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OCR for page 24
A Survey of Current Mitigation Measures
INTRODUCTION
The many causes and forms of land subsidence have led to a broad variety of efforts to
mitigate subsidence problems in the United States (Figure 17~. These efforts address problems
in areas that are either already developed or proposed for development. The following survey
of these efforts demonstrates that no single approach is applicable to aB cases. Approaches that
are in use include voluntary and educational measures such as public information and mapping
programs to raise public consciousness, regulatory schemes that require subsidence prevention
or control, land-use management and building codes to reduce damage, market-based methods
to transfer the costs of subsidence to the parties causing it, and insurance programs to distnbute
cost more equitably.
PUBLIC INFORMATION PROGRAMS
Many problems related to land subsidence are hazardous only if they are unexpected.
An informed public can minimize its exposure to financial loss and personal injury from
subsidence-related problems, even in areas where little can be done to arrest the underlying
subsidence process itself. For this reason, public information programs are under way in
most areas with major subsidence problems, ranging from very informal ones con(lucted by
local college professors to highly organized ones conducted by special-interest agencies. In
addition, many federal and state earn science agencies, such as the U.S. Geological Survey, Soil
Conservation Service, and state geological surveys, commonly publish nontechnical literature
on subsidence problems.
The objectives and scope of these programs vary. In its most restncted form, the effort
may be addressed to a narrow audience of engineers, architects, geologists, land-use planners,
code administrators, and insurers who directly confront subsidence-related problems in He
course of Heir work. Detailed technical information is disseminated Trough professional
societies, federal and state scientific and regulatory agencies, and county and municipal agencies
charged with land development, regulation, and He subsidence problem for tile public at large.
24
OCR for page 25
25
~.t . ~
`~ Wt ~ ' - ; =.i
R
t~ ^,~ , a ~ ~ - ; , ,
'
A. Mining
, , ~ \,
C. Undergrour~ fluid withdrawal
If, TV
a:
B. Sinkholes
At'
~ ~ _ - ~ ~
D. Nature compaction
~ .. . .
''DUMP ~~
E. Hydrocornpact~n F. Drainage of organic soils
LEGEND
P - Public information programs ~ - Land-use management
M - Mapping programs and building codes
R - Regulation, prevention $ - Market-based methods
and control ~ - Insurance
FIGURE 17 Summary of mitigation measures by state for each type of subsidence.
OCR for page 26
26
Information activity is generally the first activity to be undertaken In a subsidence-affected area,
beginning even before the general public may become aware that a problem exists. Similarly,
such activity persists in areas where subsidence has successfully been arrested, even after Me
issue has receded from public attention.
As the severity of subsidence increases, so does the need for greater public involvement.
~ managing a major subsidence problem, two kinds of public information are needed. First,
the public must be alerted to the specific area involved and Me nature of the hazard so that
individuals can assume an active role in managing their own exposure to personal injury
or loss. Second, the public information program must foster a general awareness of what
causes the subsidence problem and what options are available, since an informed public is
more supportive of the sometimes-costly measures needed to manage the overall problem. A
survey of information efforts under way in several subsidence areas shows that, with these
common objectives, programs in different regions rely upon a variety of methods to deliver
their message.
Mining
Illinois provides an example of the educational role state laws and insurance programs can
play in improving public awareness of subsidence problems (DuMonteDe and others, 19811.
The insurance program covers damages caused by mine subsidence, and the policy provides a
descnption of particular hazards homeowners may face. In Me 34 counties most prone to coal-
mine subsidence, insurance is automatic unless the homeowner specifically waives the policy.
This provision causes a high level of awareness of the hazards in these areas. In addition, state
law requires that homeowners receive notice 6 months before the start of mining activities
that may cause subsidence of their property. In the remaining 64 minois counties, insurance
coverage is not automatically provided, leaving homeowners with a greater responsibility for
assessing their own risks and determining whether they need to seek out the state's insurance
coverage. That residents who are not affected by mine subsidence nevertheless have a high
awareness of subsidence-related hazards is demonstrated by the fact that more than 80 percent
of claims made under the state insurance program are for damage not related to mining. The
state geological survey has programs under way to ascertain the causes of those problems that
are not related to mining and has prepared publications, brochures, billboard displays, and
radio segments to enhance public awareness of Me problems. It also works with over state and
local agencies to provide needed information in the event of subsidence damage to pipelines,
streets, foundations, and cultivated fields.
Colorado has published a special booklet for the homeowner that describes subsidence
above inactive mines (Turney, 1985~. It describes how homeowners can evaluate the subsidence
hazard and what actions they can take to minimize damage.
Sinkholes
In Florida, despite wide public awareness of the sinkhole problem fostered by the news
media, understanding of the causes of collapse remains somewhat limited. Toward this end, Me
U.S. Geological Survey, Southwest Florida Water Management District, and Florida Sinkhole
Research Institute offer circulars and lectures, slide shows for use in public schools, and an
ongoing educational program for engineers, geologists, and insurers who must deal with the
problem on a technical basis (Beck and Sinclair, 1986~. In addition, the Florida Sinkhole
Research Institute publishes a quarterly newsletter, Update.
OCR for page 27
27
Underground Fluid Withdrawal
In the Houston, Texas, area the most dangerous and potentially costly problem related to
subsidence is intermittent flooding and permanent inundation of coastal areas. Accordingly, the
Hams-Galveston Coastal Subsidence District devotes most of its public infonnation effort to
the flooding hazard. The district annually adopts a plan setting specific targets and objectives for
civic and media presentations, news releases, publications (including the quarterly newsletter
Subsidence Update) and in-school programs. This integrated program to infonn the public
of subsidence-related flood hazards has been so successful that the district reports that many
flood victims in the area identify subsidence as the proximate cause of damage even when
it is not. Thus, part of the district's program is devoted to helping residents distinguish
subsidence-related flooding from other dooding problems, such as inadequate drainage, so that
appropriate remedial actions can be taken where possible. Another aspect of the district's
program is its effort to foster an awareness of the causes of subsidence and of options for
managing the problem. In a recent survey the district found that 73 percent of area residents
had heard of subsidence and, of those, 61 percent could correctly identify the cause. This
high degree of public awareness is critical in an area where management of the underlying
problem—excessive groundwater withdrawal—calls for concerted conservation efforts and
public support for construction of surface-water treatment and transmission facilities.
