Foundation - Underpinning Alternatives.

The previous discussion has dealt with the strengthening and underpinning of the foundation in order to resist future soil movement, bypass the problem soil, or relevel the foundation. There are many other types of underpinning and soil treatment alternatives (Brown, 1990, 1992; Greenfield and Shen, 1992; Lawton, 1996). In some cases, the magnitude of soil movement may be so large that the only alternative is to demolish the structure. For example, movement of the Portuguese Bend

Landslide in Palos Verdes, California has destroyed about 160 homes. But a few homeowners refuse to abandon their homes as they slowly slide downslope. Some owners have underpinned their house foundations with steel beams that are supported by hydraulic jacks that are periodically used to relevel the house. Other owners have tried bizarre underpinning methods, such as supporting the house on huge steel drums.

An alternate method to underpinning is to treat the problem soil. Section 15.3 presents soil improvement methods that may be more effective and economical than underpinning the foundation.

For expansive soil, underpinning alternatives include horizontal or vertical moisture barriers to reduce the cyclic wetting and drying around the perimeter of the structure (Nadjer and Werno, 1973; Snethen, 1979; Williams, 1965). Drainage improvements and the repair of leaky water lines are also performed in conjunction with the construction of the moisture barriers. Other expansive soil stabilization options include chemical injection (such as a lime slurry) into the soil below the structure.

The goal of such mitigation measures is to induce a chemical mineralogical change of the clay particles that will reduce the soils tendency to swell.

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