Field Trials with Polypropylene Woven Geotextiles

Transcription

1 The First Pan American Geosynthetics Conference & Exhibition 2-5 March 2008, Cancun, Mexico Field Trials with Polypropylene Woven Geotextiles B.V.S. Viswanadham, Department of Civil Engineering, Indian Institute of Technology Bombay, India V. Satkalmi, Polymer Business Development Group, Reliance Industries Limited, Mumbai, India ABSTRACT This paper addresses field trials carried-out with polypropylene woven geotextiles in India. Firstly, a 2 km demonstration project for a part of rural road development programme in the state of Maharashtra, India involving the use of a Polypropylene woven geotextile as a separator cum reinforcement is presented. An indigenously custom designed and manufactured woven slit film polypropylene geotextile was used. To control problems associated with expansive nature of subgrade soils, lime stabilization was carried-out. Construction of the geotextile reinforced road was completed and opened to traffic in April Observations show that, in the geotextile reinforced section of the road, there are no signs of visible distress even after about 38 months. Secondly, application of woven polypropylene geotextile layer as a separator cum reinforcement layer at the interface between granular base and granular sub-base in the paved portion for the petroleum outlet units is presented. The significant influence of geotextile layer in improving the performance of road and paved portions of retail outlet units is demonstrated adequately along with the cost-benefit analysis of roads with and without geotextiles. 1. INTRODUCTION India s transport sector is large and diverse; it caters to the needs of 1.1 billion people. Good physical connectivity in the urban and rural areas is essential for economic growth. Since the early 1990s, India's growing economy has witnessed a rise in demand for transport infrastructure and services by around 10 percent a year. Roads are the dominant mode of transportation in India today. They carry almost 90 percent of the country s passenger traffic and 65 percent of its freight. The density of India s highway network -- at 0.66 km of highway per km 2 of land is similar to that of the United States (0.65) and much greater than China's (0.16) or Brazil's (0.20). Roads are significant for the development of the rural areas - home to almost 70 percent of India's population. Although the rural road network is extensive, some 40 percent of India s villages do not have access to all-weather roads and remain cut off during the monsoon season. The problem is more acute in India's with varied sub-grade and climate conditions. In such situations, one of the alternatives is to improve the ground using geosynthetics for building structures for infrastructure development. Geosynthetics have been used successfully to reinforce unpaved roads on soft subgrade for many years. Construction of reinforced temporary roads and bases for heavy machinery are examples of short-term usage of the geosynthetics, where the main goal is to save fill material (Hufenus et al. 2006). In paved roads, the adoption of geosynthetic reinforcement aims at an improvement of load carrying capacity and the survivability of the roads. Geosynthetics are installed between subgrade and road to separate or to reinforce. If migration of fines is very probable, separation is an essential function (Al-Qadi et al. 1994). Hufenus et al. (2006) reported about details of numerous field trials and full-scale laboratory investigations that illustrate the use of geosynthetics to reinforce unpaved roads on soft subgrade, improve the bearing capacity, extend the service life, reduce the necessary aggregate layer thickness, diminish deformations, and delay rut formation. Presence of geosynthetic reinforcement at the interface of subgrade and aggregate layer or within the aggregate layer provide two modes of action: They are (i) Confinement effect and (ii) Tensioned membrane effect. Firstly, in confinement effect, a vertical load (i.e. in the form of wheel load) induces lateral forces, which spread the aggregate and leading to deformations of the aggregate layer. Due to frictional interaction and interlocking between the aggregate layer and the geosynthetic (especially geogrid), the aggregate particles are restrained at the interface between the subgrade and the aggregate layer. The reinforcement can absorb additional shear stresses between the subgrade and the aggregate layer, which would otherwise be applied to the soft subgrade (Houlsby and Jewell, 1990). This improves the load distribution on the subgrade and reduces the necessary