Study On Mechanism And Treatment Of Landslide In Heifangtai

Geological disasters frequently caused by the action of water in the loess area,due to the special physical and mechanical properties of loess.There are a lot of loess landslides caused by agricultural irrigation in the Loess Plateau of Northwest China,posing severe threats to the lives of the local people and their properties.Heifangtai,located in Yongjing Country of Gansu Province,is a typical case of which having experienced numerous irrigation-induced landslides.In this paper,the geological conditions and the characteristics of the loess landslide in Heifangtai are analyzed by investigation and monitoring,and the seepage field of groundwater near a landslide edge is studied by field testing.The periodic recurrence and scale-expansion mechanism of loess landslide are found,and the evolution process of loess landslide is revealed.Based on the cognition of loess landslide in Heifangtai,a treatment of siphon drainage technique and shallow blind drains is employed to improve the stability of slope.The following achievements can be drawn from this study:(1)According to the field investigation and monitoring,the periodic recurrence mechanism of loess landslide are found.Many loess landslides that occurred around the Heifangtai platform typically have steep slip cliffs and armchair-shaped edges.These landslides have the characteristic of in situ succession,i.e.,the slope would fail repeatedly at the same site and the failures would recur in a short time,and the landslide trailing edge would gradually move to the inner of platform.Almost all shear outlets of loess landslides in Heifangtai are located at the interface of the loess and the alluvial clay layer,and a lot of groundwater discharge at the slope toe.The slumping soil will be converted into mud flow due to the sustained outward flow of groundwater and flow downward the slope,leading to the landslide deposits quickly disappear and make the free surface,which is the basic factor of the periodic recurrence of loess landslide.(2)The concave topography formed by past landslides provide an advantage for groundwater convergence,this observation is consistent with the results of a field test.The landslide is closely related to the discharge of groundwater,and the spring water yield at a landslide site is far greater than it is at the other natural platform borders.The field test of groundwater flow characteristic showed that the concave topography formed by landslides made groundwater flow to the concave center.After a landslide occurs,the groundwater gradually changes direction and flows into the failure site,and it is discharged from the toe of the trailing edge of the cliff created by the landslide.(3)The evolution of the loess landslides in Heifangtai is revealed by a numerical simulation.The seepage undercutting at slope toe is the most critical factor of the landslide.With the sustained groundwater seepage undercutting,a cavity which has a gradually increasing tendency is formed,thus the overlying soil collapse due to lose the support and the groundwater discharge will further increases.Finally the deposits will be transported by surface runoff and move away from the slope toe,producing favorable condition for the recurrence of landslide.The numerical simulation shows that the initial landslide presents the vertical sliding surface and the small scale destruction characteristics.With the extension of the landslide to the platform,the scale of the landslide gradually expands,and gradually presents the circular arc sliding surface.(4)The effectiveness of siphon drainage technology is proven by a field tests.Lowering the platform groundwater table is the basic need for the landslide prevention,which can reduce the penetration and discharge of groundwater at slope toe.The field test of siphon drainage applicability showed that the siphon drainage technology can overcome the disadvantage climate and geographical conditions in Heifangtai area.The groundwater table in the siphon borehole maintain at a low level for a long time in the vicinity of the siphon limit lift,and the groundwater table in monitoring boreholes generally drop 0.2 to 0.5 m,which can effectively improve the stability of the slope.(5)A treatment consist of siphon drainage technology and shallow blind drains is put forward,which can fill the blank of prevention and control measures in the study area.Limited by the low permeability coefficient of loess,the 2-3 m spaced boreholes can control the rising of groundwater table,but not enough to inhibit the groundwater discharge at the slope toe.An analysis base on planar potential flow theory shows that,simply rely on siphon drainage to realize the control of seepage undercutting at slope toe,the space of borehole should be reduced to approximately 0.2 m(50 mm internal diameter of borehole).Therefore,the treatment with siphon drainage and shallow blind drains is employed to control the groundwater table and the seepage undercutting at slope toe,which has proven to be a practicable and efficient countermeasure for the loess landslide in Heifangtai.