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Proceedings of Indian Geotechnical Conference December 15-17,2011, Kochi (Paper No. N-267)
SOME STUDIES ON THE BEHAVIOUR OF BACK-TO-BACK GEOSYNTHETIC
REINFORCED SOIL WALLS
Bharat Hemant Katkar, PG student, Dept. of Civil Engg., IIT Bombay, Mumbai, India. Email: bharatkatkar@iitb.ac.in
B.V.S. Viswanadham, Professor, Dept of Civil Engg. IIT Bombay, Mumbai, India. Email: viswam@civil.iitb.ac.in
ABSTRACT: Back to Back reinforced soil wall is used in bridge approaches, ramp ways, rock fall protection systems,
earthen dams, levees and noise barriers. The interaction of the failure surface between the walls needs to be studied in
greater detail to optimize the reinforcement spacing and strength. In this paper the behavior of back-to-back reinforced soil
walls, especially with reference to development of failure surfaces is studied. By keeping the length and type of
reinforcement as constant, the horizontal distance between walls is varied. A special case of connection of reinforcement in
back to back reinforced soil wall is also evaluated. For this purpose, an attempt has been made to analyze the performance
of back to back reinforced soil using a geotechnical finite element code, PLAXIS 2D. Finally, the effect of horizontal
distance between reinforced soil walls on the performance of back-to-back reinforced soil walls is presented.
INTRODUCTION
Retaining wall is a structure used to confine material from
spreading to its natural angle of repose. This can be
achieved by using various methodologies such as concrete
retaining wall, geosynthetic reinforced soil wall etc
depending upon the available space, required wall height
and available funds. Nowadays, reinforced soil walls are
preferred over conventional RCC retaining structures due to
their economy. Reinforced soil retaining structures nearly
cost 50 to 70% of the conventional RCC retaining
solutions. Reinforced soil walls are commonly used for
construction of bridge abutments, highway embankments,
retaining slopes, interchanges with access ramp way etc.
There are basically two configurations of back to back
walls are in vogue and they are (i) tiered facing and (ii)
vertical facing system. In this study the vertical facing
system is considered.
Conventionally limit equilibrium methods are used which
include stability checks for internal as well as external
stability. Finite element method (FEM) has emerged as an
efficient tool to analyze different complex geotechnical
structures. Behaviour of reinforced soil wall and its
structural elements can be easily analyzed with the help of
FEM at significantly low cost and less time as compared to
conventional limit equilibrium methods.
Back to back reinforced walls are used in ramp way and
bridge approaches. Depending on the requirement of the
roadway the distance between the walls is varied, but the
length of reinforcement is generally kept as 70% of height
of the wall. Hence, when the walls come closer due to
smaller roadway deck dimensions, the reinforcement layers
may get overlapped, and also the failure surfaces from
either wall may intersected. This aspect of intersection of
failure surfaces need to be considered for evolving
appropriate design guidelines. Also, the effect of
connecting reinforcement layers on the deformation
behaviour of walls further need to be studied.
Back to back reinforced soil walls are generally designed as
single individual reinforced soil wall. The length and
spacing of reinforcement are calculated accordingly.
Though this approach yields to a safer design, it does not
guarantee the economy of the structure. It has been seen
that as the distance between the walls reduces, the effective
thrust on the reinforcement is also reduced. While
considering both sides of the wall the FHWA Guidelines
[1] and [4] are followed, which suggest a reduction of
active thrust on the reinforced zone of the wall, while
considering external stability.
Fig 1 Design considerations for back to back reinforced soil
walls (Modified after [1]).
Where, X = horizontal distance between the ends of
reinforcement layers; H = Height of the wall; L = Length of
reinforcement layers; D = H tan(45 - /2), and = Backfill
soil friction angle.
Following cases are considered in FHWA Guidelines [1]
Case 1: When X D, full active thrust is mobilized; hence
each wall can be designed individually (Refer Fig.1).
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Bharat Hemant Katkar & B.V.S. Viswanadham
Case 2: When X < D, the failure surfaces in active zone
intersect in the backfill zone, hence active thrust is reduced
on reinforced zone.
Case 3: When X = 0 and an overlap of 0.3H is present in
reinforcement from either wall, then no active thrust exists
on reinforced zone.
