Deep Foundation Pit’s Fluid-Solid Coupling Analysis of Seepage Under Non-Saturated Conditions
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Deep Foundation Pit’s Fluid-Solid Coupling Analysis of Seepage Under Non-Saturated Conditions
Physical and Numerical Simulation of Geotechnical Engineering 1st Issue, Sep. 2010 Fluid-Solid Coupling Analysis of Deep Foundation Pit’s Seepage Under Non-Saturated Conditions CHENG Wei, HE Xiang School of Civil Engineering and Architecture, Wuhan Polytechnic University, P.R. China, 430023 [email protected] ABSTRACT: The phenomenon which micro-artesian seepage effects on foundation Pit with rainfall is a typically non-saturated seepage process. However, underground water seepage and rainfall infiltration during the deep foundation pit’s excavation and support exert a significant impact upon the stability of engineering itself. The authors have given some countermeasures after modeling and calculating of this phenomenon and make an explanation of problems which confronted during deep foundation pit’s excavation and support by relevant theories. Conclude the flow path and velocity around foundation pit through calculation under non-saturated conditions. Then it combined the predictions and practical applications, proposed correspondingly protective measures. KEYWORDS: Fluid-solid coupling, Deep foundation pit’s seepage, Non-saturated conditions 1 INTRODUCTION Since the elastic modulus of soft ground is small and the intensity of the soil itself relatively low, it will be easily lead to foundation pit’s global instability during its excavation and support process. Yet during the excavation in domestic coastal areas, disposal of underground water and waterproof or diaphragm retaining walls remains to be improved. Particularly rainfall and its infiltration during excavation and support of foundation pit. It will significantly decrease the shear strength of rock and soil, and then effect on the stability of engineering itself. And the effect that seepage and deformation have on the bottom of the foundation pit will cause the bottom of foundation pit swell or piping erosion [1], and threaten to the stability of foundation pit. Foundation pit with different geological conditions have a great difference of permeability and boundary conditions, thus the pore pressure of foundation pit and effective stress with the action of its pore pressure is different. The influencing factors of foundation pit are variety, such as design depth of diaphragm retaining walls and block about underground water during the excavation and support of foundation pit is very important. Foundation pit’s excavation is meanwhile a stress release process. Excavate a foundation pit from a geological balance plain will change the stress structure of around environment. In order to avoid the advert effects to the surrounding production and construction after long period of foundation pit’s excavation, it’s necessary to observe and experiment of the seepage field and stress field in time, and seek out its change law. Because of the underground water seepage effect, there will be deformation around foundation pit. It is useful to control and ameliorate the poor state of foundation pit that make a fluid-solid analysis to the seepage and stress deformation around foundation pit. Nowadays, the main issues of domestic deep foundation pit focus on: (1) Dynamic process of stress release; (2) The deformation for seepage, precipitation stability of foundation pit. (3) Soil liquefaction for dynamic load, and the uneven sedimentation under effect of layer seepage. © ST. PLUM-BLOSSOM PRESS PTY LTD The analysis on the seepage field and stress field which micro-artesian seepage effects on foundation Pit with rainfall under non-saturated conditions is based on the analytic calculation of correlation model. 2 THEORY ANALYZE 2.1 The mechanism of groundwater migration’s impact on the stability and deformation of foundation pit Between two impermeable layers, as soon as the foundation pit was excavated, confined water have a water head. The groundwater starts to migrate under the pore pressure effect. This effect made hydraulic pressure gradually strengthen along with the increase of foundation pit depth directly. The water head was loss as well. Since the pore water pressure is lower than steady water pressure in certain area out of the foundation pit, the situation in the bottom of foundation pit is reverse. And this caused the groundwater seepage into the inside of the foundation pit. At the same time, there will be an opposite trend of water pressure and flow way of underground