Hy(lrocompaction
Public information efforts to reduce damaging hydrocompaction have been limited pri-
manly to popular serials of state geological surveys, for example, California Geology and New
Mexico Geology. Articles in these serials have both identified where problems with hydro-
compaction are occurring and described proper construction practices. These serials, widely
distributed among practicing professionals such as engineering geologists and geotechnical
engineers, help alert them to areas where special precautions need to be taken.
Organic Soils
The public information role of local government is illustrated in southern Louisiana, where
the Terrebonne Parish Consolidated Govemment has an intensive education campaign under
way to provide information about He problems of flooding, land loss along the shoreline
and interior canals, and saltwater encroachment in cultivated areas. In addition to conducting
detailed technical studies, the parish develops circulars, brochures, billboard displays, and slide
shows aimed at a wider audience. Included in the eighth-grade curriculum in area schools is a
course of instruction covering the mechanics of subsidence and possible remedies.
MAPPING PROGRAMS
The hazards and economic costs associated with land subsidence depend upon its proximity
to populations, manmade structures, and water bodies. For this reason, mapping programs are
an important element in efforts to identify and manage subsidence problems. Such programs
are frequently an early step in subsidence-hazard-mitigation efforts. Depending upon the type
of subsidence involved, the scope and objectives of these programs vain, as does the degree
of interaction among federal, state, and local agencies. This section presents representative
examples of subsidence mapping programs carried out in recent years.
OCR for page 28
28
Mining
In 1977, Congress passed the Surface Mining Control and Reclamation Act (SMCRA) to
mitigate Me impact of past surface and subsurface coal mining and to regulate future mining
activities. As part of this act, revenues from a tax on current coal operations are deposited
in the Abandoned Mine Reclamation Fund, and the Office of Surface Mining Reclamation
and Enforcement (OSMRE) was created to administer the fund. To determine the reclamation
needs of the states, OSMRE contracted with many states to map general locations of abandoned
mines.
As an example of the studies carried out at the state level, the state of Washington conducted
an inventory and identified 10 problem areas where abandoned mines posed immediate hazards
or the danger of subsidence. Additional subsidence emergencies outside the inventoried areas
led to a 1984 cooperative agreement between Washington and OSMRE to conduct a more-
exhaustive inventory and mapping effort. The objectives of the agreement were to map
abandoned mines by county and by quadrangle; to categorize the sites by severity of hazard,
accessibility, and proximity to population; and to rank the sites for remedial action. By 1985,
84 problem areas were identified and mapped, of which 26 were assigned priority for remedial
action and 5 were targeted for more detailed mapping and assessment.
Similar activities have been camed out in other states. For example, Colorado has
conducted extensive statewide mapping of real and potential subsidence problems at scales
ranging from ~ :2,400 to ~ :50,000. About 900 abandoned coal mines and over 7,000 abandoned
metal mines were inspected, and results were summarized in a state map published at a
I:l,000,000 scale (Bucknam, 1982~. Indiana and Wyoming each mapped underground coal
mines at 1:24,000 scales. Montana inventoried its abandoned mines statewide on a scale of
1 :250,000.
As follow-up to the SMCRA inventories, some states have initiated special subsidence
mapping studies. For example, the Montana Department of State Lands prepared detailed maps
for a I,600-ha mining area in the northeast corner of the state, which has historically been
prone to subsidence problems.
In addition to the mapping program sponsored by SMCRA, other efforts are under way
at the federal, state, county, and municipal levels. In IlDinois, the state geological survey has
published maps in connection with the state subsidence insurance program enacted! in 1979.
These maps, at a scale of I :100,000, show mined-out areas along win shafts and other features
and provide information about susceptibility to, as well as the history of, subsidence problems.
In Iowa, mapping efforts date back to the 1890s, when the Iowa Geological Survey first
published maps of active and abandoned lead and zinc mines in Dubuque County. At present,
the survey keeps over 1,450 coal-mine maps on file. In addition to its ongoing mapping
efforts, the survey provides assistance to county and municipal authorities concemed about
future mine-subsidence problems.
In addition to routine mapping programs, special mapping studies are sometimes initiated
in response to immediate environmental concerns. In 1979, for example, congressmen from
the tri-state mining area of Oklahoma, Missouri, and Kansas sought assistance from the U.S.
Bureau of Mines. Over a period of several years, the bureau worked with the state geological
surveys to map abandoned mine sites, appraise subsidence hazards, and propose remedial
actions.
OCR for page 29
29
Sinkholes
Mapping efforts in areas susceptible to sinkhole collapse range from compilations of
existing sinkholes and geologic conditions favorable for sinkhole development to detailed
studies Hat consider the potential for sinkhole development.
The smaldest-scale map available is that by Davies and others (1976), who mapped areas
in the conterminous United States underlain by cavernous limestone and marble. The map,
published at a scale of 1:7,500,000, identifies broad areas where there is a potential for
catastrophic subsidence.
Several states have undertaken mapping at larger scales. The Alabama Geological Survey,
in cooperation with the U.S. Geological Survey and the State Department of Transportation,
has supported mapping of catastrophic subsidence from sinkhole colBapse. The program has
produced detailed maps of subsidence features for 38 counties and identified potential areas
of subsidence and triggering mechanisms. This effort has been the basis for resolution of a
number of court cases involving subsidence, as well as the implementation of a state insurance
program. Mapping at an even more local scale is ilDustrated by the city of Huntsville, Alabama,
which is planning a series of special maps, building design criteria, and codes to be used in
subsidence-prone areas within the city limits.
