Analysis And Evaluation Of Hole Wall Stability Of Bored Cast-in-place Pile For A Large Bridge In Guizhou Province

With the country’s emphasis on transportation construction,more and more super Bridges are built in the city,but the construction of these infrastructure needs a good foundation.With the rapid development of industrial technology,the design method,production process,bearing capacity and other aspects of pile have been well developed.Bored pile has the advantages of strong adaptability,simple construction,low noise,high bearing capacity of single pile,and has good social and economic benefits,so it has been widely used in China.In the process of construction,hole collapse and diameter shrinkage of bored pile are easy to happen,so the hole wall stability of bored pile becomes a problem that cannot be ignored.The instability of hole wall will not only affect the construction progress and increase the project cost,but also affect the quality and bearing capacity of cast-in-place pile.Therefore,the research on the stability of borehole wall of bored pile has social demand and important engineering significance.Based on an actual proj ect in Guizhou province,this paper studies the stability of borehole wall and the sensitivity of caving factors of borehole wall under different working conditions by means of experimental analysis,numerical simulation and system analysis,and proposes a comprehensive evaluation method of borehole wall collapse risk.The main research contents and achievements are as fo llows:(l)Based on the results of the indoor physical properties test of the filled soil at the construction site,the relevant parameters of the numerical simulation of hole wall stability,the sensitivity analysis of hole wall collapse factors and the comprehensive evaluation of pile hole wall collapse risk were determined.(2)Based on numerical simulation,a three-dimensional finite element drilling model was established to analyze the hole wall stability of bored pile under different conditions of mud wall protection,drilling diameter,drilling depth and filling thickness.It was found that when mud weight was 10k N/m~3,10.5k N/m~3,11k N/m~3,11.5k N/m~3and 12k N/m~3,compared with no mud wall protection,the maximum lateral displacement of hole wall was reduced by 94.30%,94.91%,95.48%,96.00%and 96.44%respectively.Under the same mud weight condition,the maximum lateral displacement of hole wall decreases by 74.7%,75.83%,76.99%,78.14%and79.12%,respectively,when the slurry level is lifted from the bottom of the casing to the surface.When the aperture is 1200mm,1400mm,1600mm,1800mm and 2000mm,the maximum transverse displacement of the hole wall increases by 34.32%,52.86%,68.11%and 84.75%,respectively.At the filling soil layer,the lateral displacement of the hole wall increases with the increase of the drilling depth,but the lateral displacement curves of different drilling depths almost coincide at the same stage of the hole depth.When the filling thickness is 5m,6m,7m,8m and 10m,the maximum lateral displacement of hole wall increases by 69.33%,141.90%,239.15%,337.91%and 575.81%respectively,and the maximum lateral displacement of hole wall occurs in the lower part of the filling soil layer.(3)Based on grey correlation theory,the sensitivity analysis model of hole wall collapse factors is established.The correlation degree of each cave-causing factor is calculated by the model,and the correlation degree between each factor and hole wall collapse is analyzed.Finally,the main and secondary factors leading to hole wall collapse in this site are determined.The results show that the ratio of the thickness of the filling soil to the depth of the design hole,the thickness of the filling soil and the average particle size of the filling soil are closely related to the hole wall collapse.Multiple linear regression is carried out with the ratio of the thickness of the filling soil to the depth of the design hole,the thickness of the filling soil and the average particle diameter of the filling soil as independent variables,and the rate of hole collapse as dependent variables.The significance test of the independent variable coefficient shows that it is consistent with the conclusion of grey correlation analysis.It is necessary to pay close attention to these indicators in the subsequent drilling construction,and this method provides a reference for the prevention and control of hole wall collapse of bored pile in this area.(4)Based on the theory of hierarchical analysis and grey correlation,a comprehensive evaluation method of pile hole wall collapse risk was established,and a large bridge project in Guizhou was evaluated.The weight of each evaluation index is calculated by analytic hierarchy process,and the ideal index sequence is formed by analyzing each index.The correlation degree between each pile position and the ideal index sequence is calculated by grey correlation degree.The hole wall collapse risk assessment was carried out on 26 pile positions of the project,and it was found that the hole collapse risk level of pile positions 1#,2#,4#,6#,9#,10#,15#and 16#was high,and special attention should be paid to the drilling construction at these positions.This method predicted the hole wall collapse risk of bored pile in this site.At the same time,it also provides reference for the prevention and control of hole collapse of bored pile.