Study On The Application Of Prestressed Anchorage Cable To The Reinforcement Of The Bedding Slope At The Tunnel Portal

China is a mountainous country and the bedding rock mass slope is widely distributed in our country.In recent years,infrastructures in our country are continuously developed.Therefore,more and more expressways are being built.When the tunnel is excavated in the strata of bedding rock mass,the terrain and geological conditions are complicated.So some difficulties will bring about in tunnel excavation.Generally,the buried depth of tunnel portal is shallow and the surrounding rock weathers and breaks.Coupled with the special geological structure of bedding rock mass,the landslide incident is easily caused.Therefore,the slope stability method,of tunnel portal must be deep studied and efective reinforcement measures must be taken.Only under this circumstance,the construction safety can be ensured.In this paper,a new tunnel of Baohan highway is taken as the example.The stability analysis method and reinforcing measures of the bedding rock mass slope at the tunnel portal are studied by means of theoretical analysis,numerical simulation and field test.The main conclusions are as follows:(1)According to diff-erent slope failure modes,differe.nt bedding slope stability analysis methods are analyzed and summarized.By comparing and analyzing the application conditions,advantages and disadvantages of different stability analysis methods,the transfer coefficient method and the Sarma method are more appropriate for the stability analysis of bedding slope in natural state.(2)When the traditional transfer coefficient method is used to analyze the stability of the anchorage bedding slope,the anchorage force is regarded as a concentrated force performing at the bottom of the soil bar where the sliding surface is located through the anchor cable,an.d the anchoring force of the remaining soil bar is zero.This calculation method can easily lead to a sharp increase of stress at the slip surface,which does not conform to the actual stress state of the soil bar.So the calculation precision of the stability safety factor at the side bedding slope will be influenced.In view of the disadvantages of the traditional transfer coefficient method in the calculation of the anchoring force after the slope plus anchorage,an improved transfer coefficient method is proposed in this paper.The anchoring force of the anchor cable is regarded as a distribution force distributed uniformly in the slippery zone on the bottom of the bar,rather than seeing as a concentration force,which only acts on the bottom of the bar where the anchor cable and the sliding surface intersect.The improved algorithm improves the stress concentration of the slip surface,and can better reflect the actual stress of the bar in the rock mass.Also the calculation accuracy of the bedding slope stability with anchorage is improved.(3)The traditional transfer coefficient method,the improved transfer coefficient method and the Sarma method are applied to a bedding slope example with anchorage.The example is at a specific tunnel of the Baohan highway.The calculated safety factors of slope after the anchorage are 1.26,1.38 and 1.39 respectively.The results show that the safety coefficient calculated by the traditional transfer coefficient method is smaller and this method is relatively conservative.So the method is not beneficial to saving the engineering cost.The improved transfer coefficient method not only has a simple and quick calculation process,but also the calculation results of it is an important basis for determining the anchorage parameters of prestressed anchor cable.The calculation results by improved transfer coefficient method is close to the results by Sarma method,which verifies the correctness of the improved transfer coefficient method.So the improved transfer coefficient method can optimize the anchoring parameters and guide the engineering practice.(4)Based on a road tunnel of Baohan highway,the tunnel excavation stage is simulated by the finite element analysis software(MIDAS/GTS).According to the special engineering survey in the anchorage bedding slope,the contact unit method and different structure method are used to simulate the contact surface among different lithologies during the establishment of the bedding slope model.The tunnel excavation process is simulated making the finite element model closer to the actual project.Comparing the calculated results of finite element analysis models with on-site monitoring data after anchoring,it is concluded that the total displacement of the bedding slope after anchoring is close to on-site monitoring results.Also the results do not exceed the allowable values of the specification and meet the requirements of safety construction.It also verifies the accuracy of the improved transfer coefficient method in the stability analysis of tunnel portal with anchorage and the feasibility of reinforcement measures using the prestressed anchor cable.