Response Of Mudstone Around Hammer-Driven Pile And Evaluation Method Of Bearing Capacity Of Single Pile

The bearing capacity of hammer driven piles in mudstone is often anomalous,which damages the economic and social benefits.For the irreversible deterioration for the strength of mudstone around the pile by driving and the difficulty of preparing model foundations from mudstone.In this paper,uniaxial compression tests and triaxial compression tests were carried out on the mudstone by taking samples from the mudstone around the pile.Based on the drilled mudstone,an laboratory model test device for driven piles was developed,which was used to carry out driven pile model tests and needle penetration tests were carried out on the mudstone around the piles after driving.The strength deterioration law of the mudstone around the pile is analyzed and the mechanical response characteristics of the mudstone around the hammer-driven pile are clarified.Based on the test results,the modified factor of undisturbed strength and undisturbed elastic modulus of the mudstone around pile were proposed,and the evaluation method of vertical bearing capacity of single hammer-driven pile into mudstone is established.Numerical simulations of static load tests were carried out to compare with the field test results to verify the accuracy of the bearing capacity evaluation method.This paper focuses on the mechanical response of the mudstone around hammer driven piles and optimizes the calculation of the load carrying capacity of driven piles in mudstone.The results of the study show that:(1)The mechanical response characteristics of the mudstone at 0.2 m around the pile in the field were clarified by uniaxial and triaxial compression tests.The test results show that the compression strength(R_c)and elastic modulus(E)of mudstone at 0.2 m around pile are reduced by 28.2%and 41.4%on average.The shear strength of the mudstone around the pile deteriorated after driving,with cohesion(c)=217.2 k Pa and an internal friction angle(φ)=21.6°,φbeing only 49.1%of the undisturbed strongly weathered mudstone and 51.4%of the undisturbed fully weathered mudstone in the same area.(2)The laboratory hammer-driven pile model tests and needle penetration tests on the mudstone around the pile were carried out to verify the feasibility of the developed test device,analyze the penetration characteristics of piles and reveal the strength characteristics of the mudstone around the pile.The results show that the test device is stable and reliable,that the piles are driven with high variability in penetration resistance characterized by the number of hammer blows,and that the mechanical properties of the mudstone are not uniform.The R_c of the mudstone around the pile gradually decreased from far to near within approximately 2d(d is the pile diameter)after pile driving,with an average decrease of 63%at 2.5mm around the pile compared with 20 mm around the pile.In contrast,R_c is poorly regular within the pile end cross-section after driving,with R_c at the pile end 11.1%higher than at 2d of the pile circumference,and R_c increases by a factor of 1 on average in the pile end cross-section compared to the mudstone at the same location in the cross-section above the pile end.(3)In-depth analysis of the test results clarifies the response characteristics of the strength and elastic modulus of the mudstone around the hammer-driven pile,proposes the modified factor for the undisturbed R_c and undisturbed E of the mudstone around the pile,and establishes the evaluation method for the vertical bearing capacity of hammer-driven piles in mudstone.The study showed that the average deterioration gradient in R_c was 66%,40.5%,17%and 7%,and the average deterioration gradient in E was 80.5%,54.5%,26.5%and 11%for each 0.5d interval from far to near within 2d for mudstone around the pile after pile driving.The mudstone within 2d around the pile was divided into 4 modified zones at 0.5d intervals and the vertical bearing capacity of the hammer driven pile was evaluated by correcting the undisturbed strength and undisturbed elastic modulus of the mudstone around the pile.(4)Comparing the field results of static load tests with the numerical simulation results,the accuracy of the bearing capacity evaluation method was verified.The numerical simulation results compared with the 3 test piles in the field respectively,the bearing capacity of the unmodified results was 42.9%to 66.7%higher and the final settlement was 46.6%to 59.2%lower.The modified results,however,differ from the test piles by 0 to 7.1%in bearing capacity and-10.4%to 17.5%in final settlement.The numerical simulation results compared with the measured average values of the field test piles,the unmodified results showed a 50.0%higher bearing capacity and a 53.5%lower final settlement.The modified bearing capacity is consistent with the measured average of the test piles and the final settlement differs by only2.0%.This demonstrates that the method is scientifically accurate and provides an effective method for resolving hammer driven pile bearing capacity anomalies in mudstone.