Proceedings of Indian Geotechnical Conference December 15-17,2011, Kochi (Paper No.J-049 )

BEHAVIOUR OF GEOSYNTHETIC REINFORCED SAND BED

UNDER CYCLIC LOAD

M.V.S. Sreedhar, Assistant professor, Osmania University, Hyderabad-500007. E-mail: mvs_sreedhar@yahoo.com

A. Pradeep Kumar Goud,M.EStudent, Osmania University, Hyderabad-500007. E-mail: pradeepgoudalwala@gmail.com

ABSTRACT: An attempt was made in this project to investigate the static and cyclic behaviour of Sand reinforced with

geosynthetic products by conducting load tests in a large size tank of 1200 x1200 x900 mm internal dimensions. The static

behavior was ascertained in terms of “Bearing Capacity Ratio (BCR)” and the dynamic response in terms of ‘Coefficient of

Uniform Compression, Cu’. The sand bed of 500mm thick was compacted at a relative density of 90% in dry state and is

reinforced with four geosynthetic products viz., Polymeric Woven Geotextile, uni-axial geogrids of two different capacities

and a coir geotextile. The size, shape, depth of placement and the surcharge were maintained same. A square model footing

of 100mm size was used as the loading unit. Static and Cyclic Plate Load tests are carried out separately on unreinforced

sand and sand reinforced with each type of geosynthetic product. The results indicated that, the geosynthetic products with

higher mobilized tensile strength have shown better improvement in BCR and Cu values. Based on the results, within the

scope of this project, a mathematical model was constituted for prediction of BCR and Cu values.

INTRODUCTION

The infrastructure development in India has tremendously

increased the applications of geo-synthetics in the areas of

pavements, retaining walls, solid waste disposal etc. The

design philosophies are mainly based on the behaviour of

reinforced earth subjected to static loads. However, all

these structures are prone to cyclic loads due to an earth

quake, traffic load, machine foundations etc. It is therefore

necessary to understand the behaviour of geosynthetic

reinforced soil subjected to cyclic loads.

LITERATURE REVIEW Limited research was done on studying the behaviour of

reinforced sand under cyclic loading. A.K.Verma and Bhatt

(2008),[1] have studied behaviour of reinforced sand under

cyclic loading. They reported that the reinforced sand has

more damping capacity than that of unreinforced sand bed

and the percentage damping capacity decreases with

increase in load intensity.

Shvets and Nazha (2000),[2] investigated the influence

exerted by anisotropy of the deformation properties of soil

foundation beds on the elastic characteristics used in

dynamic analyse of machine-bearing foundations. They

suggested that correction factors dependent on the degree

of anisotropy of the soil, which should be used in the

analyses.

STATEMENT OF THE PROJECT

The objective, necessity and scope of this project are as

given below

Objective

The primary objective of this project is to determine the

bearing capacity and coefficient of elastic uniform

compression of geosynthetic reinforced sand by conducting

static plate load test, cyclic plate load test respectively. And

the objective includes the determination intersurface

friction between sand and geosynthetics based on pull out

test.

Necessity

The design of machine foundation based on Cu , it is

related to remaining properties. Cu can be improved with

geosyntheic reinforced material. Having higher the Cu

value, resilience is enhanced.

Scope

Scope of the project is bounded to the observing the

mechanism in macroscopic level in terms of pressure and

settlement. Static , cyclic plate load test and pull out tests

are conducted using one type of sand and four types of

geosynthetics.

METHODOLOGY

The methodology includes the

Collection and characterisation on sand

Collection and characterisation of geosynthetics

Experimental programme.

Collection and characterisation on sand

The sand was used in this project collected from Krishna

River, Andhra Pradesh. The engineering properties of the

sand were as shown in Table. 1 Table 1 Properties of sand

Property Value

Specific gravity 2.52 MDD (kN/m3) 1650 OMC (%) 13.6 Cu 4.11 Cc 1.27 Classification of sand SP

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M.V.S.Sreedhar & A.Pradeep Kumar Goud.

