Application Study of FLAC in Analysis of Slope Stability

Transcription

1 Application Study of FLAC in Analysis of Slope Stability HAO Fengshan, WANG Lei College of Civil and Traffic, Liaoning Technical University, Fuxin, Liaoning Abstract: FLAC is a numerical analysis software which applied widely in geotechnical engineering. Theoretical basis and specific calculation steps of FLAC are introduced. The problems like model establishment of FLAC numerical analysis software, numerical calculation, result analysis and so on are discussed combine with a loess landslide in Liaoning province. The paper offered the proposals on the slope control. Keywords: FLAC, Rock slop, High slop reinforcement, Calculation mode, Stability 1 Introduction Hongtoushan copper mine is located in Qingyuan county, Liaoning province. The company is an affiliated enterprise in CNMC company that founded in As the scale of the production increased, the enterprise needs to build the new factory. After repeated site selection and argumentation, the place was seleted in Hongtoushan town. Its leagth from north to south was about 300m and its width from east to west was about 140m. It is builded smoothly after excavating and piling. According to preliminary scheme, the height of cut slope reaches 62m and the cut slope belongs to extra-high rock slopes. There are 220 -million -volts UHV transmission lines on the rock slope and there are auxiliary production facilities at the bottom of the rock slope. The stability and security of the rock slope is very important for the operation safety of the copper mine. Therefore, the study of stabilization after excavation of the extra-high rock slope is of great significance. This paper calculates stability and supporting force of the rock slope by using the transmitting coefficient method and analyses reinforced stabilization of the rock slope with the FLAC, a kind of finite element numerical analysis software. The research result provides reliable references for slope reinforcement and I hope to promote the application of FLAC in the extra-high rock slope engineering through this paper [2]. 2 Program Introduction FLAC is a numerical modeling code for advanced geotechnical analysis of soil, rock, and structural support in three dimensions. FLAC is used in analysing, testing, and designing by geotechnical, civil, and mining engineers. It is designed to accommodate any kind of geotechnical engineering project where continuum analysis is necessary [3]. FLAC3D utilizes an explicit finite difference formulation that model complex behaviors are not readily suited to FEM codes, such as problems that consist of several stages, large displacements and strains, non-linear material behavior and unstable systems (even cases of yield/failure over large areas, or total collapse). 3 Engineering Geological Conditions The rock slope is located in North China platform. Geology: tectonics is a mid-late Paleozoic uplift of the land into the North China platform. The altitude of the rock slope is between 289~308m. It intends to choose the spine natural slope gradient of the places averages about between 25~35. The geology topography and drilling result showed that the places of rock slope are the more complex geology. The cladding of the surface soil is bacially consist of plow plant soils and gravel soils. Topographical features see figure 1. There are three main rock from left to right of the bedrock beneath: metamorphic siltstone (T3zh-4); carbonaceous siltstone (T3zh-3): carbonaceous phyllite (T3zh-2); sand phyllite (T3zh-1).By method Investigation, plant measurement and study on documents, the wide of fault zone (F1) is about 1m with 3

2 3) EASTERN ACADEMIC FORUM the grayish-black carbonaceous gouge. Although the slope is rock, the slope is mainly consists of carbonaceous siltstone and carbonaceous phyllite. In addition, folds and joints also developed in the slope. Each layer of physics and mechanics parameter selection see table 1. The unloading rock deformation and loose was seriously influenced by pressure or weight of the slope. A large scale of dynam-relaxed rock lies in the slope site. So, such key matter as the stability of the slope and safety for transmission lines and others are very important [3]. Table 1 The physical and mechanical parameters of saturated rock slope Rock Type Unit Weight Compression Poissen's Ratio Shearing Indicators Cohesion Angle γ(kn/m E S (GPa) µ C(kPa) Φ( ) T3zh T3zh T3zh T3zh F Slope Stability 4.1 Computional model According to the preliminary design blueprint of the rock slope, the design slope ratio is 1:0.75 and the height of the slope is 62m. Roughly divided into three steps : firstly, the maximum vertically height of the slope excavation is about 22m. Secondly, the vertically height of the slope excavation is about 20m. Thirdly, the vertically height of the slope excavation is also about 20m. The altitude of the slope bottom is consistent with Planning altitude. The upper and partial terrace of the rock slope has big density and small extensions. Its structure is dense and its cutting surface is weak. The lower terrace of the rock slope has big density and small extensions. Its structure is dense and its cutting surface is soft qualitative. Therefore, it is appropriate [4] to use the slip curve surface pattern to calculate the stability of slopes. It uses the transmitting coefficient method to compute the safety factor, surplus decline force and supporting force. Its theoretical formula as follows: R j ψ j+ R Pi = wi sinαi+ Pi 1ψ i 1+ αwi cosα i n i= 1 j= 1 k = (1) ( wi cosαi αwi sinαi Ui) tanϕi+ cili (2) T ψ + T F j j n i= 1 j= 1 In this formulation: α earthquake influence coefficient k safety factor n sum of potential sliding w weight R and T resist slippery and surplus decline force P and Ψ decline force and delivers coefficient s 4