The Virginia Division of Mineral Resources (Hubbard, 1983) recently completed a mapping
study of karst features in the northern part of the state and has a similar study under way in
the southern part. Susceptibility to future catastrophic subsidence in Florida has also been
addressed in recent mapping (Sinclair and Stewart, 1985~.
Underground Fluid Withdrawal
Mapping plays an important role in bow identification and management of subsidence
problems caused by fluid withdrawal. Maps of water levels in aquifers are commonly used
for prediction and monitoring purposes, both in areas where subsidence is actively occurring
(for example, Houston, Texas) and in areas where subsidence is arrested (for example, Santa
Clara Valley, Califomia). Maps of subsidence determined from releveling of geodetic control
networks are the basic too! used to study the evolution and areal distribution of subsidence
(Figure 181. For example, in the Houston area, maps of changes of surface elevation dating
back to 1906 have been used. The National Geodetic Survey has performed comprehensive
relevelings atregularintervals, including recent relevelings in 1973, 197S, and 1987. These
are supplemented with limited releveling projects, such as those carried out in 1976 and
1983. This frequent updating of the maps permits monitoring of the performance of remedial
programs by the Harris-Galveston Coastal Subsidence District and ensures that residents will
have up-to-date information about the flood hazards they face. Other maps prepared by the
district include predicted subsidence for up to 40 years (Harris-Galveston Coastal Subsidence
District, 1985~.
Maps at scales ranging from 1:24,000 to 1:250,000 of ground ruptures associated win
land subsidence are available in several areas, including parts of southern Arizona; Fremont
Valley, California; Houston, Texas; and Las Vegas ValDey, Nevada. Three of the areas include
cities Phoenix, Houston, and Las Vegas that have undergone explosive growth in the last
two decades. Availability of maps in these two areas has improved public awareness of the
hazard and encouraged voluntary efforts to avoid construction on these damaging surface
ruptures.
~ .
OCR for page 30
30
y5% ~~ ~
~£~Y . county
'45' of
'\ ~ L-if~-'
j~, ~A~Prt: ~0t't;7Y
' ~ /
ace _
,' ~ 9'~''J - '45
i''' '' ma.
~ WAVE -I_ _ ~
~ ~ ER5 !) COUNT
r.n'I~E~.
2~5G' -
EXPLANATlON
—TO— LINE OF EQUAL LAND-SURFACE SUBSIDENCE-
lnterval' I O ond 0 5 foot (O 3 ond 015 meter)
Contours loosed on 0 limited amount of dote
3~> in"
,
/-, . N\ . \
~ Denb~
(t, . . rot of Boll
~ ~ e~ros~otel1 494~ba_ ~
Am_ i ~ ........ -.- - --'-''I
,_
( ' -
~ rid
sac
a.
O ~ * :2 '6 Ze 2. 2e SO ~:L~d~ rep.
am 7~.;5GS 1~70;~..t 4~—lit>
era,.
,, ,,,,,- ~ _ ~ ·- ?g~;/,:
1 airs
~ r
9~
FIGURE 18 Map of land subsidence in the Houston-Galveston, Texas, area from 1906 to 1978. At least 12,200
km: has subsided 15 cm or more. Note that the eastern part of subsidence bowl underlies Galveston Bay. (From
Gabrysch9 1982.)
OCR for page 31
31
Hydrocompaction
Mapping of areas prone to hydrocompaction ranges from general efforts to identify the type
and distribution of collapsible soils to more specific efforts to locate and quantify subsidence
hazards. In the latter case, the susceptibility to subsidence hazards is determined by combining
data about soil distribution with laboratory data on soil properties. The result is a map of
"collapse probabilities."
This mapping approach has been applied in Iowa, where one-third of the state is covered
by loess deposits subject to collapse when saturated. The state map of collapse probability
was prepared by combining maps of clay-content contours with laboratory data relating clay
content to collapsibility (Handy, 19731.
In Arizona the U.S. Soil Conservation Service has worked with state authorities to develop
a state map of major soils, identifying those most susceptible to colBapse (Soil Conservation
Service, 1975~. In New Mexico a general assessment and mapping program is under way,
following incidents of soil collapse in the northern part of the state. The Utah Geological and
Mineral Survey has mapped a 8,100 ha area with collapsible soils in the vicinity of Cedar
Creek (Kaliser, 1978~.
Organic Soils
Nationwide recognition of the ecologic significance of wetlands, the provenance of organic
soils, has led to their extensive mapping at both local and national scales. Not all wetlands,
however, are underlain by peat and muck, the type of organic soil that is prone to subsidence
when drained. Hence, use of these maps for assessing subsidence potential is limited. Soil
surveys prepared by the Soil Conservation Service of the U.S. Department of Agriculture are
widely available and potentially provide more-useful information, because they identify soil
types in wetland areas including peat and muck, technically known as histosols. Soil surveys,
however, are based on shallow excavations and do not map total organic soil thickness, and
thus are of limited use for estimating potential magnitudes of subsidence. Nevertheless, these
surveys provide a useful starting point for areal investigations of subsidence.
The thickness of a peat and muck deposit is probably the most useful parameter for
assessing subsidence potential. Accordingly, maps of peat and muck thickness have been a
common element of areal organic soil subsidence investigations (Snowden and others, 1980;
Newmarch, 1981~.
REGULATION OF RESOURCE AND LAND DEVELOPMENT—
PREVENTION AND CONTROL
Regulation of the activity that causes subsidence is the most direct approach to subsidence
mitigation. Approaches to preventing or controlling subsidence to minimize damage vary
widely. In the case of resource extraction, they range from banning resource extraction to
controlling how materials are removed. In the case of land development practices that cause
subsidence, they range from banning development to regulating construction practice.
Mining
Subsidence damage resulting from mineral extraction can be prevented or controlled by
leaving some material behind for support or totally refilling mined-out volumes. In active
OCR for page 32
32
mines pillars of unmined material can be designed to support the overlying strata, pillars
can be constructed to replace the mined material, or the mine can be filled with lower-cost
materials.
