Indian Journal of Geo Marine Sciences

Vol. 46 (10), October 2017, pp. 2137-2145

Experimental and numerical investigation on bearing capacity of

granular soil affected by particle roundness

Babak Karimi Ghalehjough1*

, Suat Akbulut2 & Semet Çelik

1

1Department of Civil Engineering, Engineering Faculty, Ataturk University, Erzurum, 25240, Turkey.

2 Department of Civil Engineering, Engineering Faculty, Yildiz Technical University, İstanbul, 34210, Turkey.

[E.Mail: karimi.babak@gmail.com]

Received 24 April 2017 ; revised 07 July 2017

Main objective of this study is investigation effect of roundness of particles on ultimate bearing capacity (UBC) of soil. A

strip footing was modeled and UBC has been gained at three roundness groups of angular, rounded and well-rounded soil. Two

other factors of size and relative density were added to tests for better understanding of soil behavior at different conditions.

Tests have been done at 3 roundness groups, 6 size groups from 0.30mm to 4.75mm and 3 relative densities of 30%, 50% and

70%. On next step direct shear tests done at all 54 different conditions. Values from shear box test were compared with

experimental results. Comparison showed acceptable relations between theory and experimental results. UBC decreased by

increasing roundness of particles. Changing angular soil to well-rounded shape decreased the UBC about 33% at relative density

of 30%. This decrease was about 40% at Dr=50%. Ultimate bearing capacity of well-rounded sample was 45% less than angular

samples at Dr=70%. It means effect of roundness on UBC was more considerable at bigger relative densities. Same behavior

was seen at bigger sizes. Finally changing shape of particles from angular to rounded soil had more effect on decreasing UBC

than changing rounded to well-rounded soil.

[Keywords: Granular Soil, Ultimate Bearing Capacity, Particle Roundness, Strip Footing]

Introduction

Ultimate bearing capacity of granular

materials is a classic issue in civil and

geotechnical engineering1. Lots of studies have

reported by researchers by using different

materials such as calcareous2. Reinforcement of

fill can improve both ultimate bearing capacity

and settlement of the foundation3-9

. On the other

hand particles shape is an important factor on

shear behaviors of granular materials10-12

. There

are some researches that studied effect of particles

shape or size on soils behavior.

On the study of shear strength behavior of

composite soils affected by grain size distribution

and particle size has been investigated13

. Two

different samples of fine (clay and silt) and

natural coarse (river sand and crushed gravels)

have been prepared and direct shear box test has

been done on samples at this study. Terms that

used to describe particles shape can be named as

texture, sphericity, roughness and roundness14, 15

.

Recent researches have showed that increasing

angularity of materials cause increase in

minimum and maximum void ratio (emax and emin)

and so increasing roundness of particles will lead

to a decrease on maximum and minimum void

ratio15-18

.

Holtz and Gibbs19

research showed that

increasing angularity of a mixture and angular

materials have much higher shear strength than

sub-angular or sub-rounded materials. From

previous researches it is clear that many of

investigation has been done on effect of particle

shape on granular soils properties like sands, but

experimental investigation about particles shape

and size distribution on granular materials are

rare, even if they are most frequent geo-materials

in engineering13

. By using different image

analysis techniques, many investigations have

been done on relationships between particle

shapes or grain-size distributions and mechanical

properties of geo-engineering materials20,21,22

.

Many studies have been done on bearing capacity

of strip footing on different kinds of soils such as

normal or reinforced sands, clay, silt or other

soils. Load-settlement behavior of a shallow

INDIAN J. MAR. SCI., VOL. 46, NO. 10, OCTOBER 2017

foundation on iron ore tailings has been

investigated on other study23

. At the same time

different studies have been done on different

kinds of footings or columns on different 24-28

.

Cascone and Casablanca29

have been studied

on dynamic bearing capacity of shallow

foundations. Ultimate bearing capacity (UBC) of

strip footing has been analyzed on reinforced soil

by Chen and Abu-Farsakh30

. At that study, an

analytical solution has been developed for

estimating UBC of strip footing on reinforced

soil. Results showed that underlying unreinforced

soil and relative density of reinforced soil layer

has effect on punching shear failure zone.

