Study on the Elasto-plastic Behavior of Open or Closed end Pile

1. 随着管桩直径的增大，管桩的承载力具有静不确定性。随着荷载的增大，单位面积力呈现出随桩厚径比变化的趋势，形成了桩端阻力在桩端长度为桩径两倍的范围内向桩上部传递的机制，但只要荷载较小，单位面积力可能随桩径变化。通过有限元分析模拟了开桩和闭桩的行为，认为桩顶附近的极限摩阻状态导致主应力方向形成锥体，主要由桩在锥体下沉降。(1)桩内土体随桩沉降，除桩端外的内摩擦可以忽略不计，但在桩端，由于砂粒的拱起，外摩擦或内摩擦大于其他部分。(2)在开桩情况下，近端砂土不发生失稳，但近端砂土发生失稳。(3)在闭桩情况下，桩端以下5m处被混凝土堵塞的刚性层与桩径之间的距离为桩径的2倍时，桩端附近的砂土破坏，从裂缝到上部的摩擦力达到了极限值。(4)从上述砂土的极限摩擦状态和破坏中，依次观察到通常的屈服荷载和Kishida和Takano命名的“第一极限荷载”。为了改进该方法中砂土破坏后的摩擦单元和卸载单元，必须引入位移控制或柔性方法。砂尖附近的应力值已达到颗粒破碎范围，因此也必须改进本构律。必须研究在对称轴附近的周向应变误差或应力和应变的精确定义。针对上述改进，建立了端部无摩擦片的三轴试验方法，采用应变控制进行试验。少

1. The bearing capacity of pipe piles is statically indeterminate on account of increasing in diameter. The force per unit area shows a tendancy to vary as the ratio of thicknes to diameter as the load is made larger and the mechanism, which transfers end resistance to the upper part of piles between the extent which is twice as long as diameter at the tip ,is formed, but it may vary as the diameter as long as the load is small.2. The behaviours of open or closed pile were simulated by the F.E.M analysis, and then it is presumed that the ground will be settled by the pile mainly under the cone formed by the principal stress direction due to the ultimate state of friction near the pile top.(1) The inner soil settles with the pile so that the inner friction is negligible except the pile tip At the pile tip, however, outer or inner friction is larger than other part owing to arching of sand particles.(2) In the case of the open pile, sand near the tip did not fail but near the top failed. … More (3) In the case of the closed pile, in which the rigid stratum at 5m under the tip which plugged by the concrete between the extent which is twice as long as pile diameter, the friction reached the ultimate value in order from the rip to the upper part and the sand near the tip was failed.(4) From the above mentioned ultimate state of friction and the failure of sand, the usual yielding load and 'the first ultimate load', named by Kishida and Takano, were observed in order.3. For improvement of friction element and unloading element of sand after failure in the method, the displacement control or flexibility method must be introduced. The stres values of sand near the tip reach the particle crushing range so that the constitutive law must be improved also. Errors of circumferential strain near the axis of symmetry or the precise definition of stress and strain must be investigated.4. For the above mentioned improvement, triaxial test method with end frictionless pieces was established and the test will be conducted by strain control. Less