Removal of Water and 

Rearrangement of Particles 

During the Compaction of 

Clayey Sediments Review 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 497-B 

Removal of Water and 

Rearrangement of Particles 

During the Compaction of 

Clayey Sediments Review 

By ROBERT H. MEADE 

MECHANICS OF AQUIFER SYSTEMS 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 49 7-B 

Review of the pertinent literature on the 

factors influencing the water content and 

clay-particle fabric of clayey sediments 

under increasing overburden pressures 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1964 
UNITED STATES DEPARTMENT OF THE INTERIOR 

STEWART L. UDALL, Secretary 

GEOLOGICAL SURVEY 

William T. Pecora, Director 

First printing 1964 

Second printing 1967 

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

Washington, D.C. 20402 - Price 25 cents (paper cover) 
CONTENTS 

GlossarY------------------------------------------- Abstract __________________________________________ _ 

Introduction ______________________________________ _ 

Purpose of study ______________________________ _ 

Acknowledgments ______________________________ _ 

Forces related to clay-mineral surfaces ________________ _ 

Forces between clay-mineral particles and water ___ _ 

Hydration of clays _____ --------- ___________ _ 

Properties of adsorbed water ________________ _ 

Forces betwee:p. adjacent clay-mineral particles ____ _ 

Repulsion between particles: double-layer forces_ 

Explanation in terms of osmosis _________ _ 

Attraction between particles ________________ _ 

Van der Waals forces and their relation to 

double-layer repulsion ________________ _ 

Edge-to-face attraction _________________ _ 

Factors influencing the removal of water during the early 

stages of compaction _____________________________ _ 

Particle size ___________________________________ _ 

Clay minerals ______________________ ------- ____ _ 

Interstitial electrolyte solutions __________________ _ 

N aCl-montmorillonite-water mixtures ________ _ 

NaCl-illite-water mixtures---------------------

Other electrolyte-montmorillonite-water mixtures------------------------------------

Page 

IV 

Bl 

1 

2 

2 

2 

2 

2 

2 

3 

3 

3 

4 

4 

5 

5 

5 

5 

7 

7 

9 

9 

Factors in1luencing the removal of water, etc.-Con. 

Interstital electrolyte solutions-Continued 

MgC12 (or CaC12)-illite-water mixtures ________ _ 

Exchangeable cations ________ ---- __________ ---- __ 

Removal of water during the later stages of compaction_ 

Fabric of clays and its changes under pressure _________ _ 

Factors, other than pressure, that influence the 

arrangement of clay particles __________________ _ 

Electrolyte concentration ___________________ _ 

Cations and anions _________________________ _ 

AciditY------------------------------------

Organic matter ___ -------- _________________ _ 

Physical conditions of deposition _____________ _ 

Fabric of uncompacted natural clays ____________ -_ 

Influence of pressure on the fabric of clays _______ --

Fabric of clays compressed in the laboratory_ - _ Unconfined compression _______________ -_ 

Compression of laterally confined clays __ -_ 

Influence of electrolyte concentration under 

pressure ____________________________ _ 

Fabric of naturally compacted clayey sediments_ 

SummarY------------------------------------------

References-----------------------------------------

ILLUSTRATIONS 

FIGURE 1. Explanation, in terms of osmosis, of difference in swelling of clay aggregates in electrolyte solutions of low and 

BlO 

11 

12 

13 

14 

14 

15 

15 

15 

16 

16 

17 

17 

17 

17 

18 

18 

20 

21 

Pap 

highconcentrations-------------------------------------------------------------------------------- B4 

2. Forces between basal (001) surfaces of adjacent clay-mineral particles, as a function of interparticle distance____ 4 

3. Influence of particle size on void ratio of fine-grained sediments------------------------------------------- 6 

4. Influence of clay minerals on relation between void ratio and compacting pressure---------------------------- 6 

5. Clay particles and their envelopes of adsorbed water---------------------------------------------------- 7 

6. Influence of NaCl concentration on relation between void ratio and pressure in montmorillonite________________ 8 

7. Relations between NaCl concentration, interparticle spacing, and pressure in montmorillonite__________________ 8 

8. Influence of NaCl concentration and particle size on relation between void ratio and pressure in Fithian illite ____ - 9 

9. Experimental and theoretical relations between interparticle spacing and pressure in mixture of montmorillonite 

and 2X tO-' N CaC12 solution ______________ -------- __ ------ ___ ---- __ -------------------------------- 10 

10. Influence of MgC12 concentration on relation between void ratio and pressure in <2.0-micron fractions of illite___ 10 

11. Influence of exchangeable cations on relation between void ratio and pressure in montmorillonite______________ 11 

12. Influence of exchangeable cations on relation between void ratio and pressure in kaolinite-------------------- 12 

13. Pressure-temperature curves for dehydration reactions-· of montmorillonite minerals saturated with sodium, calcium, and magnesium ______________________________________________________ -_-_---- __ -_-_-------- 13 

14. Idealized arrangements of clay-mineral particles in sediments-------------------------------------------- 13 

15. Horizontal preferred orientation of montmorillonite in unconsolidated clayey sediments from three core holes 

in San Joaquin Valley, Calif------------------------------------------------------------------------ 19 

m 
GLOSSARY 

Compaction. "* * * decrease in volume of sediments, as a result of compressive stress, usually resulting from 

continued deposition above them" (American Geological Institute, 1957, p. 58). This process is called 

consolidation in the soil-mechanics literature. 

Consolidation. "* * * any or all processes whereby loose, soft, or liquid earth materials become firm and coherent" (American Geological Institute, 1957, p. 62). This includes cementation as well as compaction. 

Fabric. "* * * the physical constitution of a * * * [sediment] as expressed by the spatial arrangement of the 

solid particles and associated voids. Since * * * fabric is concerned with spatial arrangement it is described in terms of the orientation and distribution patterns of the primary particles, compound particles, and 

voids" (Brewer and Sleeman, 1960, p. 173). 

Turbostratic. A type of fabric in which clay-mineral particles are in aggregates within which the orientation is 

preferred and between which the orientation is random (Aylmore and Quirk, 1960, p. 1046; adapted from 

Biscoe and Warren, 1942, p. 370). 

Void ratio. Ratio (e) of the volume of pore space to the volume of solid particles in a sediment (American Society 

of Civil Engineers, 1958, p. 40). Its relation to percent porosity (n) is expressed in the following formula: 

n e=-- 1-n 

In all the studies discussed in this review, the void ratio is a volumetric index of the water content. 

IV 
MECHANICS OF AQUIFER SYSTEMS 

REMOVAL OF WATER AND REARRANGEMENT OF PARTICLES DURING THE COMPACTION 

OF CLAYEY SEDIMENTS-REVIEW 

By ROBERT H. MEADE 

ABSTRACT 

Among the factors that influence the amount of water held 

in clayey sediments under overburden pressures between 0 and 

about 50 kg per cm2 are particle size, clay minerals, exchangeable cations, and interstitial-electrolyte concentration. The 

amount of water held in clayey sediments varies inversely with 

particle size through a wide range of pressures and chemical conditions. The influence of different clay minerals on water content is related mainly to their particle size. The influences of 

exchangeable cations and electrolyte concentration also vary 

with particle size: in fine-grained clays (montmorillonite, for 

example), water content varies inversely with the valence of exchangeable cations and the concentration of monovalent electrolyte; in coarse-grained clays (kaolinite, for example), water 

content varies directly with the cation valence and the 

electrolyte concentration. 

Under overburden pressures greater than about 50 kg per 

em ll' the major influences on the water content of clayey sediments seem to be particle size, clay minerals, and temperature. 

As temperature increases, less pressure is needed to compact 

the clay. Variations in exchangeable cations and in electrolyte 

concentration do not seem to influence the water content under 

higher pressures, but they do affect the clay-particle fabric. 

Although preferred and turbostratic orientations of claymineral particles can be produced by compressing clays in the 

laboratory, there is little evidence to show that they are formed 

readily during natural compaction. Experimental evidence 

suggests that preferred orientation of particles, normal to overburden pressures, would be most likely to form in sediments that 

have large clay-mineral particles, low concentration and low 

acidity of interstitial electrolyte, and low valence of exchangeable cations. The formation of a preferred orientation also 

seems to be favored by the presence of organic material and 

the existence of a partly oriented fabric at the onset of compaction. The formation of a turbostratic fabric (particles 

oriented in domains which are oriented at random with respect 

to other domains) instead of generally preferred orientation 

might be favored by high electrolyte concentration and high 

cation valence. 

Among the factors whose influence on water removal and 

particle rearrangement might be significant, but about which 

little is known, are the anions and organic matter in the interstitial solutions. Also, very little is known of the relative effect 

of each of the several factors when their influences are combined 

under complex natural conditions. 

INTRODUCTION 

The advances in clay technology and soil mechanics 

within recent years-especially in the decade 1953-62-

make possible a new look at the compaction of finegrained sediments. Instead of considering compaction 

as a simple relation between pore volume and over- . 

burden pressure, one can now take into account the 

influence of other physical and chemical factors. This 

review outlines a current set of questions that geologists 

and hydrologists can ask while investigating the compaction of clayey sediments. 

This is a summary of the pertinent knowledge as of 

1962 of the water in clays, the removal of water under 

pressure, and the spatial arrangement of clay-mineral 

particles in sediments. Most of the information is taken 

from publications in fields other than geology-soil 

science, soil mechanics, ceramics, and physical and colloidal chemistry. Except for the theoretical discussion 

of forces in the first section, this review is concerned 

mainly with reported evidence and is only secondarily 

concerned with speculations nnd conclusions. Papers 

that contain little or no evidence to support their conclusions are not reviewed. Furthermore, the experimental evidence on clay behavior that has little direct 

counterpart in nature-the work on the influence of 

lithium and of cesium electrolytes, for example-is not 

treated fully. Finally, I have tried to point out the conspicuous gaps in the present body of knowledge. 

In this review I assume that the reader is already 

generally familiar with the structure and composition 

of the common clay minerals and with their more distinctive properties, such as minute size, large surface 

area, platy habit, and ion-adsorption and ion-exchange 

capacity. For a review of these subjects, see the recent 

and concise summary by Grim (1962, p. 7-51) or the 

earlier standard works by Grim (1953) and by Jasmund (1955). I also assume that the reader is already 

familiar with the general pattern of the reduction of 

pore volume of clays under increasing overburden loads, 

Bl 
B2 MECHANICS OF AQUIFER SYSTEMS 

which was treated in the classic paper by Hedberg 

(1936) and which was reviewed more recently by 

Engelhardt ( 19.60, p. 35-50), by Lomtadze ( 1955, 1956) 

and by Weller (1959, p. 274-294). 

PURPOSE OF STUDY 

The U.S. Geological Survey is making an investigation directed toward determining the principles controlling the com paction of aquifer systems resulting 

from change in grain-to-grain load (effective stress) 

induced by major decrease of internal fluid pressure. 

This project is under the direction of J. F. Poland, research geologist. 

As one phase of the overall investigation, I am studying the physical and mineral characteristics of the 

water-bearing sediments that are compacting, the conditions under which the sediments were deposited, and 

the processes involved in their compaction. As background for the specific studies of these compacting sediments, this paper presents a review of what is currently 

known on the removal of water and the rearrangement 

of particles during the compaction of clayey sediments. 