FHWA Guidelines [1] suggest interpolation for magnitude
of active thrust from full thrust to no thrust between the
Case (1) and Case (3) given above, on each wall for D>X>0
(or overlap less than 0.3H) with unconnected
reinforcement. However, Han and Leshchinsky [2]
investigated the effect of the width to height ratio of the
wall and the quality of backfill material on the critical slip
surface, the required tensile strength of reinforcement, and
the active thrust on the reinforced zone. It was shown that
even when X = D the failure surfaces intersected from
either walls, which lead to reduction in active thrust on
reinforced zone. Also the distribution of maximum tensile
strength of the reinforcement layers was slightly affected by
change in aspect ratio, hence it was suggested that it is
conservative to ignore the influence of the width to height
ratio on the overall maximum tension in the internal
stability analysis of the back to back soil wall.
This paper addresses the deformation behaviour of back to
back reinforced soil walls along with the force development
in geosynthetic layers with depth. The intersection of the
failure surfaces was also confirmed for various
combinations of distances between reinforcement from
either wall to height ratio (i.e. X/H) using computing
software PLAXIS 2D 2010 [3]. Finally an attempt has been
made to consider the effect of variation of the backfill
friction angle on forces generated in geosynthetic layers.
ANALYSIS OF BACK TO BACK REINFORCED
SOIL WALLS
Model configuration
The configuration of the back to back reinforced soil walls
was modelled same as reported by Han and Leshchinsky
[2]. The dimensions of the wall were kept constant with
height 6m and foundation depth as 1.5m. The length of the
geosynthetic reinforcement was kept constant at 0.7H
(4.2m). The distance between the walls was varied.
Table 1 Cases formulated for finite element analysis
Sr.
No. X/H ratio Reinforcement
= 25 = 34
1 0 Connected [a] [b]
2 0 Not connected [c] [d]
3 0.6 Not connected [e] [f]
4 1.6 Not connected [g] [h]
Four cases were formulated based on the variation of
distance between the back to back walls (X/H ratio)
keeping length of geosynthetic layers as constant and two
different friction angles of the backfill soil were used, as
indicated in Table 1.
A weakened zone of soil of width 0.3m and height 0.4m
was introduced at the base of the facing element to
channelize the failure through the base of the soil wall. The
connection of reinforcement was made to the interior of the
facing element. Geosynthetic reinforcement was adopted
with axial stiffness of 60kN/m. No external load was
applied for the analysis. A special case was evaluated by
connection of reinforcement from both sides of the walls in
X/H=0 model case to evaluate the deformation behaviour
and force generation in geosynthetic reinforcement.
Fig. 2 Connectivity plot of Back to Back Reinforced soil
wall
Material properties:
Soil properties for all model cases as shown in Table 2 were
used in the analysis. The elasticity modulus E = 1x105
kN/m2 and Poisson`s ratio of 0.3 were assumed for all
soils.
Table 2 Soil properties adopted for finite element analysis
Sr.
No. Element
Unit
weight
(kN/m3)
Cohesion
c
(kPa)
Friction
angle
( )
1 Backfill soil 18 0 25/34
2 Block facing soil 18 1000 34
3 Weakened zone 18 0 25/34
4 Foundation soil 18 1000 0
Outline of numerical simulation
A finite element program PLAXIS 2D 2010 [3] was used to
model the back to back reinforced soil wall section. The
soil element was selected as 15 noded triangular element
and the geosynthetic reinforcement element was
correspondingly obtained as 6 noded triangular element [3].
Average mesh size was kept as 0.153m.
Advanced mode of analysis with Bishop`s definition of
effective stress was adopted for evaluation of all models. A
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Some studies on Back-to-Back Geosynthetic Reinforced Soil Wall
common node was entitled between the facing element and
geosynthetic element to have a connection between them.
Vertical fixity was provided to the horizontal surface of the
foundation to avoid foundation failure. Horizontal fixity
was provided along vertical faces of foundation. Interfaces
were provided as shown in Fig. 2. The interface stress
transfer coefficient (Rinter) was taken 0.97 for backfill soil-
geosynthetic and foundation soil-geosynthetic interface. For
initial stress generation in the soil due to self weight the K0
procedure was adopted. It was verified for each model that
no plastic stress points were generated in the initial stage.
