used in the statistical analysis, were 

estimated by linear interpolation between the data 

points listed in the table.

The effective (grain-to-grain) loads computed for the 

sands and gravels, however, probably do not represent 

the effective loads in the adjacent silts and clays because 

of a time lag in the adjustment of the finer sediments 

to the rapid declines in artesian pressure. The artesian 

head in the sands and gravels has been falling at rates 

as great as 15 feet per year because of the excessive 

pumping of confined ground water. The less permeable 

silts and clays cannot equilibrate immediately to such a 

rapid change. Residual excess pore pressures remain in 

the finer sediments and decay at rates that are controlled 

by sediment thickness and permeability. These excess 

pore pressures cause the void ratios of the silts and clays 

to be larger than the void ratios that represent a state 

of equilibrium with the effective overburden pressures 

in the adjacent sands and gravels.

Another effect that may limit the use of the void-ratio 

measurements is the expansive rebound that probably 

takes place when the overburden load is removed from 

the sediments as they are cored (Poland, 1963) MThe actual amount of rebound involved, however, is not easily 

determined. For laboratory tests made of compression 

and rebound of 48 of the fine clayey sediments from the 

San Joaquin and Santa Clara Valleys, rebound from the 

simulated field effective load to the unloaded condition 

was accompanied by an average increase in void ratio 

of about 0.1 (from 0.56 to 0.67). This rebound, however, 

took place in the presence of excess water over rather 

long periods of time. Saturated clays must imbibe water

in order to swell, and the silts and clays whose void 

ratios are used in this study were not exposed to excess 

water long enough to swell to their full rebound capacity 

during the coring process. I suspect, therefore, that the 

increases in void ratio related to rebound experienced by 

these samples are considerably less than the 0.1 observed 

in the laboratory compression and rebound tests.

The relation between void ratio and the logarithm of 

effective overburden load is assumed to be linear. This 

assumption is based on the data of Hedberg (1936, p. 

256, 262) and Storer (1959, p. 520-523), which are 

summarized together by Engelhardt (1960, p. 39-42). 

These data suggest that the relation between void ratio 

and the logarithm of depth is linear in the depth range 

between 1,500 and 10,000 feet. Using these data to support the assumption of a linear relation between void 

ratio and the logarithm of effective load in the sediments 

of the San Joaquin and Santa Clara Valleys involves 

two further assumptions: (1) that the relation between 

depth and effective overburden load in the sediments 

studied by Hedberg and Storer is linear and (2) that 

the relation between void ratio and the logarithm of 

depth is also linear at depths shallower than 1,500 feet.

Relations between void ratio and the logarithm of 

effective overburden load in the selected groups of 

alluvial sediments are shown in figure 11. The sediments 

from the Los Banos-Kettleman City area are segregated 

into four groups (A through D, fig. 11) by particle 

size in order to minimize the effects of the strong correlation between size and void ratio that is also found 

in these sediments (fig. 13). The 23 sediments from the 

Santa Clara Valley were treated in one group (E, fig. 

11). Although the correlation between size and void 

ratio in this group is also strong (fig. 13), there are too 

few suitable samples from the Santa Clara Valley to 

permit any further segregation by particle size. Most 

of the sediments that were sampled in the Santa Clara 

Valley are either heterogeneously bedded or too full of 

calcite cement to be included in the analysis of pore 

volume. The solid lines through the scatter diagrams 

are the regression lines that best represent the observed 

change in void ratio with overburden load. The dashed 

lines represent approximately the 95-percent confidence 

limits: 19 void ratio observations in 20 should fall within 

these limits. Details of the regressions illustrated in 

figure 11 are listed in the upper part of table 2.

The conspicuous gap in the plots for groups A through 

D, between loads of 25 and 35 kg per cm2, shows the 

manmade change in effective load across the principal 

layer of fine sediment (the lacustrine Corcoran Clay 

Member of the Tulare Formation) that confines the 

artesian aquifer system in the San Joaquin Valley. 

Samples that plot to the left of the gap are from the
D20 MECHANICS OF AQUIFER SYSTEMS

1.0

0.5

50

40

30

10 20 40 80

Clay

GROUP A 

SILTY SANDS

Sand Silt

1.0

0.5

10 20 40 80

Clay

GROUP B 

SANDY SILTS

Sand Silt

1.0

O 0.5

10 20 40

50

40

30

80

Clay

GROUP C 

CLAYEY SILTS

Sand Silt

1.0

0.5

10 20

i i i i i

50

40

30

40 80

Clay

GROUP D 

SILTY CLAYS

Sand Silt

1.0 r0.5

-I 50

40

30

i I I I i I i

10 20 40

Clay

GROUP E 

SILTS AND CLAYS

Sand Silt

EFFECTIVE OVERBURDEN

LOAD, IN KILOGRAMS 

PER SQUARE CENTIMETER

FIGURE 11. Simple relations between void ratio and effective overburden load in fresh-water-bearing alluvial 

sediments. Groups A through D from Mendota, Cantua Creek, and Huron cores from Los Banos-Kettleman 

City area; group E from Sunnyvale and San Jose cores from Santa Clara Valley. Note different range of 

effective load for group B. Particle sizes in each group are represented by triangular sand-silt-clay diagrams, 

which are subdivided according to the system proposed by Shepard (1954).
COMPACTION OF SEDIMENTS UNDERLYING AREAS OF LAND SUBSIDENCE IN CENTRAL CALIFORNIA D21

semiconfined aquifer system above the confining layer; 

those to the right are from the confined artesian system. 

The gap represents the increase in effective overburden 

load caused by excessive pumping of the artesian water.

The correlation between void ratio and load in the 

silty sands (group A) is not entirely as represented in 

figure 11. The void ratio also correlates significantly 

with QDj (see bottom of table 2 and figure 16), and 

the correlation between load and QD^ may be significant (r=0.29, P=0.92). That is, the sorting becomes 

progressively poorer ( QD^ increases) at greater depths, 

and this accounts for part of the apparent decrease in 

pore volume with increasing load. The line representing the actual change of void ratio under increasing 

load, therefore, should be less steep than the regression 

line in figure 11.

The silts and clays of groups B, C, and E show the 

most unequivocally significant relations between void 

ratio and load. The slopes of the regression lines in 

groups B and C should represent the relation between 

pore volume and load fairly clearly, as no simple correlation was observed between load and the particlesize variables in these groups. Note, in table 2, that the 

equations for the regression of void ratio on load are 

virtually identical for groups B and C.

In the silty clays of group D, the decrease in void 

ratio with increasing load is only fairly significant 

(/> =0.88). The wide scatter of samples in this group 

may be related to variability in several other factors. 

Some sediments in this group, for example, contain 

aggregates of fine silt and oriented clay particles surrounded by a poorly sorted and disoriented sand-siltclay matrix (Meade, 1967, p. 8); whereas other clays 

in this group contain few of these aggregates or none 

at all. Considering that the geometry of the pore space 

within the aggregates may be different from that of 

the pore space in the rest of the sediments, the variable 

proportions of aggregates may cause some of the wide 

scatter of points in group D. Another possible cause of 

the scatter is the influence of variations in particle size: 

The correlation between void ratio and median diameter 

(Mdt) within group D is probably significant (P= 

0.96). Still another cause might be differences in the 

elastic rebound of the clays between the time that 

they were cored and the time that their pore volume 

was measured in the laboratory, although, as stated 

previously, I doubt that this is a strong enough effect to 

account for the wide scatter in group D. And finally, 

considering also that the rate of decrease in the void 

ratio in group D is anomalously low, the scatter of

TABLE 2. Details of significant regressions of void ratio on effective overburden load, median particle diameter, and quartUe deviation of

particle-size distribution

[e=void ratio, log i=logarithm (base 10) of effective overburden load (in kg per cm2 ), Af<f0=median particle diameter (<A units, see Inman, 1952, p. 133), QZ»«=quartile

deviation (particle sorting, <i> units, see Krumbein, 1936, p. 102-103)]

Group

Designation in

figures

CorrelaNumber tion coefof samples ficient (r)Probability of

significance

of correlation

coefficient (P)

Standard error

Regression equation of estimate

of void ratio

Simple regressions of void ratio on effective overburden load

Los Banos-Kettleman City__ ____

Silty sand __ ___________ __

Sandy silt- _ _ _ ____ ____

Clayey silt__ __ __ _ ________

Silty clay _ __ _ __

Santa Clara___________ ______ __

___________ A-D

___________ A

.____-____. B

__________ C

___________ D

__________ E

135

39

28

40

28

23

-0.48

-.40

-.68

-.62

-.30

-.63

>0.99

>.99

>.99

>.99

.88

>.99

e=1.15-0. 28 log L

e=1.00-0. 23 log L

e=1.33-0. 39 log L

e=1.35-0. 40 log L

e=1.00-0. 15 log L

e= 0.90-0. 21 log L

0.11

.12

.07

.08

.12

.08

Simple regressions of void ratio on median particle diameter

Los Banos-Kettleman City___

Santa Clara.. ___ _ _ ____

Tulare-Wasco __ __ _______________ _.