aggregate thickness. The effectiveness of the reinforcement not only depends on the adequate load transmission to the aggregate material (through friction and interlocking), but also is improved by the higher stiffness of the geosynthetic layer (Cancelli et al. 1996). Secondly, in tensioned membrane effect, if an unpaved road is pre-rutted during construction, a geosynthetic reinforcement at the aggregate-subgrade interface gets tensioned and it acts like a tensioned membrane, with the pressure on the subgrade being smaller than the pressure applied to aggregate on the upper, concave side. Geosynthetic reinforced unpaved roads on soft subgrade (CBR 2) have been shown to reduce the necessary aggregate layer thickness by approximately 30 %. However, knowledge pertaining to the application of woven geotextiles as separator-cum-reinforcement layer in roads is limited, especially in India. Considering the above-cited potential, this paper presents field trials carried-out with polypropylene woven geotextiles. Firstly, a 2 km demonstration project for a part of rural road development programme in the state of Maharashtra and thereafter application of woven polypropylene geotextile layer as a separator cum reinforcement layer 1112

2 at the interface between granular base and granular sub-base in the paved portion for the petroleum outlet units constructed in Gujarat state are presented. 2. DEVELOPMENT AND ASSESSMENT OF GEOTEXTILE PROPERTIES A woven geotextile made of ultra-violet treated polypropylene tapes with a tenacity ranging from g/denier was manufactured. The polypropylene tapes are weaved by adopting a twill weaving method. Tapes are arranged in such a way that the manufactured geotextiles will have identical tensile strength-strain behavior in the both machine and crossmachine directions. In addition, the geotextile is observed to have a mass per unit area of 267 gsm and an Apparent Opening Size (AOS) equivalent to 1.18 mm. Table 1 summarizes properties of the woven fabric RPW3. Table 1. Summary of properties of woven geotextile used in field trails. S.No Property Value 1. Mass per unit area 268 gsm 2. Thickness at 2kPa mm 3. Grab Tensile strength Ultimate Ultimate Machine direction 1.1 kn 19.6 % Cross-machine direction 1.05kN 13.6 % 4. Wide width tensile strength Ultimate Ultimate Machine direction kn/m 14.6 % 5. CBR Burst 3693 kpa 6. Apparent Opening Size (AOS) 1.18 mm Cross-machine direction kn/m 16.8 % 3. DESIGN DETAILS OF GEOTEXTILE REINFORCED ROAD ALONG MDR Background of the road status along MDR82 Figure 1 presents a typical road stretch along MDR82. As shown in Figure 1, on an average, rut depths were observed to range from mm. District public works development authorities informed that along this particular stretch, every year new road is constructed. The possible reasons for the distress could be: (i) Presence of a swelling sub-grade, like black cotton soil, (ii) Inadequate drainage, (iii) Seasonal heavy traffic with higher axle loads, etc. During the process of severe rut formations, a non-uniformity in load spreading phenomena occurs. This will result in inadequate sub-grade reaction at one side (where thinning of the aggregate layer occurs) and more than adequate sub-grade reaction at other side (i.e. where thickening of aggregate layer occurs). This type of situations reduce the design life of the road, affect the riding quality and increase maintenance cost. In such situations, one of the viable alternatives is to strengthen the road by introducing a reinforcement layer in the form of geotextile at the interface of a granular sub-base layer and prepared sub-grade. For the roads constructed on weak sub-grade, the inclusion of geosynthetics, like geotextiles or geogrids can lead to less deformation of the road and can reduce the thickness of base materials needed. In many cases, this can be cost-effective, as the savings in importing the base material and in repairs to the road can offset the cost of reinforcement. A geosynthetic layer placed at the interface between the aggregate base course and the subgrade functions as a separator and helps in preventing two dissimilar materials (sub-grade soils and aggregates) from intermixing. Geotextiles and geogrids perform this function by preventing penetration of the aggregate into the subgrade. In addition, goetextiles prevent intrusion of sub-grade soils into the base course aggregate. Localized bearing failures and sub-grade intrusion only occur in very soft, wet, weak sub-grades. Therefore, separation is important to maintain the designed thickness and the stability and load carrying capacity of the base course. The stabilization of roads on weak sub-grade with a geosynthetic material is primarily attributed to the basic functions of separation of the base course layer from the sub-grade soil, and a reinforcement of the composite system. Geosynthetics are thus a great boon for ease in construction over soft soil as well as long-term performance of roads. 