The document discusses land subsidence, defined as the gradual or sudden sinking of the earth's surface, detailing its processes, causes (both natural and man-made), and effects. Key factors influencing subsidence include soil composition, depth, organic matter content, and climatic conditions. It also outlines measures to mitigate subsidence, such as controlling groundwater extraction and implementing environmentally-friendly development practices.

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