Cicek et al.31

studied effect of reinforcement

length on strip footing behavior. At this study

some laboratory tests has been done on

unreinforced and reinforced soils. Geogrid has

been used for reinforcing the soil and different

types and number of reinforcements has been

investigated to find if these parameters had effect

on optimum reinforcement or not. Results showed

that for achieving optimum reinforcing, length of

reinforcement was needed.

Cure et al.32

were generated both model and

analytical tests on strip footing near a sandy

slope. Effect of ultimate load has been

investigated at this study on both centrally and

eccentrically loaded surface. Results showed that

by increasing eccentricity ultimate load

decreased. Amount of decreasing of ultimate

capacity increased by increasing eccentricity.

Arasan et al.33

has been studied on Balast,

Calcareous, abrasive and bearing balls. By

considering roundness values of particles they

classified tested soils to 6 roundness classes. For

this purpose, the shape properties of particles

were calculated and the effect of particle shape on

the geotechnical properties of aggregate was

investigated by analyzing images of basalt,

calcareous and steel balls (abrasive and bearing

balls). Calcareous has been changed from angular

calcareous to well-rounded calcareous by using

Los Angeles machine at specified revolution.

Although there are lots of studies on strip

footing and granular materials at different

conditions but there are rare studies about particle

roundness or shape and their effect on mechanical

properties and behavior of aggregate. Effect of

size and relative density of soil can be founded at

literature, but effect of roundness of particles on

bearing capacity of soils was not founded. The

originality of this study is that, roundness of

particle has been added to two other parameters

of size and relative density, and effect of

combination of these three parameters on shear

strength and UBC of granular soil have been

studied. At present experimental study, a strip

footing has been modeled at three kinds of

angular, rounded and well-rounded soil, 6 size

groups and three different relative densities and

effect of these parameters has been studied on

shear strength and ultimate bearing capacity.

Materials and Methods

Granular crashed Calcareous has been used at

this study. It has been gained from Ergunler

Company in Erzurum, Turkey. According to

ASTM D 854-1434

, the specific gravity of soil

determined 2.7. Soil has been washed and dried at

laboratory temperature and sieved. Size of soil

was from 0.30mm to 4.75mm and it has been

divided to 6 different size groups. Sieves used for

soil division were 0.30mm, 1.18mm, 1.40mm,

2.36mm, 2.80mm, 3.35mm and 4.75mm. Totally

18 kinds of samples have been prepared and

tested at 3 different roundness classes of angular

(AC), rounded (RC) and well-rounded calcareous

(WRC) and 6 size groups. Each kind of samples

has been tested at three different relative densities

of 30%, 50% and 70%. So altogether there were

54 different tested conditions. According to

unified soil classification system, ASTM D 2487-

1135

, samples were classified as SP. Grain size

distribution gained according ASTM D 6913-0436

has been shown at ‎Fig 1.

Fig.1- Grain Size Distribution of Soil

(AC: Angular Calcareous, RC: Rounded Calcareous and

WRC: Well Rounded Calcareous)

Roundness value of soil determined using

Cox17

equation and Power37

chart. By calculating

this value, the soil gotten from Ergunler Company

was classified as angular soil. Roundness values

and shape of particles by Cox17

has been shown

on Fig.2.

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GHALEHJOUGH et al.: BEARING CAPACITY OF GRANULAR SOIL AFFECTED BY PARTICLES ROUNDNESS

Fig 2: Roundness Values and Particles Shape by Cox (1927) [17, 38]

Los Angles Rattler Machine without balls has

been used for changing angular calcareous (AC)

to rounded calcareous (RC) and well-rounded

(WRC) that explained in Arasan16, 38

. Angular soil

has been rounded for 50000 and 100000 of

revolution on Los Angles machine for changing

to the shape to rounded and well-rounded soil

respectively16

. After rounding angular soil to

rounded and well-rounded, they have been

washed, dried and sieved at mentioned size

groups. Roundness values and groups of soil have

been shown at Table 1.

Table 1- Roundness Values and Classification of Soils 33, 38

Sample Roundness

Value Roundness Class

Angular Calcareous

(AC) 0.693-0.744

Angular-Sub

Angular

Rounded

Calcareous (RC) 0.786-0.803

Rounded-Well

Rounded

Well-Rounded

Calcareous (WRC) 0.834-0.854 Well-Rounded

Shape of calcareous at three different

roundness classes has been showed on Fig.3. As

seen at this picture roundness of particles

increased by going from angular calcareous

toward well-rounded calcareous and it is even

visible by eye.