The soil displacement was set to zero at the end of initial
stress generation stage. Finite element calculations were
performed after completion of initial stage.
RESULTS OF ANALYSIS AND DISCUSSION
Finite element analysis
Finite element analysis was performed for the cases
formulated in Table 1 and the displacement diagrams,
maximum tension variation in geosynthetic and
displacement of facing element was obtained. A strength
reduction procedure was adopted to induce failure in the
model [3].
Figures 3a-3d represent the resultant displacements for the
case [c], [a], [e] and [g]. It was observed from the resultant
displacement diagrams that the soil wall with lower X/H
ratio had lesser deformations as compared to that of higher
ones. Also both the walls deformed symmetrically with
same displacement pattern. Connecting reinforcement at
mid-section reduced the overall displacements as compared
to other unconnected cases. The displacements mainly
occurred due to failure of the weaker zone leading to
deformation of the facing element.
(a) Deformed mesh for Case [c]
(b) Deformed mesh for case [a]
(c) Deformed mesh for case [e]
(d) Deformed mesh for case [g]
Fig. 3 Resultant displacement diagrams.
(Magnified by 5 times)
The location and shape of critical failure surfaces of the
back to back walls for all cases were determined based on
contours showing displacement greater than zero are
presented in Figs. 3a-3b.
(a) Displacement contours for case [a]
(b) Displacement contours for case [g]
Fig. 4 Identification of failure surface from displacement
contours
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Bharat Hemant Katkar & B.V.S. Viswanadham
The failure surfaces were found to be independent of each
other for model case [g] and [h] i.e. the walls behaved
independent of each other, but for remaining cases the
failure surfaces intercepted each other at varying depths and
found to be in agreement with those reported by [2].
Figures 5a-5b show the variation of maximum geosynthetic
forces mobilized with depth for soil = 25 and soil = 34
cases. As can be noted from Fig. 5a, distinct difference in
the pattern of geosynthetic forces with depth was not
observed for soil = 25 , also a small variation of forces was
observed for soil = 34 . In comparison, for the case [c] and
[d], the mobilized geosynthetic reinforcement forces were
predominantly higher than in the case [a] and [b]. This is
attributed to entering of failure zones of one wall into
reinforced zone of other wall. Due to presence of weaker
zone at the base of model the bottom-most geosynthetic
was incapable of developing significant amount of force;
hence it was not included in results. It was observed that the
forces are lesser in connected reinforcement case than the
unconnected case. This is due to pullout from the middle of
model is not possible due to connection.
(a) soil =25
(b) soil = 34
Fig. 5 Distribution of max. tension in reinforcement layers
Resultant displacements of the facing elements were plotted
using nodal displacements of the same element, as shown in
Figures 6a-6b. The displacement pattern indicates the
importance of using higher frictional material as backfill.
For backfill soil =25 bulging of wall occurred at mid-
height of wall whereas for =34 the displacements were
found to be uniform in nature. In this paper, an attempt has
been made to understand the deformation behaviour of back
to back reinforced soil walls. However, further work in this
direction is warranted.
(a) soil=25
(b) soil=34
Fig. 6 Resultant displacement of facing elements
CONCLUSIONS
Based on analysis and interpretation of results, following
conclusion can be drawn:
1) Connection of reinforcement in back to back walls
attracts more force in geosynthetic, but reduces
displacement of wall.
2) Maximum tension in geosynthetic was nearly
independent of the distance between the back to back walls.
3) Wall displacements were drastically reduced with the use
of higher friction angle of the backfill.
4) Less force in the bottom most geosynthetic underlines
the importance of proper connection to facing elements.
REFERENCES [1] FHWA (2001), (Federal Highway Authority, USA)
Guidelines. (FHWA-NHI-00-043)
[2] Han, J., Leshchinsky, D. (2010), Analysis of back-to-
back mechanically stabilized earth walls, Geotextiles
and Geomembranes, 28(3) 262-267.
[3] PLAXIS 2D (2010), http://www.plaxis.com/ and user’s
manual, Balkema Publishers, The Netherlands
[4] Elias, V., Christopher, B.R., Berg, R.R. (2001),
Mechanically stabilized earth walls and reinforced soil
slopes design and construction guidelines. (FHWA-
NHI-00–043)
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