Settlement and moisture movement in collapsible soils

The infiltration of ponded water beneath a footing on collapsible soil is monitored. Soil suction, water content, and footing settlement measurements are made. Laboratory infiltration tests are used to predict the extent of wetting in the field. Close agreement between observed and predicted extent of wetting is obtained. The in situ values of soil suction are quite high, corresponding to apF value of about 5.26 (1,820 m of water) and degree of saturation of about 15%. In response to ponding, the soil suction drops by a factor of about 10 behind the wetted front, but remains high, withpF 4-4.5 (100-316 m of water) and degree of saturation averaging about 50%. Laboratory collapse tests show that partial wetting produces only partial collapse, with 50% saturation producing about 85% of full collapse strain for the soil tested. Partial wetting is considered in making footing settlement predictions, and the predicted settlement exceeds the observed settlement by only about 12%. It appears reasonable to expect close agreement, as in this case, when the extent of wetting is known in advance or well-predicted.