Subsidence due to active coal mining is regulated by the SMCRA. To ensure compliance
with these regulations, operators are required to post a bond. This act requires coal mine
operators to submit a Subsidence Control Plan as part of their permit application. In the plan
the operator must identify the mineral extraction methods to be used and plans for subsidence
control or methods to be used to prevent material damage resulting from subsidence. The plan
must spell out the measures to be taken for reducing the probability of subsidence, such as
backfilling, stowing, or supports, as well as measures to be taken on the surface to prevent
material damage to structures or reductions in land values or plan for possible future land use.
The specific mitigation measures, several of which are identified in the regulations, are left to
the discretion of the operator.
Leaving unmined pillars suitable for long-te~m support of the overlying strata results
in incomplete use of the resource being mined. In addition, if the long-tenn supports are
underdesignetl, subsidence will eventually occur. Thus, full-extraction mining with planned
subsidence is permitted by SMCRA.
Subsidence can be prevented or controlled if some suitable support is provided in lieu of
the removed material. This can be accomplished in several ways, the most feasible of which
is to fib the voicis with low-cost solid material. Hydraulic Filing is the most widely practiced
method of mine filling. It involves transporting or flushing the fill material with water through
pipelines and boreholes to the point of stowage in the mine.
Hydraulic filling of abandoned underground coal mines with subsidence problems is
supported by a tax imposed by SMCRA on each ton of coal mined. Revenues from this tax
may be used only to reclaim abandoned mine lands. Subsidence is addressed on a priority
basis, with the most potentially dangerous situations receiving highest priority. This reclamation
program is not aimed at preventing the initial subsidence of lands over mines; its purpose is to
mitigate subsidence of undermined lands and to impede future subsidence of such lands.
In addition to government subsurface stabilization programs, commercial and industrial
developments in mining areas have included subsidence prevention or control measures. For
example, in a suburban Pittsburgh shopping mall, the site grading plan accounted for a mineci-
out coal seam by placing the mall at the base of the mined coal and using the excavated material
from above the mine to develop parking areas around the mall. The excavation involved 4.6
million m3 of soil and rock.
Sinkholes
Catastrophic subsidence associated with the formation of sinkholes is most commonly
triggered by either groundwater-level declines caused by pumping, or diversion of surface
runoff. Thus, the occurrence of catastrophic subsidence can be prevented in principle by
controlling these two activities. To date, attempts in the United States to control activities
that trigger subsidence have been limited, and no controls have been required by regulation.
Catastrophic subsidence problems are usually dealt with by after-the-fact maintenance. Design
and construction of the Pellissippi Parkway extension in Tennessee, however, offer an exception
(Moore, 1984~. Special efforts were made to divert runoff along the parkway from entering the
underlying cavernous system and eroding overlying sediment. These efforts included paved
OCR for page 33
33
ditches and asphalt curbs in areas of potential colBapse and improvement of flow into natural
drainage depressions to minimize erosional enlargement.
Filling underground voids with grout has been successfully used to prevent catastrophic
subsidence (Ryan, 1984~. Grouting is commonly used in general engineering practice to
strengthen foundations and stop the flow of underground water. Its application to sinkhole
prevention differs from the more common applications in that larger volumes of grout are
required, and the strength of the grout is lower. Particularly because of the volume, the
~ ~ _ .,
~ ~ _ ~ _ ~ A _ ~ ~ ~ _ ~ ~ ~ ~ ~ ~ · · · ~
economic viability of the technique is limited in most situations to major engineering works,
and the technique has been used sparingly (Ryan, 1984~.
Dynamic compaction, a soil-improvement technique that consists of the repeated dropping
of a heavy steel weight, has been successfully used in Flonda and Soup Africa to prevent
postconstruction catastrophic subsidence (Guyot, 1984~. Even where the method does not
cause collapse of the preexisting voids, analysis of the resultant surface deformation may aid
their detection.
Underground Fluid Withdrawal
Several alternatives are available to control subsidence caused by withdrawal of under-
ground fluids. Prevention or control measures include repressunng the withdrawal zone by
injection or enhanced recharge and reducing the amount of fluid withdrawn.
Operation of the Wilmington of] field in Long Beach, California, provides a well-
documented example of subsidence control through repressuring by injection (Mayuga, 1970~.
Subsidence, which ultimately reached almost 9 m, was first recognized in 1941. Repressuring
by injection of water started in 1954 under the threat of litigation; major injection (57,000 m3
per day) began in 1958. By 1966, repressuring had arrested subsidence throughout most of the
subsidence area. The experience win He Wilmington oil field led to passage of the California
Subsidence Act of 1958. The act provides for arresting or ameliorating subsidence caused by
petroleum withdrawal by requiring the repressuring of subsurface formations. The law applies
only to coastal areas subject to flooding or inundation.
Repressuring also has been practiced in the Santa Clara Valley, California, by construction
of special aquifer-recharge facilities along stream beds around the margin of the valley where
the aquifer system is unconfined. Although the primary purpose of the recharge is to mitigate
basin overdraft, it has the collateral benefit of arresting subsidence.
Reduction of groundwater withdrawal in the Houston-Galveston, Texas, area provides
an example of subsidence control by regulation. In 1975, the Texas legislature authorized
connation of the Harris-Galveston Coastal Subsidence District for the purpose of mitigating
flooding. Authority to regulate groundwater withdrawal through a permit process was delegated
to the district. The district has imposed cutbacks of pumping that are really selective in order
to stop subsidence in the coastal area, where the flood hazard is greatest. The district is
subdivided into large zones, based on amounts of subsistence that have taken place anal on
the potential effects of subsidence if withdrawal continues. In each of these zones a goal has
been set for reducing groundwater use. For example, in Zone 1, which is adjacent to He
coast, groundwater pumpage is to be reduced by 1990 to 10 percent of the total water use in
the area; in Zone 8, which is the most inland area, pumpage may continue unabated, but no
exports out of the area are permitted. The district also reduces groundwater use by encouraging
conservation of water and voluntary use of surface water that has been made available by local
OCR for page 34
34
water agencies. A shortcoming to the distnct's authority is that it cannot restrict groundwater
use where surface-water supplies are unavailable.
