Main thesis Version FINAL

MSc Geotechnical Engineeing

FRICTIONAL SURFACE BETWEEN CONSTRUCTION MATERIAL

AND GRANULAR SOILS

(EFFECT OF SURFACE ROUGHNESS ON SHEAR STRENGTH)

Sepehr Aghamehdi

1067376

Dissertation submitted is in partial fulfilment of the degree of Master of

Science.

School of Civil Engineering

University of Birmingham

1st May 2015

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Abstract

Assessments of interface friction parameters between soil aggregates and

construction materials (which in this study steel mesh was used) were conducted to

measure the shear strength of the soil and look into how the shear stress and

frictional resistance of the soil can be effected by using different void ratios and

densities.

This study will look into how also the material surface roughness affects the soil

frictional resistance of the soil.

In order to determine the shear stress and hence the angle of friction, whether

internal or interface, direct shear tests were conducted using variety of normal

stresses, 50kpa, 100kpa and 200kpa, to evaluate how the soil’s shear strength is

affected by different normal stresses. The soil used was construction aggregates

with grain sizes of 2mm used to carry out the tests.

The construction material as mentioned were steel meshes which spanned 10mm x

10mm and an overall span of 100mm x 100mm which could fit into the shear box.

These meshes had a variety of depths of 2mm, 4mm and 6mm which was made

available by welding the steel meshes together.

By evaluating and observing the graphs made by author, of shear stress-horizontal

displacement considering how by keeping void ratios and normal stresses constant

and variable at different points, it came to the attention that by doing to shear

strength of the soil is affected almost substantially by these factors, for instance:

From shear stress and horizontal displacement graphs considering constant

void ratio and variable normal stresses, it was understood that as the normal

stress increases, shear stresses increase.

However by rearranging the graphs by considering constant normal stresses

and variable void ratios, it could be observed that shear stress increases as

the void ratios decrease (density increases), hence the density has a massive

impact on shear stress and frictional resistance manipulation.

From the shear envelopes, it was noticed that by decreasing the friction angle

(although it fluctuates from time to time) the shear stress increases.

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The shear strength of soil is always higher compared to the interface friction

angle between the soil and construction material.

However, although that this study has helped to understand, how different factors

affects the shear strength and frictional resistance of the soil, one of which is surface

roughness and how important and vital it is, future work is required for more

information on, for example, how much of steel mesh’s depth need to be increased

(rougher surface), so that the shear failure plane could get pushed back into the soil,

which provides a higher shear strength as the soil always has the highest shear

strength. Other factors that can be taken into account are moisture content and

variety of grain sizes and different angularity of the grains.

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Contents Abstract ................................................................................................................................................... i

Acknowledgements ............................................................................................................................. iv

1. Introduction .................................................................................................................................... 1

1.1. Objectives .............................................................................................................................. 1

1.2. Outline of study ..................................................................................................................... 1

2. Literature Review ......................................................................................................................... 2

3. Experimentation (Direct Shear Test) ......................................................................................... 6

3.1. Soil properties and classification: ...................................................................................... 6

3.2. Sieve Analysis ....................................................................................................................... 9

3.3. Type of construction material ........................................................................................... 11

3.4. Direct Shear test procedure .............................................................................................. 14

4. Discussion of Results ................................................................................................................ 20

4.1. Constant void ratio vs variable normal stress ................................................................ 21

4.2. Constant normal stress vs variable void ratio ................................................................ 26

4.3. Shear failure envelope ....................................................................................................... 32

4.4. Dilatency of tested granular material ............................................................................... 36

4.5. Void ratio (e) and friction angle (ϕ) ....................................................................................... 46

5. Conclusion ................................................................................................................................... 48

6. Appendices .................................................................................................................................. 49

A. Relationship between the shear stress and the horizontal displacement for all

soil/material interactions with e = 0.5 as well as a graph for the comparison of all curves

with similar void ratio. .................................................................................................................... 50

B. Relationship between the shear stress and the horizontal displacement for all

soil/material interactions with e = 0.8 as well as a graph for the comparison of all curves

with similar void ratio. .................................................................................................................... 55

C. Graphs to show the comparison of all relationships between the shear stress and

the horizontal displacement for all soil/material interactions with similar normal stress and

variable void ratios. ........................................................................................................................ 60

D. Relationship between the vertical movement and the horizontal displacement for all

soil/material interactions with e = 0.5 as well as a graph for the comparison of all. ............ 63

E. Relationship between the vertical movement and the horizontal displacement for all

soil/material interactions with e = 0.8 as well as a graph for the comparison of all. ............ 69

F. Coulomb failure envelopes for all soil/material interactions with e = 0.5. ...................... 75

G. Coulomb failure envelopes for all soil/material interactions with e = 0.8. .................. 77

7. Reference .................................................................................................................................... 79

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Acknowledgements

I would like to express my gratitude to Dr. Ghataora for his invaluable support and

guidance throughout this research program. Also I would like to thank my family for

their patience and their unconditional love, who without them this study would not

have been possible. Also my good friend/ colleague Hamid Sadeghi for his

irreplaceable assistance throughout this project.

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1. Introduction Interaction between the construction materials and the soil is of a particular

importance in Geotechnical engineering, this has a major significance in soil

structure interaction problems such as retaining structures, foundations whether

deep or shallow, soil samplers, soil and geomembrane interface strength and the

stability of mechanically stabilised structures.

The strength of this interface known as shear surface between the two materials is

governed by the type of finish on the concrete surface or other construction

materials; whether it is smooth or rough which will have a massive impact on the

friction angle that describes the ability of the material to shear which is essential for

the stability of the structure such as foundations.

Granular materials are preferred for structural fill because they are strong, drain

water rapidly, and settle relatively little (Edil and Benson, 2007) and these are some

of the reasons why granular soil need to be investigated further to widen the

understanding of its shear behaviour.

The most important factors that need to be evaluated which influence the shearing

behaviour of the soils are the internal friction angle, cohesion, interface friction angle

as well as the adhesion factor, soil properties, such as void ratio, density, particle

size distribution (PSD), gradation and the soil grain shapes and sizes, not forgetting

moisture content, soil composition, surface roughness, and normal load.

