Research Report

on

Performance Evaluation of Geosynthetic Reinforced Unpaved Roads

(CIST Phase III/CIST0026/2012-2013)

Submitted to:

CiSTUP Indian Institute of Science

Bangalore 560 012

Investigator: Prof. Gali Madhavi Latha

Department of Civil Engineering Indian Institute of Science

Bangalore – 12

APRIL 2013

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Centre for infrastructure, Sustainable Transport and Urban

Planning

Indian Institute of Science, Bangalore – 560 012

FORMAT FOR PROJECT REPORT

1. Title of the Project: Performance Evaluation of Geosynthetic Reinforced

Unpaved Roads

2. Scheme Code No: CIST0026

3. Principal Investigator-Name & Department.

Gali Madhavi Latha

Department of Civil Engineering

Indian Institute of Science

Co-Investigator (If any)-Name & Department.- Nil

4. Date of Commencement – 01-04-2011

Project Duration – Two Years

Ending Date of the Project – 31-03-2013

5. Discussion/Summary of work carried out (Explaining Deliverables, Implementation etc. with List) ENCLOSED

Performance evaluation of geosynthetic reinforced unpaved roads

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SPECIFIC AIM/ OBJECTIVE OF THE PROJECT:

To understand the beneficial effects of geosynthetics in reinforcing the unpaved roads through laboratory experiments, field studies and numerical simulations SUMMARY OF WORK CARRIED OUT: Performance of geosynthetic reinforced unpaved roads was studied through systematic series of experiments and numerical simulations. Laboratory CBR (California Bearing Ratio) tests on reinforced and unreinforced soil-aggregate systems were carried out, varying the type, quantity, form and location of reinforcement, water content of the soil and thickness of aggregate layer to understand the effect of all these parameters on the bearing resistance of soil-aggregate systems. Large scale cyclic triaxial tests were carried out on reinforced subgrade materials to understand the influence of reinforcement on the cyclic loading response of the subgrades. Field tests with a vehicle passing over the unreinforced and reinforced unpaved roads were carried out to compare the relative performance and the traffic benefit ratio of various geosynthetic reinforced roads. Numerical simulations of geosynthetic reinforced unpaved roads were carried out in FLAC (Fast Lagrangian Analysis of Continuum) program and parametric studies were carried out to bring out the effects of different parameters on the cyclic load bearing capacity of unpaved roads. Finally design guidelines are given for geosynthetic based on the experimental and numerical studies. Keywords: Unpaved roads, geosynthetics, cyclic loading, CBR tests, cyclic triaxial tests, field tests, numerical analysis Deliverables -Bearing resistance of reinforced soil-aggregate systems in terms of load-penetration response and CBR tables -Cyclic deformation characteristics of subgrade materials in terms of stress-strain graphs and modulus tables -Results from field tests in terms of number of vehicle passes vs. deformation of road sections and Traffic benefit ratio comparisons -Results from numerical simulations -Design guidelines for geosynthetic reinforced unpaved roads Results, discussion and outcome of the project are presented in following sections in the order of the deliverables listed above.

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CBR TESTS ON REINFORCED SOIL- AGGREGATE SYSTEMS

INTRODUCTION

A systematic series of unsoaked CBR tests were carried out on unreinforced and reinforced soil-aggregate systems in the conventional CBR mould of 150 mm internal diameter and 175 mm total height. These tests were carried out as per ASTM D1883-07. The total height of the prepared soil or soil-aggregate systems was 125 mm in all the cases. A surcharge weight of 5 kg was applied through a steel plate of 50 mm thickness in all the tests. A plunger of 50 mm diameter was used for applying the load. The resistance offered by the sample to the penetration of the plunger was measured using a load cell.

MATERIALS USED IN EXPERIMENTS

In these experiments, two types of subgrade soils 1 and 2 (SS1 and SS2) are used for preparing the soil layer and aggregate (A1) is used as sub-base course. For filling the soil and aggregates in the mould, modified Proctor compaction effort was used. Various reinforcing materials used in the experiments are geotextile, strong geogrid and geonet. Physical and mechanical properties of these materials are discussed below.

Subgrade Soil SS1

The grain size distribution curve of SS1 soil is shown in Fig. 1. SS1 soil is classified as clay of low plasticity (CL) according to the Unified Soil Classification System. Properties of SS1 are given in Table 1. Compaction curves for SS1 soil from both standard and modified Proctor tests are shown in Fig. 2. The California Bearing Ratio of the SS1 soil is computed as per ASTM D 1557 –07. SS1 soil has unsoaked and soaked CBR values of 30 % and 19 % respectively corresponding to modified Proctor effort (optimum moisture content of 12.5 % and maximum dry unit weight of 18.3 kN/m3).The load-penetration curve of the SS1 soil from unsoaked and soaked CBR tests is shown in Fig. 3. The shear strength properties of the soil were determined from consolidated undrained (CU) triaxial compression tests on the soil samples compacted to modified Proctor effort. The stress-strain response of SS1 soil in CU test at three different confining pressures of 50, 100 and 150 kPa is shown in Fig. 4. The soil showed effective cohesion of 45 kPa and effective friction angle of 25.5° as determined from CU test.

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Fig. 1 Grain size distribution of the subgrade soils used in the experiments

Fig. 2 Compaction curves and zero air void line for subgrade soil 1

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Fig. 3 Load-penetration curves from unsoaked and soaked CBR tests on SS1

soil

Fig. 3 Stress-strain response of SS1 soil from consolidated undrained triaxial compression (CU) test

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Subgrade Soil 2 (SS2) The grain size distribution of SS2 soil is shown in Fig. 1. SS2 soil is classified

as clay of low plasticity (CL) according to the Unified Soil Classification System. SS2 soil showed maximum dry unit weight of 18.24 kN/m3 at an optimum moisture content of 15.5 % determined from standard Proctor test and the corresponding compaction curve is shown in Fig. 5. The SS2 soil has an unsoaked CBR value of 19 % corresponding to standard Proctor effort. Properties of SS2 soil are summarized in Table 1.

Fig. 5 Compaction curve and zero air void line for subgrade soil 2

Table 1 Properties of subgrade soils used in the experiments

Type of Soil SS1 SS2

Colour Reddish Brown

Reddish Brown

Specific gravity 2.7 2.71 Soil classification CL CL Liquid limit, % 36 36 Plastic limit, % 22 24 Maximum dry unit weight (kN/m3) from standard Proctor test 17.2 18.24

Optimum moisture content from standard Proctor test (%) 15.5 15.5

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AGGREGATE

Aggregate layer was used to simulate the base/sub-base course layer in the unpaved road and was placed on top of the subgrade soil in experiments. Three different types of aggregates (A1, A2 and A3) were used in the experiments and these were obtained from a nearby quarry. All the aggregates were grey in colour. Major difference between different types of aggregates is the variation in their grain size distribution. The following section describes the aggregates used in experiments.

Aggregate 1 (A1) A1 comprises of aggregates passing through 6.3 mm sieve and retained on

4.75 mm sieve. The grain size distribution curve of this aggregate is shown in Fig. 6. The specific gravity of A1 type aggregate determined using pycnometer according to ASTM C 128 -2012 was 2.65. CBR test was performed on this aggregate by filling it in 5 layers in the CBR mould and it was observed that the aggregate has a CBR value of 23 %. The maximum unit weight achieved by A1 aggregate during CBR test (at modified compaction effort) was 16 kN/m3. The load-penetration response from CBR tests on A1 aggregate is shown in Fig. 7. The irregularities in the load versus penetration response indicate crushing of the aggregate during the test.