Angle of shear resistance at 90% RD

410

Collection and characterisation of geosynthetics

Total four types of geosynthetics were used in this work.

The properties of all productes were shown in Tables 2 - 5.

All the geosynthetics are shown in Fig. 1

Table 2 Properties of woven GT (SKAPS W300)

Property Value

Tensile strength (kN/m) 40 Puncture resistance(kN) 533 Brust strength 4134 Weight ( g/m2 ) 203

Table 3 Properties GG (STRATA SG 200)

Property Value

Tensile strength MD* ( kN/m) CD** ( kN/m )

52.5 30

Creep reduction factor 1.46 Grid aperature size MD ( mm) CD (mm )

17 18

Creep limited strength MD (kN/m)

35.9

*MD: Machine direction. ** CD: Cross direction. Table 4 Properties GG (STRATA SG 700)

Property Value

Tensile strength MD ( kN/m) CD ( kN/m )

172.2 30

Creep reduction factor 1.46 Grid aperature size MD ( mm) CD (mm )

50 19

Creep limited strength MD (kN/m)

117.9

Fig.1 Image of geosynthetics

Table 5 Properties of coir GT (charankattu CCM900)

Property Value

Tensile strength MD kN/m

27

CD kN/m 10 Thickness mm 8.59 Ultimate strain (%) MD mm

31

CD mm 45.6 Weigth (g/m2) 900

EXPERIMENTAL PROGRAMME

Experimental programme consists the sample preparation

and then and static , cyclic plate load test and pull out test

were performed.

Test setup All the static and cyclic plate load test were conducted in

size of 1200mm X 1200mm X 900mm concrete tank.

Fig. 2 schematic diagram setup

The model putting used for the was square in shape, 20mm

thickness 100mm width. A screw jack was being used to

apply the pressure. Schematic diagram of test setup was

shown in Fig. 2

Preparation of sand bed

The sand was prepared by raising technique, was

compacted at 90% relative density in lifts of 50mm thick

each duly controlling the dry density of each lift.

Reinforcement was introduced at a depth of 50mm below

the footing. Surcharge in the form of pre-cast CC blocks

was applied at foundation level to simulate an overburden

of 2.40 B thick, where B is width of the model footing.

Preparation of sample shown in Fig. 3

Fig. 3 Sample prepatration

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Behaviour of geosynthetic reinforced sand bed under cyclic load.

Test Procedure

Static plate load tests were conducted as per IS 1888-1982 ,

on sand bed reinforced with four geosynthetic products in

strain controlled mode, to determine the bearing pressure

and settlement corresponding to failure .

The bearing pressure corresponding to failure is divided

into five stages for conducting the cyclic plate load test.

Each stage of loading is applied till the rate of loading is

applied till the rate of settlement is than 0.02 mm/hr and

then gradually unloaded and the settlement is in unloading

stage is also recorded. The next higher stage of loading is

applied and removed as part of cyclic . The procedure is

repeated till the last cycle where the bearing pressure

corresponding to failure is applied.

Cyclic plate load tests were conducted as per

IS:5249:1992,

on sand bed reinforced with four geosynthetic products in

stress controlled mode, bearing Pressure v/s settlement

and Pressure v/s elastic rebound graphs were plotted .

Fig. 4 Model graph for pressure Vs Elastic rebound

then coefficient of elastic uniform compression was

calculated with following formula.

Cu= P/Se

Where

P: Pressure at specific increment level

Se: Elastic rebound corresponding to specific pressure

RESULTS AND DISSCUSSIONS

Having conducted all static ,cyclic plate load tests and pull

out test ,results were obtained.

Results of Static Plate Load Tests

All the pressure Vs settlement of static plate load test were

shown in Fig. 5

Fig. 5 Pressure Vs Settlement curves of static plate load

test

Cyclic Plate load test results

All the pressure Vs settlement curves of cyclic plate load

test were shown in following Fig. 6

Fig. 6 Pressure Vs settlement curves of cyclic plate load

test.

