An analysis of long-term seasonal interaction between slab-on-ground foundation and expansive soils

Date

1997-12

Journal Title

Journal ISSN

Volume Title

Publisher

Texas Tech University

Abstract

In general, a good foundation design is one that provides sufficient bearing capacity to counteract the structural loading without excessive settlement. However, should the foundation be placed on expansive soil, a geotechnical engineer faces a major challenge: the soil can respond with a change in volume (shrink/heave), i.e., it can respond not only to the structural loading but also to a change of soil moisture condition. In other words, a soil volume change can be expected even if structural loading is well anticipated in the foundation design. The unique property of expansive soil is the change in volume when it absorbs moisture from its environment and vice versa. Moisture movement is thus the thrust of any study in expansive soil. Incorporating the energy concept of moisture retention from soil sciences, studies have shown that the interaction among the three phases of soil (i.e., solid, gaseous, and liquid phases) generates within an unsaturated soil mass a unique negative force field that tends to attract and retain water. This soil's affinity of water is usually expressed as water potential in soil science or soil suction in geotechnical engineering. The moisture movement within a fine-grained, unsaturated soil is controlled by a soil suction gradient. Soil moisture always tends to migrate from a point of low soil suction (wet) to a point of high soil suction (dry). Due to the capability of the soil to retain water, the migrated water is stored within the soil and introduces a swelling volume change problem in an expansive soil. If the swelling is restrained, the resulting swellpressure can be as high as 20,000 psf (Holtz and Kovacs, 1981). Lightly loaded structures such as houses, apartments, and pavements may be lifted by this swelling pressure. Compounding the problem, these large swelling pressures do not occur uniformly within the soil and thus results in differential soil movement. Several design procedures are available to mitigate the structural damages due to the differential soil movement. These include design procedures proposed by Building Research Advisory Board {1969), Walsh (1974), Eraser and Wardle (1975), Post-Tensioning Institute(1996), and Wire Reinforcement Institute(1982). To employ the above design procedures, the geotechnical engineer needs to predict an expected differential swelling and other parameters that cannot or seldom be performed in a soil laboratory. It has been shown that the climatic condition of a site is a major factor controlling the magnitude of the differential soil movement. The climatic condition of a site will detemiine the active zone, the possible maximum seasonal changes of soil moisture condition and thus the wet and dry soil suction profiles. These seasonal changes of soil moisture or soil suction will dictate the expected swelling pressures as well as differential soil movement. Thus, it is of great interest for a geotechnical engineer dealing with expansive soil to have a knowledge of the soil suction distribution in the soil. If the seasonal soil suction distribution for a site with expansive soil is known, its engineering behavior (i.e., heave/shrinkage) can be managed.

Description

Keywords

Concrete slabs, Foundations, Swelling soils, Soil dynamics

Citation