Subpart B
Photo of a landslideGEOLOGIC
HAZARDS

92 NATURAL HAZARDS

GEOLOGIC HAZARDS 93
Introduction
N
on-seismic ground failures involving 
landslides, land subsidence, and 
expansive soils are significant hazards 
that affect life and property in the 
United States. These hazards receive limited 
public attention and funding for hazard 
assessment and research. However, the combined 
average annual damage may be comparable 
to that caused by floods, earth-
quakes, and volcanoes. 
The occurrence of geologic hazards is often 
interrelated with other natural phenomena: 
heavy rainstorms prompted by atmospheric 
hazards can lead to flooding which can cause 
debris flows; land subsidence can exacerbate 
flooding; droughts can provoke shrinking of 
expansive soils; and ground failures often 
occur during earthquakes and volcanic activity. 
PhotoPhoto: JCP Geologists, Inc. 


Photo of a landslide
96 NATURAL HAZARDS: GEOLOGIC HAZARDS
Chapter Summary
S
everal human and natural factors may contribute to 
or influence landslides. Understanding how these 
factors interrelate is important in analyzing land-
slide hazards. The principal human factors are mining and 
construction of highways, buildings, and railroads. The 
principal natural factors are topography, geology, and precipitation. 
Landslides occur in every State and in Guam, Puerto Rico, 
the U.S. Virgin Islands, and American Samoa. They are 
most common in the coastal ranges of California, the 
Colorado Plateau, the Rocky Mountains, and the 
Appalachian Mountains. During the past 20 years, land-
slides have resulted in 38 disaster declarations, 15 of them 
in California. According to a 1985 study, roughly 40 per-
cent of the U.S. population is exposed to direct and indirect 
effects of landslides. 
Landslides have damaged or destroyed roads, railroads, 
pipelines, electrical and telephone lines, mines, oil wells, 
buildings, canals, sewers, bridges, dams, seaports, airports, 
forests, parks, and farms. Landslides often are triggered 
by other natural events such as floods, earthquakes, and 
volcanic eruptions. The damage caused by landslides 
often is attributed to the triggering events. The best estimates 
of annual losses resulting from landslides in the 
United States are 25 to 50 lives and $1 to $2 billion in 
property damage. 
Successful mitigation programs have been undertaken at 
the local level, but the Federal effort is relatively under-
funded. Recent efforts involved identification of land-
slide-prone areas, anticipating landslide events, and implementation 
of warning systems. Hazard reduction efforts 
involve reducing the frequency of landslides, reducing the 
likelihood that they will cause damage, and minimizing 
damage. 
PhotoPhoto: Red Cross 

LANDSLIDES 97
Photo of a landslidePhoto: Lynn Forman 

98 NATURAL HAZARDS: GEOLOGIC HAZARDS
HAZARD IDENTIFICATION 
"Landslide" is used to describe the downward and out-
ward movement of slope-forming materials reacting 
under the force of gravity. The term covers a broad category 
of events, including mudflows, mudslides, debris 
flows, rock falls, rock slides, debris avalanches, debris 
slides, and earth flows. Landslides may consist of natural 
rock, soil, artificial fill, or combinations of these 
materials. Earthquakes trigger many landslides, as does 
heavy and prolonged rains which lead to saturated conditions. 
Landslides are classified by type of movement and type 
of material (Varnes, 1978). The types of movement are 
slides, flows, lateral spreads, and falls and topples 
(Varnes, 1978; Pearce and others, 1993; Fleming and 
Varnes, 1991). The types of material are bedrock and 
soils, where soils are described as predominately coarse 
or predominately fine. A combination of two or more 
of the principle types of flows is referred to as a "complex 
movement." 
 Slides. Slides of soil or rock involve downward displacement 
along one or more failure surfaces. The 
material from the slide may be broken into a number 
of pieces or remain a single, intact mass. Sliding can 
be rotational, where movement involves turning 
about a specific point. Sliding can be translational, 
where movement is downslope on a path roughly parallel 
to the failure surface. The most common example 
of a rotational slide is a slump, which has a strong, 
backward rotational component and a curved, 
upwardly-concave failure surface. 
 Flows. Flows are characterized by shear strains distributed 
throughout the mass of material. Flows are 
distinguished from slides by high water content and 
the distribution of velocities resembles that of viscous 
fluids. Debris flows are common occurrences in 
much of North America. These flows are a form of 
rapid mass movement in which loose soils, rocks, and 
organic matter, combined with air and water, form a 
slurry that flows downslope. The term "debris 
avalanche" describes a variety of very rapid to 
extremely rapid debris flows associated with volcanic 
hazards, and is discussed in Chapter 18. The term 
"avalanche," if unmodified, normally refers to slope 
movements of snow and ice, and is discussed in 
Chapter 6. Mudflows are flows of fine-grained mate-
rials, such as sand, silt, or clay, with a high water con-
tent. A subcategory of debris flows, mudflows contain 
less than 50 percent gravel. 
 Lateral Spreads. Large elements of distributed, lateral 
displacement of materials characterize lateral 
spreads. They occur in rock, but the process is not 
well-documented and the movement rates apparently 
are very slow. Lateral spreads can occur in fine-
grained, sensitive soils such as quick clays, particularly 
if remolded or disturbed by construction and 
grading. Loose, granular soils commonly produce 
lateral spreads through liquefaction. Liquefaction can 
occur spontaneously, presumably because of changes 
in pore-water pressures, or in response to vibrations 
such as those produced by strong earthquakes. 
 Falls and Topples. Falls occur when masses of rock 
or other material detach from a steep slope or cliff and 
descend by free fall, rolling, or bouncing. 
Movements are rapid to extremely rapid. Rock falls 
commonly are triggered by earthquakes. Topples 
consist of the forward rotation of rocks or other mate-
rials about a pivot point on a hillslope. Toppling may 
culminate in abrupt falling, sliding, or bouncing, but 
the movement is tilting without collapse. Data on 
rates of movement and control measures for topples is 
sparse. 
A few nationwide studies of landslides and debris flows 
have been conducted. The studies attempt to assess 
areas of the United States where landslides have 
occurred and areas that are susceptible to landslides. 
Baker and Chiuruzzi (1958) performed the first regional 
evaluation of landslides in the United States. They 
based their evaluation on the results of questionnaires 
completed by State and Federal agencies, companies, 
and consultants, and a review of 267 landslide articles 
and texts published prior to 1950. A list was developed 
of the most destructive types of landslides, along with 
estimates of the sediment volume and a small-scale map 
showing the areas of major, medium, minor, and non-
existent landslide intensity. 
Radbruch-Hall and others (1976) conducted a survey of 
the distribution of landslide deposits and materials susceptible 
to landsliding in the United States. Krohn and 
Slosson (1976) independently prepared a map of land-
slide potential as part of a comprehensive survey of natural 
hazards. 
Wiggins and others (1978) combined topographic, geologic 
and rainfall information from maps prepared by 
Baker and Chiuruzzi (1958) and Radbruch-Hall and 
others (1976). The combination map yielded landslide 
potential based on adverse formations associated with 
past landslide activity (Map 9-1). 

LANDSLIDES 99
U.S. Map 9.1
100 NATURAL HAZARDS: GEOLOGIC HAZARDS
Radbruch-Hall and others (1982) prepared Map 9-2 by 
evaluating formations and groups of formations shown 
on a geologic map of the United States (King and 
Beikman, 1974) and classifying them as having high, 
moderate, or low landslide incidence (number of land-
slides), and high, moderate, or low susceptibility to 
landsliding. Map units or parts of map units with more 
than 15 percent of their area involved in landsliding 
were classified as high incidence. Those with 1.5 to 15 
percent of their area involved in landsliding were classified 
as moderate incidence. Those with less than 1.5 
percent of their area involved were classified as low 
incidence. High, moderate, and low susceptibility were 
defined by the same percentages. Alluvial fans and 
earthquake influences were not evaluated. 
Several natural and human factors may contribute to or 
influence landslides. How these factors interrelate is 
important in analyzing the hazard. The three principal 
natural factors are topography, geology, and precipitation. 
The principal human activities are cut-and-fill 
construction for highways, construction of buildings 
and railroads, and mining operations. 
Topography and geology are related. Topography influences 
stream erosion and other energy sources that, in 
turn, influence slope angle and gradient. The steeper a 
slope, the more gravity plays a role in a landslide. The 
strength of rocks, measured in terms of their resistance 
to weathering, is a basic geologic factor in the landslide 
process. Certain bedrock formations or rock (soil) 
types appear to be more susceptible than others to land-
slide activity. 
Precipitation has a pronounced effect on the morphology 
of the landscape. Slope development is influenced 
by precipitation that runs off the slope by way of established 
drainage courses and may have the capacity to 
erode and undermine slope surfaces. Precipitation that 
is absorbed increases pore water pressure and lubricates 
inherently weak zones of rock or soil. 
RISK ASSESSMENT 
Landslides often are involved in or triggered by other 
natural hazards. For example, the safety of a dam can 
be severely compromised by upstream landsliding or 
collapse of slopes bordering the reservoir or dam abutments. 
Landsliding and flooding are closely related 
because both involve precipitation, runoff, and ground 
saturation. Debris flows usually occur in small, steep 
stream channels and often are mistaken for floods. 
Landslides often result from seismic activity as experienced 
during the 1964 Alaska earthquake, and volcanic 
activity such as occurred after the 1980 eruption of 
Mount St. Helens. The simultaneous or sequential 
occurrence of interactive hazards may produce cumulative 
effects that differ significantly from those expected 
from any one of the components. 
Alger and Brabb (1985) listed 6,500 references or 
sources that either incidentally mentioned or discussed 
in depth the subject of landslides. Brabb and Harrod's 
(1989) state-by-state analysis of landslides was the first 
of its kind, and included a 4-year reconnaissance of all 
States, the U.S. Virgin Islands, and Puerto Rico, as well 
as extensive conversations and cooperative programs 
with geologists and engineers in State geological surveys 
and State departments of transportation. 
Probability and Frequency 
Maps 9-1 and 9-2 are combinations of inventory maps 
showing areas of known landslides, and susceptibility 
maps showing areas that are likely to experience slope 
failures. The incidence of, and susceptibility to, 
landsliding are rated qualitatively as high, moderate, or 
low. Ratings are assigned on the basis of the number of 
known landslides and the potential for future landslides. 
Frequency of occurrence or probability of exceedance 
are not associated with the information presented. 
Jager and Wieczorek (1994) describe a methodology for 
estimating the spatial probability of a landslide in a 
given area. This regional model of landslide susceptibility 
(probability) was developed for the Finger Lakes 
Region in New York. The occurrence or non-occurrence 
of landslides based on inventory maps and field 
inspections in 270-ft (90-m) grid cells (GIS database) 
was identified throughout the study area. Logistic 
regression analysis was used to develop a model to predict 
the susceptibility (probability) of landslides in a 
given grid cell as a function of soil type, land slope, and 
historic lake levels. Three levels of susceptibility were 
defined: low (probability < 0.02); moderate (0.02 < 
probability < 0.05); and high (probability > 0.05). 
The method developed by Jager and Wieczorek (1994) 
could be used in any area of the United States where 
adequate landslide inventory maps are available. Note, 
however, that the probability associated with the susceptibility 
map is simply the probability of a landslide 
occurring in a given grid cell and has no implication for 
repetitive occurrence over time. 
Exposure 
Landslides occur in every State and in Guam, Puerto 
Rico, the U.S. Virgin Islands, and American Samoa. 
They have damaged or destroyed roads, railroads, 
pipelines, electrical and telephone transmission lines, 
mining facilities, petroleum wells and production facil-