_________ A-D

_________ E

F

135

23

48

0.38

.60

.52

>0. 99

>.99

>.99

e=0.55+0.024 Md*

e=0.33+0.048 Md+

6=0.37 + 0.054 Md+

0.11

.08

. 10

Multiple regressions of void ratio on effective overburden load and average particle diameter

Los Banos-Kettleman City __ _ _ __ __

Santa Clara.______ _ ____ _ _

_________ A-D

E

135

23

0.58

.78

>0.99

>.99

e =0.99 -0.26 log L

L + 0.020 Md<, 

e= 0.61 -0.18 log L

+ 0.038 Md*

0.10

.06

Simple regression of void ratio on quartile deviation

Silty sand. 39 -0.44 >0. 99 6=0.81-0.099 QD* .11
D22 MECHANICS OF AQUIFER SYSTEMS

points above the regression line may show the effect of 

residual excess pore pressures in the thicker silty clays 

of low permeability.

EFFECT OF PARTICLE SIZE

The decrease in pore volume that corresponds to increasing particle size is studied in 3 groups of alluvial 

sediments: those included in groups A through D (fig. 

11) from the Los Banos-Kettleman City area, those in 

group E from the Santa Clara Valley, and a group of 

48 alluvial-fan sediments from the 2 cores of the 

Tulare-Wasco area. These 48 samples were all taken 

from below the water table; excluded from the group 

are sediments that contain calcite cement or conspicuous 

soil cavities of the types shown in the earlier chapter 

(Meade,1967,fig.l8).

Void ratio and median diameter are the dependent 

and independent variables used in this part of the analysis of pore volume. The median diameters are taken 

from particle-size analyses that were made in the Hydrologic Laboratory of the Geological Survey. They 

are expressed logarithmically in <f> units, wherein <f> is 

the negative logarithm to the base two of the diameter 

in millimeters. (See Inman, 1952, for further explanation of the <f> notation.) Note that a numerical increase 

in Md$ corresponds to a decrease in median diameter.

Whether the numercial relation between void ratio 

and Md$ is linear is uncertain. From most of the studies 

that are summarized in figures 1A and 4, one might expect the relation to be nonlinear. Those studies, however, were done on sediments that were under overburden loads less than 1 kg per cm2 . The large initial void 

ratios of sediments should be expected to be reduced 

rapidly under increasing overburden loads to values 

on the order of 1.5 or less (as shown in fig. 11). The 

question then becomes: Would such a reduction of void 

ratio involve a transformation of the numerical relation between void ratio and Md^ from a nonlinear 

relation to one that is essentially linear within the sandsilt-clay size range?

Considering that the fresh-water-bearing alluvial 

sediments of the San Joaquin and Santa Clara Valleys 

are all under significant overburden loads, one might 

suppose that the relations between void ratio and Md$ 

could be linear. This supposition is tested, but not conclusively, by plotting the residuals of the void ratioload equations against Md$ in 2 groups of sediments, 

namely the 135 samples of .groups A through D and the 

23 samples of group E (fig. 12). The residual is the 

difference expressed in void-ratio units between the 

measured value of the void ratio and the value predicted 

from the equation for the regression of void ratio on the 

effective load. (See the first and sixth equations listed

10

0.4

0.3

0.2

0.1

-0.1

-0.2

-0.3

Groups A-D

16 62 250

10 8

0.2

0.1

-0.1

-0.2

Group E

4 16

MEDIAN DIAMETER, 

IN MICRONS

62

FIGURE 12. Relations between median diameter and the 

residuals of the void ratio-load regressions (observed 

void ratio minus the void ratio predicted by the regression equation) in sediments of groups A through 

D and group E. See figure 11 and table 2 for identification of groups.
COMPACTION OF SEDIMENTS UNDERLYING AREAS OF LAND SUBSIDENCE IN CENTRAL CALIFORNIA D23

in table 2.) The residual is an indication of the variation in void ratio that is unaccounted for by variation 

in load. If its relation to Md^ is linear, then presumably 

the relation between void ratio and Md^ also should be 

linear. Unfortunately, the scatter iri the graphs of 

figure 12, reflecting the influence on void ratio of sampling error, and of factors other than load and average 

particle size, is so great that one cannot draw firm conclusions about the nature of the relation. These graphs 

do show that the pore volume decreases with increasing 

particle size. This, however, is shown more clearly in 

the simple correlations between void ratio and Md^ 

illustrated in figure 13.

Figure 13 shows the simple relations between void 

ratio and the logarithm of median diameter in 3 groups 

of sediments: groups A through D taken together, 

group E, and the 48 samples from the Pixley and Richgrove cores of the Tulare-Wasco area which are designated group F. As in figure 11, the solid and dashed 

lines represent the regression equation and the 95-percent confidence limits. Pertinent statistical details are

10

i.o

0.5

Groups A-D

I____I____I____I

16 64

- 50

250

0.5 -

32

1.0

0.5

Group F

4 16 64 

MEDIAN DIAMETER, IN MICRONS

FIGURE 13. Simple relations between void ratio and median 

particle diameter In fresh-water-bearing alluvial sediments. 

Groups A through D and E as in figure 11. Qroup F from Pixley 

and Richgrove cores of Tulare-Wasco area.

listed in table 2. In all three groups, the decrease in void 

ratio with increasing particle size (decreasing Md$ ) is 

clear. The relation between void ratio and Md^ seems to 

be approximately linear.

COMBINED EFFECTS OF OVERBURDEN LOAD AND 

PARTICLE SIZE

After treating separately the relations between pore 

volume and the two factors, overburden load and particle size, we may now consider the relations in combination with each other. This is done in two ways: (1) 

in qualitative terms of the influence of particle size on 

the relations between void ratio and load, and (2) in 

the numerical terms of multiple-regression analysis.

The regression lines from figure 11 are grouped together in figure 14 for comparison with each other 

and with similar curves derived by Skempton (1953, 

p. 55). The curves in the upper graph in figure 14 are 

segments of the more complete curves shown in figure 

IB. They represent the most complete synthesis of information of this sort that has been published so far. 

They show that, with decreasing particle size in the

1.5 -

1.0

0.5

**/.

- 60

50

40

30

10 20 40 80

1.5

1.0

0.5

50

40

30

10 20 40 80

EFFECTIVE OVERBURDEN LOAD, IN 

KILOGRAMS PER SQUARE CENTIMETER

FIGURE 14. Influence of particle size on relations between 

void ratio and effective overburden load. Upper: As 

modified from summary by Skempton (1953, p. 55). 

Lower: As observed in San Joaquin and Santa Clara 

Valleys; letters correspond to sample groups in figure 

11; dashed lines are from upper graph by Skempton.
MECHANICS OF AQUIFER SYSTEMS

silt-clay range, the void ratio becomes greater at any 

given load, and it decreases more rapidly with increasing load. The sediments from the San Joaquin and 

Santa Clara Valleys (fig. 14, lower graph), however, 

do not behave exactly as predicted from Skempton's 

curves.

Before comparing Skempton's results with those observed in California, one must understand that Skempton's size nomenclature does not coincide with the nomenclature used in this report. His nomenclature is 

based on Atterberg limits; mine is based on particle-size 

analyses and the Shepard (1954) system. A basis for 

comparison of the two terminologies is the Atterberg 

limits of the California sediments, determined and reported by Johnson, Moston, and Morris (1967). The 

Atterberg limits show that the silts of groups B and C 

probably correspond to Skempton's "silty clays," and 

that the silts and clays of groups D and E probably 

correspond to Skempton's "clays." The silty sands 

(group A) presumably correspond to Skempton's 

"silts."

Of the five groups of sediments, only those of group E 

have void ratios that might have been predicted from 

Skempton's synthesis. The void ratios of the sediments 

in groups A through D are larger than expected. Why 

this might be so is uncertain. The sediments in all five 

groups have similar clay-mineral assemblages (fig. 10) 

and appear to have been in fairly similar chemical surroundings. The greater amount of sodium adsorbed by 

the clay minerals in the sediments of the Los BanosKettleman City area may contribute to their larger 

void ratios: experimental studies by others (fig. ID) 

and evidence presented later in this chapter suggest 

strongly that the pore volume of montmorillonite-rich 

sediments is a direct function of the amount of sodium 

(versus cations of larger valence) adsorbed by the montmorillonite. However, this would not account for the 

large void ratios in the silty sands (group A) which presumably do not contain enough clay to reflect strongly 

the influence of the adsorbed cations. Another difference 

between the sediments from the two areas of California 

is the almost complete lack of mica in the Santa Clara 

Valley and its relative abundance in many of the sediments of the Los Banos-Kettleman City area. As figure 

7 shows, the presence of mica may influence the pore 

volume of sands and silts under loads as great as 10 kg 

per cm2 . Perhaps it also influences the pore volume of 

sediments under larger loads.

Whereas the slopes of the regression lines for groups 

A, B, and C correspond approximately to the slopes of 

Skempton's curves for "silts" and "silty clays", the regression line for group D is anomalous. If the analogy 

with Skempton's curves is valid, the line for group D

should be steeper than and entirely above the lines for 

groups B and C in figure 14. This anomaly could be accounted for by at least two possible factors the downward increase in the proportion of adsorbed sodium or 

the residual excess pore pressures both of which could 

cause the void ratios to be larger than anticipated in the 

artesian aquifer system below the principal confining 

layer.