-
.
a, ~
Hydrocompaction
No regulations are in force to control or prevent damage from hydrocompaction, although
mitigation techniques are available. Damage can be prevented by diverting surface drainage
from structures or controlled by precompacting foundations. A few examples of paving
areas to divert surface water from structures have been reported (Peck and Peck, 1948~.
Preconstn~ction wetting of foundations, known as prewetting, has been extensively practiced in
the United States to compact foundations beneath hydraulic structures such as dams and canals
where postconstruction percolation of surface water into the subsurface is difficult to prevent
(Lofgren, 1969~. Alternative techniques that have been tried primarily on an experimental
basis, but which offer promise, are vibroflotation and dynamic compaction (Lovelace and
others, 1982~. These melons, which density the soil by dynamic forces, are specialized
mesons that require some degree of proprietary equipment and knowledge of application
(Figure 19~.
Organic Soils
Organic soils presently cannot be developed for agriculture or urban use if drained without
incurring subsidence. Rates can be slowed, however, by controlling water-table depths and
practicing good land management. Unfortunately, regarding agricultural use, crop yield studies
indicate that water-table depths of 30 to 60 cm for pasture grass and 60 to 90 cm for most
brush and field crops are desirable in temperate climates. Thus, at optimum drainage levels for
good production of these crops, subsidence occurs at undesirable rates. Seasonal Tootling can
reduce subsidence, as will growing water-tolerant crops such as nce.
Bamng a breakthrough in the science of organic soil conservation, subsidence wiD continue
on organic soils. Meanwhile, these steps could be taken to obtain maximum agricultural use of
organic soils and minimize subsidence: keep water tables as high as crop and field conditions
permit and put drained soils into productive use as soon as possible. In areas of urban
development, dewater~ng should be minimized as much as possible. Suitable lightweight fill
should be added at required intervals, which would mitigate the need for ongoing water-table
lowering.
LAND-USE MANAGEMENT AND BUILDING CODES
Land-use management and regulations in the presence of real or potential subsidence is
an alternative to regulating resource development. Land-use planning and zoning, specialized
building codes, official maps, and constraints for public utilities accomplish ~is.
The appropriate land-use planning response to subsidence depends on He nature of He
subsidence. For example, conventional local land-use planning and zoning, which apply to
land areas or districts of relatively small extent, have limited applicability in dealing with broad
regional types of subsidence from fluid withdrawal. For small tracts of organic soils, regulatory
schemes such as zoning may be feasible. For large tracts, such as the San loaquin-Sacramento
River Delta, the Florida Everglades, and the Mississippi River Delta, historical experience has
taught that careful preplanning based on land capability and economic opportunities is essential
for successful development. Agencies with regulatory powers then help implement He plans
OCR for page 35
5 :~ DYNAMO
COM CACTI ON
FIGURE 19 Experiment conducted by New Mexico State Highway Department to test feasibility of precom-
pacting collapsible soils by dropping a heavy weight on the ground. (From Lovelace and others, 1982.)
OCR for page 36
36
and building codes to control construction techniques. In the case of localized subsidence
in karst areas and over mines, which is even less predictable in location and in time, the
cost-effectiveness of land-use regulation can be questioned.
Building codes are an alternative to land-use regulation for some situations. Buildings and
other facilities sometimes can be designed to accommodate subsidence movements. Although
building codes are rarely used in the United States to mitigate subsidence, they often recognize
special problems that require investigation and evaluation of subsidence problems before site
development.
Mining
Although planning and zoning authorities in most communities underlain by mines are
well aware of the potential for subsidence, local governments seldom incorporate this potential
into land-use plans or zoning ordinances. There are probably two principal reasons for this.
The first reason is that while broad areas subject to localized collapse usually can be identified,
the likelihood of collapse at any particular location during a given period of years is relatively
slight. The second reason is that in some communities the undermined area is so large relative
to the few incidences of damage per year that special controls may not be cost-effective.
Also, depressed communities working to revitalize their local economies, such as those in
northeastern Pennsylvania, are not likely to advertise their susceptibility to subsidence while
seeking investments in the area.
Land-use planning authorities in other countries have incorporated mine-subsidence po-
tential into the planning process. In Great Britain, mining in many areas is incorporated as
an element of the development permission process. Most mining in these areas is carried out
by the longwald technique, with which subsidence is predictable. New development can either
be delayed until after the subsidence takes place or designed to minimize damage. In the city
of Whangerei, New Zealand, a zoning scheme has been developed that requires actions to
minimize subsidence damage depending on the degree of subsidence nsk. In Zone I, removal
of the subsidence hazard is required before approval will be given for subdivision of land or
for new construction. In Zone 2, special construction measures are required for subsidence
damage resistance.
Subsidence-resistant design could be incorporated into building codes in areas of aban-
doned coal mines as wed, so as to permit fun extraction without serious structural damage in
areas of active mining. The goal of resistant designs is to minimize damage, since prevention
of damage is not always cost-effective. The difficulty for the designer is selection of reasonable
parameters of movement or force Tat will effectively minimize damage without prohibitively
increasing cost. Chen and others (1974) summarize allowable ground deformations for active
· ~ ~ ~ ~
mmlng In Europe ant Japan.
The two basic approaches in subsidence-resistant design are to use either a flexible or
ngid slab. Basements or other projections below ground level are discouraged. Support of
structures on slabs at the ground surface is structurally desirable because it allows the ground
to move freely below the structure.
