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2378 Journal of Applied Sciences Research, 9(3): 2378-2385, 2013 ISSN 1819-544X This is a refereed journal and all articles are professionally screened and reviewed ORIGINAL ARTICLES Experimental Study of Bearing Capacity for Egyptian Soils Reinforced by Geotextiles 1 M.H. Elshakankery, 1Alsaid Ahmed Almetwally and 2K.A. Tawfik 1 2 Textile Eng. Dept., National Research Center, Dokki, Cairo, Egypt Faculty of Engineering, Cairo University, Cairo, Egypt ABSTRACT Nonwoven geotextiles have been successfully used for reinforcement of soil to improve bearing capacity. The purpose of this study was to quantify the benefit of using geotextile materials as reinforcement of different Egyptian soils. Laboratory California bearing ratio (CBR) tests were performed to investigate the load – penetration behavior for different three soils types with different weights of geotextiles. Soil samples are tested for CBR with and without reinforcement .i.e. geotextile materials. The result of these tests shows that hard soil with high weight of geotextile samples increases the CBR slightly. Furthermore, bearing ratio of reinforced other soil samples with geotextile increases. The relation between the apparent opening sizes of a geotextiles and particles size of soil is taken into consideration. Key words: Nonwoven geotextiles, CBR Tests, Soil, and Reinforced ratio Introduction In Egypt, there are many hard solid, changeable sand and poor soils in many areas as a result of which the subgrades will shrink and swell with variations in water and moisture content in the soil. This shrinking and swelling movement causes irregular road surface and road deterioration, resulting in a need for premature rehabilitation of the pavement road. In recent years, there are many available technologies to improve the mechanical properties and performance of soil by using geotextiles material. Conventional road structure: The road is usually built up in several layers, each have some special functions. As shown in fig.1 a typical road cross-section consists of: Fig. 1: Road structure a) A graded and compacted subgrade. b) Aggregate base course to cover the subgrade. c) Road base layer to cover the sub-base course layer. d) Tope wearing surface or surfacing layer. Over weak subgrades, a sub-base between the aggregate layer and the soil subgrade is commonly found to be economical. Corresponding Author: M.H. Elshakankery, Textile Eng. Dept., National Research Center, Dokki, Cairo, Egypt E-mail: [email protected] 2379 J. Appl. Sci. Res., 9(3): 2378-2385, 2013 The subgrade strength used in road construction does not stay the same during its life; the change in the subgrade strength may be due to the following causes: - Regression of the crushed stone aggregate to the subgrade layer. - Variation in soil moisture content. Regression is the main cause of road structure failure; for that to obtain soil stabilization and extended life of road structure there are three important factors play a role in the design of such a system: 1- Suitability of subgrade soil as identified by its CBR (California Bearing Ratio) 2- Thickness of aggregate layer is determined by the subgrade layer and the type and volume of traffic to be cared on the road. 3- Type of wearing surface as determined by the type and volume of traffic as well as environment conditions. Geotextiles: The uses of geotextiles in many engineering applications have become more apparent and have proven to be an effective means of soil improvement. The history of geotextiles is relatively short. The South Carolina (USA) Highway Department placed a heavy woven cotton fabric in several rural roads in 1926. With data from at least eight separate field tests, they reported in 1935 that the fabric reduced cracking and failures in the road. Giroud and Noiray (1981) first introduced an analytical approach to the design of geotextile-reinforced unpaved roads. The bearing capacity of the soft subgrade is considered to increase from πcu to (π+ 2)cu with the inclusion of a geotextile; where c is the inexpert shear strength of the soil and u is recommended coefficient for the soilconfining effect with stabilization fabrics. The value of u can adjust slightly (1.95-2.05) for different acceptable rut depth. Additional improvement due to membrane action is considered to be a function of the geotextile strength and allowable rut depth. Another technique proposed by Milligan et al. (1984) is based on a small rut depth concept and does not consider membrane action. Using the Milligan et al. technique, the bearing capacity of the subgrade is improved and increases from (π/2 + 1) cu to (π/2+ 2) cu by placing a geotextile layer at the subgrade-granular fill interface. Resl and Werner (1986) carried out the laboratory tests under an axisymmetric loading condition using nonwoven, needle-punched geotextiles. The results showed that the nonwoven layers placed between subsoil and subgrade can significantly increase the bearing capacity of soft subgrades. Houlsby and Jewell (1990) extended the Milligan et al. (1984) technique to circular loadings by increasing the subgrade bearing capacity from 3.07cu for an unreinforced soil layer to 5.69 cu for a reinforced soil layer. Fannin and Sigurdsson (1996) carried out a full-scale field trial to study the performance of different geosynthetics in unpaved road construction over soft