The second approach to the description of the combined relation of overburden load and particle size to 

pore volume is by way of multiple-regression analysis. 

Multiple-regression statistics for 2 groups of sediments, 

namely the 135 samples from the Los Banos-Kettleman 

City area and 23 samples from the Santa Clara Valley, 

are given near the bottom of table 2. Taking the correlation coefficients (the square root of the coefficient of determination see glossary, p. D36) at face value, the 

combined influence of effective overburden load and 

average particle size accounts for 61 percent of the observed variance in void ratio in the sediments of the 

Santa Clara Valley and only 34 percent of the variance 

in void ratio observed in the Los Banos-Kettleman 

City area. This leaves approximately one-third and 

two-thirds of the variances in void ratio, respectively, 

to be accounted for by other factors.

EFFECT OF PARTICLE SORTING

Experimental studies of the influence of particle 

sorting on the pore volume of artificial sand mixtures 

(fig. 5) confirm the intuitive expectation that wellsorted sediments have larger pore volumes than poorly 

sorted sediments of the same average particle size. 

The numerical relations involved, however, are uncertain. Figure 5A shows the relation between void 

ratio and quartile deviation ( QD$ ) to be linear over a 

short range of QD# values. The nature of the relation 

over a wider range of values in natural sands, silts, and 

clays is not known.

In most of the central California sediments, simple 

correlations between void ratio and particle sorting are 

obscured by the more prominent relations of void ratio 

to overburden load and particle size. To minimize this 

difficulty, the sorting is compared to the variation in 

void ratio that is left over from or "unexplained" by the 

combined influence of load and particle size. That is, 

QD$ is graphed against the residuals (expressed in 

void-ratio units) of three regression equations: the two 

multiple-regression equations for groups A through D 

and group E and the equation for the regression of void 

ratio on median diameter for group F (table 2). Inasmuch as these equations assumed linear conditions, 

any conclusions that are drawn from the graphed relations between the residuals and QD$ involve the
COMPACTION OF SEDIMENTS UNDERLYING AREA'S OF LAND SUBSIDENCE IN CENTRAL CALIFORNIA D25

0.3

0.2

0.1

-0.1

-0.2

-0.3

Groups A-D

0.2

0.1

-0.1

Group E

0.4 r0.3

0.2

0.1

-0.1

-0.2

-0.3

Group F

123 

QUARTILE DEVIATION (QD<t>)

PIGUEB 15. Relations between quartile deviation and 

the residuals of void ratio-load-Jlfd * and void ratioMdj, regressions (observed void ratio minus the void 

ratio predicted by the regression equation) in sediments of groups A through. D, E, and F.

assumptions that the relations of void ratio to Md^ and 

the logarithm of the effective load are linear. The 

graphs in figure 15 confirm the expectation that pore 

volume decreases with decreasing degree of sorting (increasing QD^}. The graphs for groups A through D 

and group E also suggest that a sediment whose quartile deviation is l<f> might have a void ratio that is about 

0.2 larger than the void ratio of a sediment whose 

quartile deviation is 3<£.

A significant simple correlation between void ratio 

and QDj was found in the silty sands of group A (fig. 

16), apparently because the range of QD^ values in this 

group was especially large. The simple regression equation given at the bottom of table 2 and illustrated in figure 16, however, does not represent the influence of sorting alone. The degree of sorting also decreases systematically with depth, so that the simple regression coefficient 

of either factor, load or QD^ reflects some of the 

effects of the other factor.

The wide scatter in the graphs of figure 15 indicates 

that even when the relations with load, Md^ and Q,D$ 

are accounted for much of the variance hi void ratio 

remains unexplained. It reflects the influence of other 

properties of the sediments or characteristics of their 

surroundings, some of which are evaluated below.

COMBUSTED EFFECTS OF SELECTED PHYSICAL 

AND CHEMICAL FACTOBS

For several reasons, the sediments of the Bichgrove 

core provide a good opportunity to evaluate some of 

the factors other than load that might influence the 

pore volume. In the first place, the apparent relation 

of the pore volume to depth and load is anomalous: the 

pore volume increases with increasing depth (fig. 17). 

The influence of other factors is more strongly expressed 

than that of load. Secondly, the variations in some of 

the other factors the clay-mineral assemblage, adsorbed cations, and other chemical factors are much 

greater in the sediments of the Bichgrove core than in

SORTING (So)

1.0

w

i£

OZ)

CL _l 

O

1 2 3 

QUARTILE DEVIATION (QD0)

FIGURE 16. Simple relation between void ratio and 

quartile deviation in silty sands (group A) from 

Los Banos-Kettleman City area. See bottom of 

table 2 for details of regression equation.
D26 ME'CHANIOS OF AQUIFER SYSTEM'S

any of the other groups of sediments. These variations 

can be tested statistically to see if they correlate with 

variations in pore volume.

Although the downward increase in pore volume in 

the Eichgrove core can be related qualitatively and 

approximately to changes in the type of sediment and 

the particle size (fig. 17), these changes do not account 

for all the observed features of the distribution of pore

0.5

VOID RATIO

1.0 1.5

500

1000

1500

2000

Alluvial fan

 a 

 .- *

Flood plain

Shallow marine

30 40 50 60 

POROSITY, IN VOLUME PERCENT

1.5

g

K 1.0

Q

O

0.5

'_ »

_ 

h * 

0

9 

- ..* *_

 4»

-.» * * *% *

- * * 

,,,*,, ,*,,,,,"

60 u

O

cc

u

QUJ

50 1

O

z

40 >f

O

30 g

0.

10 20 40 80

EFFECTIVE OVERBURDEN

LOAD, IN KILOGRAMS 

PER SQUARE CENTIMETER

FIGURE 17. Relations of void ratio of fine sediments in 

Richgrove core (median diameters finer than 62 microns) to depth of burial, type of sedimentary deposit, 

and effective overburden load.

volume with regard to depth. The finest of the nonmarine sediments above 760 feet are generally coarser 

than the fine-grained marine siltstones below 760 feet 

(fig. 10), so one might expect the marine siltstones to 

have generally larger pore volumes than the nonmarine 

sediments. The gradualness of the increase in pore 

volume between 600 and 1,300 feet, however, compared 

with the more abrupt change in the type of sediment 

between 745 and 785 feet, suggests strongly that other 

factors are influencing the pore volume.

Tw.enty samples of the fine sediments in the Richgrove core were selected for analysis of a number of 

factors that might influence the pore volume. These 

factors are, in addition to overburden load and particle 

size:

1. Clay minerals, which are represented numerically by 

the percentage montmorillonite in the clay-mineral assemblage.

2. Exchangeable cations, which are represented numerically by the equivalent percentage of sodium in 

the exchangeable-cation assemblage.

3. Soluble salts, which are represented by the total dissolved solids leached by 500 ml (milliliters) of hot 

water from 10 g (grams) of sediment.

4. Acidity, represented by the pH of a mixture of 10 ml 

distilled water and 1 g of sediment.

5. Orientation of clay-mineral particles, represented by 

the montmorillonite orientation ratio.

The details of the procedures used in determining the 

clay minerals, exchangeable cations, soluble salts, and 

pH are given in the previous chapter (Meade, 1967, 

p. 65-72). The procedure for determining the orientation 

ratio is outlined in the next section. Also included in 

the analysis is the amount of diatom skeletal remains in 

the sediment; this amount was determined by point 

counts (300 points each) of thin sections of the sediments. Results of these analyses are listed in table 3. 

Multiple-regression techniques are used to sort out 

the important factors. All the measures that are listed 

for the 20 samples in table 3, with the exception of 

Md$, register simple correlations with void ratio that 

are significant at the 99-percent level. Furthermore, 

many of them register significant simple correlations 

with load a fact indicating that they change systematically with depth or with each other. Analyses of 

simple correlations and regressions would be fruitless, 

therefore because of the complex and perhaps fortuitous interrelation of the factors. Multiple-regression 

techniques must be used with the hope that the variables 

that seem to be the most significant numerically do in 

fact represent the most important influences on pore 

volume. I assume for convenience that the relations 

between void ratio and the numerical indicators of each
COMPACTION OF SEDIMENTS UNDERLYING AREAS OF LAND SUBSIDENCE IN CENTRAL CALIFORNIA D27

TABLE 3. Selected properties of fine sediments in Richgrove core

Sample >

59CAL319____

327_.__

333____

341____

349. ___

356. ___

363. ___

369____

381___-

385_.__

389. ___

390. _._

394____

396___-

397. __ .

401. ___

403___-

404. ___

410_-__

412___.