LANDSLIDES 101
U.S. Map 9-2
102 NATURAL HAZARDS: GEOLOGIC HAZARDS
ities, residential and commercial buildings, canals, sewers, 
bridges, dams, reservoirs, port facilities, airports, 
forests, fisheries, parks, recreation areas, and farms. 
Damage caused directly by landslides is largely undocumented 
or often is mis-reported. The devastating 
effects of landslides often are attributed to a triggering 
principal event, such as a flood, earthquake, volcanic 
eruption, hurricane, or coastal storm. The magnitude of 
landslide-related financial losses may exceed all other 
types of damage commonly attributed to the principal 
event (Brabb and Harrod, 1989). The best estimates of 
the direct and indirect costs of landslide damage in the 
United States is $1 to $2 billion annually. Deaths related 
to landslides are estimated to be 25 to 50 annually 
(Schuster and Fleming, 1986; Wold and Jochim, 1989). 
Brabb and Harrod (1989) provided an estimate of the 
number of people per square kilometer who are exposed 
to the effects of landslides in each State. These estimates 
were developed by constructing a slope index 
map of the United States and determining the area of 
mountainous, hilly, and steep valley terrain, where 
landslides are likely to occur. The 1985 population by 
State was superimposed on the mountainous, hilly, and 
steep valley areas to determine the number of people 
per square kilometer. This analysis indicated approximately 
108 million people, or more than 40 percent of 
the 1985 population, were exposed to direct and indirect 
effects of landslides in the United States and its territories. 
Consequences 
Public and private economic losses from landslides 
include not only the direct costs of replacing and repairing 
damaged facilities, but also the indirect costs associated 
with lost productivity, disruption of utility and 
transportation systems, and reduced property values. 
Some indirect costs of landslides are difficult to evaluate, 
thus estimates are conservative or ignored. If indirect 
costs were rigorously determined, they likely 
would exceed direct costs (Schuster and Fleming, 
1986). 
Much of the economic loss is borne by Federal, State, 
and local agencies responsible for disaster assistance, 
flood insurance, and highway maintenance and repair. 
Private costs involve mainly damage to land and structures. 
A severe landslide can result in financial ruin for 
affected property owners because landslide insurance 
(except for debris flow coverage) or other means of 
spreading the costs of damage are unavailable. 
The most financially devastating slope failures in the 
United States were associated with the Alaska earth-
quake of 1964, the 1980 landslides in southern 
California, the eruption of Mount St. Helens in 
Washington, the 1982 landslides in the San Francisco 
Bay area, and the 1983-84 landslides in Utah. 
Combined, these events caused estimated damages in 
excess of $2 billion. 
Ground failure caused approximately 60 percent of the 
$311 million total damage from the 1964 Alaska earth-
quake (Youd, 1978). About one-third of the cost of the 
$1.5 billion 1980 eruption of Mount St. Helens can be 
attributed to landslides (Schuster, 1983). That same 
year, total losses in six southern California counties 
resulting from landslides triggered by high-intensity 
rainfall approximated $500 million (Slosson and 
Krohn, 1982). 
The intense storms of early January 1982 in the San 
Francisco Bay region triggered thousands of debris 
flows, resulting in 25 deaths and $66 million in property 
damage. Following the San Francisco catastrophe, 
930 lawsuits and claims totaling nearly $300 million 
were filed against city and county agencies in the 
region, an amount several times greater than the total 
property losses (Smith, 1982). 
Total direct costs of landslides in Utah in the spring of 
1983 were estimated at $250 million (Anderson and 
others, 1984), while estimates of the 1984 events were 
as high as $50 million (Schuster and Fleming, 1986). 
During the past 20 years, landslides have resulted in 38 
disaster declarations15 of them in California, and the 
rest scattered among 15 other States. While many 
States have adopted programs to investigate and resolve 
problems related to landslides, the total effort is relatively 
minor. 
RESEARCH, DATA COLLECTION, AND 
MONITORING ACTIVITIES 
The level of effort directed to landslide research and 
hazard assessment is small compared to some other 
hazards. At the Federal level, Brabb and Harrod (1989) 
concluded that the USGS devoted no more than 20 per-
son-years each year to landslide research. According to 
Sangrey and others (1985), USGS spent the bulk (about 
one-fourth) of the $10 million allocated for landslide 
research by all Federal agencies. 
Information about landslide work conducted by other 
Federal agencies is sparse. However, FEMA, the 
Federal Highway Administration, U.S. Bureau of 
Reclamation, U.S. Forest Service, and the National 
Science Foundation have all contributed. 

LANDSLIDES 103
In response to the severe rainstorm in January 1982 that 
triggered more than 18,000 debris flows and other land-
slides in the San Francisco Bay area, USGS and NWS 
developed and currently operate a system for warning 
the public when conditions reach levels sufficient to 
trigger debris flows. During the rainy season (October 
through April), USGS monitors more than 50 radiotelemetered 
rain gauges in the network coordinated by 
NWS. 
The rainfall data and measurements of soil moisture at 
a study site in the hills south of San Francisco are used 
to estimate moisture level of soils throughout the Bay 
area. Once the soils have reached a sufficient moisture 
level, USGS monitors NWS forecasts and uses real-
time rainfall data from the gauge network to determine 
the potential for imminent debris flows. 
USGS developed thresholds that describe the minimum 
rainfall rates that may trigger abundant debris flows on 
natural slopes in the San Francisco Bay region. NWS 
broadcasts warnings over weather radio and as an emergency 
broadcast system announcement over many radio 
and television stations. 
MITIGATION APPROACHES 
Successful and cost-effective landslide mitigation pro-
grams can be implemented. Such programs exist in 
other countries, including Japan. Although there have 
been some impressive and successful local demonstrations 
of landslide control programs, information has not 
been widely disseminated. This is characteristic of the 
scattered and diffused state of landslide knowledge in 
the United States. As noted by NRC (1985), there is no 
recognized national leadership or systematic basis for 
communication. 
Landslides stand out as a severe hazard, yet mitigation 
efforts are relatively underfunded. Practical application 
of land-use zoning measures, based on appropriate 
research and enforced by local regulations, can lead to 
dramatic loss reductions. This has been demonstrated 
in the Los Angeles area, where 92-97 percent reductions 
in losses were achieved for new construction 
(Slosson and Krohn, 1982). 
There are two distinct components to reducing the cost 
associated with landsliding: emergency management 
and response, and long-term hazard reduction (NRC, 
1985). Emergency management includes: anticipation, 
prediction, and issuance of warnings of the impending 
occurrence of life- and property-threatening landslides; 
response that is required when landslides occur; identification 
of landslide-prone areas; and planning, training, 
and other preparatory measures necessary to ensure 
effective warning and response. 
Long-term hazard reduction focuses on reducing the 
frequency of landslides, reducing the likelihood that 
landslides will cause damage, and minimizing damage 
when landslides do occur. Landslide losses can be 
reduced in two ways: reduce the occurrence by requiring 
that excavation, grading, landscaping, and construction 
be carried out in ways that do not contribute to 
slope instability; and minimize the damage when land-
slides do occur by restricting development in landslide-
prone terrain and by protecting buildings and other 
structures from landslide damage (NRC, 1985). Wold 
and Jochim (1989) provide additional details for reducing 
long-term losses. 
Although NRC (1985) identifies insurance as a long-
term hazard-reduction measure, Wold and Jochim 
(1989) note that insurance does not reduce losses, but 
provides financial protection to individual owners, 
Although insurance for landslides is not available, dam-
age from debris flows (mudflows) is covered under the 
NFIP. Campbell and others (1985) describe flows and 
slides that are and are not covered under the NFIP. 
Kockelman (1986) discusses additional techniques for 
reducing landslide losses: 
 Through land-use planning, discourage new developments 
in identified hazard areas by informing and 
educating the public and posting warnings of potential 
hazards; 
 Remove or convert existing development through 
acquiring, exchanging, or removing susceptible properties 
and discontinuing non-conforming uses; and 
 Provide financial incentives or disincentives by 
adopting lending policies that reflect risk of loss or 
conditioning Federal and State financial assistance. 
RECOMMENDATIONS 
Several programs have been proposed to reduce the cost 
of landslide-related damage in the United States. The 
NRC Committee on Ground Failure Hazards (1985) 
recommended: more effective land-use regulation; 
building codes; research on landslide initiation and 
processes, landslide hazard delineation, mapping, and 
control; technology transfer; landslide insurance 
(exclusive of debris flows, for which insurance already 
exists); national leadership; and legislation to direct a 
governmental or private program to reduce landslide 
losses. 

