岩溶路基加筋防塌治理的简化设计方法

Simplified design method of reinforcement treatment for karst subgrade collapse

近年来岩溶地区呈高发态势的路面塌陷灾害不断威胁城市交通安全。用回填压实法治理塌陷时,埋入土工织物所形成的加筋垫层,能增强土体稳定性,防止二次塌陷。然而加筋治理方案中筋材的锚固长度分析及受力计算理论相对缺失,现存的加筋体设计方法也较少考虑塌陷发生后加筋治理铺设范围的选择,这些不足严重制约了土工合成材料在岩溶路基塌陷中的工程应用。针对这一现状,提出了一种岩溶路基塌陷后加筋治理的设计方法。该方法采用条分法对筋材上方土拱以下范围的填土进行受力分析,结合筋−土的作用特点,给出了塌陷区和稳定区加筋体拉力的计算公式,进而推导了加筋治理合理锚固长度的计算公式,并梳理了加筋体锚固长度设计流程;在此基础上,基于相关文献的模型试验结果及前人的计算方法对设计方法进行合理性验证,并进一步讨论塌陷宽度、加筋体最大挠度对加筋体拉力和合理锚固长度的影响。结果表明:(1)提出的加筋设计方法计算简洁,所得结果与试验结果更为吻合,由此建立的土工织物加筋垫层治理岩溶区城市路面塌陷的筋材拉力计算理论,对于工程实践具有一定的指导意义;(2)加筋体拉力在塌陷边缘处达到最大,塌陷宽度才是影响加筋体受力和合理锚固长度的主要因素,而加筋体最大挠度对合理锚固长度的影响较小。

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 thesettlement stability of the subsidence area, a calculation formula for the reasonable anchoring length of thereinforcement 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 calculationmethod of adding reinforcement tension and of the reasonable anchoring length was verified, on this basis of which theparameter design was conducted according to the relevant experimental results. It was assumed that the vertical stressin the stability area was roughly distributed in a decreasing exponential form. Finally, the effects of collapse width andmaximum 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 treatingurban road collapses in karst areas has been established in this study. In order to ensure the reliability of the proposeddesign method of reinforcement, the methods of determining values involving the vertical stress distribution functionP(x) of the tensile force reinforcement should be taken into account, one is the value determination by model tests ornumerical means;another is the value estimation according to the classical earth arch theory. Meanwhile, whendetermining the anchoring length, it is necessary for us to introduce the corresponding factor of safety in combinationwith the actual situation to ensure the project quality. Secondly, the calculation results by the design method in thisstudy are more consistent with the test results of other methods, which suggests that the proposed method not onlyincreases the safety of preventing karst subgrade collapse, but also effectively improves the utilization effect ofreinforcement. The result is of referential value for engineering practice. Moreover, the design method in this studyalso shows that both the anchoring length and the tensile force of reinforced body demanded by geotextile are big whenthe range of subgrade collapse is large. Thirdly, the collapse width is the main factor affecting the force of thereinforced body and the reasonable anchoring length. But the maximum deflection of the reinforced body has littleeffect on the reasonable anchoring length. Therefore, it is necessary to choose the anchoring length of the geosyntheticsfor the subgrade with a high standard required by the tendency to road deformation.