Depth Effective 

below overburden 

land load 2 

surface (kg per cms) 

(feet)

148

232

290

371

444

517

583

649

764

844

917

_ 1, 037

1, 156

_ 1, 240

1, 364

. 1, 447

_ 1, 527

1, 689

_ 1, 827

1, 912

7.9 

11.2 

13.2 

16.0 

18. 5 

20.9 

23. 1 

25. 0 

28.7 

30.9 

33. 1 

36.4 

39.4 

41.3 

44.0 

46. 1 

48.3 

52.0 

55.7 

57.9

Void Median Mon 

ratio s diameter 3 (pen 

(Md$) mint

0.62 

. 52 

.67 

.50 

.51 

. 55 

.63 

.71 

.98 

. 96 

.88 

1.01 

1.16 

1.45 

1. 11 

1.58 

1. 35 

1.01 

1.08 

.60

8.7 

4.6 

5.2 

4.0 

5. 0 

5. 8 

5.7 

7.2 

7.6 

6. 5 

6.6 

7.8 

6. 1 

6.6 

5.8 

6. 1 

6.2 

6.7 

7. 0 

3.6

tmorillonite 

:ent of clay- 

;ral fraction) -

71 

55 

57 

59 

52 

56 

68 

62 

70 

89 

83 

68 

80 

76 

84 

86 

83 

89 

89 

83

Adsorbed cations * '. 

(adjusted percent of so 

equivalents) in

Na

3 

3 

8 

0 

0 

0 

0 

0 

9 

10 

16 

20 

19 

25 

23 

30 

33 

26 

32 

18

Ca+Mg

97 

97 

92 

100 

100 

100 

100 

100 

91 

90 

84 

80 

81 

46 

77 

42 

20 

74 

31 

55

H

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

29 

0 

28 

47 

0 

37 

27

rotal disIved solids 

. leachate 5 

(ppm)

13 

11 

24 

9 

6 

24 

12 

21 

74 

43 

62 

94 

41 

173 

47 

286 

300 

69 

226 

98

Diatom 

Montmoril- remains 

pH * lonite orienta- (percent of 

tion ratio « sediment)

8.3 

8.7 

8.7 

8.9 

8.7 

9. 1 

8.6 

8.7 

8.4 

8.7 

8. 5 

8. 5 

8.6 

4.5 

8.7 

4.0 

4.0 

7.5 

4. 1 

5. 3

1. 1 

1.3 

1.0 

1. 1 

. 8 

.8 

1.1 

1.5 

1. 1 

1. 5 

1.8 

2.0 

1.8 

1.7 

1. 3 

1.4 

1.6 

1.6 

1.8 

1.0

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

3 

12 

3 

6 

12 

0 

0 

0

1 Numbers assigned by Hydrologic Laboratory.

2 Estimated from hydrologic data by B. E. Lofgren.

' From analyses made in Hydrologic Laboratory. 

4 Determined by H. C. Starkey.

5 Determined by Claude Huffman and A. J. Bartell.

6 Determined with assistance of J. B. Corliss.

of the factors are linear, even though no evidence is 

available to support this assumption for any of the 

factors other than load and average particle size. It 

must be understood that multiple regression is used 

here only to sort out the factors and not to derive any 

descriptive equations that can 'be applied generally to 

all sediments.

The factors are sorted out by deriving regression 

equations for several combinations of variables, noting 

the level of significance of the partial correlation coefficients in each equation, and eliminating the variables 

that seem to be the least significant. The statistics on 

which this process is based are given in table 4. All the 

"significant" partial correlations (P>0.75) represented 

in the table are positive. At first inspection of the significance levels of the partial correlation coefficients,

one may eliminate the following variables from further 

consideration: load, percentage of montmorillonite, and 

(although only evaluated once) orientation ratio. Although the evidence is not as clear, I suspect that pH 

may also be eliminated.

If the diatoms are ignored for the moment, particle 

size, absorbed sodium, and soluble salts are the remaining factors. Of these, particle size and adsorbed sodium 

seem to be the most important. Soluble salts seem to be 

of subsidiary importance: note that the large significance levels for soluble salts are registered only when 

adsorbed sodium is left out of the analysis. Judging 

from the multiple correlation coefficient of 0.89 (third 

line from the bottom in table 4), these three factors 

together may account for about 80 percent of the observed variance in void ratio (./?2 =0.79).

TABLE 4. Selected details of multiple regressions of void ratio on different combinations of variables, representing properties of selected

fine sediments in Richgrove core.

Number of 

samples

20 _____

20 ___ __

20

20 __ ____

20

20

20

15

15-_______

Number of Coefficient of en 

independent multiple corre- mt 

variables lation (R)

8 

6 

5 

4 

4 

3 

3 

3 

3 

4 

2

0.94 

.94 

.89 

.89 

.89 

.86 

.88 

.86 

.89 

.94 

.91

Standard Probability of significance (P) 1 of coefficient of partial correlation between void ratio and 

ite of void 

ratio Logio load

0. 14 (2 ) 

. 14 (2 ) 

. 18 (2) 

.17 -__-__

.17 ______

.18 ______

.17 ______

.18 ______

.16 ______

.08 _______

.10 _______

Percentage of Percentage of 

Md$ morttmoril- adsorbed Na 

lonite

0.85 <0. 75 

.96 < . 75 

. 88 (3 ) 

. 90 (3 ) 

.87 ________

.82 <.75 .

Q4-

.82 ________ _

QD

.98 _-_-_._:

