Model box tests on response of deformation of sand and clay layer under draining-recharging condition

The field monitoring data of borehole in land subsidence region indicate that the compressive strain is significant in clay layers above saturated sand layers. To study this phenomenon, a small sand-clay interbed model box is built, in which the distributed strain monitoring fiber, water table and settlement mark are embedded along the vertical direction. Three draining-recharging test cycles are carried out, and the variations of strain of soil layers, height of water table and settlement amount are monitored. The test results show that the overall permeability of soils decreases, and the soil layer is compressed during the three draining-recharging cycles. The delay phenomenon of water pressure variation exists in the clay layers. In the draining tests, the negative pressure of pore water is observed in the clay layers, and significant compressive strain is also observed in the same region. Considering the previous studies, the monitoring data of borehole and the results of model tests are compared and discussed. It is concluded that the release and supply of water in the clay layers is not equibrium when draining in the sand layers, which leads to the emergence of negative pressure, and the extra effective stress and compressive strain are generated in the clay layers. With the fluctuation of groundwater table, this effect may result in the continuous slow compression in the clay layers above the sand layers.