Fig.3- Pictures of Calcareous at Three Different Roundness

Classes of AC, RC and WRC

This study involves series of tests done at

laboratory in a model tank. A tank with

dimension of 1mx1mx0.10m has been used for

modeling a strip footing. A Plexiglas plate has

been used in front of tank to see the soil and

footing. Strip footing used at this study was made

of Polyamide with the dimension of 9.8cm length,

5cm width and 4cm of height. It was rigid

enough that footing acted as rigid foundation. At

this study the length of model footing was almost

equal to the width of the tank in order to maintain

plane strain conditions. A hydraulic jack has used

for loading the footing. It has been fixed to a

strong horizontal beam of the frame that could

carry thrust developed by hydraulic jack without

any deformation. Loading was applied in small

incensement so speed of displacement was

2<mm/min. A load-cell with capacity of 50kN

was placed between the footing and hydraulic

jack for measuring applied load. A shaft has been

used that transferred the load to footing. It has

been placed between load-cell and footing. Single

point load has been applied to footing by use of

ball bearing that has been placed between shaft

and footing. As rigid footing has been used at this

study, uniform load has been applied to soil. For

preventing contact between walls of tank and

footing a 1mm wide gap has been placed between

them. At the same time two sides of footing and

walls of tank were coated with petroleum jelly for

minimizing end friction effects. Two linear

variable displacement transducers (LVDT) were

used at two cross corners of footing for measuring

vertical settlement of footing. Average of these

two LVDSs has been considered as footing

settlement. Picture and schematic draw of test box

have been shown on ‎Fig. 4 and Fig. 5

respectively‎. Load cell and LVDTs connected to a

data logger for transferring data to computer.

Fig.4- Pictures of Text Box

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INDIAN J. MAR. SCI., VOL. 46, NO. 10, OCTOBER 2017

Fig.5- Schematic Picture of Test Box

Samples have been classified to 3 roundness

groups of angular, rounded and well-rounded

shape. Each roundness group has been divided to

6 different size groups too. Tests have been done

at three relative densities of 30%, 50% and 70%

(Dr=30%, 50% and 70%) for each roundness and

size groups. Altogether there were 54 different

types of tests done. Each test has been done at

least three times for ensuring the results. Tests

program has been showed at Table 2. Minimum

and maximum void ratio (emin and emax) of each

sample has been found according to ASTM

D4253-1639

and ASTM D4254-1640

and weight of

soil for each tested sample has been calculated

and placed in tank at three relative densities by

considering soil physical characteristics and tank

volume.

Table 2- All 54 Different Conditions of Tests

Dimension (mm) Roundness Class Relative

Density (%)

0.30-1.18 Angular, Rounded,

Well-Rounded 30 – 50 - 70

1.18-1.40 Angular, Rounded,

Well-Rounded 30 – 50 - 70

1.40-2.36 Angular, Rounded,

Well-Rounded 30 – 50 - 70

2.36-2.80 Angular, Rounded,

Well-Rounded 30 – 50 - 70

2.80-3.35 Angular, Rounded,

Well-Rounded 30 – 50 - 70

3.35-4.75 Angular, Rounded,

Well-Rounded 30 – 50 - 70

Results and Discussion

After filling the tank, strip footing has been

placed on the soil and loaded. Data have been

transferred to computer. Average of settlements at

two cross corners of footing has been considered

as settlement of footing. Bearing capacity has

been gained by dividing load to area of footing.

Load-Settlement graphs have been drawn and

ultimate bearing capacity has been gained from

these graphs 41,42

.

Ultimate bearing capacity (UBC) of strip

footing at three roundness classes (Angular,

Rounded and Well Rounded Calcareous) founded

and compared together at three different relative

densities of 30%, 50% and 70%. Results have

been shown at Fig. 6, Fig. 7 and Fig. 8. UBC has

been decreased by increasing roundness of

samples. Effect of particles roundness on bearing

capacity is more considerable at size or bigger

relative densities. By changing angular soil to

well-rounded shape, ultimate bearing capacity

decreased about 33% at relative density of 30%.