104 NATURAL HAZARDS: GEOLOGIC HAZARDS
The USGS proposed elements of a landslide loss-reduction 
program. At FEMA's request, USGS prepared a 
feasibility study for a nationwide landslide mapping 
effort (Campbell, 1985). 
BIBLIOGRAPHY AND REFERENCES 
Alger, C.S., and E.E. Brabb. 1985. Bibliography of 
United States Landslide Maps and Reports. U.S. 
Geological Survey Open-File Report 85-585. 
Anderson, L.R., J.R. Keaton, T. Saarinen, and W.G. 
Wells, II. 1984. The Utah Landslides, Debris Flows, 
and Floods of May-June 1983. National Research 
Council. Washington, DC: National Academy Press. 
Baker, R.F., and R. Chiuruzzi. 1958. "Regional 
Concept of Landslide Occurrence." Highway Research 
Board Bulletin. No. 216, pp. 1-17. 
Brabb, E.E., and B.L. Harrod. (Eds) 1989. Landslides: 
Extent and Economic Significance. Proceedings of 
the 28th International Geological Congress: 
Symposium on Landslides. Washington, DC, 
Rotterdam: Balkema. 
Campbell, R.H. (Ed.). 1985. Feasibility of a 
Nationwide Program for the Identification and 
Delineation of Hazards from Mud Flows and Other 
Landslides. U.S. Geological Survey Open-File Report 
85-276A. 
Fleming, R.W., and J.D. Varnes. 1991. "Slope 
Movements," Chapter 9 in The Heritage of 
Engineering Geology: The First Hundred Years, 
Kiersch, G.A. (Ed.) Centennial Special Volume 3. 
Boulder, CO: Geological Society of America. 
Fleming, R.W., and F.A. Taylor. 1980. Estimating the 
Costs of Landslide Damage in the United States. U.S. 
Geological Survey Circular 832. 
Harp, E.L., and R.W. Jibson. 1995. Inventory of 
Landslides Triggered by the 1994 Northridge, 
California, Earthquake. U.S. Geological Survey 
Open-File Report 95-213. 
Jager, S., and G.F. Wieczorek. 1994. Landslide 
Susceptibility in the Tully Valley Area, Finger Lakes 
Region, New York. U.S. Geological Survey Open-File 
Report 94-615. 
King, P.B., and H.M. Beikman. 1974. Geologic Map 
of the United States (exclusive of Alaska and Hawaii). 
Scale 1:2,500,000. U.S. Geological Survey. 
Kockelman, W.J. 1986. "Some Techniques for 
Reducing Landslide Hazards." Association of 
Engineering Geologists Bulletin. Vol. 33, No. 1, pp. 
29-52. 
Krohn, J.P., and J.E. Slosson. 1976. "Landslide 
Potential in the United States." California Geology. 
Vol. 29, No. 10, pp. 224-231. 
National Research Council. 1985. Reducing Losses 
from Landsliding in the United States. Washington, 
DC: National Academy Press. 
Pearce, L., H. Hightower, B. Konkin, S. Megalos, and 
J. Pernu. 1993. British Columbia: Hazard, Risk and 
Vulnerability Analysis. Vol. 1. The Disaster 
Preparedness Resources Center, The University of 
British Columbia. 
Radbruch-Hall, D.H., R.B. Colton, W.E. Davies, I. 
Lucchita, B.A. Skipp, and D.J. Varnes. 1982. 
Landslide Overview Map of the Conterminous United 
States. Scale 1:7,500,000. U.S. Geological Survey 
Professional Paper 1183. 
Radbruch-Hall, D.H., R.B. Colton, W.E. Davies, B.A. 
Skipp, I. Lucchita, and D.J. Varnes. 1976. Preliminary 
Landslide Overview Map of the Conterminous United 
States. Scale 1:7,500,000. U.S. Geological Survey 
Miscellaneous Field Studies Map MF-771. 
Sangrey, D.A., and others. 1985. U.S. Geological 
Survey Landslide Research Program's Report of the 
Committee for Review of the U.S. Geological Survey 
Ground Failure and Construction Hazards Program. 
U.S. Geological Survey Administrative Report. 
Schuster, R.L., and R.W. Fleming. 1986. "Economic 
Losses and Fatalities Due to Landslides." Association 
of Engineering Geologists Bulletin. Vol. 23, No.1, pp. 
11-28. 
Slosson, J.E., and J.P. Krohn. 1982. "Southern 
California Landslides of 1978 and 1980." Storms, 
Floods and Debris Flows in Southern California and 
Arizona, 1978 and 1980: Proceedings of a 
Symposium. Washington, DC: National Academy 
Press. 
Smith, T.C. 1982. "Lawsuits and Claims Against 
Cities and Counties Mount After January 1982 Storm." 
California Geology. Vol. 35, No. 7, pp.163-164. 

LANDSLIDES 105
Toppozada, T., G. Borchardt, W. Haydon, M. Peterson, 
R. Olson, H. Lagorio, and T. Anvik. January 1995. 
Planning Scenario in Humboldt and Del Norte 
Counties, California, for a Great Earthquake on the 
Cascadia Subduction Zone. California Department of 
Conservation, Division of Mines and Geology, Special 
Publication 115. 
Varnes, D.J.. 1978. "Slope Movement Types and 
Processes," in Landslides: Analysis and Control, 
Schuster, R.L., and R.J. Krizek (ed.). National 
Academy of Sciences, National Research Council, 
Transportation Research Board. Special Report No. 
176, pp. 11-33. 
Wiggins, J.H., J.E. Slosson, and J.P. Krohn. 1978. 
Natural Hazards: Earthquake, Landslide, Expansive 
Soil Loss Models. Institute of Behavioral Science. The 
University of Colorado, Boulder, CO. 
Wold, R.L. Jr., and C.L. Jochim. 1989. Landslide Loss 
Reduction: A Guide for State and Local Government 
Planning. Denver, CO: Colorado Geological Survey, 
Department of Natural Resources. 
Youd, T.L. 1978. "Major Cause of Earthquake Damage 
is Ground Failure." Civil Engineering. American 
Society of Civil Engineers. Vol. 48, No. 4, pp. 47-51. 



108 NATURAL HAZARDS: GEOLOGIC HAZARDS
Chapter Summary
L
and subsidence, the loss of surface elevation due to 
the removal of subsurface support, ranges from 
broad, regional lowering of the land surface to 
localized collapse. The primary causes of most subsidence 
are human activities: underground mining of coal, ground-
water or petroleum withdrawal, and drainage of organic 
soils. This hazard affects parts of at least 45 States. 
However, because of the broad range of causes and 
impacts, there has been limited national focus on this hazard. 
Regional lowering of land elevation occurs gradually over 
time. It may aggravate flooding potential, particularly in 
coastal areas. Collapses, such as the sudden formation of 
sinkholes or the collapse of an abandoned mine, may 
destroy buildings, roads, and utilities. 
Generally, subsidence poses a greater risk to property than 
to life. The average annual damage from all types of subsidence 
is conservatively estimated to be at least $125 mil-
lion. Damage consists primarily of direct structural dam-
age and property loss and depreciation of land values, but 
also includes business and personal losses that accrue during 
periods of repair. 
Current efforts to address subsidence include improved 
hazard identification, public information programs, map-
ping, regulation of subsurface resources and land development, 
land-use management and building codes, and insurance. 
Mitigation measures generally are designed for specific 
situations. 

