Land Subsidence Land subsidence is defined as the lowering of the land surface. Many different factors can cause the land surface to subside. Subsidence can occur rapidly due to: a sinkhole or under ground mine collapse, or during a major earthquake. Land subsidence is the lowering of the landsurface elevation from changes that take place underground. Common causes of land subsidence from human activity are: pumping water, oil, and gas from underground reservoirs; dissolution of limestone aquifers (sinkholes); collapse of underground mines; drainage of organic soils; and initial wetting of dry soils (hydrocompaction). 1
DAMAGE CAUSED BY LAND SUBSIDENCE Land subsidence causes many problems including: (1)changes in elevation and slope of streams, canals, and drains; (2)damage to bridges, roads, railroads, storm drains, sanitary sewers, canals, and levees; (3)damage to private and public buildings; and (4)failure of well casings from forces generated by compaction of fine-grained materials in aquifer systems. WHAT Is KARST? Karst is the name applied to landforms that develop in areas underlain by comparatively soluble rocks such as: limestone, gypsum, and salt. Karst terrain is characterized by solution features such as: caves, sinkholes, depressions, enlarged joints, fractures, and internal drainage that can have a negative impact on use of the land. 2
The name was derived from the Karst region of Slovenia (part of the former Yugoslavia), which is underlain by limestone. The passage of water through soluble rocks results in the formation of cavities in the rock. If the ceiling of a cavity collapses, a sinkhole may form at the ground surface. Karst terrain is commonly characterized by: highly uneven depths to bedrock; residual red, clay-rich soil; and surface drainages that disappear underground. Voids in bedrock can capture surface-water flow and disrupt the surface drainage system. Soil and other surficial material may be washed Soil and other surficial material may be washed into the underground network of cavities. 3
Hazards from karst include the formation of: sinkholes or collapse pits, as well as cracking of walls, foundations, roads, and other structures. Less obvious but equally important are the impacts karst can have on water quality. Networks of interconnected caverns and voids allow contaminants such as: sewage, landfill leachate, or hazardous chemicals to travel unimpeded into shallow aquifers that may supply drinking water. There are, however, geologic characteristics that are common to most collapse locations. These are, (1) )groundwater greater than 15 m (50 ft.) below the surface, (2) thick and porous residual soils, (Soil materials dominated by permeable, thick ( 9m thick) residual silty-clay soils of ten with relict bedrock structure, ) (3) Highly weathered underlying limestone or dolomite bedrock, and, (4) Nearby active sinkholes, or within a losing stream valley. 4
Collapse in residential neighborhood, Farmington, Missouri. This collapse had been repaired with soil fill only, but it reactivated. Dissolution Water in the atmosphere can dissolve small amounts of carbon dioxide (CO 2 ). This results in rain water having a small amount of carbonic acid (H 2 CO 3 ) when it falls on the Earth's surface. As the water infiltrates into the groundwater system and encounters carbonate rocks like limestone, it may start to dissolve the calcite in the limestone by the following chemical reaction: 5
CaCO 3 + H 2 CO 3 = Ca +2 + 2HCO 3-2, which states that calcite reacts with carbonic acid to produce dissolved Calcium ion plus dissolved Bicarbonate ion. This reaction takes place as the water moves along This reaction takes place as the water moves along fractures and other partings or openings in the rock. This results in dissolution of much of the limestone if the reaction continues to take place over a long period of time. 6
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Cargill sink formed as a result of solution mining of salt near Hutchinson, Kansas. The sudden collapse of bedrock and near-surface, water-saturated sands into the underground brine-filled cavity created the sinkhole and left the Missouri-Pacific railroad tracks suspended 6 meters in the air. The volume of the crater was estimated at 70,000 cubic meters. Subsidence can also take place slowly, becoming evident over a time span of many years. In either situation, subsidence can result in millions of dollars of damage. Sinkhole Collapse (Karst) Mine Collapse Fluid Removal Earthquake Induced 8
Sinkhole Collapse (Karst) Sinkholes are a naturally occurring, roughly circular depression in the land surface, formed most commonly in areas of carbonate bedrock. Carbonate rocks include limestone and dolostones. Both limestone and dolostone are composed of the highly reactive mineral calcite (CaCO3), which readily dissolves in the presence of slightly acidic ground water. 9
In areas of humid climate, rain water percolates downward d through h the soil cover into openings in the limestone and dolostone bedrock, gradually dissolving the rock matrix. Void spaces in the subsurface will eventually form, and over time may develop into a surface depression called a sinkhole. A sinkhole develops from dissolution and collapse of the underlying carbonate bedrock and overlying weathered material or regolith. Sinkhole development typically occurs over an extended period of time, but sudden and catastrophic failures have occurred, especially during extreme fluctuations of groundwater levels from prolonged wet and dry periods. 10
Sinkholes vary from small cylindrical openings to large conical or parabolic basins which collect and funnel runoff into underlying aquifer. The two most common types of sinkholes are: cover-subsidence and cover-collapse. A cover-subsidence sinkhole is formed where: overburden is relatively thin (a few feet to tens of feet), more permeable, and composed of a greater percentage of silt and sand, thus it lacks the cohesiveness to form a significant "bridge" across the void. 11
