IGC. 50 th. 50 th INDIAN GEOTECHNICAL CONFERENCE FINITE ELEMENT MODELLING OF FULL-SCALE DOUBLE-FACED VERTICAL REINFORCED SOIL WALL

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1 50 th IGC 50 th INDIAN GEOTECHNICAL CONFERENCE 7 th 9 th DECEMBER 205, Pune, Maharashtra, India Venue: College o Engineering (Estd. 854), Pune, India FINITE ELEMENT MODELLING OF FULL-SCALE DOUBLE-FACED VERTICAL REINFORCED SOIL WALL Anubhav, Prabir K. Basudhar 2 ABSTRACT Reinorced soil structures (slopes, walls etc.) are generally analyzed by suitably modiying the Limit Equilibrium Methods originally developed or unreinorced slopes and walls. However, with the use o such method it is not possible to predict the displacement and stress distribution within the reinorced soil mass. In order to predict the deormation o reinorced soil walls and slopes numerical techniques such as Finite Element Methods and Finite Dierence Methods are commonly adopted. In present study, inite element simulation o ooting on ull-scale model double-aced wall has been done using plane strain sotware Plaxis. For this purpose mode test results as available in literature (Tatsuoka et al. (992) or a ull-scale 5 m high geosynthetic reinorced soil walls subjected to ooting pressure were used. One side o the embankment consisted o continuous rigid cast-in-situ acing while other side acing was constructed using discrete panels. The reinorcements were placed near the wall aces over the entire height o the wall. The vertical spacing o the reinorcement layer was 300 mm and length was 2 m on either side. 3 layers o reinorcement were placed rom one end to the other (double aced wall). Data rom plane strain tests have been used to simulate soil behaviour in the inite element model. The backill sand and bottom sand cushion was modeled as a homogenous isotropic, linearly elastic material with Mohr-Coulomb ailure criterion with non-associated low rule. The reinorcement was modelled using elastic geogrid elements. Continuous rigid concrete acing has been simulated with linearly elastic 5-noded triangular elements. Plate elements have been used to simulate the discrete acing elements. These discrete concrete panels were laterally supported during construction phase. The lateral supports (props) have been modelled using Fixed end anchors (springs) or each concrete panel. The geosynthetic reinorcement, concrete acing and discrete acing elements were modeled as linearly elastic material. Footing is placed on top using plate element and on top o the plate; load is applied by creating distributed load in geometry model. The construction o the wall was done in stages, which is simulated by building model wall layer by layer and generating gravity stress till the inal height is reached. Ater construction, ooting is placed on top using plate element and on top o the plate; load is applied by creating distributed load in geometry model. Dy. General Manager, Solapur Super Thermal Power Project, NTPC Ltd., Solapur, India, anubhav@ntpc.co.in 2 Proessor, Civil Engineering, GLA University, Mathura, India, pkbd@iitk.ac.in

2 Anubhav & Prabir K. Basudhar Fig. Deormed Mesh at 400 kpa Footing Pressure The inite element analysis predictions are compared with the experimental results reported in the literature. FEM based predictions o load-deormation behaviour o ooting on large-scale model reinorced soil wall are in excellent agreement with experimental data. The predictions o deormation o acings are ound to be conservative. The pattern o tilting o ootings obtained rom the developed FE model, was also in good agreement with the experimental observations. Keywords: Geosynthetics, reinorced soil wall, strip ooting, inite element method, acing element