This decrease affected by roundness of particles

was about 40% at relative density of 50%.

Ultimate bearing capacity of well-rounded

samples was 45% less than angular samples at

Dr=70%.

Fig.6- Ultimate Bearing Capacity of Strip Footing at Three

Roundness Groups (Dr=30%)

Fig.7- Ultimate Bearing Capacity of Strip Footing at Three

Roundness Groups (Dr=50%)

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GHALEHJOUGH et al.: BEARING CAPACITY OF GRANULAR SOIL AFFECTED BY PARTICLES ROUNDNESS

Fig.8- Ultimate Bearing Capacity of Strip Footing at Three

Roundness Groups (Dr=70%)

By looking to results it found that, changing

angular soil to rounded soil has more effect on

ultimate bearing capacity than changing rounded

to well-rounded soil. On the other word

differences on ultimate bearing capacity while

angular soil changed to rounded soil is more than

differences on UBC while rounded soil changed

to well-rounded. Changing angular to rounded

soil caused an average decrease more than 40%

on ultimate bearing capacity at relative density of

70%. While rounded soil changed to well-

rounded this value was less than 20%. This

decrease on ultimate bearing capacity is more

considerable at bigger relative densities. For

example at relative density of 70%, changing

angular soil to rounded shape caused at least 40%

of decrease on ultimate bearing capacity that this

value at Dr=30% was less than 20%.

There was an increase on bearing capacity by

increasing the size of soil from 0.30mm to

4.75mm too. This increase was 2.3, 2.5 and 2

times on angular, rounded and well-rounded soils

respectively while relative density of samples was

30%. At relative density of 50% (Dr=50%)

increasing value was 2.9, 3.2, and 2 times for

three kinds of angular, rounded and well-rounded

samples. Finally effect of increasing of

roundness and size of particles from 0.30 to

4.75mm was, 1.8, 2.2 and 2.2 times on angular,

rounded and well-rounded soils at Dr=70%.

Fig. 9 shows ultimate bearing capacity of soil

at all 54 different conditions of tests at three

different roundness groups and relative densities.

As seen at this figure, ultimate bearing capacity of

soil decreased by changing shape of particles

from angular to rounded and well rounded.

Roundness of particles and less friction of soil on

well rounded samples are the reasons of this

decreasing. Changes on bearing capacity at a

same roundness class are more considerable on

bigger sizes than smaller.

Fig.9- Ultimate Bearing Capacity at Three Roundness

Classes and Relative Densities

By looking on Load-Settlement graphs it

found that at relative densities of 30%, 50% and

70%, soil had punching, local and general shear

failure respectively. As an example it has been

shown for angular calcareous soil with size of

1.40 to 2.36mm at Fig. 10. Samples showed to

some extent same behavior at all angular, rounded

and well-rounded classes. Soil failure

mechanisms on failing moment have been shown

below on Fig. 11, Fig.12 and Fig.13.

Fig.10- Load-Settlement Graphs at 3 Different Relative

Densities for angular Calcareous for material with

Dimension of 1.40 to 2.36mm

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INDIAN J. MAR. SCI., VOL. 46, NO. 10, OCTOBER 2017

Fig.11- Picture of Angular Calcareous at Dr=30% on Failing

Moment

Fig.12- Picture of Angular Calcareous at Dr=50% on Failing

Moment

Fig.13- Picture of Angular Calcareous at Dr=70% on Failing

Moment

There was an increasing on ultimate bearing

capacity by increasing dimension or relative

density of aggregate. Increasing of bearing

capacity because of increasing size, has been

shown for well rounded calcareous (WRC) at

relative density of 30%, 50% and 70% on Fig.14,

Fig.15 and Fig.16 respectively. As seen at these

graphs, increasing size from 0.30 mm to 4.75 mm

caused an increase of 2 times on ultimate bearing

capacity. Approximately 3 times of increase has

been happened while relative density increased

from 30% to 70%.

