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Abstract Zygophyllum xanthoxylon and Caragana korshinskii are shrubs planted separately in in-situ shear test devices designed and worked out by the researchers. The dynamic variation on the shear resistance，shear strength of the soil without roots and root-soil composite systems are observed from two degrees of slope via in-situ shear test on soil without roots，Zygophyllum xanthoxylon root-soil composite system and Caragana korshinskii root-soil composite system. At the same time，the time effect of slope protection for shrubs is evaluated；and the accuracy of the in-situ shear test is proved by indoor direct shear tests. The results show：(1) At the same degree of slope，the shear resistance and shear strength of root-soil composite system are larger than those of soil without roots，i.e. plant roots can significantly improve the shear capability of slope soil. The shear resistance and shear strength of Zygophyllum xanthoxylon root-soil composite system are reflected in tension or shear of broken roots，friction between soil and pulled-out roots，anchor and friction of slipped roots，while those of Caragana korshinskii root-soil composite system from the anchor of roots and friction between roots and soil. The shear resistance and shear strength of soil without roots and shrub root-soil composite systems decrease with the slope degree increase. (2) The shear resistance of soil without roots reaches its peak value after three seconds，while that of root-soil composite system reaches the peak values after 69.8 to 168.2 seconds. The shear strength of soil without roots reaches the peak value when the displacements are within one millimeter，while that of root-soil composite system reach the peak values when the displacements are from 23.13 to 83.13 millimeter，i.e. the peak time of shear resistance and peak displacement of shear strength for root-soil composite system appear later than those of soil without roots. Those findings reflect that the tensile capability of roots and the friction between roots and soil during the process of shearing gradually transform to be the shear ability which can delay the slope deformation and failure. (3) To prove the accuracy of in-situ shear test，indoor direct shear tests on soil without roots and shrub root-soil composite systems are conducted. The shear strength values of indoor direct shear tests approach the values of the in-situ shear test，showing that the method and the results of in-situ shear test are rational. Those achievements have theoretical and practical values in constructing ecological engineering，protecting ecological environment and preventing geological catastrophes，such as soil and water loss，shallow landslide in Xining basin and the loess slopes of northeast Qinghai—Tibet plateau.
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Received: 15 October 2012
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2013, Vol. 32 
