an assessment of the selected reinforcements of motorway pavement subgrade

8/13/2019 An assessment of the selected reinforcements of motorway pavement subgrade

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Advances in Transportation Geotechnics Ellis, Yu, McDowell, Dawson & Thom (eds) 2008 Taylor & Francis Group, London, ISBN 978-0-415-47590-7

An assessment of the selected reinforcements of motorway pavement

subgrade

D. Wanatowski

Nottingham Centre for Geomechanics, School of Civil Engineering, The University of Nottingham, UK

A. Florkiewicz & W. GrabowskiInstitute of Civil Engineering, Faculty of Civil and Environmental Engineering, Poznan University of Technology,Poland

ABSTRACT: Thawing fine-grained soils are often saturated and have extremely low bearing capacity. In many

countries with long-lasting winter seasons such weak soils may cause serious problems in the maintenance ofroad pavements. Consequently appropriate selection of the most efficient method of subgrade reinforcement isone of the most important factors in enhancing the performance and extending the service life of roads. In thispaper, some of the most common methods, such as removal and replacement, cement stabilisation or the use ofgeotextile, used for reinforcement of motorway subgrade in Poland are analysed. Advantages and disadvantagesof selected reinforcement examples with regard to environmental conditions are discussed.

1 INTRODUCTION

Thawing fine-grained soils are often fully saturatedand have extremely low bearing capacity. In manycountries with long-lasting winter seasons such weaksoils may cause serious problems in the maintenanceof road pavements. An appropriate selection of themost efficient method of subgrade reinforcement istherefore one of the most important factors in enhanc-ing the performance and extending the service lifeof roads. This is particularly significant in the caseof motorways, which are of the utmost importance tothe development of any country. Owing, however, tosystematic traffic of heavy vehicles and frost action

in subgrade, motorway pavements can suffer seriousstructural damage during the spring thaw, and thus canlast considerably less longer than expected.

In order to protect motorway pavements from dam-aging frost action and increased moisture content dur-ing the spring thaw, the subgrade must have sufficientbearing capacity. For example, in Poland, according tothe criteria of the General Directorate for Public Roads(GDDP 1997), a frost-susceptible motorway subgradeis required to have an elastic modulus E2 120 MPaand a relative compaction RC 103%. The elasticmodulus E2is determined by the plate-loading test and

the relative compaction by the standard Proctor test.If these requirements are not fulfilled the subgradehas to be reinforced by one of the available methods,

e.g. removal and replacement, cement stabilisation orgeosynthetic reinforcement.

In this paper selected methods of reinforcement ofa motorway pavement subgrade are compared. Theadvantages and disadvantages of each method arepointed out. Practical aspects of the real technicalsolutions used in Poland with regard to environmentalconditions are also discussed.

2 SELECTED REINFORCEMENT METHODS

It is well known that different reinforcement aspectsneed to be considered for roads built on embank-

ments and in cuttings. These two cases are illustratedschematically in Figure 1. It can be observed fromFigure 1 that in the case of embankment constructedon a weak subgrade the reinforcement is normallylocated at its base (Fig. 1a) whereas in the case ofcutting the reinforcing layer is located immediatelybelow the pavement (Fig. 1b). In addition, the naturalsubgrade beneath embankment is normally modi-fied by one of the deep improvement methods suchas precompression, grouting, or in situ soil mixing(Fig. 1a).

It should also be pointed out that the soil used as

a fill in an embankment is generally remoulded, wellcompacted and has good drainage characteristics. Onthe other hand, the soil beneath a pavement of the

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Figure 1. Reinforcement of a weak subgrade: (a) embank-

ment; (b) cutting (modified after Florkiewicz & Grabowski1999).

Figure 2. Flexible pavement structure considered in the

study.

road in cutting is normally in its natural undisturbedstate. Furthermore, the road in cutting often requiresa more complex drainage system compared with theroad on embankment. These two situations will there-fore normally require separate consideration (Brown1996, Florkiewicz & Grabowski 1999).

