International Journal of Geosciences, 2013, 4, 844-849
http://dx.doi.org/10.4236/ijg.2013.45078 Published Online July 2013 (http://www.scirp.org/journal/ijg)
Influence of Recycled Aggregate Composites on the Factor
of Safety of Earthen Structures
Md. Zakaria Hossain
Department of Environmental Science and Technology, Mie University, Mie, Japan
Received April 19, 2013; revised May 23, 2013; accepted June 22, 2013
Copyright © 2013 Md. Zakaria Hossain. This is an open access article distributed under the Creative Commons Attribution License,
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
ABSTRACT
In this study, six composite reinforcements such as cement composite made of Abandoned Cell Husks (ASH), Stones,
Wood chips, Concrete and Bricks have been used along with control specimen. It is known that the material used in
earth reinforcement applications must be safe against tension failure and adhesion failure for its effective utilization in
the field and reliable design of earth structures. Single type of material can provide limited reinforcement capability in
reinforced earth structures due to its low frictional resistance and poor cohesion. For an optimal response, therefore,
composite reinforcement, that fulfils both the requirements such as possess adequate tensile strength and adequate fric-
tional resistance, is getting considerable attention. Slope stability analyses containing six types of reinforcement have
been performed. Stability of the slope has been quantified using minimum factor of safety corresponding to critical slip
surface. It was observed that the composite reinforcement whose surface treated by brick aggregate enhanced the factor
of safety significantly. The paper also depicted the design aids of reinforced slope in terms of embedding lengths and
spacing of reinforcements.
Keywords: Earthen Structures; Stability Factor; Composite Reinforcement; Recycled Aggregate
1. Introduction
The embankments, dams, foundations, abutments and all
earth fill structures must be stable under all static and
dynamic loadings during construction and on-service [1].
Collapse in earth fill embankments occurs at the critical
slip surface when the factor of safety (Fs) decreases due
to weathering, erosion, seepage, changes of surface and
subsurface water, earthquake and many other natural
calamities [2-8]. In order to obtain a necessary factor of
safety for a given slope, it must be reinforced to improve
the stability above the safety level. There are various
earth reinforcing materials worldwide. Among them, the
geosynthetic or geogrid are conventional reinforcements
that are commonly used for earth reinforcement applica-
tions. It is evident that the conventional reinforcements,
used for reinforcing earth fill structure, contain only one
type of material such as geogrid, geosynthetic or wire
mesh etc. The material used in earth reinforcement ap-
plications must be safe against tension failure and adhe-
sion failure for its effective utilization in thefield and safe
design of earth structures [9-11]. For a given situation,
single type of material can provide limited reinforcement
capability in reinforced earth structures due to its low
frictional resistance and poor cohesion. For an optimal
response, therefore, different types of reinforcement that
fulfils both the requirements such as possess adequate
tensile strength and adequate frictional resistance, is get-
ting considerable attention lately [12,13].
Stability analysis is usually performed to find out the
factor of safety of the earth fill structures [14-18]. Refer-
ring to Figure 1, the composite reinforcement provides
resisting force thereby increases the factor of safety of
the fill embankments. In this paper, slope stability analy-
ses containing six types of composite reinforcement have
been performed. Thin reinforced-mortar composite con-
sisting of evenly distributed fine mesh as the reinforce-
ment and cement-sand mortar as the matrix showed en-
hanced performance because of its synergetic action of
mesh with mortar and mortar with soil [19-23]. Consid-
ering the facts given above, slope stability analyses for
reinforced embankments containing various types of
composite reinforcements have been conducted using
simplified method. Six composite materials were used
Surface treatment for five types of composite specimens
were made by using Abandoned Cell Husks (ASH),
Stones, Wood chips, Concrete and Bricks. The control
C
opyright © 2013 SciRes. IJG

