bearing resistance of geosynthetic reinforced soil

Bearing Resistance of Geosynthetic Reinforced Soil-Aggregate Systems

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BEARING RESISTANCE OF GEOSYNTHETIC REINFORCED SOIL-AGGREGATE SYSTEMS

Asha M. Nair Research Scholar, Department of Civil Engineering, IISc, Bangalore–560 012, India. E-mail:[email protected] G. Madhavi Latha Associate Professor, Department of Civil Engineering, IISc, Bangalore–560 012, India. E-mail:[email protected]

ABSTRACT: This paper summarizes the results from laboratory CBR tests on reinforced and unreinforced soil-aggregate systems. Soil used in the studies is clay of intermediate plasticity (CI) with liquid limit of about 35%. The average size of the aggregate used is around 5.5 mm. The soil layer is prepared in four lifts in the CBR mould and the aggregate layer is placed on top of the soil layer in one lift. In tests with geosynthetic reinforcement, the reinforcing layer is inserted at the interface of the soil and aggregate layers. A high strength geotextile and a biaxial geogrid are used as reinforcement in these tests. The soil-reinforcement-aggregate systems are subjected to standard penetrating load in the CBR test. The relative performance of different geosynthetics and the effect of soil density on the performance of geosynthetic reinforced soil-aggregate systems are brought out from this work. 1. INTRODUCTION

Application of geosynthetics in the field of transportation, geo-environmental and geotechnical constructions has increased in the recent years. Geotextiles and geogrids are extensively used in paved and unpaved roads to reinforce the subgrade or base layer to improve the pavement performance.

The performance of soil fabric aggregate systems were studied by a number of researchers, e.g., Bender & Barenberg (1979), Giroud & Noiray (1981), Gregory & Bang (1994), Fannin & Sigurdsson (1996), Giroud & Han (2004), Hufenus et al. (2006). The reinforcing action of geosynthetics have been studied by many researchers by conducting laboratory experiments and model studies. e.g., Love (1984), Miura (1990), Alenowicz & Dembicki (1991), Elvidge & Raymond (1999), Gurung (2003), Raymond & Ismail (2003), Yetimoglu et al. (2005), Naeini & Mirzakhanlari (2008).

In the present study, reinforced and unreinforced soil aggregate systems were prepared in the laboratory CBR mould. The reinforcing materials were placed at the interface of soil aggregate system. Experiments were conducted to study the effect of soil consistency and density on the penetration resistance of reinforced and unreinforced soil aggregate systems.

2. MATERIALS

2.1 Soil

Soil used in the test is classified as clay of intermediate plasticity (CI) according to IS 1498 (Part 1)—1987. Table 1 summarises the properties of the soil used in this study.

2.2 Aggregate

The aggregate used in the test is gray in colour and was obtained from a nearby quarry. The size of the aggregate used in the tests ranged from 4.75–6.3 mm.

Table 1: Properties of Soil Colour Reddish brown

Specific gravity 2.69 Soil classification CI Liquid limit & Plastic limit, % 36 & 22 Maximum dry unit weight (as per modified compaction), kN/m3

18.75

Optimum moisture content, % 13 CBR at OMC and MDD, (as per modified compaction) %

11

Undrained cohesion at OMC and MDD, cu, kPa

40

2.3 Reinforcing Materials

In preparing reinforced soil aggregate systems, geotextile and biaxial geogrid were used as the reinforcement. The woven geotextile used in experiments is white in colour and has pore size less than 0.075 mm. The geotextile is made of polypropylene and the ultimate tensile strength obtained from wide width tension test is 55 kN/m at an axial strain of 38%. The biaxial geogrid is a stiff grid with square openings of size 30 mm × 30 mm, and has an ultimate tensile strength 40 kN/m in both longitudinal and transverse directions at a failure strain of 17%.

IGC 2009, Guntur, INDIA


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The geosynthetic layer was placed at the interface of soil and aggregate layer in the tests. In experiments with biaxial geogrid, a layer of geotextile is used below the grid as a separator to prevent the mixing of soil and aggregate layers.

3. EXPERIMENTAL STUDIES

Series of unsoaked laboratory CBR studies were carried out in the conventional CBR mould of 150 mm internal diameter and 175 mm total height. CBR tests were carried out for soil alone, soil aggregate and reinforced soil aggregate systems. Soil was filled in the CBR mould in 4 lifts and aggregate was filled in one lift. All these layers were compacted using a rammer of 4.89 kg weight, falling from a height of 450 mm. A metal plate of 6mm thickness and 1.25 kg weight was used while compacting the aggregate layer to avoid the spilling of the aggregate. The total thickness of the prepared soil and soil aggregate systems was 125 mm in all the cases. The thickness of individual layers of soil and aggregate in each lift was maintained as 25 ± 2 mm in all the cases. Figure 1 shows the schematic sketch of the prepared soil aggregate systems. Experiments were carried out at OMC (13%) and at higher water contents of 14.5%, 16% and 17.5%. In all the cases the aggregate layer was compacted to a unit weight of 15.4 kN/m3.