ACKNOWLEDGMENTS 

I want to thank the following people for stimulating 

discussions and for reviewing early drafts of the manuscript: R. R. Compton, W. R. Dickinson, Kurt Servos, 

and G. A. Thompson, of Stanford University; J. K. 

Mitchell of the University of California; Wolf von 

Engelhardt, of the University of Tiibingen; W. B. Bull, 

B. E. Lofgren, R. E. Miller, H. W. Olsen, J. F. Poland, 

Julius Schlocker, and H. A. Tourtelot, of the U.S. 

Geological Survey. H. van Olphen of Shell Development Co., and Prof. von Engelhardt supplied copies 

of manuscripts that were in press during the preparation of this review. The papers by Ts. M. Raitburd 

were translated from the Russian by Alice A. Parcel, 

of the U.S. Geological Survey. 

FORCES RELATED TO CLAY-MINERAL SURFACES 

Because of the large surface areas of clay minerals, 

the response of water-saturated clayey sediments to 

compacting pressures must be thought of in terms of 

the forces associated with clay-mineral surfaces. In 

very fine grained clays, such as montmorillonites, in 

which the surface areas may be as great as 800m2 per g 

(square meters per gram) , the forces related to the 

clay-particle surface are more pertinent to the understanding of compaction than are the gravitational 

forces associated with the particle mass. This section of the review is an introductory and largely 

theoretical discussion of the surface forces. The role 

of these forces in compaction processes is one of the 

main subjects of the subsequent sections of the review. 

FORCES BETWEEN CLAY-MINERAL PARTICLES AND 

WATER 

HYDRATION OF CLAYS 

The affinity between water and clay is well known. 

Dry clay will adsorb water vapor from the atmosphere. 

It will soak up large quantities of liquid water, if available, and swell to many times its dry vol~me. The 

cause of this attraction seems to be the negative charge 

on the surfaces of clay-mineral particles that arises 

from imperfections and substitutions within the mineral structure. The charge attracts water in at least 

two ways: directly, by attracting the protons of the 

polar water molecules; and indirectly, by attracting 

cations which in turn attract water molecules. The 

hydration properties of the cations are signific~t, as 

the intensity and extent of the clay-water attractiOn are 

different for different adsorbed cations. 

Several series of experiments have shown fairly conclusively that the water closest to the surfaces of 

montmorillonite or vermiculite particles is adsorbed or 

desorbed in molecular-layer units (Barshad, 1949; 

Collis-George, 1955 ; Foster and others, 1955, p. 299-

300· Mooney and others, 1952; Norrish, 1954; Rowland 

and' others 1956 ; Walker, 1956) . That is, if one allows 

a limited ;mount of water or water vapor to equilibrate 

with an originally dry clay, the water will first cover all 

accessible clay surfaces in a layer one molecule thick 

(about 2.5 A 1 ). A second molecular layer will be 

adsorbed only after all accessible surfaces have been 

covered with the first layer. The pattern is reversed 

during drying or compaction: the layer closest to the 

clay-mineral surface is desorbed only after the complete removal of the next closest layer. Although the 

formation or removal of the third, fourth, and subsequent water layers varies with the type of clay mineral and adsorbed cation, the stepwise pattern of hydration and dehydration generally holds true for a distance 

from the particle surfaces equivalent to several layers 

of water. 

PROPERTIES OF ADSORBED WATER 

The water adsorbed on the surfaces of clay minerals 

has properties that differ from those of ordinary liquid 

water because it exists in a force field. Although this 

difference has been known for many years (see Terzaghi, 

1925, p. 746), and although hundreds of experiments 

have been run to define quantitatively such properties 

as the density and viscosity of adsorbed water, very 

little more can be stated with certainty. Measurements 

of the characteristics of adsorbed water necessarily must 

be made on clay-water mixtures, and the presumed 

influence of the solid clay particles on the measure1 A=angstrom unit=lO-B em. 
REMOVAL OF WATER AND REARRANGEMENT OF PARTICLES DURING COMPACTION B3 

ments is subtracted from the whole to leave what is 

presumed to be the effect of adsorbed water. Interpretation of the experimental data therefore must rest on 

theoretical or conjectural assumptions whose validity 

is uncertain. The significant experimental work on 

adsorbed water was reviswed thoroughly by Low (1961) 

and Martin (1962). Significantly, these two authorsboth of whom are highly qualified to review the subject-reached different conclusions about the structural 

arrangement of water on clay-mineral surfaces. 

Low favored the idea that adsorbed water has a high 

degree of structural order and rigidity, making it more 

resistant than ordinary water to normal and shear 

stresses. Martin, on the other hand, favored a twodimensional-fluid model in which the water is essentially 

fluid (perhaps more so than ordinary water) in directions parallel to the mineral surface and is essentially 

solid in the direction normal to the mineral surface; 

such an arrangement would make the water more resistant to normal stress but perhaps less resistant to 

shear stress than is ordinary water. 

FORCES BETWEEN ADJACENT CLAY-MINERAL 

PARTICLES 

Another way of considering the behavior of claywater mixtures is in terms of the forces that operate 

between adjacent clay particles. Interparticle forces 

have received increasing attention and study by soil 

scientists and engineers since 1948, when Verwey and 

Overbeek published a theoretical treatment of the forces 

that operate between particles in colloidal suspensions 

(see also Overbeek, 1952). Their theoretical concepts 

were introduced to soil mechanics by Jimenez Salas and 

Serratosa ( 1953), Bolt ( 1956), and Lambe ( 1958a, p. 

4-12 ; 1958b, p. 8-11) and have been useful in understanding the differences in behavior that seem to arise 

from variations in the chemical and physical components of soils. Because Verwey and Overbeek's 

theory was devised for dilute colloidal suspensions, its 

application to compacted sediments is limited. No 

theory has yet been devised that accounts for all the 

phenomena observed during compaction, and the available theory provides models that are useful as long as 

one keeps in mind their limitations. 

The following discussion is taken from the papers 

cited above, plus Michaels (1958) and Parry (1960, p. 

3.1-3.26). See also the critical discussion by Leonards 

(1962, p. 85-107). 

REPULSION BETWEEN PARTICLES: DOUBLE-LAYER 

FORCES 

The attraction between clay particles and adsorbed 

exchangeable ions in a clay-water mixture is opposed 

by the tendency of the ions to diffuse and distribute 

themselves evenly in water. The result is a diffuse cluster of ions about a clay particle. This system can be 

considered a diffuse electric double layer: one layer is 

formed by the negative charge on the surface of the 

particle, and the other layer is formed by the concentration of exchangeable cations near the particle surface. The entire double layer-comprising the clay 

particle, exchangeable ions, and water-is an electric 

field, and the water in the field is attracted by an induced electrical force. 

The particles in a clayey sediment repel each other 

because the outer parts of the double layers associated 

with the particles have the same net electrical charge 

(usually positive). The range and effectiveness of the 

forces of repulsion between particles are controlled by 

the thickness of the double layers, which probably 

ranges betwean 50 and 300 A in most natural clayey 

materials and which varies systematically with certain 

factors. For example, the thickness of the double layer 

can be decreased by increasing the concentration of 

electrolytes in the water that occupies the pore space 

between clay particles. Increasing the concentration of 

electrolytes in the pore water inhibits the tendency of 

the adsorbed ions to diffuse and allows them to be held 

more closely to the surfaces of the clay particles. This 

added proximity, in turn, decreases the thickness of the 

double layer and shortens the distances over which the 

repulsive forces between particles are effective. Less 

pressure is then needed to compact the clay. The 

thickness of the double layer can be decreased in other 

ways: by replacing the adsorbed cations with cations of 

higher valence, or by increasing pressure. 

EXPLANATION IN TERMS OF OSXOSIS 

A convenient, but oversimplified, way to imagine the 

interaction of double layers is in tenns of osmotic pressure. Because a clay particle in water attracts and 

holds cations, it serves the same function as the semipermeable membrane in the classic osmosis experiment. 

The concentration of ions on the surfaces of clay particles relative to the concentration of ions in the bulk 

of the clay-water system determines the magnitude and 

direction of the osmotic pressure. When an aggregate 

of clay particles having ions attached is wet by water 

in which the concentration of electrolytes is low, the 

osmotic pressure is great and forces water between the 

clay particles, and the aggregate swells (fig. 1A). If 

the concentration of electrolytes is high (fig. 1B)-the 

difference between the cation concentration at the claymineral surfaces and that in the bulk solution is less--

the osmotic pressure therefore is less, and the clay has 

less tendency to swell. The osmotic or swelling pressure is the force that must be overcome to compact the 

clay. 
B4 MECHANICS OF AQUIFER SYSTEMS 

+ 

+ 

A 

+ + + 

+ + + + 

+ 

--

+ + + + Strong ~ + + + ~osmotic 

+ + + + 

pressure 

~~ 

+ + + + 

+ + + 

+ 

Low electrolyte concentration 

+ 

+ 

+ 

+ 

B 

+ 

+ 

+ 

+ + 

+ 

+ + + + 

-~ 

+ + + + 

+ 

+ + + + Weak 

~ + + + ~osmotic 

+ + + + pressure 

~~ 

+ + + + 

+ + + + + 

+ 

+ + 

High electrolyte concentration 

FIGUBII 1.-Jilxplanation, in terms of ~osis, of difference in swelling of clay aggregates in electrolyte solutions of 

(A) low and (B) high concentrations. Clay-mineral plates, viewed edgewise, represented by dark rectangles; 

cations and anions, represented by + and -, respectively. 

Double-layer repulsion and the osmotic analogy, however, explain only part of the behavior of clay particles 

in water because they ignore the attractive forces that 

are effective when particles approach one another 

closely. 

ATTRACTION BETWEEN PARTICLES 

VAll DER W .AALS FORCES A:ND THEIR RELATION TO 

DOUBLE-LAYER REPULSION 

Attractive van der Waals forces become significantly 

effective as the distance between clay-mineral particles 

is reduced to 10 or 20 A. The intensity of these forces 

is an inverse function of a high power (fifth to sixth 

#,, 

l' 

I 

I 

I 

#_ ... ------

t-van der Waals 

: attr.action ,,r 

DISTANCE BETWEEN PARTICLES~ 

power, probably, in most fine clays) of the interparticle 

distance. That is, the attractions are very strong at the 

particle surface and decrease rapidly in intensity away 

from the surface, as illustrated schematically by the 

dotted curve in figure 2.A. Repulsive double-layer 

forces, represented by the dashed curve in figure 2.A, 

are less intense at the particle surface, but their decrease with increasing distance between particles is less 

rapid. The resultant of the two types of forces (solid 

curve, fig. 2.A) is net repulsion when particles are some 

distance apart and net attraction when they are very 

close together-on the order of 5 to 10 A. 

B 

t 

DISTANCE BETWEEN PARTICLES~ 

FIGURJD 2.-Forces between basal (001) surfaces of adjacent clay-mineral particles, as a function of interparticle distance. Modified after Overbeek (1952, p. 272). .A., Combination of repulsive double-layer and attractive van der 

Waals forces; B, effects of low, intermediate, and high concentration of electrolyte on net force between particles. 
REMOVAL OF 'VATER AND REARRANGEMENT OF PARTICLES DURING COMPACTION B5 

The electrolyte concentration, as discussed above, influences the thickness of the double layer. If the electrolyte concentration is great enough, it 1nay suppress 

the repulsive forces to a distance that lies within the 

distance range of net attraction and thus cause the clay 

particles to attract one another. This is shown schematically in figure 2B and is a currently favored explanation for the influence of electrolyte concentration on 

the flocculation of clay particles. 