In this study, a flexible motorway pavement con-structed in a 1 m deep cutting is considered. A standardpavement structure selected from the Polish catalogue(GDDP 1997) is analysed. The pavement consists ofthree asphalt concrete layers and has a total thick-ness of 36 cm, as shown schematically in Figure 2.According to the Polish requirement (GDDP 1997),such motorway pavement requires a foundation withan elastic modulus E2 120 MPa and a relative com-paction RC 103%. It is obvious that such parameterscannot normally be obtained for fine-grained soil sub-grade so, in order to meet the required parameters, fivedifferent reinforcement techniques were used:

a) Removal and replacement of the weak soil,b) Geotextile-reinforced well-graded gravel,

c) Mechanically-stabilised crushed aggregate,d) Geogrid-reinforced crushed aggregate,e) Cement stabilisation.

Figure 3. Transformation used for the pavement withoutgeotextile reinforcement.

3 SUBGRADE

A very soft sandy clay (symbol CL, according to theUnified Soil Classification System) in a plastic statewas chosen as an example of a weak, frost-susceptiblesubgrade. The clay has the liquidity index IL = 0.50,the undrained shear strength cu = 37 kPa, the Califor-nia Bearing Ratio, CBR= 1.6%. The assumed elasticmodulus E0 = 10CBR= 16 MPa.

The groundwater table at the depth of 1 m from thebottom of the pavement structure and the depth of frostpenetration hz = 0.90 m were assumed. According to

the Polish standard (GDDP 1997), this situation is con-sidered to be a bad groundwater condition.As a result,the total thickness of the pavement structure, includingthe reinforcing layer, must be at least 0.85hz, which inthis case equals 0.76 m.

4 DESIGN METHODOLOGY

Three different methods were used to design therequired thickness of reinforcing layers. In all threemethods the same design criterion was used, that is, to

obtain E2 120 MPa at the top of the reinforcement.As mentioned earlier, according to the Polish require-ment (GDDP 1997) this condition must be fulfilled forall motorway pavements.

All conventional solutions (i.e. without geotextilereinforcement), such as removal and replacement,mechanically-stabilised aggregate and cement stabili-sation, were designed by the use of a method of equiv-alent modulus based on the elasticity theory (Yoder &Witczak1975).The principle of this method is to trans-form a multi-layer elastic system into an equivalenttwo-layer system to which Burmisters equations (Bur-

mister 1943) can be applied (Fig. 3). In the equivalentmodel, the pavement material is assumed to be homo-geneous, isotropic and elastic, and is characterised by

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Figure 4. Transformation used for the geotextile reinforcedpavement.

Poisons ratio,, and the equivalent elastic modulus,

Ee, whichcan be obtained from thefollowing equation:

where hi and Ei are the thickness and the elasticmodulus of single layer, respectively.

The subgrade soil is also assumed to be homoge-neous, isotropic and elastic and is characterised byPoissons ratio, 0, and the elastic modulus, E0, asshown in Figure 3.

A geotextile reinforcement of the weak subgradewas designed by the modified method of equiva-lent modulus described by Wojtowicz (1994) andBugajski & Grabowski (1999). This method is basedon the assumption that the elastic properties of thegeotextile-reinforced pavement will not be lower thanthose of the conventionally-reinforced pavement (i.e.without geotextiles). The principle of this method issimilar to that of the previous method, i.e. to trans-form a multi-layer elastic system into the equivalenttwo-layer system. The modified method, however,also accounts for the tensile properties of the geotex-tiles, in terms of the tensile modulus, K, as shownschematically in Figure 4.

A geogrid reinforcement of the subgrade wasdesigned with Tensar geogrids characterised by a hightensile stiffness. A method developed at the Universityof Hannover in Germany (Golos 2005) was adopted.This method is based on Boussinesqs theory and full-scale experiments carried out by Kennepohl et al.(1985). The method accounts for the interlock createdbyTensar geogrids and the compacted aggregate and ispresented as a set of design curves for different valuesof the equivalent elastic modulus (Ee) required at thetop of the reinforcement. In this study, the curves forthe Ee = 120 MPa were used (Fig. 5).