.99

0.81 

.83 

.93 

.94 

.95

>.99

.95 

.96

Logio total 

dissolved 

solids

<0.75 

<.75 

<.75 

<.75 

<.75 

>.99

~~~>.~99 

<.75 

.85 

>.99

Montmoril- Percentage of 

pH lonite orienta- diatoms 

tion ratio

<0. 75 <0. 75

<.75 -____-__

<.75 ________ 

<.75 ________

.91 ________

>0. 99 

.99

1 One-sided significance test used for logio load, Md$, percentage of montmorillonite, 

and percentage of adsorbed sodium; two-sided test used for other variables. (See Dixon 

and Massey, 1957, p. 97-99.)

2 Positive (and therefore insignificant) partial correlation with void ratio.

3 Negative (and therefore insignificant) partial correlation with void ratio.
D28 MECHANICS OF AQUIFE.R SYSTEMS

The influence of the diatoms cannot be evaluated 

properly by these methods because only five samples 

contain diatoms. The fact, however, that these five 

samples have the five largest void ratios suggests 

strongly that the diatom skeletons contribute significantly to the pore volume. This suggestion is supported 

by multiple-regression analyses of the 15 diatom-free 

samples (bottom two lines, table 4), in which the removal of the diatomaceous samples seems to improve 

the degree of multiple correlation with the other variables and lessen significantly the standard error of 

estimate of the void ratio.

The specific effect of the diatom remains in the preservation of pore volume under load is uncertain. Perhaps 

the large open spaces in the skeletons themselves account for the larger void ratios. Or perhaps, as suggested by Hamilton (1964, p. 4263), the diatom 

skeletons contributed material to an incipient lithification process that kept the pore space open under load. 

That is, perhaps a little of the silica in the skeletons was 

dissolved and later reprecipitated at the contacts between sediment particles. Perhaps this slight reprecipitation of silica cemented the particles together but 

preserved the greater pore volume that was characteristic of a shallower depth of burial. No petrographic 

evidence of this process could be seen in thin sections of 

the siltstones; but one should not expect to be able to see 

such evidence because of the difficulty of resolving 

minute amounts of cementing material at contacts between fine particles. Hamilton's suggestion does seem 

paradoxical, however, in that it calls upon cementation, 

a process that eventually fills pore space, to account for 

anomalously large pore volumes.

The conclusion in any event is that particle size, the 

proportion of sodium adsorbed by the clays, and the 

amount of diatom skeletal material are the main factors 

influencing the pore volume of this group of sediments. 

This conclusion is supported by the studies of the 

sediments in the other cores. The influence of particle 

size is confirmed by the statistical studies summarized 

in figures 12 and 13. The influence of the downward 

increase in adsorbed sodium was suggested as a possible 

cause of the anomalously low slope of the void ratioload curve for group D (p. D24). And the influence of 

diatom skeletons is suggested by the large void ratios 

of the upper lacustrine layer in the Mendota core (the 

Corcoran Clay Member, at depths between 600 and 

700 feet see fig. 10), which also contains large proportions of diatom skeletons. But one must keep in mind 

that the variables are interdependent: the montmorillonite content, pH, and soluble-salt concentration, although they seem to be insignificant or only marginally 

significant, probably have an indirect influence on the

void ratio through their influence on the amount of 

adsorbed sodium. Although variations in the amount 

of montmorillonite do not seem to correspond to variations in void ratio, the very presence of montmorillonite 

with its large cation-exchange capacity probably makes 

the influence of the exchangeable sodium large enough 

to be discernible. Furthermore, the proportion of sodium adsorbed by the montmorillonite (see Meade, 

1967, fig. 26) is closely related to the concentration of 

soluble salts and to pH.

FABRIC OF THE SEDIMENTS AND ITS RELATION 

TO OVERBURDEN LOAD AND OTHER FACTORS

ORIENTATION OF CLAY-MINERAL PARTICLES

The fabric of the fine sediments was studied with the 

aim of discovering any systematic changes that might 

be related to an increase in overburden load or to 

variations in other factors. The results showed no variation in fabric that could be related unequivocally to 

variation in load. The fabric does seem to vary significantly, however, between the different types of deposits. 

The best developed preferred orientation was found in 

lacustrine deposits; whereas little or no preferred orientation was found in alluvial-fan deposits, regardless of 

the depth of burial below the land surface.

OBSERVATION AND MEASUREMENT

The orientation of clay-mineral particles was observed 

in thin sections and was measured by a method involving the use of X-ray diffraction. The thin sections were 

prepared from samples of clayey sediments (whose natural moisture had been sealed in at the coring site) that 

had been impregnated in the laboratory with a waxlike 

polyethylene glycol compound according to the method 

described by Tourtelot (1961).

The X-ray diffraction method for measuring preferred orientation, described in detail in an earlier 

paper (Meade, 1961a), involves a numerical comparison 

between the intensities of the basal (001) reflection and 

the nonbasal (020) reflection from montmorillonite. In 

the diffraction patterns from each of several sections 

of air-dry clay, the intensities of these reflections are 

measured and recorded. From these intensities a peakheight ratio, which is the height of the (001) trace on the 

diffractometer chart divided by the height of the (020) 

trace, is computed. Peak-height ratios are measured in 

at least three mutually perpendicular sections one 

parallel, and the other two perpendicular to the bedding 

direction. (In some samples, six sections were scanned 

the three mutually perpendicular sections plus sections 

at angles of 22°, 45°, and 67° to the bedding direction.) 

The peak-height ratios are combined into another ratio 

which is the quotient of the peak-height ratio of the
COMPACTION OF SEDIMENTS UNDERLYING AREAS OF LAND SUBSIDENCE IN CENTRAL CALIFORNIA D20

bedding-direction section divided by an average peakheight ratio for the two sections perpendicular to the 

bedding. This quotient is called the orientation ratio. 

Values of the orientation ratio near 1.0 signify random 

orientation of montmorillonite particles with regard 

to the bedding direction. Progressively larger values 

signify progressively greater degrees of orientation 

parallel to the bedding.

The general range of orientation ratios or their equivalents measured by X-ray diffraction in sediments and 

sedimentary rocks is shown in the following listing.

Approximate range of 

orientation ratio

Devonian shales, Alberta *__________________________ 6-13

Pennsylvanian clays and claystones, Illinois: 2

Black shales____________________________ 10-20

Gray shales.__________________________________ 6-10

Underclays__________________________________ 1- 3

Cretaceous mudstones, Texas 3 ____________-___-____- 2-30

Cretaceous shales, Alberta and Saskatchewan 1 ________ 2-5

Pliocene to Recent sediments, California:

Lacustrine clays.______________________________ 1. 5- 3

Marine silts and siltstones______________________ 1. 5- 2

Flood-plain clays______________________________ 1- 2

Alluvial-fan clays._____________________________ «1

 Computed from data of Kaarsberg, 1959, p. 469-470.

2 Data from O'Brien, 1963, p. 12-15.

3 Data from Beall, 1964, p. 154-158.

The main advantage of the X-ray diffraction method 

is that it provides a numerical expression of orientation that can be used to compare the different amounts 

of orientation in different samples. The main disadvantages are (1) that it measures the orientation only 

with regard to a single arbitrarily selected direction 

and (2) that it measures only the bulk orientation over 

an area of 3 to 4 square centimeters and does not resolve 

small-scale orientations.

Montmorillonite, the most abundant of the clay minerals in these sediments, is the mineral whose reflections 

were used in the study of orientation. The detailed study 

of orientation was confined to the sediments of the San 

Joaquin Valley. The sediments of the Santa Clara 

Valley were not included because they contain a chlorite 

mineral whose (001) reflection interferes with the (001) 

reflection of montmorillonite and obscures the relation 

between the intensity of the reflection and the orientation of montmorillonite. The degree of orientation observed in thin sections (in the samples for which sections were available) generally confirmed the amount 

of orientation that was measured by the X-ray method.

RELATION TO DEPTH OF BTTRIAL AND TYPE OF 

SEDIMENTARY DEPOSIT

The orientation ratios of fine sediments in the four 

cores from the Los Banos-Kettleman City area are 

plotted against depth in figure 18. Thirty-five of these

measurements were presented and discussed in an earlier paper (Meade, 1961b). Figure 18 contains another 

17 measurements, in sediments from the Cantua Creek 

and Huron cores, that were made since the earlier results were published. All samples were taken from sediments that had been below the water table and had 

presumably been saturated with water in their natural 

states. In most of the samples, sand and silt appeared 

to be "suspended" in clay. That is, the coarser particles 

did not touch one another, and the clay fraction must 

have borne any load that was placed on the sediment.

Ignoring the types of deposits for the moment, the 

results plotted in figure 18 give little indication of progressive development of preferred orientation with increasing depth of burial. The range between 0.9 and 1.2 

of most of the orientation ratios indicates that very 

little orientation of montmorillonite developed parallel 

to the bedding either before or during compaction.

Comparison of the orientation ratios to the type of 

deposit in which they were measured sheds more light 

on the observed variations in orientation. The best developed preferred orientation is found in the lacustrine 

deposits in the upper parts of the Oro Loma and Mendota cores the Corcoran Clay Member of the Tulare 

Formation although the orientation in the lacustrine (?) beds at the bottom of the Mendota core seems 

to be random. The alluvial-fan deposits consistently 

show the most random orientation. The orientation in 

the flood-plain deposits ranges from random to fairly 

well preferred (orientation ratios between 1.0 and 2.0).

The differences in the orientation of montmorillonite 

particles in the different types of deposits perhaps 

reflect different means of deposition. In the still waters 

of a lake, the particles may have an opportunity to 

settle out individually into a well-oriented arrangement. 

On an alluvial fan, on the other hand, clayey materials 

are deposited rapidly, often in a heterogeneous mixture that includes an assortment of coarser particles. 

