To investigate the influence of degree of saturation on small-strain stiffness of soil under evaporation conditions

an experimental device for measuring small-strain stiffness of unsaturated soils was developed. The device consists of three components: relative humidity controlling system

deformation monitoring system

and wave velocity measuring system. The relative humidity generator is used to modulate the relative humidity inside the environmental chamber. The weight change of soil sample is recorded through the weighing module. A digital camera is used to take photos of the upper surface and the side of the soil sample at the same time. The sensors measuring compression and shear wave velocities are designed and fabricated by using piezoelectric ceramic elements. The small-strain stiffness of soils is then determined by wave velocities. The experimental results show that the system can stably control the relative humidity of the environmental chamber

indirectly measure the volume change of soil sample

and monitor the variation of wave velocities of soil in real time. On this basis

the dependencies of wave velocity and small-strain stiffness of soils on degree of saturation can be observed. Shear wave velocity and small-strain shear modulus of soil samples gradually increase with decreasing the degree of saturation

while compression wave velocity and small-strain volume modulus first decrease and then increase with decreasing the degree of saturation. The test system is simple in structure and easy to operate

providing a powerful tool to investigate the effect of degree of saturation on the mechanical properties of soil.