Aggregate 2 (A2) A2 type of aggregate comprises of aggregates passing through 12.5 mm sieve

and retained on 6.3 mm sieve. The average size of A2 type aggregate is 10 mm. The grain size distribution curve of this aggregate is shown in Fig. 7. This aggregate has a specific gravity of 2.67. Photograph of A1 and A2 type aggregates is shown in Fig. 8.

Fig. 6 Grain size distribution of A1 and A2 type aggregates

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Fig. 7 Load-penetration response for A1 aggregate from CBR test

Fig. 8 Photograph of A1 and A2 type aggregates (equal weight samples)

Aggregate 3 (A3) Both A1 and A2 type of aggregates are clean aggregates without fines.

Ministry of Rural Road Development (MRRD), Specification for Rural Roads, India, 2004 has given three gradations for selection of material for construction of granular sub-base or base course for rural roads. A3 aggregate corresponds to the Gradation III among the MRRD specified gradations. A3 is a mixture of aggregates and granular fines of different sizes, obtained by mechanical mixing of materials of different gradation. Fig. 9 shows the grain size distribution of selected gradation for A3 along with the upper and lower limits prescribed by MRRD for Gradation III.

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Photograph of granular materials of various size ranges which are mechanically blended to obtain the selected gradation is shown in Fig. 10.

Fig. 9 Grain size distribution of A3 type aggregate

Fig. 10 Photograph of granular materials of various sizes blended to obtain A3 type aggregate (equal weight samples)

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The maximum dry unit weight achieved by A3 aggregate using wet method was 20.6 kN/m3 (bulk unit weight of 21.96 kN/m3) at a water content of 6.7 %. The maximum dry unit weight so achieved was 20.5 kN/m3 (bulk unit weight of 21.4 kN/m3) at a water content of 4.5 %.

Geotextile The geotextile used in the experiments is a polypropylene multifilament

woven fabric. The individual multifilaments are woven together in such a manner so as to provide dimensional stability relative to each other. The properties of geotextile are given in Table 2. Ultimate tensile strength of the geotextile was determined by the wide-width strip method as per ASTM D 4595 –01. The geotextile has an ultimate tensile strength of 55.16 kN/m in the warp direction. The mobilized tensile strength of the geotextile material corresponding to 2% strain is 3.02 kN/m, and the corresponding secant modulus is calculated as 151 kN/m. The load-elongation response of the geotextile in the warp direction is shown in Fig.11.

Table 2 Properties of the geotextile

Breaking strength: warp 55.16 kN/m

weft 46.0 kN/m

Elongation at break warp 38% weft 21.3%

Thickness 1 mm

Mass per unit area 230 gm/m2

Fig. 11 Load – elongation response of the geotextile from wide-width tension test

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Geogrids Two varieties of biaxial geogrids (biaxially oriented integrally extruded

geogrids with rigid junctions and stiff ribs), made of polypropylene (PP) are used in the present study. They are designated as strong biaxial geogrid (SG) and weak biaxial geogrid (WG) in this thesis based on their ultimate tensile strength. Fig. 12 shows the nomenclature used to describe the dimensional details of a typical biaxial geogrid. Dimensions of both the biaxial geogrids as per the nomenclature used in Fig. 12 are presented in Table 3. The tensile properties of the geogrids were obtained from standard multi-rib tension test (as per ASTM: D 6637-01). The tensile strength of both these biaxial geogrids with respect to strain as obtained from standard multi-rib tension test is presented in Fig. 13.

Fig. 12 Dimensional details of bi-axial geogrid

Fig. 13 Load – elongation response of strong and weak geogrids

from multi-rib tension test

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Table 3 Dimensions of bi-axial geogrids

AL AT WLR WTR tJ tLR tTR Unit weight (Kg/m2)

SG 30 30 2.6 3.0 5.8 2.2 1.4 0.53

WG 35 35 2.3 3.0 4.1 1.4 1.1 0.22

Geonet The geonet (GN) used in the tests is an extruded polymeric flexible mesh with square openings of size 1.5 mm ×1.5 mm, typically used for insect screens and is gray in colour. The load elongation response of the geonet obtained from wide width tension test is shown in Fig. 14. The geonet has sustained a peak tensile load of 7.6 kN/m at 2.4 % strain at which it failed. Table 4 summarizes the properties of various geosynthetic materials used in experiments.

Fig. 14 Load – elongation response of geonet

Table 4 Properties of different geosynthetics used

Property Geotextile WG SG Geonet Aperture size (mm) - 35×35 30×30 1.5×1.5 Ultimate tensile strength (kN/m) 55.16 26.4 38.1 7.6

Yield strain, % 38 16.50 16.7 2.40 Secant modulus at 2% strain (kN/m) 151 219 588 319

Mass per unit area (g/m2) 230 220 530 125

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EXPERIMENTAL SET-UP Schematic sketch of the reinforced soil-aggregate system is shown in Fig. 15.

In these experiments, subgrade soil 1 (SS1) is used for preparing the soil layer and the aggregate 1 (A1) is used as sub-base course. For filling the soil and aggregates in the mould, modified Proctor compaction effort was used. A total of 52 CBR tests were conducted under 13 different series and many of them were repeated to check the repeatability of the test results. Details of different test series are presented in Table 5. Tests in each series were conducted at four different water contents of the soil layer i.e., 12.5 % (corresponding to Optimum moisture content), 14.5 %, 16.5 % and 18.5 %.

Fig. 15 Schematic sketch of the reinforced soil-aggregate system

prepared in conventional CBR mould

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Table 5 Details of different series of CBR tests carried out

Series No Description of test series Notation

Sche-matic sketch

1 Soil alone S

2 Soil and aggregate SA

3 Soil and aggregate with geotextile at the interface SAGT

4 Soil and aggregate with geogrid at the interface SABG

5 Soil and aggregate with geonet at the interface SAGN

6 Soil and aggregate with geotextile and geogrid at the interface SAGTBG

7 Soil and aggregate with geotextile and geonet at the interface SAGTGN

8 Soil and aggregate filled in geocells made of geogrid with geotextile at the interface SAGTGCBG

9 Soil and aggregate filled in geocells made of geonet with geotextile at the interface SAGTGCGN

10 Soil and aggregate with two layers of geogrid, one at the interface and the other within the aggregate layer

SABG2

11 Soil and aggregate with two layers of geonet, one at the interface and the other within the aggregate layer

SAGN2

12 Soil and aggregate filled in geocells made of geogrid with geogrid at the interface SABGGCBG

13 Soil and aggregate filled in geocells made of geonet with geonet at the interface SAGNGCGN

RESULTS AND DISCUSSION

California Bearing Ratio Plunger of 50 mm diameter was made to penetrate the unreinforced and

reinforced soil-aggregate systems at a uniform rate of 1.25 mm/ min. The load –penetration readings were recorded at regular intervals and based on this the CBR

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value of the unreinforced and reinforced systems were estimated. In case of soil alone systems the CBR value at 2.5 mm penetration was higher than that at 5 mm penetration whereas for unreinforced and reinforced soil-aggregate systems, the CBR at 5 mm penetration was higher than that at 2.5 mm penetration CBR values of various systems computed at four different water contents are summarized in Table 6.