EDGE-TO-FACE ATTRACTION 

The above conceptual framework for the interaction 

between double-layer and van der vVaals forces, intellectually satisfying as it is, unfortunately cannot account for all the forces observed in clay-waterelectrolyte mixtures, mainly because it assumes that all 

particles are arranged so that their basal surfaces are 

parallel and because it ignores the forces associated 

with the nonbasal edges of the particles. The existence 

of positive charges on the edges of clay-mineral particles is strongly supported by several lines of evidence 

(most of them enumerated by Lambe, 1958a, p. 8), the 

most convincing of which is the edge-to-face arrangement of particles seen in electron micrographs of clayey 

sediments (Rosenqvist, 1962, pl. 5). The positively 

charged edges apparently are attracted to the negatively charged basal surfaces. 

FACTORS INFLUENCING THE REMOVAL OF WATER 

DURING THE EARLY STAGES OF COMPACTION 

"The water content which exists within a clay mass 

at any given time represents a balance between the urge 

of the clay minerals to suck in water and the tendency 

of applied loads to squeeze out water" (Lambe and 

Whitman, 1959, p. 39). Among the significant factors 

that influence the tendency of clay to retain water under small overburden loads-0 to about 50 kg per cm2 

(kilogra1ns per square centimeter)-are the size and 

spatial arrangement of the particles, the clay-mineral 

constituents and their adsorbed cations, and the type 

and concentration of the interstitial electrolyte solutions. These factors are singled out here, not necessarily because they are the most significant influences, 

but because the literature contains enough material 

about them to make discussion profitable. The influence of other factors that may be significant-organic 

material, for instance-is almost completely unknown. 

The influence of particle arrangement on water retention when the clay is under pressure is treated in 

more detail in the later sections of this review. 

The present knowledge of the influence of the several 

factors, except for particle size, is based mainly on laboratory experiments. As nearly all these experiments 

have been performed on selected size fractions of 

monomineralic clays in contact with distilled water or 

simple electrolytes, their results cannot be used to quantitatively predict the compaction of complex natural 

clay-water-electrolyte mixtures. In most experiments, 

furthermore, a single factor has been so isolated for 

study that one cannot tell whether the factor might be 

significant or inconsequential when it operates in concert with other factors in nature. The experiments, 

however, do indicate what the effects of some of the factors might be, and they provide questions that might 

be asked in any study of the early stages of natural 

compaction. 

PARTICLE SIZE 

The water content and compaction rate of finegrained sediments are inverse functions of particle size. 

Figure 3A. shows the decrease in void ratio (a measure 

of water content in saturated sediments) with increasing particle size in unconsolidated sea-bottom and lake 

sediments. Part of the variation in the void ratio of 

these groups is related to their different depths of 

burial, which are listed below. 

Feet below 

sediment-water 

interface I. Lake ~ead ____________________________________ 0 

II. Lake ~aracaibO-------------------------------- 0-18 

III. Reservoirs in United States _____________________ 0-10( ?) 

IV. Atlantic Ocean _________________________________ 0-9 

V. Pacific and Arctic Oceans----------------------- 0-7( ?) 

Most of the sediments in group III were dried at least 

once between the time they were deposited and the time 

they were sampled; the sediments in the other groups 

were never dewatered in their natural state. Figure 3B, 

which Skempton (1953, p. 55) derived from many different sediments in the United States and in Great Britain, shows that the relation between void ratio and 

particle size persists and that the fine sediments are 

compacted more rapidly than the coarser sediments under moderate overburden loads. The influence of particle size is also well demonstrated in the different void 

ratios and compaction rates of the different size fractions of illite shown in figure 8. 

CLAY MINERALS 

The difference in the void ratios of the three principal 

clay minerals under a wide range of pressures is shown 

in figure 4. Because the void ratio of these clays is a 

measure of their water content, the figure shows that 

under any given pressure in the range from 1 to 10,000 

kg per cm2, montmorillonite retains more water than 

illite, and illite, in turn, retains more water than kaolinite. The differences in water content are related principally to differences in particle size or, more precisely, 
B6 MECHANICS OF AQUIFER SYSTEMS 

8 A 

6 

o~----,---------~--------~--------~ 1 10 100 1000 

MEDIAN DIAMETER, IN MICRONS 

2 

0 

~ a:: 

c 

~ 1 

OT---------r-------~~------~-

0.1 1 10 . 100 

EFFECTIVE OVERBURDEN PRESSURE, 

IN KILOGRAMS PER SQUARE CENTIMETER 

FIGURID 3.-Influence of particle size on void ratio of fine-grained sediments. A1 Unconsolidated sediments. I, Lake Mead 

(Sherman, 1953, p. 399)'; II, Lake Maracaibo (Sarmiento and Kirby, 1962, p. 719),; III, Reservoirs in Wyoming and 

other States (Hembree and others, 1952, p. 39) ; IV, Atlantic Ocean deep-sea deposits (Richards, 1962, p. 16); V, Pacific 

and Arctic Oceans, shallow and deep-sea deposits (data from Shumway, 1960, p. 454-463, and Moore, 1962, table 1). 

B1 Generalized relation between size, void ratio, and effective overburden pressure. After Skempton (19G3, p. GG). 

to differences in specific surface (surface area per unit 

mass). The following data, from Bower and Goertzen 

(1959), Diamond and Kinter (1958), and Kinter and 

Diamond (1960), show the range of the specific surface 

of pure clay minerals: 

The effect of specific surface on water sorption is shown 

schematically by Lambe's conception of typical sodiumsaturated clay-mineral particles isolated in electrolytefree water (fig. 5) : a fairly large particle of kaolinite 

and a much smaller particle of montmorillonite, each 

adsorbing a water envelope of the same thickness. The 

montmorillonite particle thus adsorbs much more water 

relative to its mass than does the kaolinite particle. 

8pecifto aur/ace 

(ml per 1/) 

Montmorillonite ------------------------ 600-800 

Illite ---------------------------------- 65-100 

Kaolinite ------------------------------ 5-30 

30 A 3 B 3 c 

22o 0 

i=2 Q2 ~ c( 

a:: a:: ~ a:: 

c c c ~ 10 ~1 §?1 

0 0 0 0.1 1 10 1 100 10,000 1 100 10,000 

PRESSURE, IN KILOGRAMS PER SQUARE CENTIMETER 

FIGUBI!l 4.-Infiuence of clay minerals on relation between void ratio and compacting pressure. Note dl1ferent 

void-ratio and pressure scales in A. A1 At low pressure, saturated with 10-a N NaCl. After Mitchell 

(1960, figs. M-3, M-7). B1 At moderate to high pressure, saturated with distilled water. After Chlllngar 

and Knight (1960, p. 104) ; modifled by conversion of moisture contents to void ratios (assuming specific 

gravities : illite, 2. 75 g per em• ; montmorillonite and kaolinite, 2..6 g per cm1 ) and by conversion of psi to 

kg per em•. 0, At moderate to high pressure, saturated with water (Ionic composition not reported). 

After Krlukov and Komarova (1954) ; modified by conversion of moisture contents as in B. 
REMOVAL OF WATER AND REARRANGEMENT OF PARTICLES DURING COMPACTION B7 

(~ B 

FIGURE 5.-Clay particles and their envelopes of adsorbed water. After 

Lambe (1961, p. 559). A, Typical montmorillonite particle, 1,000 

by 10 A; B, typical kaolinite particle, 10,000 by 1,000 A. 

Differences in chemical composition within the major 

mineral types may have a secondary influence on the 

water contents of clay minerals under pressure. Foster 

(1953, 1955) found a negative correlation between free 

swelling and the substitution of iron and magnesium 

for aluminum in the octahedral positions of 12 different 

montmorillonite samples. The exchange positions of 

each montmorillonite sa.mple were sa.turated with 

sodium to standardize the effects of adsorbed ions. 

The swelling correlated much better with octahedral 

substitution than with either tetrahedral substitution 

or cation-exchange capacity. The differences do not 

seem to be due to charge deficiencies brought about by 

the substitution, because magnesium and trivalent ir.on 

caused about the same reduction in swelling volume. 

Rather the deficiencies seem to be due to changes in 

internal energy and bonding relations related to the 

differences in polarizing power between iron and magnesium on the one hand and aluminum on the other. 

Although Foster measured only the free swelling of 

the clays and not their behavior under increasing pressure, the different degrees of swelling should reflect 

at least qualitatively the degrees of resistance that the 

clays might offer to compaction. 

INTERSTITIAL ELECTROLYTE SOLUTIONS 

The interstitial electrolyte solutions seem to affect the 

response of clayey sediments to overburden loads in the 

range between 0 and about 10 kg per cm11• The effect 

is a complex function of the type and concentration of 

electrolyte and of the size of the clay particles. In 

recent years, attempts have been made to rationalize 

these complexities in terms of the diffuse double-layer 

theory, and the theory seems to correspond fairly closely 

to the experimental observations made on mixtures of 

N aCl solutions and montmorillonite. Attempts at 

using the theory to predict the compaction and swelling 

behavior of other clay-water-electrolyte mixtures, however, have been only partly successful or entirely unsuccessful. 

NaCl-MONTMORILLONIT.E-W AT.I!IR MIXTURES 

Because it provides an experimental verification of 

the diffuse double-layer theory, the NaCl-montmorillonite system probably has been given more attention 

than other electrolyte-clay mixtures. Experiments by 

Mitchell (1960), for example, showed some of the effects 

that the theory predicts (fig. 6) : the greater interparticle distances (larger void ratios) at a given pressure 

are associated with the smaller electrolyte concentrations, and the presence of nonclay particles dilutes but 

does not otherwise influence the response of montmorillonite to pressure (note in fig. 6B that the void-ratio 

scale is different from that in fig. 6A). Although the 

agreement between the theoretical and experimental results of Mitchell's study of unfractionated montmorillonite is not particularly close, other experiments on 

very fine grained montmorillonite ( <0.2 micron) 

showed rather close agreement. (See fig. 7 A ; see also 

Bolt, 1956, p. 91, and Warkentin and Schofield, 1960, 

1962.) 

The agreement between observed and theoretical results suggests that NaCl-montmorillonite systems satisfy the assumptions made in applying the double-layer 

theory to the compaction of clays. Some of these assumptions are worth examining because they do not 

seem to be satisfied by other electrolyte-clay systems. 

In addition to the validity of the Gouy-Chapman diffuse 

double-layer theory and the van't Hoff osmotic equation, 

which are used to compute the thickness of the double 

layer for a given pressure (see Bolt, 1956, p. 89-90), one 

must assume a model of the clay-water system in which 

the basal planes of all particles are parallel to one 

another a.nd norma.l to the compa.cting stress and in 

which the distance between particles is uniform for any 

given pressure and electrolyte concentration. 