Flexible structures having a pinpointed steel frame win cladding designed to move relative
to the frame offer many advantages in a subsidence-prone area. Since 1956 a form of flexible
construction known as CLASP has been used for over 2,000 buildings in Great Britain. Bell
(1978) reviewed Me performance of buildings built win the CLASP system over a 15-year
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37
THE NETHERLANDS - A MILLENNIUM OF SUBSIDENCE MANAGEMENT
One of the oldest records of man-induced subsidence appears to be on the organic soils of the
old grassland polders in the western part of the Netherlands. The low-moor peat soils in these old
polders, with elevations initially near sea level, were reclaimed between the ninth and thirteenth
centuries. Initially excess water was drained by gravity through sluice gates that were opened at low
tide. Around the beginning of the thirteenth century, drainage problems that were in part attributable
to subsidence led to the installation of hand- and horse-powered pumps. This system of drainage
continued until the fifteenth century, when the land surface had subsided to such an extent that
windmills were required to pump out the excess drainage water (Figure 20~. By the nineteenth
century, total subsidence ranged from 1 to 2 m. About 1870, pumping stations powered by steam,
and later by diesel and electric motors, were placed into operation. The increased drainage caused
subsidence to accelerate. Whereas the first 1 to 2 m of subsidence occurred in a millennium, the
next 0.5 m took only a century.
The complex development of land and water resources in the Netherlands required water
resources management that could operate effectively' swiftly, and fairly. In the twelfth century, when
the first regional waterworks came into operation, the first management groups, called waterboards,
were established. From the fourteenth through sixteenth centuries, region after region developed
its own waferboard. With time, the more powerful boards assimilated smaller local boards. They
also increasingly assumed responsibilities, formerly belonging to villages and farmers, for the
maintenance and repair of dikes, dams, and sluices. These waterboards possessed great powers in
the Middle Ages. They were responsible for all justice in the area of water administration and could
even impose capital punishment on offenders.
About 1840, changes in the Dutch constitution gave the provincial governments power to
recognize these boards and create new laws and regulations for them. This led to a more democratic
structure within the boards and the election of representatives by landowners. Today, about 200
waterboards staffed with expert technical and administrative personnel manage these water resources
of the Dutch polders.
FIGURE 20 Dutch windmills have played an important role in organic soil subsidence in Holland because
they have been used to facilitate drainage of areas underlain by organic soils. (Photograph courtesy of John
C. Stephens.)
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38
period (1957-1971) In the Nottinghamshire area and concludes that it worked well. Such
designs have not been used in the United States.
Rigid structures are another approach to subsidence. Usually, the foundation consists of a
thick slab or raft stiffened by thick shear wales. Such structures may be provided win facilities
for jacking them level if tilting should occur. In some cases, rigid structures have been used
in conjunction with a three-point support system. The structure can tilt without distress and
be jacked back into position (National Coal Board, 1978~. Cochran (1971) estimates that a
0.15-m-thick reinforced concrete slab would add 4 percent to We purchase price of the average
new house built in western Pennsylvania.
The Institution of Civil Engineers (1977) provides considerable information on how to
design structures, transportation networks, and utilities that may be subjected to subsidence.
Yokel and others (1981) and Baker (1974) also suggest construction procedures in subsidence
areas.
Design criteria for flexible structures that substantially reduce the risk of damage rarely
exceed 5 percent of a building's cost (Wardell, 1969~. However, the use of rigid designs to
protect a building may add significantly to its cost (MieviBe, 1971~.
Sinkholes
Catastrophic subsidence associated with sinkholes is usually not specifically considered
in land-use plans or zoning regulations in the United States. Assessment of the potential
for site-specific catastrophic subsidence is difficult and expensive. In addition, the reliability
of site-specific assessments tends to decrease as the area under investigation increases. The
usual approach is voluntary subsurface investigations usually for large buildings and taking
structural measures when the risk is found to be high. An innovative attempt is being made
by Ike city of Huntsville, Alabama, which is planning a series of special maps, building-
design criteria, and codes to be used in subsidence-prone areas. Siting of hazardous-waste
facilities in Florida offers another exception. State regulations there require that the potential
for catastrophic subsidence be considered in the site-selection process.
Although building codes have not been used to mitigate catastrophic subsidence, some
foundation designs are Darticulariv well suited for sinkhole areas (Sowers. 19751. Dnlled
. ,., , , · , , · · ,, , . ~ · ~ ~ a ·~ ~ ·~
piers to solid rock are extensively used In sinkhole areas to bypass voids and collapsible soil
and rock. Reinforced mats can be used to resist failure from cavity collapse. If cavities are
small, they may be bridged or straddled with batter piles. Because of the large range of
potential subsurface conditions even within smald areas, however, it is clear that consideration
of this type of subsidence by building codes should allow for engineering judgment on a
case-by-case basis. These areas also require special exploration techniques, including test
drilling, logging, geophysics, and remote sensing to properly locate areas of potential collapse.
This comprehensive approach win special exploration and engineering is illustrated by route
selection and construction of a natural gas pipeline in central Alabama (LaMoreaux and
Newton, 1986~. Geologic, geophysical, and hydrological surveys were conducted to find He
route with the least potential for catastrophic subsidence, and then construction employed
special design features to tie the pipeline to stable bedrock in case collapse occurred.
Underground Fluid Withdrawal
Land-use planning and zoning are potentially applicable to mitigating at least two of
the hazards from land subsidence caused by fluid withdrawal flooding and ground rupture.
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Flood-plain zoning, in fact, already is used in many subsidence areas, although the motivation
for its use has been solely to address flooding. Subsidence complicates flood-plain zoning
because the area affected by flooding increases with time as the land subsides. The time-
dependent increase in flood potential in and adjacent to zoned areas has not been seriously
considered in subsidence areas to date and is a troublesome complication that needs to be
considered. Land-use planning and zoning also can be used to mitigate ground rupture, since
susceptible areas can be identified. An example of how this might be accomplished is the
A1quist-Priolo Special Studies Zones Act of 1972, codified in Califomia Public Resources
Code as Division 2, Chapter 7.5. Under the act, the Califomia State Geologist must delineate
"special studies zones" along earthquake faults that pose the Great of surface rupture. The act
provides for public safety in areas subject to surface-fault rupture by requiring developers to
prepare geologic reports, cities and counties to recognize fault hazards in approving projects,
and selders of real estate or their agents to disclose fault hazards. An even more comprehensive
example, related to public information, is a Santa Clara County, Califomia, ordinance that
enforces preconstruction geologic investigations that require all sellers of real estate within
flood, landslide, or fault-rupture zones to provide buyers with a written statement of He
geologic risk.