All the materials used in the design of reinforcinglayers are summarised in Table 1 (Rolla 1987). A cir-cular tyre imprint with the diameter D= 0.313 m and

Figure 5. Design curves for reinforcement of aggregatelayers using Tensar geogrids (modified after Golos 2005).

Table 1. Summary of the materials used in reinforcinglayers.

Elastic modulus Symbol inMaterial E (MPa) Fig. 6

Asphalt concrete 0/20 mm 1500

Asphalt concrete 0/25 mm 1500

Asphalt concrete 0/31.5 mm 1500

Cement-stabilised soil 600

Crushed-stone aggregate 400

Well-graded gravel 200

a contact pressure p= 0.65 MPa were assumed in allthe calculations. This corresponds to a 50 kN singlewheel load (100 kN single axle load).

5 ASSESSMENT

5.1 Proposed reinforcement solutions

All the proposed reinforcements of the weak subgradeare presented in Figure 6. In addition to the materialssummarised in Table 1, the following geosyntheticswere considered in the design:

A woven geotextile with a minimum tensile strengthof 45 kN/m (Fig. 6b),

Tensar SS30 geogrid with the tensile strength of30 kN/m (Fig. 6d)

An unwoven geomat acting as a separation layer attheinterphase of thesubgradeand thereinforcement(Figs 6a, c, d, e).

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Figure 6. Proposed reinforcement solutions for the sub-grade with CBR= 1.6% (all dimensions in cm): (a) removal

and replacement; (b) geotexile reinforced well-gradedgravel; (c) mechanically-stabilised crushed-stone aggre-gate; (d) geogrid-reinforced crushed-rock aggregate; (e)cement-stabilised soil.

Figure 6 shows that in addition to the reinforcinglayer required by the soft subgrade, a capping layerof 15 cm was provided for all the structures. Accord-ing to the Polish requirement (GDDP 1997) such acapping layer is required for all the roads that carrylarge volumes of heavy traffic in order to provide aworking platform for heavy machinery during con-

struction of the pavement layers. The Polish standard(GDDP 1997) specifies that the capping layer shallbe at least 10 cm thick. The capping layer should be

Figure 7. Total surface deflections.

made of crushed aggregate or cement-stabilised soilof CBR 40%.

5.2 Total surface deflection

It is well known that most of thetotal surface deflectionis caused by the elastic compression of the subgradelayer (Yoder & Witczak 1975). Furthermore, deflec-tions are simply the mathematical integration of thevertical strain with depth. It can, therefore, be assumedthat the same factors that affect vertical strains in thesubgrade also affect the surface deflection. As a result,the values of total surface deflection were used inthis study to assess the effectiveness of the proposedreinforcement solutions.

Deflection values werecalculated byuse of thesolu-tion of the two-layer system proposed by Burmister(1943). In this paper, it is assumed that thesurface layerrepresents all the pavement layers and is characterisedby the equivalent modulus, Ee. The underlying layerrepresents the natural subgrade and is characterisedby the elastic modulus, E0 (Fig. 3).

Total surface deflection, T, for the two-layersystem was calculated by means of equation:

where p= contact pressure; a= radius of tireimprint; E0 = elastic modulus of the subgrade;F2 = dimensionless factor depending on the ratios ofEe/E0 and h/a.

The total surface deflections calculated for eachreinforcement method are compared in Figure 7. Itshould be noted that letters on the horizontal axiscorrespond to the solutions shown in Figure 6.

It can be observed from Figure 7 that the calcu-lated values of total surface deflection are in the rangeof 0.580.70 with the lowest obtained for the removal

and replacement method and the highest for geogrid-reinforced crushed-stone aggregate. Figure 7 alsoshows that the geotextile reinforcement of well-graded

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gravel causes a slight reduction in the deflection com-pared with the well-graded gravel without geotextile.On the other hand, the use of Tensar geogrid togetherwith crushed aggregate reduces the pavement deflec-tion significantly when compared with the layer ofcrushed aggregatewithout any geogrid.This is becausethe interlockmechanismbetween the granular materialandthe geotextile canonlybe created when the geogridreinforcement is provided. When a typical woven geo-textile is used, the mechanism of interlock cannot becreated.

It canalso be observed from Figure 7 that thedeflec-tionobtainedfor the cement stabilisation is muchlowerthan that for the conventional removal and replacementmethod even though thethickness of the former layer ismuch smaller than that of the latter. This suggests thata greater reduction in the deflection will be obtainedby increasing the modulus or rigidity of the reinforcing

layer rather than by increasing its thickness.

5.3 Shear stresses in the subgrade

The preceding section showed that different values ofthe total surface deflection will be obtained for dif-ferent reinforcement solutions. It was demonstratedthat the pavement deflection could be reduced byincorporation of more rigid reinforcing layers and/orincreasing their thickness. As the pavement layers,however, become stiffer and provide increased loadspreading capability, shear stresses within the pave-ment and the subgrade will change. Therefore, it isimportant to verify whether the shear stresses devel-oped in the subgrade do not exceed the shear strengthof the subgrade soil.

Similarly to the total surface defections, the shearstresses in the natural subgrade were calculated by useof Burmisters theory. The maximum shear stress inthe subgrade, max, was calculated under the edge ofthe tyre.The value ofmaxwas determined from nomo-graphs based on the ratios Ee/E0and h/D, and the shearstrength of the subgrade soil (Rolla 1987).

The maximum shear stresses in the subgrade

obtained for each reinforcement are compared inFigure 8. Although all the maximum shear stressesshown in Figure 8 are smaller than the undrained shearstrength of the subgrade soil, cu = 37 kPa, differencesbetween the values of max obtained for each rein-forcement are noticeable. The highest shear stress of24.0 kPa was obtained for the removal and replace-ment, whereas the lowest shear stress of 16.6 kPa wasobtained for the cement stabilisation.

It can also be observed from Figure 8 that anincrease in the reinforcement stiffness causes a reduc-tion in the shear stress in the subgrade. In other words,

the effect of stiff reinforcing layers such as crushedaggregate with geogrid or the cement-stabilised soil ispronounced. Similar observationshave also been made

Figure 8. Maximum shear stresses in the subgrade.

for two stiffer subgrade soils with CBR of 2.5% and3.9%. The differences between shear stresses in thesubgrade obtained for different reinforcements were,

however, much smaller (Wanatowski & Florkiewicz2000).From the values of total surface deflection and shear

stresses developed in the subgrade it can be concludedthat the design of geogrid reinforcement or cementstabilisation can guarantee the highest bearing capa-city of the subgrade. The reinforcement with the useof geotextile or mechanically-stabilised aggregate canprovide a good bearing capacity. Finally, the replace-ment of the soft soil with well-graded gravel providesa satisfactory bearing capacity of the subgrade.

5.4 Drainage

As mentioned earlier, the capping layer of 15 cm wasprovided for all proposed motorway pavement struc-tures. According to the Polish standard (GDDP 1997),a crushed aggregate or cement-stabilised soil withCBR 40% can be used for such layers. In order toimprove long-term drainage in the pavement, how-ever, the material used for the capping must also havea sufficient permeability. Therefore, it is proposedthat an aggregate with the coefficient of permeabil-ity k 0.01 m/s rather than a cement-stabilised soil

should be used for the capping layer in all the pro-posed solutions. In addition, the capping layer shouldbe connected to a deep drainage system in the formof drains installed under the ditches. Such a drainagesystem will provide an excellent way of removing alltroublesome water from the pavement structure. Anexample of the motorway drainage system used in thecase of removal and replacement of soft soil is shownin Figure 9.