Table 6 CBR values for various soil-aggregate systems

Test Series CBR values (%) at water content %

OMC 14.50 16.50 18.50

S 29.80 10.40 2.70 1.30

SA 36.70 13.20 5.60 2.80

SAGT 40.20 14.60 6.50 3.90

SAGTBG 47.60 16.90 8.40 4.60

SAGTGN 42.70 15.40 7.40 3.90

SAGN 37.70 13.40 6.50 3.70

SAGN2 40.20 14.90 7.40 3.90

SAGNGCGN 32.70 11.90 6.90 3.70

SAGTGCBG 50.60 17.90 8.70 4.70

SABG 37.70 13.50 8.30 4.71

SABG2 54.10 20.70 9.20 5.40

SABGGCBG 60.50 23.30 10.20 5.60

SAGTGCGN 33.20 12.60 6.50 3.60 CBR Improvement Factor To quantify the improvement in the CBR value of the soil-aggregate systems due to reinforcement, a dimensionless parameter called Improvement Factor, F is introduced. The CBR improvement factor F is defined as the ratio of the CBR of the reinforced soil-aggregate system to that of the unreinforced soil-aggregate system. F = CBRr / CBRu

(1) Where CBRr is the CBR value of the reinforced soil-aggregate system and CBRu is the CBR value of the unreinforced soil-aggregate system at the same water content and density. CBR improvement factor for various reinforced systems was estimated and the values are summarized in Table 7. An improvement factor of 1 implies that there is no extra benefit in using geosynthetic reinforcement, whereas a value less than 1 indicates that the performance of reinforced system is inferior compared to the unreinforced system. From Table 7 it is observed that the improvement factors increase with water content for all the cases. Improvement factors below 1 for soil-aggregate systems reinforced by geonets in geocell form at lower water contents

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showed that these systems had an inferior performance compared to the unreinforced systems. The reason for this low improvement factor could be attributed to the rupture of geocells in these cases.

Table 7 Improvement factors for various reinforced soil-aggregate systems

Test Series Improvement factor at water content % OMC 14.5 16.5 18.5

SAGT 1.10 1.11 1.16 1.39

SAGTBG 1.30 1.28 1.50 1.64

SAGTGN 1.16 1.17 1.32 1.39

SAGN 1.03 1.02 1.16 1.32

SAGN2 1.10 1.13 1.32 1.39

SAGNGCGN 0.89 0.90 1.23 1.32

SAGTGCBG 1.38 1.36 1.55 1.68

SABG 1.03 1.02 1.48 1.68

SABG2 1.47 1.57 1.64 1.93 SABGGCBG 1.65 1.77 1.82 2.00

SAGTGCGN 0.90 0.95 1.16 1.29 Effect of Water Content

To understand the effect of water content of the subgrade soil on the beneficial

effects of geosynthetic reinforcement, CBR tests were carried out on soil alone, unreinforced and reinforced soil-aggregate systems by varying the water content of the underlying soil. Fig. 16 shows the variation in the CBR and improvement factor for geotextile reinforced systems with respect to water content. It was observed that geotextile reinforced soil-aggregate system had a higher CBR value than unreinforced system at all water contents. At optimum moisture content, SAGT system had a CBR value of 40 %. With an increase in water content of 2%, the CBR value of SAGT system reduced to 14 %, the percentage reduction in CBR value being 65 %. The improvement factor is high at higher water contents as seen in Fig. 16. This emphasises the fact that the benefit of geosynthetic reinforcement is more significant at high water contents of subgrade soil.

The effect of water content on SAGT series is shown in Fig. 17. From the

figure it is seen that, at a penetration of 15 mm, soil-aggregate system reinforced with geotextile had a load resistance of 13.8 kN at optimum moisture content and only 1.7 kN at a water content of 18.5 %. Similar trend was observed for other series. This emphasises the fact that the decrease in load carrying capacity of the soil-aggregate systems with the increase in water content of the subgrade soil follows a nonlinear relationship. Hence, maintenance of optimum moisture content is very important during road construction.

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Fig. 16. Variation of CBR value and improvement factor

with respect to water content

Fig. 17. Load versus penetration response for geotextile

reinforced soil-aggregate systems at various water contents

Effect of Type of Reinforcement Fig. 18 compares the load penetration response for unreinforced and various

geosynthetic reinforced soil-aggregate systems at optimum moisture content and at a water content of 18.5 %. Most of the reinforced soil-aggregate systems exhibited

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better performance compared to unreinforced soil-aggregate system at both the water contents. However, this difference in performance became evident only after a penetration of 2 mm. At higher penetration levels, the load-penetration response of these systems is observed to be certainly better than in the unreinforced systems as observed in Fig. 18.

Fig. 18 Load-penetration response for unreinforced and planar reinforced soil-

aggregate systems at OMC and at a water content of 18.5%

The load-penetration response of the SAGN series was found to be on par with SAGT series at low levels of penetration. For the test series carried out at OMC (Fig. 18a), after a penetration of 5 mm, SAGN exhibited inferior performance even compared to unreinforced soil-aggregate system because the geonet got punctured during the test due to its low tensile strength and in this case it was sandwiched

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between the aggregate layer and stiff soil layer compacted at OMC. At higher water contents, the soil layer became softer and no puncture was observed in the geonet layer. Hence, the performance of SAGN series is better when compared to unreinforced soil-aggregate system at higher water contents as seen in Fig. 18b. However, at higher water contents, the geonet was getting clogged. The photographs of the exhumed geonets after the tests are shown in Fig. 19. It is clearly seen in the figure that the geonet is punctured in the test with OMC and remained intact in tests with higher water content.

Fig. 19 Photographs of geonet exhumed after the tests

(a) at OMC (b) at a water content of 18.5 % Since biaxial geogrid and geonet have apertures, there is a possibility of

subgrade soil squeezing into the aggregate layer during loading. This was clearly observed by exhuming the reinforcing layers after the test. Inclusion of geotextile layer as a separator can improve the effectiveness of such reinforcements. Hence experiments were carried out with biaxial geogrid and geonet underlain by geotextile (SAGTBG and SAGTGN) at all the four water contents. When geotextile was used as a separator along with biaxial geogrid and geonet, the performance was improved. Even in the case of SAGTGN series, at low water contents, geonet was punctured because of its low tensile strength However, because of the reinforcing and separation action of the geotextile, its performance was far better when compared to that of the SAGN series, as shown in Fig. 20. The benefit of geosynthetic reinforcement depends upon its tensile modulus and also on the water content of the soil subgrade. To quantify the benefit of geosynthetic reinforcement, Koerner (1999) has used reinforcement ratios. Reinforcement ratio is defined as the ratio of the load sustained by reinforced system to that sustained by unreinforced system at any particular level of displacement. Hence, to quantify the benefit of geosynthetic reinforcement, reinforcement ratios were estimated at different settlements at optimum moisture content and at 18.5 % water content for SAGT, SABG, SAGN, SAGTGN and SAGTBG series and are summarised in Table 8. From the table it is seen that reinforcement ratios are high at higher water contents of the subgrade. A plot of variation of reinforcement ratio with penetration levels at two different water contents used in the tests is shown in Fig. 21. Even a weaker reinforcement like geonet is found to have a reinforcement ratio greater than 1 at higher water contents. The use of geotextile as a separator for geogrid and geonet reinforced systems was also found to be effective.

