Analysis Of Cracks And Subsequent Preferential Flow Caused By Uneven Settlement In Clay Barrier

The waterproofing properties of compacted clay barrier at landfill site would be significantly weakened by crack which was induced by the waste’s differential settlement, and the leachate would increase subsequently, which would lead to serious engineering accident, such as instability of waste body, pollutant dispersion and so on.A three-point bending test was carried out to reveal the tensile cracking character of compacted clay barrier that caused by differential settlement, the load was controlled by mid-span displacement and the strain was measured by PIV technology during the test. This study was mainly focused on the cracking process of clay beam and the influence of water content on tensile cracking character. On the other hand, in order to reveal the effects of preferential flow on the performance of cracked barrier, the crack was treated as high-permeable porous medium, and a simplified saturated/unsaturated seepage model of the barrier with single crack was developed to study the process of preferential flow. In addition, the influence of crack position, depth, width and permeability coefficient on the performance of the barrier, include the water breakthrough time and steady flux was comparatively analyzed. The test and numerical simulation results show that:(1) The waterproofing performance is greatly influenced by crack position. The bottom crack has little influence on the barrier’s water breakthrough time and steady flux, but both of them are affected by fully penetrated crack seriously; besides, the water breakthrough time would significant shortened by upper crack. So, it is very important to control the upper crack, and strictly prohibited for fully penetrated crack during the barrier’s service.(2) For the upper crack, with the increase of crack depth, width and permeability coefficient, the water breakthrough time would be rapidly shortened and the steady leakage would increase respectively; moreover, the water breakthrough time would be controlled by the barrier without crack while the permeability of crack is high enough to the barrier, and the upper crack can be treated as infiltration boundary. That’s to say, for the upper crack, the crack width and depth, especially for the depth should be restricted effectively.(3) The anti-cracking performance of clay was significantly improved with the increase of water content, the strain at crack initiation increased from2.4%to4.1%along with the moisture content from32.6%to37.0%, but the tensile strength and elastic modulus were significantly reduced. Therefore, it is very necessary to keep the barrier’s water content in the process of construction and service.(4) The tensile strain at the bottom of compacted clay barrier is closely related to the differential settlement of cover system. In addition, the crack would extend fast as soon as it appear, and then followed by the slower pace of expansion; crack width is linear growth with the mid-span displacement, but crack depth is stepwise growth with that. Hence, the uneven settlement should be effectively controlled or reduced in the design of landfill.(5) The cracking character was greatly influenced by water content. With the increase of water content, the fracture characteristics of clay beam would transform from brittle into ductile gradually, and the plastic area in crack tip was enlarged.(6)The character of strain distribution during crack, as well as the strain development at crack location, can be well revealed by PIV (Particle Image Velocimetry) technology.