3.2 Soil properties Table 2 presents the summary of soil properties. An average free swell of the soil sample collected is obtained as 93 %, which indicates a high degree of expansion. The sub-grade soil is of a black cotton soil and expanding in nature. Potentially expansive soils, such as, black cotton soils are montmorillonite clays and are characterized by their extreme hardness and deep cracks when dry and with tendency for heaving during the process of wetting. Roadbeds made up of such soils when subjected to changes in moisture content due to seasonal wetting and drying or due to any other reason undergo volumetric changes leading to pavement distortion, cracking and general unevenness. A proper design incorporating the following measures may considerably minimize the problems associated with expansive soils. As per 1113

3 IRC: , one of the alternatives is to stabilize the soil using quick lime extending over the road formation width along with measures for efficient drainage of the pavement section. Figure 1 Status of road along MDR 82 in Table 2 Summary of properties of expansive soil along MDR82. Property Value Hygroscopic water content, w g [%] 9.9 Specific gravity, G s [ -] 2.80 Liquid Limit, LL [%] 69 Plastic Limit, PL [%] 33 Plasticity Index, PI [%] 36 Size fractions Gravel [ > 4.75 mm ] (%) Sand [ mm] (%) Silt [ mm] (%) Clay [<0.002 mm] (%) 62 Soil classification (USCS) Max. dry unit weight γ d [kn/m 3 ] CH Optimum moisture content [%] 31.5 *Undrained Cohesion [kn/m 2 ] 52.5 California Bearing Ratio CBR [%] -unsoaked -soaked 3.3 Design details of geotextile reinforced road Free Swell [%] 93 The design of the geotextile reinforced road was carried out as per the procedure outlined by Giroud and Noiray (1981). In the design, a standard axle load of 80 kn was considered with 10,000 passes. By taking the properties mentioned in Tables 1 and 2, the pavement block sections with and without geotextile layer were arrived. Keeping in view of the expansive nature of the sub-grade, the sub-grade is pre-treated with lime and saturated with water trickling. Figure 2 presents a schematic cross-section of geotextile reinforced road. The design particulars include the following: unreinforced aggregate thickness with traffic h 0 is equivalent to 0.72 m and whereas reinforced aggregate thickness with traffic h R works out to be 0.51 m. The reduction of aggregate thickness, Δh, resulting from the use of a geotextile, is 0.21 m. This results in 29 % percentage savings in the aggregate requirement. These values were calculated for CBR = 1. However, in this study, in order to evaluate the performance of geotextile reinforced road along with adjacent stretches constructed without any geotextile, it was decided to provide an aggregate layer of 520 mm thickness above the geotextile layer. A 100 mm soft murrum cushion layer above and below the geotextile layer are included in an aggregate layer of 520 mm thickness above the geotextile layer. The construction methodology involves trimming of the existing road surface and widening to obtain the road formation width for the entire 2 km stretch along MDR82. This is followed by treating the sub-grade in two stages: (i) by forming lime + black cotton soil mix trenches on both sides and (ii) spreading of lime on top of the prepared sub-grade and trickling with water (Figure. 2). Figure 3 shows a typical view of 1114

4 the road during geotextile fabric laying. As can be seen from the Fig. 3, the geotextile is overlaid on a compacted soft murrum base and pegged along the edges to keep it in position. After laying the geotextile fabric, a thin layer of soft murrum was laid to cushion the geotextile fabric. This is adopted to protect the geotextile from any damages during laying, installation and post-construction stages. The laying of the fabric was completed in April 2004 and is currently in the monitoring stage. Figure 2 Schematic cross section of geotextile reinforced road. Figure 3 View of the road during installation of geotextile. Figure 4 shows a typical view of an un-reinforced road along MDR82 as on July 30, The surface of the road shown in Fig. 4 was built on the sub-grade of almost identical conditions but without any treatment or geotextile layer. The initiation of a rutting can be noted in Fig. 4 and distress in road can be noted. Contrary to this, geotextile reinforced stretch was observed to behave well with reduced deformations and rutting (Fig. 5). This shows the significant influence of a geotextile layer along with a lime treatment in enhancing the performance of the road stretch along MDR82. In order to assess the riding quality of reinforce and un-reinforced road stretches, falling weight deflectometer/benkelmann beam tests are being planned. Fig. 4 View of an un-reinforced road as on July 30,