Laboratory tests play a vital role for the study of the mechanical behaviour of soil-

structure interfaces. Laboratory friction tests with obvious boundary conditions can

provide fundamental information.

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1.1. Objectives

Main objectives of this research are as follow:

Evaluation of internal friction angle of the soil and the interface friction angle

between the soil and the material used.

Behaviour of the soil under shear stress and determining the behaviour of the

shear envelopes between the material used and the soil interface.

Determination of different void ratio of soil and the variables such as density

and its effect on the interface frictional resistance.

Establish the relationship between the horizontal displacement and the normal

stress for soil-soil interface, soil-steel mesh.

Effect of surface roughness on shear strength.

1.2. Outline of study

This research paper discusses the shear strength of soil and the shearing resistance

of the soil-steel mesh interface and the layout is as follows:

Section 1: This is the introduction of the study as well as the main objectives of this

research.

Section 2: Presents the literature review and previous studies, which were carried

out and about the shear strength and frictional resistance.

Section 3: Illustrates and explains the methods used to obtain information such as

using direct shear box test and the procedure and methodology of the tests.

Section 4: The results from the direct shear box test as well as analysing the data

will be presented here.

Section 5: Discussion of the findings from previous chapter.

Section 6: Provides conclusions for the study and further work.

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2. Literature Review

Das (2005) defined shear strength as follow:

“The shear strength of a soil is the internal resistance

per unit area that the soil mass can offer to resist failure

and sliding along any plane inside it”

Essentially the shear strength is the cohesion and frictional resistance

between solid particles of the soil and the failure occurs in a scenario where the

particles of the soil are able to roll and/or slide past one another and it is the

measurement of the soil resistance to deformation by continuous displacement of its

particles.

Shear strength is determined by several laboratory tests, which can as follow

(Das, 2005):

Direct Shear Test (Shear Box Test)

Triaxial Test

Direct Simple Shear Test

Torsional Ring Shear Test

Although there are variety of tests to determine the shear strength of soil, but

direct shear test is the most common tests to evaluate the soil.

Gireesha and Muthukkumaran (2011) determined the frictional resistance of

granular soils by carrying out number of direct shear test on the soil as well as

looking at the interface friction resistance and how the surface roughness of different

construction material impacts the shear strength of the soil.

In order to illustrate this phenomenon, number of construction material such

as concrete, steel and wood were used, with different degree of surface roughness

(smooth and rough). The results that were obtained showed that by increasing the

relative density (Dr) of the samples, interface friction angle (𝛿) and internal friction

angle (ϕ) also increased.

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Hence by doing so and considering the ratio of 𝛿/ϕ for all three material, it was

found out that concrete produces higher internal friction angle comparing to the rest,

this comparison and values can be seen in Table 1.

The tests were carried out for both well graded and poorly graded similarly

maximum, minimum and 50% relative density were considered for the comparison;

consequently it showed that the friction angle is affected substantially by different

gradation, grain sizes, relative density (Packing) as well as surface roughness of the

construction materials.

Concrete

Dr Well Graded Poorly Graded

ϕ 𝛿 𝛿/ ϕ ϕ 𝛿 𝛿/ ϕ

Max 40.1 32.1 0.8 36.1 28.8 0.79

50% 38.2 30 0.78 34.8 27.5 0.79

Min 36.6 28.1 0.76 33 25.9 0.78

ϕ 𝛿 𝛿/ ϕ ϕ 𝛿 𝛿/ ϕ

Max 40.1 31.5 0.78 36.1 28.7 0.79

50% 38.2 29.6 0,77 34.8 27.2 0.78

Min 36.6 27.5 0.75 33 25.6 0.77

ϕ 𝛿 𝛿/ ϕ ϕ 𝛿 𝛿 / ϕ

Max 40.1 30.7 0.76 36.1 28.4 0.78

50% 38.2 29 0.75 34.8 27 0.77

Min 36.6 26.7 0.72 33 25.4 0.76

Table (1): Interface and internal friction angle for different construction material

(Gireesha and Muthukkumaran, 2011).

As it can be seen the gradation, different density and surface roughness has a

massive influence on the final value of the friction angle and essentially the shear

resistance of the soil. For this fact and importance, in 1961, Potyondy used similar

construction material as Gireesha and Muthukkumaran, but with a difference of

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determining the values in strain and stress controlled environment (Shear box), the

stress controlled shear box had a shearing area of 80cm2 and as for the strain

controlled box, it had an area of 36cm2.

The tests carried out by Potyondy, were mostly for determining the frictional

resistance for saturated and dry aggregates with varying grain sizes of maximum

7.5mm and 2.5mm for rough and smooth surfaces respectively.

By doing so, Potyondy acquired values for internal (ϕ) and interface (𝛿) friction angle

through putting the sand under normal stresses of nearly 50 and 150Kpa with dense

packing of the grains, void ratio of 0.66 and the moisture content of 0.8%.

The summary of the findings can be found in the table 2 below.

Materials

50 Kpa 150 Kpa

Dry Saturated Dry Saturated

ϕ 𝛿 ϕ 𝛿 ϕ 𝛿 ϕ 𝛿

Smooth Steel 44 24 39 24 43 24 37 23

Rough Steel 44 34 - - 43 33 - -

Wood parallel to

grain 44 35 39 33 43 33 37 33

Wood at right angle

to grain 44 39 39 34 43 38 37 34

Smooth Concrete 44 39 39 34 43 38 37 33

Rough Concrete 44 44 - - 43 42 - -

Table (2): Interface and internal friction angle for different construction material

(Potyondy, 1961).

As moisture content, normal stress, surface roughness, gradation and etc are

the factors affecting the shear strength of soil; shearing rate of which tests are being

taken has a significant role and impact on shear strength and that was clearly

demonstrated by Al-Mhaidib (2006), in which tests were conducted to evaluate the

effect of shearing rate on the frictional resistance between steel and sand, with range

of shearing rate on the specimen of soil varying from 0.9mm/min down to 0.0048

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mm/min and normal stresses of 50kpa, 100kpa and 150kpa of which these tests

were done using a 100mm x 100mm shear box.

