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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