So as subsurface solution occurs, the land surface gradually subsides into the void space below typically forming a shallow parabolic basin. Cover-subsidence sinkholes typically develop more gradually than covercollapse sinkholes. Aerial picture of Florida sinkhole lakes. 12
In some areas, the carbonate bedrock is not directly exposed at the surface, but is covered by a variable thickness of clay, silt and sand. A thicker clay-rich overburden may bridge subsurface cavities for long periods of time. Eventually a catastrophic collapse of the overburden into the subsurface cavity may occur, forming a cover-collapse sinkhole. Typically, cover-collapse sinkholes form steep sided d cylindrical i l openings. A cover-collapse sinkhole usually develops in a short period of time with no prior indication of its pending existence, thus having the potential to cause damage to property and structures. 13
Earthquake Induced Land Subsidence In 1811 and 1812, a series of strong earthquakes rattled northeastern Arkansas. These earthquakes initiated significant liquefaction that drastically changed the ground surface. Land subsidence was one of the major effects of these earthquakes. Large tracts of land subsided during the earthquakes and were flooded d with water. Lakes such as Big Lake and St. Francis Lake formed in areas where the land surface has significantly subsided. 14
Land subsidence can occur in various ways during an earthquake. Movement that occurs along faults can be horizontal or vertical or have a component of both. As a result, a large area of land can subside drastically during an earthquake. Land subsidence can also be caused during Land subsidence can also be caused during liquefaction. Liquefaction can result in the settling and compacting of unconsolidated sediment in an event of a major earthquake. This can result in the lowering of the land surface. 15
REDUCING FUTURE SUBSIDENCE In some areas where ground-water pumping has caused subsidence, the subsidence has been stopped by switching from ground-water to surface-water supplies. If surface water is not available, then other means must be taken to reduce subsidence. Possible measures include reducing water use and determining locations for pumping and artificial i recharge that t will minimize i i subsidence. Optimization models coupled with ground-water flow models can be used to develop such strategies. To detect the presence of nearsurface karst To detect the presence of near-surface karst, several geophysical methods can be employed. These include: ground-penetrating radar, electrical resistivity, spontaneous potential, gravity, and magnetic surveys. These methods rely on differences in physical properties between the caverns or their filling materials versus the surrounding rock. 16
Engineering Methods for Detecting Sinkholes Soil borings or other direct testing Borings can be reduced by reconnaissance scannings using the following methods: Electromagnetics (EM) and DC Resistivity: detect variations in subsurface electrical properties related to anomalously thick or wet soils (electrical conductivity highs similar to our use of moisture meters in homes), or voids in the electrically conductive clay soil mantle (electrical conductivity lows) Spontaneous Potential (SP): detects naturally-occurring minute electrical currents or potentials commonly associated with concentrated vertical water infiltration (Streaming potentials) Micro-gravity: detects minute variation in gravity (subsurface voids create missing mass and lower gravity) Seismic Refraction: profiles the top-of-rock which may display conical depressions of a type associated with subsidence sinks or deep gouges or cutters which represent sinkhole-prone lineaments. Ground-penetrating radar. Mitigation The simplest method to mitigate a sinkhole or depression is to fill it in. However, this method works only if the sinkhole is inactive and is already mostly filled in at depth. If a sinkhole is still open at depth and connected to other voids, surface material may continue to wash into the voids. Filling an active sinkhole may be only a temporary solution. In any case, surface drainage should be directed away from karst features to avoid piping or collapse. 17
Mitigation of sinkhole collapse Mitigation of sinkhole collapse is of particular concern in areas of new urban development. Traditionally, many development projects have enhanced the threats t associaed with sinkholes. Development necessarily reduces the amount of surface area that can absorb precipitation, thereby increasing stormwater runoff and directing it into limited areas. Real estate developers should make every effort to plan developments in a manner that avoids sinkholes and least disturbs natural drainage patterns. Ultimately, the best mitigation for sinkhole collapse is to reduce the volume of water passing into a sinkhole. Here are some other measures that can reduce the threat associated with sinkhole collapse: Use steel reinforcement in the foundation footers and construction Use peers and caissons set on bedrock and constructed of concrete and steel for all large multi-story buildings Keep water away from foundations Direct gutter downspouts in a manner to drain water away from structures Eliminate ditches or low spots close to structures Ensure that no leaks exist in water supply lines and sewer lines Purchase sinkhole collapse insurance (currently Kentucky insurance companies are not required to offer such policies, and such insurance is difficult to find) 18
Temporal Sinkhole Triggers Following a period of heavy or prolonged rain (washing-in supporting soils) Following a period of drought (lowering the water tables, leaving cavities) Following a period of housing development (adding pressure on supporting soils) Over pumping existing water supply wells, or drilling of additional wells in an area (lowering the aquifer) Diverting surface water from a large area and concentrating it in a single point Artificially creating ponds of surface water 19