The results showed that by increasing the rate of shear, for both smooth and

rough surface, the maximum shear stress increases. This can be seen in figure 1 in

which it can also be noted that by manipulating the surface roughness, soil will

behave in a way which has a higher shear stress.

Figure 1: Coulomb failure envelopes between sand and Steel, rough and smooth

surfaces, (Al-Mhaidib, 2006).

Having said that, by taking into account the work, which was completed by

Laskar and Dey (2011), it could be clearly seen that by using different surface

roughness; R1, R2 and R3 which is accounted for values of 0.1, 0.2 and 0.3; with

constant normal stress, the shear stress will be influenced and gives a higher value.

The surface roughness values in this study were defined as the ratio of the

steel surface roughness to the mean diameter of grain size D50. The results of these

test and physical properties of the sand used can be seen in the figure 2 and table 3

respectively.

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Sand Properties Value

Particle diameter, D50 0.5

Maximum void ratio 0.88

Minimum void ratio 0.62

Relative density 0.85

Specific gravity 2.65

Table 3: Physical Properties of sand (Laskar and Dey, 2011).

Figure 2: Shear stress/strain curves with different surface roughness factors (Laskar

and Dey, 2011).

3. Experimentation (Direct Shear Test)

In this section direct shear test procedure, type of material used in the

process, soil properties and classification will be discussed in details.

3.1. Soil properties and classification:

Dry aggregates were prepared, throughout the tests conducted to find out the

properties of the soil. The soil came across as very angular and low sphericity.

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Figure 3: Soil grain angularity and sphericity table.

It was revealed that actual maximum and minimum values for void ratios are

0.44 and 0.82 respectively which was calculated by using the value of the maximum

and minimum density of the aggregates of which were 1892kg/m3 and 1495kg/m3

respectively.

𝛾 = 𝜌 𝑥 𝑔

𝑒 = 𝐺𝑠 𝑥 𝛾𝑤

𝛾𝑑− 1

Note: 𝛾 (Unit weight of the soil) = 𝛾𝑑 as the specimen is dry.

g = Gravitation (9.81m/s2)

γw = Unit weight of water

GS = Specific gravity (2.67)

However, the void ratios used to conduct the tests were of two different values

to clearly show the effect of densities on the shear strength of soil. These were 0.5

for the densest and 0.8 for loose state and the values were used to obtain the

maximum and minimum relative density for the aggregates, which produced values

of 0.842 and 0.053 for the densest and loosest state of each test specimen.

𝐷𝑟(%) = 𝑒𝑚𝑎𝑥 − 𝑒

𝑒𝑚𝑎𝑥 − 𝑒𝑚𝑖𝑛

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Where:

emax = soil maximum void ratio

emin = soil minimum void ratio

e = actual void ratio used for the tests

In order to achieve such densities, before tests began, the void ratios were

controlled by having the volume of the soil in the shear box fixed in order to reach

the ideal density in the box and changing the weight of the soil to get the

corresponding void ratio where appropriate.

The apparatus used was 100mm x 100mm direct shear box with a volume of

300mm3, therefore, the method of calculation used to estimate the amount of soil

that can be fitted into the box in occasions whether the construction material are or

not being present, is as follow:

𝐺𝑠 𝑥 𝛾𝑤 𝑥 (1 + 𝑤)

(1 + 𝑒)

Where:

W = weight of soil

V = Volume available in the box for the soil to be fitted into

GS = Specific gravity (2.67)

e = Void ratio used

Void ratio (e) Soil weight without

construction material (g)

Soil weight with

construction material (g)

Loose state (0.8) 400 178

Dense state (0.5) 450 214

Table 4: Soil weights in shear box for different void ratios.

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3.2. Sieve Analysis

Sieve analysis was undertaken in order to examine the particle size and how

the distribution of the soil is in reality.

The test was analysed by using four different sieves, 5mm, 3.35mm, 2mm and

1.18mm and at the bottom a solid pan was put in order to gather all the leftovers.

Sieves were shaken for 10 minutes for thorough separation and they

contained a sample of 500 g as it can be seen in the figure below.

Figure 4: Sieve analysis

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Hence, by doing so, the percentage of grain finer table was produced,

therefore, used to acquire the particle size distribution (PSD) curve, and the result of

the analysis is illustrated in the table below.

Sieve Size

Retained Cumulative

Pass Percent % Retained

Weight (g)

Retained

Percent %

Cumulative

Weight (g)

Cumulative

Percent %

5 mm 0.00 0.00 0.00 0.00 100.00

3.35 mm 29.40 5.88 29.40 5.88 94.12

2 mm 407.60 81.52 437.00 87.40 12.60

1.18 mm 62.50 12.50 499.50 99.90 0.10

Pan 0.50 0.10 500.00 100.00 0.00

Table 5: The percentage of grain finer.

The Particle Size Distribution curve in figure 5 presented, to illustrate the grain

distribution and hence using the graph to obtain values for D60, D30 and D10 of which

assist the author to classify the soil being used in this study.

Figure 5: Aggregates Particle Size Redistribution curve.

0.1 1 10

Grain Size (mm)

Sieve Analysis

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Moreover, using the curve above, the following information can be obtained:

D60 = 2.6mm

D30 = 2.3mm

D10 = 1.9mm

Taking these into account the values for Cu and Cc can be found using:

Cu = D60

1.9= 𝟏. 𝟒 < 6

Cc = 𝐷30

𝐷60 𝑥 𝐷10=

2.6𝑥1.9= 1 < 𝟏. 𝟎𝟕 < 6

Considering British standard, the classification that can take place here is of

Multi graded according to British standards (BS EN ISO, 2004).

3.3. Type of construction material

The type of construction materials used for this thesis were steel meshes

which spanned 10mm x 10mm and an overall spanning of 100mm x 100mm which

could fit in the shear box and they sat on top of wooden blocks in order to form the

bottom half of the shear box.

Steel meshes used to for which to evaluate the interface friction angle

between soil and structures were arranged in three different thicknesses; 2 mm, 4

mm and 6 mm deep; figure 6 below demonstrates the difference in thickness.

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The way that construction material were being prepared for tests were to

place the wooden blocks at the bottom of the shear box then placing each steel

mesh for each test.

