Ground Settlement and subsidence.
In geotechnical engineering , settlement is defined as the vertical movement of the ground, generally caused be changes in stresses within the earth. Subsidence is a term often used to described ‘caving in’ or sinking of the ground, that may not be associated within changes in soil stresses. Excessive settlement or subsidence can lead to distortion and damage to structures, services and infrastructure that are founded on the material subject to the movement. The settlement can be almost instantaneous or can take years or decades to occur, depending on the underlying soil conditions and cause of the movement.
Settlement is most likely to occur when increased vertical stresses are applied to the ground on or above soft or loose soil strata. It is also possible that lowering the ground water, migration of soil fines, deep voids, underground excavation for tunnels, induced ground vibrations and seismic events will cause significant settlement.
Development of an appropriate ground model requires investigation that extends to an adequate depth, identifies any voids, uncontrolled fill, soft clay ground or loose granular material that may be cause unacceptable settlement or subsidence. Where such materials are identified it is essential that the geotechnical risks, as well as the potential causes and consequences of settlement, are considered and adequately addressed.
In some circumstances it may be necessary to carry out ground improvement works, to strengthen, stiffen, over consolidate and densify the ground, to reduce the magnitude of the likely settlement. In other situations, it may be necessary to incorporate deep foundations, or to underpin existing or new structures, such that the foundation stresses are imposed below the ground strata that is subject to settlement.
In areas of deep soft, recently deposited, clayey strata there is a significant risk of short-medium term consolidation settlement, associated with any increase if vertical effective stress, as well as long term secondary, or creep, settlement that can occur without the application of increased vertical stresses. It must also be remembered that, if the depth of soft or loose material varies significantly in plan location, such as where recent paleo-channels are encountered, there will be significant differential settlements at the surface and associated lateral ground movements.
The CWM team has significant experience in the identification, analysis, design, specification and monitoring of project solutions associated with high ground settlements. We can help identify and resolve project challenges associated with construction works, and existing structures or services, that are impacted by potential ground settlements and identify suitable remediation measures to help manage this aspect of geotechnical risk.