超长灌注桩桩-土界面剪切试验研究

Experimental research on pile-soil interface shear behaviors of super-long bored pile

为研究超长灌注桩桩侧与土体接触界面的剪切力学行为,采用大型界面剪切仪,开展混凝土与粉质黏土、粉细砂土接触界面剪切试验。针对钻孔灌注桩泥浆护壁施工特点及后注浆工艺的应用,在混凝土与粉细砂土界面设置膨润土泥皮或膨润土泥皮与水泥浆,以研究泥皮及存在泥皮时注浆对界面剪切性状的影响。试验结果表明:界面剪切应力随剪切位移增加逐步达到极限值,之后,剪切应力保持基本不变或出现软化现象;土体类型及法向应力大小对界面剪切力学行为具有较大的影响;粉细砂土-混凝土界面摩擦角与土体有效内摩擦角相近,存在泥皮时,界面摩擦角降低达40%,泥皮的润滑作用较大地削弱了界面的剪切性能;存在泥皮情况下,注浆后界面摩擦角较泥皮界面提高近1倍,水泥浆的注入不仅消除了泥皮产生的不利影响,也有利于进一步改善界面剪切性能,提高其抗剪强度;泥皮及注浆对界面剪切应力与剪切位移关系亦有较大的影响;界面剪切作用对土体具有一定的影响范围,并在接触界面附近土体中逐渐形成剪切破坏带,剪切过程中界面呈现出由剪切位移阶段逐步过渡至剪切滑移阶段;此外,在剪切过程中,不同界面类型的土体变形存在较大差异。

A series of shear tests is carried out on the interfaces between concrete and silty clay, fine sand to study pile-soil interface shear behaviors of super-long bored pile using large-scale interfacial shear apparatus. Bentonite mudcake or bentonite mudcake and cement slurry are installed on the interfaces between fine sand and concrete to study the influence of mudcake and post grouting on the shear behavior of pile-soil interface. The results show that the shear stress increases nonlinearly with the increase of the shear displacement. After the shear stress reaches an ultimate value, the shear stress retains mainly a constant value or shows a softening tendency. The shear behaviors of the interface are significantly influenced by the soil type and the normal stress on the interface. The friction angle of interface between fine sand and concrete is close to the effective internal friction angle of the soil. The friction angle of the interface reduces 40% when the mudcake exists, showing that the lubrication action of the mudcake can greatly weaken the shear performance of interface. By grouting the cement slurry into the interface with mudcake, the friction angle of the interface increases nearly by one time that of the mudcake interface. It is shown that grouting not only can eliminate the adverse effects of mud cake, but also can further improve the shear properties of the interface. Furthermore, the relation between shear stress and shear deformation of the interface is closely related to mudcake and cement slurry. Due to the interface shear in the soils, a shear band is found to form gradually in soil near the interface. The phase of shear displacement gradually transforms into the phase of shear slipping during shear process. Moreover, there is great difference in the shear deformations of soils with different types of interface during shearing.