Study On Bearing Characteristics Of Uplift Short Piles In Weathered Sandstone

With the large-scale development of infrastructure in the western region of China and the rapid development of power industry,more and more transmission lines are traveling in mountainous areas.In engineering practice,the code method can't satisfy engineering requirements.For the calculation of the uplift bearing capacity of the pile foundation embedded in the bedrock of the “upper soil and lower rock” pile end of the transmission line,the calculation results are conservative,resulting in larger quantities of foundation engineering;or considering the unreasonable rock-socketing effect,there are potential safety hazards.At present,the research on uplift piles is mainly aimed at soil conditions,and a few rock-socketed uplift piles are also based on field tests under specific conditions.The design and calculation of uplift piles are mainly based on the design method of compressive piles,the uplift coefficient is introduced.There is still no uplift bearing capacity design method of pile foundations with "upper soil and lower rock" under rock-socketed conditions in domestic and foreign pile foundation standards.Based on the 220 k V transmission line project of Zhaohua-Guangyuan traction station of the State Grid Project,this paper conducts field destructive test of 19 uplift piles and centrifugal model test of 20 uplift piles.Based on the analysis of test data,the bearing characteristics of rock-socketed uplift piles are analyzed,and the calculation method of ultimate uplift bearing capacity is put forward.The load-displacement curve characteristics of uplift piles in field test are studied.The applicability of mathematical model method,graphic method and displacement control method to determine the ultimate bearing capacity of uplift piles in weathered sandstone is analyzed.The main contents and conclusions are as follows:1.Through field test and centrifugal model tests,the failure modes,axial force distribution,lateral resistance distribution and the influence factors of ultimate uplift bearing capacity of rock-socketed short piles are clearly understood.(1)Centrifugal model tests show that the failure modes of even section piles are cylindrical(static pressure installation method)and composite(non-interference installation method).The failure modes of belled piles are all trumpet-shaped.Through field test,it is considered that the test pile and the surrounding rock mass are relatively slippery,and the surrounding rock mass is sheared or crushed.(2)The distribution of axial force of pile body is mainly affected by the nature of rock and soil layer and pile type.The attenuation speed of axial force of pile body in rock layer is much higher than that of overlying soil layer,and the attenuation speed of axial force of pile body in the bell is higher than that of pile body with even section.Under ultimate load,the maximum lateral resistance of even section piles are generally within 1.0 m above the pile bottom,while the peak value of lateral resistance of belled section piles are located at the bell.In centrifugal model test,the maximum lateral resistance of pile body with even section is within 1.0m of embedded rock,and the maximum lateral resistance of the pile is continuously moved down from the surface of the rock until the enlarged head as the uplift load increases.Under the ultimate load,the lateral resistance of even section piles in rock strata is inverted rectangular as a whole,and the belled section piles are rectangular as a whole.(3)The ultimate top displacement of the pile is 1.3%?5.2% of the even section piles,the average is 3.0%(18.0?30.0 mm),and the ultimate displacement of the belled section pile is 0.6%?2.8% of the diameter of the pile,the average is 1.9%(15.2 mm).The ultimate relative displacement of the silty clay,the strongly weathered sandstone and the moderately weathered sandstone are 2.5?4.0 mm,8?18 mm and 20?25 mm,respectively.(4)Field test and centrifugal model tests results show that the ultimate uplift bearing capacity of even section piles and belled section piles increases linearly with the increase of rock-socketed depth.The ultimate uplift bearing capacity of even section piles increases linearly with the increase of pile diameter.Compared with even section piles,belled section piles not only significantly improve the ultimate uplift bearing capacity,but also significantly reduce the ultimate pile top displacement.The ultimate uplift capacity of the test pile installed without interference is higher than that of static pressure installation or excavation backfilling installation.2.The differences between field test and centrifugal model tests make the failure mechanism of test piles different,and different failure modes of rock and soil are obtained.Based on the results of the two test kinds,the calculation methods of ultimate uplift bearing capacity are proposed in this paper.(1)Based on centrifugal model tests,a hypothetical model is proposed to calculate the ultimate uplift bearing capacity of even section piles and belled piles under the condition of complete rock stratum.The relative error between the calculation results of even section piles and centrifugal model test results is 3.3%?8.4%,and the relative error between the calculation results of belled section piles and centrifugal model test results is 0.5%?6.3%.(2)Based on field tests,a cylindrical calculation method for calculating medium cross section piles in weathered sandstone stratum is proposed.The method includes two parts:lateral resistance and self-weight of pile body.The average errors between the two calculation methods results and the experimental values for determining the lateral resistance of even section piles are 10.7% and 20.3%,respectively.The method includes three parts:lateral resistance of even section pile body,resistance provided by side of bell conical platform and self-weight of pile body.The average errors between the two calculation methods results and the test results for determining the lateral resistance of the even section of belled section piles are-0.6% and 11.6%,respectively.It shows that the calculation method of ultimate bearing capacity of uplift piles in weathered sandstone layers proposed in this paper is reasonable.(3)Based on the load-bearing and failure mechanism of even section piles and belled piles in field tests,it is proposed that the ultimate lateral resistance of even section piles in rock is equivalent to the shear strength of rock,or it can be calculated by the formula of uniaxial compressive strength of rock.The average ultimate pile side resistance of weathered sandstone layer is power function relationship with uniaxial compressive strength of rock.3.Based on the load-displacement curve of field test,the ultimate bearing capacity of uplift piles is determined by different failure criteria.This paper studies the mathematical model method,graphic method and displacement control method respectively,and analyses the applicability of each method to determine the ultimate bearing capacity of uplift piles in weathered sandstone.(1)In the mathematical model method,the hyperbolic model has the highest fitting accuracy for the uplift load-top displacement curve of medium even section piles and belled piles in weathered sandstone.The ratio of predicted and measured ultimate bearing capacity of even section piles is 1.11?1.58,with an average value of 1.25 and a standard value of 1.32.The ratio of predicted and measured ultimate bearing capacity of belled section piles is1.16?1.45,with an average value of 1.27.Or the lower limit curve function of the normalized load-displacement curve hyperbolic model can be used to calculate the bearing capacity of uplift piles in weathered sandstone layer.(2)The results obtained by the double line intersection method are 85.2%?98.5% of the measured values,with an average of 94.8%.(3)The ultimate displacement of short uplift piles with medium cross-section in weathered sandstone layer is 25 mm,and the ultimate displacement of short uplift piles with bell is 15?20 mm.