water between two sides of the wall. This will be made an induced cause to unstable foundation that is excavation. Combined with the flow characteristics of underground water, if the foundation located in river or lack where contain abundant water, the flow size will be ascent, and there should be some appropriate emergency measures. Moreover, with different terrain, the characteristic and strength of soil must be different, and the impression that water seepage to the foundation stable should be under the premise that doing a serials of cogent and detail experiment previously. While the basic principle is similar, are due to the underground water flow for water head difference, and then cause the dynamic excavation problems of deep foundation pit. Because of timely supplement rainfall, the underground water level is not determined. With different rainfall intensity, the quantity of underground water float range will Fluid-Solid Coupling Analysis of Deep Foundation Pit’s Seepage Under Non-Saturated Conditions DOI: 10. 5503/J. PNSGE. 2010. 01.007 be different. And the circumstance that near river or lake area, must be considered independently. When it rains at certain time after the foundation pit have been excavate, there will be a great impression on the integrated support of the foundation. It is prone to piping if we didn’t have reinforce interface that initial stress release or some correspondingly drainage treatment of it, meanwhile, consider the alter of stress during this non-saturated process, and the change of underground water seepage and pore pressure that results from it, there will be a certain deformation along with it in a significant area. And in the process of raining, there will be hint of remain rainfall and seepage water in the foundation pit, and it is the micro-artesian. The seepage process under foundation pit is complicated under the micro-artesian effect. u a is pore air pressure; and u w is pore water pressure; (u a u w ) is soil suction; is a constant between 0 and 1. b) Oberg and Sallfors proposed the shear strength forecast equation of non-saturated soil as follows[6]: c ' ( u a ) tan ' (u a u w )( S ) tan ' (5) And S is the saturation in this equation. 2.2.3 Considered non-saturated condition, the calculate equation of permeability along with pore pressure change is as follows[7]: K rs* K r ( S ) K rs Kr 2.2 Relevant theories of non-saturated soil 2.2.1 The relationship between soil suction and its saturation. Because of rock and soil is porous media, the total pore pressure must be lower than zero under non-saturated condition, and the negative value of the total pore pressure reflect the soil suction. The saturation of soil maintains in a range for the effect results from soil suction. Assignation the absorption and dehydration curve previously, and the alter law between these two. The inside seepage of non-saturated soil obey Darcy’s law, the solid and liquid coupling by strain and pore pressure. The coupled mathematical equations of saturated-unsaturated seepage of rock and soil are as follow after referenced by multiphase seepage theory of porous media[2,3,4]: ij' , j (Sp ij ) j f i 0 pore ' ij ij is valid stress; pressure; saturation; kk fi is is volume strain; derivation of the x coordinate; derivation of time t; v F k H H (K x ) (K y )Q 0 x x y y S Q is water flow. 2.3.2 Transient seepage equation H H H (k x ) (k y ) ct (9) x x y y t K x , K y are permeability in x and y directions, unit is m / s ,H is total water head, H ( x, y, t ) . H ( x, y, t ) C H 0 ( x, y, t ) is p is H 0 ( x, y, t ) the ( )j stands for the ( ) stands for the (10) is given water head boundary, with t=0, initial water head is H 0 ( x, y) . 2) Water flow boundary k is Darcy’s velocity; and the calculate equation of c is as follow: ns ds (3) k ' dp ' And c is storage constant; n is porosity; k modulus; s is the function of pore pressure p ; (8) 2.3.3 Boundary conditions 1) Water head boundary is kronecker constant; force; (7) 2.3.1 Steady seepage equation (2) volume (S S r ) 3 1 3/ 2 2 (1 S r ) 3 p 1 F 2.3 2-D seepage equations[8] (1) S vkF,k cp 0 (6) H n C q(h, x, y, t ) (11) 3 SEEPAGE-STRESS ANALYSIS OF DEEP FOUNDATION PIT UNDER INFLUENCE OF MICRO-ARTESIAN c is bulk 3.1 Rainfall infiltration boundary conditions Mein and Larson[9] using rainfall intensity q, the soil allowable infiltration capacity f P , and the permeability 2.2.2 The strength theory and reference computer equations of non-saturated soil are as follows: a) In 1959, Bishop proposed the strength equation of non-saturated soil to single stress state variables [5]: when soil is saturated K rs . When it meet the condition: f P q K rs , all of the rainfall is infiltration c ' [( u a ) (u a u w )] tan ' (4) ' In the equation, is shear strength; c stands for ' valid cohesion; stands for valid internal friction angle; into soil and the capacity is decrease along with the depth increase. 