砂的静力加卸载试验和应力释放模型研究

采用室内静力模型试验研究了某种石英砂的加卸载和应力释放特性，对比了在砝码加卸载条件下圆筒玻璃容器和矩形铁箱容器中砂中不同位置处的应力变化，明确了砂的加卸载应力变化规律，并在此基础上提出了针对砂土的摩擦型散粒体材料的应力释放模型。研究表明：砂中应力在加载阶段呈线性增加；而在卸载初期时应力无变化或减小量远小于卸去荷载，卸载后期砂中应力先是慢慢释放，之后在某一级卸载时突然快速释放，有较明显的分界点。分析认为，砂中加卸载规律不同的主要原因在于砂中的内摩擦作用，当卸去的荷载“作用”大于砂中内摩擦阻力时，才会产生应力释放。应力释放与否和释放大小不仅与内摩擦阻力大小有关，还与卸载水平和卸载前的应力状态有关。

Static loading-unloading test of sand and stress release model

A series of small-scale model tests were carried out to measure stresses developed in testing quartz sand bed during loading and the corresponding variation during unloading in glass cylinder and steel rectangular box. A new conceptual stress release model was proposed based on the model test results by considering the fundamental behavior of granular materials. It was found that stress in sand increased linearly with applied load. There is almost no change of stress at the beginning of unloading; significant stress release took place in later period of unloading and shown an obvious boundary. It is analyzed that the difference between loading and unloading was attributed to the internal friction of sand. Stress can only release when “removed load” is larger than internal friction resistance. Whether or not and how much stress will release not only depends on the internal friction resistance, but also is related with magnitude of unloading and stress state prior to unloading.