Test And Theory Study On Shaft Resistance Softening Of Large Diameter And Super-Long Piles

With the construction of super-high buildings and the wide-span bridge, the large diameter and super-long piles (LDSLP) have been used widely due to their high bearing capacity. Aiming at the bearing characteristic, the deformation mechanism and the shaft resistance softening of LDSLP, the study has been expanded from the field tests, normal indoor tests, centrifuge tests and theory analysis.The static load tests of LDSLP were carried out. The pile top and bottom settlement, the shaft stress and strain have been observed during the course. Baesd on the test results, the effect of load level, the shaft geometry, the bearing stratum characteristic and the shaft compression to the bearing and deformation mechanism of LDSLP have been sdudied. The exerting rules of shaft resistance and end bearing were obtained. Their exertion is asynchronous and coupling. The shaft resistance softening was discovered in LDSLP which embedded in deep soft soil and the typical load-settlement curve was obtained.The normal indoor tests and triaxial tests of mudcake and soil between piles which sampling from the locale were carried out and the physical and mechanical indexes and deformation characteristic were obtained. The results show that mudcake is more prone to softening than the soil. The centrifuge tests of nine single piles and four pile groups which considering the mudcake or not were carried out. The effect of mudcake to pile shaft resistance and capacity were analyzed. The bearing and deformation mechanism of LDSLP were studied furtherly. The effect of pile length and diameter to shaft resistance were obtained.The shaft resistance softening was studied and compared with the field results. The effect of pile space to pile group and the group efficiency coefficient were studied. The shaft resistance between single pile and pile group were compared.The cylindrical cavity contraction was analyzed using the unified strength theory and three fold-lines model. Based on the Drucker-Prager model, the effect factors of soil around pile to structural softening were analyzed. The effects of different friction models and the principal stress axial rotation on shaft resistance have been studied. The results show that the characteristic of soil around the pile and pile-soil interface, the load level, the geometry and compression characteristic will influnce the shaft resistance softening. Based on the induction of centrifuge test and field test results, the function of normalized shaft resistance and pile-soil relative displacement was set up and the calculating method of shaft resistance in different stage was put forward.A generalized parabolic model which considering the shaft resistance softeningwas put forward. Based on the load transfer method, coupling with the Mindlin solution and the Rusch model which considering the settlement produced by the shaft resistance and the plasticity of concrete under heavy load respectively, the analysis model of LDSLP was set up. Its rationality was proved by the contrast of the calculating, the field and centrifuge test results. The effects of shaft geometry, load level, concrete elastoplasticity and shaft resistance softening on bearing and deformation characteristic of LDSLP were studied by using this model. The optimum length and elastic modulus were put forward. The changing regularities of axial rigidity and end bearing ratio of LDSLP were obtained.Based on the finite element method, the analysis model of pile group, which considering the soil strain softening and pile-soil interface friction, was set up. The rules of group pile shaft resistance exertion, effect of pile length and space to pile group bearing characteristic were studied by using this model.