Simplified design method of reinforcement treatment for karst subgrade collapse

Karst areas are widely distributed in China, especially in South China. With the proposal of the Belt and Road Initiative, a large amount of infrastructure has to be built in the karst area. However, the diversity of structure and mechanical properties of karst make it difficult for us to construct roads on karst foundations. In recent years, the frequent occurrence of settlement and even collapse of the road surface caused by karst foundations under soil layers of roads has threatened the safety of urban transportation and the development of economy. Therefore, the treatment of karst collapse has become an urgent engineering problem. For the controlling of karst collapse under a complex urban road environment, the treatment of karst collapse by the method of backfilling and compacting is economical in construction and can be finished in short time, but collapses are prone to happen again after treatment. In this case, the treatment can be carried out with geosynthetics, namely filling the geotextile in the subsidence area during the backfilling and compacting process. This kind of treatment utilizes not only the reinforcement effect of geotextiles to achieve the self-stability of the entire reinforced soil system, but also utilizes the filtration performance of geotextiles to prevent the loss of soil particles backfilled, thus addressing both the symptoms and root causes of the collapse. However, the interface interaction between reinforcement and soil is so complex in this method that the corresponding design method of reinforcement is still not clear. In previous studies, the design method of reinforcement focuses on the prevention of collapse before its occurrence. In this kind of method, reinforcement materials are generally used with sufficient anchoring length for full paving. As for the design method of treatment after collapse, there are few studies on the range selection of paving reinforcement. In addition, the maximum tensile force of geotextile will affect its strength design and then impact the overall strength of the reinforced body. Therefore, the previous studies on anchoring length and the lack of theories on calculating reinforcement force have restricted the application of reinforcement in engineering practice.

In order to accurately calculate the maximum tensile force of the reinforcement and to study the anchoring length of the geotextile, a design method for reinforced treatment of karst roadbed collapse was proposed. Firstly, the simplified Bishop's strip method was used to analyze the soil force. According to the characteristics of tension action between reinforcement and soil, the calculation formula of the reinforcement force in subsidence areas and stability areas was deduced by assuming that the deformation of the reinforcement was catenary. Meanwhile, with the assumption that the anchoring force of the reinforced body was greater than the maximum tensile force to ensure the settlement stability of the subsidence area, a calculation formula for the reasonable anchoring length of the reinforcement was worked out and the relevant design procedure of the anchor length was sorted out. Additionally, based on the test results of existing models and the comparison with previous calculation methods, the calculation method of adding reinforcement tension and of the reasonable anchoring length was verified, on this basis of which the parameter design was conducted according to the relevant experimental results. It was assumed that the vertical stress in the stability area was roughly distributed in a decreasing exponential form. Finally, the effects of collapse width and maximum deflection of the reinforcement on the tensile force and on reasonable anchoring length were analyzed.

The results show that firstly, a design method of anchoring length of geotextile reinforced cushion for treating urban road collapses in karst areas has been established in this study. In order to ensure the reliability of the proposed design method of reinforcement, the methods of determining values involving the vertical stress distribution function P(x) of the tensile force reinforcement should be taken into account, one is the value determination by model tests or numerical means; another is the value estimation according to the classical earth arch theory. Meanwhile, when determining the anchoring length, it is necessary for us to introduce the corresponding factor of safety in combination with the actual situation to ensure the project quality. Secondly, the calculation results by the design method in this study are more consistent with the test results of other methods, which suggests that the proposed method not only increases the safety of preventing karst subgrade collapse, but also effectively improves the utilization effect of reinforcement. The result is of referential value for engineering practice. Moreover, the design method in this study also shows that both the anchoring length and the tensile force of reinforced body demanded by geotextile are big when the range of subgrade collapse is large. Thirdly, the collapse width is the main factor affecting the force of the reinforced body and the reasonable anchoring length. But the maximum deflection of the reinforced body has little effect on the reasonable anchoring length. Therefore, it is necessary to choose the anchoring length of the geosynthetics for the subgrade with a high standard required by the tendency to road deformation.