3 50 th IGC 50 th INDIAN GEOTECHNICAL CONFERENCE 7 th 9 th DECEMBER 205, Pune, Maharashtra, India Venue: College o Engineering (Estd. 854), Pune, India TITLE: FINITE ELEMENT MODELLING OF FULL-SCALE DOUBLE-FACED VERTICAL REINFORCED SOIL WALL Anubhav, Dy. General Manager, NTPC Ltd., anubhav@ntpc.co.in Prabir K. Basudhar, Proessor, GLA University, pkbd@iitk.ac.in ABSTRACT: In present study, inite element simulation o ooting on ull-scale model double-aced wall has been done using plane strain sotware Plaxis. The construction o the wall was done in stages, which is simulated in the model wall. Ater construction, ooting is placed on top using plate element and load is applied. The inite element analysis predictions are compared with the experimental results. FEM based predictions o load-deormation behaviour o ooting on large-scale model reinorced soil wall are in excellent agreement with experimental data. The predictions o deormation o acings are ound to be conservative but the pattern o tilting o ooting was in good agreement with the experimental observations. INTRODUCTION Use o geosynthetics as reinorcing elements in reinorced soil structures gained momentum, due to the problem o corrosion associated with metallic reinorcement. Geosynthetics are chosen primarily due to their ability to be engineered chemically, physically and mechanically to suit variety o geotechnical applications. With increase in height o the structure the cost o conventional type retaining walls (gravity, RCC etc.) increases very rapidly, as compared to the mechanically stabilized earth walls. Reinorced soil structures can tolerate much larger settlement and thus these walls can also be constructed on poor oundation soils. Being lexible structures, such walls can withstand seismic loading better than conventional retaining walls. Finite element method (FEM) and Limit equilibrium method (LEM) are the common techniques used to analyze reinorced soil structures (slopes, walls etc.). Using FEM it is possible to predict both lateral and vertical deormations and the associated stress distribution within the reinorced soil wall system, which is not possible with LEM. Some o the FEM studies on reinorced soil walls during construction and under surcharge loading are due to [], [2], [3], [4], [5]. FDM based sotware, FLAC, was used by [6], [7] or numerical modelling o GRS walls. A FEM analysis o ull-scale walls subjected to ooting loads with rigid concrete continuous acing and discrete panel acing or working loads was presented by [8]. The response o two tire geosynthetic reinorced segmental retaining wall under working loads was studied by [9]. Plaxis (FEM) was used by [0] or modelling o GRS wall under surcharge loading. For simulating behaviour o reinorced soil walls subjected to strip loading, Plaxis was also used by []. Tatsuoka et al. [2] presented a model study on a ull-scale 5 m high geosynthetic reinorced soil walls subjected to ooting pressure. The reinorcements were placed near the wall aces over the entire height o the wall and 3 layers o reinorcement were placed rom one end to the other (double aced wall). In the present study, Plaxis sotware was used to analyze the behaviour o this ull scale double aced reinorced soil wall loaded by ooting. The results o FE studies were

4 Anubhav & Prabir K. Basudhar compared with the experimental results available in the literature. PROBLEM STATEMENT Prototype (ull-scale) reinorced earth embankments were constructed using sand with 6% ine as backill material. For the construction o embankments, the sand was compacted to a dry unit weight o 4.62kN/m3 with 5.3% water content. The embankments were reinorced with a grid which consisted o members made o polyester with rectangular cross section o 0.9mm x 3mm and an aperture o 20 mm covered with PVC to increase durability [2]. One side o the embankment consisted o continuous rigid cast-insitu acing while other side acing was constructed using discrete panels (Fig. ). The continuous concrete acing is 300mm thick at the top and 570mm thick at the bottom. Fig. GRS Walls Test Section [2] The discrete acing was made using 6 mm thick pre-cast concrete units o size 600 mm x 600 mm. The vertical spacing o the reinorcement layer was 300 mm and length was 2 m on either side. Plain strain conditions were maintained in the embankment section. Embankment was 5.0 m high and 6.9 m wide and constructed by placing engineered backill soil in layers with gabions placed near ace o wall at each layer. Continuous rigid acing o delayed cast-in-place unreinorced concrete with two construction joints were then constructed. For other side, discrete concrete panels were laterally supported and placed in advance and then soil layer was compacted. Ater attaining the inal height lateral supports were removed. The embankment was loaded to ailure with a 2m 3m size ooting located at the centre o top surace. The model test or ooting load on ull-scale GRS wall briely described above has been modelled using Plaxis and results were compared with the experimental results reported in literature. FINITE ELEMENT MODELLING & ANALYSIS Construction o Numerical Model o Wall The geometry o the reinorced wall as shown in Figure has been modelled using input module. During the experiments plane strain conditions were maintained [2], thereore the same conditions have been simulated in the FE modelling and analysis. The boundaries o the dierent materials o wall were kept same as the prototype dimensions. Footing is placed on top using plate element and on top o the plate, load is applied by creating distributed load in geometry model. The discrete panel wall acing has been modelled using plate elements. These discrete concrete panels were laterally supported. The lateral supports (props) have been modelled using Fixed end anchors (springs) or each concrete panel. Rigid Concrete Facing Segment g Gabions 2m Footing 2m Reinorcement 2m Segmental Facing Segment h Props Fig. 2 Modelled Geometry and Finite Element Mesh The mesh is generated considering all elements in place by automatic mesh generation capability o the sotware. 5-noded triangular elements were used or modelling the oundation soil, backill soil, gabions and continuous rigid acing. 6-noded linear elements were used or geotextile and plate elements. The modelled geometry o the wall