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Fig. 20 Load-penetration response of unreinforced system and

geonet reinforced system underlain with and without geotextile at OMC

Table 8 Reinforcement ratios for various reinforced soil-aggregate systems

Reinforcement ratio at optimum moisture content

Series Penetration, mm

2.5 5.0 7.5 10.0 12.5 14.0

SAGT 1.17 1.08 1.05 1.13 1.12 1.18

SAGN 1.02 1.03 0.89 0.93 0.95 1.02

SABG 1.04 1.03 1.03 1.09 1.15 1.22

SAGTGN 1.22 1.18 1.08 1.00 1.00 1.11

SAGTBG 1.39 1.26 1.24 1.22 1.22 1.37

Reinforcement ratio at 18.5 % water content

Series Penetration, mm

2.5 5.0 7.5 10.0 12.5 15.0

SAGT 1.25 1.39 1.44 1.49 1.58 1.62

SAGN 1.31 1.26 1.41 1.48 1.56 1.62

SABG 1.41 1.70 1.80 1.89 1.85 1.81

SAGTGN 1.28 1.39 1.47 1.67 1.71 1.79

SAGTBG 1.53 1.58 1.84 1.91 2.04 2.10

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Fig. 21 Reinforcement ratio for reinforced systems at optimum moisture

content and at a moisture content of 18.5 % Effect of Quantity of Reinforcement

In order to understand the effect of the quantity of reinforcement on the bearing capacity of reinforced soil-aggregate systems, series of tests were carried out with a single layer and two layers of biaxial geogrid and geonet. From the test results summarized in Table 6 it was observed that an increase in the quantity of reinforcement led to an increase in CBR value at all water contents. However, the difference was quite significant at lower water contents.

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Though the use of two layers of reinforcement improved the performance of soil-aggregate systems significantly when compared to the use of a single layer of reinforcement, it did not double the bearing resistance when compared to a single reinforcement layer as shown in Fig. 22. For example, at OMC, at a penetration of 14 mm, the load resistance of a single layer of biaxial geogrid is 13.5 kN whereas that of two layers of biaxial geogrid is 20.3 kN (Fig. 4.14a). Similarly, in tests with a water content of 18.5 %, at a penetration of 15 mm, the unreinforced soil-aggregate system could sustain a load of 1 kN, whereas it could sustain a load of 1.7 kN when reinforced with a single layer of geonet and 2 kN when reinforced with two layers of geonet (Fig. 22b). Hence, it can be understood that the beneficial effect of reinforcement is not directly proportional to the quantity of reinforcement used.

Fig. 22 Effect of quantity of reinforcement on the load-penetration response of reinforced soil-aggregate systems (a) geogrid reinforcement at OMC (b) geonet

reinforcement at a water content of 18.5%

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Effect of Form of Reinforcement

For understanding the effect of form of reinforcement on the load-penetration response, tests were carried out with biaxial geogrid and geonet in planar and cellular forms. Fig. 23 compares the performance of geosynthetics viz., biaxial geogrid and geonet in both planar layer and geocell forms at OMC. Fig. 23a compares the performance of geocells made of biaxial geogrid, with a geotextile or biaxial geogrid layer at the base, at OMC. From the figure, it is seen that the geocells perform better compared to the single planar layer of reinforcement and similar trend is followed at all the water contents. Geocells with geogrid basal layer are more effective when compared to geotextile basal layer. This is because the geogrid basal layer is connected to the cells, and hence acts as an integral part of the geocell layer in providing all-round confinement unlike the geotextile layer, which is not connected to the geocell layer. At higher water contents, the performance of the geogrid geocell layer was almost on par with double layer geogrid because the full tensile strength of the geocell is not mobilized due to the low shear strength of the soil.

Fig. 23 Load-penetration response for soil-aggregate systems

reinforced with biaxial geogrid and geonet in different forms at OMC

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Fig. 23b summarizes the results obtained from the test series with geonet in planar and geocell forms at OMC. The bearing resistance of the soil-aggregate system was the maximum with geocell reinforcement. However, the difference in the performance of the planar layers and geocells is not as significant as in the case of biaxial geogrid reinforcement because the reinforcement is very weak and was ruptured during the tests. Since there is no covering material, the load was directly applied to the geocell joint and the geonet being weak it, got ruptured at very low levels of penetration. The geocells were subjected to tearing along the joints, and rupture initiated at the central portion and propagated outwards, showing the failure of joints at several places. Exhumed geocells showed damage at many places. The base geonet got punctured at lower water contents, as explained in earlier sections. The performance of the geocells was quite similar at all water contents and at higher water contents the geocells made of geonet were clogged. Photographs of geocells made of geonet exhumed after the tests are shown in Fig. 24. These photographs clearly show the rupture of the geocells at low water contents and the clogging of geocells at higher water contents. Geonet failed to act as a separator and there was some mixing of the aggregate and soil layers at higher water contents. Tests with geotextile basal layer showed better performance because it was intact during loading and also acted as a separator. For reinforcing unpaved roads, stronger grids which do not rupture within the service load limits should be used.

Fig. 24 Photographs of geocells made of geonet exhumed after the tests at water

contents (a) OMC (b) 14.5 % (c) 16.5 % (d) 18.5 %

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Effect of Soaking Soaked CBR tests were carried out on unreinforced and reinforced soil-aggregate systems at optimum moisture content to understand the effect of type of reinforcement on the soaked CBR value. For this purpose, a total of 5 tests were carried out viz., S, SA, SAGT, SABG and SAGN under soaked condition. The results of soaked CBR tests carried out on unreinforced and reinforced soil-aggregate systems are presented in Fig. 25. It is clear that the bearing resistance of soaked test specimens is less compared to that of unsoaked test specimens. The exhumed geonet sample after the soaked test on soil-aggregate reinforced with geonet (SAGN) showed a rupture similar to the unsoaked test on an identical system. The increasing order of performance of the various soaked tests were SAGN, SAGT & SABG. The performance of SABG in soaked CBR test is almost similar to that of SAGT. The order of performance improvement has not differed much in soaked and unsoaked CBR tests.

Fig. 25 Load-penetration response for unreinforced and

reinforced soil-aggregate systems under soaked condition

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CYCLIC TRIAXIAL TESTS ON REINFORCED GRANULAR SUB-BASE

INTRODUCTION

Experimental results from static and cyclic triaxial tests carried out on large diameter granular sub-base samples reinforced with geosynthetics are presented in this section.

EXPERIMENTAL SET-UP

The cyclic triaxial test set up used in this study is designed to be used with a triaxial cell with a maximum working pressure of 1000 kPa for specimens with diameter range of 38-300 mm. It is provided with a 100 kN load frame fitted with a cyclic actuator which can apply cyclic loading up to 10 Hz frequency. Photograph of the cyclic triaxial testing facility is shown in Fig. 26. The basic system consists of the following components:

a) 100 kN capacity load frame with cyclic actuator b) Hydraulic power pack for the load frame c) Dynamic control system for data acquisition and control d) 1000 cc/ 2 MPa advanced pressure and volume controller e) Pneumatic regulator for cell pressure f) Triaxial cell

Fig. 26 Cyclic triaxial set up used

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Sample Preparation and Testing The triaxial specimen of 300 mm diameter and 600 mm height is prepared by the compaction of the granular material inside the split mould placed around the cell base. After placing the porous plate on the cell base, the 2 mm thick latex membrane is fitted to the triaxial cell base pedestal using clamps. The split mould is placed over the cell base and the membrane is stretched such that it fits the inner side of the split mould. Once the membrane is fully stretched it is held tightly to the walls of the split mould using clamps. Then the granular material is placed in layers and compacted. To achieve the maximum density (at standard compaction effort), if the sample is compacted in 5 layers, each layer is to be given 245 blows (using a rammer of 4.7 kg falling from a height of 450 mm). For static and cyclic triaxial tests A3 type aggregate was used. 1/4th of the standard compaction effort was applied while preparing the sample. The compacted sample had a bulk unit weight of 19 ±0.5 kN/m3 at a water content of 4.5±0.5 %. The sample was prepared in five equal lifts and compaction was done using rammer of 4.7 kg weight falling from a height of 450 mm. The triaxial cell with sample inside is shown in Fig. 27b.

