Cause analysis of exfoliation of side walls for Karez tunnel and seepage control reinforcement

Abstract: The Karez is one of the ancient water conservancy projects with advanced technology, which well adapts to the unique climatic conditions, geomorphology and engineering geological conditions in piedmont basin of arid areas. The Karez is mainly distributed in the Turpan Basin in China, which has been decaying at an alarming rate of about 20 each year in the last few decades. Exfoliation collapse of kanat and tunnel is one of main reasons to the declining of the number of Karez. The water distribution characteristics, moisture sources and soil parameters of Karez's wall were obtained by field measurement. The main causes for exfoliation of the Karez wall were discussed through the failure characteristics and soil moisture distribution of wall in Karez tunnel. The finite element analysis of unsaturated steady-state seepage was carried out based on GEO-SEEP/W calculation module, and the wetting soil water characteristic curve (SWCC) obtained by the filter paper method was utilized. In the meanwhile, the effect of groundwater level on the unsaturated seepage characteristics was analyzed by using the identical method. The results show that the main source of soil moisture in the Karez's wall is capillary water that was migrated from the culverts canal and also was regarded as the primary cause for exfoliation damage. A zone significantly affected by capillary water was observed at a depth of 0-0.6 m at the side walls of the Karez. The sum of silt and clay content is 68.3% of the total mass for the Karez soil, which is far greater than the critical proportion of 10% that is generally taken to classify whether or not soil is the frost heaving soil. When the underground water level is shallow, the water is fully recharged by the Karez, but there is no significant direct recharging relationship between the Karez and the groundwater when the groundwater level is below 10.0 m. Groundwater level has larger influence on soil volumetric water content below the free water surface of the Karez, but the effect on that above the free water surface of the Karez was minor or even negligible. The wetting SWCC is more suitable for unsaturated flow analysis of the Karez tunnel, which is in line with the moistening process of the Karez wall. The extent of the exfoliation in wall is much greater than that of the vault of the Karez, and even a lot of Karez vaults can still maintain the initial outline. The characteristic height of 1.0-1.5 m corresponding to the serious exfoliation section is basically the same as the height of 1.1 m for the initial water content of frost-heaving. However, the above features do not exist in the deeper kanat and the effective cohesion of low confining pressure decreases significantly after freeze-thaw cycling. So it is clear that freeze-thaw is the main cause for the exfoliation of the Karez wall. Because the exfoliation damage of the side walls occurs at the initial stage of the tunnel collapse, preventing the exfoliation of the Karez wall is the most practical method for protecting the Karez tunnel. The surveying results indicate that the depth of the negative temperature section is about 8.0-10.0 m for the typical Karez section. Both the impervious sections of 11.16 and 1.04 m are laid out both inside and outside the kanat outlet, which can play an effective anti-frost heaving function for a typical Karez section. Finally, the engineering reinforcement measures of local impervious lining were put forward for the negative temperature section of the tunnel. This method has the advantages of anti-frost heave of wall, convenient construction and maintenance, and effective protection of the historical original appearance of the Karez. Meanwhile, the work carried out here has certain theoretical and technical value in preserving similar historical heritages.