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.