International Conference on GEOTECHNIQUES FOR INFRASTRUCTURE PROJECTS 27th & 28th February 2017, Thiruvananthapuram
Numerical Modelling of MSE Wall Using Pseudodynamic Method Shilpa S Vadavadagi
Chidanand M Jadar
Asst. Professor, Civil Engg. Dept. Alvas Institute of Engineering and Technology Mijar, India
[email protected]
Asst. Professor, Civil Engg. Dept. Acharya Institute of Technology Bengaluru, India
[email protected]
Abstract— MSE wall falls under the category of internally stabilized walls. The engineering essence of providing MSE wall is to give lateral support to the unstable soil and by reinforcing material into the soil strength can be increased. MSE walls have the advantage of being cost effective, reliable and constructible. The reinforcing material may be geo synthetic material or metallic strips. In the present study MSE wall has been analyzed for pseudo dynamic condition with two different soil backfills. The soil backfills used are c-phi soil and silty soil. Geosynthetic material geogrid has been used as reinforcing material into both the backfills. Spacing of the geogrid stiffness is varied from 0.25m to 1m with the geogrid spacing interval of 0.25m. For the same geogrid stiffness are also varied from 500kN/m to 2500kN/m with geogrid stiffness interval of 2500kN/m. The software used for MSE wall analysis is Plaxis V 8.2. Horizontal, Vertical and Total displacements for both the backfills have been analyzed and compared the same with both. Percentage reduction graphs again for both backfills are drawn and compared. Reduction in displacements with respect to the unreinforced backfills to the reinforced backfill are analyzed and compared. Keywords— MSE analysis; Plaxis V8.2.
Wall;
Geosynthetics;
Pseudo-dynamic
I. INTRODUCTION Development of soil retention systems have been evolved since four or five decades. Gravity retaining walls, reinforced concrete cantilever etc. are classified as externally stabilized retaining walls. Whereas Metal strip walls or geotextile reinforced retaining walls as internally stabilized retaining walls. In the former case, structural wall itself takes the surcharge loads as well as the weight of the backfill. In the latter case soil reinforcement is given by horizontal laying of the reinforcing materials. The lateral pressure is taken as by the interaction between soil and reinforcement. Stiffness of the soil reinforcement can be varied as required. Thus, here the volume of the concrete can be minimized [1]. In case of Pseudo static case of analysis of MSE wall, the cohesion and adhesion increases coefficient of seismic passive pressure increase but
decreases with increase in unit weight of the soil, surcharge and height of wall [12]. there was an increase in seismic passive earth pressure with increase in adhesion, soil friction angle, cohesion and wall friction angle [11]. [6], calculated seismic active and passive coefficients. Both are found to be safe approach for retaining wall design against the devastating effect of earthquake. Formulated new formula to calculate seismic earth pressure on the inclined retaining wall using horizontal slice method and some limit equilibrium assumptions [10]. Retaining wall with relieving plate have high stability and require less masonry quantities. It is suggested to use the soil with low specific gravity with high cohesion and internal friction angle [8]. For unreinforced retaining wall, Translation is observed in the pseudo dynamic case and rotation is observed in pseudo static case [14]. Behavior of MSE walls with several geo-synthetic straps is compared with the metallic one and it is found that the use of geo-synthetic straps induces more deformation of the wall but a higher safety factor, [2]. Geotextile reinforcement use may be used to improve the embankment constructed over ghat-road and combined use of geotextile reinforcement and light weight fill for improved performance of embankment over hilly terrain [5]. Increase in number of reinforcing layers decreased the dynamic earth pressure and lateral displacement. Increase in length of the reinforcing material also decreased the earth pressure for both the cases [4]. Geotextile reinforced retaining wall perform well for the earthquake excitation of less than .5g, [10]. Increase in cohesion leads to decrease in displacement of the wall [15]. Increase of cohesion led to the decrease in permanent displacement of earthquake induced retaining wall [3]. MSE walls with Geo-synthetic reinforcement are the cheapest of all the types of reinforcements considered and the RCC walls were found to be the costliest walls. II. DEFINITION OF PROBLEM Two tier wall of height is 22m with each tier height 11m. Two soil backfills used are c-ϕ soil and silty soil. Geosynthetic material geogrid as a reinforcement is being reinforced with the
Vadavadagi & Jadar spacing interval of 0.25m spacing from 0.25m spacing to 1m spacing. And with the geogrid stiffness from 500kN/m to 2500kN/m with stiffness interval of 500kN/m. The uniform surcharge load of 20kN/m2 is distributed over the wall with soil backfill and georgrid reinforcement. The MSE wall analysis is carried in plaxis 2D. Horizontal, vertical and Total Displacements for bothe backfills are analysed and compared. Percentage reduction for both the backfills are calculated.
