Behavior of Geotextile Reinforced Sand on Weak Subgrade

7/25/2019 Behavior of Geotextile Reinforced Sand on Weak Subgrade

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Behavior of Geotextile Reinforced Sand on Weak Subgrade


Talal O. Al-RefeaiCivil Engineering Department, College of Engineering

King Saud University

P.O. Box 8, !iyad" ##$%#Saudi &ra'ia

Abstract. The use of nonwoven geotextiles in unpaved roads provides at least two basic contributions,namely separation and reinforcement functions, therefore reducing rutting and maintenance cost

and greatly improves the overall performance of reinforced unpaved roads !vailable results of

studies on the resilient behavior of subgrade"geotextile"base system are limited and inconsistent

! series of cyclic triaxial tests were carried out to study the effect of nonwoven geotextile on theresilient and plastic behavior of a subgrade"geotextile"base system The results suggest that the

presence of the geotextile did not significantly increase the resilient modulus #increase of only

$%&' (owever, there was a ma)or reduction in the permanent deformation #*+& reduction'

Introduction

Soft clayey and silty subgrades under cyclic loadings exhibit large deformations#rutting' leading to failure of pavement and the pumping of fine"grain subgrade soils intothe graded base course The use of geotextiles in the construction of roads on soft

subgrade is widely practiced and becoming popular especially for unpaved roadsSeveral theoretical #$, ', laboratory #$ - .' and field studies #/,0' are reported on thebehavior of geotextile reinforced aggregate on soft subgrade under repeated or trafficloads Geotextiles placed between soft subgrade soil and the aggregate base layerenhance the load"carrying capacity and improve the performance of unpaved roads

essentially by providing separation and reinforcement benefits The geotextile asseparator prevents intermixing of the aggregate material and the subgrade under theaction of repeated loads, where intermixing reduces the shear strength and stability of the

aggregate layer 1n the reinforcement function the geotextile affects the behavior ofaggregate"geotextile"subgrade mainly by restraint of soft subgrade and confinement ofaggregate layer Robnett and 2ai #3' reviewed the general performance characteristics of

aggregate"geotextile"soil system They concluded that use of an interlayer of geotextilein an aggregate"surface road can lead to either better performance or to substantialreductions in aggregate layer thickness 1t is also shown that the behavior of aggregate"geotextile"soil is complex and difficult to analy4e with theoretical models

5ifferent empirical design methods based on the performance of full scale road

or laboratory studies have been proposed for unpaved roads reinforced with geotextile(ausmann #$+' has assessed and compared them to one another The well known Giroud


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and 7oiray8s method #$$' is based on Boussines9 elastic theory similar to flexiblepavement design methods To develop a more rational design method, the dynamicnonlinear stress"strain behavior of aggregate"geotextile soil system under repeated loadsmust be ade9uately predicted The available literature dealing with soil"geotextile"aggregate system is limited and in some cases, results are even contradictory Raad #$'performed finite"element analysis on a two"layer system that consists of a granular base

over a soft clay subgrade, and found that prestressed geotextile would increase the

resilient modulus of the subgrade below the geotextile :riedli and !nderson #$;'conducted cyclic triaxial tests to study the effects of placing two types of wovengeotextiles between fine and coarse soils Their results showed that the resilient moduliof samples with geotextile were higher by $+ to *+ percent than unreinforced samplesRao #$%' reported that nonwoven geotextile separating silt and sand significantly

improved the resilient modulus in cyclic triaxial testing Saxena and


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for combination of deviator stress #d' and confining stress #;' by the following

e9uationA

@R d?r

where r resilient #recoverable or elastic' axial strain

The !!S(T6 T3% test procedure proposed separate steps for conditioningand testing of granular and cohesive soils as shown in Table $

Table $A Summary of conditioning and loading se9uences #!!S(T6 3%'

T#!e $ soil %&!si' d &!si' (o. of c#cles

conditioning $* $* $+++

testing ;*$+

$*+

;, ., 3*, $+,$*

$+, +, ;+

$+, $*,;+$*, +, %+

$++ each

T#!e ) soil %&!si' d &!si' (o. of c#cles

conditioning . % $+++

testing .;+

, %, ., 0, $+, %, ., 0, $+, %, ., 0, $+

$++ each

$ psi .03 k>a

once!t of *aborator# Testing +rogra,

This study consists of repeated triaxial loading test carried out on specimens of$++ mm in diameter and ++ mm high The lower half of the specimen was the softsubgrade soil and the upper half was the granular base layer !lthough such specimen

does not represent field conditions especially regarding scaling it represents an easymethod to evaluate the resilient behavior of the system under repeated loadingconditions, that represent the stresses in pavements sub)ected to moving loads 5ue tothe si4e of the specimen, it was decided to use scaled"down Cmodel8 base material in thetests instead of aggregate material normally employed in the full scale roads in Saudi