The clay particles have little opportunity, therefore, to 

develop a preferred fabric. On flood plains, the physical conditions of deposition probably range between 

those visualized in lakes and those on alluvial fans.

Differences in the water content of the sediments, immediately following their deposition and during their 

burial beneath the first few tens of feet of overburden, 

may account for the differences in the orientation of 

montmorillonite. This supposition is based on results of 

the experimental work of Martin and O'Brien (reviewed on p. D7) which suggested (1) that the preferred 

orientation, if it develops at pressures lower than about 

100 kg per cm2 , forms at pressures near 1 kg per cm2
D30 MECHANICS OF AQUIFE.R SYSTEMS

and (2) that the amount of water in the clay at this 

early stage may determine the degree of development 

of the orientation. The lacustrine sediments must have 

remained saturated with water as long as the lake in 

which they were deposited remained in existence, and 

preferred orientation presumably could develop during the deposition of successive layers of sediment on 

the lake bottom. The alluvial-fan sediments, on the 

other hand, were dried out soon after they were deposited, and they did not become saturated with water 

again until they were buried below the water table. 

Judging from modern conditions in the Los BanosKettleman City area, the alluvial-fan sediments must 

have been covered with several hundred feet of overburden before they reached the water table. By the time 

that the sediments met the water table, the volume and 

geometry of their pore space were such that preferred 

orientation could not develop. As with the conditions 

of deposition discussed in the previous paragraph, one 

might expect the degree of water saturation in floodplain sediments to have been intermediate between that 

in lacustrine sediments and that in alluvial fans.

The orientation of montmorillonite particles in sediments from the Eichgrove core of the Tulare-Wasco 

area is shown in figure 19. The 4 samples nearest the 

land surface may not have been saturated with water at 

the time that they were cored; the other 16 samples,

taken from depths below 400 feet, were below the water 

table. The alluvial-fan sediments represented in figure 

19 are coarser than those represented in figure 18. The 

sand and silt grains in the alluvial-fan sediments of 

the Kichgrove core are not "suspended" in a clay matrix, and the sand-and-silt skeleton probably supports 

most of the overburden load. This textural arrangement was found also in many of the samples of marine 

siltstone.

The orientation seems to be random in the alluvialfan sediments that make up the uppermost 600 feet of 

the Kichgrove core. Orientation ratios in the samples 

from this interval range between 0.8 and 1.3. In the 

marine siltstone layers, on the other hand, most of the 

orientation ratios fall into the range between 1.3 and 

2.0, indicating a degree of orientation greater than in 

the alluvial-fan sediments but less than in the lacustrine Corcoran Clay Member in the Los Banos-Kettleman City area. Within each type of deposit at Eichgrove, there is little indication of a progressive increase 

in preferred orientation with depth. Such an increase 

should not be expected, however, in view of (1) the textural arrangement of sand and silt grains that prevents 

the load from being borne directly by the clay and (2) 

the observation (fig. 17) that the pore volume does not 

decrease systematically with increasing depth.

MONTMORILLONITE ORIENTATION RATIO 

123 12

500

1000

AF

LA

FP

ORO LOMA 

CORE

1500

AF

FP

LA

 FP

AF

LA(?)

MENDOTA CORE

2000

AF

FP

FP

CANTUA CREEK 

CORE

2000

AF

AF

Deltaic

HURON CORE

FIGURE 18. Relations of mantmorillonite-particle orientation to depth of burial and type of deposit represented by fine sediments cored in Los Banos-Kettleman City area. Orientation ratios determined with assistance of J. B. Corliss. AF, alluvialfan deposit; FP, flood-plain deposit; LA, lacustrine deposit.
COMPACTION OF SEDIMENTS UNDE.RI.YING AREAS OF LAND SUBSIDENCE IN CENTRAL CALIFORNIA D31

RELATION TO PARTICXJB SIZE 

AND CHEMICAL FACTORS

The previous work reviewed earlier in this paper 

showed that certain physical and chemical factors 

might be expected to help or hinder the tendency of a 

clay to develop preferred orientation under load. The 

relation of some of these factors particle size, electrolyte concentration, exchangeable cations, and pH to 

the orientation ratio is examined in the 11 samples of 

marine siltstone from the Richgrove core (depths between 764 and 1,827 feet below land surface). These 

samples were selected because (1) their relatively uniform lithology suggests approximately uniform conditions of deposition, (2) the orientation ratio and the 

other factors are variable enough to show differences 

that might be correlative, and (3) the data are already 

available from the analysis of pore volume (table 3).

Multiple-regression techniques are used, in the same 

way that they were used in a precedingsection (table 4), 

in an attempt to sort out the important factors. The 

montmorillonite orientation ratio is used as the dependent variable, and its numerical relation to each of the 

measures of the independent variables is assumed to be 

linear. The pertinent correlation and regression staMONTMORILLONITE 

ORIENTATION RATIO TYPE OF 

DEPOSIT

500

1000

1500

2000 2000 -

Alluvial fan

Flood plain

Marine 

(siltstone)

Marine 

(sand)

FIGURE 19. Relations of montmorillonite-particle orientation 

to depth of burial and type of deposit represented by fine 

sediments in Richgrove core. Orientation ratios determined 

with assistance of J. B. Corliss.

tistics are listed in table 5. The significant partial correlations between the orientation ratio and each of tho 

first three listed independent variables effective load, 

Md^^ and percentage of absorbed sodium are positive. 

Partial correlations between the orientation ratio and 

the concentration of soluble salts are negative. The statistics listed in table 5 suggest that particle size, adsorbed sodium, and soluble salts may have some influence on the degree of preferred orientation in the 

marine siltstones. The multiple-correlation coefficient of 

0.48 for the regression relation in which these three are 

used as the independent variables (bottom line, table 5) 

suggests that they only account for about one-fourth of 

the observed variance in orientation ratio (#2 = 0.23). 

These conclusions are rather tentative, however, mainly 

because too few samples were available.

DOMAINLIKE AGGREGATES

Many of the fine alluvial-fan sediments of the Los 

Banos-Kettleman City area have a fabric that consists 

of sand-sized aggregates of clay-mineral particles in a 

matrix of poorly sorted and randomly oriented sand, 

silt, and clay. (See Meade, 1967, fig. 4.) The orientation of clay minerals within these domainlike aggregates is highly preferred, but the orientation of the 

aggregates with regard to each other or to any plane 

through the sediment is random. As the orientation is 

preferred in small areas only (a millimeter or so across) 

it was not detected by the X-ray diffraction method 

which integrates the bulk orientation over an area of 

3 to 4 square centimeters. The oriented aggregates were 

apparently deposited as such, either as shale fragments 

or as fragments of previously deposited clay layers on 

the alluvial fan. These aggregates were observed in the 

alluvial-fan sediments of the Los Banos-Kettleman 

City area only; none were seen in the alluvial sediments 

of the Tulare-Wasco area or Santa Clara Valley.

Whether or not this kind of orientation is enhanced 

by increasing overburden loads is not clear from the 

evidence visible in thin sections. Even though these

TABLE 5. Selected details of multiple regressions of orientation 

ratio on different combinations of variables representing properties 

of marine siltstones in Richgrove core

Number of independent 

variables

Coefficient

of multiple

correlation

(B)

Standard

error of

estimate of

orientation

ratio

Probability of significance (P) > of 

coefficient of partial correlation between 

montmorillonite orientation ratio and 

Logio 

load

Percent- Logic

age of total pH

adsorbed dissolved

sodium solids

0.61 

.48 

.36 

.48

0.27 (2) 0.88

.30 ...... .77

.30 0.76 <.75

.28 ...... .81

0.87 

.81

.85

0.87 

<.75 

<.75

.77

1 One-sided significance test. (See Dixon and Massey, 1957, p. 97-99.)

2 Negative (and therefore insignificant) partial correlation with orientation ratio.
D32 MECHANICS OF AQUIFER SYSTEMS

aggregates are much larger than the submicroscopic 

domains that supposedly characterize turbostatic orientation, perhaps their role during compaction is the 

enhancement of a fabric that is at least analogous to the 

one represented in figure 3B. The development or enhancement of turbostatic orientation in these sediments 

is possible on several counts. The main clay-mineral 

constituent is calcium-montmorillonite, which according to experimental evidence reviewed earlier may be 

susceptible to this kind of arrangement under pressure. 

Furthermore, perhaps the deposited aggregates provide nuclei for further orientation of clay-mineral 

particles under pressure. No evidence, however, was 

found in the sediments themselves to support or reject 

this possibility. A fruitful approach might be to 

measure the internal and external surface areas of the 

sediments (Diamond and Kinter, 1958) or their poresize distributions (Aylmore and Quirk? 1962, p. 109- 

115; Barrett and others, 1951) and look for changes 

under increasing overburden loads.

DISTRIBUTION OF MONTMORELLONITE ORIENTATION 

WITHIN THE SEDIMENTS

The distribution of the orientation of montmorillonite particles within some of the sediments from the 

Huron and Eichgrove cores is shown in figure 20. In 

this figure, the orientation is indicated by the ratio of 

the heights of the diffractometer peaks that represent 

the reflections from montmorillonite at 15 A and 4.4 A. 

Larger values of the peak-height ratio, within any one 

sediment, correspond to greater degrees of preferred 

orientation. Determinations for each sample were made 

for five planes of reference from 0° to 90° from the bedding direction. The peak-height ratio should not be confused within the orientation ratio used in figures 18 and 

19, which is computed from the peak-height ratios of 

different sections.

Within the alluvial-fan and the deltaic sediments 

(left and center columns, fig. 20), the distribution of 

the orientation from section to section varies in a random way from sample to sample. In the marine sediments from the Eichgrove core (right column, fig. 20), 

the peak-height ratios seem to decrease at fairly regular 

rates with increasing angular distance from the direction of bedding. This indicates that the preferred orientation is not strictly confined to the plane of the bedding, but only reaches a maximum there.