The threat of flood damage to structures in areas undergoing subsidence also can be
addressed Trough building codes by requiring that structures be built at higher-than-normal
grades. Examples that were required by codes include structures in flood-prone areas built on
piers (Figure 21) in the Houston-Galveston, Texas, area and placement of fill at higher-~an-
normal elevation in anticipation of additional subsidence in Long Beach, California.
Hydrocompaction
Land-use controls and building codes are particularly wed suited to mitigating damage
in urban areas from hydrocompaction, although they have not been applied for this purpose.
Identification of areas underlain by collapsible soils commonly is feasible on the basis of
laboratory and field investigations (Curtin, 1973), and special building requirements can be
stipulated. For example, surface runoff can be diverted from structures, precompaction can be
required, or structural designs similar to those that are applicable to mining or sinkhole areas
can be mandated.
Organic Soils
Land-use planning and zoning regulations affecting development in wetlands are common.
These restrictions, however, are aimed primarily at preserving wetlands and controlling runoff
and flooding, rather than preventing subsidence damage, but they have the effect of reducing
subsidence damage. State laws in Massachusetts, Connecticut, New Hampshire, and others
prohibit development in wetlands. A regional approach is taken in the San Francisco Bay
area, where He Bay Conservation and Development Commission regulates development in
organic "bay mud" areas. A very large number of cities and counties prohibit the develop-
ment of wetlands through zoning; these include such widely dispersed communities as Coon
Rapids, Michigan; Dartmouth, Massachusetts; Little Silver, New Jersey; Richmond, California;
Broward County, Florida; and Hempstead, New York.
The New Oricans, Louisiana, area provides a classic case history Hat documents where
in the absence of land-use controls specific to subsidence, much greater costs were incurred
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FIGURE 21 Beachfront house built on suits in the Houston-Gal~reston area to reduce potential for damage Tom
storm surges. Damage is from high winds caused by Hurricane Alicia, 1983. (Photograph courtesy of H. Crane
Mill=.)
(Earle, 1975; Mumphrey, 1975~. Much of Me present problem resulting from differential
subsidence within the city of New Orieans could have been avoided Trough ordinances. The
following three-tiered system of regulatory and management guidelines for development in
organic soils regions has been proposed to mitigate subsidence hazards (Mumphrey and others,
1976; Mumphrey and Brooks, 1978~:
Comprehensive Zoning Ordinance. This is an all-encompassing local land-use control
to determine whether or not development will be allowed in an area sensitive to land
subsidence and related flooding.
2. Subdivision Regulations. This second level of land-use control would govern the par-
titioning of subsidence-prone land into lots for sale. These regulations would include
requirements as to development densities, lot sizes and configurations, road design and
construction standards, minimum drainage requirements, and minimum flood elevations
(Imperial Calcasieu Regional Planning and Development Commission, 1974; Mumphrey
and Brooks, 1978~.
3. Building Codes. The third level of local land ordinances applies at the more cietaile~i level
and regulates all aspects of on-site construction, including lot grade, piling and foundation
preparation, utility installation, structural specifications, and product design and perfor-
mance standards (Imperial Calcasieu Regional Planning and Development Commission,
1974; Mumphrey and Brooks, 1978~.
Predevelopment strategies can be most effective in avoiding subsidence-related hazards in
areas of organic soils. If possible, permitted uses of land in subsidence-prone areas or organic
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soils should be restricted to agricultural, recreational, and limited public purposes. Use for
permanent residences and industry should be severely restricted where possible.
The state of Florida has mitigated water-resource problems that include flooding associated
with organic soil subsidence by establishing about six large water-resource management districts
that cover almost the entire state. These districts regulate land use, zoning, drainage, and
irrigation and control the construction and operation of canals, dams, and other structures that
affect water levels. The oldest and largest of these governing bodies is the Central and South
Florida Water Management District, which covers the southeastern part of the state, including
the Everglades and adjacent coastal land.
Building codes can be used to address problems of differential subsidence in organic soil
areas. Several elements of the problem can be addressed:
2.
3.
4.
Site drainage, clearing, and filling techniques should be carefully reviewed. Improper
clearing practices, such as burying tree roots and stumps, can cause serious homeowner
problems through differential foundation settlement. If a site is to be cleared, the vegetation
should be removed from the site, not buried.
Drainage and filling methods should provide for the greatest degree of safety from flooding
and groundwater contamination. A "Modified Wet Method" and "Modified Fill Method"
have been recommended to substantially reduce the amount of initial and continued
subsidence (Kaiser Engineers and Burk and Associates, 1976; Mumphrey and Brooks,
1978~. In many coastal regions, these reclamation methods depress the water table below
mean sea level but provide for sufficient quantities of fill material to raise the land surface
elevation above normal flood levels.
Utility lines, including water, gas, telephone, electricity, and sewers, can be laid in
trenches with special underground cradle structures to support manholes (Mumphrey and
Brooks, 1978~. Because of the high potential for failure of utility lines due to differential
subsidence, threaded connectors between gas mains and house feeder lines should be
prohibited (Earle, 1975~. In existing developments, stress-resistant natural gas connections
should be used.
Building piles should be driven to a specific depth or to a point of refusal to further
penetration. Precautions must be taken to avoid drainage that will lower the water table
below the base of any pilings, thus undermining their support, strength, and stability.
Foundations need to be evenly supported once pilings are in place. The most commonly
used foundation in subsidence-prone areas is the slab-on-pilings technique. While this
technique provides for a stable foundation, the surrounding area continues to subside, pro-
ducing differential subsidence and the problems inherent to this process. It is particularly
important that fill material to remediate for differential subsidence be more permeable than
the underlying organic sediments so that combustible gases release to the atmosphere. To
further minimize the chance of explosions, additional fill material should not be placed
under concrete slab foundations if voids are created by subsidence, as this replacement
has been known to impede the escape of methane gas from broken gas mains or from the
decay of organic soils.
