International Journal of Engineering Trends and Technology (IJETT) – Volume 38 Number 1- August 2016 ISSN: 2231-5381 http://www.ijettjournal.org Page 50 Geosynthetic Reinforcement of Pavement Subgrade with Optimum and High Water Content Erhan Burak Pancar#1 #Asst. Professor, Department of Architecture, Ondokuz Mayıs University, Turkey Abstract In this study geocell and geotextile reinforcement techniques were investigated for pavement clayey subgrade with optimum water content and high water content by 10% increasing the optimum water content. These two reinforcement techniques were made solely and at the end together. For this purpose, a large scale plate load test was designed and utilized. Loading-settlement curve was achieved and modulus of subgrade reactions and bearing capacities for 8 different states were determined. It was detected that although promising results were obtained on stabilization of the pavement subgrade when both techniques were used together, only one state among 8 states gave the sufficient result for modulus of subgrade reaction. Keywords — Geocell, geotextile, geosynthetic, reinforcement, pavement, subgrade I. INTRODUCTION Bearing capacity of the base soil is generally affected from soil type, water content and compaction degree. It is required to stabilize the base soils that are not appropriate for road superstructure by improving them adequately. With the improvement of the base soil, bearing capacity is increased, settlements are decreased and therefore surfacing thickness is decreased and surfacing performance is increased. Among soil improvement techniques, geosynthetic reinforcement has been increasingly used as basal reinforcement since it facilitates rapid construction at low costs. Latha and Somwanshi [1] compared the relative performance of different forms of geosynthetic reinforcement (i.e. geocell, planar layers and randomly distributed mesh elements) in foundation beds by using same quantity of reinforcement in each test. Both the experimental and numerical studies demonstrated that the geocell is the most advantageous form of soil reinforcement technique of those investigated, provided there is no rupture of the material during loading. It was also determined that geocell reinforcement is more desirable than planar reinforcement [2-3]. As seen at Figure 1, cellular confinement systems (geocell, geoweb, neoweb etc.) is a network having a high resistance that was developed with the aim of stabilizing the soil by taking it under control and formed from three-dimensional cells interconnected with nodes in the shape of a honeycomb made from polyethylene. The cellular load bearing systems expands in the construction field and filled with soil. The filling material completely covers the cell walls and confined the entire environment in the soil. Therefore, it increases load-deformation behavior and resistance of the soil by taking vertical loading stresses at the cell walls and soil resistance at the adjacent cells [4]. Figure 1. Application of cellular confinement system in the field [5] Zhao et al. [6] reviewed the geocell-reinforced layers under embankments and suggested that the main geocell layer functions in three aspects: lateral resistance effect, vertical stress dispersion effect and membrane effect. In their study, Zhang et al. [7]determined that “lateral resistance effect” of geocell reinforcement has no direct effect on increasing the bearing capacity of subgrade soil. The bearing capacity increment on the foundation soil can be made up of “vertical stress dispersion effect” and “membrane effect”. The loads from the embankment deflect the geocell reinforcement thus generate a further tension force, as shown in Figure 2. The vertical component of the tension force in the reinforcement is helpful to reduce the pressure on the subgrade soil. This is the “membrane effect” of geocell reinforcement [7]. Geotextile reinforcement also reduces the pressure on the subgrade by “membrane effect”.International Journal of Engineering Trends and Technology (IJETT) – Volume 38 Number 1- August 2016 ISSN: 2231-5381 http://www.ijettjournal.org Page 51 Figure 2. Membrane effect of geocell reinforcement It is determined that increasing the reinforcement width more than 4.2 times of footing width for the geocell would not provide much additional improvements in bearing pressure nor additional reduction in footing settlement [2]. Sireesh et al. [8] and Dash et al. [3] also detected the efficient width of the geocell as 4.9 and 5, respectively. The improvement in bearing capacity due to the provision of reinforcement is frequently estimated at an unrealistically high range of footing settlement level, up to 40-50% of footing width [3, 8, 9], whereas this range of settlement level is not acceptable (the amount of settlement must not be large) for the design of the footings in most practical circumstances. The value of footing settlement equals 12% of footing width (s/B) is considered an absolute upper limit. For s/B>6% (higher settlement levels) geotextile inclusion increases the values of improvement factor in bearing pressure of footing and percentage reduction in footing settlement, significantly [2]. It is detected that planar geogrid at the base of the geocell mattress with h/B (height of the geocell layer/width of footing) = 1.2 could bring an improvement in bearing capacity as high as 30% more than