Fig. 27 Photograph of (a) sample prepared within the membrane and

(b) triaxial cell with sample inside

The reinforcing materials used in the triaxial tests were strong biaxial geogrid and geocell. The strong geogrid was cut into circular discs of 25 mm diameter and these discs were used as planar reinforcing layers. The diameter of the geogrid reinforcement was kept slightly less than the diameter of the sample to avoid puncture of the membrane due to the sharp edges of the geogrid. The geocell reinforcement was made by stitching the geotextile into circular shape, forming a geocell of 298 mm diameter. A total of 11 tests were carried out on

29

unsaturated large diameter granular sub-base samples out of which 6 were static tests and 5 were cyclic tests. All the tests were carried out at a confining pressure of 50 kPa. Details of the experiments carried out are summarised in Table 9.

Table 9 Details of large diameter triaxial tests carried out

Type of the test Notation

Unreinforced (static & cyclic) UR 2 layers of strong geogrid (static & cyclic) SG-2 3 layers of strong geogrid (static & cyclic) SG-3 4 layers of strong geogrid (static & cyclic) SG-4 5 layers of strong geogrid (static) SG-5 Geocell enclosed sample (static & cyclic) GC

Static unconsolidated undrained (UU) triaxial tests were carried out on unreinforced and strong geogrid reinforced large diameter triaxial samples. Multiple layers of geogrid reinforcement were placed in between and the stress-strain responses were compared. The arrangement of geogrid reinforcement in the different tests is shown in Fig. 28. For constructing sections reinforced with 4 layers of reinforcement, the sample was prepared in 4 lifts instead of 5 lifts to avoid reinforcement slippage. Unconsolidated undrained test was also carried out on geocell enclosed granular sub-bases. In all the tests, the loading was done at a strain rate of 0.5 mm/min and the stress strain plots are compared.

RESULTS AND DISCUSSIONS

Static Triaxial Tests The stress-strain curve from static triaxial tests (unconsolidated undrained tests) is compared to understand the relative improvement in stress-strain behaviour with reinforcement. Fig. 28 compares the deviator stress versus axial strain for the unreinforced and geogrid reinforced systems. From the figure it is seen that the use of geogrid reinforcement has improved the peak deviatoric stress of the granular material. When the number of layers of reinforcement is increased, the maximum deviatoric stress measured for the sample also increased as expected. However, the quantitative improvement in the maximum deviatoric stress observed within the test limits has not proportionately increased with the quantity of reinforcement. The configuration of two layers of geogrid reinforcement has given the maximum benefit to the quantity of reinforcement used. With the increase in quantity of reinforcement, the increase in the peak stress sustained decreased. It is also observed from the figure that the unreinforced system and systems reinforced with 2 layer and 3 layers of geogrid reached the ultimate axial stress at 4.5%, 7% and 9% axial strain respectively, whereas the systems reinforced with 4 or 5 layers of geogrid reinforcement did not reach the ultimate stress within the test limits.

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Fig. 28 Deviator stress versus axial strain for unreinforced

and geogrid reinforced granular sub-bases

The geocell reinforcement exhibited an inferior performance at low levels of strain as seen in Fig. 29. The beneficial effect of geocell reinforcement was evident only after a strain level of 2%. Though the parent material used for making geocell has a high tensile strength of 55 kN/m, the performance of geocell reinforced samples depend upon its seam strength, which is low. The list of various systems in the increasing order of peak deviatoric stress is UR, SG-2, GC, SG-3, SG-4 and SG-5.

Fig. 29 Deviator stress versus axial strain for unreinforced

and geocell enclosed granular sub-bases

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The improvement in performance by geosynthetic reinforcement is quantified in terms of reinforcement ratio, which is defined as the ratio of the deviator stress of reinforced system to that of unreinforced systems at any particular strain level. Fig. 30 shows the reinforcement ratio for the various geogrid reinforced systems at varying levels of strain. From the figure it is seen that for all the reinforced systems, the reinforcement ratio has increased with the axial strain.

Fig. 30 Reinforcement ratio for different geogrid reinforced systems

Cyclic Triaxial Tests Displacement controlled cyclic triaxial tests were carried out on unreinforced and reinforced granular sub-base. Fig. 31 shows the variation of deviatoric stress and pore water pressure with number of cycles in a typical test. The slope of the secant line connecting the extreme points on the hysteresis loop is the dynamic young’s modulus, Edyn and is defined as follows:

Edyn = d (2)

where σd is the deviatoric stress at extreme points of hysteresis loop and ε is the amplitude of the applied axial strain.

32

Fig. 31Typical measurements during strain controlled cyclic triaxial test

Variation of the dynamic modulus with respect to the number of cycles for unreinforced and reinforced granular sub-bases is shown in Fig. 31 and Fig. 33. All

33

the system showed higher modulus in the initial cycles (up to 2000) after which the modulus dropped drastically. From the figures it is seen that the reinforced systems had higher modulus compared to unreinforced systems. Among the reinforced systems, geocell reinforced system has the least modulus. The 2 and 3 layer geogrid reinforced systems had high modulus in comparison to 4 layer geogrid reinforced system.

Fig. 31 Variation of dynamic modulus with respect to number of cycles

for unreinforced and geogrid reinforced samples

Fig. 32 Variation of dynamic modulus with respect to number of cycles for unreinforced and geocell reinforced samples

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FIELD TESTS ON REINFORCED UNPAVED ROAD SECTIONS INTRODUCTION

The objective of field tests is to understand the performance of unpaved low volume roads constructed over weak subgrade using different geosynthetic reinforcements. The relative advantages of different reinforcing materials placed at the interface of subgrade and base course in terms of increase in load carrying capacity and reduction in rut depth are studied through systematic field experiments. The experimental program consisted of total seven field tests at the identified location in the campus of Indian Institute of Science, Bangalore. The first test was conducted on unreinforced unpaved test section. Remaining tests were conducted on test sections reinforced with single or multiple layers of geosynthetics such as geotextile, biaxial geogrid, uniaxial geogrid and a layer of geocells placed at the interface of the subgrade and aggregate base. The test sections were subjected to moving vehicle load and the rut depths were measured at specific grid points with increasing number of cycles up to a maximum of 250 passes. The results are analyzed to compare the relative efficiency of various reinforcement layers in reducing the formation of rut in unpaved roads.

MATERIALS AND TESTING

The site chosen for constructing the model road section measured 2 m 1 m. The soil at this location was never subjected to any building load in past and hence thought to be ideal for simulating the condition of a newly constructed forest road or a village road.

Subgrade Soil The Subgrade Soil, which was the natural soil available at site, was reddish

brown in colour. The in-situ bulk unit weight of the soil was 18 kN/m3 and the natural water content in dry season was around 7.5 %. But in order to simulate the rainy season, which is the worst condition for trafficking, the top 10 cm thickness of soil was mixed with excess amount of water to make it slushy. The average bulk unit weight after mixing water was found to be 17 kN/m3 and the water content was 30 %. The Specific gravity of the subgrade soil was found using density bottle tests and the average value was found to be 2.61. The distribution of different grain sizes in the subgrade soil was determined using sieve analysis and hydrometer tests. The soil can be classified as Sandy Clay with letter symbols SC. The Maximum Dry Density (MDD) and the Optimum Moisture Content (OMC) of the soil were determined using standard proctor compaction test were found to be 1817 kg/m3 and 13% respectively. The undrained cohesive strength (cu) of the soil was determined from laboratory vane shear apparatus. The cu value for the undisturbed soil sample taken directly from the in-situ ground without mixing water was found to be 40 kPa. The cu value for the prepared subgrade after adding water and mixing was found to be 12 kPa. These values were average values for three different samples taken at three different sections along the length of the road with only 5% variation in values at different sections. CBR tests was conducted on the subgrade soil sample prepared to represent the field density and water content. The unsoaked CBR at 2.5 mm penetration for original ground was calculated as 22% whereas for the prepared subgrade, it was around 1%.