MARKET-BASED METHODS
The objective of market-based mitigation efforts is to internalize the cost of subsidence
by transferring external costs to either the parties responsible for it or the consumers of the
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products of those parties. Intemalizing subsidence costs may be done by taxes or fees on Me
parties causing the subsidence or by requiring those parties to carry out prevention measures
directly. Litigation also can be used to internalize costs. To date, market-based mitigation
methods have been applied primanly to subsidence caused by mining and withdrawal of
groundwater.
Mining
Two market-based methods have been used to mitigate mine subsidence: (1) requiring
mining companies either to try to prevent subsidence or to compensate those damaged by it and
(2) taxing extracted material to cover costs of public prevention programs or reimbursement
of victims. The first method is illustrated by the Pennsylvania Bituminous Mine Subsidence
and Land Conservation Act of 1966 that requires bituminous coal miners to take measures
to prevent "damage as the result of caving-in, collapse, or subsidence" to public buildings,
dwellings, or cemeteries. Under this act, owners of buildings that existed in 1966 can receive
compensation from mining companies for damage due to mining after that date. The second
method is illustrated by the federal Surface Mining Control and Reclamation Act of 1977,
which taxes each ton of coal mined to provide funds for reclamation of abandoned mine land.
The tax may be used only to stabilize subsiding areas and not to provide relief for subsidence
damage. This act also requires underground coal mine operators to prevent damage from
subsidence.
Sinkholes
The causes of catastrophic subsidence are generally complex, and it is difficult, although
not always impossible, to charge Me cost of dam age to the party causing it. For example,
recent litigation prompted by flooding caused by seepage from a reservoir built on limestone in
Alabama led to a $10 million settlement paid by the company that had impounded the water.
Underground Fluid Withdrawal
Resolution of subsidence problems due to the withdrawal of groundwater and other fluids
usualRy requires a regional approach. Although use of market-based methods is not widespread,
these methods are very suitable for most examples of subsidence induced by withdrawal of
underground fluids, because resource extractors can be identified. This approach is used in the
Santa Clara ValDey, California, where both surface water and groundwater are managed by the
Santa Clara Valley Water District. The district imposes a tax on groundwater pumpage that
eliminates the cost advantage of groundwater over surface water.
Hydrocompaction
For many major engineering projects where hydrocompaction damage or its prevention
is a consideration, costs are already internalized. For example, along the west side of the
San Joaquin Valley, Califon~ia, hydrocompaction caused $8 million in damage and added $15
minion to construction costs of canals. The costs, which were incurred by the canal builders
and operators, were passed on to water users.
Costs are also intemalized for many smalRer structures, since damage commonly is caused
by runoff from the structure itself. Questions of liability may arise, however, when a property
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owner incurs damage caused by drainage from adjacent property or has purchased a structure
whose design did not consider hydrocompaction.
Organic Soils
To a limited extent, costs of organic-soil subsidence already may be internalized. Con-
struction of levees and flood-contrl! works is commonly a public cost supported by bonds.
For the most part, however, the costs of subsidence damage are not distributed equitably.
INSURANCE
Insurance programs to provide relief from subsidence damage have been used in several
areas to distribute losses more equitably and encourage risk-reducing actions. Programs have
been implemented to insure against losses from coal-mine subsidence and catastrophic subsi-
dence associated win sinkhole collapse. Although it was not intended to mitigate subsidence,
me National Flood Insurance Program offers relief to those impacted by flooding aggravated
by subsidence.
Mining
Coal-mine subsidence insurance is available in Pennsylvania, Illinois, West Virginia, and
Kentucky and is under consideration in several other states. In addition, Colorado offers a spe-
cial Mine Subsidence Protection Program that is publicly managed and privately administered.
Participants must pay a one-time fee of $100 for an inspection of building conditions at the
time of enrolknent and an annual fee of $35.
These programs limit the cost of protection to the group that wiD benefit. The programs
insure only in areas subject to subsidence, share risk only among those exposed to subsidence,
and do not differentiate the degree of risk.
Although the Pennsylvania insurance program was adopted in 1961, this state-run program
has not been widely used by homeowners, since it requires purchase of a separate policy. The
Illinois program calls for subsidence coverage to be included with the basic property insurance
policies unless the homeowner specifically waives it. The result has been a much larger group
of insureds and lower rates than in Pennsylvania. The relatively new insurance programs in
West Virginia and Kentucky are pattemed after the Illinois program. Experience with the
Colorado program is limited, because it was established in l9S8.
Sinkholes
Starting in 1969, aD homeowners' policies in Florida were, if requested by the owner,
required to include sinkhole coverage. Few requests were made for the coverage, however,
and in 1973 the state Insurance Commissioner mandated that sinkhole coverage be included
in all homeowners' policies. In 1981 sinkhole coverage was extended to all structures,
although insurance companies were given an option not to provide coverage for commercial
and government buildings. Alabama also has sinkhole insurance available to the owners of
un(leveloped property. Sinkhole damage has been a small cost for the insurance industry.
Underground Fluid Withdrawal and Drainage of Organic Soil
The National Flood Insurance Program covers damage by flooding, which, as previously
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noted, can be aggravated by subsidence. The incremental cost to the program from subsidence
is not known, but it is potentially very large, as shown in this example. The Texas Gulf Coast
experienced Tree major rainfalls in 1979. As a result of the flooding from these three storms,
the federal government, Hugh the National Flood Insurance Program, received over 17,000
claims totaling $170,000,000. A large part of the developed area has subsided. In 1979 over
102,000 flood insurance policies were in force in the subsidence area in Harris and Galveston,
Texas, with $4.8 billion in flood insurance exposure.
\
Representative terms from entire chapter:
catastrophic subsidence