!rabia ! coarse sand from a wadi in Riyadh area was selected as a base material Thesubgrade soil used in the test represent a typical soft subgrade type found beneath roadsin Saudi !rabia The subgrade was obtained from a site %+ km south of Riyadh


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+RI"(TA* WOR/

Testing A!!aratus

!ll experiments were conducted in resilient modulus testing apparatus with an

electropneumatic loading frame The apparatus was made by (DE materials Research

and 5evelopment 1nc Specimens were sub)ected to a repeated deviator stress of fixedmagnitude using a haversine shaped load pulse consisting of a +$+ second load followedby a +3+ second rest period The deviator stress is measured with a load cell mountedwithin the triaxial cell, thereby eliminating load measuring errors caused by the frictionbetween the load piston and the top of the triaxial cell

!ir was used as the confining fluid in the plexiglass cell and pressure wasmeasured by an air"pressure gauge mounted at the base of the triaxial cell Fxternallymounted 2inear Eariable 5ifferential Transducers #2E5Ts' were used to measurerecoverable axial deformations >ermanent axial deformations were measured by a dialgage and an 2E5T mounted on opposite sides of the loading piston outside the triaxial

cell The resilient moduli were calculated for each loading se9uence using a personalcomputer with a data reduction and analysis program :igure $ presents a photograph of

the main components of the experimental setup

Soil and Geotextile +ro!erties

Two different soils were employed in the study, a coarse sand #base material'

and a clayey silty sand #subgrade material' Grain"si4e distribution curves for thesematerials are illustrated on :igure The characteristics of the two soils are summari4edin Table ! nonwoven geotextile was used for all tests The properties of thisgeotextile are given in Table ;

S!eci,en +re!aration

!ll samples were compacted into a split mold lined with a rubber membraneassembled on the triaxial cell base plate in layers of * mm thick with predetermined

tamping compaction effort to achieve the maximum dry density at optimum moisturecontent 1n order to attain a particular dry density, a trial and error procedure wasadopted by which the number of blows by the tamper re9uired per layer of soil to becompacted is predetermined 1n preparing composite specimens of subgrade withoutgeotextile, the subgrade soil was placed and compacted in the lower half, than the basematerial was placed and compacted in the upper half 1n preparing the specimen with

geotextile a geotextile disk having diameter of 30 mm was put hori4ontally on the top ofthe subgrade before compacting the base layer


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:igure $ >hotograph of the experimental setup


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0 . 0 0 $0 . 0 $ 00 . $ 0 0$ . 0 0 0$ 0 . 0 0 0$ 0 0 . 0 0 0

G r a i n 1 i a , e t e r 2 & , , '

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Table A roctorA @55H #k7?m;'

6@a 517 *;0** $+


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!t the conclusion of each test, the permanent deformation was determined !ll reportedtest values are averaged results from duplicate specimens

1t is important to note that according to !!S(T6 T"3% #$', base andsubgrade soils used in this study should be classified as Type materials for not meetingthe criteria of less than /+& passing the 7o $+ sieve and +& maximum passing the 7o

++ sieve Fxploratory tests conducted in this study indicated that !!S(T6 method of

testing for Type material, produced excessive deformation when using 4ero confiningpressures The present testing method has three combinations of confining stress levelsof ., % and psi whereas !!S(T6 method of testing for Type soil #Table $' has ., ;,and + psi, it is believed that the used method of testing is much appropriate for mostpractical purposes

RSa 1t is well known from the literature that fine grained soils usually exhibit stress"dependent behavior, it is evident from this figure that when increasing the deviatorstress, the resilient modulus of subgrade soil decreased significantly especially at highconfining stress 6n the other hand, increasing confining pressure was found to increase

the resilient modulus as shown in :igure ;

The resilient behavior of subgrade"base samples is shown in :igure % The @Rvalues of composite samples #subgrade"base' were found to be higher than the subgradesamples alone The typical curves presented in :igure % show similar influence of thedeviator stress obtained for subgrade samples (owever, increasing confining stress can

greatly increase the resilient modulus at any level of deviator stress These results are

logical, where resilient modulus of granular soil always increases with increasingconfining stress which is referred to as Cstress"hardening8 behavior