Penetration tests were carried out in the CBR mould by keeping a surcharge of 4.85 kg on top of the prepared soil and soil aggregate systems. The plunger was made to penetrate the prepared specimen at a rate of 1.25 mm/minute. Load and penetration values were recorded and the CBR values were computed from the load-penetration curves.

Fig. 1: Schematic Sketch of the Prepared Soil-Reinforcement-Aggregate System

4. RESULTS AND DISCUSSIONS

The load-penetration curves plotted from the CBR tests were analysed to compare the relative performance of the reinforced soil aggregate systems at various densities.

4.1 Effect of Water Content

Figures 2–5 shows the load-penetration plots for soil alone, soil aggregate and geosynthetic reinforced soil aggregate

systems. In these figures, S represents soil alone, SA represents soil-aggregate system, SAGT represents Soil-aggregate-geotextile system and SAGG represents soil-aggregate-geogrid-geotextile system. As observed in these figures, replacement of the top soil layer with the aggregate layer resulted in improvement in bearing resistance at all the water contents.

The load penetration response for reinforced soil-aggregate systems is not significantly deviating from that of the unreinforced soil aggregate systems in the initial stages. The reason for this is that the reinforced systems require an initial strain to mobilise the tension in them. However, as the strains developed, the geosynthetics start functioning as reinforcing tensile elements in the system, the load penetration response improved rapidly. The ultimate loads of the reinforced soil aggregate systems are very high when compared to that of the unreinforced ones as observed from Figures 2–5.

Also, there is a distinct improvement in the CBR value with the addition of the reinforcement in the soil aggregate systems. Table 2 summarises the CBR values at various water contents for different tests. The improvement of CBR value is found to be the maximum at OMC for all the cases.

Fig. 2: Load Penetration Curves at OMC

Fig. 3: Load Penetration Curves at w = 14.5%

Fig. 4: Load Penetration

Curves at w = 16% Fig. 5: Load Penetration

Curves at w = 17.5%

Surcharge Aggregate layer Geosynthetic layer Soil layer compacted in 4 lifts


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Table 2: Summary of CBR Results at various Water Contents

CBR Value, % Water

content (%)

Soil alone

Soil- aggregate

Soil- aggregate- geotextile

Soil-Aggregate-geotextile-

geogrid

OMC (13) 11.0 17.5 20.5 31.0

14.50 7.5 8.5 14.0 15.5

16.00 4.5 6.0 7.5 8. 0

17.50 3.0 3.5 5.0 6.0

4.2 Effect of Reinforcement

Figures 6–9 shows the plot of load-penetration for soil alone, soil-aggregate and geosynthetic reinforced soil-aggregate systems for a range of water contents. In case of soil alone and unreinforced soil-aggregate, the declination in performance is proportional to the increase in the percentage water content. However, the performance in case of reinforced soil-aggregate systems is different from that of the unreinforced systems. Though the bearing resistance has come down with the addition of water, the declination in performance is not proportional to the increase in water content as observed from the load-penetration plots. Especially for the case of geogrid reinforced soil-aggregate systems, there is clear drop in the performance with just 1.5% increase in water content from OMC and the difference is only marginal from thereafter, with addition of more water.

Fig. 6: Load Penetration Curves for Soil Alone

Fig. 7: Load Penetration Curves for Soil Aggregate

System

4.3 Comparison of Secant Modulus for Water Contents

Secant modulus in CBR test is defined as the load in kPa at a penetration of 5 mm to the penetration of 0.005 m. Figure 10 shows the variation in the secant modulus with respect to water contents for various test systems. From this figure it is clear that decrease in the secant modulus with increase in water content is drastic at OMC and at higher water contents, this effect is not significant.

Fig. 8: Load Penetration Curves for Soil Geotextile

Aggregate System

Fig. 9: Load Penetration Curves for Soil Geogrid

Aggregate System

Fig. 10: Variation in Secant Modulus with Water Content for

various Soil-Aggregate Systems


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The findings from these experiments provide clear guidelines for the design of reinforced soil-aggregate systems in terms of relative efficiency of various reinforcement types and the effect of water content on the load-penetration behaviour.

5. CONCLUSIONS

The following major conclusions are drawn from this study: • Reinforced soil-aggregate systems performed better than

unreinforced ones in terms of increase in CBR value and decrease in penetration at any specific load.

• Highest improvement is achieved with soil-aggregate-geogrid-geotextile system at all water contents tested.

• As the water content is increased, the performance of the unreinforced as well as reinforced soil-aggregate systems dropped. The effect is more pronounced for geogrid reinforced systems.

• The decrease in the secant modulus with increase in water content for various soil systems is drastic at OMC and the variation is not pronounced at higher water contents.

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bearing resistance of geosynthetic reinforced soil

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