The reason behind using meshes with different depth is that, when soil is

being sheared on its own, it has the highest friction angle, which provides a higher

shear resistance. Therefore, by using so, these test will tend to push the failure line

back into the soil, in which it will generate a higher friction angle and hence greater

shear strength. This will be discussed more in detail in the following sections.

Figure 6: Types of steel mesh

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The formation of material inside the box, using different steel meshes can be

seen in figures 7, 8 and 9 below for comparison.

Figure 7: 1 steel mesh.

Figure 8: 2 steel meshes.

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Figure 9: 3 steel meshes.

3.4. Direct Shear test procedure

Direct shear is the simplest method to determine the angle of shear resistance

of soils, which can be evaluated by containing the specimen in a split box and apply

normal stress in order to compress the sample and as the two halves slide on top of

each other, the vertical movement and horizontal movement are being measured in

order to evaluate the shear strength of the soil.

As direct shear tests are normally conducted in drained conditions, the tests

carried out in this study will undergo a same maintained condition.

The shearing rate at which these tests were measured was 1 (mm/min) as it

was considered due to the general guidance for sand and suggestion made by

Bolton (1979), also note that if tested quickly the results will be meaningless.

The test procedure for direct shear test which was taken by the author is as

follows:

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1. Take the split box and unscrew the bolts which hold the parts together, this

can be seen in figure 10 blow.

Figure 10: set up procedure of shear box.

2. Place the steel plate at the bottom of the bottom half, depending on

whether it is a test where the soil is being placed on its own for configuring the

internal friction angle or soil is being tested with construction material present to

determine the interface friction angle.

The amount of soil weight, which was calculated with regards to what void

ratio is being used for the test (as it was shown in table 4 in section 3.1) can be

poured into the bottom of the shear box.

As the tests were recorded for two separate void ratio, the manner of which

the soil is being put into the box changes. As of the dense packing sample to be test

after pouring the soil and ensuring it has filled in almost all the gaps in the corner and

edges of the box the by using a compactor.

The compactor is placed on top of the soil and it should be pressed against

the soil to attain the densest state.

On the other hand, for the loose state of the specimen, the soil is being

poured into the box from a small height above the box to make a condition for the

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loose state and following this, the box can be shaken with a gentle manner for the

soil to fill the unnatural gaps inside the box.

3. As the soil is dry, it does not require any porous stone or such to be placed

in the box. Once completed the lid of the box can be placed on the top, which is used

to make the normal stress apply at the very centre of the box, which then distributes

the load uniformly then putting the support bolts back in and tighten, as it can be

seen in the figure 11.

Figure 11: Assembly of direct shear apparatus.

4. Take the box to the direct shear machine, then place the piston on top of

the lid, which is connected to the lever, which is used to for the load to be put on and

transferred to the sample as a normal vertical stress, then unscrew the bolts and

take them out.

5. Once everything is in order, then the gauges for vertical and horizontal

movement as well should be set to zero in order to obtain the displacements, before

the test begins. As it is shown in the figure 12 below.

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Figure 12: Reading gauges set to zero and ready for test to begin.

6. After preparing everything, turn on the machine and set the appropriate

shearing rate, in this case 1mm/min, and as of the readings for these test, the

vertical dial gauge and load dial gauge were being recorded for every 0.2mm of

horizontal movement, in order to obtain the vertical movement and the shear

strength respectively.

Figure 13: Shearing rate set to 1mm/min.

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7. The normal loads applied for these tests were, 50kpa, 100kpa and 200kpa.

8. As the tests start to run, the shear stress increase quite rapidly dependant

on the normal stress applied. The shear stress start to rise and as it gets close to the

peak, it starts to fall towards the constant state or flattens, the test should not be

stopped right after the maximum shear stress is attained as it make a massive error

towards the final result, as the apparatus is operated without a computer and the

readings are being recorded by hand, each test needs to be let go until the end.

9. Once completed the rest of the tests for the specimen can be carried out at

different normal load, which then by inputting all the figures, a Coulomb line, shear

stress versus horizontal displacement and vertical movement versus horizontal

displacement can be drawn to establish and illustrate the behaviour of the sample.

As it can be seen in figures 14 and 15 for instance, for the dense state of aggregates

itself with 50kpa as a normal stress.

Figure14: shear stress versus horizontal displacement.

0 2 4 6 8 10 12 14

Horizontal displacement (mm)

50 KPa

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Figure 15: vertical movement versus horizontal displacement.

Simple calculation which was used with regards to the direct shear box:

𝛾 = 𝜌 𝑥 𝑔

𝑒 = 𝐺𝑠 𝑥 𝛾𝑤

𝛾𝑑− 1

Note: 𝛾 (Unit weight of the soil) = 𝛾𝑑 as the specimen is dry.

g = Gravitation (9.81m/s2)

Calculate:

Horizontal displacement: by multiplying the dial reading by 0.01mm.

Vertical movement: by multiplying the dial gauge reading by 0.002mm

y = 0.1549x - 0.4177

0 2 4 6 8 10 12

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Shear stress: by (multiplying the load dial reading by the specific gravity) dividing by

[(shearing area [100mm2] x 106)/103].

4. Discussion of Results

This paper will mostly focus on how the factors such as density and surface

roughness of construction material, with three normal loads applied (50kpa, 100kpa

and 200kpa) has an effect on shear strength of the soil.

In addition, all the graphs and charts in the discussion will concentrate on

these factors pertinent to the change in shear strength of the soil and the interface

frictional resistance.

The tests in which the soil was subjected to the mentioned normal stresses,

were conducted with two different void ratios 0.5 and 0.8. Moreover, four different

materials (which have different surface roughness) were considered:

1. Soil/Soil

2. Soil/1 Steel Mesh

Hence, by manipulating the depths of steel meshes, different rate of

roughness for the surfaces were achieved.

The system in which the discussion of results will be presented, is to look into

the effect on shear stress-displacement curve for different normal stresses and

different void ratios for the same construction material first and then for different

construction materials, with the main purpose of comparison.

All the factors affecting the shear strength and frictional resistance will be

compared together, so it could be found out which version has the most and closest

strength and resistance similar to of a soil itself. This study shows how the frictional

resistance varies with different depths of steel mesh, which will illustrate how it

affects the shear resistance of the soil.