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Aggregate Grey coloured stone chips obtained from a near-by quarry were used as the base course aggregate for the tests. The average size of the aggregate was 12 mm. An aggregate layer of 10 cm thickness was placed directly over the subgrade soil in case of unreinforced test sections or over the geosynthetic layer in case of reinforced test sections. The unit weight of aggregate layer was maintained at 13 kN/m3 for all tests. Surface Course A leveled surface course layer of 5 cm thickness was placed above the base course aggregate for better rideability of vehicle in experiments. The surface course layer was prepared by placing in-situ dry soil and then rolling with sufficient quantity of water to avoid dust and ravelling when a vehicle passes over it. Reinforcing Materials Three different types of geosynthetics, namely, a woven geotextile, a biaxial geogrid and a uniaxial geogrid were used in different reinforced test sections. Two tests were conducted with geocell layer reinforcement. In these tests, geocell layers were formed at site using biaxial geogrid. In one test, tyre shreds were used as reinforcing layer. Geotextile The woven geotextile used in experiments is white in colour with negligible pore size of <0.075 mm. The geotextile is made of polypropylene and the ultimate tensile strength is 55 kN/m at an axial strain of 38%. Biaxial Geogrid The biaxial geogrid used in experiments is made from High Density Polyethylene (HDPE). It is a stiff grid with square openings of size 30 mm 30 mm and the ultimate tensile strength is 40 kN/m in both longitudinal and transverse directions at an axial strain of 10%. Uniaxial Geogrid The uniaxial geogrid used in experiments is made from high strength polyester yarns with black PVC coating. It is a stiff grid with rectangular openings of size 220 mm 17 mm and ultimate tensile strength of 60 kN/m in longitudinal direction and 40 kN/m

in transverse direction at an axial strain of 10%. Loading Vehicle

A 4-stroke, 102 cm3, single cylinder scooter of dimensions 1765 715 1130 mm with 1235 mm wheel base and ground clearance of 145 mm was used in experiments. The weight of the vehicle was 106 kg.

CONSTRUCTION OF UNPAVED ROAD SECTIONS In all these experiments, initially the unpaved road was constructed in stages.

First stage involved the preparation of soil subgrade to the required density and water content. In the second stage, aggregate layer of 10 cm thickness was prepared to the required density above the subgrade for unreinforced cases. For reinforced cases, prior to the placement of aggregate layer, the reinforcing layer was placed above the

36

subgrade. After the base course was placed and leveled, a surface course of 5 cm thickness was constructed using in-situ dry soil and rolled with sufficient quantity of water and leveled. These different stages are illustrated in following subsections. The original soil at the location was mixed with excess amount of water and made slushy for a depth of 10 cm and leveled. This bed was left as such for at least 24 hours so that the soil attained uniform consistency. Once the bed was ready, undisturbed tube samples were taken at three different sections of the soft bed to determine the placement water content and unit weight. For all the experiments, the water content and unit weight were maintained as 30% and 17 kN/m3 respectively. Most of the times, it required 2-3 trials of mixing to achieve these uniform values for all the tests. For tests involving reinforced road sections, reinforcing layers were placed above the soft soil subgrade before placing the aggregate base course. In case of geotextile and geogrids, a geosynthetic layer was cut from the rolls and placed over the test section, covering the entire test section. The longitudinal direction of geosynthetic layer was coinciding with the length direction of the road for all the tests to achieve maximum benefit.

In case of geocell reinforcement, initially a geotextile layer was placed over the subgrade. A layer of geocells was constructed in diamond pattern at the site to a size of 2 m 1 m using biaxial geogrid and anchor pins of 6 mm diameter and 10 cm effective height and placed above the geotextile as shown in Fig. 33. Geotextile layer was needed for this case to separate the subgrade and base course and to avoid mixing of layers during vehicle passage. Tests were done with geocell layers of two different geometries. The area of biaxial geogrid used to prepare the layer of geocell was 5.85 m2 in one case and 2 m2 in the other case.

Fig. 37 Geocell layer constructed in field

The aggregate was placed over this bed directly (in case of unreinforced tests)

or over the geosynthetic layer placed on top of the leveled soil subgrade (in case of reinforced tests). The total quantity of material required to obtain the desired unit weight of 13.05 kN/m3 for 10 cm thickness was divided into three portions and after spreading each portion, it was compacted using a hand roller and leveled. In case of tests with geocell reinforcement, aggregate was filled in geocells itself at the required

37

density. Care was taken not to fill any geocell to the total height until the adjacent cell was at least filled to half of the height, to ensure the proper shape of the geocell layer with aggregate infill. The in-situ dry soil was mixed with 10% water and placed over the aggregate layer to prepare a comfortable riding surface. The thickness of this layer was maintained as 5 cm and it was levelled using a drop hammer of 5 kg mass falling from a height of 450 mm on a square base plate of 150 mm 150 mm in size.

RUT DEPTH

ASTM: E 1703/E 1703M – 95, defined rut depth as the maximum measured perpendicular distance between the bottom surface of the straightedge and the contact area of the gage with the pavement surface at a specific location. This is shown in Fig. 38. When vehicle is passed over the prepared road surface, the surface gets deformed, forming ruts. Arrangements were made to measure the rut depth at 11 equally spaced grid points across the width of the road at three sections spaced uniformly along the length of the road. The schematic diagram showing the layout of the grid points marked on the plan of the road section is shown in Fig. 39.

Fig. 38 Rut Depth as defined by ASTM:E 1703/E 1703M – 95

Fig. 39 Layout plan of grid points for measuring rut depth

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Testing Procedure

A scooter of mass 106 kg was driven by a person weighing 55 kg along the centerline of the finished roadbed. The speed of the vehicle was maintained as 18 to 20 km/hr and the vehicle was passed in one direction only. The rut depths were measured at all grid points after every 20 passes until 200 passes were completed. Then it was passed continuously for 50 times more and the final rut depths were noted. If the vehicle started skidding in any point of time, the test was stopped at that particular stage and the corresponding number of passes and rut depths were noted. The testing arrangement is shown in Fig. 40.

Fig. 40 Test arrangement

TEST RESULTS The test results at Section 1 and Section 3 showed slightly similar responses.

The cross section profile and the rut depth measured at those two sections were almost more in majority of the cases when compared to Section 2.

Fig. 41 illustrates cross section profile of the road at various sections for

unreinforced road section. At the central portion of the road, the rut depth was the maximum since the vehicle was passed only at the central 10 cm width of road. When the vehicle is passed on this section, within 17 passes, a maximum depression of 95, 57 and 132 mm were observed when compared to the initial ground surface in Sections 1, 2 and 3 respectively. Afterwards, the vehicle started skidding because of softer slushy subgrade. Hence the test was stopped at 17 vehicle passes. From the cross section profile it is observed that the central cross section showed a better response when compared to the other two cross sections. The unreinforced road section is taken as the Control Section for all the reinforced test sections for comparing the results.

39

Fig. 41 Cross section profile at various sections for unreinforced road. In the first reinforced section, only a layer of geotextile was placed at the

interface of subgrade and base course. When this section is tested with moving vehicle, rut was formed in the central section and the passage of vehicle became impossible after 100 passes because of skidding. This test section showed a maximum depression of 96, 99 and 64 mm when compared to the initial ground surface in Sections 1, 2 and 3 respectively. In this case, Section 1 and Section 2 showed comparable responses with respect to depression below the initial ground surface as shown in Fig. 42.

