Flexible Pavement Reinforced with Planer Reinforcement – Experimental Study G Narendra GoudAssistant Professor

Flexible Pavement Reinforced with Planer Reinforcement – Experimental Study
G Narendra GoudAssistant Professor, Dept. of CE-MVSR Engg. College, and Doctoral Student, Dept. of Civil Engineering, IIT- Hyderabad Telangana. India
B UmashankarAssociate Professor, Department of Civil Engineering, Indian Institute of Technology Hyderabad, Kandi, Medak, Telangana 502285, India
E-mail: [email protected]; [email protected]
ABSTRACT: Reinforcing flexible pavement with planer reinforcement in the form of geogrid and hexagonal steel wire mesh (HSWM) in flexible pavements can improve their performance compared to unreinforced pavements. Quality aggregates for constructing base and subbase layers of pavements are not readily available at all the construction sites and the introduction of reinforcement can provide a sustainable solution in such sites. In this study, large-scale model experiments are performed to obtain (a) load-settlement response of reinforced flexible pavements, and (b) interface properties of reinforcement-pavement base/subbase materials. The load improvement factor of reinforced pavement with respect to unreinforced pavement can be used in the design of reinforced flexible pavements. While the interaction coefficients of reinforcement with pavement materials can be used in numerical modeling of reinforced pavement systems. The load improvement factors are found to vary from 1.1 to 1.9 for the two reinforcement types considered in the study corresponding to different settlement ratios. The interaction coefficients range from 0.73 to 1.45 for geogrid and HSWM reinforced interfaces under normal stresses ranging from 30 kPa to 90 kPa. The interface shear modulus of different interfaces considered in this study range from about 19,773 kPa/m to 57,337 kPa/m corresponding to a normal stress equal to 90 kPa.

Keywords: planer reinforcement; geogrid; hexagonal steel wire mesh; load improvement factor; reinforced interface
Roads are one of the major infrastructure facility playing a vital role in improving the socio-economic status of any country and also it is one of the major consumers of the construction materials. Flexible pavements constitute a major portion of Indian road network, the reasons being ease of construction and comfortable riding quality. Improving the performance of flexible pavements to sustain heavy traffic loading and traffic flow, especially on soft soil subgrades under severe climatic conditions, has always been a challenging task. However, evolving alternative construction techniques and materials keeps the hope of civil engineers to cope up with the requirements of the society. One of the alternative construction technique to improve the pavement life or to consume reduced quantities of construction material for same performance is to use reinforcement in flexible pavements. Studies by researchers ADDIN CSL_CITATION { “citationItems” : { “id” : “ITEM-1”, “itemData” : { “author” : { “dropping-particle” : “”, “family” : “Perkins”, “given” : “Steven W”, “non-dropping-particle” : “”, “parse-names” : false, “suffix” : “” } , “container-title” : “Geosynthetics International”, “id” : “ITEM-1”, “issue” : “5”, “issued” : { “date-parts” : “1999” }, “page” : “347-382”, “title” : “MECHANICAL RESPONSE OF GEOSYNTHETIC REINFORCED FLEXIBLE PAVEMENTS”, “type” : “article-journal”, “volume” : “6” }, “uris” : “http://www.mendeley.com/documents/?uuid=7056d651-72ff-44aa-bd6d-bee6c36dab45” }, { “id” : “ITEM-2”, “itemData” : { “author” : { “dropping-particle” : “”, “family” : “Zornberg”, “given” : “Jorge G”, “non-dropping-particle” : “”, “parse-names” : false, “suffix” : “” }, { “dropping-particle” : “”, “family” : “Prozzi”, “given” : “J”, “non-dropping-particle” : “”, “parse-names” : false, “suffix” : “” }, { “dropping-particle” : “”, “family” : “Gupta”, “given” : “Ranjiv”, “non-dropping-particle” : “”, “parse-names” : false, “suffix” : “” }, { “dropping-particle” : “”, “family” : “Luo”, “given” : “Rong”, “non-dropping-particle” : “”, “parse-names” : false, “suffix” : “” }, { “dropping-particle” : “”, “family” : “McCartney”, “given” : “John S”, “non-dropping-particle” : “”, “parse-names” : false, “suffix” : “” }, { “dropping-particle” : “”, “family” : “Ferreira”, “given” : “J Z”, “non-dropping-particle” : “”, “parse-names” : false, “suffix” : “” }, { “dropping-particle” : “”, “family” : “Nogueira”, “given” : “C”, “non-dropping-particle” : “”, “parse-names” : false, “suffix” : “” } , “container-title” : “FHWA/tx-08/0-4829-1”, “id” : “ITEM-2”, “issued” : { “date-parts” : “2008” }, “publisher-place” : “Austin”, “title” : “Validating Mechanisms in Geosynthetic Reinforced Pavements”, “type” : “report”, “volume” : “7” }, “uris” : “http://www.mendeley.com/documents/?uuid=06ef5269-4cf9-4968-8f21-e4647a2c0615” }, { “id” : “ITEM-3”, “itemData” : { “DOI” : “10.1520/GTJ102277”, “ISSN” : “01496115”, “abstract” : “The performance of geogrid base reinforcement in pavement on weak\nsubgrade under cyclic plate load testing was studied. The performance of\ninstrumentation sensors was also evaluated to improve future\ninstrumentation programs. The tests were conducted inside a test box of\ndimensions of 2.0 x 2.0 x 1.7 m(3) using a servo-hydraulic actuator. A\n40-kN load at a frequency of 0.77 Hz was applied through a\n305-mm-diameter steel plate. The sensors used included linear variable\ndisplacement transducers, pressure cells, bondable foil strain gages,\nand piezometers. The test results showed that the inclusion of geogrid\nat the subgrade/base course layer interface can significantly improve\nthe performance of flexible pavement on weak subgrade (California\nbearing ratio=0.5 %) and that the traffic benefit ratio can be\nincreased up to 3.5 for a rutting depth of 25 mm. The test results also\nshowed that the reinforcement can redistribute the applied load to a\nwider area, thus achieving an improved stress distribution on the\nsubgrade, which will eventually reduce the permanent deformation of\nsubgrade. 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Steel-wire mesh or geosynthetic reinforcement can be installed within the subbase or base materials. In recent years, it has become a common practice to attend to asphalt pavement rehabilitation through milling and recycling technique and placement of steel-wire-mesh within the asphalt layers or at the interface of bound and unbound layers can severely hinder the pavement rehabilitation process. Very limited studies are available on reinforcing the base or subbase pavement layers using steel-wire-mesh reinforcement. Quality aggregates for constructing base and subbase layers of pavements are not readily available at all construction sites and the introduction of reinforcement can provide a sustainable solution in such sites.
Figure 2 shows the cost to the agency on the x-axis and the achievable knowledge about pavement performance on the y-axis in relative terms with respect to different approaches with which pavements can be studied. Performance studies of the constructed pavements in the field, study of test roads constructed especially to generate performance data, and accelerated pavement testing of full-scale pavements prove to be the best and reliable approaches to study the performance of the pavements with new materials or alternate materials, however, the study requires expensive budget allocations and within reasonable budget, time and effort, one of the reliable ways to study and evaluate the benefits of reinforcing the pavements is large-scale laboratory experiments. Many researchers used large-scale laboratory experiments to generate performance related data to design pavement structure. In this study, large-scale model experiments are performed to obtain (a) load-settlement response of reinforced flexible pavements, and (b) interface properties of reinforcement-pavement base/subbase materials.

Figure SEQ Figure * ARABIC 1 Financial investment by agency and associated knowledge about pavement performance (adopted from ADDIN CSL_CITATION { “citationItems” : { “id” : “ITEM-1”, “itemData” : { “author” : { “dropping-particle” : “”, “family” : “Hugo”, “given” : “Frederick”, “non-dropping-particle” : “”, “parse-names” : false, “suffix” : “” }, { “dropping-particle” : “”, “family” : “Mccullough”, “given” : “B Frank”, “non-dropping-particle” : “”, “parse-names” : false, “suffix” : “” }, { “dropping-particle” : “Vander”, “family” : “Walt”, “given” : “Barry”, “non-dropping-particle” : “”, “parse-names” : false, “suffix” : “” } , “container-title” : “Transportation Research Record”, “id” : “ITEM-1”, “issued” : { “date-parts” : “1991” }, “page” : “52-60”, “publisher” : “Transportation Research Board”, “title” : “Full-Scale Accelerated Pavement Testing for the Texas State Department of Highways and Public Transportation”, “type” : “article-journal”, “volume” : “1293” }, “uris” : “http://www.mendeley.com/documents/?uuid=89550090-7bc0-47ce-a55d-5beaea96587f” } , “mendeley” : { “formattedCitation” : “7”, “plainTextFormattedCitation” : “7”, “previouslyFormattedCitation” : “7” }, “properties” : { “noteIndex” : 0 }, “schema” : “https://github.com/citation-style-language/schema/raw/master/csl-citation.json” }7)
Planar reinforcement
Flexible pavements are reinforced using two-dimensional or planar reinforcement such as geogrids, or three-dimensional reinforcement such as geocell, or combination of both planar and geocell in the form of basal reinforcement to improve the performance or to reduce the base layer thickness without compromising the required level of service. The mechanisms through which beneficial effects of reinforcement is realized in flexible pavements are described by ADDIN CSL_CITATION { “citationItems” : { “id” : “ITEM-1”, “itemData” : { “author” : { “dropping-particle” : “”, “family” : “Perkins”, “given” : “Steven W”, “non-dropping-particle” : “”, “parse-names” : false, “suffix” : “” } , “container-title” : “Geosynthetics International”, “id” : “ITEM-1”, “issue” : “5”, “issued” : { “date-parts” : “1999” }, “page” : “347-382”, “title” : “MECHANICAL RESPONSE OF GEOSYNTHETIC REINFORCED FLEXIBLE PAVEMENTS”, “type” : “article-journal”, “volume” : “6” }, “uris” : “http://www.mendeley.com/documents/?uuid=7056d651-72ff-44aa-bd6d-bee6c36dab45” } , “mendeley” : { “formattedCitation” : “1”, “plainTextFormattedCitation” : “1”, “previouslyFormattedCitation” : “1” }, “properties” : { “noteIndex” : 0 }, “schema” : “https://github.com/citation-style-language/schema/raw/master/csl-citation.json” }1 . Figure 2 shows the expected tensile behavior of different materials potentially useful as reinforcing material. ADDIN CSL_CITATION { “citationItems” : { “id” : “ITEM-1”, “itemData” : { “author” : { “dropping-particle” : “”, “family” : “Perkins”, “given” : “Steven W”, “non-dropping-particle” : “”, “parse-names” : false, “suffix” : “” } , “container-title” : “Geosynthetics International”, “id” : “ITEM-1”, “issue” : “5”, “issued” : { “date-parts” : “1999” }, “page” : “347-382”, “title” : “MECHANICAL RESPONSE OF GEOSYNTHETIC REINFORCED FLEXIBLE PAVEMENTS”, “type” : “article-journal”, “volume” : “6” }, “uris” : “http://www.mendeley.com/documents/?uuid=7056d651-72ff-44aa-bd6d-bee6c36dab45” } , “mendeley” : { “formattedCitation” : “1”, “plainTextFormattedCitation” : “1”, “previouslyFormattedCitation” : “1” }, “properties” : { “noteIndex” : 0 }, “schema” : “https://github.com/citation-style-language/schema/raw/master/csl-citation.json” }1 Found that the improvement increases with increase in geosynthetic stiffness. Asphalt academy TG 3 ADDIN CSL_CITATION { “citationItems” : { “id” : “ITEM-1”, “itemData” : { “author” : { “dropping-particle” : “”, “family” : “Asphalt Academy”, “given” : “”, “non-dropping-particle” : “”, “parse-names” : false, “suffix” : “” } , “container-title” : “TG 3 First edition”, “id” : “ITEM-1”, “issue” : “November”, “issued” : { “date-parts” : “2008” }, “number-of-pages” : “112”, “title” : “Technical Guideline: Asphalt Reinforcement for Road Construction”, “type” : “book” }, “uris” : “http://www.mendeley.com/documents/?uuid=fccf90b5-b080-43f2-b59c-704c3c937aa7” } , “mendeley” : { “formattedCitation” : “8”, “plainTextFormattedCitation” : “8” }, “properties” : { “noteIndex” : 0 }, “schema” : “https://github.com/citation-style-language/schema/raw/master/csl-citation.json” }8 recommends use of stiff reinforcing materials exhibiting the steeper stress-elongation behavior. In the present study, three types of geogrids made up of polypropylene and a hexagonal steel wire mesh (HSWM) are selected as reinforcing materials to understand the behavior when the gradually applied load is normal to the pavement structure and also when the shear load is applied in a large-scale reinforced pavement system. Table 1 presents the properties and Figures 3 (a), (b), (c) and (d) provide the photographs of different reinforcing materials used in this study.

Figure SEQ Figure * ARABIC 2 Tensile strength-elongation behavior of different reinforcing materials (modified after Asphalt Academy )
Table SEQ Table * ARABIC 1 Properties of reinforcing materials
Material Property Material type
Aperture size / Mesh opening, mm 40 66 31 105
Percent open area, % 84 84 91
Tensile strength(MD/CMD*), kN/m 20/17 23/22 40/32 380-550 kPa*
Rib thickness (MD/CMD), mm 2.4/1.0 2.1/0.9 0.85 —
Rib width (MD/CMD), mm 2.4/3.7 4.4/5.6 —
Diameter of rod (wire mesh /transverse), mm — — — 2.4/4.4
* As per manufacturers datasheet




Figure SEQ Figure * ARABIC 3 Photographs of the reinforcing materials (a) GG1, (b) GG2, (c) GG3 and (d) HSWM
Large-scale model experiment
The performance of reinforced and unreinforced pavement structure examined through Large-scale model experimental (LSME) studies. The LSME is devised to model a full pavement structure or may be part of it at prototype scale in a manner that replicates field conditions as practical as possible.

Materials used in LSME
In this study, unpaved pavement structure overlying a medium-stiff subgrade was considered. Subgrade was prepared using locally available river sand. The maximum density of sand was found to be equal to 1.78 g/cc using the vibratory compaction method. The coefficient of uniformity, Cu, and the coefficient of curvature, Cc, were equal to 1.89 and 1.13, respectively. It is classified as poorly-graded sand (SP) as per the Unified Soil Classification System (USCS). Locally available crushed aggregates of average size equal to 6 mm were used above the sand subgrade layer to prepare a strong aggregate layer overlying a sand layer. Table 1 gives the details of physical properties of the reinforcement Figure 3 (c) GG3 and Figure 3 (d) HSWM used in the study.
Experimental test setup and Preparation of the Pavement Section
Abu-Farsakh et al. (2014) used a test tank of 1.5 m long, 0.91 m width and 0.91 m depth applied static load on the pavement section through a plate of 190 mm diameter, and Montanelli et al. (1997) used a test tank of 0.9 m x 0.9 m in cross section to apply a repeated load through 300 mm diameter plate to study the behavior of geogrid reinforced pavement. In the present study, a test chamber of dimensions equal to 1m x 1m x 1m was used to study the behavior of model pavement under 150 mm diameter circular plate. Loading was applied through a computer-controlled, servo-hydraulic actuator of 100 kN capacity. The actuator was attached to a reaction frame with a clearance height equal to 3.5 m. The detailed test bed preparation procedure has been explained in Hariprasad and Umashankar (2015). This method of sample preparation was found to produce uniform sand samples inside the test tank. The compaction of aggregates was carried out by placing the aggregates in a single layer of 100-mm thick and compacted to a relative density of 70%. Schematic view of reinforced pavement structure prepared in the laboratory is shown in figure 4. The thickness of the top aggregate layer (H1) and sandy soil (H2) was kept as 100 mm and 800 mm respectively. Tests were performed in a displacement-controlled mode with a rate of displacement of 1mm per minute, and the static test loading was terminated at a displacement equal to 50mm keeping field deformations and equipment limitations in mind. The response of load-displacement was obtained for the following test cases: a) Unreinforced aggregate base layer overlying a sandy soil subgrade, b) Biaxial geogrid (GG3)-reinforced aggregate base layer overlying a sandy soil subgrade, and c) Hexagonal steel-wire-mesh reinforced aggregate base layer overlying a sandy soil with reinforcement placed at optimum dr/B.

Figure SEQ Figure * ARABIC 4 Schematic diagram of the testbed
Results and discussion
Figure 5 shows the variation of bearing pressure under the circular plate with respect to the settlement for the three cases, one with unreinforced section and others with geogrid and hexagonal steel-wire-mesh- reinforced pavement sections. For the case of unreinforced pavement section, a peak bearing pressure equal to 403 kPa is reached within footing settlement of 25 mm followed by a plateau in the load-settlement behavior. While no such peak behavior in the load-settlement behavior was noticed for the reinforced pavement sections (both GG3 and HSWM) and the load was found to increase continuously with the settlement for footing settlements within 50mm. Load improvement factors are obtained for reinforced layered system corresponding to various settlement ratios. Load improvement factor (If) is defined as
where, qr is the bearing pressure under the footing resting on the reinforced layered system at a given settlement, s, and qo is the bearing pressure under the footing resting on the unreinforced layered system at the same footing settlement. Figure 6 shows the variation in load improvement factor with respect to different settlement ratios for both GG3 and HSWM reinforcements. The improvement was found to be higher at higher settlement ratios for both the reinforcement types confirming the mobilization of reinforcing effect at higher pavement rut depth. Load improvement factor ranges from 1.4 to 1.9 and 1.1 to 1.7 for HSWM and geogrid (GG3) reinforced pavement sections, respectively. Abu-Farsakh et al. (2014) performed studies on pavement sections reinforced with a single layer of reinforcement and reported that load improvement factors ranged from 1.04 to 1.28 at a settlement ratio of 26% for different types of geogrids considered in their study. The findings from the present study indicate that HSWM reinforcement can also be a potential material to contribute towards the reduction in the pavement crust thickness, construction, rehabilitation, and maintenance costs of asphalt pavement layers, leading to the provision of sustainable road infrastructure.

Figure SEQ Figure * ARABIC 5 Variation of bearing pressure with settlement: effect of reinforcement types for H1/B=0.66, and dr/B=0.45:

Figure SEQ Figure * ARABIC 6 Variation of load improvement factor with settlement ratio
Large-scale direct shear testing
To model the reinforced pavement system, the shear strength and the interface shear strength of pavement material and reinforcement is one of the important input parameter. Many research studies are available in the literature on the shear behavior of sands and sand mixes using direct shear apparatus, however, studies on the interaction of the HSWM reinforcement with the unbound granular pavement materials are very limited. The properties of fill material–reinforcement interface count on various factors such as the interaction mechanism between soil and reinforcement, the physical and mechanical properties of soil, and properties of the reinforcement. The soil-reinforcement interaction mode with the interface elements may be pullout or direct shear. Bergado et al. (2003) investigated the pullout or direct shear mechanisms, as well as the behavior of hexagonal wire mesh, reinforced embankment with silty sand backfill by using a numerical method. Analysis of reinforced embankment over soft soil provides the interaction mode of the elements as pullout or direct shear type (as shown in Figure 7). They observed that most of the elements are in the direct shear type of interaction mode, thus direct shear is more predominant in comparison with that of pull-out mode under working-stress condition. Hence in this study, large size direct shear apparatus is adopted to determine the interface shear properties of reinforcement and fill materials.

Center line
Center line

Figure SEQ Figure * ARABIC 7 Directions of interface shear stresses indicating appropriate soil—reinforcement interaction modes adopted from Bergado et al. (2003)
Materials used for large-scale direct shear testing
The local soil near the construction site of IIT Hyderabad, Kandi campus, Sangareddy, Medak, India, was collected and tested for various properties to use it as subgrade for large-scale direct shear testing. The liquid limit, plastic limit, free swell index, maximum dry unit weight, optimum moisture content and CBR of soil found to be 37%, 13.2%, 25%, 18.4 kN/m3, 13.5%, and 3.2% respectively. Standard Proctor compaction energy equal to 600 kN-m/m3 was used. The gravel surface (GS) and gravel base (GB) materials used in this study were selected in accordance with Indian Road Congress Specifications for Rural Roads. These materials were obtained by blending different sizes of crushed aggregate suitably to meet the required gradation as per the specification. The maximum size of aggregate in GS and GB was equal to 26.5 and 37.5 mm respectively. Figure 8 shows the gradation curves of subgrade soil, GS, and GB. Modified compaction tests were conducted according to ASTM D1557 to determine the maximum dry unit weight and optimum moisture content of the GS and GB materials and are found to be equal to 21.6 kN/m3, 8.7%, and 22.6 kN/m3, 7.7% respectively. Fig. 9 shows the compaction curves of GS, GB and subgrade soil.

Figure SEQ Figure * ARABIC 8 Gradation curves of subgrade, gravel base, and gravel surface

Figure SEQ Figure * ARABIC 9 Compaction curves of gravel base, gravel surface, and subgrade
Experimental setup and methodology
To examine the interface shear properties of subgrade soil, gravel surface and gravel base with various reinforcements, large-size direct shear apparatus of box size equal to 300 mm × 300 mm × 200 mm in length, width, and height was used. Test apparatus consisted of horizontal and vertical load cells of capacity equal to 45 kN each, and two linear variable differential transducers (LVDTs) with a range of ± 50 mm displacement. LVDTs were used to measure the horizontal and vertical deformations of the sample during the shearing process. The measurements were automated through a Data Acquisition System (DAQ). During interface testing, the reinforcement was placed at the interface between the lower and upper boxes and was tightly fixed to the lower box using a clamping system. The rate of shearing was kept at 1 mm/min in accordance with ASTM D 5321. The normal stresses selected to apply over the specimen are 30 kPa, 60 kPa, and 90 kPa. A criterion for failure was decided based on the peak shear stress or 50 mm of horizontal displacement (~ 17% of box size) considering the limitations of the equipment.

Figure SEQ Figure * ARABIC 10 Photograph of the large-scale direct shear apparatus used for interface testing
Results of large size direct shear testing
The large-size direct shear tests on the subgrade (SG) and two aggregate mixes (GS and GB) were conducted at the predetermined unit weight and moisture contents, figure 11 shows the Mohr-Coulomb shear strength envelops and the fitting parameters cohesion intercept (c) and friction angle (?) considering linear variation of shear strength with change in normal stress are presented in Table 2. The subgrade under consideration did not show clear peak but for horizontal displacement beyond 20 mm (i.e., 6.7% of box size), shear stress along the horizontal plane reaches a plateau and there is no significant increase in horizontal stress. For the case of graded aggregate mixes compacted at optimum water content using modified Proctor compaction energy, the well-defined peak was observed within 10–15 mm horizontal displacement (3.3–5%). GB consisting of higher maximum aggregate size (37.5 mm) gives higher shear strength in comparison with the GS having lower maximum aggregate size (26.5 mm). Nicks et al. (2015) reported the friction angles in the range of 40-60° for open-graded aggregates having maximum aggregate size equal to 25.4 mm using similar test setup. GB exhibited a higher value of friction angle in comparison with the values reported in the literature; it could be attributed to well-graded aggregate blend and large size of aggregate. GS exhibited higher cohesion (94 kPa) in comparison with GB (36 kPa), the contributing factor being the availability of more fines in GS.

Figure SEQ Figure * ARABIC 11 Mohr-Coulomb shear strength envelopes of Subgrade (SG), Gravel surface (GS), and Gravel base (GB)
Table SEQ Table * ARABIC 2 Shear strength properties of granular mixes and subgrade soil
Material Cohesion c, Friction angle ?, (degree)
GS 94 68
GB 36 70
SG 48 25
Interface shear properties and interaction coefficient
To ascertain the interface shear strength, interface shear tests of two geogrids (GG1 and GG2) of different aperture sizes and HSWM with different pavement materials were conducted under three normal stresses equal to 30, 60, and 90 kPa. A total 12 number of experiments were conducted on four types of reinforced interface specimens alone. The following are the four types of reinforced interfaces 1. GB-GG1-GB, 2. GB-GG2-GB, 3. GB-SWM-GB and 4. GB-GG1-SG. For the case of GB–GG1–GB interface, a peak was reached within a horizontal displacement of 5% of box size for higher normal stress, whereas no well-defined peak shear stress was observed at lower normal stresses (equal to 30 and 60 kPa) even at large horizontal displacement of the box. Mohr-Coulomb shear strength envelopes at peak for the four cases selected is shown in figure 12.
The Interface friction angle and adhesion intercept for GB–GG1–GB interface were found to be 69° and 54 kPa. It was noticed that placement of geogrid resulted in increased apparent cohesion of the material from 36 to 54 kPa; however, it decreased the friction angle slightly (70° to 69°). Similar results were reported by Tutumluer et al. (2012). The peak interface friction angle and adhesion intercept values for soil–aggregate interface reinforced with different types of geogrids observed by Sakleshpur et al. (2017) were in the range of 8.8°–35.1° and 26.3–111.6 kPa respectively. Kamalzare & Ziaie-Moayed (2011) observed interface friction angle ranging from 42° to 46° and adhesion intercept ranging from 53 to 74 kPa for clay soil–geogrid–granular soil interfaces. In this study, interface friction angle was found to vary from 42° for SG substratum to 69° for GB substratum, while adhesion intercept was ranging from 75 kPa for SG substratum to 54 kPa for GB substratum reinforced with GG1. Table 3 summarizes the shear properties of various reinforced interfaces under consideration. It was observed that shear stress of HSWM reinforced interface was higher at all the normal stresses and substantially higher in comparison with that of geogrid reinforced interface at 30 kPa normal stress, mainly due to higher percentage of open area of 91% for the HSWM compared to 84% for geogrid reinforced interface and stiffness of steel wire. The higher interface shear stress in the case of HSWM compared to that of geogrid could be mainly resulted from higher percentage open area leading to increased mechanical interlocking of aggregate particles against the lateral ribs, in addition to the tensile strength of the HSWM compared to the geogrid reinforcement. The geogrid reinforced interface shear stress curves followed similar trend because of the same percent open area (84%) and alike tensile strength (17–23 kN/m) for the geogrids GG1 and GG2.

