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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
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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.
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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.
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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.
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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.
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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. ~ .
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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.)
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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
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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
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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
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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
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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.)
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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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39 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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41 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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42 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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43 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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44 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: