Study On Uplift Bearing Characteristics And Theoretical Calculation Of Rock-socketed Pile With Shallow Overburden

With the rapid development of power industry in mountainous areas of western China,a large number of transmission lines need to be built on steep hillsides and ridges.Due to the influence of extreme weather（such as strong wind,snow,etc.）and wire tension,high requirements are imposed on the uplift resistance of the transmission tower foundation.Moreover,restricted by topography and geomorphology,the excavation depth of the foundation is limited.Secondly,the most common type of foundation for transmission line corridor is a composite stratum with upper soil and lower rock.At present,there is little research on uplift bearing characteristics of transmission tower foundation under the condition of shallow overburden rock-socketed.In addition,due to differences in geological conditions and application industries,the existing theoretical results and test data can not fully meet the urgent needs of uplift design and construction of tower foundation in this area.This paper relies on the National Grid Project"Zhaohua-Guangyuan Traction Station220 k V Line Project"and"Luping-Fule 500 k V Double-circuit Line Newly-built Engineering".Through field test,indoor model test（1g）,centrifugal model test,numerical simulation and theoretical research,etc.,the uplift bearing characteristics of rock-socketed piles are systematically studied,revealing the uplift failure mode of constant-section piles and belled piles.Furthermore,the theoretical calculation method of ultimate uplift capacity is deeply studied,and a new prediction method based on intact and non-intact load-displacement curves to determine the ultimate capacity is proposed.The main conclusions and suggestions are as follows:1.Through the field test,17 test piles（equal-section piles and pedestal piles）under different factors（such as pile diameter,rock-socketed depth）have been completed.Then,a comparative analysis of the load-displacement curve,the axial force distribution,the side friction of the pile,and the relative displacement of the pile-soil/rock were carried out.The results show that the load-displacement curve has obvious inflection points,and they all show a steep deformation failure,and the uplift failure of the constant-section pile is more rapid than that of the belled pile.The displacement of the pile top when the equal-section pile and the belled pile reach the limit state is about 2.75%d and 1.94%d（d is the pile diameter）,respectively.Compared with the constant-section pile,the ultimate uplift bearing capacity of the belled pile can be increased by about 1.2 times,and the corresponding ultimate displacement can be reduced by about 66.9%.Compared with the equal-section pile,the uplift coefficient and the utilization rate of the pile material of the belled pile increased by 112.0%and 90.6%,respectively.When the test pile reaches the limit state,the average values of the side friction resistance of silty clay,strongly weathered sandstone,and medium weathered sandstone are 77.2 k Pa,380.0 k Pa,and 1001.6 k Pa,respectively.The relationship between the side friction resistance of the sandstone layer and the corresponding rock uniaxial compressive strength（UCS）can be better expressed in the form of exponential function,namely q_r=0.213σ_c^0.5（equal-section pile）and q_r=0.205σ_c^0.5（belled pile）,which is suitable for the UCS of rock（sandstone）is about 5-20 MPa.2.Based on the similarity theory,the uplift static test of 6 model piles was completed through the indoor model test（1 g）.The failure mode,load-displacement curve,axial force distribution,and the side friction of equal-section pile and belled pile were compared when the slope effect and horizontal load component effect were considered in turn,and compared with the general conditions（horizontal ground and vertical uplift load）.Results demonstrate when the horizontal load component is 25%and the slope is 30°,the ultimate uplift capacity of equal-section pile and belled pile is reduced by 27.3%and 22.2%,11.5%and 9.2%,respectively（compared with the condition of horizontal ground and vertical uplift load）.Compared with equal-section pile under the same condition,the ultimate uplift capacity of belled pile can be increased by 61.3%（straight uplift on flat ground）,72.5%（horizontal load component is 25%）and 64.7%（slope is 30°）.When the equal-section pile reaches failure,all the sand area and part of the bedrock area on the tension side appear void,and part of the medium weathered bedrock on the compression side belongs to extrusion failure（horizontal load component is 25%）.When the belled pile reaches failure,all sand areas and part of the bedrock areas on the tension side appear to be voided,and the failure mode of medium weathered bedrock is in the inverted cone shape（horizontal load component is 25%）.The bedrock failure of the equal-section pile and the belled pile is that the large-scale rock in front of the slope in a fan-shaped overall failure,and the small-scale rock in the back of the slope in small broken（slope is 30°）.3.Based on centrifugal model test,the uplift failure test of 20 model piles（11equal-section piles and 9 belled piles）embedded in the complete bedrock was carried out,and the failure modes of equal-section piles and belled piles in complete bedrock,load-displacement curve,axial force distribution and pile side friction are obtained.Observation results show that the failure mode of bedrock of the constant-section pile is related to the characteristics of the pile-rock interface.On the one hand,when the pile-rock interface factor≥0.8,the failure mode of bedrock is complex.Moreover,the average value of the ratio of the inverted cone height of composite failure(L_cr)to the length of the rock-socketed（L_r）is 0.47,and the relationship between L_cr/L_r and L_r/d can be well expressed by exponential function.The average angle between the inverted cone