5 50 th IGC 50 th INDIAN GEOTECHNICAL CONFERENCE 7 th 9 th DECEMBER 205, Pune, Maharashtra, India Venue: College o Engineering (Estd. 854), Pune, India section and inite element mesh used in the analysis are shown in Figure 2. Model Parameter Estimation The backill sand and bottom sand cushion was modelled as a homogenous isotropic, linearly elastic material with Mohr-Coulomb ailure criterion with non-associated low rule. Plane strain compression tests on reconstituted samples (prepared at a dry density and water content nearly identical to those obtained rom the block samples extracted rom test embankments) were perormed by [3] and reported by [8]. Hyperbolic model proposed by [4] was used to simulate the response o triaxial tests. In this model tangent modulus is expressed as: E t = R σ σ 3 ( σ σ ) 3 2 n σ K Pa P 3 a () where, K is modulus number, n is modulus exponent, P a is atmospheric pressure, R is ailure ratio deined as ( σ σ 3) ( σ σ 3) ult and ( σ σ 3 ) is the deviatoric stress at ailure. Deviatoric Stress (kpa) Test Data [3] Simulated Axial Stain (%) Fig. 3 Simulated Plane Strain Compression Test Results Figure 3 shows the simulated hyperbolic curves and plane strain compression test data using Equation () with K =380, n =0.6, R =0.9, φ 0 =37.9 and Δ φ =0.5. Deviatoric Stress (kpa) P a =0kPa, Axial Stain (%) Fig. 4 Predicted Stress-Stain Response o Kalpi Sand or σ = 3. kpa n 5 In case o this ull-scale wall the average overburden pressure works out to only 3.5 kpa. Using these parameters, stress-strain curve was developed corresponding to the average conining pressure prevailing in the GRS walls (Fig. 4). The conining pressure was obtained by using Jaky s ormula ( K0 = sinφ ) with average vertical stress ( σ n ) equal to 3.5 kpa. In Plaxis, secant modulus o elasticity at 50% o strength ( E re ) is taken as input parameter. Accordingly, E 50 is obtained rom the stress-strain curve. Similar technique was used by [] or obtaining soil parameters or FEM modeling o ooting over reinorced sol wall. The model parameters or reinorcement and gabions as reported by [8] have been considered in the present analysis. The reinorcement was modelled using elastic geogrid elements. The stiness (EA) o the grid reinorcement is considered as 240kN/m. The gabions were simulated with 5 noded triangular elements and linearly elastic model was used. An elastic

