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Land subsidence

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Md Asif Hasan
Md Asif Hasan

This document was submitted as an assignment as a part of the partial fulfillment of my master's degree.

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Assignment on “Land Subsidence” Submitted by Md. Asif Hasan Roll: SH-037 Date: September 1, 2019
Land Subsidence Land subsidence is a gradual settling or sudden sinking or is the downward movement of the Earth’s surface due to movement of earth materials. Processes of Subsidence: • Compression/Compaction: Compression is the change in soil volume produced by the application of a static external load. Compaction is produced artificially by momentary load application such as rolling, tamping, or vibration (USDI 1974) • Consolidation: The gradual, slow compression ofa cohesivesoil due to weight acting on it, which occurs as water, or water and air, are driven out of the voids in the soil (Scott 198 1) • Shrinkage: The change in soil volume produced by capillary stress during drying of the soil (USDI 1974) • Oxidation: The process by which organic carbon is converted to carbon dioxide and lost to the atmosphere (Young 1980). Causes of Land Subsidence: Natural Causes of Subsidence: ✓ Cohesive soils shrinking and swelling: Cohesive soils such as clay and silt are particularly likely to cause subsidence, since they shrink and swell depending on their moisture content. This means that the weather can have an enormous impact on their composition. Clay soil is found extensively across England and Wales, so this is a problem that will affect lots of people. ✓ Non-cohesive soils being washed away: Non-cohesive soils, like sand and gravel, are less susceptible to shrinkage and growth, but they are more susceptible to being washed away by water flow (such as when a water pipe is damaged).Our UK subsidence map to shows which areas of the country are most affected by subsidence. ✓ Movement of varying soil structures: If there are different soil structures underneath a building’s foundations, then their movement can result in subsidence. ✓ Soil decomposition: Some natural soils, such as peat, can decompose and cause ground movement.
✓ Trees and other vegetation: As much as that nearby tree or bush adds to the charm of where you live or work, there is the possibility that it could be causing subsidence to occur. Vegetation near the foundations can be a cause because roots suck moisture from the soil, causing it to shrink. This is more of a risk during long periods of dry weather as thirsty plants push their roots through building foundations in search of water. Oak, willow and poplar trees all absorb a lot of moisture from the soil. Man-Made Causes of Subsidence: ✓ Damaged drains: Water escaping from damaged drains can sometimes soften, or even wash away the ground beneath the foundations, resulting in ground movement and, therefore, building movement. This issue usually affects sandy or gravelly soil. ✓ Historic mine shafts: Mines are another cause of subsidence. Areas where there was heavy coal mining will have several open areas underneath the ground, which can lead to subsidence. ✓ Nearby traffic: If a building is near to an area of heavy traffic, the resulting extended vibration can cause soil to move ✓ Improper ground preparation. Effect of Land Subsidence: Some serious effects of land subsidence include loss of levee freeboard and subsequent reduction in flood protection, change of gradient along water conveyance canals, damage to roads and bridges, and collapse of water well casings. Understanding and detecting land subsidence in very early stages is important to prevent these types of structural damage. Factors Influencing Land Subsidence: The factors that influence the rate and degree of subsidence are the following (Segeren and Smits 1980): ✓ Clay content: The water content in sediments is linearly related to their clay content; hence clayey sediments lose more water than sandy sediments. As a consequence, clay soils will subside more ✓ Depth of the layer in the profile: The loss of water in the different soil layers decreases with depth. The number of roots and their water uptake similarly decrease with depth. Beside this, deeper layers are closer to the water table and will thus be kept moist by capillary rise. So the subsidence caused by the shrinkage of the different soil layers at a given clay content decreases with depth ✓ The thickness of compressible layers: The greater the thickness of the compressible layers, the greater will be the subsidence ✓ Organic matter content: The water content depends to some extent on the organic matter content. Mineral soils containing high contents of organic matter show greater degrees of subsidence. The oxidation of the organic matter not only results in the loss of the organic matter, but also in the loss of the water associated with it ✓ Type of crop: As different crops are characterized by different evapotranspiration rates, their influence on subsidence also differs. The difference may be due to the depth of the rooting system; compare, for instance, alfalfa with its deep rooting system and grass with its shallow rooting system. The length of the growing period is another important factor: spring-sown cereals harvested in midsummer have a lower total evapotranspiration than perennial crops like grass or alfalfa; ✓ Density of the soil: Sediments with different pore volumes (and different water contents) show different water losses and hence different degrees of subsidence. Seaand lake-bottom soils have a lower density
than sea-shore deposits exposed at each low tide. During the formation of such sea-shore deposits, shrinkage already occurs and thus also subsidence ✓ Field drainage conditions: Under poor drainage conditions, which often prevail in the first years after reclamation, the shrinkage may be limited because the water table is still high and consequently the capillary stresses are low. Thus, under these conditions, the rate of subsidence will be less than in well- drained soils; ✓ Climatic conditions: The drier the climate, the more water will be lost, and hence the greater will be the subsidence; ✓ Time: Subsidence, both that caused by consolidation and that by shrinkage, is a function of time. As shrinkage is caused by the physical ripening of the soil, the rate of subsidence will decrease with time. Measures to take to control land subsidence • Control of ground water extraction, in urban, industrial and rural areas. Regulations that stop ground water extractions need to be combined with management of demand and enhanced surface water use which requires integrated water resources management and planning • Implementation of environmentally-friendly development • Control over peatland canalization • The development of a roadmap for the regional and national levels to anticipate and mitigate land subsidence in an integrated and consistent way