Differences in particle orientation probably account 

for the different degrees of agreement between observed 

and theoretical compaction behavior in the first and 

subsequent compressions of montmorillonite reported 

by Warkentin and others (1957) (fig. 7A). During 

the first compression (open circles, fig. 7 A), the observed interpa.rticle spacings were greater than those 

predicted from theory (dashed line), probably because 

the orientation of montmorillonite flakes was not parallel. The compression, however, appa.rently improved 

the orientation, and the subsequent decompressions and 

recompressions yielded values very close to those predicted from theory. Had Mitchell (1960) repeated his 

loa.ding and unloading several times, perhaps his measured values would have eventually coincided with his 

theoretical curves (fig. 6). 

That sodium montmorillonites satisfy the assumption 

of uniform interparticle distance for a given electrolyte 
B8 MECHANICS OF AQUIF~R SYSTEMS 

30 A 15 B 

20 10 

0 0 

i= 

< ~ 0::: 0::: 

c c 

~ ~ 

10 5 

0;------------------r----------------~ 0+---------------~----------------. 

0.1 1 10 0.1 1 10 

PRESSURE, IN KILOGRAMS PER SQUARE CENTIMETER 

FIGURE G.-Influence of NaCl concentration on relation between void ratio and pressure in montmorillonite. After Mitchell (1960, 

figs .. M-7, M-8). Solid lines represent experimental results; dashed lines represent theoretical curves. A1 Montmorillonite 

alone; B, montmorillonite plus silt (50 percent each by weight). 

u.i 400 

u 

z 

~en 300 en:; 

c~ 

LLJI- <3 ~ 200 

-z 

~< 

~ ~ 100 

LLJ 

..... 

z 

A 

1 10 

PRESSURE, IN KILOGRAMS 

PER SQUARE CENTIMETER 

100 

1 

200 

100 

B 

NORMALITY OF NaCI 

0.1 0.03 0.01 

~+ 

++ 0 

+ 

+ + 

+ 

OT=~,_~~-r~~~--~~~ 

0 2 4 6 8 10 

(NORMALITY OF NaCI) -Yz 

12 

40 

x 

0 

30 g: 

a.. 

::5, 

20 Q 

..... 

< 

0::: 

10 c 

5 

> 

0 

FIGURE 7.-Relations between NaCl concentration, interparticle spacing, and pressure in montmorillonite. A, Experimental and 

theoretical relations between interparticle spacing and pressure in a mixture of <0.2-micron fraction of montmorillonite 

and 10-' N NaCl. After Warkentin and others (1957, p. 496). Open circles indicate first compression; solid circles, first 

decompression. Plusses indicate second compression, second decompression, and third compression. Dashed line is theoretical curve. B, Measured relations· between normality of NaCl solution and interparticle spacing in dilute suspensions 

of <0.1-micron fractions of montmorillonite. Data indicated by open circles from Foster and others (1955, p. 302), by 

plusses from Norrish (1954, p. 124); farthest right measurement by Norrish reported by Quirk (1960, p. 2 .. 2). 

concentration was suggested by the experiments of Foster and others ( 1955) and of N orrish ( 1954; also described by Norrish and Quirk, 1954), which are shown 

in figure 7 B. Both these experiments showed an inverse 

linear relation betw~een interparticle distance (measured 

by X-ray diffraction) and the square root of NaCl concentration at water contents greater than 50 percent by 

weight (void ratio about 2.6) and at electrolyte concentrations less than 0.3 N. At lesser water contents or at 

greater electrolyte concentrations, the interparticle distance diminishes rather abruptly to 20A or less, which 

corresponds in thickness to one, two, or three molecular 

layers of water. At '-vater contents greater than 50 

percent, the diffuse double layer apparently has the 

opportunity to form fully; at lesser water contents, the 

double layer does not seem to form, and the double-layer 

theory no longer accounts for the thickness of the water 

layers adsorbed on the particle surfaces. Graphs 

showing interparticle distance plotted against water 

content in sodium montmorillonites, not reproduced 

here, are similar to figure 7 B. (See also Foster and 

others, 1955, p. 300; Hight and others, 1962, p. 506-509 ; 
REMOVAL OF WATER AND REARRANGEMENT OF PARTICLES DURING COMPACTION B9 

and either N orrish, 1954, p. 126, or N orrish and Quirk, 

1954, p. 257.) 

N&Cl•XLLrl'E-W .A.TER MIXTURES 

The influence of N aCl concentration on the compaction behavior of illite varies with particle size. This 

was shown in the results of experiments on the type 

illite from Fithian, Ill. by Bolt (1956, p. 92) and by 

Mitchell (1960) (fig. 8). In very fine grained illite 

( <0.2 micron, fig. SA), as in the NaCl-montmorillonite 

system, the greater void ratio at a given pressure is associated with the more dilute electrolyte. Illite particles of this size are apparently so small that their behavior is controlled mostly by surface forces that can 

be predicted qualitatively (but not quantitatively-note 

the difference between the theoretical and measured 

curves) from double-layer theory. The opposite relation is found in the coarser illite ( < 1.0 micron, 

fig. SB): the greater void ratios are associated with the 

more concentrated electrolyte. This effect is even more 

pronounced in the coarsest illite (fig. 80). 

The coarser illite particles are apparently large 

enough to be affected somewhat by gravitational forces 

related to their mass, and their platy shapes make them 

susceptible to rearrangement under pressure. Only a 

part of their compaction takes place by decreasing the 

thicknesses of their double layers; the rest of it comes 

about through mechanical rearrangement. More pressure is required to rearrange the coarser particles that 

are in equilibrium with a more concentrated electrolyte 

because at higher concentrations of electrolytes the particles are more likely to flocculate. Once flocculated, 

6 A 

5 

4 

0 

~ 

B 

~ 

0:::2 

0 

~ ~ 

0:::3 

0 

~ 2 

c 

0+-----r----.--

0.1 1 10 0.1 1 10 

PRESSURE, IN KILOGRAMS PER SQUARE CENTIMETER 

FIGURE 8.-Influence of NaCl concentration and particle 

size on relation between void ratio and pressure in 

Fithian illite. Solid lines represent experimental results ; dashed lines represent theoretical curves. A, 

<0.2-micron fraction. After Bolt (1956, p. 92; also 

reported by Bolt and Miller, 1955, p. 287). B, <1.0-

micron fraction. After Mitchell (1960, fig. M-5). C, 

Unfractionated. After Mitchell (1960, fig. M-3). 

they are not as free to rearrange as they w~uld be if 

they had been subject only to the repulsive interaction 

of double layers; therefore, they offer more' resistance 

to compaction. 

One compression experiment on a <2-micron fraction of kaolinite mixed with 1 N and 10-3 N solutions 

of NaCl (Mitchell, 1960, fig. M-4) showed the same relation between void ratio and N aCl concentration as 

that observed in the coarser grained illites. 

OTHER ELECTROLYTE-MONTMORILLONITE-W .A.TER 

MIXTURES 

Clays mixed with other electrolytes do not always 

respond to compacting pressures in the same way as 

do clays mixed with N aCl. Montmorillonites that are 

mixed with electrolyte solutions containing calcium, 

magnesium, or aluminum, and whose exchange positions are saturated with these cations, will not swell to 

interparticle distances greater than 9 A, regardles~ of 

the dilution of the interstitial electrolyte ( N orr1sh, 

1954 · also reported by N orrish and Quirk, 1954). Appare~tly, some minimum-energy situation (perhaps an 

appropriate balance of repulsive ~ouble-lay.er and. attractive van der Waals forces) exists at this spacrng, 

beyond which the montmorillonite crystal will not expand freely. Likewise, montmorillonites . saturated 

with potassium do not seem to expand readily beyond 

5 or 6 A with decreasing KCl concentration (Foster 

and others, 1955, p. 302-303; N orrish, 1954, P: 124) . 

This implies that the water in these clays occupies two 

different types of pore space : interlamellar spaces, 

whose thickness does not exceed 9 A, within the clay 

aggregates ; and "external" pore spaces . ?~tween t?e 

clay aggregates. The existence of two distinctly different sizes of pore space in calcium montmorillonite 

is supported by the nitrogen -sorption measurements 

reported by Aylmore and Quirk (1962, p .. 109-112). 

The compaction behavior of these clays, therefore, cannot be completely understood on the basis of doublelayer theory because one of the main assumptionsuniform interparticle distance at a given electrolyte 

concentration-is not satisfied. 

An example was provided by the experiment of 

Blackmore and Miller (1961), the results of which are 

shown in figure 9. They compressed a mixture of 

montmorillonite and 2 X 10-4 N CaCl2 solution under 

selected pressures and allowed it to reswell after rel~ase 

of pressure. The initial compressions followed fairly 

closely the pattern predicted from double-layer theory 

(dashed line). The reswelling, however,. did not follow the theoretical pattern, thus suggestrng that the 

montmorillonite flakes were compressed into aggregates 

that remained intact after the pressure was released. 

X-ray measurements supported the idea that these ag-
BIO MECHANICS OF AQUIFER SYSTEMS 

u.i 100 

~ 10 () 

z x < 0 t-en a:: ~:::!: a. co a. 

L&.Ja:: ~ 

cjlii 50 

~ 50 _(.!) t= 1-Z O:< < a:: ~~ "'..;:: 

~ ', 0 a:: 

LLJ '> 0 1- > ~ 0 0 

0.1 1 10 100 

PRESSURE, IN KILOGRAMS 

PER SQUARE CENTIMETER 

FIGURE 9.-Experimental and theoretical relations between 

interparticle spacing and pressure in mixture of montmorillonite and 2 X 10-4 N CaC12 solution. After Blackm()re 

and Miller (1961, p. 170). Theoretical curve indicated by 

dashed line. 

gregates were "domains"-packets of several parallel 

unit montmorillonite sheets-within which the interlamellar distance remained a constant 8.8 A, regardless 

of the total water content of the clay. 

MgCls (OR CaCl2)•ILLITE-W A.TER MIXTURES 

Mixtures of <2.0-micron fractions of Fithian illite 

and solutions of MgCl2 and CaCl2 were studied by 

Olson and Mitronovas (1962). The results of their 

compression studies of illite mixed with different solutions of MgCl2 are shown in figure 10 : these results 

apply also to CaCl2 solutions, as the effects of calcium 

seem to be very similar to those of magnesium. The 

experimnts showed the greatest resistance to compression at Mg012 concentration near 10-2 N. The other 

four curves in figure lOA show a decrease in void ratio 

6 A 

with decreasing concentration. The effects of different 

electrolyte concentrations seem to disappear at the 

maximum pressure of ~bout 30 kg per cm2. Therebound curves for all concentrations are essentially 

identical. 

The observed decrease in void ratio with decreasing 

electrolyte concentration is contrary to the predictions 

of double-layer theory (compare fig. lOA, B). This 

reversal corresponds to the behavior of coarse-grained 

illite in N aCl solutions (fig. 8B, 0) and can be explained 

in terms of the same model of particle rearrangement 

and flocculation. Olson and Mitronovas ( 1962, p. 193-

194) did not believe that the influence of electrolyte 

content on the void ratios of these illites was especially 

significant: they found that they could produce the 

same degree of variation in void ratio by varying other 

factors such as particle arrangement and initial water 

content. 

All these observations on the influence of electrolytes 

and electrolyte concentration were made on simple systems-mixtures of monomineralic clays with solutions 

of a single electrolyte, mixtures in which the cation 

of the electrolyte was the same as the cation adsorbed 

on the clay-mineral surfaces. In them one can find only 

suggestions of what might be the influence of mu1tipleelectrolyte solutions on multimineralic clays or of what 

one might expect in mixtures in which one cation predominates in the pore solution and another cation predominates in the assemblage adsorbed on the particle 

surfaces. 