The resilient behavior characteristics of subgrade"geotextile"base specimenswere enhanced by the inclusion of a geotextile layer as shown in :igure *


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2 3 4 5 6 7 8 9$ 0 $ 0 0

# c l i c 1 e v i a t o r S t r e s s 2 k + a

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2 3 4 5 6 7 8 9$ 0 $ 0 0 # c l i c 1 e v i a t o r S t r e s s 2 k + a

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: i g . 3 . T # ! i c a l R e s u l t s o f t h e R e s i l i e n t " o d u l u s v s . 1 e v i a t o r

S t r e s s f o r S u b g r a d e - b a s e s a , ! l e .

: i g . 4 T # ! i c a l R e s u l t s o f t h e R e s i l i e n t " o d u l u s v s . 1 e v i a t o r S t r e s s f o r S u b g r a d e - g e o t e x t i l e - b a s e s a , ! l e .

o n f i n i n g S t r e s s 2 k + a

3 $

) 7

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"odels :or Resilient "odulus

1n order to gain the most plentiful information from resilient modulus tests,several constitutive models have been proposed for describing the results of resilientmodels tests for cohesive and cohesionless soils !!S(T6 proposed two models, themodel recommended for cohesive soils is a relationship between resilient modulus and

deviator stress #d' and the model for granular soil describes @Ras a function of bulk

stress # dI ;' as shown in the following e9uationsA

cohesive soilA @R k$ #d'k #$'

granular soil @R k;#'k #'

The resilient modulus of fine"grained soil according to e9uation #$' is mainly afunction of the applied deviator stress, when single confining stress level is consideredThe main disadvantage of e9uation #' is that it does not ade9uately model the effect ofdeviator stress

The results of resilient modulus test in this study are presented in amathematical model that, directly, incorporates the stress sensitivity of the resilient

modulus value in terms of both confining and deviator stresses regardless of the soiltype >e4o #;' defined this mode by the following relationshipA

@R k$ #d'k#;'

k; #;'

where k$, kand k;are the material constants to be obtained from tests performed on the

given soil

Through non"linear regression analysis, using S>SS #%' software, values of k$,kand k;were determined for each set of data ! summary of these values with thecoefficient of determination #R' is shown in Table %

The value of k$, represents resilient modulus value #k>a' at unit deviator andconfining stresses !s kapproaches a value of 4ero, the soil is truly linear #constant @Rvalue for a given confining stress level', whereas larger knegative values imply highsensitivity #stress"softening' :or small values of k;, the resilient modulus is lesssensitive to change in confining stress, whereas large values indicate a greater degree of

nonlinearity #stress"hardening'

!s shown in Table %, the weak subgrade soil, being highly nonlinear, showsrelatively large reduction in @Rattributable to an increase in deviatoric stress 1nclusionof geotextile in the subgrade"base system greatly improved the resilient behavior of thesystem by reducing the tendency of stress"softening behavior as the value of kincreases

from - +* to -+$./ There appears to be little change in k;due to geotextile


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inclusion, where the value of k;appears to decrease slightly Thus the relative sensitivityfor the confining stress is less pronounced with the inclusion of the geotextile

! more convenient way of presenting the changes in resilient modulus with theinclusion of the geotextile is to evaluate the @Rvalues for a typical unpaved roadsection in Saudi !rabia The load used in this study was a %+ k7 single wheel load and a

tire pressure of **+ k>a !nalysis using the multilayer elastic computer program,

F2SJ@* #*' indicates that the subgrade confining pressure is typically ;. k>a and thedeviator stress is $;+ k>a around ! typical low"traffic"volume road section #+; mm ofcrushed stone' and a very soft subgrade were assumed The predicted field values ofconfining and deviator stresses were substituted in the model, then using the values oflaboratory k$, kand k;values, and the expected field resilient modulus values were

calculated and summari4ed in Table % 1nclusion of geotextile slightly increased theresilient modulus of subgrade"base system by about $%& The magnitudes of change inresilient modulus obtained in this study confirm the finding of previous studies #$., $/'that the resilient modulus of subgrade"geotextile"base system is not significantlyinfluenced by the presence of a geotextile

+er,anent 1efor,ation

npaved roads when built over weak subgrade soils, are sub)ected to severerutting, aggregate loss, and costly maintenance Thus, increasing resistance to ruttingwhich is a manifestation of improved plastic behavior could be of ma)or significance tothe overall performance and service life of unpaved roads under applied traffic loads

