(1 Yichang Institute of Geology and Mineral Resources, Yichang, Hubei, 443000; 2 Chengdu University of Technology, Chengdu, Sichuan, 6 10059)
Baijiapu landslide is located in the Three Gorges reservoir area, near the right bank of Xiangxi River in Zigui County, and enters the Yangtze River estuary. The monitoring data show that the landslide in 2004 is in a state of relative stability-deformation-relative stability, and the amount of landslide movement in flood season is obviously greater than that in non-flood season. In this paper, the engineering geological model of Baijiapu landslide is established, the groundwater dynamic field caused by rainfall infiltration is simulated by finite element method, and the stability coefficient of landslide under different rainfall intensities and durations is calculated when the backwater of reservoir reaches 135m level. The results show that the landslide is in critical equilibrium when the rainfall lasts for 3 days. At this time, the direction of groundwater flow caused by rainwater infiltration in the landslide body is basically consistent with the sliding surface; With the extension of rainfall time, the groundwater level increases obviously, and the safety factor of landslide decreases accordingly. The difference of rainfall intensity and duration is the main reason of landslide displacement change.
Baijiabao landslide; Monitoring data; Numerical simulation; Stability coefficient of rainfall and groundwater flow
1 Introduction
The influence of rainfall on landslide is a dynamic process, which is closely related to rainfall intensity, duration and soil permeability. The engineering geological model of Baijiapu landslide (Figure 2) is based on this, which is not divorced from reality, but also grasps the key to the deformation and failure of rock and soil. The model material consists of residual slope deposit, sliding body, sliding zone and bedrock; The material condition for controlling landslide deformation is sliding zone; The dynamic factors controlling landslide deformation are reservoir water level and rainfall. The establishment of Baijiapu landslide model is beneficial to the study of landslide in all aspects. Therefore, the influence of rainfall on the stability of Baijiapu landslide is beyond doubt.
Fig. 5 Correlation curve between deformation of pier A2 and rainfall during surface monitoring in 2004.
3 Numerical analysis of groundwater flow caused by rainfall
Hydrogeologists and soil scientists have studied unsaturated seepage for a long time, and the seepage of water in unsaturated soil also obeys Darcy's law. In the stability analysis of soil slope, when the local groundwater is shallow, the soil water movement in unsaturated area and groundwater movement in saturated area are interrelated, which is convenient to study them together, that is, the so-called saturated-unsaturated flow problem. In this case, according to Darcy's law and the principle of groundwater flow balance, a dynamic model describing the two-dimensional saturated-unsaturated flow problem of landslide profile can be obtained:
Essays on Geological Disaster Investigation and Monitoring Techniques and Methods
Where: H is the head, K is the permeability coefficient, and S is the specific water storage coefficient.
The boundary conditions of unsaturated seepage problems can take many forms. For example, given water content, given water head or seepage velocity on a given boundary, etc. This paper mainly studies the transient seepage field caused by rainfall, so the exposed boundary of the slope is given the infiltration velocity (Newman boundary), the bottom is the first boundary (impermeable boundary), and the two sides are the second boundary (given head boundary) [10]. Because the landslide surface is generally an irregular slope area, the water table as the upper boundary changes with time. In the process of simulating the two-dimensional phreatic flow in the profile, the triangular element finite element method is used to divide the hydrogeological model. According to the hydrogeological parameters such as soil layer structure and permeability coefficient of landslide and the infiltration intensity of atmospheric rainfall recharge, the groundwater effect of Baijiapu landslide is numerically simulated.
Based on the annual average rainfall 1028mm, the groundwater level and groundwater flow under the conditions of no rainfall and continuous rainfall days 1 day, 2 days, 3 days, 4 days, 5 days, 6 days, 8 days and 10 days are simulated and analyzed (see Figure 6). At first, the flow direction of groundwater was inconsistent with the sliding direction of landslide, and the groundwater level changed little. When the rainfall lasts 1 day, the groundwater in unsaturated soil is mainly replenished by rainfall infiltration, mainly by vertical migration, and the isoline of groundwater level formed by rainfall is consistent with the slope gradient; With the continuous rainfall, the direction of groundwater flow gradually changes from vertical to the direction consistent with the tendency of sliding surface. When the rainfall lasts for 2 days, the direction of water flow tends to be consistent with the tendency of sliding surface. When the rainfall lasts for 3 days, the direction of groundwater flow is basically consistent with the tendency of sliding surface, especially the phreatic flow appears at the bottom of sliding surface. Through the observation section of transient groundwater, it is found that the transient groundwater level will continue to rise if the rainfall continues.
4 landslide stability analysis based on rainfall conditions
Through the finite element numerical simulation of groundwater, the seepage field of slope and the seepage force of each node are obtained In the calculation of slope stability [1 1], the overall and local stability coefficients (Fs) are obtained by the Morgenstern-Price method, taking the seepage force as the external load. The block broken line is used to search the most dangerous sliding surface with overall stability, and the arc method is used to search the most dangerous sliding surface with local stability.