BCR of reinforced sand

The variation of BCR values for the un-reinforced and

geosynthetic reinforced sand as shown in Fig. 7 . It can be

observed that, the maximum BCR was observed in respect

of WGT and GG-SG700, which in general possess greater

tensile strength and better co-efficient surface friction.

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M.V.S.Sreedhar & A.Pradeep Kumar Goud.

Fig. 7 Varitation of BCR & Cu Vs geosynthetic products

Variation of Cu value

The improvement in Cu ratio for the un-reinforced and

reinforced sand are shown in Fig.7. The maximum Cu

value was observed in respect of WGT and GG-SG700,

which in general possess greater tensile strength and better

co-efficient surface friction similar to that of BCR.

It is important to note that, though the tensile strength of

GG-SG200 in machine direction is higher than the tensile

strength of WGT, the BCR and Cu are less than that due to

WGT, for the reason that, the GG is uni-axial while the load

bearing mechanism is symmmetrical to both machine and

cross direction. Hence, the low tensile strength in cross

direction of GG and the redistribution of stresses may be

responsible for the low BCR and Cu. It was demonstrated

in the form of failure of GG in cross direction.

Fig. 8 Deformation of geogrid in cross direction

Development of a Constitutive model

A Mathematical model was developed by regression

analysis for Qf & Cu. In developing the mathematical model

tesile strength , intersurface friction in both directions and

strain at faillure are considered as significant independent

variables.The intersuface friction for four geosynthetic

product were found out by laboratory pullout test. All the

value are presentd in Table 6 Table 6 Results for development of constitutive model

WGT GG200 GG700 CGT UR

Qf 208 184 206 174 120

Cu 17.2x10-4 16.7x10-4 17.5x10-4 17.2x10-4 8x10-

4

μx 0.20 0.30 0.22 0.16 ---

μy 0.16 0.24 0.17 0.12 ---

Tx 40 52.5 172.5 27 ---

Ty 40 30 30 10 ---

f 8.8 10.6 10.4 11.5 13

The constitutive models obtained from regression analysis

with R2 value of 1.0 are as given below.

Qf = 158.833μx + 0.218Tx - 2.290 Ty - 34.628 f + 570.167

Cu = 0.02μy+1.78 x 10-6TX -1.0 X 10-5TY +0.03

Where in

Qf is load at failure in kN/m2 under static loading

Cu is coefficient of elastic uniform compression in kN/m3

μx , μy are coefficient of intersurface friction

Tx,Ty are tensile strength in X, Y directions respectively

f is the strain at failure under static loading (%)

CONCLUSIONS

Based on the experimental results found during this project,

the following conclusions are made

The sand reinforced with geosythetic products has

shown phenominal improvement in BCR and Cu

values

Higher the tensile strength and intersurface

friction of the geosynthetic product, higher will be

the improvement in BCR

The load bearing mechanism of sand reinforced

with uniaxial geogrid subjected to symmetrical

stress conditions such as those of plate load test,

failure in terms of excessive deformations have

occurred in cross direction. This shows that, the

uniaxial geogrids are not appropriate in biaxial

stress conditions.

REFERENCES

1. A.K Verma (2008), Design of machine foundation on

reinforced sand”. The 12th International association for computer method and advances in geomachines, Goa,

India.

2. Shvets. N. S., Nazha. P. N. (2000). Elastic coefficients

of anisotropic foundation beds. Journal of Soil

Mechanics and Foundation Engineering, springerlink

37: 2, 29-34..

3. S.N. Moghaddas Tafreshi (2008), “Cyclic loading on

foundation to evaluate the coefficient of elastic

uniform compression of sand”. The 14th world conference on earthquake engineering, Beijing, China.

4. Verma and Bhatt (2007). “Effect of on damping

capacity of foundation soil under ring footing”,

proceedings of 5th International conference on

Seismology and Earthquake engineering, May-2007,

Tehran, Iran, SF61-pp.1-8.

5. Swami saran (1999), Soil dynamics and machine

foundation, Galgotia publications.

6. IS 1888-1982, “Method of load test on soil”.

7. IS 5249-1977, “Test for the determination of dynamic

properties of soil”.

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