3 Fig.1 The distribution feature of the bottom excavation slope According to geological structure, lithology, weathered states, terrace structure, unloading loose state and numerical calculation regress, we can receive the factor of the distribution of rupture zone. This zone may constitute a potential sliding surface. With the addition of the load of transmission lines tower, we can use the limit equilibrium and limit analysis calculation to determine the slope as a whole and every bench local potential slide. We establishe design ratio of of the slope model is 1:0.75. The calculation model of the whole slope see figure Stability analysis There are three different working condition: 1 the natural condition; 2 the natural and heavy rains condition;3 the natural and earthquake condition. The Level peak acceleration of seismic fortification is 0.15g. The effect of the vertical earthquake acceleration is not taken into account. According to The Code for Prospecting the Geological Engineering and the situation of the rock slope, when the safety factor is identified 1.25, the rock slope is analyzed with limit equilibrium method combined with FEA under conditions of natural heavy-rain and earthquake. Their Supporting forces of the rock slope are 625kN/m and 795kN/m. The Results are in table 2. Fig.2 The calculation model of the whole slope 5

4 Table 2 The stability coefficient and support force of the whole slope Supporting Decline Safety Force Condition Factor Force kn/m kn/m Strengthening method According to numerical results of its slopes formation and limit equilibrium method, compound structure of RC lattice frame beam and prestressed anchor is very applicable to practice. The prestressed anchorage is 500kN and the verticalinterval and horizontal spacing of the anchor line is designed for 5m. The verticalinterval and horizontal spacing of lattice frame in accord with the prestressed anchorage, its width is 0.3m. 5 Deformation and Numerical Analysis of Slope Stability 5.1 Simulation model This paper takes the linear structures as the research object, so using the linear computation can solve the problem existing in the flat structure. According to the mechanical process of process in rock excavation, the boundary conditions of the model uses the fixed displacement boundary condition [5]. Given the limits of computation of rock gravity, the tower gravity affects the bottom position of rock. The length of the anchor is average 30-metres, the pull in the prestressing cables is average 500N. There are four main rock from left to right of the bedrock beneath : metamorphic siltstone; carbonaceous siltstone: carbonaceous phyllite; sand phyllite. The physical and mechanical parameters of saturated rock slope see table 1. construction of geometric model see figure 3. Fig.3 The numerical modeling of slope 5.2 Stress and displacement analyses The distribution characteristics of slope stress can be see in figure 4~6. Here is that: the maximum main stress in the slope is pressure stress. Maximum principal stresses occur in the deeper of bottom border. The maximum is roughly 5MPa. The maximum main stress in the slope surface is roughly 3kPa~5kPa. Faults makes the maximum main stress of rock slope different, the maximum main stress within the fault zone could have reduce. 6

5 Fig.4 The maximum principal stress distribution of the reinforcement slope The results show that the minimum main stress includes compressive and tension stress. The maximum compressive stress intensity appears in the bottom borders of the rock slope. The maximal of compressive is 0.9MPa. The maximum tension stress intensity appears in the top border edge position. The value of the maximum tension stress is 150KPa. Such tensile stress is within prestressed anchorage control [6], and it cannot inflict tension damage. Faults makes minimum principal stress distribute widely, while minimum principal stress of faults reduce. Compared with the maximum main stress, the value changes very small. The vertical stress of the rock slope is compression. The maximum compressive stress occur in bottom border. The maximum value is roughly 5.6MPa. Fig.5 The minimum principal stress distribution of the reinforcement slope The vertical stress of the rock slope surface is roughly 0. The distribution of vertical stress express a certain rule. The vertical stress increases with depth of the rock slope. Bad connection conditions will be good for the slope stability. The crustal stress analysis indicate that the reinforced slope are no flouted. Fig.6 The vertical stress distribution of the reinforcement slop The displacement of slope see figure 6.The displacement of slope is concentrated in the top of the rock slope, which will not be a threat to the high slope stability. The maximum horizontal displacement of slope is roughly 4.8cm, and the maximum displacement is not over 5.3cm. It shows that maximal displacement is mainly composed of horizontal displacement. So, it is good to adopt bolt to support rock slope. 7

6 6 Conclusions and Suggestions Fig.7 The displacement distribution of the excavation slope Using the FDM code FLAC, the long-term behavior of rock slope is simulated, and modeling the slope in FLAC software. The engineering case analysis shows that the application of this model in evaluating slope stability can achieve good effect. The engineering example indicates that the new assessment method is reasonable and feasible, and it provides a new idea for slope assessment. Finally, the safety factor of the reinforced slope is calculated, and the result shows that the calculated indexes can meet the requirement of the specifications. References [1]. Itasca Consulting Group, Inc.FLAC (Fast Lagrangian Analysis of Continua) Online Manual [R]. Itasca Consulting Group, Inc [2]. LIU Bo, HAN Yan-hui. The principle, example and application guide of the FLAC program [M]. Beijing: China Communications Press, 2005: 5 25 [3]. GriffithsDV, Lane P A. Slope stability analysis by finite elements [J]. Geotechnique, 1999, 49(3): 387~403. [4]. CHEN Zhongyi, ZHOU Jingxing, WANG Hongjin. Soil mechanics [M]. Beijing: Tsinghua University Press, [5]. LIANG Qingguo, LI Dewu. Discussion on strength reduction FEM in geotechnical engineering [J]. Rock and Soil Mechanics, 2008, 29(11): [6]. ZHOU Cuiying, LIU Zuoqiu, DONG Liguo, et a1.large deformation FEM analysis of slopes failure [J]. Rock and Soil Mechanics, 2003, 24(4):