4 B 

0.1 1 10 100 0.1 1 10 100 

PRESSURE, IN KILOGRAMS PER SQUARE CENTIMETER 

FIGURE 10.-Infiuence of MgCis concentration on relation between void ratio and pressure in < 2.0-micron fractions of illite. 

After Olson and Mitronovas (1962, p. 195, 197). .A.~ Experimental; B~ predicted from double-layer theory. 
REMOVAL OF WATER AND REARRANGEMENT OF PARTICLES DURING COMPACTION Bll 

EXCHANGEABLE CATIONS 

Another approach to understanding the chemical influences on clay compaction is to compare the behavior 

of clays whose exchange positions are saturated with 

different ions. Most of the experimental work done so 

far has tested the effects of different inorganic cations. 

Very little is known of the influence of adsorbed anions 

on the relation between water and pressure in clays. 

Likewise, except for a few compression experiments on 

clays mixed with organic liquids (mostly alcoholssee Jimenez Salas and Serratosa, 1953, p. 196-198; 

Waidelich, 1958), very little is known of the effects of 

organic ions and molecules. 

Because of its large exchange capacity, montmorillonite has been used in most of the studies of the influence of different cations. Results of three studies of 

some of the common cations are shown in figure 11. 

Montmorillonite saturated with sodium has a consistently larger void ratio at a given pressure than does 

montmorillonite saturated with aluminum, potassium, 

or calcium (or, presumably, magnesium, whose influence 

seems to be about the same as that of calcium). This 

difference is apparently related to the ability of sodium 

montmorillonite crystals to swell freely (fig. 7B), and to 

the limited ability of the other montmorillonites to swell 

to interparticle distances greater than 9A. The difference in the void ratios at low pressures between Bolt's 

experiments (fig. 11A) and the other two experiments 

(fig. 11B, 0) is related to differences in experimental 

conditions-Bolt began using a greater initial water 

content and used a finer grained montmorillonite. 

25 

20 

Q 15 15 1-

<X: 

a:: 

0 

0 10 Q1o > 1-

< 

a:: 

0 

Thomas and Moody ( 1962, p. 154) also measured the 

water contents of montmorillonite saturated with sodium, calcium, and aluminum at pressures of 0.33 and 

15 kg per cm2 : their results, which are too brief to be 

included in the figure, are similar to those shown in 

figure 110. 

Although Samuels' work (fig. 110) suggested that 

differences in void ratio arising from differences in the 

exchangeable cations are insignificant at pressures 

greater than 10 kg per cm2 , Bolt's results (fig. 11A, 

representing very fine grained montmorillonite) suggested that these differences may be significant at somewhat greater pressures. The inference from Bolt's 

results is supported by a study of montmorillonite-rich 

sediments in the San Joaquin Valley of California 

(Meade, 1963), which suggests that the influence of 

different exchangeable cations may be significant in 

natural sediments at pressures as great as 50 kg per 

cm2 • 

Other experiments by Samuels showed that the influence of exchangeable cations on the void ratio of kaolinite are different from their influence on the void ratio of 

montmorillonite. Results of his experiments on fractionated kaolinite (80 percent finer than 1 micron, 98 percent finer than 2 microns) are given in figure 12. Comparison with figure 11 shows two differences: the influence of different cations on the void ratio of kaolinite is 

much less than their influence on the void ratio of montmorillonite, and the general relation between void ratio 

and cation valence is reversed. Because clays flocculate 

more readily in solutions that contain cations having 

B 20 c 

15 

0 

i= 

< 

a:: 10 0 

0 

> 

5 0 5 

~ 5 > 

0 0 0 

10 0.1 1 0.1 1 10 

PRESSURE, IN KILOGRAMS PER SQUARE CENTIMETER 

FIGURE 11.-lnfluence of exchangeable cations on relation between void ratio and pressure in montmorillonite. A., 

<0.2.a fraction mixed with 10-a molar chloride solutions of indicated cations. After Bolt (1956, p. 91). 

B, <1.1,a fraction mixed with hydroxide solutions (concentrations unspecified) of indicated cations. After 

Jimenez Salas and Serratosa (1953, p. 194). a, Presumably unfractionated and mixed with distilled water. 

After 'Samuels (1950; cited and described by Grim, 1962, p. 255-2.56). 
B12 ME~CS OF AQUIFER SYSTEMS 

4 

3 

0 

~ 0:: 2 

0 

~ 

0~---------------.--------------~ 

0.1 1 10 

PRESSURE, IN KILOGRAMS PER SQUARE CENTIMETER 

FIGURE 12.-lnfluence of exchangeable cations on relation between void ratio and pressure in kaolinite. After Samuels 

(1950; data also reported by Grim, 1962, p. 257). 

high valences, perhaps this reversal in kaolinite represents the effects of particle flocculation that were invoked above to explain the relation between void ratio 

and electrolyte concentration in coarse-grained illite. 

REMOVAL OF WATER DURING THE LATER STAGES OF 

COMPACTION 

Compared to the removal of water during the early 

stages of compaction, which has been studied intensively in recent years by soil scientists and foundation 

engineers, the removal of wat~r during the later stages 

of compaction has received little specific attention. 

Some general observations can be made, however, mainly 

on the basis of experiments conducted by von Engelhardt and Gaida ( 1963) and by Van Olphen ( 1963). 

Of the factors that seem to influence the amount of 

water held in clayey sediments under low pressures, the 

clay-mineral composition and particle size also seem to 

be effective at higher pressures. Apparently, the 

amount of surface area available for water sorption is 

a major governing factor over the entire range of pressures to which natural clayey sediments are subjected. 

Electrolyte concentration and exchangeable cations, on 

the other hand, do not seem to influence the relation 

between water content and pressures greater than about 

30-50 kg per cm2 • Experiments by von Engelhardt 

and Gaida (1963, p. 924-925) showed essentially no difference in the equilibrium void ratios, at any given pressure in the range from 30 to 3,100 kg per cm2 , either of 

montmorillonite mixed with N aCl solutions, CaCl2 solutions, and distilled water, or of mixtures of montmorillonite and N aCl solutions varying in concentration 

between 4.6 N and pure water. The only effect noticed 

was an increase in the rate of compaction-that is, the 

rate of water expulsion-with increasing N aCl concentration. Apparently the physicochemical influences of 

the different cation types and electrolyte concentrations 

do not affect the amount of water held by a clay unless 

the amount exceeds a certain minimum necessary to form 

diffuse double layers around the particles. This minimum amount seems to be about 50 percent by weight in 

very fine-grained ( <0.1 micron) sodium montmorillonite; it should be somewhat less in coarser grained 

and more silty clays. When overburden loads have reduced the amount of water in a clayey sediment below 

this minimum, the forces that must be overcome to compact the sediment further are more conveniently 

thought of as forces of hydration-the attractions between the clay surfaces and water or between cations 

and water-rather than as forces of repulsion or attraction between particles. 

The energiM involved in the later stages of water expulsion have been estimated from measurements of the 

temperatures and negative water-vapor pressures required to remove the last few molecular layers of water 

from clays. From published data on the desorption of 

montmorillonites in response to decreasing water-vapor 

pressure, Van Olphen (1963, p. 182) estimated that the 

work required to remove the second and last layers of 

adsorbed water from calcium-saturated montmorillonite surfaces is equivalent to compacting pressures on 

the order of 2400 and 5200 kg per cm2, respectively. 

These estimates are only approximate, but they indicate 

the enormity of the pressures required to squeeze the 

last increments of water from clayey sediments. Furthermore, these estimates are minimum values because 

they assume a parallel orientation of montmorillonite 

particles normal to the compacting stress-that is, they 

assume that the full load is borne by the water and that 

no load is borne by the solid particles. In nature, however, the particles probably will not lie in completely 

parallel positions but will touch one another at points 

of contact through which some of the load may be 

supported. 

Adsorbed water can be removed from clay surfaces at 

temperatures that are fairly low in contrast to the large 

pressures required. Figure 13 shows pressure-temperature curves for the loss of the last layer of adsorbed 

water from montmorillonite surfaces at low watervapor pressures as determined by Crowley and Roy 

(1959). Although the temperatures necessary to dehydrate the montmorillonites vary with the composition 

of the coordinated mineral lattice and with the exchangeable cations, they change only slightly with in-
REMOVAL OF WATER AND REARRANGEMENT OF PARTICLES DURING COMPACTION Bl3 

0.001 

1 

IV 

z 

I I I 

I I I 

I I I 

I I I I I I 

I I I 

tlllJI I 

::?! II I 

Ill "'' f ()/ 

I I 

I I 

I I 

n 1 

II I 

Ill 

:211 I 

I 11 

Ill I, 

II 

Iii ' . 

I II 

0 100 200 

TEMPERATURE, IN DEGREES CENTIGRADE 

FIGURE 13.-Pressure-temperature curves for dehydration reactions of montmorillonite minerals saturated with sodium, calcium, and magnesium. 

After Crowley and Roy ( 1959, p. 19). Montmorillonite represented by solid lines ; saponite by 

dashed lines. 

creasing water-vapor pressure. If one considers the 

range of temperatures and pressures to which most natural sediments are subjected) temperature might be as 

significant a factor as pressure in removing the last 

increments of water from clays. 

A 

The combined influences of pressure and temperature 

were assessed by Van Olphen (1963, p. 183-186) in experiments on the removal of the last two molecular 

layers of water from a vermiculite clay. From the desorption of the vermiculite with decreasing water-vapor 

pressure at 25°0, he estimated (and his estimates were 

partly verified by .X-ray measurements made during 

compression of the vermiculite to 2,000 kg per cm2 ) 

that the work involved in removing the second and last 

layers of water was equivalent to compacting pressures 

of 1,200 and 5,000 kg per cm2, respectively. From a 

desorption experiment at a higher temperature, 50°0, 

he estimated the same two compacting pressures at 640 

and 4,300 kg per cm2 • That is, an increase in temperature caused a noticeable decrease in the amount of pressure energy required to dehydrate the clay. 

The removal of water during the late stages of the 

eompaction of clayey sediments needs much more 

study-not only in terms of pressure and temperature, 

but also taking into account and assessing the significance of sueh factors as particle sizes, clay-mineral composition, and salinity. 