Fig.14- Load-Settlement Graphs for well-Rounded

Calcareous at Dr=30%

Fig.15- Load-Settlement Graphs for well-Rounded

Calcareous at Dr=50%

Fig.16- Load-Settlement Graphs for well-Rounded

Calcareous at Dr=70%

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GHALEHJOUGH et al.: BEARING CAPACITY OF GRANULAR SOIL AFFECTED BY PARTICLES ROUNDNESS

Direct shear strength tests have been done on all

samples and friction angle of soil at three

roundness groups and different sizes and relative

densities have been calculated. Shear tests have

been done according to ASTM D3080M – 1143

and results have been shown at Fig.17. As seen at

this figure by increasing roundness of particles,

shear strength of soil has been decreased. In

apposite increasing relative density or size of soil

increased the shear strength of samples.

Fig.17- Friction angle of Soil at different Roundness Groups

and dimensions

Smallest value of friction angle gained at well-

rounded samples with size of 0.30-1.18mm at

Dr=30% that was 28 degree and biggest value of

48 degree belongs to angular calcareous at size of

3.35-4.75mm and relative density of 70%. Like

ultimate bearing capacity, difference on shear

strength of samples was more between angular

and rounded soil than differences between

rounded and well-rounded soil. It means

difference on shear strength is more considerable

while angular soil changes to rounded.

Theory results of ultimate bearing capacity can

be gained from Terzaghi formula (Formula. 1)

that used for strip footings.

qu=CNc+γDfNq+0.5γBNγ (1)

As soil was sandy soil so the value of cohesion

waws equal to zero (C=0). Footing spaced on the

soil surface then Df = 0. Value of Nγ depended on

friction angle of soil. By calculating values for Nγ

considering γ (Unit Weight) at different

conditions of tests and knowing amount of B

(Footing width), ultimate bearing capacity of soil

can be calculated. Results gotten from theory and

experimental tests have been shown at Table. 3.

As seen at this table results are near and are in

acceptable range. On experimental shear tests,

there can be an acceptable tolerance of 2 degrees

on getting and calculating friction angle (ø) of

soil. Changing in ø will change the value of Nγ

and so the result for ultimate bearing capacity will

change. By considering this tolerance some

bigger differences on theory and experimental

results of ultimate bearing capacity will be in

acceptable rang too.

Table 3- Comparison between Theory and Experimental Result of Ultimate Bearing Capacity

qu (kN/m2)

Size (mm) Dr AC

(Theory) AC (Test)

RC

(Theory) RC (Test)

WRC

(Theory) WRC (Test)

0.30-1.18

30% 11.6 15.0 6.2 12.0 5.4 10.0

50% 24.6 40.0 12.6 28.0 12.9 25.0

70% 55.4 130.0 26.4 55.0 22.4 44.0

1.18-1.40

30% 23.2 18.0 13.9 13.0 11.9 12.0

50% 42.4 45.0 24.6 34.0 17.6 28.0

70% 80.4 130.0 45.3 80.0 38.2 72.0

1.40-2.36

30% 27.9 20.0 19.8 15.0 17.3 13.0

50% 52.0 50.0 43.8 42.0 30.9 30.0

70% 100.1 135.0 67.9 85.0 56.9 80.0

2.36-2.80

30% 41.2 24.0 29.1 22.0 17.3 16.0

50% 96.8 70.0 53.7 44.0 45.3 40.0

70% 156.7 155.0 84.1 88.0 85.7 86.0

2.80-3.35

30% 50.2 30.0 42.7 25.0 20.8 18.0

50% 96.8 75.0 66.2 55.0 45.8 44.0

70% 156.7 168.0 104.6 100.0 86.7 96.0

3.35-4.75

30% 61.5 35.0 52.7 30.0 30.1 20.0

50% 121.2 115.0 82.0 64.0 55.5 50.0

70% 252.1 250.0 130.9 120.0 107.3 100.0

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INDIAN J. MAR. SCI., VOL. 46, NO. 10, OCTOBER 2017

Conclusion Increasing each parameters of angularity, size

or relative density of aggregate will increase shear

strength and UBC of soil. Changes on shape of

angular soil are more considerable than rounded

or well-rounded soils. This effect is more

considerable at bigger sizes or relative densities.

By increasing particles size or relative density,

UBC showed more increase on angular soils than

rounded or well-rounded soils. Effect of

increasing size and relative density is more

considerable at angular soils than rounded or

well-rounded soil. Changes on shape of angular

soils especially at dense or bigger soils should be

studied carefully.

Acknowledgement

Authors thank the Ataturk University and

Geotechnical engineering division for facilities.

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