Fig. 42 Cross section profile at various sections for unreinforced road

40

The next test was with biaxial geogrid placed along with the geotextile at the subgrade and base course interface. This section was stable till 250 passes of the vehicle, where the experiment was stopped because the rut was stabilized and there was no visible displacement for increasing number of passes. The cross section profile of the road for this test is shown in Fig. 43. The maximum depression as observed in comparison to the initial ground surface after 250 passes were 92, 56 and 90 mm for Sections 1, 2 and 3 respectively. As seen from Fig. 43, the heave and subsidence were increasing with the number of passes but they were stabilized at 250 cycles in this test.

Fig. 43 Cross section profile at various sections for Biaxial Geogrid reinforced road

In the next test, uniaxial geogrid was used along with geotextile for reinforcing the road section. In this test also, the rut depths were increasing till 250 passes and beyond 250 passes, the increase in deformations were very minimum hence the test was stopped after 250 passes. The cross section profile for this test is shown in Fig. 44. This test section showed a depression of 76, 66 and 66 mm when compared to the initial ground surface in Sections 1, 2 and 3 respectively. The next two tests consisted of road section reinforced with geocell layer placed at the interface of soft subgrade and aggregate base along with the geotextile layer. Geotextile layer is used to perform the function of separator in these tests. In one test, the geocell layer was formed using 5.85 m2 of biaxial geogrid and 150 connecting pins. The aspect ratio of geocells for this case was 1. Results from this test are presented in Fig. 45. Unlike in case of geotextile and biaxial geogrid, in case of road section reinforced with geocell layer, there was no progressive change in the cross section profile with the number of passes. The maximum heave and subsidence were observed within 100 passes and afterwards remained constant from 100 passes to 200 passes. This behaviour is because the geocell layer acts as stiff reinforcing mat for the road and

41

supports the loads. Even the heave and subsidence observed for this case was relatively small compared to other reinforced sections because geocell layer allows uniform distribution of loads and reduces differential settlements. The maximum depression as observed in comparison to the initial ground surface after 200 passes were 73, 47 and 76 mm for Sections 1, 2 and 3 respectively.

Fig. 44 Cross section profile at various sections for uniaxial geogrid reinforced road

Fig. 45 Cross section profile at various sections for geocell (GC 5.85) reinforced road

42

In the next test, geocell layer was formed using 2 m2 of biaxial geogrid, which is equivalent to the area of geogrid used in test with planar biaxial geogrid. Fig. 46 shows the cross section profile observed from this test. Even in this case, the settlement was almost immediate and after that, remained constant with the increase in number of passes. However, the maximum subsidence observed in this test at the end of 250 passes was 100, 68 and 82 mm, which is relatively higher than the previous test with geocell layer made up of 5.85 m2 of geogrid. This is because the area of biaxial geogrid used for preparing this geocell is very less compared to the previous test and the aspect ratio is 0.25 for geocells, making it less stiffer compared to the geocell layer with cells having an aspect ratio of 1.

Fig. 46 Cross section profile at various sections for geocell (GC 2) reinforced section

Comparison of Rut Depth with Number of Passes

When no reinforcing material was used, in the control section, the vehicle was able to pass only 17 times and thereafter it started skidding. The road is considered as totally failed at that point. All reinforced test sections performed better than the control section in terms of sustaining more vehicle passes for the same rut depth. The test section reinforced with geotextile layer failed at 100 passes of vehicle, whereas all other sections were in operating condition even after 250 vehicle passes. Comparison of the performance of unreinforced and geotextile reinforced test sections is shown in Fig. 47. As observed in the figure, the geotextile was efficient in increasing the number of vehicle passes at failure to 100 against 17 for control section. As observed from the figure, in the first and third sections, the rut depth observed for geotextile reinforced section was significantly less compared to that observed in the control section. However, in the central section, not much different was observed in the rut depths.

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Fig. 47 Comparison of rut depth for geotextile-reinforced road section with the control section

When the road sections were excavated and seen after the test, there was

mixing of layers and intrusion of aggregate into subgrade observed for the control section. In case of geotextile-reinforced section, the layers were separate even after the test, demonstrating the role of geotextile as separator apart from providing membrane support to the road as shown in Fig. 48. Hence the beneficial effect of the geotextile layer is seen clearly in terms of arresting the mixing of layers as well as in increasing the vehicle passes.

(a) (b)

Fig. 48 Rut formation in the test sections (a) Unreinforced test section (b) Geotextile-reinforced test section

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Fig. 49 compares the performance of road section reinforced with geotextile and biaxial geogrid with the performance of control section. By observing this plot, it can be understood that the biaxial geogrid is much superior to the geotextile for reinforcement in unpaved roads. Though the tensile strength of the biaxial geogrid is less compared to that of the geotextile used in this study, it is proved to perform better in reducing the rut depths and also in improving the life period of the unpaved road. The reason for the better performance of biaxial geogrid is the interlocking mechanism of the geogrid materials, which imparts more stiffness to it. Geogrid also acts as a separator if the size of the aggregate is more than its opening size. Since the aggregate size used in this study is smaller than the opening size of the geogrid, a layer of geotextile was used along with it to act as separator. As observed from this study, corresponding to 100 passes, the biaxial geogrid reduced the rut depth almost 50% compared to geotextile in Section 2. In sections 1 and 3, the rut depth of geotextile reinforced section was comparable to that of the biaxial geogrid reinforced section for the full 100 passes. The reason for this may be due to the fact that these sections were at 0.25m from the edges; hence the full interlocking mechanism of the geogrid was not utilized. But due to interlocking mechanism, biaxial geogrid could provide greater stiffness to the road section and hence sustain the full 250 passes.

Fig. 49 Comparison of rut depth for geotextile-reinforced road section and Biaxial

Geogrid reinforced section with the control section

Fig. 50 compares the results from tests done with biaxial geogrid and geocell layer made of biaxial geogrid of area 2m2. It should be noted that the quantity of grid used in both these tests is same, i.e. 2 m2. From Fig. 50, it is observed that the geocell layer in this case was not much effective in reducing the rut depth, except in Section 1. The reason for this is the low aspect ratio (0.25) for geocells in the layer. The pocket size being more, the cells are not effective in holding the aggregate. Whereas the geogrid layer, being continuous throughout the road section, provided better support and effective friction development at the interface.

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Fig. 50 Comparison of performance of biaxial geogrid and geocell layer (Equal quantity of geogrid used in both the cases)

Comparison of Traffic Benefit Ratio Perkins (1999) defined TBR as the number of cycles to reach a particular permanent surface deformation, for a reinforced test section, divided by the number of cycles to reach this same deformation in an unreinforced test section with the same layer thicknesses. The TBR values are calculated for different reinforced test sections for different rut depths and are plotted in Fig. 51. The efficiency of various layers can be compared from the TBR vs. rut depth plot as below: For all the sections, the TBR value of geocell layer (5.85 m2) was the highest

among all the reinforcements that have been used. Hence it is the most efficient reinforcement. The TBR of geocell layer (5.85 m2) at a rut depth of 71 mm is 16.5, whereas that of biaxial geogrid is 6.5. Hence the geocell layer (GC 5.85) is around 61% more efficient than biaxial geogrid in reducing rut depth.

The TBR value of shredded tyre layer is greater than that of the geotextile layer. Hence as far as traffic benefit ratio is compared tyre shreds are a better option than geotextile. However, in case of tyre shreds, initially the TBR value was low and started increasing with the passes. The reason for this could be the initial compression of the shredded tyre. Hence shredded tyre forms more rut in the Section 1 than unreinforced section.