LAND SUBSIDENCE 109 
Photo: Bettmann 

110 NATURAL HAZARDS: GEOLOGIC HAZARDS
HAZARD IDENTIFICATION 
Land subsidence affects parts of at least 45 States. 
More than 17,000 mi2 (44,000 km2) of land has been 
lowered. Resource development and land-use practices, 
particularly underground mining of coal, groundwater 
and petroleum withdrawal, and drainage of organic 
soils, are the primary causes (NRC, 1991). Land subsidence 
due to surface faulting and liquefaction, triggered 
by earthquakes, is discussed in Chapter 16. 
Subsidence is caused by a diverse set of human activities 
and natural processes that include mining of coal, 
metallic ores, limestone, salt, and sulfur; withdrawal of 
groundwater, petroleum, and geothermal fluids; dewatering 
of organic soils; wetting of dry, low-density 
deposits known as hydrocompaction; natural sediment 
compaction; melting of permafrost; liquefaction; and 
crustal deformation. This diversity and the broad range 
of impacts probably influence lack of a national focus 
on subsidence. Instead, many industries, professions, 
and Federal, State, and local agencies are involved independently. 
Collapse Into Voids. Collapse of surficial materials 
into underground voids is the most dramatic form of 
subsidence. Most of the subsidence-related voids in the 
United States were created by coal mining. Coal-mine 
subsidence is caused by collapse of the mined-out or 
tunneled voids, and depends on the number, type, and 
extent of the voids. Abandoned tunnels and under-
ground mining of metallic ores, limestone, and salt con-
tribute to a much smaller extent, although problems 
may be severe at specific locations. For example, hundreds 
of subsidence occurrences in the Midwest have 
been associated with failures of abandoned lead-zinc 
mines. 
In the longwall method of mining coal, in which most of 
the coal seam is removed along a single face, the roof 
above the mined-out seam is allowed to collapse as the 
mining progresses. Subsidence above longwall mines is 
rapid, generally ending within a few months after 
removal of subsurface supports. 
Subsidence associated with partial extraction mining is 
usually unplanned. In this method, only a portion of the 
coal called the "rooms" is removed; the unmined portions, 
the "pillars," are left to provide support. 
Subsidence resulting from collapse into rooms may take 
years to manifest. Examples of collapses occurring 100 
years after mines were abandoned have been documented 
(NRC, 1991). 
Although most collapses are human-induced, some cavities 
in bedrock were formed prior to human activities. 
This is particularly true of carbonates such as limestone, 
because rates of solution are so low. Cavities in halite 
can be an exception because of its high solubility. For 
example, in the last 30 years several dozen sinkholes 
have formed in Kansas as a result of dissolution of salt 
beds by leaks through casings of brine-disposal wells. 
A recent example is a 200-ft (60-m) wide, 110-ft (33-m) 
deep sinkhole that formed during the summer of 1988 
near Macksville, KS (Geotimes, 1988). Catastrophic 
subsidence is most commonly induced by water-table 
lowering, rapid water-table fluctuation, diversion of surface 
water, construction, use of explosives, or impoundment 
of water. 
Sediment Compaction. Sediment compaction typically 
causes broad regional subsidence. Exceptions 
include ground rupture and hydrocompaction. Rates of 
subsidence usually are low, ranging from a few millimeters 
to centimeters per year, but total subsidence 
may reach several meters over decades. Sediment compaction 
results from underground fluid withdrawal, natural 
compaction, and hydrocompaction. 
Underground fluid withdrawal is one of the major causes 
of sediment compaction in the United States. The 
weight of the overburden above fluid reservoirs is sup-
ported by both the fluid pressures and stresses transmitted 
through the solid skeleton of the reservoir soil or 
rock. When fluids are withdrawn, fluid pressures 
decline and support of the overburden is transferred to 
the solid skeleton. If the reservoir soil or rock is compressible, 
sediment compaction and subsidence occur. 
Another type of sediment compaction occurs naturally 
as older sediment is buried by younger sediment. 
Natural subsidence is occurring most rapidly in the 
Mississippi River Delta area of southern Louisiana, 
where approximately 1,500 mi2 (3,900 km2) of land are 
subsiding. Estimated average rates of subsidence range 
from 0.3 to 0.4 in (0.8 to 1.0 cm) per century (Penland 
and others, 1988). Maximum rates measured by geodetic 
surveys are approximately 0.5 in (1.3 cm) per year. 
Hydrocompaction occurs when dry, low-density sediments 
collapse because moisture content increases. 
These sediments, known as collapsible soils, generally 
are of two types: mudflow deposits in alluvial fans, and 
wind-deposited, moisture-deficient silt called loess. 
Most collapsible soils have low densities because they 
remained moisture deficient throughout their post-depositional 
history. When water percolates through the root 
zone into these soils, the structure collapses, the soil 
compacts, and very localized subsidence may result. 

LAND SUBSIDENCE 111
Drainage of Organic Soils. Drainage of organic 
soils, particularly peat and muck, induces a series of 
processes that reduces the volume of soil. These 
processes include biological oxidation, compaction, 
and desiccation. Biological oxidation usually dominates 
in warm climates. The principal areas of organic 
soil subsidence in the United States are the greater New 
Orleans, LA area; the Sacramento-San Joaquin River 
Delta, CA; and parts of the Florida Everglades. 
Maximum observed subsidence is 21.0 ft (6.4 m) in the 
Sacramento-San Joaquin River Delta (Ireland and others, 
1984). 
RISK ASSESSMENT 
The National Research Council compiled maps showing 
the cumulative damage by State resulting from various 
types of land subsidence (NRC, 1991). These 
maps, which appear in the subsection on Consequences, 
can be used as a measure of the risk associated with 
land subsidence. The time periods for compilation of 
the cumulative costs vary from state-to-state, and the 
costs were not converted to constant dollars. In general, 
the costs indicated are considered to be conservative 
estimates. 
Probability and Frequency 
Land subsidence occurs slowly and continuously over 
time or on abrupt occasions, as in the case of sudden 
formation of sinkholes. Procedures for determining the 
probability or frequency of land subsidence have not 
been recommended. The cumulative damage land subsidence 
maps discussed later in this chapter do not 
imply probability or frequency of occurrence. 
Exposure 
Exposure of people and property is a function of the 
type and duration of subsidence, and extent of the area 
affected. 
Collapse Into Voids. Collapse of surficial materials 
into underground voids is most commonly associated 
with coal mining. Coal is found in 37 States and 
mined underground in 22 States (HRB-Singer, 1980). 
Approximately 12,400 mi2 (32,000 km2) of land is 
undermined, and it is anticipated that the area will ultimately 
increase to 62,500 mi2 (162,000 km2). 
Approximately 3,100 mi2 (8,000 km2) of the under-
mined area, most of which is in the Eastern United 
States, already has experienced subsidence. 
The U.S. Bureau of Mines estimates that 620 mi2 (1,600 
km2) of land in urban areas is threatened (Johnson and 
Miller, 1979). Seventy-one percent of this area is in 
Pennsylvania, Illinois, and West Virginia. 
Davies and others (1976) indicate more than 500,000 
mi2 (1.4 million km2 ) of land in 39 States is underlain 
by cavernous limestone and marble. More than 11,600 
mi2 (30,000 km2) of this area lies beneath Standard 
Metropolitan Statistical Areas inhabited by 33 million 
people (HRB-Singer, 1977). However, only a small 
portion is actually underlain by voids and considered to 
be at-risk of subsidence. 
The States with the greatest number of active sinkholes 
are Alabama, Florida, Georgia, Indiana, Missouri, 
Pennsylvania, and Tennessee. Newton (1986) estimates 
that more than 6,000 collapses have occurred in the 
Eastern United States since around 1950. 
Sediment Compaction. Sediment compaction subsidence 
is caused by pumping groundwater and petroleum. 
More than 30 areas in seven States have experienced 
land subsidence of this type. The two largest 
areas are in the San Joaquin Valley, CA, and Houston, 
TX, where approximately 5,200 mi2 and 4,800 mi2 
(13,500 km2 and 12,500 km2), respectively, have sub-
sided because of groundwater withdrawal. Maximum 
elevation loss from this type of subsidence has been 30 
ft (9 m) in the San Joaquin Valley from the mid-1920s 
to 1977 (Ireland and others, 1984). Petroleum withdrawal 
in Long Beach, CA, caused parts of the City's 
harbor facility to subside almost 29.6 ft (9 m) from 
1937 to 1966. 
Groundwater withdrawal in Houston, TX, caused some 
coastal areas to subside more than 6 ft (2 m). 
Approximately 30 mi2 (80 km2) of land were inundated. 
Several hundred square kilometers, including the 500-
unit Brownwood Subdivision in Baytown which was 
abandoned in 1983, were added to areas susceptible to 
flooding by storm surges. Some areas of local subsidence 
in the Houston area resulted from the extraction 
of gas and oil. 
Damaging hydrocompaction has been reported in 17 
States. The three largest affected areas are the alluvial 
slopes of the western San Joaquin Valley and loess-covered 
areas in the Missouri River basin and the Pacific 
Northwest. Other known areas of hydrocompaction 
include the Heart Mountain-Chapman Beach and 
Riverton areas in Wyoming; Hysham Bench, MT; 
Denver, CO; and the Washington-Hurricane-Cedar City 
areas in southwest and central Utah. The major impact 
has been on design and operation of hydraulic structures 
such as canals, aqueducts, and dams. Locally sig-

