Test And Theoretical Study On Bearing Capacity Behavior And Settlement Of Pile In Soft Soils

Piles have been worldwidely used as the foundations of high-rise building, high-speed railway, highway, bridge, dock, harbor and other heavy-load structures. However, the construction effects especially thickness of residues at the pile base, disturbance of pile tip soil, mudcake around pile shaft, stress relaxation of borehole, and integrity of pile were primarily responsible for the discrepancy in pile capacity for a given soil profile. The responses of a single pile and pile groups need further study using field test and numerical analysis. The observations have provided some key findings summarized as below.1. The construction technology of the over100-meter-long rock-socketed bored piles crossing about40-meter-deep huge thick gravel layer and the test scheme for large tonnage static load test in Wenzhou Lucheng Square were presented. Static load tests were conducted on the piles instrumented with strain gauges to capture the pile load-settlement response, the behavior of pile shaft compression, the mobilization of skin friction and end resistance, and the influence of the residues at the pile base on the end resistance. The measured results show that the super-long pile works as end-bearing friction pile at the maximum loading level. Under the working load, the percentage of the settlement at the super-long pile head due to the compression of pile shaft is over90%. Therefore, for practical purposes, the influence of pile quality on the settlement of pile top must be taken into account. Moreover, the settlement of super-long piles is observed to be closely associated with the compressibility of the sediment at pile end. It also can be concluded that the shaft resistance of piles has slight weakened and enhanced effect in the upper and lower soils, respectively.2. The differences in the destructive and non-destructive pile responses were analyzed using a series of field tests. It is observed that at shallow depth the skin friction of the non-destructive pile decreases from the ultimate skin resistance with increasing load as the test pile is loaded to the maximum value, whereas for the destructive pile the shaft resistance along the pile depth approaches to the limiting state and decreases from a peak value with further increase of the pile head load. In addition, the load transmission curve of the soils around the pile base corresponds to a softening model in the field tests on the destructive pile, whereas the relationship between displacement and mobilized base load developed at the pile tip can be described using a hardening model.3. Based on static load tests of super-long piles in conditions of different thicknesses of residues and different soil strengths at the pile end, the enhancement effect of the soil strength at the pile tip on the shaft resistance was analyzed. The observations have provided some key findings that the skin friction and the base resistance are not respectively independent, and the total skin friction will be increased by improving the soil or rock strength at the pile base, especially for the skin friction near the pile end. It can be concluded that for the pile with a stiff soil layer at the pile base, the arching effect of the stiff bearing stratum, the enlarged pile shaft area near the pile tip, and the density of the compacted soil near the pile tip can be enhanced by improving the soil or rock strength at the pile tip. These are the causes of the strengthening effect of the soil strength at the pile tip on the shaft resistance.4. This paper reported a well-instrumented case study on the field performance of pile-soil interaction in the design of pile groups. The reaction force of the pile at different locations of pile groups, the steel stress of base plate, and the percentage of the total load shared by the piles and the soils were analyzed. The measured data indicate that during decoration period, the measured average reaction force at pile top is more than50%of the ultimate bearing capacity of a single pile, differing from a single pile in the traditional design situation. Moreover, the percentage of the total load shared by the soils gradually decreases with increasing load and reaches about20%at the completion of decoration period. The measured average steel stress is observed to be small and far lower than the ultimate stress of the steel bar.5. Three simplified approaches for the analysis of a single pile response were presented in this paper. In the first method, the pile settlement was assumed to consist of three aspects:(i) the pile tip settlement induced by the mobilized base load;(ii) the pile shaft compression; and (iii) the settlement due to the skin friction. Assuming that the load-displacement relationship developed along the pile-soil interface and at the pile base followed a hyperbolic model and a bilinear model, respectively, a highly effective iterative computer program was developed for the nonlinear analysis of the load-settlement behavior of a single pile in the second method. Two models were adopted in the third approach. One model used a softening nonlinear relationship to simulate the degradation behavior between unit skin friction and displacement developed along the pile-soil interface, and the other model adopted a bilinear model to capture the pile end response. Based on the proposed two models and a bisection method, a computer program was proposed to analyze a single pile response. The proposed three simplified approaches can be commonly used to analyze the nonlinear response of a single pile embedded into layered soils.6. Based on the proposed simple analytical approaches for the analysis of a single pile response and the Equivalent Pier Method, the first simplified calculation approach for predicting an average settlement of pile group was proposed. The key of the first method was the adoption of a fair value of ω related to the relationship between a single pile settlement and pile groups displacement. The back-analysis of field tests on single pile and pile groups shows that the values of ω are found to be in the range0.25to0.45in silt and soft soils. In the second method, a new and simple approach was presented to analyze the interaction between piles including pile shaft and base interaction in layered soils. In this method, the interaction between piles was assumed to be in a linear elastic state, and the sheltering effect on the interaction factor between two piles was taken into account. Furthermore, in the third method, a hyperbolic model was used to capture the relationship between unit skin friction and pile-soil relative displacement developed along the pile-soil interface and the load-displacement relationship developed at the pile end. Determinations of the parameters presented in the hyperbolic model of skin friction and end resistance of an individual pile in pile groups were obtained considering interactions between piles. Based on the determinations of the parameters presented in the hyperbolic model of an individual pile in pile groups and the proposed iterative computer program developed for the analysis of a single pile response, the load-settlement response of an arbitrary pile in pile groups could be obtained.7. For static load tests with the use of reaction piles, the load transfer method and the shear displacement approach were adopted to analyze the influence of the reaction piles on the test pile behavior. The analysis shows that the settlement of an influenced test pile is smaller than that of an individual pile at the same loading level. In practical applications, the measured results of static load test conducted using reaction piles should be modified, and the safety degree of an influenced test pile will be overestimated in some degree without modification of test results.8. This paper, under the assumption that the pile-pile interaction showed elasticity, analyzed the pile-pile interaction between two dissimilar piles in layered soils using the load transfer method and the shear displacement approach, and taking the sheltering effect into account to modify the conventional interaction factor between two dissimilar piles. Parametric study shows that the interaction factor between two dissimilar piles decreases with increasing of distance between longer pile and shorter pile, ratio of length of load-free pile to loaded pile length, and shorter pile diameter, and increases with an increase in elasticity modulus ratio of pile to soil.