fracture surface and the vertical direction is 53.5°.On the other hand,when the pile-rock interface factor≤0.4,the failure mode of bedrock is cylindrical.The bedrock failure mode of belled pile is trumpet shaped,and the projection area of the fracture surface on the bedrock surface is approximately distributed in a circle or ellipse centered on the pile axis,which is about 3 to 7times of the pile diameter.In addition,the ultimate uplift capacity of belled pile can reach1.95～2.30 times of that of equal-section pile.What’s more,with the increase of pile top load,the decreasing trend of axial force changes from“inner concave-like shape”to“outer convex-like shape”.4.On the basis of verifying the correctness and reliability of the three-dimensional numerical model,the influence of pile diameter,rock-socketed length and overburden thickness on the ultimate uplift capacity of equal-section pile and belled pile was studied by using the geotechnical finite element software PLAXIS 3D.Based on the premise that the amount of concrete is equal,the relationship between the strength and weakness of the pile geometry parameters（pile diameter,rock-socketed length,expansion ratio,etc.）on the ultimate uplift capacity was compared.Finally,the influence of horizontal load component and slope on the uplift bearing characteristics of equal-section piles and belled piles was analyzed.The results show that the ultimate uplift capacity increases linearly with the increase of pile diameter,rock-socketed length,expanding ratio,interface strength reduction factor,internal friction angle and cohesion of bedrock.Under the premise of changing the equal amount of concrete,the expanding ratio has the most obvious influence on uplift bearing capacity,followed by the rock-socketed length and pile diameter.Therefore,it is suggested that the expansion ratio,rock-socketed length,and pile diameter should be given priority in uplift design.On the one hand,when the slope increases in the range from 0 to30°,the influence of the uplift capacity of equal-section piles and belled piles is smaller or linear decreases;on the other hand,when the slope increases between 30°～60°,the weakening effect of the uplift capacity of equal-section piles and belled piles is sharply highlighted.5.According to the empirical value method of pile side friction,the prediction formula of ultimate capacity of uplift pile under the condition of rock-socketed in shallow overburden is summarized,and compared with the calculation result of existing codes.Based on the limit equilibrium method,the calculation method and simplified calculation method of ultimate uplift capacity of equal-section pile and belled pile are proposed respectively.The calculation results between different strength criteria（Mohr-Coulomb,nonlinear Mohr-Coulomb,Hoek-Brown）are compared and analyzed,and the theoretical calculation results and test results are verified and analyzed.The results show that the calculation results using domestic and foreign codes（e.g.,JGJ 94-2008,GB 50007-2011,DIN 4014）are conservative,and the relative error is about 80%.The ultimate uplift capacity obtained by the empirical method in this paper is close to the field test results,and the relative error is generally less than 40%.The failure mode of the bedrock of the belled pile can be expressed by the fracture surface equation represented by the tangent function,and the undetermined parameter of fracture surface shape in intact soft rock is N=0.421～0.986（average value is0.685）;the undetermined parameter of fracture surface shape in intact soft rock is N=0.803,the undetermined parameter of fracture surface shape in strongly～medium weathered soft rock is N=1.2～1.5,and the undetermined parameter of fracture surface shape in strongly weathered extremely soft rock is N=3.0.Moreover,the integral median point is generally located in the integral interval of 0.50～0.62.The calculated value of ultimate uplift capacity based on MC or HB strength criterion is similar,and it is in good agreement with several test results.The calculation result based on the nonlinear MC strength criterion decreases with the increase of the nonlinear coefficient m,and tend to be stable.Hence,it is recommended to consider at least the nonlinear coefficient m=1.4～1.8.6.Based on the field destructive uplift static load test data,seven models（hyperbolic model,exponential model,etc.）are used to fit the intact load-displacement curve and error analysis.Further,six methods,such as displacement control method and reduction coefficient method,are used to determine the predicted value of ultimate capacity,and compared with the measured value.Based on the law of the change rate of uplift stiffness with displacement,a method for determining the ultimate capacity of non-intact load-displacement curve is proposed,and compared with the prediction results of commonly used mathematical model and test results,and has been verified by multiple engineering cases.Observation results show that it is suggested that the exponential model should be given priority in the prediction analysis of ultimate capacity,and the reduction factor method and critical stiffness method should be used to further determine the ultimate capacity.The reduction coefficient should be selected from 0.78 to 1.00（exponential model）;the standard values of the change rate of critical stiffness are 154 k N/mm（constant-section pile）and 232k N/mm（belled pile）,respectively.The relationship between the change rate of uplift stiffness and displacement can be well fitted with a power function.In addition,the proposed theoretical method greatly improves the prediction accuracy of the bearing capacity of the non-intact load-displacement curve,and the reliability is not affected by the integrity of the curve.The theoretical method is simple and applicable for the piles that are not loaded to failure state,and thus provides new insights into ultimate capacity determination of test piles.