112 NATURAL HAZARDS: GEOLOGIC HAZARDS
nificant impacts have been sustained by buildings and 
highways. Irrigation for agriculture has caused differential 
hydrocompaction that required re-leveling of 
fields. 
Drainage of Organic Soils. Approximately 
3,600 mi2 (9,400 km2) of land underlain by organic soil 
has subsided because of drainage of organic soils. An 
even larger area is susceptible to subsidence. 
Approximately 39,000 mi2 (101,000 km2) of the conterminous 
United States are covered by peat and muck 
soils (Stephens and others, 1984) and more than 10,000 
mi2 (26,000 km2) of organic wetlands are in Standard 
Metropolitan Statistical Areas (HRB-Singer, 1977). 
Consequences 
The average annual damage from all types of subsidence 
is estimated conservatively to be at least $125 
million. The estimated annual damage by type of subsidence 
is given in Table 10-1 (NRC, 1991). The practical 
impacts of land subsidence depend on the specific 
form of surface deformation. Regional lowering may 
either aggravate the flood potential or permanently 
inundate an area, particularly in coastal or riverine settings. 
Local collapse may impair or destroy buildings, 
roads, and utilities. 
The major damage results from underground mining of 
coal, withdrawal of underground water and petroleum, 
and drainage of organic soils. The costs consist primarily 
of direct structural and property losses and 
depreciation of land values, but also include business 
and personal losses incurred during periods of repair. 
Subsidence is more hazardous to property than to life, 
because of the typically low rates of surface lowering, 
and has caused few casualties. However, subsidence 
increases the potential for loss of life in flood-prone 
areas by increasing the depth of floodwaters and extent 
of areas susceptible to flooding. 
Although total annual damage associated with subsidence 
is small relative to the U.S. economy, subsidence 
imposes substantial costs on individual cities and neighborhoods. 
Cities where cumulative damage from subsidence 
exceed $100 million include Long Beach, CA; 
Houston, TX; and New Orleans, LA. 
Collapse Into Voids. Most mine-related subsidence 
damage in the United States is associated with abandoned 
coal mines over which urban growth has 
occurred. Damage in urban areas has been estimated to 
cost more than $30 million annually (HRB-Singer, 
1977). In Scranton, PA, and Seattle, WA, collapse of 
abandoned coal mines has damaged surface structures. 
In rural areas subsidence affects field drainage, reduces 
crop yields, and lowers property values. A study in 
Illinois indicated property values in rural areas affected 
by subsidence were discounted an average of 16 percent 
(Illinois Department of Energy and Natural Resources, 
1985). 
The cumulative costs of damage from subsidence 
caused by underground mining are shown in Map 10-1. 
States with cumulative damage in excess of $1 million 
are concentrated in the Ohio Valley and the Northern 
Plains States, however the most significant damage is in 
Pennsylvania and Louisiana. 
Costs of preventive measures and damage from sink-
hole activity, incurred primarily in the Eastern United 
States since 1970, are more than $170 million (Newton, 
1986). The costs are dominated by expenditures of 
$130 million at five dams to minimize or eliminate 
sinkhole activity. Sinkhole potential is usually evaluated 
during siting of major engineered structures in lime-
stone terrain, a factor that helps to minimize future 
damage. 
The cumulative costs associated with sinkholes are 
shown in Map 10-2. Texas, Florida, Tennessee, 
Kentucky, Arkansas, Louisiana and 
Pennsylvania experienced the most damage. 
TABLE 10-1.Estimated annual damage from land subsidence The Winter Park, FL, sinkhole collapse of 
May 8-9, 1981, is probably one of the better 
known incidents of land subsidence in an 
area of limestone geology. A 324-ft (100-m) 
wide, 100-ft (30-m) deep sinkhole formed in 
approximately 36 hours. The collapse was 
caused in part by the prevailing drought. 
Economic loss was estimated to exceed $2 
million, including a house, several cars, portions 
of several businesses, streets, and the 
municipal swimming pool (Hays, 1981). 
Source: National Research Council, 1991 
Type of Subsidence Damage 
Organic soils $40,000,000 
Underground fluid withdrawl $35,000,000 
Mines $30,000,000 
Natural compaction $10,000,000 
Sinkholes $10,000,000 
Hydrocompaction (Not available) 
Total $125,000,000 

LAND SUBSIDENCE 113
U.S. Map 10-1U.S. Map 10-2
114 NATURAL HAZARDS: GEOLOGIC HAZARDS
Sediment Compaction. Damage from compaction 
caused by withdrawal of underground fluids is dominated 
by large losses in a few areas. For example, 
although subsidence caused by petroleum withdrawal in 
Long Beach, CA, is under control, mitigation activities 
cost approximately $150 million from 1937 to 1966 
(Mayuga, 1970). 
Jones (1977) estimated that damage in the Houston area 
averaged $31.7 million annually from 1969 to 1974. 
Costs were nearly equally divided between decreased 
property values and actual damage, and most were associated 
with increased flood risk and permanent inundation. 
Prokopovich and Marriott (1983) estimated that 
post-construction rehabilitation of subsidence damage 
to California's Central Valley Project canals cost $34 
million. 
The only available estimate of subsidence damage to 
well fields was prepared by Roll (1967) for Santa Clara 
Valley, CA, where more than $4 million was spent to 
repair or replace well casings. 
The cumulative damage by State associated with subsidence 
due to underground fluid withdrawal is shown in 
Map 10-3. Cumulative damage exceeds $100 million 
in Texas and California. Losses from natural compaction, 
particularly in the Mississippi River Delta, are 
difficult to estimate because of the uncertain value of 
coastal wetlands. Increased flooding potential is the 
principal impact because affected areas commonly are 
low lying and naturally subject to flooding. Annual 
revenue losses are possibly on the order of millions of 
dollars (NRC, 1991). 
Nationwide, hydrocompaction damage and prevention 
expenditures are very poorly documented. The most 
costly individual impact documented was on the 
California Aqueduct along the western margin of the 
San Joaquin Valley, where an aggregate length of 96 mi 
(155 km) of canal was built on collapsible soils (James, 
1974). Investigations and presetting to compact foundations 
resulted in mitigation costs of $20 million 
(Curtin, 1973). Design modifications to the nearby 
Central Valley Project led to an $8 million mitigation 
cost (Prokopovich and Marriott, 1983). The most costly 
reported urban incident is a $3-million decrease in 
property value in Cedar City, UT, where structural 
damage totaled approximately $1 million (Kaliser, 
1982). 
Even areas with humid climates have incurred significant 
costs. For example, collapsible soils added more 
than $2.5 million in mitigation costs to interstate high-
way construction in Louisiana (Arman and Thornton, 
1972). 
The cumulative damage, by State, associated with subsidence 
due to hydrocompaction is shown in Map 10-4. 
The States with the highest damage costs are California 
and Louisiana. 
Drainage of Organic Soils. Costs associated with 
structural damage due to differential subsidence caused 
by drainage of organic soils appear to be high. HRB-
Singer (1977) estimated that approximately $30 million 
was spent annually in New Orleans to repair damage 
and maintain property. A study in New Orleans indicated 
that costs are disproportionately distributed: 45 
percent of homeowners sampled encountered problems 
and 5 percent reported serious problems (Earle, 1975). 
Increased flooding is the most serious problem associated 
with organic soil subsidence. The low relative elevation 
of organic soil areas makes them vulnerable to 
flooding even in their natural state. Their high compressibility 
makes them a poor foundation material for 
structures. Consequently, special construction practices 
that rely on piles driven to firm materials are commonly 
employed. These practices have led to subsiding 
land surfaces relative to the structures. 
The cumulative damage, by State, associated with subsidence 
due to drainage of organic soils is shown in 
Map 10-5. Cumulative damage exceeds $100 million 
in California, Louisiana, and Florida. 
RESEARCH, DATA COLLECTION, AND 
MONITORING ACTIVITIES 
The National Research Council (1991) suggests that 
primary data collection and mapping of land subsidence 
should be carried out by State geological surveys, 
USGS, and the Natural Resources Conservation Service 
(formerly Soil Conservation Service). Special research 
to define mapping criteria and to identify and solve 
complex causes of subsidence should be the responsibility 
of Federal agencies such as USGS, the U.S. 
Bureau of Mines, and the USDA Agricultural Research 
Service. 
Research on land subsidence is primarily conducted or 
funded by Federal agencies. The U.S. Bureau of Mines 
has been primarily responsible for research on mining; 
USGS for underground fluid withdrawal, natural compaction, 
sinkholes, and geologic aspects of mining; U.S. 
Bureau of Reclamation for hydrocompaction; and the 
USDA Agricultural Research Service for organic soils 
(NRC, 1991). 
USGS research on land subsidence resulting from 
groundwater withdrawal illustrates research by a 
Federal agency. Holzer (1984) describes the areas in 

LAND SUBSIDENCE 115
U.S. Map 10-3U.S. Map 10-4
116 NATURAL HAZARDS: GEOLOGIC HAZARDS
U.S. Map 10-5the Western United States where ground failure due to 
groundwater withdrawal is a problem. Ireland and others 
(1984) summarize studies of land subsidence in the 
San Joaquin Valley, CA. Hanson and Benedict (1993) 
describe how a groundwater model can be used to simulate 
potential land subsidence in the Upper Santa Cruz 
Basin in Arizona. Leake (1990, 1991) describes the 
vertical compaction component of a groundwater model 
useful in subsidence studies. 
MITIGATION APPROACHES 
The many causes and forms of land subsidence have led 
to a variety of mitigation efforts. Because of the diverse 
impacts, mitigation measures generally are designed for 
specific situations and address problems in areas that 
are already developed or proposed for development. 
The NRC (1991) discussed several approaches for dealing 
with subsidence, described below. 
 Public Information Programs. Many problems 
related to land subsidence are hazardous only if they 
are unexpected. An informed public can minimize 
exposure to financial loss and personal injury. For 
this reason, public information programs are under-
way in most areas experiencing significant problems. 
They range from very informal campaigns led by 
local college professors to highly organized efforts 
conducted by special-interest groups to publication of 
non-technical literature by many Federal, State, and 
local agencies. 
 Mapping Programs. Mapping programs are an 
important element in efforts to identify and manage 
subsidence. Such programs frequently are an early 
step in mitigation efforts. Depending on the type of 
subsidence, the scope and objectives of mapping pro-
grams vary, as does the degree of interaction among 
Federal, State, and local agencies. 
 Regulation of Resource and Land Development 
Prevention and Control. Regulation of the activities 
that cause subsidence is the most direct approach 
to mitigation of damage. Approaches for preventing 
or controlling subsidence to minimize damage vary 
widely. In the case of resource extraction, they range 
from banning extraction to controlling how materials 
are removed. In the case of land development practices 
that cause subsidence, approaches range from 
banning development to regulating construction practices. 
Subsidence caused by active coal mining is 
regulated by the Surface Mining Control and 