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4.1. Constant void ratio vs variable normal stress

The results of tests shown below, illustrate characteristics of shear stress-

displacement relationship under dry conditions at same void ratios:

Which clearly show by increasing the normal stress the shear stress hence

increases.

a. Soil/Soil

Figure 16 and figure 17 clearly show the effect of different normal stresses on

the shear strength of the soil, which demonstrate the typical behaviour of soil under

dry conditions and at the void ratios of 0.5 and 0.8 respectively

Figure 16: Shear stress versus horizontal displacement for void ratio of 0.5 for

soil/soil interface.

0 2 4 6 8 10 12 14

S/S 50Kpa S/S 100Kpa S/S 200Kpa

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Figure 17: Shear stress versus horizontal displacement for void ratio of 0.8 for soil/soil interface.

b. Soil/1 Steel Mesh

Figure 18, shows the shear stress fluctuation as the horizontal displacement

increases in the densest state and figure 19 illustrates the same behaviour for which

used a void ratio of 0.8, and both of these curves were obtained under dry

conditions.

Figure 18: Shear stress versus horizontal displacement for void ratio of 0.5 for soil/1

steel mesh interface.

0 2 4 6 8 10 12 14

S/SM1 50 Kpa S/SM1 100 Kpa S/SM1 200 Kpa

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c. Soil/2 Steel Mesh

Figure 20 and 21 demonstrate the shear stress versus horizontal

displacement and how it is affected as the normal stress increases, for samples

compacted at 0.5 and 0.8 void ratio respectively.

0 2 4 6 8 10 12 14

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d. Soil/3 Steel Mesh

Figure 22 and 23 illustrate similar behaviour as the other three construction

materials, which is by increasing the normal stress, the shear stress also increases

for the void ratios of 0.5 and 0.8 respectively. As all of these tests established a

similar trend, the relationship can be explained through using the coulomb failure

equation:

Where:

τ = shear stress

c = cohesion of soil which is zero for cohesion-less soils such as sand

σ = Normal stress

ϕ = internal friction angle

0 2 4 6 8 10 12 14

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Which by considering the coulomb failure equation, in order to calculate the

internal friction angle and/or interface friction angle, the equation can be rearranged

as follows:

ϕ = tan-1(τ/ σ)

0 2 4 6 8 10 12 14

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As discussed in this section, by looking into the state where normal stresses

were variable with constant void ratio, it could be concluded what happens to the soil

if the density is constant; and it was proven that by increasing the normal stress,

ideally the shear strength of the soil should in fact rise, which it was clearly shown by

various graphs in this section, through using different versions of steel mesh

interfacing with the soil.

4.2. Constant normal stress vs variable void ratio

Following that, in order to distinguish the effect of void ratio on shear strength

and prove that by changing the density of the soil itself within the tests carried out,

the shear resistance of the soil can in fact be manipulated and produce a higher

frictional resistance.

Hence, with the intention of illustrating that, number of tests were performed

to show that by keeping the normal stresses constant and changing the void ratios

0 2 4 6 8 10 12 14

27 | P a g e

and by looking at the result it can be concluded that almost often by decreasing the

void ratio, the frictional resistance of the soil does in fact increase; considering the

depth of the steel mesh as well as the normal stress applied to the specimen; as little

as it can be.

The following figures of 24 – 35 show the behaviour of the soil at which, it is

subjected to the constant rate of normal stress in each graph ranging from 50kpa,

100kpa and 200kpa, with variable void ratios of 0.5 (dense) and 0.8 (loose) to show

the effect of density on the shear stress. These tests have been carried out for the

soil/soil, soil/1 steel mesh, soil/2 steel mesh and soil/3 steel mesh.

Finally, individual graphs for the shear stress versus the horizontal

displacement will be provided in the appendices for more details.

Figure 24: Shear stress versus horizontal displacement.

0 2 4 6 8 10 12 14

S/S 50Kpa (Dense) S/S 50Kpa (Loose)

28 | P a g e

. Figure 26: Shear stress versus horizontal displacement.

0 2 4 6 8 10 12 14

29 | P a g e

0 2 4 6 8 10 12 14

S/SM1 100 Kpa (Dense) S/SM1 100 Kpa (Loose)

0 2 4 6 8 10 12 14

30 | P a g e

0 2 4 6 8 10 12 14

0 10 20 30 40 50 60 70

31 | P a g e

0 2 4 6 8 10 12 14

32 | P a g e

4.3. Shear failure envelope

In this study the shear failure envelopes were prepared with regards to

constant void ratios, from which by varying the normal stresses and by plotting them

on the graph, a comparison was concluded to illustrate the difference between a

different version of soil/steel-mesh tests and soil/soil, where through these tests,

different values of internal and interface friction angles were obtained.

Therefore, purely for comparison of such behaviour, the following graphs have

been provided to illustrate under which condition of soil/material interface; the soil

produces a higher value for the angle of friction and hence a higher shear strength,

these were categorised in two major graph for 0.5 and 0.8 void ratios.

Having said that, for more detail, a separate graph for each test will be

provided in the appendices.

Figure 37: Typical shear envelope for soil sample for different interfaces with a void

ratio of 0.5.

0 50 100 150 200 250

Normal stress (Kpa)

Soil/Soile=0.5

Soil/SM1e=0.5

Soil/SM2e=0.5

Soil/SM3e=0.5

Linear(Soil/Soile=0.5)Linear(Soil/SM1e=0.5)Linear(Soil/SM2e=0.5)Linear(Soil/SM3e=0.5)

33 | P a g e

Figure 38: Typical shear envelope for soil sample for different interfaces with a void

ratio of 0.8.

As it can be clearly seen, as the void ratio decreases the critical interface

friction angle increases. By examining more in detail it can be seen that, as void ratio

increases, the interface friction angle between soil with three steel meshes and the

aggregates start to show a higher value than the internal friction angle of the soil

itself, whereas in dense state this phenomena is not true and the internal friction

angle still has a highest value.