44 Physical and Numerical Simulation of Geotechnical Engineering 1st Issue, Sep. 2010 a , r are gas 3.2 Modeling and calculating K rs is This example is using 2-D finite element method to do the numerical calculation of foundation pit. All monitor area is the seepage range, or the flow region of the groundwater. The bottom, left, and right boundary and diaphragm retaining walls boundaries have a constraint. Considered the permeability of soil is changed along with the change of saturation, we defined a set of data based on the following two equations: pressure and water pressure in soil, and ar K rs [ar (br ( a r )) cr ] (S n S i ) Su Si [at (bt ( a t )) ct ] Kr permeability when soil saturated. at , bt , ct are material coefficients, saturation. S n is maximum saturation. Si ar , br , cr , is residual The breath of foundation pit is 3m, depth 20.32m, height 52.46m,using whole breath 200m, therefore can consider large range, and after excavation, the diaphragm retaining walls depth is 25m, thickness 0.1m, elastic modulus 10.8 MPa , Poisson’s ratio 0.3, cohesion c' 16 KPa , friction angle 30 degree. Model in mesh as follow: (12) (13) Figure .1 Simplified diagram of model After parameter setting, the calculation results are as follows: Figue.2 Distribution of pore pressure Vertical effective stress distribution: Figue.3 Distribution of vertical effect Horizontal and vertical displacement: 45 Fluid-Solid Coupling Analysis of Deep Foundation Pit’s Seepage Under Non-Saturated Conditions DOI: 10. 5503/J. PNSGE. 2010. 01.007 Figue.4 Horizontal displacement Figue.5 Vertical displacement Displacement vector graph: Figue.6 Velocity of flow vector graph level and the wall depth. The finite element analysis of this process will be discovered that the characteristic and saturation of soil will be change gradually along with the infiltration degree of rainfall. And it will take a gradually difference of underground water infiltrate feature, then the following change of porosity and permeability of soil. It can also predict the relative change trend graph thorough the calculating date that the stress and strain change of foundation element and node, or the valid stress path of node. Combine these date, we concluded that the stress and strain of element will be change when we give different rainfall intensity and endure time, and the result suit to the rainfall infiltration conclusion. 4.1.2 The finite element analysis of soil stress and strain change: In terms of the analysis of date that node or element displacement internal soil, we can concluded stress path of every node and total stain trend. We can also observe the total strain in different saturation and pore pressure, and the potential region of initial destruction and adverse geological phenomena, or see the impression area integrity. Finite element method will be accurately calculated the date of stress and strain of every node and element, then doing a stress and deformation analysis with these results. With the stress path that the method had been calculated, we can forecast the strain of soil under certain force circumstance. And then forecast the general trend of soil 4.1 Analysis the numerical simulation of the fluid-solid process In figure 1 and 2, the displacement in the bottom and near area of the pit is comparatively large, and hydraulic gradient in the bottom of pit is very large, the soil around the pit start to swell and its intensify is reduce for the gravity and static water pressure effect. From the vector graph, we can see the discrepancy of velocity between inside and outside foundation pit area clearly. As the main drainage orientation, the water pressure and velocity of flow inside the pit is larger than outside any more, meanwhile, inside the pit, there is a upward seepage. The effective stress decrease gradually, and in certain situation, it will evolve to piping. 