Groundwater Response Of Slope To Rainfall Sequence And Stability Prediction Of Landslides Induced By Rainfall

The response of soil moisture and groundwater to rainfall sequence is a complex dynamic process with differences in different climatic regions.In the loess plateau area of northwest China,rainfall is the only recharge source of groundwater when irrigation is neglected.When surface water passes through the vadose zone of loess to recharge deep groundwater,the response of groundwater takes a long time and is not significant.The impact of rainfall is mainly reflected in the change of wetting front.At present,the research and observation in this area are still very few.Meanwhile,in the bedrock mountain area with heavy rainfall and coastal hilly area with typhoon of China,rainfall directly recharges groundwater.However,the response law of groundwater to rainfall in different periods and different time scales is still not clear.Therefore,the soil slope under rainfall sequence in different climatic regions was taken as the research object in this paper.Based on the observation data of three in-situ observation stations located in Gansu Province,Sichuan Province,and Fujian Province,the following studies have been conducted through experiments,numerical calculations and statistical analysis:（1）These elements including rainfall,air temperature,soil moisture content,soil temperature and groundwater level were monitored for a long time through the field observation station established in the Loess Plateau of East Gansu.The seepage law and groundwater response law in the vadose zone of loess under rainfall sequence were analyzed.The results show that rainfall is low and evaporation is strong in semi-arid loess area.The vadose zone of thick loess can be divided into active zone and steady zone.In the active zone with a thickness of～7 m,piston flow is the main mode of moisture transfer in loess.Preferential flow develops locally and temporally.In the steady zone,piston flow is the only mode of moisture transfer in loess.The soil moisture content and soil temperature are relatively constant with time.The percolation velocity at interbedded loess and paleosol is constant and consistent.This result means that hundreds of years is required for soil water to pass through the vadose zone of loess to recharge groundwater.（2）These elements including climatic factors,soil moisture content,soil temperature and groundwater level were monitored for a long time through the field observation station established on the gravel soil landslide in the heavy rainfall area of Qingchuan County,Sichuan.The response law of soil moisture content and groundwater level to the rainfall sequence were analyzed.The results show that there is abundant rainfall here and has the characteristic of concentrated rainfall with single peak within the year.The response of soil moisture and soil temperature to climate factors is strong above 0.6 m.The study period can be roughly divided into the stable period,descending period from April to June,and rising period from July to September.Heavy rainfall can not alter the continuous decline of groundwater level during descending period.During the rising period,the peak value of hourly rainfall stimulates the peak of groundwater level after 2–10 h under heavy rainfall.The rise amplitude of groundwater level and maximum groundwater level has a linear positive correlation trend with rainfall intensity when the rainfall intensity is more than 21.0 mm/d.Compared with loess,the permeability of gravel soil layer is high and the groundwater level is shallow.Therefore,the groundwater responds quickly to rainfall.（3）These elements including climatic factors,soil moisture content,soil temperature and groundwater level were monitored through the field observation station established on the granite residual soil slope in the typhoon heavy rainfall area of Ningde City,Fujian.The response law of soil moisture content and groundwater level to the rainfall sequence were analyzed.The results show that there are frequent rainstorm here and has the characteristic of multiple concentrated rainfall with multiple peak.The response of soil moisture and soil temperature to climate factors is strong above 0.6 m.The study period can be roughly divided into the descending period,stable period from March to April,and rising period from May to September.The rainstorm event can not alter the continuous decline of groundwater level during descending period.During the rising period,the peak value of hourly rainfall stimulates the peak of groundwater level after 1–2 h under heavy rainfall.The rise amplitude of groundwater level and maximum groundwater level has an obviously linear positive correlation with rainfall intensity when the rainfall intensity is more than 17.8 mm/d.Compared with inland heavy rainfall area,the response speed of groundwater to rainfall is faster.（4）The response of wetting front or groundwater to rainfall sequence was simulated by saturated-unsaturated finite element method for slopes in three typical areas.The simulation results are basically consistent with the observation results.Through the simulation results and statistics of observation data,the quantitative relationship between wetting front of loess slope or groundwater level of residual soil slope and rainfall sequence was established.On this basis,the wetting front of loess slope and the groundwater depth of residual soil slope were introduced into the stability analysis model of infinite slope.The prediction model for loess landslides and residual soil landslides caused by rainfall sequence was established.The landslide cases which have occurred near these three observation stations shows that the corresponding model is feasible.（5）Based on statistical analysis of observation data including rainfall and soil moisture content from observation stations located in Gansu Province,Sichuan Province,and Fujian Province respectively,the recession constant of the corresponding region was obtained.The results show that the recession constant in Zhengning County,Gansu Province is 0.90 when the number of antecedent days is 3–30 days.The recession constant in Qingchuan County,Sichuan Province is 0.78 when the number of antecedent days is 3–7 days.The recession constant in Ningde City,Fujian Province is 0.80 when the number of antecedent days is 3–7 days.In the early warning of rainfall-induced landslides,the recession constant is a key parameter for determining the critical rainfall threshold of landslides.Therefore,the data of 146 recorded landslides and antecedent rainfall from 2011 to 2020 in Ningde City were collected.The statistical model of critical rainfall threshold for landslide was established.The fitting curve equations of average effective rainfall intensity-duration（I-D）threshold are I_0.9=433.810D^-1.382,I_0.5=247.617D^-1.382 and I_0.1=37.373D^-1.382 respectively in this area.