EXPERIMENTAL STUDY OF UNSATURATED PERMEABILITY OF CHENGDU CLAY BY INSTANTANEOUS PROFILE METHOD

Unsaturated hydraulic conductivity is the foundation of seepage analysis in soil. The theory system of unsaturated seepage is not mature enough, so the best method to obtain unsaturated hydraulic conductivity is measurement directly. There is not a few measurement technology of unsaturated hydraulic conductivity with long cycles and higher requirements on the environment. Chengdu clay is a kind of typical unsaturated expansive soil and has the characteristics of swelling and shrinkage. In the process of infiltration under confined condition, the expansion of the clay particles causes pore volume decrease, and lower permeability. Unsaturated infiltration experiments of Chengdu clay are quite difficult to be carried out. According to the instantaneous profile method, infiltration experiments were conducted to study the unsaturated permeability of Chengdu clay under confined condition. To do so, water contents and matric suctions were respectively monitored by EC-5 soil moisture sensors and MPS-2 dielectric water potential sensors. Synchronous measurements of moisture content and matrix suction, ensured the consistency of test conditions and avoided the influence of using other soil water characteristic curve. Considering the ceramic plate probe of MPS-2 is fragile and convenient components installed, a removable cuboid permeameter(50 cm long, 10 cm wide and 10 cm high) was custom-made. The dry density of the soil sample is 1.7 g·cm^-3 and initial water content is 27%.The result shows that, during water infiltration, the unsaturated hydraulic conductivity of the sample is between 1.33×10^-11 and 3.14×10^-9 m·s^-1, and it does not change linearly with matric suction. At the beginning of infiltration, i.e.when the matric suction is high, the hydraulic conductivity does not change significantly with the influence of expansion of clay particles. When the matric suction reduces to a certain value, the hydraulic conductivity increases rapidly. At the end of the experiment, the sample was near saturation, and presented an increasing trend of hydraulic conductivity because the discharge of gas in the sample is very slow and effective flow cross sectional area increases slowly. Derived from fitting the VG model to the experimental data, the α value was 0.048 kPa^-1, the n value was 1.79 and the m value was 0.48. The conclusion can be utilized for seepage analysis of Chengdu clay area.