It should be noted that although the capping layerprovides an excellent drainage of the pavement, noneof the proposed reinforcements can fully protect the

natural subgrade from surface water that can infil-trate through cracks in the pavement or at the pave-ment edges. Nevertheless implementation of a deep

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Figure 9. Example of deep drainage system of motorwaypavement (modified after Florkiewicz & Grabowski 1999).

drainage system should provide a very efficient pro-tection of the soft subgrade from further plasticising.

5.5 Frost resistance

It can be seen from Figure 6 that the total thickness ofeach pavement structure is greater than the requiredvalue of 0.85hz = 0.76 m. Therefore, all the proposedstructuresmeets the design criterion with respect to thefrost resistance (GDDP 1997). The motorway pave-ment reinforced with the use of Tensar geogrid andcement stabilisation have the smallest thicknesses of0.95 and 0.94 m, respectively (Figs 6d, e). These val-ues are very close to the depth of frost penetrationhz = 0.90 m.Therefore, it is possible that in the case ofa severe winter, the two above-mentioned pavementsmay still be subjected to damaging frost action. Thetotal thickness of the other three structures is notice-

ably larger (Figs 6a, b, c), which guarantees a verygood frost resistance of the motorway pavement.

5.6 Construction costs

Adequate analysis of the selected reinforcement meth-ods must include consideration of their constructioncosts. In this study, a simplified economic analysis wascarried out. A normalised cost index (NCI) defined byEquation 4 was used in the assessment.

For the sake of simplicity it was assumed thatall necessary materials are available within 15 km ofthe construction site. It should also be noted thatmaintenance costs were not considered in the analysis.

The normalisedcostsof the proposed reinforcementare summarised in Figure 10. It can be observed fromFigure 10 that highest construction cost was obtainedfor the conventional removal and replacement method.On the other hand, the lowest cost was calculatedfor the layer of well-graded gravel reinforced with

geotextile. Figure10 also shows that the layer ofTensargeogrid-reinforced aggregate is more cost-effectivecompared with the layer of mechanically-stabilised

Figure 10. Normalised costs of the proposed reinforce-ments.

aggregate. This is because reinforcement of the aggre-gate layer with Tensar geogrid allows the overall

construction depth to be reduced, saving on materialsand excavation.

Similar conclusions have also been drawn for thesubgrade with CBR of 2.5% and 3.9%.The differencesbetween construction costs of different reinforcementswere, however, smaller (Wanatowski 1999).

6 CONCLUSIONS

Rapid development of modern technologies and appli-cationof new materialsin highway engineering make itpossible to solve even very complicated reinforcementproblems by a variety of methods. It can be noticedthat geotextiles play a significant part in most ofthe modern reinforcement techniques. This is becausethey have proved to be among the most versatile andcost-effective ground modification materials.

Selected reinforcement methods of the subgradepresented in this paper show that the mode of operationof a geotextile in such applications is defined by threefunctions: separation, reinforcement and drainage.Depending on the application, the geotextile can per-form two or more of these functions simultaneously,e.g. a woven geotextile used for the reinforcement of

the layer of well-graded gravel (Fig. 6b) performs bothreinforcement and separation functions.

The simplified assessment of the selected reinforce-ment techniques has shown that all solutions possessboth advantages and disadvantages. For instance, theconventional removal and replacement method canprovide a very good frost protection of the subgradebut it may be relatively expensive. On the other hand,the layer of well-graded gravel reinforced with geo-textile can be very cost-effective but it does notprovide the most efficient reinforcement for the naturalsubgrade.

In the authors opinion, successful reinforcementof a weak subgrade of motorway pavement requiresa complex geotechnical analysis which should be

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included in motorway design reports. Such analysisshould consider the combined effect of technical andeconomical aspects of the proposed solutions and theactual environmental conditions on the durability andreliability of motorway pavement.

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an assessment of the selected reinforcements of motorway pavement subgrade

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