Modulus of elasticity E characterizes elastic deformation of granular beds. Modulus of elasticity E could be determined in uniaxial compression test. The first phase of unloading is use for it. Figure 1 shows the scheme of the uniaxial compression apparatus that is use in testing.
The granular material is poured into the test chamber, without vibration or other compacting action. The sample is 80 mm high and 210 mm in diameter. The bedding is loaded to the reference vertical stress sz0 of 100 kPa using universal testing machine. During compression, the top plate of the apparatus is moving down with a constant speed of 0.35 mm*min-1, while the displacement is measured with inductive transducer having accuracy of 0.01 mm. After completing compression unloading take place with the same speed of deformation until the 0 kPa of stress level is reached.
Two phases of the unloading can be distinguished. During the first phase of unloading (path BC), sample shows linear reaction, which is characteristic for elastic deformation. Total vertical deformation εz may be expressed as below: