Active zone depth and edge moisture variation distance in expansive soils

This research was conducted to investigate two fundamental expansive soil foundation design parameters known as active zone depth and edge moisture variation distance. These parameters were evaluated in terms of their historical definitions and characterized in terms of the effects of climate conditions, geologic conditions, and soil properties.; Field test sites were constructed in Colorado and Texas. In addition, laboratory and numerical investigations were conducted to investigate these parameters. The following key observations and conclusions summarize the results of this research. (1) The zone of seasonal moisture fluctuation underestimates the depth of the active zone. The depth of seasonal moisture fluctuation at CSU is approximately 5 to 7 feet. Water content increases beneath the slab have been observed as deep as 18 feet. (2) Initial increases in water content beneath slab foundations on expansive soil are due to the reduction in evaporation at the surface. This boundary change causes an increase in moisture to a depth that is generally consistent with the depth of seasonal moisture fluctuation. However, additional water content increases occur in soils deeper than the depth of seasonal fluctuation. These additional water content increases are the result of infiltration from extreme climatic events and irrigation in the uncovered soils adjacent to the slab. (3) Climate conditions, geologic conditions, and soil properties control the rate of increase in the depth of the active zone. In general, the availability of moisture is controlled by climate conditions and the addition of water from irrigation exacerbates the problem. In addition, preferred flow paths due to geologic conditions such as bedding planes also provide conduits for flow. (4) Based on model results presented herein, the long-term active zone depth for a residential foundation at the CSU test site where irrigation is applied in accordance with Northern Colorado Water Conservancy District guidelines approaches the depth of potential heave based on stress conditions, which is equal to 35 feet. (5) The edge moisture variation distance measured in this investigation is equal to half the slab width, 15 feet. Therefore a larger slab is necessary to determine the maximum extent of edge moisture variation distance.