6 Anubhav & Prabir K. Basudhar modulus o 2700 kpa and Poison s ratio o 0.3 was selected or gabions. Continuous rigid concrete acing has been simulated with linearly elastic 5-noded triangular elements. Modulus o elasticity and Poisson s ratio 8 or concrete has been considered as. 5 0 kpa and 0.7, respectively. Plate elements have been used to simulate the discrete acing elements. A 6 linearly elastic model with EA = 9 0 kn/m, EI=2700 kn-m 2 /m and ν = 0.7 have been considered or plate element. Simulation o construction and Footing Load Simulation o the construction o the wall was done in stages as ollowed during the actual construction. Ater the creation o the initial geometry, the stage construction technique is used. During actual construction, on continuous rigid acing side no temporary support acing was used except gabions whereas on discrete element panel acing side lateral supports were used till the completion o wall; there ater the supports were removed. To simulate this construction sequence irst the gabions (both sides), discrete panel (plate element) with prop (Fixed end anchor) or the irst layer were activated and gravity stresses were generated. The model wall was built layer by layer till the inal height is reached. No compaction was simulated. Ater activating last layer, concrete rigid acings were activated and props were deactivated (simulation o removal o temporary support) to complete the construction sequence. Vertical Displacement o Footing (cm) Average Footing Pressure (kpa) hexp. [2] Analysis Fig. 5 Comparison o the Observed and the Predicted Vertical Settlement o the Footing with Average Footing Pressure Ater completion o construction sequence plate element representing the ooting was activated. The load was applied in steps o 50 kpa pressure on the top o ooting. The deormations at the centre o ooting and at the middle o the wall ace on discrete element acing side were observed or each loading. Average Footing Pressure (kpa) Wall Displacement at Mid Height (cm) hexp. [2] Analysis Fig. 6 Comparison o the Observed and the Predicted Wall Displacement at Mid Height with Average Footing Pressure The predicted variation o vertical settlement o the ooting with average ooting pressures using FE has been compared with the observed data in Figure 5 and the comparison shows excellent agreement. Fig. 7 Horizontal Deormation Contours at 400 kpa Footing Pressure

7 50 th IGC 50 th INDIAN GEOTECHNICAL CONFERENCE 7 th 9 th DECEMBER 205, Pune, Maharashtra, India Venue: College o Engineering (Estd. 854), Pune, India Figure 6 shows the observed and predicted horizontal displacement o the wall at mid height with average ooting pressure. The predicted deormations o the wall were ound to be higher as compared to the observed ones. Figure 9 shows the scaled up deormed shape o mesh ater application o 400 kpa ooting pressure. In this case also it can be seen that predicted direction o settlement o the ooting is similar to the observed ones. Horizontal deormation contours or the wall have been plotted in Figure 7. It is observed that maximum deormation occurred at the top o the wall (discrete element acing side), however, during the experiments maximum horizontal deormation was observed near the bottom o the wall (Fig. 8). The deormation observation o the continuous rigid acing was not available in the literature hence it could not be compared with predicted ones. Fig. 0 Incremental Shear Strain Contours at 400 kpa Footing Pressure Incremental shear strain contours have been plotted in Figure 0. It is observed that at 400 kpa o ooting pressure a nonlinear ailure surace initiated rom edge o the ooting near discrete element acing and runs near the base o the ooting. Fig. 8 Deormations at 400 kpa Average Footing Pressure [2] Fig. 9 Deormed Mesh at 400 kpa Footing Pressure (Section g-h ) CONCLUSIONS The plane strain inite element simulations o ullscale double aced wall subjected to ooting load was done. Based on the above inite element analysis, the ollowing conclusions can be drawn:. FEM based predictions o load-deormation behaviour o ooting on large-scale model reinorced soil wall are in excellent agreement with experimental data. 2. Soil parameters obtained rom plane strain test should be used in FE model walls or good predictions. 3. The predictions o deormation o discrete acing elements are conservative. 4. The pattern o tilting o ootings obtained rom the developed FE model, was also in good agreement with the experimental observations.

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