! comparison of accumulated plastic strain for the investigated samples isshown in Table * 1nclusion of geotextile significantly reduced the permanent strain ofthe aggregate"subgrade system by about *+& This behavior was substantiated from allprevious research investigations This behavior is probably due, at least in part, to therestraint of the granular layer by the geotextile, which significantly improve strength,stiffness and permanent deformation behavior of the granular layer With separation

effect of the geotextile, integrity of the granular layer is maintained, which distribute

evenly the stress imposed on the subgrade, compared to that without geotextile

Table % Summary of model parameters and typical @Rvalues

Specimen Type k $ k k; R @RH #k>a'

SubgradeSubgrade"baseSubgrade"geotextile"base

$3/33$;3.3$;$3;

"+3$"+*"+$./

+***+.*0+.;

+03+3$+3

;*$+%3%+*.%+

H at ; ;. k>a and d $;+ k>a


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Table * Summary of >ermanent Strain

Specimen Type >ermanent Strain #&'

Subgrade

Subgrade baseSubgrade"geotextile"base

$+;

/%%;3$

onclusions

This study has been undertaken with the aim of understanding the resilientbehavior of geotextile reinforced granular layer resting on weak subgrade soil

>articular emphasis has been placed on the effect of confining and deviator stresses onthe resilient behavior of base"subgrade system reinforced with a nonwoven geotextilelayer Based on the results of this laboratory study, the following conclusions can bedrawn, which are applicable to the materials used and the test conditions adoptedA

$ The resilient modulus of subgrade"geotextile"base system increases asthe confining stress increases and decreases as a deviator stress increases

The regression analysis demonstrated that it is possible to determinethe resilient modulus of subgrade"geotextile"base system using the relationshipsuggested by >e4o #;'

; 1n contrast to the relatively minor influence of geotextile inclusion onk;, the influence of geotextile on kis significant, thus the relative sensitivity ofsubgrade"geotextile"base system for changes in deviator stress is lesspronounced than for subgrade"base system

% The models proposed by !!S(T6 #e9uations $ and ' may not be

capable of predicting the change of resilient modulus of subgrade"geotextile"base system due to variation of confining and deviator stresses

* The inclusion of nonwoven geotextile in subgrade"geotextile"basesystem showed only minor increase that amounts to about $%& in the resilientmodulus of the system

. The presence of geotextile improved the plastic behavior and reducedthe permanent deformation of subgrade"geotextile"base system by about *+&


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Ackno=ledge,ents

The study described was conducted at the laboratories of 5epartment of ractice, :ukuoka,Kapan, Eol $, pp .*3"..%

L;M =inney, T #$30' NSmall Scale 2oad Test on a Soil"Geotextile"!ggregateSystemO, >roceedings of Second 1nternational ractice, :ukuoka, Eol $, pp .*3"..%

L/M Sowers, G:, and 7oiray, 2 #$30$' NGeotextile"Reinforced npaved Road5esignO, Kournal of Geotechnical Fngineering 5ivision, !S


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L$M Raad, 2 #$30+' NReinforcement of Transportation Support Systems Through:abric >restressingO, Transportation Research Record /**, pp %3"*$

L$;M :riedli, > and !nderson, 5G #$30' NBehavior of Woven :abrics nderSimulated Railway 2oadingO, >roceedings of Second 1nternational lastic :abric for >avement >rotection

5uring :rost BreakO, >roceedings of 1nternational aris, Eol $, pp $%;"$%3

L$/M Brown, S:, >appin, KW and Brodrick, BE #$30+' N>ermanent 5eformationof :lexible >avementsO, Furopean Research 6ffice, S !rmy, niv of7ottingham, 7ottingham, Fngland, :inal Report

L$0M Thompson, @R and Raad, 2 #$30$' N:abric se in 2ow"5eformation

Transportation Support SystemO, Transportation Research Record 0$+, pp */".+

L$3M Barksdale, R5, et al #$33+' N2aboratory 5etermination of Resilient @odulusfor :lexible >avement 5esignO, 7 Report $"0, Georgia 1nstitute ofTechnology

L+M Thompson, @R and Robnett, P2 #$3/.' NResilient >roperties of Subgrade

SoilsO, :inal Report, 1llinois <


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Behavior of Geotextile Reinforced Sand on Weak Subgrade

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