4.1.1 Analysis of foundation rebound and surrounding surface subsidence Overall, since the frequently flow of underground water, there formed a large area of displacement and deformation, and red stands for positive displacement, blue area stands for comprehensive deformation region in terms of underground water flow effect. When it effect for a period of time, the stress concentrate in downside interface of the wall, inside of the foundation there are uplift deformation. The calculate result reveal that the rebound float of foundation pit relatively large, and it determined mainly by porosity of soil and the relationship between groundwater 46 Physical and Numerical Simulation of Geotechnical Engineering 1st Issue, Sep. 2010 strain. The last project we must accomplish is that reasonable projection the might phenomena during foundation pit excavation in a certain circumstance. rational answer about these questions. (1) The soil strength is significantly relevant to its pore pressure and permeability under the rainfall infiltration non-saturated condition. It is benefit for foundation pit’s devise of support program and construction that combine with rainfall situation and forecast the dynamic change of underground water. (2) Use the practical computer equation of soil under non-saturated condition enable us make a detail analysis of the dynamic change path of stress and strain during rainfall and filtration process. However, it need further research in these issues: Stability of the foundation pit under different rainfall intensity and stability in different kinds of layer and soil properties which this paper didn’t calculate. 4.2 Proposal to the deep foundation pit’s excavation and support 4.2.1 Great importance should be attached to the change of the groundwater seepage and we should avoid the rainy season and make good support when excavate foundation pit. Make a prediction about seepage property in a certain period before excavation, and seek out the best support way of this time. Similarly, make a discussion about the benefit way to excavation in different location. 4.2.2 When the aquifer thickness is comparatively large and the same as permeability, the conventional method is combine using diaphragm retaining walls and grouting curtain, but rarely used vertical back cover. And the grouting curtain is a good way to block the groundwater under the bottom of foundation pit. 4.2.3 The depth that diaphragm retaining walls into rock layer should be enough, however, for economic consideration, how to determine the best depth should be based on technology and the permeability of environment that foundation pit locate. In the bottom of the foundation pit, intensity is not necessary but we must ensure that the waterproof ability of diaphragm retaining walls is sufficient. Meanwhile, we should consider the environment property around foundation pit when it drainage. This work should be based on the ample environmental investigation in all, and then make a rational distribution. REFERENCES [1]. [2]. [3]. [4]. 5 CONCLUSION [5]. The problem that groundwater seepage during the excavation and support of foundation pit continuously the key issue of foundation pit excavation. Based on the abundant investigation of around environment, we can predict the seepage field of groundwater and its change law before construction through numerical calculation. It will provide valuable reference material to determine the excavation method in time, to choose the rational program consider economical factors. Deep foundation pit’s excavation in rainy season will form differently seepage situation. Under water seepage effect, there will be comprise displacement and sedimentation in large field, and reduce the foundation pit’s stability. We give the [6]. [7]. [8]. [9]. 47 C.Yan, X.S.Li etc. The FEM Analysis of deep foundation pit’s stability under influence of micro-artesian, Site Investigation Science and Technology, 2006, 2: 3~7 (in Chinese) S. E. Cho, S. R. Lee. Instability of unsaturated soil slopes due to infiltration [J]. Computers and Geo-technics, 2001 (28): 185-208. Fredlund, D. G. and H. Rahardjo. Soil mechanics for unsaturated soils American Society of Civil Engineers, 1993. WANG Zhi-cheng, WANG Peng and etc. Numerical Simulation of Unsaturated Soil Slope Stability Coupling under Rainfall [J]. Subgrade Engineering, 2010 (2): 26-28 (In Chinese) Bishop A.W. The principle of effective stress[M]. Teknisk Ukeblad. 1959, 106 (39): 859-863 Fredlund D.G. and Anqing Xing. Equations for the soil-water characteristic curve [J]. Canadian Geotechnical Joumal. 1994, 31: 521-532. A. Truty, A Galerk in/least-squares finite element formulation for consolidation [J]. International Journal For Numerical Methods In Engineering, 2001 (52): 763-786. Kong Xiangyan. Advanced Mechanics of Fluids in Porous Media. Hefei: University of Science and Technology of China Press, 1999: 52~56 (in Chinese) R.G., Mein and C.L., Larson. Modeling infiltration during a steady rain. Water Resource, Res.vol.9 NO.2 384~394, April 1973