| Heifangtai is a typical loess tableland in Gansu province.Due to the continuous agricultural flood irrigation for nearly 50 years,a large amount of irrigation water infiltrates into the ground,the ground water table in the loess layer is continuously rising,which induced a large number of loess landslides at the margin of the tableland,causing serious impacts on the lives and economic development of local residents.The occurrence of these landslides in Heifangtai is closely related to its groundwater system.Therefore,it is of great significance to study the infiltration characteristics of the surface water,groundwater table distribution characteristics,groundwater table change characteristics and other scientific issues for the study of the formation mechanism of landslides and disaster prevention and control in this area.Conventional means such as soil column experiment?borehole reveals and numerical simulation were given priority to groundwater system research,which has certain limitation,while the ERT as one of the most mature geophysical detection method,can effectively make up for the inadequacy of previous research work.And,it will provides a scientific basis for the further study of the groundwater system in the region,but also for the similar case studies provide scientific research methods.In this paper,the surface water infiltration,the groundwater table distribution and variation characteristics in Heifangtai were studied by using different detection schemes of ERT.Firstly,the in-situ irrigation infiltration test was carried out through the test scheme of 4D ERT,and the 3D infiltration characteristics of irrigation water were studied through the verification and analysis of sensor monitoring data.Secondly,the spatial distribution characteristics of ground water table were studied through multiple 2D ERT profile and interpolation calculation schemes combined with the verification of field hole.Then,through the 2D ERT,the regular monitoring of the fixed section in the irrigation area was carried out,and the characteristics of soil moisture content and groundwater change in the loess layer under irrigation conditions were studied and revealed.Finally,the distribution and variation characteristics of the groundwater table at the back edge of the DC-9 landslide were obtained by ERT as the initial conditions,and the variation trend of its slope stability was simulated and analyzed.The main conclusions are as follows:?1?According to the field irrigation infiltration test conducted by 4D ERT,the apparent resistivity of the irrigated area decreases layer by layer,while that of the non-irrigated area shows no change.Moreover,during the test,the pore water pressure of shaft W1,which is 1m away from the irrigated area,changes slightly within 3m of the shaft wall,indicating that the infiltration mode of surface irrigation water in the loess is mainly vertical unsaturated migration.According to the analysis,the infiltration depth of irrigation water at 1 hour,1 day,2 days,3 days,4 days and 5days after the first irrigation is 1m,1.5m,2.5m,3m,3.5m and 4m respectively,and the infiltration rate gradually decreases.After the end of the water supply?the 9th day after the irrigation begins?,the infiltration depth revealed by the three-dimensional high-density electric method is about 6 m,and the excavation is started immediately after the water supply is stopped.The infiltration depth revealed by the shaft W2located in the middle of the irrigation area is about 6.25 m.During the follow-up 7days of monitoring,the maximum infiltration depth of irrigation water is about 6.5m.?2?Experiment on the relationship between moisture content and resistivity of loess were carried out using the loess samples from Heifangtai.The experimental results show that under the same conditions?density,salinity,etc.?,the resistivity decreases sharply with the increase of water content when the water content is lower than the liquid limit of the loess,and when the liquid limit is reached,the resistivity is almost unchanged.?3?The spatial distribution characteristics of the underground water table in Heifangtai were obtained from the interpolation of 50 ERT profiles?38km in length?distributed in the central part of the platform and the side of the platform.According to the results,there are three groundwater table vaults in Heitai,where the groundwater table is shallow,only 1-12m under the ground surface,and there is a unique phenomenon here that is shallow loess salinization is very obvious.According to the field investigation,the three areas are large areas of farmland with large amount of irrigation.The groundwater seeps from the three vaults above into the surrounding area,the main seepage direction is from the west to the south and east.According to the typical ERT profile of the tableland margin landslide area,it can be seen that part of the back margin of the landslide in the study area has an obvious phenomenon of local water accumulation with a high range of 7-10m.?4?According to the monitoring results of the fixed section in the irrigated area by ERT,the periodic irrigation can significantly change the soil moisture content in the shallow surface and affect the depth of about 7.5-10m.During the 21-month monitoring period by electrical method,the groundwater table in the irrigation area fluctuates greatly,with the maximum lifting range of about 2-3m.The borehole monitoring data located near the section shows that the water table increases by about2m in the ERT monitoring period,which is basically consistent with the monitoring results by electrical method.?5?According to the monitoring results of the fixed section at the back edge of the DC-9 landslide by the ERT,the rise rate of the groundwater table in this area is about 0.7m/year.Geo-Studio calculation results show that the stability coefficient of the slope is gradually decreasing.The current stability coefficient of the slope obtained by different stability calculation methods is between 1.075 and 1.163,which is in a basically stable state and the possibility of landslide is low.As the groundwater table in this area continues to rise,the stability of the slope will continue to decrease and may be destabilized under the current water table fluctuations. |
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