FABRIC OF SANDS

This section of the report contains observations of the 

fabric of the sands and some inferences about how the 

observed fabric may be related to compaction. No 

quantitative measurements of sand fabric were made 

because the sands probably were disrupted somewhat

during the coring process. Thin sections were made, 

however, and some features were observed in them. Indirect inferences were made by relating some of the 

observable features to the experimental studies whose 

results were reviewed earlier.

Although its effects do not show in the thin sections, 

the simple movement of grains into more efficiently 

packed arrangements was probably the principal response of the sands to increasing overburden loads. 

This supposition is based mainly on the experimental 

work, reviewed earlier, on pure-quartz sands. Its chief 

modifications in polymineralic sands such as those in 

the San Joaquin and Santa Clara Valleys are the supplementary processes of distortion and bending of the 

softer and more flexible grains.

PREPARATION OF THIN SECTIONS

A group of well-sorted sands from the Mendota, 

Cantua Creek, and Huron cores of the Los Banos-Kettleman City area was selected for thin-section study. 

Their good sorting, atypical of the recovered sands 

as a whole, was a necessary requisite for the impregnating and sectioning procedures. The sands were 

sealed in wax at the coring sites to preserve their textures as well as possible. One must assume, however, 

that their original textures were disturbed during the 

coring operations. In the laboratory, they were impregnated under partial vacuum with a polyester resin 

which, when hardened, has a refractive index of 1.54. 

Thin sections were made from the impregnated sands 

along planes normal (and, in some samples, parallel) 

to the bedding. The sections were taken from near the 

center of the core to minimize the disruptive effects 

of the coring. Sieve analyses of contiguous samples of 

the sands were also made and results have been published (Meade, 1967, table 7).

DISTORTION OP COMPREISSIBUH GRAINS

The distortion of compressible grains elastic micas 

and nonelastic soft rock fragments was observed in 

some of the sections. In the San Joaquin Valley this 

process is significant in the sands, derived from the 

Sierra Nevada, which usually contain 3 to 5 percent 

mica. And many sands, especially those in the Los 

Banos-Kettleman City area that are derived from the 

Diablo Range, contain fragments of soft materials 

such as partly weathered shales or metamorphic rocks.

A few of the distorted grains are sketched in figure 

21. The distortion of the mica in figure 2L4 is the most 

extreme example observed. A more typical degree of 

mica distortion is sketched in figure 215. This represents 

a minimum distortion of this grain because one cannot 

tell how much elastic recovery may have taken place 

between the time the sand was cored and the time it
COMPACTION OF SEDIMENTS UNDERLYING AREAS OF LAND SUBSIDENCE IN CENTRAL CALIFORNIA D33

45 90

i i i i

ALLUVIAL FAN

1651

1693

DELTAIC

1801

1899

HURON CORE 

Sample from 822-foot 

depth is from Cantua 

Creek core)

ORIENTATION OF SECTION 

(DEGREES FROM BEDDING DIRECTION)

0 45 90

ALLUVIAL FAN

2.0

148

232

290

444

517

583

FLOOD PLAIN

649

2.0

2.0

1.8

1.2

2.4

1.8

1.2

1.8'

1.6

2.4

1.8

45 90

i i i i

MARINE

764

844

917

1037

1240

1364

1447

1527

1689

1827

RICHGROVE CORE

FIGURE 20. Orientation of montmorillonite in sections cut at different angles to the bedding of sediments from Huron and Richgrove

cores. Determined with the assistance of J. B. Corliss.
D34 MECHANICS OF AQUIFER SYSTEMS

was impregnated and sectioned. Neither, incidentally, 

can one be certain of how much of the distortion is due 

to natural loading and how much is due to the coring 

procedure. In general, the wide thin flakes of mica were 

more distorted than the thick books. Other factors influencing the degree of distortion were the orientation 

of the flakes with regard to the compacting pressure, 

the size of the mica flakes relative to the sizes of adjoining grains, and the nature of the contacts between flakes 

and grains. Because of the direction of orientation and 

poor contacts with adjacent grains, for instance, some 

mica flakes in the section from which figure 21A was 

sketched showed no evidence of distortion. Shale fragments were broken and squeezed by pressure exerted 

through more resistant adjoining grains. The distortion in figure 21C is as severe as any observed in the 

sand sections. The distortion of soft rock fragments 

was controlled by the same factors that controlled the 

distortion of micas.

ORIENTATION OF1 MICA PAJRTICl^ES

Two mica fabrics differing in the orientation of 

mica flakes and the size of the flakes relative to the 

nonmica grains are illustrated in thin-section views 

of sands from the Mendota core. Figure 22A shows a 

micaceous sand derived from the Sierra Nevada; figure 

225, one derived from the Diablo Range. In the sand 

derived from the Sierra, the larger mica flakes tend to 

be oriented parallel to the bedding. The long dimensions of the mica flakes are generally larger than the

FIGURE 21. Distortion of compressible sand grains. A, Bent and 

broken biotite; Mendota core, depth 1,347 feet. B, Bent biotite; 

Cantua Creek core, depth 1,551 feet. Q, Crushed and broken shale 

fragment (stippled) ; Mendota core, depth 1,347 feet.

diameters of the nonmicaceous grains. In the sand derived'from the Diablo Range (fig. 225), the long dimensions of the micas are approximately the same as the 

diameters of the nonmicaceous grains. Furthermore, the 

micas are oriented roughly at angles of 45° to the bedding. One cannot be certain whether the orientation is 

a depositional feature or a result of changes during 

compaction, but W. B. Bull (written commun., 1964) 

says that such an orientation is common in the modern 

alluvial sands in the western San Joaquin Valley.

Because of their different mica fabrics, these two 

sands should be expected to respond differently to compacting pressures. The mica flakes shown in figure 2QA 

should respond to loads by bending around adjacent 

grains, a process that enhances the compressibility of 

the aggregate. In the event that the pressure is released, 

the large mica flakes should regain some of their original shape, giving the aggregate a certain amount of 

elasticity. In the sand shown in figure 225, on the other 

hand, one should expect a greater degree of compressibility but a lesser degree of elasticity. Their size, shape, 

and orientation should allow the mica particles to slip 

more easily past the nonmica particles (or vice versa) 

into more compact arrangements; but this kind of 

movement is largely irreversible. The compressibility of 

both these sands, however, is probably greater than that 

of otherwise equivalent sands that contain no mica.

CONCLUSIONS

In the fresh-water-bearing alluvial clays, silts, and 

silty sands in the San Joaquin and Santa Clara Valleys 

of California, the loss in pore volume that results from 

compaction by effective overburden loads in the range 

between 3 and 70 kg per cm2 averages about 0.3 voidratio units or about 15 percent of the bulk volume of 

the fine sediments. When one allows for the lesser compaction of the interbedded coarser sands and gravels, 

the reduction of the total volume of the alluvial sediments amounts to about 12 percent between 10 and 70 

kg per cm2 on the west side of the San Joaquin Valley 

and about 10 percent between 3 and 33 kg per cm2 in 

the Santa Clara Valley.

The factors that directly influence the pore volume of 

the alluvial and shallow-marine sediments under these 

loads are the average particle size, the particle sorting, 

the large proportion of montmorillonite, the proportion of exchangeable sodium relative to exchangeable 

calcium and magnesium, and the presence of diatom 

skeletons. Although not shown by direct evidence, the 

proportion of mica probably also influences the pore 

volume. Factors having at least indirect influence, if 

not direct influence, are the acidity of the sediments and 

the concentration of interstitial electrolytes. The relative
COMPACTION OF SEDIMENTS UNDERLYING AREAS OF LAND SUBSIDENCE IN CENTRAL CALIFORNIA D35

importance of all these factors is not easily evaluated 

'because (1) their interrelations are often complex and 

(2) some of them are too constant (the proportion of 

montmorillonite, for example) to show variable effects 

on pore volume within this group of sediments. But one 

can distinguish with a fair degree of certainty the factors of primary importance from those of subsidiary 

importance.

The average particle size and the content of diatom 

skeletons are certainly primary factors. Under these 

overburden loads, a sediment that has a median diameter of 1 micron can be expected to have a void ratio 

greater by at least 0.2 (about 7 percent of the total 

sediment volume) than a sediment whose median diameter is 100 microns. Diatom skeletons, where they are 

present, increase the pore volume markedly.

In sands and silts, the influences of the particle sorting and the proportion of mica flakes on the pore volume are probably of subsidiary importance. Whether 

particle sorting is a primary or subsidiary factor is not 

clear. Rather uncertain statistical correlations suggest 

that one might expect a difference in void ratio near 

0.2 between sediments whose QD^ values are 1 and 3; 

but the relation of pore volume to sorting is probably 

less important and almost certainly less pervasive than 

the relation to average particle size. The ranges of mica 

content and overburden loads in the sediments and the 

experimental work carried out in previous studies indicate that the differences in the mica content probably

do not cause differences in void ratio much greater than 

0.1.

In the more clayey sediments, the content of montmorillonite and the proportion of adsorbed sodium become important. Judging mainly from the previous experimental work, the large proportion of montmorillonite must be a primary influence on the pore volume. No 

numerical estimates are made of the range of its possible 

effects, but the sediments of the San Joaquin and Santa 

Clara Valleys are certainly more porous than they 

would be if their principal clay-mineral constituents 

were illite or kaolinite. The proportion of adsorbed sodium relative to the other exchangeable cations is probably of subsidiary importance because (1) it is closely 

related to the pH and concentration of the interstitial 

electrolytes and (2) its total effect depends on the 

amount of clay-mineral surface available for cation 

sorption the amount of montmorillonite, in these 

sediments.

The degree of preferred orientation of the montmorillonite particles parallel to the bedding direction 

is consistently related to the depositional types of the 

fine sediments. Lacustrine sediments show the greatest 

degree of orientation; alluvial-fan sediments, the smallest degree; flood-plain and shallow-marine sediments 