FABRIC OF CLAYS AND ITS CHANGES UNDER PRESSURE 

The discussion of the fabric of clayey sediments is 

in three steps. First discussed are some of the factors 

other than pressure that influence the arrangement of 

clay-mineral particles. Then follmYs a summary of the 

available information on the fabric of uncompacted 

natural sediments and, finally, a summary of experic 

f·.·.·.·.·.·.·.·t 

D, .... ·.· .. ·.·.·3 ~ 

~~~··= 

~~· 

~L:''······!i! ~~·· . 

~.~ ...... : .. :·.·J· ~ 

FIGURE 14.-ldealized arrangements of clay-mlneral particles in sediments. A, Flocculated, edge to face, in salt-free water. After 

Lambe (1958a, p .. 11). B, Flocculated, face to face, in salt solution. After Lambe (1958a, p. 11). a, Flocculated, edge to 

face. After Tan (1958, p, 87; 19:59, p. 92). D, Preferentially oriented. E, •Turbostratic. 
B14 MECHANICS OF AQUIFER SYSTEMS 

mental and field observations on the influence of pressure on fabric. 

Some of the more probable arrangements of platy 

clay-mineral particles in sediments are illustrated in 

figure 14. Schofield and Samson (1954, p. 142-143) 

and Lambe (1958a, p. 11) inferred the two types of 

flocculation shown in figure 14A, B from considerations 

of interparticle forces. In concentrated electrolyte 

solutions, the net force between particles may be attraction, resulting in a flocculated arrangement as shown in 

figure 14B. In electrolyte-deficient water, the principal attractive forces may be between the negatively 

charged basal surfaces and the positively charged particle edges, resulting in an edge-to-face arrangement as 

shown in figure 14A. A three-dimensional model of 

edge-to-face arrangement as visualized by Tan (1958, 

p. 87; 1959, p. 92) is shown in figure 140. Preferred 

orientation of clay particles is represented in figure 14D. 

"Turbostratic" arrangement (name suggested by Aylmore and Quirk, 1960) is illustrated in figure 14E. It 

consists of domains-also called "tactoids," "packets," 

or "clusters"-of clay sheets within which the preferred 

orientation is nearly perfect and between which the orientation may be entirely random. The sequence A-BD in figure 14 represents qualitatively a progression 

from random orientation to preferred orientation of 

particles. 

FACTORS, OTHER THAN PRESSURE, THAT INFLUENCE 

THE ARRANGEMENT OF CLAY PARTICLES 

Which of the factors that constitute a sedimentary 

environment might influence the arrangement of particles in a clayey sediment~ On the basis of work published through 1962, the following chemical factors seem 

to be significant: electrolyte concentration, types of 

associated cations and anions, acidity or alkalinity, and 

associated organic matter. Mechanical factors of particle deposition also seem to be significant, but they have 

received less attention than have the chemical influences. 

The available information on the relation between 

these factors and clay fabric is limited. Most of it is 

based on experiments in which the influence of individual factors has been isolated for detailed scrutiny; the 

combined influence of all factors in complex natural 

sediments is yet to be evaluated. Furthermore, most of 

these experiments recorded the influence of various factors at atmospheric pressure and only provided indirect 

suggestions of the influence that the factors might have 

in sediments under overburden loads. And, finally, the 

arrangements of particles in most of these experiments 

was inferred indirectly from observations of sedimentation volume and from the tendencies of the clays to 

aggregate or disperse in different chemical solutions. 

In very few of these experiments has the fabric been 

measured directly by X-ray, optical, or other means. 

ELECTROLYTE CONCENTRATION 

Electrolyte concentration seems to have different 

effects on the fabrics of different clay minerals. It influences illite and kaolinite in one way, montmorillonite 

in another. Rosenqvist (195.5, p. 53-62) experimented 

with the sedimentation of 10-percent suspensions of illite 

in N aCl solutions ranging in concentration between 

0.002 and 0.5 N and found that the equilibrium volume 

of settled-out sediment increased with increasing concentration of N aCI. Similar results were obtained by 

Hsi and Clifton ( 1962, p. 272) in their experiments with 

1- to 2-percent suspensions of kaolinite and metahalloysite that had flocculated in solutions of N aCl, CaCl2, 

FeCl2, and AlCl3 ranging in concentration from 0.0003 

to 0.02 N. If one assumes that the volume of the sediment reflects the degree of disorder of the particle arrangement (random orientation of the particles with 

regard to each other), these results suggest that the 

tendency toward random orientation of illite and kaolinite particles is a direct function of the electrolyte 

concentration. The arrangement of particles flocculated by the more concentrated electrolytes is presumed 

to be as shown in figure 14B. 

In montmorillonite, on the other hand, the relation 

between sedimentation volume and electrolyte concentration seems to be either inverse or nonexistent, 

depending on the cation involved. Hofmann and Hausdorf (1945) experimented with 0.4- to 4.0-percent suspensions of montmorillonite in N aCl and KCl solutions 

ranging in concentration from 0.004 to 2.0 N and found 

that the equilibrium volume of the sediment that 

settled out decreased with increasing electrolyte concentration. In CaC12 and MgC12 solutions, the sedimentation volume and interparticle distance (9-10 A) 

remained essentially constant at all concentrations between 0.004 N and 2.0 N. These observations are similar to those cited in the discussion of the influence of 

different electrolytes on the removal of water from 

montmorillonites under low pressures (figs. 6, 7, 9) and 

are perhaps best explained in the same terms--confonnity or lack of cvnformity with the predictions of the 

double-layer theory. 

The spatial arrangement of montmorillonite particles 

in dilute N aCl solutions-if it is in fact a function 

of the double-layer repulsion between particles-should 

be similar to the arrangement illustrated in figure 14D. 

That is, the particles are probably dispersed in the solution in an array that approaches parallel orientation, 

the interparticle distance varying with electrolyte concentration as shown in figure 7B. Because the sedimentation volume and interparticle distance of mont-
REMOVAL OF WATER AND REARRANGEMENT OF PARTICLES DURING COMPACTION B15 

morillonite do not vary with the concentration of CaCl2 

and MgCl2, the arrangement of particles in these solutions probably is not a salt-flocculated arrangement but 

something similar to the collection of parallel aggregates 

illustrated in figure 14E. 

These interpretations of the influence of electrolyte 

concentration on clay fabric are based on several assumptions-different assumptions, moreover, for montmorillonite and for the other clay minerals-and should 

be· regarded as tentative. The spatial arrangement of 

clay minerals in different solutions needs to be studied 

by some means more direct than inference from the 

sedimentation volume. 

CATIONS AND ANIONS 

In the experiments of Hsi and Clifton (1962) on the 

flocculation of kaolinite and halloysite in solutions of 

NaCl, CaC12, and AlCls, the volumes of the settled-out 

suspensions in the different electrolytes increased in the 

order Na+1 , Ca+2, Al+3• This increase suggests that the 

higher valence cations are associated with a more random particle arrangement; the lower valence cations, 

with a greater degree of preferred orientation. The 

suggestion is supported by Samuels' observations on kaolinite under pressure (fig. 12) but is contradicted by 

the observed influence of cations on the void ratio of 

montmorillonite under pressure (fig. 11). Apparently, 

as is suggested by the experiments on electrolyte concentration, the different cations have a different influence on the different clay minerals. 

Properties of the cation other than the valence also 

seem to influence the settling volume, as shown by experiments of Rosenqvist ( 1955, p. 59; 1959, p. 40) on 

illites sedimented in 0.75 N concentrations of N aCl and 

KCI. He reported that the volume of the sediment 

that settled in the KCl solution was about 30 percent 

greater than the volume that settled in the N aCl solution. Experiments with illites that settled in 0.75 N 

solutions of LiCl (smaller settling volume than in 

NaCl) and CsCl (greater settling volume than in KCl) 

indicate further that valence is not the only cation property that influences the arrangement of particles. 

Lambe ( 1958a, p. 7, 9) suggested, on the basis of doublelayer considerations, that this influence on the settling 

volume is an inverse effect of the size of the hydrated 

ion, which seems to decrease in the order Li > N a > 

K > Cs. That is, the smaller hydrated ions allow the 

illite particles to approach one another more closely, to 

flocculate more readily, and to orient more randomly. 

The influence of different anions on the arrangement 

of clay particles is essentially unknown. Hsi and 

Clifton (1962) studied the influence of NaCl, Na2SO,, 

and Na:C03 on the flocculation of kaolinite and halloysite, and Longenecker ( 1960, p. 188) measured the 

settling volumes of a montmorillonite-rich clay in different concentrations of N aCl and N a2SO,. Except 

for the observation that the montmorillonite-rich clay 

has a larger equilibrim sedimentation volume inN a2SO, 

solutions than in N aCl solutions of equivalent normality, little more can be said than that the anions are 

likely to influence the fabric. 

ACIDITY 

Schofield and Samson ( 1953, 1954) found an apparent relation between acidity and flocculation in 10-

percent suspensions of kaolinite. Purified sodium 

kaolinite flocculated spontaneously in distilled water 

but deflocculated on addition of small amounts of 

N aOH. Measurements of the adsorption and desorption of chloride ions by the kaolinite led Schofield and 

Samson to conclude ( 1) that the flocculation resulted 

from attractions between positively charged particle 

edges and negatively charged particle faces (fig. 14A), 

and (2) that the positive edge charges (and hence the 

tendency to flocculate) were strongest under acid conditions, diminished with increasing pH, and eventually 

became negative. 

Sisler (1960, p. 192) measured the sedimenation volume of samples of mud from the bottom of Lake Mead 

after he allowed them to settle in HCl or N aOH solutions adjusted to a series of pH values between 2 and 12. 

Judging from Rolfe's (1957) analyses of sediments 

from the same part of Iake Mead (Boulder Basin), 

the mineral constituents of the mud are probably quartz, 

illite, and kaolinite, plus minor amounts of montmorillonite and feldspar. The largest sedimentation volume-1 percent greater than the volume at pH 7.2 

(approximately the natural pH)-was observed at pH 

2. The smallest volume-22 percent less than at pH 

7.2-was observed at pH 10. These results suggest that 

the tendency for clays to assume an open flocculated 

arrangement is substantially less at strongly alkaline 

pH than at neutral or strongly acid pH. 

ORGANIC MATTER 

Organic matter probably influences the arrangement 

of clay particles in sediments. Studies by Bloomfield 

(1956), Lawson and Keilen (1951}, and SOderblom 

( 1960), among others, indicated that kaolinite or illite 

can be flocculated or deflocculated by solutions of some 

of the organic materials that are likely to be associated 

with sediments-aqueous extracts from tree leaves, bark, 

wood pulp, or peat-but these studies contain no direct 

indications of the arrangement of particles. 

Ingram ( 1953) observed a direct correlation between 

the presence of organic material and the preferred 

orientation of clay-mineral particles in 50 clayey rocks 

from Colorado, Iowa, and Wisconsin. The rocks 

ranged in age from Ordovician to Eocene. Ingram 
B16 MECHANICS OF AQUIFER SYSTEMS 

made his observations from thin sections of the rocks. 

He noted (p. 873-875) that organic stain was associated 

with clays that showed preferred orientation and that 

it was absent frmn clays in which the orientation was 

random. In one rock sample, this relation was expressed in alternate organic and nonorganic clayey 

layers. Organic matter also seemed to influence the 

fabric of the clay minerals when the rocks were disaggregated, suspended, and then sedimented from suspensions by adding N aCl. Clay minerals from rocks 

that contained organic material settled out of suspension in a pronounced parallel arrangement, whereas 

the arrangement of sedimented particles from the nonorganic rocks was mostly random. 

PHYSICAL CONDITIONS OF DEPOSITION 

In addition to being influenced by chemical factors, 

the fabric of a clayey sediment must also be affected 

