沉井侧壁摩阻力室内试验研究

Model test on sidewall friction of open caisson

沉井侧壁摩阻力的大小和分布是沉井结构设计以及助沉方法选择的依据。目前,其计算方法带有较强的经验性,计算结果与实际情况往往存在较大差异。为了进一步认识和掌握其分布规律,研制了能够直接测量沉井侧壁阻力的微型摩阻力仪,并对其准确性、稳定性和可靠性进行了验证;然后,利用该仪器以及静态应变测试系统等电测设备开展沉井下沉阻力的模型试验。试验表明:随着入土深度的增加,沉井侧壁摩阻力的变化特征可分为3个阶段,在第1阶段,侧壁摩阻力随深度的增加接近线性增加;在第2阶段,随着深度的增加,侧壁摩阻力总体仍是上升的,但其上升速率则有所降低,在下沉一定深度后,摩阻力达到峰值;在第3阶段,侧壁摩阻力随入土深度的增加而逐渐减小。运用极限分析理论,对上述变化规律的原因进行了分析。上述试验及分析成果可作为现场原位测试数据的补充,两者共同为沉井下沉阶段的侧壁摩阻力计算研究提供基础数据支持。

The magnitude and distribution of sidewall friction of open caisson are very important to caisson structure design and the selection of sinking-aid methods. However, the computation of sidewall friction is still quite empirical, which often leads to significant deviations from real situation. In order to further understand and master the distribution rule of the sidewall friction of open caisson during the process of sinking, a new type of micro-friction-sensor is invented to measure the sidewall friction directly; and its accuracy, stability and reliability are verified. Then, it is used in model test along with static strain test system. The result shows that the variation characteristics of sidewall friction can be divided into three phases with the increase of sinking depth. In phase one, the increase of sidewall friction and depth is close to linear relation; in phase two, the sidewall friction keeps increasing with depth, yet the rate of increase drops and the value reaches the peak at a particular depth; in phase three, the sidewall friction drops gradually with the increase of depth. The mechanism of this phenomenon is analyzed with theory of limit analysis. The test result can be treated as a complement of in-situ monitoring, and both provides fundamental data for analytical and numerical computation for sidewall friction of open caisson.