show intermediate degrees of orientation. However, 

there is no clear identification of the specific properties 

of the sediments or characteristics of the depositional 

environments that control the degree of orientation.

FIGURE 22. Sections of micaceous sands from Mendota core. Sections cut normal to bedding, which is parallel to top of 

page. A, Flood-plain sand from 833 feet; Sierra Nevada source. Mainly quartz, potassium feldspar, twinned plagioclase 

(light cleavage), biotite, shale fragments (strippled), and a little hornblende. B, Flood-plain sand from 1,076 feet; 

Diablo Range source. Mainly quartz, potassium feldspar, "dirty" twinned plagioclase (light cleavage), shale and other 

rock fragments (strippled), biotite, chlorite, and dark accessory minerals. Optical relief of grains exaggerated in 

both sections.
D36 MECHANICS OF AQUIFER SYSTEMS

Experimental work by O'Brien and the inferred postdepositional histories of the sediments suggest that the 

water content of the sediments during the very early 

stages of compaction may be a critical factor. A limited 

regression analysis suggests that the development of 

preferred orientation in the shallow-marine sediments 

may be favored by decreasing particle size, decreasing 

concentration of interstitial electrolyte, and increasing 

proportion of sodium adsorbed by the montmorillonite. 

Only the influence of electrolyte concentration is well 

supported by experimental studies.

The most certain conclusions from the study of clay 

fabric are negative: the degree of preferred orientation 

of the montmorillonite does not increase with decreasing pore volume or with increasing depth of burial beneath overburden loads as great as 70 kg per cm2.

GLOSSARY OF STATISTICAL TERMS

Most of the statistical terms used in this report and in 

another report (Meade, 1967) in this series are defined below. 

Many of these definitions were taken, with little or no modification, from James and James (1949). References to further 

explanation and discussion are given after most of the definitions.

Coefficient of determination. The percentage to which the variance in the dependent variable is determined by the independent variable or variables. The square of the coefficient 

of linear correlation.

Correlation coefficient or coefficient of linear correlation. A 

number between 1 and +1 that indicates the degree of 

linear relationship between two or more setsi of numbers. 

See Coefficient of determination. The simple correlation 

coefficient (r) indicates the degree of linear correlation 

between two sets of numbers. The multiple correlation coefficient (R) indicates the degree of linear correlation between one set of numbers (values of the dependent variable) and a combination of two or more sets of numbers 

(values of the independent variables). The partial correlation coefficient indicates the degree of linear correlation 

between the dependent variable and one of the independent 

variables, while eliminating the linear tendency of the remaining independent variables to obscure the relation. See 

Ezekiel and Fox (1959, p. 127-129, 190-197), Fisher (1950, 

p. 175-210), Moroney (1956, p. 271-320), and Waugh (1952, 

p. 446-449, 508-509). 

Cumulative frequency. The sum of all preceding frequencies, a

certain order having been established.

Dependent variable. A quantity that takes on a value corresponding to every value or set of values of another variable 

or set of variables (called independent variables). For example, in the regression equations

or

where y is the dependent variable, x and z are independent 

variables; and 6 and c are regression coefficients. See 

Ezekiel and Fox (1959, p. 47-48) and Fisher (1950, p. 129- 

130). 

Independent variable. See Dependent variable.

Mean. The sum of observed values, divided by the number of 

values observed.

Measures of central tendency. Values loosely referred to as 

"averages". Measures of central tendency used in these reports are the mean, median, and mode. See Moroney (1956, 

p. 34-55), and Waugh (1952, p. 61-125).

Median. The midpoint of a group of measurements; the point in 

a distribution above and below which lie half the values.

Mode. The value that occurs most frequently.

Normal distribution or normal frequency distribution. The 

distribution is described by

where n is the mean, <r the standard deviation of the distribution, and e is the base of Naperian logarithms. When 

graphed, the curve of the normal distribution is bell shaped, 

symmetrical about the mean, and extends infinitely far in 

both the positive and negative directions. See Dixon and 

Massey (1957, p. 48-66), Krumbein and Petti John (1938, p. 

252), Moroney (1956, p. 106-119), and Waugh (1952, p. 155- 

186).

Percentiles. In a particle-size distribution, the 99 points that 

divide the distribution into 100 segments in such a way that 

the segments contain equal amounts (by weight) of particles. A given percentile is the value which divides the 

range of particle sizes into two parts such that a given 

percentage (by weight) of the particles is coarser than this 

value. See Waugh (1952, p. 77-78).

Probability of significance. Probability that the assumed model 

is correct; probability that the observed relation could 

not have occurred by chance. See Ezekiel and Fox (1959. 

p. 293-298) and Moroney (1956, p. 216-237).

Quartiles. The .three points that divide a distribution into four 

equal parts. The 1st and 3d quartiles correspond respectively to the 25th and 75th percentiles. The 2d quartile 

corresponds to the 50th percentile, or median. See Waugh 

(1952, p. 75-76).

Quartile deviation. A measure of dispersion: one-half the distance between the first and.third quartile. See Krumbein 

(1936, p. 102-103).

Regression coefficient. The coefficient of an independent variable 

in a regression equation. In simple regression (two variables 

only), the slope of the regression line. See Dependent 

variable. See also Ezekiel and Fox (1959, p. 134-140, 147- 

150) and Fisher (1950, p. 126-136).

Regression equation or equation of linear regression. The 

equation of the straight line used to estimate one variable 

(the dependent) from one or more other (independent) 

variables. It is computed by the method of least squares 

which minimizes the sum of squares of the residuals between the observed values of the dependent variable and the 

values computed by the regression equation. See Ezekiel 

and Fox (1959, p. 55-68, 170-187), Moroney (1956, p. 276- 

320), Waugh (1952, p. 301-316), and Williams (1959, p. 

10-52).

Residual. The difference between the observed value of a quantity and the value predicted from the regression equation. 

See Ezekiel and Fox (1959, p. 119-121).

Skewness. A measure of the asymmetry of a distribution. See 

Waugh (1952, p. 200-206).

Standard deviation. A measure of dispersion: the square root of 

the arithmetic mean of the squares of the deviations from
COMPACTION OF SEDIMENTS UNDERLYING AREAS OF LAND SUBSIDENCE IN CENTRAL CALIFORNIA D37

the mean. For a normal distribution, approximately twothirds of the samples fall within one standard deviation, 

plus or minus, of the mean; approximately 95 percent fall 

within two standard deviations of the mean. See Moroney 

(1956, p. 61-62) and Waugh (1952, p. 135-147).

Standard error of estimate of the dependent variable. A measure of how close the observed values fall to the regression 

line: The standard deviation of the residuals between the 

observed values and the corresponding values computed by 

the regression equation. See Ezekiel and Fox (1959, p. 119- 

121, 147-150).

Variance. A measure of dispersion: the arithmetic mean, of the 

squares of the deviations from the mean. The square of the 

standard deviation.

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D38 MECHANICS OF AQUIFER SYSTEMS

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COMPACTION OF SEDIMENTS UNDERLYING AREAS OF LAND SUBSIDENCE IN CENTRAL CALIFORNIA D39

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APPENDIX A

Samples used in statistical studies

The groups of alluvial sediments shown in figures 11 to 15 

consist of the following samples. The numbers were assigned by 

Johnson, Moston, and Morris (1967). 

Group A : 

57 GAL : 

8, 11, 27, 37, 40, 57, 62, 76, 78, 79, 81, 82, 84, 85, 90, 108, 

113, 115, 136, 139, 142, 152, 155, 162, 176, 177, 186. 

58 GAL: 

1, 4, 7, 28, 41, 47, 74, 75, 76, 81, 82, 93. 

Group B: 

57 GAL: 

7, 29, 31, 50, 64, 65, 83, 121, 130, 149, 154, 166, 168, 169, 

174, 178, 181. 

58 GAL : 

2, 20, 33, 42, 49, 52, 59, 65, 72, 73, 90. 

Group G : 

57OAL: 

25, 39, 48, 51, 54, 55, 56, 58, 59, 61, 86, 88, 114, 123, 124, 

146, 147, 150, 159, 161, 164, 167, 173, 175, 185, 187, 188, 

189, 190. 

58 GAL: 

5, 9, 10, 12, 26, 31, 50, 53, 64, 67, 84. 

Group D : 

57 GAL *' 

9, 10, 12, 13, 14, 52, 107, 109, 111, 112, 131, 135, 137, 148, 

151, 157, 163, 165, 183. 

58 GAL: 

23, 30, 32, 38, 46, 48, 51, 57, 60. 

Group B : 

60 GAL: 

16, 17, 18, 23, 24, 29, 34, 35, 38, 42, 44, 48, 49, 57, 64, 72,

73, 74, 79, 83, 92, 93, 95. 

Group P: 

58 GAL: 

102, 104, 105, 110, 111, 112, 113, 114, 115, 116, 117, 118, 

119, 120, 121, 122, 123, 125, 126, 127, 128, 129, 131, 132, 

133, 134, 135, 136, 137, 138, 140, 141. 

59 GAL: 

350, 351, 352, 353, 354, 355, 356, 357, 361, 362, 363, 364, 

365, 366, 367, 369.

APPENDIX B

Effective overburden loads at different depths in the cored sections 

at time of coring

[Computed by K. E. Miller (Mendota, Cantua Creek, and Huron), B. E. Lofgren 

(Pudey and Kichgrove), and J. H. Green (Sunnyvale and San Jose)]

Depth below land Effective overburden load

(psi) (kg per cms)

Mendota 1957

120 104 7. 3 

625 308 21. 6 

700 475 33. 4 

1, 500 832 5a 5

Effective overburden loads at different depths in the cored sections 

at time of coring Continued

Depth below land Effective overburden load

(psi) (kg per cm2)

Cantua Creek 1958

192 170 12. 0 

565 305 21. 4 

575 475 33.4 

1, 720 970 68. 2 

2, 000 1, 090 76. 6

Huron 1957

120 105 7. 4 

730 305 21. 4 

747 540 3a 0 

1, 640 915 64. 3 

2, 200 1, 155 81. 2

Pixley 1958

120 86 6. 0 

285 165 11. 6 

296 213 15. 0 

760 413 29. 0

Richgrove 1959 1

180 135 9. 5 

533 306 21. 5 

744 400 28. 1 

1, 053 524 36. 8 

1, 175 568 39. 9 

1, 205 578 40. 6 

1. 377 629 44. 2 

1, 384 634 44. 6 

1, 743 760 53. 4 

1, 818 790 55. 5 

1, 900 819 57. 6 

2, 200 950 66. 8

Sunnyvale 1960

150 64 4. 5 

185 130 9. 1 

215 144 10. 1 

415 235 16. 5 

680 360 25. 3 

1, 000 503 35. 4

San Jose 1960

137 120 8. 4 

245 166 11. 7 

475 270 19. 0 

785 418 29. 4 

815 432 30. 4 

865 449 31. 6 

895 464 32. 6 

930 481 33. 8 

1, 000 513 36. 1

1 Pore pressures in fine-grained beds in 1959 were assumed equivalent to waterlevel conditions of 1921. This assumption probably is reasonable for pressures m the 

recently tapped (1946+) , slow draining, thick marine siltstcme beds bet wecm depths 

of 744-1,900 feet. However, computed effective loads between depths of 180-^44 feet

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