These tests suggest that by having a loose state of the soil and increasing the

depth of the steel meshes, it does in fact assist the failure line to be pushed into the

soil to generate a higher value for interface friction and angle and hence a higher

frictional and shear resistance.

Also curves for shear stress ratios versus normal stress were prepared for

each interface test, with different void ratios in order to display how the shear stress

ratios are affected by considering different void ratios. The graphs 39 – 42, hence

visualised this behaviour, for every material interface test with the soil.

0 50 100 150 200 250

Normal stress (Kpa)

Soil/Soile=0.8

Soil/SM1e=0.8

Soil/SM2e=0.8

Soil/SM3e=0.8

Linear(Soil/Soile=0.8)Linear(Soil/SM1e=0.8)Linear(Soil/SM2e=0.8)Linear(Soil/SM3e=0.8)

34 | P a g e

Figure 39: shear stress ratio versus normal stress for soil/soil with different void

ratios.

Figure 40: shear stress ratio versus normal stress for soil/1-steel-mesh with different

void ratios.

0 50 100 150 200 250

τ /σ

normal stress (kpa)

Soil/Soil e=0.5

Soil/Soil e=0.8

Linear(Soil/Soile=0.5)

Linear(Soil/Soile=0.8)

0 50 100 150 200 250

τ /σ

normal stress (kpa)

Soil/SM1 e=0.5

Soil/SM1 e=0.8

Linear(Soil/SM1e=0.5)

35 | P a g e

void ratios.

0 50 100 150 200 250

τ /σ

normal stress (kpa)

Soil/SM2 e=0.5

Soil/SM2 e=0.8

0 50 100 150 200 250

τ /σ

normal stress (kpa)

Soil/SM3 e=0.5

Soil/SM3 e=0.8

Linear (Soil/SM3e=0.5)

Linear (Soil/SM3e=0.8)

36 | P a g e

As it can be demonstrated, these graphs show how the shear stress ratio

drops as the normal stress increases. Although it has been evidently proven that the

soil/soil interface has higher value of the ratio compared with other interfaces,

however, soil/3-steel-meshes does corresponds to such behaviour in similar manner

as the soil/soil and has the closest value to the soil shear stress ratios.

All other charts prepared for the relations for shear envelopes for individual

soil/material interfaces are provided in the appendices. It is noticed that all the

results produce a rather similar trends.

4.4. Dilatency of tested granular material

As it was mentioned the packing and initial density of soil has a massive

influence on how the soil behaves whilst shearing, hence by considering the granular

material that has been used in this study, an increase in volume can be observed.

For dense and slightly dense packing coarse soil; the soil undergoes grains

contracting at the start of shearing as the normal stress compacts the soil initially, as

this occurs, there will be a point where the interlocking of granular particles will

prevent further contraction and since there will be no more shearing due to this,

therefore, the soil has to dilate (expand in volume) in order to roll/slide pass one

another and shear.

As additional shear force is required to dilate the soil, peak strength occurs.

Once the peak is reached and has been overcome by continued shearing, the soil

reaches a state where the applied shear reduces and there will be no further

changes in volume of the soil, this is called strain softening.

In this section to show this behaviour of soil, the soil was subjected to a

number of different normal stress with different void ratios of 0.5 and 0.8, for different

interfaces to assess the performance of the soil.

The graphs 43 – 50, show the volume change of the soil subjected to 50kpa,

100kpa and 200 kpa for each soil/material interfaces with different packings.

37 | P a g e

Figure 43: Vertical movement and horizontal displacement curve for soil/soil with

void ratio of 0.5.

Figure 44: Vertical movement and horizontal displacement curve for soil/1-steel-

mesh with void ratio of 0.5.

0 2 4 6 8 10 12 14

38 | P a g e

0 2 4 6 8 10 12 14

39 | P a g e

Figure 47: Vertical movement and horizontal displacement curve for soil/soil with

void ratio of 0.8.

0 2 4 6 8 10 12 14

40 | P a g e

0 2 4 6 8 10 12 14

41 | P a g e

As it can be undoubtedly seen the recorded results of the tests, suggest that

by increasing the normal stress and the load applied to the soil the volume change

and dilation of the soil decreases.

All the other charts which present the results for each individual test carried

out for all the soil/material interfaces will be accessible in the appendices for more

details.

Considering the results and that through shearing the soil in the shear box, the lid

moves upwards at an angle of dilation (ψ), therefore, in order to calculate the dilation

angle of each soil the following method was carried out:

Ψ = tan-1 (-dεv/dγ)

Which was obtained by using the trend-line for each test and each normal

stress to produce the exact dilation angle.

Since the dilation of the soil, has an impact on the friction and shearing

resistance, then the actual shearing resistance mobilised will be ϕ’current which is

made up of two component:

ϕ’current = ϕ’crit + ψ

However, Bolton (1986), showed that in plane strain, the contribution of

dilation is better represented by:

ϕ’current = ϕ’crit + 0.8ψ

This correction to the ϕ’current, will decrease the value to the critical state value ϕ’crit.

Tables 6 – 13 present the values for the initial internal and interface friction angles as

well as the dilation angles, hence, the altered internal and interface friction angles for

each soil/material interfaces and normal stress for different void ratios.

42 | P a g e

Soil/Soil

e = 0.5

Normal stress

(σ) (kpa)

Shear stress

(τ) (kpa) ϕ’crit (ᵒ) 0.8ψ (ᵒ) ϕ’current (ᵒ)

50 62 51.11 5.29 56.4

100 121.75 50.6 7.1 57.7

200 216.54 47.27 5 52.27

Table 6: Interface friction angle for soil/soil with e = 0.5.

Soil/Soil

e = 0.8

Normal stress

(σ) (kpa)

Shear stress

50 73.96 55.94 7.45 63.4

100 110.3 47.8 4.8 52.6

200 206.4 45.9 3.97 49.87

Table 7: Interface friction angle for soil/soil with e = 0.8.

43 | P a g e

Soil/1 Steel Mesh

e = 0.5

Normal stress

(σ) (kpa)

Shear stress

50 60 50.2 6.88 57.08

100 111 47.98 5.78 53.76

200 200.5 45.1 4.48 49.58

Table 8: Interface friction angle for soil/1 steel mesh with e = 0.5.