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Land subsidence

  • 1. Assignment on “Land Subsidence” Submitted by Md. Asif Hasan Roll: SH-037 Date: September 1, 2019
  • 2. Land Subsidence Land subsidence is a gradual settling or sudden sinking or is the downward movement of the Earth’s surface due to movement of earth materials. Processes of Subsidence: • Compression/Compaction: Compression is the change in soil volume produced by the application of a static external load. Compaction is produced artificially by momentary load application such as rolling, tamping, or vibration (USDI 1974) • Consolidation: The gradual, slow compression ofa cohesivesoil due to weight acting on it, which occurs as water, or water and air, are driven out of the voids in the soil (Scott 198 1) • Shrinkage: The change in soil volume produced by capillary stress during drying of the soil (USDI 1974) • Oxidation: The process by which organic carbon is converted to carbon dioxide and lost to the atmosphere (Young 1980). Causes of Land Subsidence: Natural Causes of Subsidence: ✓ Cohesive soils shrinking and swelling: Cohesive soils such as clay and silt are particularly likely to cause subsidence, since they shrink and swell depending on their moisture content. This means that the weather can have an enormous impact on their composition. Clay soil is found extensively across England and Wales, so this is a problem that will affect lots of people. ✓ Non-cohesive soils being washed away: Non-cohesive soils, like sand and gravel, are less susceptible to shrinkage and growth, but they are more susceptible to being washed away by water flow (such as when a water pipe is damaged).Our UK subsidence map to shows which areas of the country are most affected by subsidence. ✓ Movement of varying soil structures: If there are different soil structures underneath a building’s foundations, then their movement can result in subsidence. ✓ Soil decomposition: Some natural soils, such as peat, can decompose and cause ground movement.
  • 3. ✓ Trees and other vegetation: As much as that nearby tree or bush adds to the charm of where you live or work, there is the possibility that it could be causing subsidence to occur. Vegetation near the foundations can be a cause because roots suck moisture from the soil, causing it to shrink. This is more of a risk during long periods of dry weather as thirsty plants push their roots through building foundations in search of water. Oak, willow and poplar trees all absorb a lot of moisture from the soil. Man-Made Causes of Subsidence: ✓ Damaged drains: Water escaping from damaged drains can sometimes soften, or even wash away the ground beneath the foundations, resulting in ground movement and, therefore, building movement. This issue usually affects sandy or gravelly soil. ✓ Historic mine shafts: Mines are another cause of subsidence. Areas where there was heavy coal mining will have several open areas underneath the ground, which can lead to subsidence. ✓ Nearby traffic: If a building is near to an area of heavy traffic, the resulting extended vibration can cause soil to move ✓ Improper ground preparation. Effect of Land Subsidence: Some serious effects of land subsidence include loss of levee freeboard and subsequent reduction in flood protection, change of gradient along water conveyance canals, damage to roads and bridges, and collapse of water well casings. Understanding and detecting land subsidence in very early stages is important to prevent these types of structural damage. Factors Influencing Land Subsidence: The factors that influence the rate and degree of subsidence are the following (Segeren and Smits 1980): ✓ Clay content: The water content in sediments is linearly related to their clay content; hence clayey sediments lose more water than sandy sediments. As a consequence, clay soils will subside more ✓ Depth of the layer in the profile: The loss of water in the different soil layers decreases with depth. The number of roots and their water uptake similarly decrease with depth. Beside this, deeper layers are closer to the water table and will thus be kept moist by capillary rise. So the subsidence caused by the shrinkage of the different soil layers at a given clay content decreases with depth ✓ The thickness of compressible layers: The greater the thickness of the compressible layers, the greater will be the subsidence ✓ Organic matter content: The water content depends to some extent on the organic matter content. Mineral soils containing high contents of organic matter show greater degrees of subsidence. The oxidation of the organic matter not only results in the loss of the organic matter, but also in the loss of the water associated with it ✓ Type of crop: As different crops are characterized by different evapotranspiration rates, their influence on subsidence also differs. The difference may be due to the depth of the rooting system; compare, for instance, alfalfa with its deep rooting system and grass with its shallow rooting system. The length of the growing period is another important factor: spring-sown cereals harvested in midsummer have a lower total evapotranspiration than perennial crops like grass or alfalfa; ✓ Density of the soil: Sediments with different pore volumes (and different water contents) show different water losses and hence different degrees of subsidence. Seaand lake-bottom soils have a lower density
  • 4. than sea-shore deposits exposed at each low tide. During the formation of such sea-shore deposits, shrinkage already occurs and thus also subsidence ✓ Field drainage conditions: Under poor drainage conditions, which often prevail in the first years after reclamation, the shrinkage may be limited because the water table is still high and consequently the capillary stresses are low. Thus, under these conditions, the rate of subsidence will be less than in well- drained soils; ✓ Climatic conditions: The drier the climate, the more water will be lost, and hence the greater will be the subsidence; ✓ Time: Subsidence, both that caused by consolidation and that by shrinkage, is a function of time. As shrinkage is caused by the physical ripening of the soil, the rate of subsidence will decrease with time. Measures to take to control land subsidence • Control of ground water extraction, in urban, industrial and rural areas. Regulations that stop ground water extractions need to be combined with management of demand and enhanced surface water use which requires integrated water resources management and planning • Implementation of environmentally-friendly development • Control over peatland canalization • The development of a roadmap for the regional and national levels to anticipate and mitigate land subsidence in an integrated and consistent way