黄土地基湿陷时桩的负摩阻力最大值出现深度研究

A Study on the Position of Maximum Negative Skin Friction on Piles in Collapsing Loess Ground

负摩阻力及中性点的定义表明,负摩阻力沿桩身(从桩顶两侧到中性点位置)的变化并不均匀,其间应存在一个最大值,最近的黄土地基浸水湿陷试验证明了这一点。本文利用该试验的资料,从理论上对桩的负摩阻力最大值出现深度进行分析研究,结果表明负摩阻力随深度的变化受到某一深度(如负摩阻力最大值处)以上土体有效重量和该深度桩周土体的有效沉降量这两个主要因素的制约,并由此推导出不同沉降性能土体湿陷时桩的负摩阻力最大值出现的深度。通过与实测资料的对比可见,本文提出的确定桩的负摩阻力最大值出现深度的理论是可信的,由此得到的计算黄土地基湿陷时桩身总的负摩阻力的方法更具合理性。

The definitions of negative skin friction (NSF) and neutral point show that the variation of NSF from both sides of the tope of pile to the neutral point is not uniform, within which there might be a maximum value of NSF. This has been confirmed by recently obtained data from in situ tests in collapsing loess ground. It is necessary to determine the position of the maximum NSF on pile in order to obtain the theoretic curve of the variation of NSF versus depth, which may help to understand how the NSF takes place and calculate the value of NSF. To date, there have been few studies on this problem. Using the data obtained from tests in the collapsing loess ground, this paper studies the position of the maximum NSF by theoretical analysis. It shows that the variation of NSF with depth is likely to be influenced by the effective weight of the soil body above certain depth (such as the depth of maximum NSF) and the settlement of the soil body around the pile at that depth in the collpasing loess ground. Furthermore, the principle is derived, which determines the position of maximum NSF on piles in loess ground with different water col-lapsibility. Compared with in situ measurements, the method proposed in this paper for the determination of the position of maximum NSF is theoretically reasonable and the estimation of the total NSF on piles with this method is more rational than other applications using the effective stress method (ESM), with which the neutral point is usually taken as the computing depth for the calculation of the NSF value and the oversimplication of the computing depth violates the difinition of the nertral point, where the NSF value equals zero.