Study On The Groundwater System And Its Effects On Landslides In Heifangtai Loess Terrace,northwest China

The Loess Plateau is one of the key region for the implementation of the Western Development Strategy in China.However,a large number of loess landslides occurred in this region,which has affected the regional development.Heifangtai is a typical loess terrace,where the agricultural irrigation has been conducted for over fifty years due to the local drought.The excessively irrigated water infiltrated into the bottom of the loess layer and increased the local groundwater level,which induced a large number of loess landslides at the edge of the terrace.It is worthy note that the study on the local groundwater system is urgent,which can not only reveal the mehanism of the groundwater on the irrigated loess terrace,but also raise the sciencefic suggesstions for the local geohazards alleviation.The field investigation was conducted for the basical geological conditions and landslides characteristics in Heifangtai.To clarify the mechanism of groundwater system in study area,firstly,the temperature-vegetable dryness index(TVDI)was analyzed to support a broad land use division;the specific partitioning planting characteristics of different crops was analyzed by employing the remote sensing image.Secondly,a series of infiltration tests were condubted to select a proper method to study the infiltration in loess,the tests includes the variable infiltration test,loess column test,double-ring infiltration test and irrigation test monitored by electronic resistivity tomography(ERT).Thirdly,a field infiltration test was carried out using the optimal method,and the coupling infiltration mechanism of preferential flow and matrix flow in loess was analyzed by numerical simulation.Then,the ERT method was used to reveal the distribution of groundwater level in the terrace,and the discharge of all springs along the edge of terrace was monitored based on triangle-weir.At the same time,based on the centrifuge test of the undisturbed loess the failure mode of the slope under the increase of groundwater level and spring drainage was analyzed.Finally,based on the conservation of water mass in the groundwater system of the terrace,a long-term human-land harmony mechanism was proposed,and the suggestions for the regional disasters alleviation were put forward.The main conclusions of this study can be boiled down as follows:(1)The recharge of irrigation water on the terrace has portioning characteristics.The total amount of annual irrigation volume on the terrace has been kept above500×10~4m~3 for over fifty years,and there are mainly spring and summer irrigation period and winter irrigation period.The distribution of irrigation volume in the terrace can be characterized by the portioning of different crops with different water requirements.The low TVDI area,namely irrigation concentration area,is mainly distributed in the middle,southern and northern of the terrace,where fruit trees,vegetables and wheat are planted,respectively.The object-oriented classification results of optical remote sensing images show that the crops in terrace can be divided into vegetable area,mixed-used area,wheat area and fruit tree area.The irrigation volume of the four regions decreases sequentially in recent years,and hence the recharge of irrigation water can be divided according to the portioning planting of crops.(2)The method of time-lapse ERT data on monitoring infiltration combing a single ring infiltrometer is aoolicable for reproducing the infiltration process in loess.In each infiltration test,the permeability coefficient of loess decreases linearly with the increase of dry density.The double-ring infiltrometer result shows the largest permeability coefficient,while the remolded sample with variable head test shows the smallest result.The infiltration pattern of the remolded loess in colume test infiltrates with the uniform saturated matrix flow.The field irrigation test monitored by ERT shows that the water infiltration includes uniform matrix flow in the surface and preferential flow in the bottom,which was verified by the in field exploration wells.Therefore,the in field double/sigle-ring infiltration test monitored by time-lapse ERT can effectively detect the infiltration process in loess.(3)The cracks and hiden cracks at the edge of terrace can provide the preferential flow channel.The single ring infiltrometer results showed a high relative reduction of electrical resistivity(>80%)due to matrix flow(MF)and preferential flow(PF)within0–0.5 h of the test.Then,the process followed by a transformation of preferential flow to a more matrix-dominated unsaturated flow with continuous downward progression into the deeper subsurface(greater than 1.2 m)while MF stagnated at the near surface(about 0.4 m).The results suggested that the presence of even a near-surface preferential path significantly accelerate the infiltration into the deeper loess by the means of PMF,as compared to observed slow infiltration of pure matrix flow in previous studies.The numerical simulation results of the dual permeability model show that under the high intensity and short duration irrigation condition of 25mm/h,the preferential flow dominants and the water can quickly infiltrate into the soil bottom and cause the water level increasing,while the top matrix flow is unsaturated infiltration.Under the low intensity and long duration irrigation condition of 2mm/h,the matrix flow dominants and infiltrates with the saturated state,while the preferential flow infiltrates with the unsaturated state without recharge groundwater.(4)The distribution of groundwater level in terrace is mainly controlled by water recharge and spring discharge.The buried depth of the groundwater table(GWT)varies from 1.19 m to 40.50 m.The areas with GWT burried less than 10 m are mainly distribute in the north side of and the center of the terrace,while the buried depth of the groundwater level is generally deep at the edge of the terrace.The fruit tree region has the deepest GWT,while vegetable region has the shallowest GWT.The greater the discharge flow,the deeper the buried depth of GWT.In the middle of the terrace the buried depth of GWT is controlled by the type of crops,while at the edge of the terrace,the buried depth of GWT is mainly controlled by the discharge of springs.(5)The failure mode of the loess slope is mainly affected by the GWT and the drainage.The slope evolution includes four stages in the centrifuge test.Firstly,the cracks occurs at the toe of slope due to collapsibility,erosion.Secondly,a small collapse develops at the bottom of the slope.Thirdly,a greater collapse occurs in the upper position of slope.Finally,a loess cave occurs at the top of slope due to the inner erosion and settlement.The failure includes two collapses at the edge of slope and one cave at the top of slope,which is similar to the field tracking investigation.It is proved that the failure of GWT induced loess landslide has a retrogressive evolution mode.(6)The solution based on the local groundwater system is crucial for the landslides alleviation in Heifangtai.Based no the calculation of water conservation in the groundwater system of the terrace,the difference of?Q(accounting for 16%of the total recharge)between the recharge(irrigation and rainfall)and the excretion(spring drainage and evaporation)is positive.The?Q causes the increase of GWT,and it is close to the water volume for the actual increased water level with an error of 6.7%.The local human land relationship includes groundwater system,water-human and water-soil relationship.The water and social system relationship is the key to maintain a local balanced state.Thus,the water resource management,land resource management and engineering governance should be considered for the alleviation of local geological disasters.