by physical and mechanical factors involved in deposition: rate of deposition, state of agitation or quiescence 

of the water in which the sediment is deposited, particlesize distribution and concentration of the sediment 

being deposited, and drying of the sediment between 

deposition and burial. 

The concentration of the sediment in the depositing 

medium, for example, seems to influence the degree to 

which the particles assume whatever equilibrium fabric 

is favored by the existing chemical conditions. Raitburd ( 1960, p. 110) reported that, whereas kaolinite 

particles settled out of 2- to 5-percent suspensions to 

form a preferentially-oriented fabric (measured by Xray diffraction), no predominant orientation of particles was observed in kaolinite that had settled out of a 

50-percent (by weight~) suspension. In the experiments of Olson and Mitronovas {1962, p. 192), a calcium illite sedimented from a very dilute suspension 

maintained a larger void ratio under pressures in the 

range from 0 to 10 kg per cm2 than did one sedimented 

from a 34-percent (by weight) suspension. This result 

suggests that the illite sedimented from the more dilute 

suspension assumed a more open flocculated arrangement, which influenced its response to pressure. 

Results of experiments by Isaac Barshad (oral communication, 1963) suggested that drying may have irreversible effects on the fabric of some montmorillonites. 

In his experiments, well-dispersed montmorillonites 

were first saturated with sodiu1n, potassium, calcium, 

or magnesium and sedimented by centrifuging. The 

montmorillonites then were dried, dispersed again mechanically, and sedimented once n1ore by centrifuging 

under the same conditions as before. The amounts of 

water retained by the sedimented clays are expressed 

volumetrically in the following void ratios : 

Void ratio 

After first centri- After drying and 

Exchangeable cation fuging recentrifuging 

Sodium or potassium_____________ ,...,40 ,...,40 

Calcium or magnesium____________ 18-25 6-8 

These results suggested that drying irreversibly increases the size of the domains into which calcium and 

magnesium montmorillonites are aggregated-an effect 

similar to the irreversible compression of calcimn montmorillonites that was observed by Blackmore and 

Miller (1961) and is represented in figure 9. The wet 

volmne (and presumably the fabric) of sodium and 

potassium montmorillonites, en the other hand, does not 

seem to be influenced significantly by intermediate 

drying. 

FABRIC OF UNCOMPACTED NATURAL CLAYS 

Most of the specific information on the fabric of recently deposited and unconsolidated clays comes from 

the electron micrographs of illitic clays from Scandinavia made by Rosenqvist ( 1958, p. 441 ; 1959, p. 39; 

1962, pl. 5). In the marine clays that he examined, 

Rosenqvist {1959, p. 38) found an openwork arrangement of particles dominated by contacts between corners and planes, corresponding "to an astonishing 

degree" with the arrangement as visualized by Tan (fig. 

140). Rosenqvist reported further (1962, p. 25) that, 

in about a hundred stereoscopic electron micrographs 

of unconsolidated and unweathered marine clays from 

Scandinavia, he had "* * * never observed anything 

resembling a domain structure." In fresh-water clays, 

on the other hand, he has found a greater degree of 

parallel orientation between clay particles-in arrangements approaching that shown in figure 14D. Rosenqvist related the difference between the fabrics of 

marine and fresh-water clays to the flocculating effect 

of the electrolytes in sea water. In fresh water, the 

illite particles apparently did not aggregate readily but 

settled individually out of suspension into more efficiently packed arrangements. 

Rosenqvist's observations on the different fabrics of 

marine and fresh-water clays are partly corroborated 

by observations made in other parts of the world in 

sediments buried under a few tens of feet of overburden. At the.~:~e depths, presumably, the fabric re· 

fleets its original state more than it reflects the influence 

of pressure. Raitburd {1960, p. 113, 115) reported 

completely random orientation-measured by X-ray 

diffraction-of illite and kaolinite particles in a Quaternary marine clay from the northeastern part of the 

Black Sea. Mitchell (1956), frmn his thin-section 

study of 14 marine and nonmarine clays from North 

America, reported generally better formed preferred 

orientation in the nonmarine clays. Wu (1958), on the 
HEMOVAL OF WATER AND REARRANGEMENT OF PARTICLES DURING COMPACTION B17 

other hand, found different degrees of random and preferred orientation in glacial-lake clays from the southern Great Lakes area that could not be related to different types or concentrations of ions in the interstitial 

waters. 

An indirect indication of the conditions under which 

clays might be flocculated at fairly low salinities is 

given by evidence from studies of the sediments transported by the Colorado River and deposited in Lake 

Mead. Sherman ( 1953, p. 396-400) compared the settling velocities of fine-grained lake sediments in the lake 

water with those of the same sediments in solutions of 

a dispersing agent and found that many of the particles 

in nature were aggregated into floccules. Howard 

( 1960, p. 104) found the same type of floccules suspended 

in the waters of the Colorado River at Grand Canyon, 

145 river miles upstream from Lake l\fead. Sherman 

also compared the pore volumes of natural lake sediments with those of sediments that had been dispersed 

artificially. The natural sediments having median 

diameters of about 1 micron had an average void ratio 

of about 6 (curve I, fig. 3A) ; sediment of the same size 

that had been dispersed with a chemical agent and 

allowed to settle out of suspension had a void ratio of 

about 2. Although these observations give no indication 

of the degree of orientation, they suggest that the arrangement of fine particles transported by the Colorado 

River and deposited on the bottom of Lake l\fead might 

be less preferred than are the orientations observed in 

Scandinavian lakes by Rosenqvist. The clay minerals 

in Lake Mead and presumably in the river sediment are 

illite, montmorillonite, and kaolinite, in proportions that 

vary from one part of the lake to another (Rolfe, 1957). 

The pH of slurries of the lake sediment ranges from 

7.0 to 7.6 (Rolfe, 1957, p. 380). The principal ions 

dissolved in the lake and river water are Na+t, Ca+2, 

S04+2

, and HCOa-1 , and the total salinity usually ranges 

between about 300 and 1200 parts per million (Howard, 

1960, p. 108). Expressed as normality, this concentration range is about 0.005 to 0.02 N, which was the range 

of concentrations of the sodium and calcium salts that 

was sufficient to flocculate kaolinite minerals in some, 

but not in all, of the experiments of Hsi and Clifton 

(1962). 

In addition to most of the fabrics observed in clayey 

sediments, clayey soils exhibit some peculiar particle 

arrangements of their own. Oriented coatings on silt 

and sand grains, oriented films on the walls of cracks 

or pores, and domainlike aggregates seem to be formed 

in soils by elutriation, repetitive drying, and leaching 

processes. For details of these processes and fabrics, 

see Brewer (1956), l\finashina (1959), and Rosenqvist 

( 1962, p. 23-25). . 

INFLUENCE OF PRESSURE ON THE FABRIC OF CLAYS 

The removal of water under the influence of pressure 

implies that the solid particles in a clayey sediment must 

move closer together into a more efficiently packed arrangement. At least two special arrangements-preferred orientation and turbostratic orientation (fig. 

14D, E)-may be formed during compaction. These 

two fabrics, and particularly the preferred orientation, 

have received most of the emphasis in compaction studies. One should keep in mind, however, that other responses to pressure such as the bending and crumpling 

of clay particles are also possible. 

FABRIC OF CLAYS COMPRESSED IN THE LABORATORY 

Laboratory experiments on the rearrangement of particles in water-saturated clays under uniaxial pressure 

can be divided into two groups: those in which the sides 

of the clay are not confined but are allowed to spread 

freely, and those in which the sides of the clay are rigidly confined. Neither condition is an exact replication 

of the distribution of pressures during natural compaction, but the laterally confined experiments probably 

represent nature more closely. 

UNCONFINED COMPRESSION 

Preferred orientation is produced readily during unconfined compression. Aggregates of Georgia kaolinite 

were subjected to uniaxial compressions of 0.22 and 0.55 

kg per cm2 by Buessem and Nagy (1954). X-ray powder photographs showed that the orientation of kaolinite 

flakes in the uncompressed aggregates was random and 

that compression increased the orientation of the flakes 

normal to the direction of applied load. Williamson 

(1947) produced preferred orientation by repeatedly 

dropping a ball of illite-kaolinite clay onto a glass plate 

from a height of about 1 foot; he produced the some orientation by squeezing a similar ball of clay between 

parallel glass plates in a hand press. Raitburd ( 1958, 

p. 791) compressed a cylinder of kaolinite paste between 

glass plates to one-third its original thickness and produced a clearly detectable (by X-ray diffraction) preferred orientation. Popov ( 1944) observed preferred 

orientation in fluid montmorillonite pastes (water, 40-

70 percent by weight) that he produced by squeezing 

the pastes between glass slides. The combination of 

compression and lateral flow apparently favors the formation of preferred orientation. 

C01MPRESSION OF LATERALLY CONFINED CLAYS 

Some clays form preferred orientation under low 

pressure when they are laterally confined. Mitchell 

(1956) mixed an illite-quartz powder with sea water, 

placed it in a rigid-walled cylinder, and subjected it to 

a uniaxial load of 4 kg per cm2 • A thin section of the 

compressed clay showed that the illite particles, initially 
B18 MECHANICS OF AQUIFER SYSTEMS 

in random orientation, were in preferred orientation in 

the plane normal to the compressive stress. He also 

stu9-ied the effects of uniaxial pressures of 2 kg per cm2 

in a group of 14 marine and nonmarine clays from 

North America. In the marine clays, in which the 

predmninant clay mineral was illite, the compression 

caused an increase in the degree of preferred orientation over large areas as well as within small domainlike aggregates (generally less than 0.5 mm in diameter). In the nonmarine clays, most of which contained 

substantial proportions of montmorillonite, the results 

of the compression were not consistent; the degree of 

preferred orientation was increased in some samples 

and decreased in others. 

Domains also seem to form in some clays under low 

to moderate pressures. From measurements of the 

sharpness of X-ray reflections from compressed calcium 

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

inferred that the number of montmorillonite unit 10-A 

thick sheets per domain increased progressively with 

pressure, ranging from about 5 unit sheets at 0.5 kg per 

cm2 to nearly 8 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. 

Preferred orientation seems to form readily under 

great pressures applied in laboratory experiments. 

Engelhardt and Gaida (1963, p. 925-926) observed 

an increase in the degree of preferred orientation of 

montmorillonite and kaolinite particles with increasing 

pressures between 80 and 800 kg per cm2• Olson (1962, 

p. 34, 37) produced an extreme degree of preferred orientation by cyclically compressing and decompressing 

calcium illite 15 times at pressures between 4.4 and 

4,500 kg per cm2 •. Raitburd (1960, p. 111-113) made a 

series of uniaxial compression experiments on pastes of 

different clay minerals-presumably kaolinite and 

montmorillonite. He did not state the range of pressures used in these experiments, but one can infer from 

other experiments described in his paper that the pressures may have been as great as 5,000 or 10,000 kg per 

cm2• When thin pastes (1.5 mm) were compressed, a 

homogeneous orientation formed normal to the compressive stress. When thicker pastes (2 em) were compressed, orientation formed in two directions: one 

normal to the compression, and the other along slip 

planes at about 45° to the direction of compressive 

stress. The particle orientation in all these highpressure experiments was measured by X-ray diffraction. 

Evidence of bending and crumpling of fine-grained 

clay particles under pressure is scarce. Tan (1959, p. 

93) stated that montmorillonite particles may be broken 

under heavy local stress concentrations, but he did not 