that with geocell alone. The beneficial effect of this planar reinforcement layer becomes negligible at large heights of geocell mattress [9]. The overall goal of this study was to demonstrate the benefits of geocell and geotextile reinforcement soleley and together for pavement clayey subgrade with optimum water content and with high water content. A laboratory model loading tests were conducted by considering results obtained from literature to determine the most efficient sizes of the plate loading test aparatus and type of testing. Loading-settlement curves were drawn for eight different states and modulus of subgrade reactions and bearing capacities of soil for these eight different states were determined. The results obtained at the end of the study compared each other and the affect of water content on reinforcement types was researched. II. MATERIAL AND METHOD In this paper, experimental studies were conducted on clayey soil with optimum water content and high water content respectively. After sieve analysis, consistency limit experiments and hydrometer analyses, respectively were done on the soil, it was classified according to AASHTO and unified soil classification system. In order to determine optimum water content and dry unit weight of the clay material, also modified proctor experiments were conducted. The experiments that were conducted up to here were done with the aim of determining class and specifications of the soil. In this paper, the experiment model that was mainly wanted to be conducted was plate loading experiment. With this aim, model plate loading experiments were conducted on the mixtures that were prepared from optimum water content (25%) and high water content (10% more than optimum water content). In these experiments, the stabilization was done by the way the soil was reinforced with geocell and geotextile sole and together for two different water content soils. The sieve analysis of soil used as a subgrade are shown in Table 1. The results detected by liquid limit and plastic limit experiments for natural soil were 57 and 27, respectively. Table 1. Wet sieve analysis Sieve analysis Sieve no Sieve Diameter mm Residue of sieving (gr) Sieved (gr) Sieved percent, % 3/8'' 9,53 0 420 100 4 4,76 42,7 377,3 90 10 2 30,1 347,2 83 40 0,42 18,73 328,47 78 100 15,4 313,07 75 200 0,074 11,5 301,57 72 Pan 301,57 As per ASTM D2487 [10], the soil used as a subgrade was classified as clay with high plasticity (Class CH). The sand used as an infill material for geocell in this investigation was dry. It was used as a base layer for unreinforced test section. The effective particle size ( ) was 1.2 mm, coefficient of uniformity (Cu) was 2.25, specific gravity was 2.64, coefficient of curvature (Cc) was 1.05. It is classified as poorly graded sand (SP) according to unified soil classification system [10]. The void ratio of the sand was 0.42 and internal friction angle was 37º. The geocell and planar reinforcemet used in this study were both made and supplied by the same company. The type of geotextile was non-wowen. The engineering properties of this geotextile, as listed by the manufacture, are in Table 2.International Journal of Engineering Trends and Technology (IJETT) – Volume 38 Number 1- August 2016 ISSN: 2231-5381 http://www.ijettjournal.org Page 52 Table 2. Technical properties of non-woven geotextile Properties Values Unit weight (gr/m²) 500 Thickness (mm) 4 Tensile strength (kN/m) 27-29 Breaking elongation (%) 50-80 Statical puncture resistance (N) 5500 Dynamical puncture resistance (mm) 3 Water permeability (m/sn) 0.025 Characteristic aperture size (mm) 0.1 The engineering properties of the geocell, as listed by the manufacture, are in Table 3. There were also drainage holes having 10 mm diameter at geocell cell walls. Table 3. Technical properties of geocell Properties Values Density (gr/cm³) 0.94 Welding size (cm) 40 Cell length (mm) 300 Cell width (mm) 250 Thickness (mm) 2 Cell depth (cm) 20 A laboratory model loading tests were conducted to compare the influence of geocell and geotextile reinforcement and lime stabilization on increasing the bearing capacity of clayey soil in a steel box. The overall inner dimensions of the box were 1.2 m long, 1.2 m wide, and 1.2 m height as seen in Figure 3. Unpaved road test sections were constructed inside the box. The pocket size (d) of the geocell is taken as the diameter of an equivalent circular area of the pocket opening. This diameter was 25 cm in this study. Pocket diameter/ Footing width (d/B) is reported around 0.8 times of footing width which is found to be the one that gives maximum performance improvement [3]. Due to this reason, the diameter of circular footing was determined as 30 cm in this experimental tool. The footing was loaded with a hydraulic actuator and the circular footing was 30 cm in diameter and 3 cm thick. 1 cm thick rubber pad was attached to the bottom of the loading plate to ensure full contact and minimize stress concentrations at the edge of the plate. The peak load was selected to simulate a single wheel load of 40 kN (equivalent to an axle load of 80 kN and a tire contact pressure of 550 kPa). Figure 3. Schematic diagram for the set-up of the plate loading test. The test box was filled with clayey soil with optimum water content and high water content (35% water) by 10% increasing the optimum water content. The soil was used as a subgrade and the depth was 75 cm. The subgrade soil was placed in 3 layers with 25 cm thickness for each layer. The placed layers were compacted in lifts inside a box using a vibratory plate compactor. After preparing the subgrade, three strain gages were installed on the top of the subgrade. 5 pressure cells were installed on the surface of the subgrade at the center, 15 cm, and 30 cm away from the center of the loading plate, respectively. A linear variable differential transducer (LVDT) was also placed on the footing model to provide the value of footing settlement during the loading (Figure 3). Eight unpaved road test sections were prepared in the test box. Experiments were conducted on one (subgrade with 25% water content and unreinforced base), one (subgrade with 25% water content and geotextile reinforced base), one (subgrade with 25% water content and geocell reinforced base), one (subgrade with 25% water content and geocell+geotextile reinforced base), one (subgrade with 35% water content and unreinforced base), one (subgrade with 35% water content and geotextile reinforced base), one (subgrade with 35% water content and geocell reinforced base), one (subgrade with 35% water content and geocell+geotextile reinforced base). Reinforced and unreinforced bases were all 23 cm thick. Unreinforced bases consisted of clayey soil. After installation of pressure cells and strain gages, a layer of geotextile was placed on top of the subgrade and the geocells were placed on top of geotextile for geosynthetics (geotextile+geocell) reinforced sections. The geocell used in this experiment was 20 cm thick, top of the geocell mattress was at a depth of 3 cm from the bottom of the footing and the geocell width was 1.18 m as Moghaddas Tafreshi and Dawson [2] and Dash et al. [9] detected the ratios between footing width,International Journal of Engineering Trends and Technology (IJETT) – Volume 38 Number 1- August 2016 ISSN: 2231-5381 http://www.ijettjournal.org Page 53 geocell height and geocell width to get optimum test results. III.RESULTS AND DISCUSSION With the plate loading experiments done in the laboratory, effects of geotextile reinforcement, geocell reinforcement, geosynthetics (geotextile + geocell) reinforcement on a clay soil with optimum water content (25%) and with high water content (35%) were separately reviewed in Figure 4. Figure 4. Loading-settlement curve for different reinforcements and water contents While the maximum settlement in soil with 35% water content was 45 mm at 550 kPa pressure, this settlement was decreased to 41 mm when soil had 25% water content, 36 mm when soil (35% water content) was reinforced with geotextile, 32 mm when soil (25% water content) was reinforced with geotextile, 26 mm when soil (35% water content) was reinforced with geocell, 23 mm when soil (25% water content) was reinforced with geocell, 16 mm when soil (35% water content) was reinforced with geosynthetics (geotextile+geocell), 8 mm when soil (25% water content) was reinforced with geosynthetics (geotextile+geocell). The settlement in soil with 35% water content was about 1.3 times, 1.7 times and 2.7 times the settlement when this soil was reinforced with geotextile, geocell and geosynthetics, respectively. The settlement in soil with 25% water content was about 1.3 times, 1.8 times and 4.1 times the settlement when this soil was reinforced with geotextile, geocell and geosynthetics, respectively. The affect of using geotextile and geocell solely on settlement was almost similar for soils with optimum water content and high water content (10% more than optimum water content). But when they are used together as a geosynthetics reinforcement, the affect changes. Geosynthetics reinforcement on soil with optimum water content becomes 1.5 (4.1/2.7) times more effective than the geosynthetics reinforcement on soil with high water content. Using geotextile and geocell together as a soil reinforcement is better alternative than using them solely for soils with different water contents. Modulus of subgrade reaction values (k) were calculated with the help of Figure 4 by determining the inclinations of loading-settlement curves. These values are listed in Table 4. Table 4. Modulus of subgrade reactions for different states States Modulus of subgrade reaction (k) (kN/m³) 35% water content 6.450 25% water content 8.214 Geotextile + 35% water content 12.105 Geotextile + 25% water content 13.269 Geocell + 35% water content 17.037 Geocell + 25% water content 21.905 Geosynthetics + 35% water content 24.211 Geosynthetics + 25% water content 60.000 As it is seen from Table 5, “k” value was 6.450 kN/m³ for the soil which had 35% water content and 8.214 kN/m³ for the soil which had 25% water content. Modulus of subgrade reaction value was 60.000 kN/m³ for the soil which had 25% water content and reinforced with geocell+geotextile. According to Highways Technical Specifications in Turkey, this value is to be no less than 55.000 kN/m³ and only one reinforcement state (geosynthetics + 25% water content) met this requirement among eight states. It is known that half of the stress corresponding 10 mm settlement at load-deformation curve obtained from plate loading experiment gives bearing capacity of the base soil. By starting from this information, half of the stresses corresponding 10 mm at load-deformation curves were calculated and bearing capacity values