MD. Z. HOSSAIN 845
specimen was prepared without any surface treatment.
The analyses were performed in such a way so that the
most critical slip surface which showed Minimum Factor
of Safety (Fs min) was identified based on random search
technique. The factors of safety for various slope having
different slope inclination (30˚, 45˚ and 60˚) and differ-
ent angle of internal have been reported in tabular and
graphical form as a ready reference for design of rein-
forced slope containing composite reinforcement. Rela-
tionships between the slope inclination and number of
layers of composite reinforcements and layer spacing are
given as a ready reference to ease in the design of rein-
forced embankments.
2. Materials and Methods
The conventional soil reinforcement materials are shown
in Figures 2 and 3. The composite reinforcements used
in this study are shown in Figures 4-9.
Figure 1. Placement of composite reinforcement for slope
stability.
Figure 2. Geogrid mesh (basalt mesh).
Figure 3. Geosynthetics (basalt material).
Figure 4. Control specimen (43 × 35 × 1.5 com).
Figure 5. ASH (Abandoned shell husk).
Figure 6. Stone cement composite.
Figure 7. Wood cement composite.

MD. Z. HOSSAIN
846
Figure 8. Concrete cement composite.
Figure 9. Brick cement composite.
3. Determination of Shear Resistances of
Composite Reinforcements
Details of soils-reinforcements interaction studies can be
found elsewhere [21-23]. The apparatus used in this stu-
dy is shown in Figure 10 which is capable of performing
direct shear tests and pullout tests. The panels were
clamped in the box in such a way that the embedded
length of the panel is 38.0 cm in the loading direction
and 31.6 cm in the transverse direction. After embedding
the composite reinforcement on the lower box; the upper
part was set on the panel, and then the sand was rained in
the upper box. The tests were carried out in such a way
that the panel along with lower box was pushed out from
the san in the upper box. The shear speed was 1mm per
minute. However, it can be fixed at any slower/faster
speed. The shear force and the displacements were
measured at the lower box by means of LVDTs and the
data were recorded in a computer system directly. Re-
sults obtained from the experiment are given in Table
1.
4. Slope Stability Analyses with and
without Reinforcements
A brief description is given in this paper and detailed
procedure can be found elsewhere (Hossain et al., 2012).
External and internal forces acting on the slope and a
slice are shown in Figure 11. Here, Ea and Ta are hori-
zontal and vertical external driving forces acting at upper
face of the slope. The external horizontal and vertical
resisting forces Eb and Tb are acting at the toe of the slope.
The other vertical and horizontal forces caused by sur-
charge due to external loadings and body forces are
shown by P and Q, respectively.
The horizontal and vertical boundary forces E and T or
t are acting at a height ht from the base of the slice. The
differences of forces for slice (width ΔX) are ΔE and ΔT
and the difference of forces for external loading are ΔQ
and ΔP. The parameter U is the water force acting up-
ward at the base of the slice. The σ and τ are normal and
shear stresses acting beneath the slice.
The factor of safely is calculated based on the pa-
rameters A and B, resisting and driving forces, respec-
tively and is given the following equation:
VDT
Shear boxHDT
Load cell
Figure 10. Apparatus used for testing shear and pullout
resistances of composite reinforcements.
Table 1. Surface roughness of reinforcements and resis-
tance coefficient.
Specimen Roughness, Degree Resistance
Coefficient
Toyoura Sand 30.00 1.00
Geosynthetic (Basalt Cloth) 16.00 0.53
Geogrid (Basalt Mesh) 27.00 0.90
Composite Reinforcement
(Control) 32.25 1.14
Composite with Abandoned
Cell Husks (ASH) 35.50 1.19
Composite with Stone 36.12 1.26
Composite with Wood Chips37.47 1.33
Composite with
Recycled Concrete 38.63 1.38
Composite with Brick Aggregate39.73 1.44

MD. Z. HOSSAIN 847
Figure 11. Forces acting on a slope and a slice.
s
F
ab
A
EE B


(1)
where,:
The apparent resisting moment A is the function of
cohesion, frictional resistance and resisting external forces
acting on the slice and is given by:
A
An




tan
(2)
where:
A
cp


tu x   

n
(3)
The

is a parameter given by frictional resistance,
inclination of slip surface along with factor of safety
which is given by the following equation:

2
tantan
1tan
s
11F
n




()tanptx

 (4)
And the driving moment B is the function of external
force, water force and angle of slip surface which is gi-
ven by the following equation:
BQ

 

tan
(5)
All the above equations are for fill embankments wi-
thout any reinforcement. Considering the facts for im-
proving the factor of safety of fill embankments, the fol-
lowing equations have been proposed.
A
cpt
ux
 

(6)

2
tantan
an
11
1t
s
F
n





 (7)
The vertical component of load/weight of slope is de-
fined by p and the average pore water pressure is defined
by u. The other parameters in the above equations are
conventional such as cohesion and angle of internal fric-
tion are defined by c and φ. The parameter β is defined as
coefficient of frictional resistances which is employed in
the proposed equations due to the additional resisting
forces come from the embedment of composite rein-
forcements.
The analyses were performed using the programming
in Microsoft excels spreadsheet as given in Figure 12.
The program starts with an initial slip surface and calcu-
lates the Fs for it. It then performed repeated analyses for
several trial slip surfaces in order to search the critical
slip surface and obtained final slip surface of which the
Fs is minimum.
The following steps were followed to obtain the com-
plete convergence of the factor of safety values.
Step 1 Input date for a given slope such as soil pa-
rameters, slope properties, boundary conditions, external
and internal forces etc. (marked by red color).
Step 2 Calculating nα, A and Fs repeatedly until the
convergence of Fs become 1.5 (marked by green color).
Step 3 Substituting the Fs obtained in Step 2 in equa-
tion nα and calculating the Fs again considering the boun-
dary forces of the slice (marked by blue color).
Step 4 Calculating the E and T from Step 3 and then
calculating the final Fs from this (marked by blue color in
Step 4).
5. Results and Discussion
The minimum factors of safety (Fs min) along with coef-
ficient of frictional resistance (β) calculated for the unre-
inforced and reinforced embankments with different
slope inclinations (α) are given in Table 2.
It is observed that the Fs min for unreinforced em-
bankments for slope inclination of 30˚, 45˚ and 60˚ are
1.19, 0.84 and 0.56 respectively indicating that the Fs min
Table 2. Factor of safety values for unreinforced embank-
ments with φ = 40˚.
Minimum factor of safety
α = 130˚ Α = 45˚ Α = 60˚
Unreinforced slope 1.11 0.084 0.056
Reinforced slope
with geosynthetics 1.12 0.55 0.44
Reinforced slope with geogrid1.18 0.85 0.62
Reinforced slope with
control composite 1.35 0.95 0.64
Reinforced slope with
ASH composite 1.40 1.00 0.66
Reinforced slope with
stone composite 1.48 1.06 0.70
Reinforced slope with
wood composite 1.56 1.11 0.73
Reinforced slope with
concrete composite 1.62 1.16 0.76
Reinforced slope with
brick composite 1.68 1.20 0.79

MD. Z. HOSSAIN
IJG
848
Step 3
o
Fs
1.44
Step ２
Sum BoSum A
130.85 187.83
Step 1
cφEa Eb
0.00
0.23
18.00 0.000.00
Y
tp=Xi/slope
r
Ytp -Yb
XYYtp hij1/2(hij1+hij2)r
19.00 15.00 15.000.00tan  X
Pc
57°20’
Slice#
Ea = 0