Soil/1 Steel Mesh

e = 0.8

Normal stress

(σ) (kpa)

Shear stress

50 63.5 51.78 3.62 55.4

100 113.7 48.67 1.57 50.24

200 199.5 44.93 1.76 46.69

44 | P a g e

Soil/2 Steel Mesh

e = 0.5

Normal stress

(σ) (kpa)

Shear stress

50 60.3 50.33 5.92 56.25

100 120.2 50.24 5.16 55.4

200 203.45 45.5 4.24 49.74

Soil/2 Steel Mesh

e = 0.8

Normal stress

(σ) (kpa)

Shear stress

50 60 50.2 2.11 52.31

100 103.6 46.01 1.98 47.99

200 201 45.14 1.93 47.07

45 | P a g e

Soil/3 Steel Mesh

e = 0.5

Normal stress

(σ) (kpa)

Shear stress

50 59.8 50.1 5.96 56.01

100 127.9 51.98 4.86 56.84

200 205.3 45.75 3.2 48.95

Soil/3 Steel Mesh

e = 0.8

Normal stress

(σ) (kpa)

Shear stress

50 68.62 53.92 4.08 58

100 123.62 51.03 2.28 53.31

200 208.5 46.2 1.84 48.04

46 | P a g e

4.5. Void ratio (e) and friction angle (ϕ)

As mentioned previously, the internal and interface friction angle increase as the

void ratio decreases. In this section the average values of friction angles for all

materials will be compared.

Figure 51 shows the typical relation between the friction angle and void ratio

and it can be observed that while void ratio increases, the friction angle drops. This

can be evidently demonstrated by the slope for the soil/soil interaction as has a

higher slope, hence drop.

Figure 51: Relationship between void ratio and friction angle for all soil/material

interfaces under dry conditions.

It should be noted that the interface friction angle for soil interfaced with three steel

meshes does not change significantly with the change of void ratio.

0.45 0.5 0.55 0.6 0.65 0.7 0.75 0.8 0.85

e (ᵒ

Void Ratio (e)Soil/Soil Soil/SM1 Soil/SM2 Soil/SM3

Linear (Soil/Soil) Linear (Soil/SM1) Linear (Soil/SM2) Linear (Soil/SM3)

47 | P a g e

It is noticed from all the curves that the relation is rather linear and as the void

ratio increases the friction angle decrease.

Therefore, table 14 presents the average values of friction angle for all

material with all normal stresses combined and considered, for different void ratios

(0.5 and 0.8).

Interfaces Void ratio (e) Friction angle

(ϕ) (ᵒ)

Soil/Soil 0.5 47.1

0.8 44.9

Soil/1-steel-mesh 0.5 44.8

0.8 44.5

0.8 44.7

0.8 45.72

Table 14: Overall friction angle for soil/material interfaces, with different void ratios.

48 | P a g e

5. Conclusion

Direct shear tests were conducted in order to illustrate the effect of surface

roughness of construction material; which in this study steel meshes with variety of

depths were used; on the shear strength of the soil, also the effect that different void

ratios and densities have on this matter.

The soil used was construction aggregates with grain sizes of 2mm used to carry out

the tests. To determine the shear stress and hence the angle of friction, whether

internal or interface, direct shear box apparatus was used, with normal stresses of

50kpa, 100kpa and 200kpa on dry conditions.

The materials used to evaluate the interface friction angle were steel meshes that

were prepared with depths of 2mm, 4mm and 6mm which was made available by

welding the steel meshes together.

Through carrying out these test, the study here draw the following conclusions:

From shear stress and horizontal displacement graphs considering constant

void ratio and variable normal stresses, it was understood that as the normal

stress increases, shear stresses increase.

However by rearranging the graphs by considering constant normal stresses

and variable void ratios, it could be observed that shear stress increases as

the void ratios decrease (density increases), hence the density has a massive

impact on shear stress and frictional resistance manipulation.

From the shear envelopes, it was noticed that by decreasing the friction angle

(although it fluctuates from time to time) the shear stress increases.

The shear strength of soil is always higher compared to the interface friction

angle between the soil and construction material.

Future work can be carried out, to evaluate and observe the behaviour of the soil by

adding up to the depth of the steel meshes and see what sort of differentiation they

make to the shear strength and frictional resistance of the soil, also considering the

change of moisture contents, grain sizes and angularity to evaluate how these have

an effect on the shear strength of the soil.

49 | P a g e

6. Appendices

soil/material interactions with e = 0.5 as well as a graph for the comparison of

all curves with similar void ratio.

C. Graphs to show the comparison of all relationships between the shear stress

and the horizontal displacement for all soil/material interactions with similar

normal stress and variable void ratios.

D. Relationship between the vertical movement and the horizontal displacement

for all soil/material interactions with e = 0.5 as well as a graph for the

comparison of all.

E. Relationship between the vertical movement and the horizontal displacement

comparison of all.

F. Coulomb failure envelopes for all soil/material interactions with e = 0.5.

G. Coulomb failure envelopes for all soil/material interactions with e = 0.8.

50 | P a g e

0 2 4 6 8 10 12 14

Soil/soil,50 Kpa,e=0.5

0 2 4 6 8 10 12 14

Soil/soil, 100 Kpa, e=0.5

0 2 4 6 8 10 12 14

Soil/soil, 200 Kpa, e=0.5

51 | P a g e

0 2 4 6 8 10 12

Soil/SM1, 50 Kpa, e=0.5

0 2 4 6 8 10 12 14

52 | P a g e

0 2 4 6 8 10 12

0 2 4 6 8 10 12 14

53 | P a g e

0 2 4 6 8 10 12

0 2 4 6 8 10 12 14

54 | P a g e

0 2 4 6 8 10 12 14

Horizontal displacement

S/S 50Kpa S/S 100Kpa

S/S 200Kpa S/SM1 50 Kpa

S/SM1 100 Kpa S/SM1 200 Kpa

55 | P a g e

0 2 4 6 8 10 12

Soil/Soil, 50 Kpa, e=0.8

0 5 10 15

56 | P a g e

0 5 10 15

0 2 4 6 8 10 12

0 5 10 15

57 | P a g e

0 5 10 15

0 2 4 6 8 10 12

0 5 10 15

58 | P a g e

0 5 10 15

0 2 4 6 8 10 12

0 5 10 15

59 | P a g e

0 5 10 15

0 2 4 6 8 10 12 14

Horizontal displacement (mm)S/S 50Kpa S/S 100Kpa S/S 200Kpa

60 | P a g e

C. Graphs to show the comparison of all relationships between the shear stress

and the horizontal displacement for all soil/material interactions with similar

normal stress and variable void ratios.