LAND SUBSIDENCE 117
Reclamation Act of 1977, which requires coal mine 
operators to submit Subsidence Control Plans as part 
of permit applications. Around Houston, the Harris-
Galveston Coastal Subsidence District regulates 
groundwater withdrawal through a permit process. 
 Land-Use Management and Building Codes. 
Land-use management and regulation in the presence 
of real or potential subsidence is an alternative to regulating 
subsurface resource development. 
Techniques include land-use planning and codes, specialized 
building codes, official maps, and constraints 
on public utilities. 
 Market-Based Mitigation Efforts. The objective of 
market-based efforts is to internalize the cost of subsidence 
by transferring those costs to the parties 
responsible for it or to the ultimate consumers. 
Internalizing subsidence costs may come about 
through taxes or fees on the parties causing the subsidence 
or by directly requiring those parties to carry 
out prevention measures. 
 Insurance. Insurance programs to provide relief 
from subsidence damage have been used in several 
areas to distribute losses more equitably and to 
encourage risk reduction actions. Programs have 
been implemented to insure against losses from coal-
mine subsidence and catastrophic subsidence associated 
with sinkhole collapse. The NFIP offers insurance 
in areas impacted by flooding aggravated by 
subsidence. Coal-mine subsidence insurance is avail-
able in Pennsylvania, Illinois, West Virginia, and 
Kentucky, and is under consideration in other States. 
In 1981, sinkhole insurance coverage in Florida was 
extended to all structures, although insurance companies 
were given an option not to provide coverage for 
commercial and government buildings. 
RECOMMENDATIONS 
The National Research Council (1991) recommended 
efforts in three general areas to address subsidence 
problems: 
 Collection of a broad range of earth science data in 
order to assess the incidence and potential impact 
from each type of subsidence, both locally and 
regionally; 
 Technical research to improve the capability to predict 
the time, rate, magnitude, and place of subsidence, 
and to develop engineering designs that are 
resistant to subsidence damage; and 
 Evaluation of methods of subsidence mitigation for 
cost effectiveness and suitability for each type of subsidence. 
The NRC (1991) expressed concern that efforts to 
reduce the Federal deficit, which have jeopardized the 
small but effective subsidence research programs in 
Federal agencies, will halt scientific and technical 
progress. Data and research results from Federal pro-
grams historically have complemented State and local 
efforts by establishing much of the technical basis for 
recognition of subsidence potential, as well as its prevention 
or mitigation. The NRC supported the maintenance 
of Federal budgets in order to provide data and 
techniques required by State and local governments and 
industry to mitigate subsidence in a timely and cost-
effective manner. 
BIBLIOGRAPHY AND REFERENCES 
Arman, A., and S.I. Thornton, 1972. Collapsible Soils 
in Louisiana. Louisiana State University Engineering 
Research Bulletin No. 111. 
Curtin, G. 1973. "Collapsing Soil and Subsidence." In: 
D.E. Moran and others (Eds.), Geology, Seismicity, and 
Environmental Impact. Association of Engineering 
Geologists. 
Davies, W.E., J.H. Simpson, G.C. Ohlmacher, and G.J. 
Newton. 1976. Map Showing Engineering Aspects of 
Karst in the United States. Scale 1:7,500,000. U.S. 
Geological Survey Open-File Report 76-625. 
Earle, D.W. 1975. Land Subsidence Problems and 
Maintenance Costs to Homeowners in East New 
Orleans, Louisiana. Ph.D. Dissertation. Louisiana 
State University. 
Geotimes. 1988. Kansas Sinkholes. Vol. 33, No. 11, 
p. 15. 
Hanson, R.T., and J.F. Benedict. 1994. Simulation of 
Groundwater Flow and Potential Land Subsidence, 
Upper Santa Cruz Basin, Arizona. U.S. Geological 
Survey Water, Resources Investigations Report 93-
4196. 
Hays, W.W. 1981. Facing Geologic and Hydrologic 
Hazards: Earth Science Considerations. U.S. 
Geological Survey Professional Paper 1240-B. 
Holzer, T.L. 1989. "State and Local Response to 
Damaging Land Subsidence in United States Urban 
Areas." Engineering Geology. Vol. 27, pp. 449-466. 

118 NATURAL HAZARDS: GEOLOGIC HAZARDS
Holzer, T.L. 1984. Ground Failure Induced by 
Groundwater Withdrawal from Unconsolidated 
Sediments. In: T.L. Holzer (Ed.), Man-Induced Land 
Subsidence. Geological Society of Amercia, Reviews 
in Engineering Geology. Vol. 6. 
HRB-Singer, Inc. 1977. Nature and Distribution of 
Subsidence Problems Affecting HUD and Urban 
Areas. U.S. Department of Housing and Urban 
Development. National Technical Information 
Services. Report No. PB80172778. 
HRB-Singer, Inc. 1980. Technical and Economic 
Evaluation of Underground Disposal of Coal Mining 
Wastes. Report prepared for Bureau of Mines. 
Contract No. J0285008. 
Illinois Department of Energy and Natural Resources. 
1985. The Economic Effects of Underground Mining 
Upon Land Used for Illinois Agriculture. Illinois 
Department of Energy and Natural Resources 
Document No. 85/01. 
Ireland, R.L., J.F. Poland, and F.S. Riley. 1984. Land 
Subsidence in San Joaquin Valley, California, as of 
1980. U.S. Geological Survey Professional Paper 437-I. 
James, L.B. 1974. The Role of Engineering Geology 
in Building the California State Water Project. 
Centenaire de la Societe Geologique de Belgique 
Colloque. Geologie de L'Ingenieur, Liege. 
Johnson, W., and G.C. Miller, 1979. Abandoned Coal-
mined Lands: Nature, Extent, and Cost of 
Reclamation. U.S. Bureau of Mines Special 
Publication No. 6-79. 
Jones, L.L. 1977. External Costs of Surface 
Subsidence: Upper Galveston Bay, Texas. 
International Association of Hydrological Sciences 
Publication No. 121. 
Kaliser, B.N. 1982. "Hydrocompaction Affecting 
Cedar City, Utah." Proceedings of the 19th 
Engineering Geology and Soils Engineering 
Symposium. Pp. 145-160. 
Leake, S.A. 1990. "Interbed Storage Changes and 
Compaction in Models of Regional Groundwater 
Flow." Water Resources Research. Vol. 26, No. 9, pp. 
1939-1950. 
Leake, S.A. 1991. "Simulation of Vertical Compaction 
in Models of Regional Groundwater Flow." Land 
Subsidence, Proceedings of the Fourth International 
Symposium on Land Subsidence. International 
Association of Hydrological Sciences Publication No. 
200. 
Mayuga, M.N. 1970. "Geology and Development of 
California's Giant-Wilimington Oil Field." Geology of 
Giant Petroleum Fields. American Association of 
Petroleum Geologists Memoir No. 14. 
National Research Council. 1991. Mitigating Losses 
from Land Subsidence in the United States. 
Washington, D.C.: National Academy Press. 
Newton, J.G. 1986. Natural and Induced Sinkhole 
Development in the Eastern United States. 
International Association of Hydrological Sciences 
Publication No. 151. 
Penland, S., K.E. Ramsey, R.A. McBride, J.T. 
Mestayer, and K.AA. Westphal. 1988. Relative Sea 
Level Rise and Delta-Plain Development in 
Terrebonne Parish Region. Louisiana Geological 
Survey. Coastal Geology Publication Technical Report 
No. 4. 
Prokopovich, N.P., and M.J. Marriott. 1983. "Cost of 
Subsidence to the Central Valley Project, California." 
Association of Engineers Geologists Bulletin 20. Vol. 
3, pp. 325-332. 
Roll, J.R. 1976. "Effect of Subsidence on Well Fields." 
American Water Works Association Journal. Vol. 59, 
No. 1, pp. 80-88. 
Stephens, J.C., L.H. Allen Jr., and E. Chen. 1984. 
Organic Soil Subsidence. In: T.L. Holzer (Ed.), 
Man-Induced Land Subsidence. Geological Society 
of America, Reviews in Engineering Geology. Vol. 6. 
Turney, J.E. 1985. Subsidence Above Inactive Coal 
Mines: Information for the Homeowner. Colorado 
Geological Survey Special Publication No. 26. 