Q
20.0013.0015.002.00 2.00 1.0018.000.000.00
21.0011.8014.132.33 1.20 1.0039.010.000.00
22.0010.9012.401.50 0.90 1.0034.520.000.00
23.0010.1010.670.57 0.80 1.0018.650.000.00
23.609.70 9.70 0.00 0.67 0.60 5.130.000.00
φBo A'o n 
Ao
11.5636.00 28.080.63 44.34
21.5646.81 60.850.94 64.55
31.5631.07 53.861.09 49.36
41.5614.92 29.091.14 25.56
51.56 2.05 4.801.19 4.03
Step 4
 B1 A1 Fs =
124.05 185.99 1. 50
Eo Eo / 
 Eo
0.0 0
5.115.11
1.846.95
-3.31 3.64
-2.89 0.75
-0.75 0.00

x
tant ht T1  T1 t1B1 A'1 n A1

E1 E1(Ea = 0)
0.000.00 0.000.00
Slice#5.11 3.48 1.33 0.67-4.50-4.50-4.5027.0021.060.6234.19-86.95-86.95
16.95-0.741.090.78-8.14-3.64 -3.6442.4455.17 0.9259.88-157.17-244.12
23.64 -3.10 1.180.50 -5.842.312.3133.1557.451.0753.71-145.90-390.02
30.75 -2.27 1.110.19 -1.274.574.5718.5736.221.1232.42-89.50-479.52
40.00-1.260.98 0.00 0.001.27 2.122.90 6.78 1.17 5.78-16.38-495.90
4.56 4.38
16.37 15.72
19.79 19.01
13.19 12.67
4.45 4.28
Figure 12. Factor of safety obtained by slope stability analyses.
decreased with the increase of slope inclination. This is
obvious due to the driving forces for unreinforced slope
with the increase of slope inclination. For improving the
stability of the slope, composite reinforcements were
impregnated. The factor of safety values were increased
with the increase of composite reinforcements. This is
expected because the reinforcement embedded inside the
soil resulted more stress-transfer ability of the compos-
ites thereby increased the resisting forces. It is also no-
ticed that the brick treated cement composites exhibits
the maximum enhancement in the factor of safety values.
This is apparent owing to the synergy between the two
materials in the hybrids such as synergetic action of mesh
with mortar and brick treated mortar with soil. The time
effects of different reinforcements have not been studies
in this present research because the data noted in this
paper were obtained by short term experiment. It is rec-
ommended to perform long term durability tests of dif-
ferent reinforcements for better comparison.
Figure 13. Length and spacing of reinforcements.
optimization attempts using surface treated composite rein-
forcement are obviously warranted.
For convenient design of reinforced fill embankments,
relationships between factor of safety and inclination of
slope along with required length of reinforcement to be
embedded and spacing between the reinforcement are
shown in Figure 13. Form this figure one can easily de-
sign stable embankments by selecting necessary rein-
forcements, embedding length and spacing. As can be
seen, the composite reinforcement surface treated with
wood, concrete and brick provides factor of safety 1.5 or
more. In this case, the length of the embedment should
be more than 1.7 m and spacing should be 1.0 m. A
closer inspection of the plotted results revealed that the
increment trend of the safety factor containing composite
reinforcements are somewhat different from the safety
factor that were observed in case of unreinforced or con-
ventional reinforcements embankments thereby, further
6. Conclusion
In this research work, frictional resistance and the effec-
tiveness of composite reinforcement on slope stability
have been studied. The results obtained and the observa-
tions made clearly revealed that the addition of small
amount of recycled aggregate not only increased the re-
sisting forces of cement composites but also significantly
improved the stability of reinforced soil. It has been
demonstrated that the conventional reinforcement made
of single material provided limited shear resistance and
the shear performances of soil-structure interaction were
enhanced by using the composite reinforcements. Among
the six categories of composites tested in this study, the
brick treated cement composites appeared to be more
effective than the individual ones and the others. This

MD. Z. HOSSAIN 849
study further suggests that the simple soil-structure in-
teraction tests can effectively be used in characterization
of shear behaviour of the surface treated mesh-reinforced
cement composites.
7. Acknowledgements
The present study is partly supported by the Research Grant
No. 22580271 with funds from Grants-in-Aid for Scientific
Research, Japan. The writers gratefully acknowledge these
supports. Any opinions, findings, and conclusions ex-
pressed in this paper are those of the authors and do not
necessarily reflect the views of the sponsor.
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