Experimental and analytical assessment of soil settlement due to groundwater rising in the framework of unsaturated soil mechanics

In the present study, with the help of experimental studies and based on the basic relationships of unsaturated soil mechanics theory, an analytical model is proposed to estimate soil settlements due to groundwater level rise after construction and its importance in accurately estimating the final soil subsidence. To determine the geotechnical parameters used in the design, experiments are performed either under saturated conditions or at natural moisture, and the effects of changes in moisture; in other words, drying and wetting paths are not considered. However, in the event of rainfall, the burst of water and sewage pipes, leakage from absorption wells, rising groundwater levels, and similar phenomena, the soil becomes wet and its behavior changes, eventually leading to softening and imposing additional settlement which can cause damage to the surface structures. In traditional laboratory studies, this wetting is partially evaluated using the double consolidation test with the calculation of the collapsibility index. This assessment method is limited to soil with fully saturation in a one-dimensional and uncontrolled matric suction device. The results are qualitative and without enough accurate for use in settlement calculations in wetting paths for unsaturated soils. In this study, the behavior of silty soil under consolidated isotropic tests under saturated and unsaturated conditions as well as controlled wetting of consolidated specimens at constant loading with the help of an unsaturated triaxial device was investigated; Afterward; using the results of laboratory studies with presenting a analytical model based on the basic relationships of unsaturated soil mechanics and the theory of calculation of terzagi consolidation, the effect of water level rising on the shallow foundation settlements under constant loading was investigated. The results showed that the amount of soil settlements due to this phenomenon is related to the initial density and soil consolidation properties in unsaturated conditions, volumetric change characteristics in the wetting paths, dimensions and intensity of foundation loading, initial groundwater level depth and ascent rising rate. In the present study, for a square foundation with dimensions of 10 to 20 meters with constant loading located on the silty soil mass, the depth of the initial groundwater level of 25 meters and its rise from 1 to 10 meters, the settlement amount of about 5.5 mm ( For minimum foundation width and minimum water level rise) up to 49.5 mm (for maximum foundation width and maximum water level rise) was calculated. The results also show that the amount of settlement due to soil wetting is more dependent on the depth of the initial groundwater level and the rate of rising comparing with the dimensions and intensity of loading of the foundation.