Land Subsidence_Engineering Geology & Geomorphology

1. LAND SUBSIDENCE
ENGR. MD. SHAMIM AHMED
LECTURER
DEPT. OF CIVIL ENGINEERING
SOUTHERN UNIVERSITY BANGLADESH

2. WHAT IS LAND SUBSIDENCE?
Subsidence is the sudden sinking or gradual downward settling of the
ground's surface with little or no horizontal motion.
The definition of subsidence is not restricted by the rate, magnitude, or
area involved in the downward movement. It may be caused by natural
processes or by human activities.

3. COMMON CAUSES OF 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.
• 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).
• Movement of varying soil structures- If there are different soil structures underneath a
structure/building’s foundations, then their movement can result in subsidence.
• Soil decomposition- Some natural soils, such as peat, can decompose and cause
ground movement.

4. COMMON CAUSES OF SUBSIDENCE
Natural Causes of Subsidence
• Trees and other vegetation- Vegetation near the foundations can be a cause because
roots suck moisture from the soil, causing it to shrink.
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.

5. COMMON CAUSES OF SUBSIDENCE
Man-Made Causes of Subsidence
• Improper ground preparation- If a building or structure has been built on compressible
clay soil and no ground preparation has been carried out, this will often lead to
subsidence.
• Bad foundations- Incomplete/poor quality foundation works can cause subsidence
problems.

6. COMMON CAUSES OF SUBSIDENCE
Man-Made Causes of Subsidence
• Improper ground preparation- If a building or structure has been built on compressible
clay soil and no ground preparation has been carried out, this will often lead to
subsidence.
• Bad foundations- Incomplete/poor quality foundation works can cause subsidence
problems.

7. WHAT CAN YOU DO TO EITHER PREVENT OR MITIGATE
THE IMPACT OF SUBSIDENCE?
Following steps are to reduce the likelihood of being affected by subsidence.
 Remove trees and bushes planted near the house. Trees are one of the most common
causes of subsidence as the roots withdraw moisture from the soil supporting the
foundations.
 If trees can not be removed, maintain them with regular pruning. This should curb their
growth and limit their search for water close to the house.
 Carry out regular inspections of the property, paying particular attention to pipework,
gutters and drainage systems in case of leaks and/or blocks.
Homes can also be affected by subsidence because of location, such as old mining
towns, and soil type in the area. Clay soils in particular are susceptible to subsidence as
the soil dries and shrinks, or absorbs water and expands.

8. THANK YOU