Figure SEQ Figure * ARABIC 12 Mohr-Coulomb shear strength envelopes of various reinforced interfaces
Table SEQ Table * ARABIC 3 Shear properties of different interfaces
Interface Adhesion intercept ca, kPa Interface friction angle ?, deg.
GB-GG1-GB 54 69
GB-GG1-SG 75 42
GB-GG2-GB 93 65
GB-SWM-GB 230 42
The interaction coefficient of reinforcement with soil may be defined as the ratio of the shear strength at the soil-reinforcement interface to the shear strength of the soil without reinforcement at the same overburden condition Umashankar et al. (2015). Equation 1 is used to evaluate the interaction coefficient of the reinforcement.

?=?reinforced?unreinforced (2)
where ? is the interaction coefficient of reinforcement with soil at a specified normal stress, ?reinforced is the shear strength of reinforced soil, and ?unreinforced is the shear strength of unreinforced soil. An interaction coefficient more than one represents the beneficial effect of reinforcement with effective interlocking in reinforced pavement systems. Table 4 presents the interaction coefficients of various reinforced interfaces at normal stress equal to 30, 60, and 90 kPa. For the case of geogrid reinforced interfaces, the interaction coefficient was higher at 60 kPa normal stress in comparison with other two normal stresses, while for the case of HSWM reinforced interface, the interaction coefficient decreased with increase in normal stress.

Table SEQ Table * ARABIC 4 Interaction coefficients for different reinforcements
Interaction coefficient
Normal Stress ?n = 30kPa ?n = 60kPa ?n = 90kPa
Interface GB-GG1-GB 0.73 0.95 0.87
GB-GG2-GB 0.78 1.16 0.82
GB-SWM-GB 1.45 1.23 0.95
Coulomb Friction Model for Interfaces
Coulomb friction model is generally used in modeling of the interfaces on either side of the reinforcement. The interface shear modulus, GI (Eq. 3) expressed as the slope of the elastic portion of the shear stress-displacement curve of interface resulted from the direct shear test Perkins et al. (2004).

GI=?maxEslip (3)
where ?max is the maximum shear stress, and Eslip is the interface shear displacement parameter. Table 5 presents the interface shear modulus values calculated for different interfaces based on interface tests performed on various reinforcement types and infill materials using Eq. 3. Eslip is exacted from the plot of shear stress and shear displacement for a normal stress equal to 90 kPa.

Table SEQ Table * ARABIC 5 Interface Shear Modulus of various interfaces at 90 kPa normal stress
Interface Peak shear stress (kPa) Eslip (mm) Interface shear modulus, GI (kPa/m)
GB-GG1-GB 288 12.0 24004
GB-GG1-SG 158 8.0 19773
GB-GG2-GB 270 5.0 54100
GB-SWM-GB 315 5.5 57337
The inclusion of reinforcement in the form of geogrid and steel-wire-mesh reinforcements within the aggregate layer for reinforcement placement depth ratio equal to 0.45 resulted in load improvement factor ranging from 1.1 to 1.7 and 1.4 to 1.9 for the two reinforcement types at various settlement ratios of the footing.

Friction angle and cohesion intercept of the gravel base aggregate mix and the gravel surface aggregate mix at optimum water content, compacted with modified Proctor compaction effort, were found to be 70° and 36 kPa, and 68° and 94 kPa, respectively.
Interface shear stress curves of GG1 and GG2 reinforced interfaces followed the similar trend when infill material was granular base, mainly due to same percent open area of geogrids.

The peak interface shear strength of hexagonal-steel-wire mesh reinforced interface was higher than that of the geogrid reinforced interface with similar test conditions.

For the geogrid reinforced interfaces the interaction coefficients range from 0.73 to 1.16, whereas it varies from 0.95 to 1.45 for HSWM reinforced interface when the normal stress was in the range of 30 to 90 kPa.

Interface shear modulus of different interfaces considered in this study ranges from 19,773-57,337 kPa/m for a normal stress equal to 90 kPa.

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