0 2 4 6 8 10 12 14

61 | P a g e

0 2 4 6 8 10 12 14

62 | P a g e

0 2 4 6 8 10 12 14

63 | P a g e

D. Relationship between the vertical movement and the horizontal displacement

comparison of all.

Soil/soil e = 0.5 50kpa

y = 0.1549x - 0.4177

0 2 4 6 8 10 12Ve

y = 0.1559x - 0.4217

0 2 4 6 8 10 12 14

64 | P a g e

Soil/1-Steel-mesh e = 0.5 50kpa

y = 0.1092x - 0.3371

0 2 4 6 8 10 12 14

y = 0.1551x - 0.2296

0 2 4 6 8 10 12

65 | P a g e

y = 0.1268x - 0.32

0 2 4 6 8 10 12 14

y = 0.0981x - 0.2878

0 2 4 6 8 10 12 14

66 | P a g e

y = 0.13x - 0.2328

0 2 4 6 8 10 12

y = 0.1286x - 0.2275

0 2 4 6 8 10 12 14

67 | P a g e

y = 0.0923x - 0.205

0 2 4 6 8 10 12 14

y = 0.1307x - 0.2129

0 2 4 6 8 10 12

68 | P a g e

y = 0.1287x - 0.2054

0 2 4 6 8 10 12 14

y = 0.0688x - 0.1574

0 2 4 6 8 10 12 14

69 | P a g e

E. Relationship between the vertical movement and the horizontal displacement

comparison of all.

y = 0.164x - 0.3673

0 2 4 6 8 10 12

y = 0.1053x - 0.2799

0 2 4 6 8 10 12

70 | P a g e

y = 0.0867x - 0.2952

0 2 4 6 8 10 12

y = 0.079x - 0.195

0 2 4 6 8 10 12

71 | P a g e

y = 0.0342x - 0.1583

0 2 4 6 8 10 12 14

y = 0.0384x - 0.2143

0 2 4 6 8 10 12 14

72 | P a g e

y = 0.13x - 0.2328

0 2 4 6 8 10 12

y = 0.1286x - 0.2275

0 2 4 6 8 10 12 14

73 | P a g e

y = 0.0923x - 0.205

0 2 4 6 8 10 12 14

y = 0.0886x - 0.1836

0 2 4 6 8 10 12

74 | P a g e

y = 0.0498x - 0.1242

0 2 4 6 8 10 12 14

y = 0.04x - 0.2288

0 2 4 6 8 10 12 14

75 | P a g e

F. Coulomb failure envelopes for all soil/material interactions with e = 0.5.

Soil/Soil e = 0.5

Soil/1-Steel-mesh e = 0.5

y = 1.0769x + 5.842

0 50 100 150 200 250

Normal stress (kN/m2)

Coulumb envelope

Linear (e=0.5)

y = 0.9917x + 6.1

0 50 100 150 200 250

Coulumb envelope

Linear (e=0.5)

76 | P a g e

y = 1.0116x + 7.47

0 50 100 150 200 250

Coulumb envelope

Linear (e=0.5)

y = 1.0264x + 8.44

0 50 100 150 200 250

Coulumb envelope

Linear (e=0.5)

77 | P a g e

G. Coulomb failure envelopes for all soil/material interactions with e = 0.8.

Soil/Soil e = 0.8

y = 0.9977x + 10.364

0 50 100 150 200 250

Coulumb envelope

Linear (e=0.8)

y = 0.9821x + 8.24

0 50 100 150 200 250

Coulumb envelope

Linear (e=0.8)

78 | P a g e

y = 0.9901x + 4.52

0 50 100 150 200 250

Coulumb envelope

Linear (e=0.8)

y = 1.0253x + 10.472

0 50 100 150 200 250

Coulumb envelope

Linear (e=0.8)

79 | P a g e

7. Reference

Al-Mhaidib, Abdullah I. "Influence of Shearing Rate on Interfacial Friction between

Sand and Steel." Engineering Journal of the University of Qatar 19 (2006): 1-16.

Bolton, M. D., (1979). “A guide to Soil Mechanics” Macmillan Press Ltd, London.

British Standard, (2004). “Geotehcnical investigation and testing identification and

classification of soil: Part 2: Principles for a classification”, EN ISO 14688-2:2004

Das, Braja M., (1997). “Advanced soil mechanics”, Second Edition, California State

University, Sacramento.

Das, Braja M., (2005), “Fundamental of Geotechnical Engineering”, 4th Edition,

California State University, Sacramento

Forst, J.D. and Han, J., 1999, Behaviour of interfaces between fiber-reinforced

polymers and sands, Journal of geotechnical and geoenviromental engineering,

125:633-640.

Gireesha, T. and K. Muthukkumaran. "Study on Soil Structure Interface Strength

Property." International Journal of Earth Sciences and Engineering 4.6 (2011):

89-93. Web

Laskar, Anowar H. "A Study on Deformation of the Interface between Sand and Steel Plate under Shearing." Proceedings of Indian Geotechnical Conference (2011): 895-898. Web.

Potyondy, J. G., 1961, “Skin Friction between Various Soils and Construction

Materials”. Geotechnique 11.4: 339-353.

Rinne, N.F., 1989, Evaluation of interface friction between cohesionless soil and

common construction materials, the University of British Columbia, Canada.

Uesugi, M. and Kishida, H., 1986, Frictional resistance at yield between dry sand

and mild steel. Soil Foundation.

Yoshimi, Y. and Kishida, T., 1981, Friction between sand and metal surfaces. In:

Proceedings of 10th International Conference on Soil Mechanics and Foundation

Engineering, Stockholm, Sweden, vol. 1.

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