cite evidence or give an idea of the magnitude of pressures he had in mind. Norton and Johnson (1944) 

said that kaolinite particles averaging 0.32 micron in 

diameter and 0.04 micron in thickness were distorted 

by pressures as low as 40 kg per cm2 ; but again no 

direct evidence was cited. 

INFLUENCE OF ELECTROLYTE CONCENTRATION UNDER 

PRESSURE 

Von Engelhardt and Gaida (1963, p. 926) used X-ray 

diffraction to measure the bulk degree of preferred orientation of montmorillonite compressed under 800 kg 

per cm2 in three solutions of NaCl ranging in concentration from 0.16 to 1.1 N. They found an inverse relation between the concentration of N aCI and the degree 

of preferred orientation at right angles to the compacting pressure. They found also that, whereas the void 

ratio remained constant at about 0.5 in all three 

samples, the permeability (as measured by the rate of 

compaction) increased with increasing N aCl concentration. These observations can be explained by a greater 

degree of preferred particle arrangement (fig. 14D) at 

smaller concentrations and by a more turbostratic arrangement (fig. 14E) at larger concentrations. Figures 

14D and 14E are drawn with approximately equal void 

ratios and show the effect of fabric on permeabilitywater moves more readily through large pore spaces. 

In addition to providing tho most satisfactory model 

to explain the above observations, a turbostratic fabric 

seems reasonable in a mixture of montmorillonite and 

1.1 N NaCl when one recollects studies that were discussed earlier in this review. The experiments illustrated in figure 7 B indicated that, at N aCI concentrations greater than about 0.3 N, montmorillonites do not 

expand readily to interparticle spacings greater than 

9 A. Montmorillonites mixed with calcium electrolytes 

do not expand readily beyond interparticle spacings of 

9 A at any concentration. Figure 9 and other observations suggest that particles of calcium-saturated 

montmorillonite under load are compressed irreversibly 

into domains. Perhaps montmorillonite in contact 

with concentrated NaCl solutions behaves in the same 

way. 

FABRIC OF NATURALLY COMPACTED CLAYEY 

SEDIMENTS 

Only a few detailed studies have been made of the 

orientation of clay-mineral particles in shales, and, with 

the exception of one study, there is little evidence to 

support the formation of oriented fabrics-either preferred or turbostratic-during natural compaction. 

In a recent study of a 2,000-foot-thick section of unconsolidated nonmarine sediments (Meade, 1961b), 

X-ray diffraction measurements showed no progressive 

increase of the degree of orientation o:f montmorillonite 
REMOVAL OF WATER AND REARRANGEMENT OF PARTICLES DURING COMPACTION B19 

particles parallel to the bedding with increasing depth 

(fig. 15). The only significant preferred orientation 

was found in a fresh-water-lake deposit, which suggests 

that the fabric might not have been produced by compacting pressures but by the slow settling of individual 

clay particles in the still waters of the lake. In the 

rest of the sediments, mainly alluvial-fan and floodplain deposits, no preferred orientation seems to have 

formed either during deposition or as a result of compaction. 

If one considers that von Engelhardt and Gaida 

(1963) were able in the laboratory to produce an increase in the degree of preferred orientation of pure 

montmorillonite with increasing pressures between 80 

and 800 kg per cm2, the lack of preferred orientation in 

the alluvial sediments represented in figure 15 may be 

due to one or more of the following reasons: 

1. Perhaps not enough pressure has been exerted on the 

natural sediments to cause any discernible preferred orientation in the montmorillonite; the 

maximum effective load to which these sediments 

have been subjected is 60 to 70 kg per cm2• 

2. Perhaps the presence of silt and sand particles inhibits the formation of preferred orientation in 

the natural sediment. 

3. Perhaps the montmorillonites have been compressed 

into domains (which are not detectible by the Xray method) rather than into a homogeneous 

parallel fabric. 

1- 0 LIJ 

LIJ 

LL. 

~ 

ui 

(.) 500 <( 

LL. 

0:: 

:::> 

en 

~ 1000 <( 

....1 

3: 

0 

....1 

LIJ 

a:l 

J: 

1-

a.. 

LIJ 

0 

MONTMORILLONITE ORIENTATION RATIO 

123 12 123 

• 

o.........-...._.......__J 

500 

1000 

1500 

• 

• • • 

• 

• ... • 

0 

500 

1000 

1500 

• 

• 

• • 

• 

• 

• 

• 

• 

• 

• 

FIGURE 15.-Horizontal preferred orientation of montmorillonite in unconsolidated clayey sediments from three core holes 

in San Joaquin Valley, Calif. Montmorlllonite orientation 

ratio computed from X-ray-diffraction peak heights (explained 

by Meade, 1961a) : the larger the ratio, the greater the degree 

of preferred orientation parallel to the bedding. Ratio near 

1.0 signifies random ortentatlon. Lacustrine desposits c:rossbatcbed. 

The formation of domains is a likely possibility for 

two reasons: ( 1) The principal exchangeable cation is 

calciun1, and (2) domainlike aggregates that were deposited as such (shale fragments and other aggregates) 

are available for nucleation of further material. 

Kaarsberg ( 1959) found, in flat-lying shales buried 

under 2,700 feet or more of overburden in several parts 

of the United States and Canada, that the preferred 

orientation of illite increased with the depth of burial. 

Illite is the predominant clay mineral in all the natural 

sediments he studied. Many of them contain chlorite; 

only a few have even minor amounts of montmorillonite. He used X-ray diffraction and sound velocities 

to measure preferred orientation, and he used dry bulk 

density as a measure of compaction. A sample of his 

measurements is tabulated below. Using a method described elsewhe~e (Meade, 1961a), I computed the "orientation ratio" from X-ray diffraction measurements 

that Kaarsberg had plotted in a graph on page 469 of 

his report. The larger the ratio, the greater the degree 

of preferred orientation of illite particles parallel to the 

bedding. Other X-ray diffraction measurements plus 

sound velocities support Kaarsberg's conclusion (p. 

470-471) "* * * that the degree of preferred orientation of the basal planes of the illite particles parallel 

to the bedding increases with compaction." 

Results of Kaarsberg' s study (1959) 

[Orientaton ratio computed bY! Meade} 

Present 

depth beZow Bulk demitu Orientation 

Type o/ sample 

Recent clay ______________ _ 

Cretaceous shale __________ _ I>o __________________ _ 

])() __________________ _ 

])() __________________ _ 

Slate ____________________ _ 

surface (/t) (g per cm3 ) ratio 

0 

4,340 

5, 136 

6, 776 

9, 120 

1.9 

2.4 

2. 5 

2. 6 

2. 7 

2. 9 

1. 5 

2. 8 

2. 2 

3. ? 

5.0 

32. 0 

In many older shales, however, increasing degree of 

preferred orientation with increasing depth of burial 

has not been observed. White (1961, p. 564), for example, stated that the fissility (which is presumably 

a reflection of fabric) of Paleozoic shales in Illinois does 

not vary systematically with the depth of burial. In 

the same group of Paleozoic shales, Grim and others 

(1957) noted a relation between preferred orientation 

of clay minerals and particle size; preferred orientation 

was better formed in fine-grained shales (in which 

most of the particles were smaller than 2 microns) 

than in coaser grained shales that contained appreciable 

amounts of nonclay" mineral grains. The influence of 

natural compaction on the fabric of clayey sediments 

apparently needs to be evaluated in conjunction with 

other likely influential factors. 
B20 MECHANICS OF AQUIFER SYSTEMS 

SUMMARY 

The removal of water and the rearrangement of 

particles in clayey sediments under pressure are complex functions of particlb size, clay minerals and associated ions, interstitial electrolyte concentration acid- . ' Ity, temperature, and the arrangement of particles at 

the onset of compaction. Other factors such as associated organic material may influence the compaction 

processes, but very little is known about them. 

Particle size is perhaps the most significant factornot only in its consistently inverse relation to pore 

volume under a wide range of pressures, but also in 

the sense that it influences the nature and degree of the 

influence t~at the other factors have on the progress 

of compactiOn. In very fine grained clays, which have 

large areas of particle surface available to interact 

with interstitial water and dissolved ions, the influence 

?f electrolytes and exchangeable ions can be expressed 

In terms of the forces of repulsion or attraction related 

to the particle surfaces. In coarser grained clays, on 

the other hand, the surface forces become less significant, and one must give more consideration to the rearrangement of particles in response to gravitational 

forces associated with the particle mass. 

The different influences of cation type and electrolyte 

concentration under low overburden pressures seem to 

be as follows: 

1. In fine-grained clays ( montmorillonites and finegrained illite) : · 

(a) In sodium solutions less concentrated than 

about 0.3 N, the water content at equilibrium (and perhaps the degree of preferred orientation of the particles~) 

increases with decreasing concentration. 

(b) In sodium solutions more concentrated than 

about 0.3 N or in potassium, calcium, 

magnesium, or aluminum electrolyte solutions, the variation of water content with 

electrolyte concentration is less apparent 

because of the tendency of the clays to 

aggregate irreversibly into oriented domains in which the interparticle distance 

does not exceed 9 A. This tendency is reinforced by overburden pressure during 

compaction. 

2. In coarse-grained clays (kaolinite and coarsegrained illite}, the water content at equilibrium 

and the randomness of particle orientation increase 

with increasing electrolyte concentration or with 

increasing cation valence. This relationship apparently results because the coarse clays flocculate 

more readily in the more concentrated solutions or 

in higher valence cation solutions to form openwork structures that resist the compacting effects 

of small overburden pressures. 

These observations, however, were made on singlecation, single-electrolyte, single-mineral systems and 

have only suggested what might be observed in more 

complex systems. 

The response of clayey sediments to compacting pressures greater than about 50 kg per cm2 seems to be influenced mainly by the particle size, clay minerals 

(whose influence is related mainly to their size), and 

temperature. Under these pressures the cation type 

and . electrolyte concentration may influence the clay 

fabric, but they do not seem to influence the total 

amount of interstitial water held in the sediments. 

The main effect of increasing temperature is to reduce 

the amount of pressure necessary to remove the interstitial water. 

Although preferred and turbostratic orientations can 

be pr~du?ed by .compressing clays in the laboratory, 

there IS httle evidence to show that these fabrics are 

formed readily and generally during natural compaction. ~{ainly on the basis of experin1ental evidence the 

following factors might see1n to encourage the fo;mation of preferred orientation under overburden loads: 

1. Existence of a partly oriented fabric at the onset of 

compaction. If preferred orientation is already 

incipient in the sediment, it would seem more 

likely to form to a greater degree during compaction than if the particles were in some other kind 

of arrangement. 

2. Increasing size of clay-mineral particles. J(aolinite 

and illite semn to be oriented more readily under 

pressure than is montmorillonite. 

3. Decreasing concentration of interstitial electrolyte. 

This decrease should reduce the tendency of the 

coarser clays to flocculate and the tendency of the 

finer clays to aggregate into domains. 

4. Decreasing cation valence also should reduce the 

tendency of the clays to flocculate or to form domains. 

5. Decreasing acidity. 

6. Presence of organic material. 

The formation under pressure of a turbostratic fabric 

instead of a general degree of preferred orientation 

might be favored by increasing electrolyte concentration and increasing cation valence. 

The influence of all these factors on compaction 

processes needs further study. Especially needed are 

studies of the combined influences of the factors and 

their relative significance under a wide varietv of natural conditions. " 
REMOVAL OF WATER AND REARRANGEMENT OF PARTICLES DURING COMPACTION B21 

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B22 MECHANICS OF AQUIFER SYSTEMS 

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*U.S. GOVERNMENT PRINTING OFFICE: 1967 0-241-118