LAND SUBSIDENCE 119



122 NATURAL HAZARDS: GEOLOGIC HAZARDS
Chapter Summary
S
oils and soft rock that tend to swell or shrink due to 
changes in moisture content are commonly known 
as expansive soils. Changes in soil volume present 
a hazard primarily to structures built on top of expansive 
soils. The most extensive damage occurs to highways and 
streets. 
In the United States, two major groups of rocks serve as 
parent materials of expansive soils, and occur more commonly 
in the West than in the East. The first group consists 
of ash, glass, and rocks of volcanic origin. The aluminum 
silicate minerals in these volcanic materials often 
decompose to form expansive clay minerals of the smectite 
group, the best known of which is montmorillonite. 
The second group consists of sedimentary rock containing 
clay minerals, examples of which are the shales of the 
semiarid West-Central States. Because clay materials are 
most susceptible to swelling and shrinking, expansive soils 
are often referred to as swelling clays. 
The effects of expansive soils are most prevalent in regions 
of moderate to high precipitation, where prolonged periods 
of drought are followed by long periods of rainfall. The 
hazard occurs in many parts of the Southern, Central, and 
Western United States. Recent estimates put the annual 
damage from expansive soils as high as $7 billion. 
However, because the hazard develops gradually and seldom 
presents a threat to life, expansive soils have received 
limited attention, despite their costly effects. 
The best means of preventing or reducing the damage from 
expansive soils is to avoid building on them. When that is 
not possible, commonly applied engineering practices 
include removal of the soil, application of heavy loads to 
offset the swelling pressure, preventing access to water, 
presetting, and chemical stabilization. 
Photo: JCP Geologists, Inc. 

EXPANSIVE SOILS 123 
Photo: JCP Geologists, Inc. 

124 NATURAL HAZARDS: GEOLOGIC HAZARDS
HAZARD IDENTIFICATION 
"Clay" is defined as a natural, earthy, fine-grained mate-
rial that develops plasticity when mixed with a limited 
amount of water. A swelling clay, according to the 
American Geological Institute Glossary of Geology 
(Bates and Jackson, 1980), is a "clay that is capable of 
absorbing large quantities of water, thus increasing 
greatly in volume " 
Dry clays that are capable of absorbing water, if unconfined, 
will increase in volume in an amount proportional 
to the amount of water absorbed. The amount of 
water absorbed and the degree of expansiveness are 
dependent on several variable and interrelated factors. 
The increase in soil volume causes damage to foundations 
and structures. These same clays may also be a 
source of engineering problems due to shrinkage resulting 
from loss of moisture. 
Dry swelling clays absorb much larger quantities of 
water before becoming plastic than do dry, non-swelling 
clays. They also remain plastic over a wider range of 
moisture content, referred to as the plasticity index (PI). 
The PI is expressed as the numerical difference between 
the plastic limit (the percent moisture content at which 
clay passes from the solid to the plastic state) and the 
liquid limit (the percent moisture content at which clay 
passes from the plastic to the liquid state). The PI bears 
a direct relation to the amount and type of clay minerals 
present and to the orientation and size of clay particles. 
Other factors remaining constant, the PI increases with 
amount of clay minerals, decreases with degree of parallel 
orientation of the clay minerals, and decreases with 
clay particle size. 
The plasticity index is generally a good indicator of 
swelling potential. Seed and others (1962), who found 
the PI to be the single most useful indicator of swelling 
potential, noted ". . . this parameter alone can provide an 
assessment of swelling that is probably accurate to with-
in 35 percent." Sowers and Kennedy (1967) found the 
PI to be "the most reliable working tool" in identifying 
potentially troublesome clays in the humid coastal 
plains of the southeastern United States. 
Expansive soils can be recognized either by visual 
inspection in the field or by conducting laboratory 
analyses. Shales, clay shales, weathered volcanic rocks, 
and residual soils containing smectite often have a characteristic 
"popcorn" texture, especially in semiarid 
areas. 
The most successful methods of recognizing expansive 
soils involve laboratory analysis of the clay-mineral 
content in solid and soft rock, including X-ray diffraction, 
differential thermal analysis, and microscopic 
examination. The most common methods used to identify 
expansive soils on the basis of physical characteristics 
related to volume change are free swell tests, plasticity 
tests, and direct measurements of volume change. 
Swelling potential refers to the amount of volume 
increase due to swelling that is possible in a clay in its 
natural environment. Swelling potential is influenced 
by many factors, some of which are inherent to the clay 
and others that are related to its environment. Inherent 
factors determine the maximum increase in volume that 
can take place under optimum conditions, and include 
clay mineral composition, amount of non-clay material 
present, density, void ratio, size and orientation of clay 
particles, cementation, macrostructure, size and thickness 
of clay body, and depth below ground surface. 
Volume changes in clay caused by variations in moisture 
content occur within approximately 30 ft (10 m) of 
the ground surface (Jones and Holtz, 1973). Most 
changes that cause engineering problems occur at 
depths less than 10 ft (3 m) (Hamilton, 1963; Gromko, 
1974). 
Clays beneath the water table have no swelling potential 
because they are completely saturated with no capacity 
for moisture absorption. Clays above the water table are 
generally unsaturated and will have capacities for moisture 
and swelling that will differ according to their 
degree of saturation. Generally, saturation levels are 
high and swelling potentials are low for clays just above 
the water table, because they have access to abundant 
moisture due to capillary action (Olive and others, 
1989). 
Clays in the weathering zone, which may extend to 
more than 30 ft (10 m) below the surface, usually have 
minimum moisture contents that are determined by climate. 
Variations in moisture content and volume 
changes are greatest in clays found in regions of moderate 
to high precipitation, where prolonged periods of 
drought are followed by long periods of rainfall. It is in 
these regions, which include many of the Southern, 
Central, and Western States, that swelling of clays 
resulting from climatic fluctuations cause the most 
severe engineering problems. 
RISK ASSESSMENT 
Probability and Frequency 
The risk associated with expansive soils is related to 
swelling potential in a qualitative manner: high, moderate 
to slight, and little to no swelling potential. 
Swelling clay potential is defined in a spatial sense with 

EXPANSIVE SOILS 125
no implications over time. Probability and frequency 
analyses have not been prepared because of the nature 
of occurrence of this hazard. This is consistent with 
other geologic hazards that occur rarely or slowly over 
time. 
Exposure 
Olive and others (1989) developed a map of the conterminous 
United States to show the distribution and 
potential of swelling clays (Map 11-1). Geologic units 
as shown on the geologic map of the United States 
(King and Beikman, 1974) were classified according to 
the amount and swelling potential of clay they contain. 
The availability of data on expansive soils varies greatly. 
In or near metropolitan centers and at dam sites, 
abundant information on the amount of clay generally is 
available. However, for large areas of the United 
States, little information is reported other than field 
observations of the physical characteristics of clay of a 
particular stratigraphic unit. Therefore, fixed criteria 
for determining the swelling potential have not been 
devised. The method adopted by Olive and others 
(1989) is largely subjective and the classification presented 
was based on the authors' appraisal of pertinent 
data that, for a single clay body or geologic unit, may 
vary considerably in quantity and quality from one area 
to another. 
The swelling-clay classification of Olive and others 
(1989) was based mostly on numerous published 
descriptions of the physical and mineralogic properties 
of clays. Other data, often unpublished, were obtained 
from communication with practicing engineering geologists 
and geotechnical engineers. The map developed 
by Olive and others (1989) is consistent with maps 
developed by Patrick and Snethen (1976) and a map 
published by the American Society of Civil Engineers 
(ASCE) in Civil Engineering in October 1978. 
Compared to Olive's map, another expansive soil map 
by Wiggins and others (1978) has similar patterns but 
some noticeable differences as well. The differences 
are due to the different approaches for identifying 
swelling clay potential. Wiggins used soil taxonomy 
obtained from the Soil Geography Unit of the U.S. Soil 
Conservation Service, while Olive related clay swelling 
potential to geologic units. 
Consequences 
Most engineering problems caused by volume changes 
in swelling clays result from human activities that modify 
the local environment. They commonly involve 
swelling clays beneath areas covered by buildings and 
slabs or layers of concrete and asphalt, such as those 
used in construction of highways, canal linings, walk-
ways, and airport runways. 
Damage to the built environment results from differential 
vertical movement that occurs as clay moisture con-
tent adjusts to the changed environment. In a highway 
pavement, differential movement of 0.4 in (1 cm) with-
in a horizontal distance of 20 ft (6 m) is enough to pose 
an engineering problem if high standards for fast travel 
are to be maintained (Williams, 1965). 
Buildings are capable of withstanding even less differential 
movement before structural damage occurs. 
Generally, a differential movement of 0.25 in (0.61 cm) 
between adjacent columns will cause cracking in load-
bearing walls of a 20-ft wide (6-m wide) bay. With differential 
movement of 1.5 in (3.7 cm) over a span of 20 
ft (6 m), beams are likely to be structurally damaged 
(Skempton and McDonald, 1956). 
Houses and one-story commercial buildings are more 
apt to be damaged by the expansion of swelling clays 
than are multi-story buildings, which usually are heavy 
enough to counter swelling pressures. However, if constructed 
on wet clay, multi-story buildings may be dam-
aged by shrinkage of the clay if moisture levels are substantially 
reduced, such as by evapotranspiration or by 
evaporation from beneath heated buildings. 
The most obvious manifestations of damage to buildings 
are sticking doors, uneven floors, and cracked 
foundations, floors, walls, ceilings, and windows. If 
damage is severe, the cost of repair may exceed the 
value of the building. 
Probably the greatest amount small building damage 
has impacted those constructed when clays were dry, 
such as during a drought, followed by soaking rains that 
prompt swelling of clays. Other reported cases of dam-
age involve volume increases due to moisture from broken 
or leaking water and sewer lines, watering of lawns 
and shrubbery, and modifications of the surface that 
produce ponding (Olive and others, 1989). 
Jones and Holtz (1973) estimated the total annual 
expansive soil-related damage throughout the United 
States to be just under $2.5 billion (Table 11-1). This 
study represents a conservative estimate of the total 
damage attributed to expansive soils. Many problems 
are not recognized as being expansive-soil related or 
may be considered nuisances and not repaired. Also, 
these figures probably do not reflect the increased costs 
attributed to over-design of structures in areas either 
highly or moderately at risk for expansive soil problems 
(Wiggins and others, 1978). 

