DESIGN OF PIER FOUNDATIONS ON EXPANSIVE SOILS

摘要

For sites with high expansion potential the most reliable foundation system is the pier and grade beam foundation. The design of the piers requires careful characterization of the soil profile and appropriate computation of the required pier length. This, in turn, requires prediction of the free-field heave of the foundation soil and/or bedrock. Current design procedure generally considers the maximum ultimate predicted heave that will occur at a site. This is appropriate for sites with low to moderate expansion potential. However, for sites with high to very high expansion potential, it may be impractical to design for the ultimate heave. Therefore, design of foundations for buildings on expansive soils must consider the migration of the wetting front and the associated heave that will occur during the design life of the structure. This paper presents the procedure for predicting free-field heave and heave of straight shaft, belled, and helical piers within the design life of the structure. The effect of overexcavating and replacing the upper few meters of expansive soil in combination with installing piers is discussed. The results of the calculations indicate that the required pier length can be reduced significantly if the rate of heave within the design life of the structure is considered.