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Understanding Common Soil Movement Problems, Part 1:

Real estate in Southern California is a valuable commodity. Even after the housing bust, the value of land in the southland far exceeds most areas of the country and world. Since most of the easy land has already been built on, Developer's are forced to build their product in areas that may cause problems after homes are constructed. Two common problems that cause soils movement are subsidence of soil or the expansion of soil under a building. Understanding these terms are the basis to understanding soil movement problems.

Construction of buildings frequently involves grading, which is essentially done to create a level building pad. If the natural ground has a slope, grading might be performed by cutting into the slope and partially removing it. Alternatively, additional soils can be brought to the building site to build up the pad elevation at the low lying areas. Soil that is not natural to the building area, but that is used to raise an existing grade is called fill soil. A building or structure may become distressed when the fill soil compacts or subsides following completion of construction. When the fill soil consolidates, the building settles. Settlement can lead to foundation cracks, wall cracks, difficulty in the opening of windows or doors. The severity of the fill settlement can also depend on the depth of the fill. The deeper the fill, the greater the capacity for consolidation, and the greater possibility for building movement.

Not all buildings constructed on fill soils will settle to the extent of causing damage to the structure. Common problems that lead to fill settlement are the improper placement, or compaction of the fill soils. Fill needs to be compacted to at least 90% during the grading process to meet the minimum building code requirements. If the grader fails to compact the soil properly, there is a greater risk of building settlement. Another factor that may lead to settlement is the use of improper material in the fill. Organic materials or the use of too much rock in the fill can lead to soil settlement.

In contrast to damage caused by buildings settling or subsiding, damage can also occur when the soil beneath a structure expands or heaves. Another name for expansive soil is clay. Homes built on clay frequently experience problems due to soil movement. Clay soils will expand when they get wet. Clay soils will shrink when they dry out. This cycle of expansion and contraction can wreak havoc on a building foundation system. It is not uncommon for soils to become saturated during a rainy season, or even from frequent watering of the landscape placed around a new home. Dry summers, drought, and water rationing can result in the soil drying out, and shrinking. These cycles of saturation and drying out can cause a foundation to heave, and then shrink, and this cycle can result in damage to the structure. Cracking of pavement, slabs, stucco are all possible symptoms of expansive soil.

A transition lot is perhaps the most susceptible to damage from soil movement. A transition lot occurs when only part of the lot contains fill, and the rest of lot is on native ground. In the area where a transition from fill to cut or native soil occurs, it is not unusual to experience building distress. This can occur because the fill soil and the native soil respond differently to the introduction of water, and other mechanisms. When the fill side expands or contracts at a rate different than the native soil side, damage can result.

Cracking of walls, floors, slabs, and driveways or other flatwork are all indications of movement of the underlying soils. These problems should be evaluated immediately to determine the cause of the problem, and the extent to which these problems will get worse in the future. 

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