Fig.1, Typical numerical modelling of MSE wall in pseudo-dynamic condition
B. Case-1: c- ϕ soil
Horizontal Displacement (mm)
c-ϕ soil Mohr Coulomb -
EA (kN/m) EI (kNm2) γdry (kN/m3) Poison’s ratio Cohesion (kNm2 Angle of internal friction, ϕ o Young’s modulus (kN/m2)
Silty soil Mohr Coulomb -
-
-
18 0.33 10 25
16 0.3 1 30
6000
8000
MSE wall Elastic 1.050E+1 0 8.7505E+ 8 7.2 0.150 -
Table.2, Geogrids parameters Parameter
Geogrid
Stiffness (kN/m) 2500 2000 1500 1000 500
From fig.2, it is noted that, at EA=1500kN/m, for the increase in the spacing from 0..25m to 0.50m, 8.8% of increase is found in the horizontal displacement. It is also being noted that decrease in the stiffness of the geogrid increases the displacement in the horizontal direction. At 1.0m of geogrid spacing, for the decrease of EA=2000kN/m to 1500kN/m, horizontal displacement is increased for about 2.1%.
60 40 500 1000 1500 2000 2500
20 0 0.25 0.5 0.75
1
Spacing (m)
Fig.2, Variation of horizontal displacements at kh=0.3, kv=0.3 for c- ϕ soil 250 0
Vertical Displacement (mm)
Parameters Model type
80
Stiffness of geogrid (kN/m)
Table.1 Material properties for c-ϕ soil [16], Silty soil [9], for MSE wall[5]
2500 2000 1500 1000
80
60 40 500 1000 1500 2000 2500
20 0 0.25 0.5 0.75 Spacing (m)
1
Stiffness of geogrid kN/m
A. Modelling and analysis Plaxis 2D allows to use the basic geometry line to draw the soil layers as backfill in the present case. Plate material can be used to simulate the retaining wall. Geogrid itself can be used as reinforcing material. Material properties can be assigned for the materials used as shown in Table.1 and Table. 2. For pseudo dynamic analysis of MSE wall, seismic horizontal acceleration coefficient kh=0.3 and seismic vertical acceleration coefficient kv= kh=0.3 is used.
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200 0
Fig.3, Variation of vertical displacements at kh=0.3, kv=0.3 for c- ϕ soil Similarly, from fig.3, at EA=2000kN/m, 6.5% of increase is noted in the vertical displacement for the increase of spacing from 0.25m to 0.50m. Decrease of stiffness of geogrid also increases the displacement in vertical direction. At 1.0m spacing, 1% of increase in vertical displacement is noticed for the decrease in the stiffness from 1500kN/m to 1000kN/m.
2
0 0.25 0.5 0.75 Spacing (m)
1
20
Again from fig.4 at EA=2000kN/m, 4.1% of total displacement is increased for the increase in the spacing from 0.50m to 0.75m. And at 1.0m spacing, 2.0% of increase in total displacement for the decrease in the stiffness of from 1500kN/m to 1000kN/m. Fig.2, Fig.3 and Fig.4 show the Displacements in horizontal, vertical and total displacements for c-phi soil as backfill material respectively. From all the shown above it is read that increase in geogrid spacing from 0.25m to 1m with the spacing interval of 0.25m there is increase in horizontal, vertical and total displacements. Again, with increase in geogrid stiffness from 500kN/m to 2500kN/m with stiffness interval of 500kN/m there is decrease in horizontal, vertical and total displacements.
40
500 1500 2500
20 0 0.25
0.5 0.75 Spacing (m)
1
Stiffness of geogrid (kN/m)
Horizontal Displacement (mm)
60
0 0.25
0.5 0.75 Spacing (m)
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1
Fig.6, Variation of vertical displacements at kh=0.3, kv=0.3, silty soil. 2500 2000 1500
80
1000
60
500
40
500 1000 1500 2000 2500
20 0 0.25
0.5
0.75
1
Spacing (m) Fig.7, Variation of total displacements at kh=0.3, kv=0.3, silty soil. Again, there is decrease in horizontal, vertical and total displacements with increase in geogrid stiffness from 500kN/m to 2500kN/m with stiffness interval of 500kN/m.
C. Case-2: c- ϕ soil
80
500 1000 1500 2000 2500
Stiffness of geogrid (kN/m)
40
Fig.4, Variation of total displacements at kh=0.3, kv=0.3 for c- ϕ soil.
2500 2000 1500 1000 500
60
Stiffness of geogrid (kN/m)
500 1000 1500 2000 2500
2500 2000 1500 1000 500
80
Vertical Displacement (mm)
50
2500 2000 1500 1000 500
Total dispplacement (mm)
100
Stiffness of geogrid kN/m
Total Displacement (mm)
Vadavadagi & Jadar
Fig.5, Variation of horizontal displacements at kh=0.3, kv=0.3, silty soil. Fig 5, Fig. 6 and Fig 7 show the Displacements in horizontal, vertical and total displacements for silty soil as backfill material respectively. Similar to the Fig 2, Fig 3 and Fig 4 here also it is seen that there is increase in horizontal, vertical and total displacements with increase in geogrid spacing from 0.25m to 1m with the spacing interval of 0.25m.
