【作者】 辛公锋；

【导师】 夏唐代； 张忠苗；

【作者基本信息】 浙江大学 ， 岩土工程， 2006， 博士

【摘要】 随着高层、超高层建筑及大跨桥梁等建(构)筑物的建设,大直径超长桩因具有单桩承载力高等优点逐渐得到广泛应用。本文针对软土地基中大直径超长桩承载变形机理和试验中发现的桩侧阻软化现象,从现场试验、室内常规试验与离心模型试验、理论分析三方面开展了研究工作。 对深厚软土地基中的大直径超长桩进行了同时观测桩顶、桩端沉降和桩身应力应变的静载荷试验,研究了荷载水平、桩身几何参数、桩端持力层性质、桩身压缩变形特性对其承载变形性状的影响;得到了侧阻与端阻的发挥规律,二者的发挥是一个异步且互相耦合的过程。研究发现深厚软土中的大直径超长桩存在侧阻软化现象,在实测基础上给出了侧阻软化后桩的典型Q-S曲线。 对现场取样的桩侧泥皮土和桩间土进行了室内常规试验和三轴试验,得到了其物理力学性能和变形特性。试验结果表明,泥皮土比桩间土更容易发生应变软化。对9根不同桩长、桩径、持力层的单桩和4组群桩进行了考虑与不考虑泥皮效应的离心模型试验,分析了泥皮对桩侧摩阻力和承载力的影响,进一步研究了软土中大直径超长桩承载变形性状,得到了桩径、桩长对桩侧摩阻力的影响规律,研究了侧阻软化效应并与现场测试结果进行对比;分析了桩距对大直径超长群桩承载性状的影响以及承载力的群桩效应,对比了单桩与群桩侧阻发挥的异同。 采用统一强度理论和三折线软化模型分析了柱孔收缩问题,基于Drucker-Prager模型分析了桩侧土体的结构性软化影响因素,研究了不同摩擦本构关系以及桩侧土体主应力轴旋转对侧摩阻力的影响,分析了长径比对桩身压缩量及桩侧摩阻力的影响。结果表明,桩周土体性状、桩土界面性状、荷载水平、桩身几何与压缩变形特性都将对侧阻软化产生影响。通过对离心试验结果与现场实测结果的归纳分析,建立了归一化后的桩侧摩阻力与桩土相对位移函数关系,提出了侧阻在不同阶段的计算方法。 提出了一个广义双曲线函数来考虑侧阻软化效应,基于荷载传递法,通过耦合Mindlin解来考虑桩侧摩阻力引起的桩端沉降,采用Rusch模型来考虑高荷载水平下桩身混凝土的弹塑性特性,建立了大直径超长单桩分析模型。通过与现场实测结果和离心试验结果的对比,验证了其合理性。采用该分析模型,分析了桩身几何参数、桩顶荷载水平、混凝土弹塑性和桩侧阻软化效应对大直径超长单桩承载变形性状的影响。提出了最优桩长和最优弹性模量概念,得到了大直径超长桩轴向割线刚度和端阻比的变化规律。 利用有限元方法,建立了群桩分析模型,模型中考虑了桩侧土体的应变软化以及桩土界面的摩擦性状,对大直径超长群桩进行了研究,得到了群桩侧摩阻力更多还原

【Abstract】 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.更多还原