were determined. The bearing capacity values are given at Table 5 and Figure 5. Table 5. Bearing capacities of different states States Bearing capacity (kN/m²) 35% water content 28 25% water content 37 Geotextile + 35% water content 60 Geotextile + 25% water content 69 Geocell + 35% water content 85 Geocell + 25% water content 101 Geosynthetics + 35% water content 138 Geosynthetics + 25% water content 275International Journal of Engineering Trends and Technology (IJETT) – Volume 38 Number 1- August 2016 ISSN: 2231-5381 http://www.ijettjournal.org Page 54 Figure 5. Comparision of bearing capacities of different states under plate loading The bearing capacity of soil with 35% water content increased from 28 kN/m² to 138 kN/m² (4.9 times) and the bearing capacity of soil with 25% water content increased from 37 kN/m² to 275 kN/m² (7.4 times) by geosynthetics reinforcement. While the bearing capacity of soil with 25% water content was 1.3 times the bearing capacity of soil with 35% water content, this ratio became to 2 after geosynthetics reinforcement. When the soil with 35% water content is reinforced with geotextile and geocell, the bearing capacitiy becomes 2.1 and 3 times, respectively. When the soil with 25% water content is reinforced with geotextile and geocell, the bearing capacitiy becomes 1.9 and 2.7 times, respectively. IV.CONCLUSIONS In this study, the effects of geotextile and geocell reinforcement of clayey soil with optimum water content (25%) and high water content (35%) were investigated by using these two materials solely and together. Eight different states were examined and model plate loading experiments were done for this purpose. Settlements of soil under 550 kPa which represents the ultimate tire pressure on road, modulus of subgrade reactions and bearing capacities of these eight different states were calculated. The results were compared each other and this comparision has not been done before in other studies under these circumstances. In this study, it is determined that geocell reinforcement is better than geotextile reinforcement and using these materials together is the best alternative for soil reinforcement. Geosynthetics (geotextile+geocell) reinforcement affect on settlement decreases when the water content increases in the soil. Settlement of soil which has 10% more water content than optimum water content is 1.5 times the settlement of soil with optimum water content under 550 kPa tire contact pressure when they are both reinforced with geosynthetics. Only geosynthetics reinforcement of the soil which had 25% (optimum) water content met the modulus of subgrade reaction requirements of Highways Technical Specifications among eight different states examined in this study. Although the affect of geotextile and geocell on bearing capacity of the soil with optimum water content is slightly less than the soil with 10% more water content, geosynthetics reinforcement is more effective for the soil with optimum water content. As a result of this study, it is concluded that geosynthetics reinforcement is a good alternative to increase the bearing capacity of the soil. But water content is very important in this treatment. It is desired to bring this content to optimum level to meet the Highways Technical Specifications. If it is difficult to do that especially for watery areas, lime stabilization of the soil with geosynthetics reinforcement can be examined to get better results. REFERENCES [1] G.M. Latha, A. Somwanshi, “Effect of reinforcement form on the bearing capacity of square footings on sand,” Geotextiles and Geomembranes, 27(6), pp. 409-422, 2009. [2] S.N. Moghaddas Tafreshi, and A.R. Dawson, “Comparison of bearing capacity of a strip footing on sand with geocell and with planar forms of geotextile reinforcement,” Geotextiles and Geomembranes, 28(1), pp.72–84, 2010. [3] S.K. Dash, S. Sireesh, T.G. Sitharam, “Model studies on circular footing supported on geocell reinforced sand underlain by soft clay,” Geotextiles and Geomembranes, 21(4), pp.197–219, 2003. [4] A. Emersleben, and N. Meyer, “Bearing capacity improvement of gravel base layers in road constructions using geocells,” The 12th International Conference of International Association for Computer Methods and Advances in Geomechanics (IACMAG), 1-6 October, India, pp.3538-3545, 2008. [5] O. Kief, Y. Schary, S.K. Pokharel, “High-Modulus geocells for sustainable highway infrastructure,” Indian Geotechnical Journal, 45(4), pp. 389-400, 2015. [6] M.H. Zhao, L. Zhang, X.J. Zou, H. Zhao, “Research progress in two-direction reinforced composite foundation formed by geocell reinforced mattress and gravel piles,” Chinese Journal of Highway and Transport, 22(1), pp.1-10, 2009. [7] L. Zhang, M. Zhao, C. Shi, H. Zhao, “Bearing capacity of geocell reinforcement in embankment engineering,” Geotextiles and Geomembranes, 28(5), pp.475–482, 2010. [8] S. Sireesh, T.G. Sitharam, and S.K. Dash, “Bearing capacity of circular footing on geocell-sand mattress overlying clay bed with void,” Geotextiles and Geomembranes, 27(2), pp. 89–98, 2009. [9] S.K. Dash, N.R. Krishnaswamy, K. Rajagopal, “Bearing capacity of strip footings supported on geocell-reinforced sand,” Geotextiles and Geomembranes, 19(4), pp.235- 256, 2001. [10]ASTM, Standard practice for classification of soils for engineering purposes (unified soil classification system), D2487, West Conshohocken, PA, 2006.