The increase/decrease change of water content is one of important factors for shear strength of unsaturated soils. Direct shear tests are done on the compacted soils under continue drying conditions. The results show that the behavior of shear strength changes significantly during the drying conditions. The peak value of shear strength occurs under lower water content conditions. And the strain softening characteristic is demonstrated during the shear process

whereas the strain strengthening characteristic is shown under higher water content conditions. The shear strength model is developed by combining the soil-water characteristic curve (SWCC) and suction stress characteristic curve(SSCC)

which can be used to predict the shear strength of unsaturated soil accurately. Compared to the measured data

the model can predict the evaluation of suction strength under a continue drying condition only using the determined soil-water retention curve and shear strength parameters under the fully saturated state. The measurement can be simplified and the tested time can be saved greatly when the model is used in practice. Furthermore

the nuclear magnetic resonance (NMR) technique is used to determine the distribution of pore water in soils on the same samples for shear strength tests. The measured data are used to explain the strength evolution of clayey soils under different water contents from microscopic analysis.