126 NATURAL HAZARDS: GEOLOGIC HAZARDS
U.S. Map 11-1
EXPANSIVE SOILS 127
TABLE 11-1.Estimated annual soil-related damage costs 
According to Jones and Holtz, the greatest amount 
of damage from expansive soils is to highways and 
streets. In response to a questionnaire submitted to 
highway departments in 48 States, Puerto Rico, 
and the District of Columbia, 36 departments 
acknowledged the occurrence of swelling clays. 
Nineteen departments ". . . recognize swelling soils 
as part of their pavement design criteria" (Lamb 
and Hanna, 1973). According to Lytton (1974), ". 
. . general pavement roughness . . . due to swelling 
clays, is one of the most costly single elements in 
any pavement design." 
For all types of building construction, Jones and 
Holtz (1973) estimated annual losses of $740 mil-
lion. Furthermore, it was reported that of the more 
than 250,000 homes built each year on expansive 
soils, 10 percent sustain significant damage during 
their useful lives, some beyond repair, and 60 per-Source: From Jones and Holtz, 1973 
cent sustain minor damage. 
Location Cost 
Single-family homes $300,000,000 
Commercial building $360,000,000 
Multi-story buildings $80,000,000 
Walks, driveways, parking areas $110,000,000 
Highways and streets $1,140,000,000 
Buried utilities and services $100,000,000 
Airport installations $40,000,000 
Involved in urban landslides $25,000,000 
Other $100,000,000 
TOTAL $2,255,000,000 
Wiggins and others (1978) estimated that the 1970 
nationwide damage to single and multi-family 
dwellings totaled $1.13 billion. Their estimate was 
based on: an expansive soils map; data on damage to 
single-family foundations in Dallas County, TX, and in 
California located in high, moderate, and low expansive 
soil zones; and a breakdown by soil zone and State of 
the population living in single and multi-family 
dwellings. 
These estimates are significantly higher than the $740 
million annual losses for all types of buildings made by 
Jones and Holtz (1973). Using projections in population, 
Wiggins estimated that the total losses to all types 
of building will be on the order of $2.7 billion by the 
year 2000 (expressed in 1970 dollars). Adjusted for 
inflation, this value would be over $10 billion in 1995 
dollars. More recent estimates of annual damages from 
expansive soils are as high as $7 billion (Krohn and 
Slosson, 1980). 
MITIGATION APPROACHES 
The best means to prevent or reduce damage from 
expansive soils is avoidance. When other choices are 
not possible, engineering practices are necessary (Hays, 
1981). The most commonly applied engineering practices 
are removal of the soil, application of heavy loads, 
preventing access to water, presetting, and stabilization. 
Removal of expansive soils and replacement with non-
expansive soils is sometimes possible. Usually, expansive 
soils extend to such a great depth that complete 
removal and backfill are not economical. Although 
most engineering problems occur due to moisture 
changes in the first 10 feet of soil depth, the amount of 
excavation and backfill needed to prevent the occurrence 
of destructive volume change is site specific. 
Backfill with non-expansive material must be of sufficient 
depth to provide the necessary weight to resist the 
uplift of the surrounding expansive soil. 
Swelling can be prevented by loading an expansive soil 
so that the confining pressure is greater than the 
swelling pressure developed by the soil. Loads can be 
applied to a foundation soil by means of an embankment 
or blanket of non-expansive soil or by construction 
of large buildings. 
Water entering expansive soils is usually surface water 
that moves downward. In semiarid areas, water often 
moves upward from the groundwater table by means of 
capillary flow. Methods for isolating expansive soils 
from moisture include installation of ditches or pipes to 
carry away surface water, use of sand and gravel to 
break the continuity of capillary flow, and enveloping 
expansive soil masses with impermeable membranes. 
Concrete slabs and bituminous pavements on clay soils 
in semiarid areas inhibit the normal evaporation of capillary 
water, increasing the moisture content near the 
surface. The potential for damage can be reduced by 
presetting the underlying soils to the moisture contents 
expected while the slab and pavement are in service. 
Chemical stabilization has been used successfully to 
prevent or minimize volume change of expansive soils. 

128 NATURAL HAZARDS: GEOLOGIC HAZARDS
The ionic character of the soil and water combination 
can be modified by the addition of certain chemicals, 
such as hydrated lime, to prevent volume change. This 
approach is based on studies that show that the ionic 
character of water has a major effect on volume change. 
RECOMMENDATIONS 
In contrast to earthquakes and landslides, expansive soil 
hazards are slow to develop and do not usually pose 
risks to public safety. Because of low visibility, this 
hazard has received relatively little attention in proportion 
to the projected annual damage. 
Considerable data on swelling soils exist from construction 
in and around certain metropolitan areas, at 
dam sites, and other areas of construction of critical 
facilities. These data could be used to develop more 
quantitative methods of estimating the potential of 
future damage from expansive soils. 
In areas where expansive soils create significant problems, 
procedures for control of new construction could 
be instigated in grading and building regulations, 
including proper slab design and emplacement procedures. 
Structural damage due to expansive soils is not 
covered by most insurance. 
BIBLIOGRAPHY AND REFERENCES 
Bates, R.L., and J.A. Jackson (Eds). 1980. Glossary of 
Geology. 2nd Edition. Washington, DC: American 
Geological Institute. 
Building Research Advisory Board, National Research 
Council. 1968. "Criteria for Selection and Design of 
Residential Slab-on-ground." Federal Highway 
Administration Report No. 33. National Academy of 
Sciences Publication No. 1571. 
Gromko, G.J. 1974. "Review of Expansive Soils." 
Journal of the Geotechnical Engineering Division. 
American Society of Civil Engineers. Vol. 100, No. 
GT6, pp. 667-687. 
Hamilton, J.J. 1963. "Volume Changes in Undisturbed 
Clay Profiles in Western Canada." Canadian 
Geotechnical Journal. Vol. 1, No. 1, pp. 27-42. 
Hays, W.W. (Ed). 1981. Facing Geologic and 
Hydrologic Hazards: Earth-Science Considerations. 
U.S. Geological Survey Professional Paper 1240-B. 
Jones, D.E., and W.G. Holtz. 1973. "Expansive 
Soilsthe Hidden Disaster." Civil Engineering. Vol. 
43, No. 8, pp. 49-51. 
King, P.B., and H.M. Beikman. 1974. Geologic Map 
of the United States (Exclusive of Alaska and Hawaii). 
Scale 1:2,500,000. U.S. Geological Survey. 
Krohn, J.P., and J.E. Slosson. 1980. "Assessment of 
Expansive Soils in the United States." Proceedings of 
the Fourth International Conference on Expansive 
Soils. Volume 1, pp. 596-608. 
Lamb, D.R., and S.J. Hanna. 1973. Summary of 
Proceedings of Workshop on Expansive Clays and 
Shales in Highway Design and Construction. 
Prepared for the Federal Highway Administration, 
Report No FHWA-RD-073-72. Washington, DC: U.S. 
Government Printing Office. 
Lytton, R.L. 1974. "Expansive Clay Roughness in the 
Highway Design System in Active Clays in 
Engineering and Construction Practice." Association 
of Engineering Geologists Bulletin. Vol. 11, No. 4, pp. 
399-419. 
Olive, W.W., A.F. Chleborad, C.W. Frahme, J. 
Shlocker, R.R. Schneider, and R.L. Schuster. 1989. 
Swelling Clays Map of the Conterminous United 
States. U.S. Geological Survey Map I-1940. 
Patrick, D.M., and D.R. Snethen. 1976. Expansive 
Earth Materials a Survey by Physiographic Areas 
of Their Occurrence and Distribution. Vicksburg, 
MS: U.S. Army Corps of Engineers, Waterways 
Experiment Station. 
Seed, H.G., R.J. Lundgren, Jr. 1962. "Prediction of 
Swelling Potential for Compacted Clays." Journal of 
Soil Mechanics and Foundations Division. American 
Society of Civil Engineers. Vol. 88, No. SM3, pp.53-
87. 
Skempton, A.W., and D.H. McDonald. 1956. "The 
Allowable Settlement of Buildings." Institute of Civil 
Engineers Proceedings. Vol. 5, Pt. 3, pp. 727-784. 
Sowers, G.F., and C.M. Kennedy. July 1967. "High 
Volume Change Clays of the Southeastern Coastal 
Plain." Pan American Conference on Soil Mechanics 
and Foundations Engineering, 3rd (Caracas). 
Proceedings: Socieded Venezolana de Mecanica del 
Sueloe Ingeneria de Fundaciones. Vol. 2, pp. 99-120. 
Wiggins, J.S., J.E. Slosson, and J.P. Krohn. 1978. 
Natural Hazards: Earthquake, Landslide, Expansive 
Soil Loss Models. Institute of Behavioral Science. the 
University of Colorado, Boulder, CO. 

EXPANSIVE SOILS 129
Williams, A.A.B. 1965. "The Deformation of Roads 
Resulting from Moisture Changes in Expansive Soils in 
South Africa." Moisture Equilibria and Moisture 
Changes in Soils Beneath Covered AreasA 
Symposium in Print (Aitchison, G.D., ed.). Sydney, 
Australia: Butterworths.