5 Fig. 5 View of a Geotextile reinforced road as on July 30, GEOTEXTILE REINFORCED PAVED AREAS FOR PETROLEUM RETAIL OUTLETS 4.1 Details of Reinforced paved areas The majority of petroleum retail outlet units are constructed on filled-up areas or agricultural fields along national highways/expressways in India. Paved areas within petroleum retail outlet units are subjected to a sustained traffic volume, which is about 10 % of traffic along particular national highways/expressways. Out of these, several retail outlet units were observed to experience distress; mainly they are reported to be in the form of rutting and displacement of paved blocks or wearing course. This necessitates periodical maintenance and will be un-economical and also hinder usage of a retail outlet unit. This section presents a field trial involving the use of a woven geotextile layer as a separatorcum-reinforcement layer placed within the granular base of pavement section for a retail outlet unit in Gujarat state of India. Up to 2.5 m from the existing ground level, it was noticed that there is layer of soil having silt and clay fraction > 75% with a liquid limit equal to 58 % and plasticity index equal to 28%. Thereafter, it is followed by another layer of clay upto 5.1 m. Virgin soil at the site is classified as CH according to Unified soil classification system. The soaked CBR of a virgin soil was found to be 2.8 % in the laboratory with a coefficient of permeability equal to 6.8 x m/s. Estimated traffic volume in this retail outlet unit is about 5 msa (million standard axles). As per IRC:37 (2001), pavement block thickness shall have to be equal to 595 mm. A geotextile reinforced pavement section was designed by considering properties of geotextile given in Table 1. Table 3 gives split of the pavement sections with and without geotextile. Geotextile layer is placed at the interface between the granular sub-base and the prepared sub-grade and a 50 mm reduction in granular base thickness could be provided. This was adopted to alleviate the cost-economics of geotextile reinforced pavement section. In order to safeguard the geotextile layer from puncturing failure, a 50 mm thick cushioning layer of comprising of predominantly sandy soil was provided. Figure 6 depicts status of paved areas of a retail outlet unit as on August 22, Construction of this outlet unit was completed in May Significant influence of geotextile layer in enhancing the performance of paved areas could be demonstrated quantatively only. Some of the retail outlet units without any geotextile layer were observed to experience distress immediately after the monsoon in Table 3 Details of pavement sections for paved area in retail outlet units. Details Un-reinforced pavement section Reinforced pavement section Wearing Course 25 mm BC 40 mm BC Binding Course 50 mm DBM 50 mm DBM Granular Base 250 mm 200 mm Granular Sub-base 270 mm *270 mm Total Pavement Thickness 595 mm 560 mm *Including 50 mm thick cushioning layer above geotextile; BC = Bituminous Concrete; DBM = Densie Bituminous Macadam. 1116

6 Paved area Paver blocks Fig. 6 View of paved areas of a retail outlet unit as on August 22, COST-BENEFIT ANALYSIS OF GEOTEXTILE REINFORCED ROADS In this section, cost-benefit analysis of road with and without geotextile layer is presented. Generally, road stretches are designed for a period of 15 years. During its life time, it is expected that in addition to regular maintenance and there is a requirement of relaying wearing course or replacing pavement blocks. In case of road on soft soil sub-grade formation, maintenance of the road will become menace and incur expenditure. In case of geotextile reinforced road, reduction in maintenance can be envisaged and which subsequently improves the riding quality of paved section. The following benefits can be identified as a result of laying a geotextile as a reinforcing layer between the aggregate layer and the sub-grade: i) Reduction of the thickness of the aggregate layer and ii) Reduction in the rut formation as function of trafficking, increasing the serviceable life of the road. Economic advantages of