But comparing both the backfills c-phi soil (Fig.2, Fig.3, Fig.4) and silty soil (Fig.5, Fig.6, Fig.7), more horizontal, vertical and total displacements are found in case of silty soil as backfill material than compared to c- ϕ soil as backfill material. Fig.8, shows the percentage reduction for horizontal displacements. Percentage reduction has been plotted in ordinate and spacing of the geogrid in abscissa. From the graph, it is found that percentage reduction decrease with the increase in the geogrid spacing from 0.25m to 1.0m. It is also found that percentage reduction has been increased for the geogrid stiffness from 500kN/m to 2500kN/m. The maximum horizontal percentage reduction is found for the 2500kN/m geogrid stiffness with geogrid spacing interval of 0.25m is 30%. The minimum horizontal percentage reduction is found for the 500kN/m geogrid stiffness with geogrid spacing interval of 1m is 5%.
3
Vadavadagi & Jadar In the Fig 10, The maximum horizontal percentage reduction is found for the 2500kN/m geogrid stiffness with geogrid spacing interval of 0.25m is 23%. The minimum horizontal percentage reduction is found for the 500kN/m geogrid stiffness with geogrid spacing interval of 1m is again 6%.
2500 kN/m 2000 kN/m 1500 kN/m 1000 kN/m 500 kN/m
35 30 25 20 15 10 5 0 0.25
0.5
0.75
1
Spacing (m)
25 20 15
2500 kN/m 2000 kN/m 1500 kN/m 1000 kN/m 500 kN/m
% Reduction
30 25 20
5 0 0.25
0.75
Spacing (mm)
21
1
Fig.9, Percentage reduction for vertical displacements at kh=0.3, kv=0.3, c- ϕ soil. 25
EA=2500 kN/m
20
EA=2000 kN/m
% Reduction
0.75
Spacing(m)
1
EA=2500 kN/m EA=2000 kN/m EA=1500 kN/m EA=1000 kN/m EA=500 kN/m
24
10 5
18 15 12 9 6 3 0 0.25
0.5
0.75
1
Spacing (mm)
15
Fig. 12, Percentage reduction for vertical displacements at kh=0.3, kv=0.3, silty soil
10 5 0.25
EA=2500 kN/m EA=2000 kN/m EA=1500 kN/m EA=1000 kN/m
20 0.5 Spacing (m)0.75
1
Fig.10 Percentage reduction for total displacements at kh=0.3, kv=0.3, c- ϕ soil. Fig.9, shows the percentage reduction for horizontal displacements. Similar to the Fig. 8, Percentage reduction has been plotted in ordinate and spacing of the geogrid in abscissa. From the graph, it is found that percentage reduction decrease with the increase in the geogrid spacing from 0.25m to 1.0m. It is also found that percentage reduction has been increased for the geogrid stiffness from 500kN/m to 2500kN/m. The maximum horizontal percentage reduction is found for the 2500kN/m geogrid stiffness with geogrid spacing interval of 0.25m is 24%. The minimum horizontal percentage reduction is found for the 500kN/m geogrid stiffness with geogrid spacing interval of 1m is again 5%.
17
Axis Title
% Reduction
0.5
Fig. 11, Percentage reduction for horizontal displacements at kh=0.3, kv=0.3, silty soil
15
0.5
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10
Fig.8, Percentage reduction for Horizontal displacements at kh=0.3, kv=0.3, c- ϕ soil.
0 0.25
2500 kN/m 2000 kN/m 1500 kN/m 1000 kN/m 500 kN/m
30
% Reduction
% Reduction
III. COMPARISON AND CONCLUSIONS
14 11 8 5 0.25
0.5
Axis Title
0.75
1
Fig. 13, Percentage reduction for total displacements at kh=0.3, kv=0.3, silty soil. In the Fig.11, The maximum horizontal percentage reduction is found for the 2500kN/m geogrid stiffness with geogrid spacing interval of 0.25m is 26%. The minimum horizontal percentage reduction is found for the 500kN/m geogrid stiffness with geogrid spacing interval of 1m is 2%.
4
Vadavadagi & Jadar Fig.12, The maximum horizontal percentage reduction is found for the 2500kN/m geogrid stiffness with geogrid spacing interval of 0.25m is 21%. The minimum horizontal percentage reduction is found for the 500kN/m geogrid stiffness with geogrid spacing interval of 1m is 6%.
[8]
In the Fig.13, The maximum horizontal percentage reduction is found for the 2500kN/m geogrid stiffness with geogrid spacing interval of 0.25m is 20%. The minimum horizontal percentage reduction is found for the 500kN/m geogrid stiffness with geogrid spacing interval of 1m is 7%.
[10]
[9]
[11]
[12]
For both the backfills with the same geogrid stiffness and same geogrid spacing, for all the combinations of horizontal, vertical and total displacements, more deformations are found to be in the case of silty soil as backfill material than compared to the cphi soil as backfill material. That is c-phi soil has experienced lesser displacements. Percentage reductions are found to be more for the c-phi soil as backfill material tan compared silty soil as backfill material. Hence it is concluded in the present analysis that c-phi is more advantageous than silty soil as backfill material.
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