Influence of Recycled Aggregate Composites on the Factor of Safety of Earthen Structures

doi:10.4236/ijg.2013.45078

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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

Email: [email protected]

Received April 19, 2013; revised May 23, 2013; accepted June 22, 2013

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

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

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.

F

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:









tan

(2)

where:





tu x



 



(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:



tantan

1tan

11F







()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:



 



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.

cpt

ux







(6)



tantan











 (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

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

tp=Xi/slope

Ytp -Yb

XYYtp hij1/2(hij1+hij2)r

19.00 15.00 15.000.00tan 

X

57°20’

Slice#

Ea = 0



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 

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

0.0 0

5.115.11

1.846.95

-3.31 3.64

-2.89 0.75

-0.75 0.00



tant

ht T1

 T1

t1B1 A'1

n



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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