Variation of Soil Suction with Depth in Dallas and Fort Worth, Texas

Abstract

The variation of soil suction and the estimate of constant soil suction with depth is used to design many slab-on-grade foundations and pavement moisture barriers. The Post-Tensioning Institute’s design procedures for slab-on-grade foundations and design of vertical pavement moisture barriers use the constant suction at depth to predict differential soil movements that influence shear, deflection, and moment magnitudes and the effective barrier depth. Constant soil suction estimates can be correlated to the climate or long-term weather conditions at any given site by using the Thornthwaite moisture index (TMI), which estimates the amount of net moisture surplus or deficit from precipitation and evapotranspiration of moisture from the ground surface. On the basis of the empirical curves, the constant value of total soil suction for the Dallas–Fort Worth, Texas (DFW) area is about 246 kPa based on an average TMI of 0. Analysis of more than 1,200 total soil suction laboratory tests performed on developed and undeveloped lots indicates that the measured average total soil suction value in the upper 6 m is closer to 979 kPa for the DFW area ranging between 55 kPa and 11,246 kPa during the 1995–1997 period. Some hypothesized reasons for the difference between the empirical and measured equilibrium (constant) soil suction values are amounts of clay, clay origin, variable plasticity indexes, soluble salt content, and equilibration curve differences.