Correlational Study On The Compressive Capacity Between The Designed Test Pile On The Ground And The Foundation Pile At The Foundation Pit Bottom

Pile foundation is widely used in various fields such as highways,railways,port wharves and high-rise buildings.But due to the limit of various influence factors,plenty of problems still exist in the actual process.How to evaluate the characteristic value of bearing capacity of the foundation pile at the foundation pit bottom according to the pile test data on the ground,and fully utilize the bearing capacity under the guarantee of the structure safety,are the main issue of this article.According to the static load test data,the influence of excavation section on the vertical compressive bearing capacity of single pile is analyzed by theoretical analysis and numerical simulation methods.The compressive capacity relationship between the test pile on the ground and the foundation pile at the pit bottom in three different strata are compared,and the main conclusions are as follows:(1)When the gravity is considered,the variation trend of vertical compressive stiffness of single pile with pile length,pile diameter,material modulus,soil modulus,Poisson’s ratio and subgrade coefficient is consistent with that ignoring weight,but its value is lower.It is worth noting that the stiffness of single pile under gravity is not fixed,but first raises with the increase of the pile top load and then tends to a certain value.(2)The stiffness method of single pile and finite element method are used to compare the bearing capacity of test piles on the ground and foundation piles at the pit bottom under three geological conditions.The results shows that the characteristic value of bearing capacity of foundation piles at the pit bottom after excavation is related to the soil layer state before excavation.When the soil conditions before excavation are that the upper soil is hard and lower soil is soft,or the soil near the pile is uniform distributed,the characteristic value of bearing capacity of the foundation pile at the pit bottom is less than that of the ground test pile.(3)The single pile stiffness method and finite element method are used to analyze the influence of excavation depth,effective pile length and subgrade coefficient at the pile end on the characteristic value of bearing capacity of foundation piles at the pit bottom.The calculation results show when other conditions are the same,the smaller the excavation depth,the higher the characteristic value of bearing capacity of foundation pile at the pit bottom after excavation;The larger the effective pile length,the higher the characteristic value of the bearing capacity of the foundation pile;The larger the subgrade coefficient of the pile end soil,the higher the characteristic value of bearing capacity of the foundation pile.(4)The key to the empirical parameter deduction method is whether the physical and mechanical parameters of soil are reasonable.The general empirical parameter deduction method ignores the changes in pile side resistance and pile end resistance caused by the shortening of pile length,therefore a correction factor ω is introduced in order to appropriately adjust the formula.Eventually,to prove the applicability of each method,a pile test on the ground and another pile test at the pit bottom are selected.Both cases show that the results of the stiffness method of single pile,the finite element method and the deduction method are consistent,which coincide with the actual test data.