INVESTIGATION AND REMEDIATION OF LOESS SLIDES IN GEO-ENVIRONMENTALLY FRAGILE ZONES OF NORTHERN SHAANXI

In geo-environmentally fragile Northern Shannxi quite a few tensile and shear fissures were discovered in areas of filling during large scale excavation and filling operations for a 330kV substation resulting in work stoppage and ensuing investigations for the cause. After extensive engineering geological survey, borehole drilling, test-pitting, trenching as well as multi-approach laboratory shear tests and stability analyses, it was found that the selected site is located on a large, low stability landslide whose slide-bed is rectilinear and coincides with the plane of contact between the slide-mass loess and sandy mudstone, i.e.,the direction of sliding and dip angle of the slide-bed are the same as those of the monoclinic beds of rock (300°～310°,∠8°～12°). The sandy mudstones,outcropping at the forefront of the site,are not original-state bedrocks. Instead,they are slide debris. They obstructed and hampered drainage of the slide-mass and contributed to the formation of wide-spread weak soft layers on the slide-bed. As a triggering effect, the large-scale excavation and filling operations destroyed the state of equilibrium of the old landslide. As a result, parts of the old slid were reactivated and three unstable engieering-related landslides came into being. The lessons that can be drawn from a case of this kind are :in geo-environmentally fragile zones siting investigation should be emphasized; it is feasible to use residual shear strength parameters from soil samples tested in saturated state (c=6.6kPa,φ=5.6°) in stability analysis and computation for loess slides which are in direct contact with underlying monocliclic sandy mudstones with rectilinear slide-beds; and remediation approaches which coordinate use of silt arresters for preservation and increase of tillable land as well as environomental protection and use of retaining structures can have prospects of wide application.