46

Fig. 51 Traffic Benefit Ratios for different reinforcing layers

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NUMERICAL SIMULATIONS USING FLAC INTRODUCTION Numerical analysis of geosynthetic reinforced unpaved road sections is carried out using finite difference program FLAC (Fast Lagrangian Analysis of Continua) Version 6.0 FLAC FLAC uses an explicit finite difference formulation to find solutions to the dynamic equations of motion for the specific problem to be analyzed. This process cycle of FLAC in arriving at a solution to the problem is repeated until force equilibrium is reached (Fig. 52).

Fig. 52 FLAC calculation cycle (FLAC Manual, 2011) SIMULATIONS IN FLAC Behavior of unreinforced and reinforced unpaved road sections with and without geosynthetic reinforcement subjected to static loads is analyzed in FLAC. The road section is considered as a two layer system, consisting of a base layer made of sand resting on a subgrade soil of low bearing capacity. Plane strain analysis is carried out

48

using large strain mode. The concept of using large strain analysis is justified because the large strain represents the deep ruts that are allowed in unpaved roads. In case of reinforced road sections, soil-geogrid and geogrid-base contacts are governed by an interface that has a behavior, elastic perfectly plastic of Mohr-Coulomb. Boundary conditions of the problem are given in Fig. 53.

Fig. 53 FLAC model for reinforced unpaved road section

Material properties used for numerical analysis are given in Table 10.

Table 7.1 Material properties used in numerical simulations

Subgrade Properties

Unit weight, kN/m3 15.6

Poisson’s ratio 0.33

Young’s modulus, MPa 10

Undrained Cohesion, kPa 30

Base Properties

Unit weight, kN/m3 22

Poisson’s ratio 0.25

Young’s modulus, MPa 50

49

Cohesion, kPa 0

Friction angle, degrees 40

Dilation angle, degrees 20

Reinforcement Properties

Reinforcement material Weak geogrid Geonet

Modulus of reinforcement, MPa 183 1151

Thickness, mm 1.1 0.275

Poisson’s ratio 0.3 0.3

Interface Properties

Reinforcement material Weak geogrid Geonet

Stiffness per unit area k, (kN/m3) 3.58E+04 3.65E+04

Cohesion, kPa 0.1 21.38

Interface friction angle, Degrees 28 33

Numerical Analysis with Flac Grid used for the plane strain analysis in FLAC is shown in Fig. 54.

Fig. 54 Flac grid used for numerical analysis

50

To achieve a rut, downward velocity is imposed to the 4 gridpoints representing the static wheel load. The geogrid was modeled as a structural beam. The beam adopted has zero inertia, to characterize the membrane effect of the geogrid. RESULTS Results of numerical analysis of two layered unpaved road sections simulated in FLAC using large strain analyses are shown in Fig. 55. The results presented in Fig. 55 show the load-displacement simulation of the unreinforced, geogrid reinforced and geonet reinforced road sections. Regarding the improvement in load bearing capacity of the structure, provided by the reinforcement, and according to the results obtained with Flac simulations, the improvement is about 120% for geogrid reinforced section and about 80% for the geonet reinforced section. This demonstrates that the geogrid reinforcement has a better effect on increasing the load bearing capacity of a two-layer system.

-20

-18

-16

-14

-12

-10

-8

-6

-4

-2

00 100 200 300 400 500 600 700

Normalized Rut Depth, /B (%)

Bearing pressure (kPa)

Unreinforced

Weak Geogrid

Geonet

Fig. 55 Load bearing capacity of unreinforced and reinforced unpaved road sections

Parametric Studies Parametric numerical studies are carried out on the unpaved road section, varying the stiffness of the reinforcement, number of reinforcing layers and undrained cohesion of the soil layer below. Results are presented in Figs. 56-58. From these results, it is clear that the stiffness of the reinforcement has significant effect on the load carrying capacity of pavement sections. It is the most influencing parameter in the analysis.

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Fig. 56 Effect of reinforcement stiffness on the maximum tension developed in the reinforcement

Fig. 57 Effect of undrained cohesion of soil layer on the load bearing ratio

52

-20

-17.5

-15

-12.5

-10

-7.5

-5

-2.5

00 200 400 600 800 1000 1200

Rut Depth Ratio, /B (%)

Bearing pressure (kPa)

UnreinforcedN=1N=2N=3

Fig. 58 Effect of number of geogrid layers on the load bearing pressure of the road

section

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DESIGN GUIDELINES FOR REINFORCED UNPAVED ROAD SECTIONS

Based on the results of experimental and numerical studies carried out on unpaved road sections and the materials involved, the following guidelines are arrived at for their design. o Geosynthetic reinforcement improves the load resistance of soil-aggregate systems.

The beneficial effect of reinforcement is evident at higher water contents because the tensile strength of the reinforcement is mobilized better in systems with higher water contents. Hence geosynthetics can be successfully used in water clogged road sections for improving the load bearing capacity.

o The improvement in performance is not directly proportional to the tensile strength of the reinforcing material but is a function of its tensile stiffness. While selecting the reinforcement, care should be taken to get the right stiffness rather than high ultimate tensile strength.

o Geocell reinforcement was found to be more effective than planar reinforcement. Geocells could be conveniently used to derive immediate and long term increase in load bearing capacities.

o Increasing the quantity of geogrid reinforcement resulted in improvement in stress-strain behaviour but this improvement is not significant beyond certain level (3 layers of geogrid in this case).

o None of the samples failed during triaxial testing. At the end of cyclic tests, samples became compact and dense and stood stiff on their own, demonstrating the effectiveness and sustainability of these materials under cyclic loads.

o The maximum tension in the geogrid continues to increase proportionally with the increase of the stiffness. Optimal choice of the geogrid depends on the expected traffic loads and allowable costs.

REFERENCES

1. Koerner, R. M. (1999). Designing with Geosynthetics. 4th Edition, Prentice

Hall Inc, New Jersey, 761p.

2. Perkins, S. W. (1999). Mechanical response of geosynthetic-reinforced

flexible pavements. Geosynthetics International, Vol. 6, No. 5, pp. 347-382.

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LIST OF PUBLICATIONS BASED ON THE STUDY

Journal Publications

Asha M Nair and Madhavi Latha, G. (2011) "Bearing resistance of reinforced soil-aggregate systems", Ground Improvement, Proceedings of the ICE, Vol. 164, GI2, pp. 83-95.

Asha M Nair and Madhavi Latha, G. (2012) "Taming of large diameter triaxial setup", Geomechanics and Engineering, Techno Press, Vol. 4, No.4. pp. 251-262.

Asha M Nair and Madhavi Latha, G. (2012) "Cyclic loading behaviour of reinforced soil-aggregate bases”, Ground Improvement. Proceedings of the ICE, Accepted.

International Conference

Asha M. N. and Madhavi Latha G (2012). Strength behaviour of reinforced soil-aggregate systems under repeated and cyclic loading, ASCE Geotechnical Special Publication, GeoCongress - 2012, March 25-29, 2012, Oakland, CA, pp. 1513-1522.

National Conferences

Asha M. N. and Madhavi Latha G (2011). Performance of geosynthetic reinforced soil aggregate systems under cyclic loading, Proc. of Indian Geotechnical Conference -2011, Kochi, Vol. 1, pp. 537-540.

Asha M. N. and Madhavi Latha G (2012). Model studies on geosynthetic reinforced unpaved road sections, Proc. of Indian Geotechnical Conference -2012, Delhi, Vol. 1, pp. 219-222.