geosynthetic reinforcement lay primarily in the possibility to reduce the thickness of the aggregate layer or to limit the amount of subgrade to be removed. This saves on the use of aggregate materials and the amount of unsuitable material for removal and deposition elsewhere, which has both economical and ecological aspects, and may be useful for construction of roads laid without paved surfaces. Reduction in aggregate thickness will be a great boon for those areas where availability of stones (or aggregates) is scarce. The following paragraphs elucidate cost-benefit analysis of roads with and without geotextile layer. Consider the initial cost of a flexible pavement section as x and r m is the maintenance cost of an un-reinforced pavement, which equals to r m /100 times initial cost of a flexible pavement [expressed in Currency units/(lane-km)]. Equation (1) below gives cost of an un-reinforced pavement for a period of 15 years, which includes maintenance cost and number of overlays (n OL ) in its design life (and y is the cost of each over lay per lane and km). Similarly, equation (2) is for a reinforced road, with r mr is the maintenance cost of a reinforced pavement, which equals to r mr /100 times initial cost of a flexible pavement. Generally, due to accrued benefits of reinforcement pavement section, r mr < r m. In equation (2), initial cost of geotextile per km-lane is considered and savings due to reduction in aggregate layer procurement and laying costs is subtracted. With an anticipation of reduction in maintenance costs, requirement of number of overlays during its design life can be possible, this in turn reduces the cost of a reinforced pavement section up to the end of its design period. Finally, net cost savings (NCS) were calculated as given in equation (3). Using geotextile properties similar to those given in Table 1, NCS for a life period of 15 years was estimated approximately as 14,500 US $ per kmlane. This brings out the significance of the use of geotextile layer as a separator-cum-reinforcement layer not only as a performance enhancer but also as a viable cost-economic alternative. C C rm = 1 + x nol y 100 [1] rmr = 1 + x + nol y + xg xδ h 100 [2] UR + R NCS = C UR - C R [3] 6. CONCLUSIONS In this paper, an attempt has been made to present observed performance of reinforced stretches using woven polypropylene geotextile layers as a separation-cum-reinforcement layer. Due to non-availability of riding quality test and instrumented data, the performance results are based on the observed features. With the completion of 3 years period, 1117

7 it is proposed to evaluate riding quality of reinforced pavement sections and control sections by conducting Benkelmann beam tests. The following benefits can be identified as a result of laying a geotextile as a reinforcing layer between the aggregate layer and the sub-grade: i) reduction of the thickness of the aggregate layer and ii) reduction in the rut formation as function of trafficking, increasing the serviceable life of the road. Economic advantages of geosynthetic reinforcement lay primarily in the possibility to reduce the thickness of the aggregate layer or to limit the amount of subgrade to be removed. ACKNOWLEDGEMENTS The authors would like to thank Public Works Department (PWD), Pune, Government of Maharashtra, India and Reliance Engineering Associates Limited (REAL), Mumbai for coming forward to undertake the field trials and their cooperation throughout the study is highly appreciated. M/s Techfab India, Mumbai, India are acknowledged in manufacturing the geotextile used in this study. REFERENCES Al-Qadi, I.L., Brandon, T.L., Valentine, R.J., Lacina, B.A., and Smith, T.E. (1994). Laboratory evaluation of geosyntheticreinforced pavement sections, Transportation Research Report, 1439, pp Cancelli, A., Montanelli, F., Rimoldi, P., and Zhao, A. (1996). Full-scale laboratory testing on geosynthetic reinforced paved roads, Proc. Int. Symp. On Earth Reinforcement, Fukuoka, pp Giroud, J. P. and Noiray, L. (1981). Geotextile-reinforced unpaved road design., Journal of Geotechnical Engineering, ASCE, Vol. 107, No. 9, pp Houlsby, G.T., and Jewell, R. A.(1990). Design of reinforced unpaved roads for small rut depths. Proc. Fourth Int. Conf. on Geotextiles, Geomembranes, and Related Products, Vol. 1, pp Hufenus, R., Rueegger, R., Banjac, R., Mayor, P., Springman, S.M., and Brönnimann, R. (2006). Full-scale field tests on geosynthetic reinforced unpaved roads on soft subgrade, Geotextiles and Geomembranes, 24, IRC: 37 (2001). Guidelines for the design of Flexible Pavements Indian Roads Congress, New Delhi, India. 1118