When the depth of groundwater table in the observation section is H ′ > 36m, the landslide is in a stable state, but the safety reserve is not high. When the buried depth of water level in the observation section is H ′ = H0 = 36m (continuous rainfall for 3 days), the landslide is in a state of limit equilibrium. When the buried depth of water level in the observation section h ′ = 33 ~ 35m, the whole landslide is in a state of limit equilibrium and local deformation occurs. When the buried depth of water level in the observation section is H ′ < 33m, the whole landslide is in an unstable state.
Fig. 6 Isogram of groundwater and flow vector diagram after continuous rainfall.
Through the observation section of transient groundwater, it can be found that the overall and local stability coefficient of landslide decreases because the groundwater level increases obviously with the duration of rainfall: 8 days before continuous rainfall, the most dangerous sliding surface of the observation section is between 92 and 95 m; The local most dangerous sliding surface of the observation section is between 69 and 73m. This phenomenon matches the two-layer slip zone in ZK 1. After continuous rainfall, the shallow landslide is deformed, and the most dangerous sliding surface becomes shallow after corresponding rainfall 10 (see table 1).
Table 1 Rainfall Duration and Landslide Stability Table
In nature, rainfall is random, and the groundwater level of landslide changes with the infiltration of rainfall, so the overall stability of landslide is also dynamic. When the buried depth of groundwater level is h' < h, the fs of landslide is less than 1, and the landslide begins to deform and displace. When the depth of groundwater table is h ′ > h, the landslide is fs > 1, and the landslide returns to a stable state.
Because of this fluctuation, the displacement velocity of landslide may change, which is the deep-seated reason that Baijiapu landslide is relatively stable-deformed-relatively stable. This deformation phenomenon of Baijiapu landslide should be regarded as the external manifestation of the development cycle of landslide under the influence of external conditions (mainly rainfall) [12].
5 conclusion
According to the analysis of monitoring data, it is found that the change rhythm of Baijiapu landslide has a certain corresponding relationship with the rainfall time, and the amount of material movement of landslide in flood season is obviously greater than that in non-flood season.
The engineering geological model of Baijiapu landslide is established, and the influence of rainfall on groundwater seepage field and landslide stability is analyzed.
The research results of (1) show that the landslide is in a critical equilibrium state when the rainfall lasts for 3 days. At this time, the flow direction of groundwater generated by rainwater infiltration in the landslide body is basically the same as that of the sliding surface, and there is phreatic flow at the bottom of the sliding body.
(2) The transient groundwater level increases obviously with the rainfall duration, and the safety factor of landslide decreases.
(3) Baijiapu landslide presents a relatively stable-deformation-relatively stable rhythm, and its fundamental reason is the change of groundwater caused by rainfall.
refer to
[1] Yao Hailin, Zheng Shaohe, Li Wenbin, et al. Parametric study on the influence of rainfall infiltration on the stability of unsaturated expansive soil slope [J]. Journal of Rock Mechanics and Engineering, 2002,21(7):1034 ~1039.
Lin Lusheng, Jiang Gang. Discussion on slope stability analysis method considering rainfall infiltration [J]. Journal of Wuhan University (Engineering Edition), 200 1, 34 (1): 42 ~ 44
Chen Xiaoting, Huangbailin. Preliminary analysis on deformation monitoring of Baijiapu landslide in Xiangxi River basin [J]. South China Geology and Mineral Resources, 2004, (2): 55 ~ 58
Zhou pinggen Hydrogeological structural types of landslides [J]. chinese journal of geological hazard and control, 1998, (9) (Suppl.): 207 ~ 2 13.
Yan Echuan, Liu Guangrun. On the basic geological model of landslide [J]. Journal of Engineering Geology, 2004,12 (1): 21~ 24
Bing Xu. On engineering geological model [J]. Journal of Engineering Geology,1997,5 (3):199 ~ 204.
Zhang Junjie, Jiao Junjie, Yang Junjie. Study on In-situ Rainfall Infiltration on Slopes in Hubei Province [J]. Engineering Geology, 2000,57: 31~ 38.
Xing Jianmin. Analysis on the Relationship between Landslide and Rainfall in Accumulation Layer [J]. Journal of Yangtze University of Staff and Workers, 1999, 16 (3): 5 1 ~ 52
Lin, people. Study on the coupling relationship between landslide and rainfall [J]. Disaster Science, 200 1, 16 (2): 87 ~ 92.
Li, Zou Lizhi. Groundwater dynamics [M]. Changchun: Jilin University Press,1995:146 ~162.
Liu Hongxing Wang Yongping. Slope stability analysis with seepage force [J]. People's Yangtze River, 2002,33 (6):19 ~ 27.
[12] Chen shanxiong, chen shouyi. Systematic analysis method of unsaturated soil slope stability considering rainfall [J]. Geotechnical Mechanics, 200 1, 22 (4): 447 ~ 450.