Subsidence

Subsidence is the motion of a surface  as it shifts downward relative to a datum such as sea-level. The opposite of subsidence is uplift, which results in an increase in elevation. Ground subsidence is of interest to land owners and householders. Surveyors measure these relative levels from time to time.

Factors Effecting subsidence

On the macro level we have natural occurrences of earthquakes, isostatic compensation, dissolution of rocks and soils and on a man made level we such activities as mining for minerals and gases, water table change due to extraction and redirection of surface waters which then permeate to lower levels.

On the micro level the householder or landowner is affected by such things as leaking drainage pipes and leaking taps.  These are the common problems which cause localised problems to the homeowner or building manager.  Wetting down of the soil causes a change in the bearing capacity of the soil and so one can get local subsidence of the building footings form 1 to 10 cm.  This type of subsidence can be very crostly in terms of repair and rectification work.

Small subsidence in driveway asphalt

Costly Subsidence due to inadequate engineering of soil characteristics and pier design.

Many soils contain significant proportions of clay and due to the very small particle size are affected by changes in soil moisture content. Seasonal drying of the soil results in a reduction in soil volume and a lowering of the soil surface. If building foundations are above the level to which the seasonal drying reaches they will move and this can result in damage to the building in the form of tapering cracks. Trees and other vegetation can have a significant local effect on seasonal drying of soils. Over a number of years a cumulative drying occurs as the tree grows, this can lead to the opposite of subsidence, known as heave or swelling of the soil, when the tree declines or is felled. As the cumulative moisture deficit is reversed, over a period which can last as many as 25 years, the surface level around the tree will rise and expand laterally. This is often more damaging to buildings unless the foundations have been strengthened or designed to cope with the effect.

 

Where buildings are built on lowlands or coastal plains all reclaim land such as landfill sites, there is a great capacity for subsidence to occur if adequate foundation work is not performed correctly.  Building in these areas generally requires piling down to the original natural level of soil. silly, she was going to buy new home in new development or a new hotel it is worth checking out the original ley of the land before the development and so avoided any future issues of land subsidence. also worth knowing is if any gas extraction isgoing on in the area. [More than half of NSW is presently under exploration lease for natural gas]

If natural gas is extracted from a natural gas field the initial pressure (up to 60MPa (600 bar)) in the field will drop over the years. The gas pressure also supports the soil layers above the field. If the pressure drops, the soil pressure increases and this leads to subsidence at the ground level. Major subsidence of the Mississippi River Delta due to oil and gas extraction has caused the ocean to rise and flood over 34 square miles (88 km2) of marshes and land each year. Since 1930, Louisiana has lost 1,200,000 acres. Since exploration of the Slochteren (Netherlands) gas field started in the late 1960s the ground level over a 250 km² area has dropped by a current maximum of 30 cm.

 

Road subsidence

Coal long wall mining is another culprit in causing large areas to subside.  Note the south-west of Sydney, where rivers have disappeared roads have buckled and houses that had to be demolished due to settlement causing severe cracking.

Below is a repair to site subsidence. Piers were only 3m in length on a fill section of site and were not founded on rock. Over a period of time soil settles, moves down hill under gravity and is subject to expansion and contraction of clay when ever the weather changes. Rectification involved a reinforcing pier structure founded on rock with capping beams. A new non moving deck can now be installed.

References: http://en.wikipedia.org/wiki/Subsidence

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