ground. Several papers have examined the reinforcement of soil (Bergado et al., 2001; Raymonda and Ismail, 2003; Park and Tan, 2005; Yetimoglu et al., 2005; Patra et al., 2005; Varuso et al., 2005); current research work mainly emphasize on the strength, mechanism and bearing capacity at the reinforced soil (Haeri et al., 2000; Michalowski, 2004; Zhang et al., 2006; Latha and Murthy, 2007; and Senthil Kumar et al., 2012;). CBR tests are also conducted by introducing geotextiles and geogrid in hard soil (Naeini and Mirzakhanlari, 2008; Duncan et al., 2008; Naeini and Moayed, 2009; and Dhule et al., 2011). Further, based on CBR test, the influence of geotextile, geogrid and geonet are investigated in clay with low or medium compressibility (Love et al, 1987; Srivastava et al., 1995; Giroud et al., part I and Part II, 2004; Naeini and Moayed, 2009; Nair and Latha, 2010; Moayed and Nazari, 2011; and Nair and Latha 2011) as soft subgrade in an unpaved road system. Hence, in this study the effect of non-woven geotextiles placed between two different layers (subsoil & aggregate) on the CBR strength by the comparison between three types of soil bearing capacity in dry condition, under axisymmetric loading were investigated. Experimental Work: The materials used, experimental set-up, tests conducted and the experimental procedures have been presented as follows. Material: Soil: Three different Egyptian compressible soil samples obtained are used for the present experimental investigations. The required properties of them were determined and are presented in Table 1. The particles Distributions are showed in Figure 2. 2380 J. Appl. Sci. Res., 9(3): 2378-2385, 2013 Geotextiles Properties: Five different needle-punched polyester geotextiles were tested: G1, G2, G3, G4, and G5, respectively. Table 2 gives the geotextiles physical and mechanical properties. All of the samples from the geotextiles listed in Table 2 were tested with the length direction (parallel machine direction) and crosswise. Table 1: Soils Properties Particulars Soil Specific Gravity Tons/m3 Maximum Dry Density Tons/m3 Optimum Moisture Content % Porosity % Particles D90 mm Soil Class CBR ratio % at 0.1 CBR ratio % at 0.2 Table 2: Geotextiles Properties Physical property Fiber Dinner Fiber Length (mm) Mass per unit area (g/m2) ASTM D 5261 Thickness (mm) ASTM D 5199 Wide Width Strength (kN/m) ASTM D 4595 (200 mm) Machine direction/ Crosswise Grab strength (N) ASTM D 4632 Machine direction/ Crosswise Elongation at break (%) ASTM D 4632 Machine direction/ Crosswise Puncture resistance (N) ASTM D 4833 Equivalent opening size (μm) ASTM D 4751 Soil I 1.91 1.42 12.6 47 0.096 Solid or hard 66.71 84.0 Soil II 2.03 1.82 18.3 32.2 0.154 Medium 54.8 54.54 Soil III 2.88 1.74 32.2 38 0.25 Very soft 27.5 25.2 G1 G2 G3 G4 G5 6 128 6 128 9 128 9 128 9 128 250 300 400 500 600 2.12 2.56 3.78 4.22 4.82 10.5/17.3 13.3/20.4 15/24.5 18.8/31.3 20.334.2 508/610 635/756 882/964 996/1330 1236/1840 92/82 85/81 75/62 65/60 62/54 420 688 866 950 1200 125:177 105:125 74:88 74:88 >74 Aggregate: Aggregate is a broad category of coarse particulate material used in construction, including sand, gravel, crushed stone, slag and recycled concrete. Aggregates are used as base material under foundations, roads, and railroads. The grain size distribution of the aggregate shown in Figure 2 indicated that it was somewhat finer than the allowable gradation specifications, the aggregate used in the CBR test is classified as well graded gravel. Fig. 2: Particle size distribution of the aggregate and deferent soils 2381 J. Appl. Sci. Res., 9(3): 2378-2385, 2013 Cbr Test Procedure: The California bearing ratio (CBR) is a penetration test for mechanical strength evaluation of road subgrades and base courses with standard circular piston at the rate of 1.25 mm/min. The CBR test is made according to ASTM Standards D1883-07 (for laboratory-prepared samples). Samples for the CBR were compacted according to ASTM Standards D 698 (Methods B and D). The samples were placed in three layers at the standard mould. First layer was a soil which compacted for 5 inches height by 56 a rammer dropped blows from 12 inches. Second layer was a geotextile sample which was placed between first layer and aggregate final layer which was compacted for 2 inches as shown in figure 3. Fig. 3: Schematic arrangement of the Soil-Aggregate in the CBR mould with geotextiles Results And Discussion Different soils with different geotextile weight materials are tested and results are obtained. The results obtained by taking the average of three trails for each penetration levels are plotted as stress with geotextiles weight. The variation of penetration stress curves for different soil samples and aggregate without geotextiles material are shown in figure 4. The variation of penetration stress curves for different soil samples and aggregate with geotextiles material are shown in figures 5, 6 and 7. Soil I Soil II Soil III Penetration stress psi 1400 1200 1000 800 600 400 200 0 0 0.025 0.05 0.075 0.1 0.125 0.15 0.175 0.2 0.3 Penetration depth In Fig. 4: Penetration stress of different soil samples-Aggregate without geotextiles material Soil I: 0.4 0.5 2382 J. Appl. Sci. Res., 9(3): 2378-2385, 2013 Fig. 5: Penetration stress of soil I -Aggregate with different geotextiles weight Table 4: Multiple regression analysis of soil I for penetration stress R= .74039580 R²= .5481859 Adjusted R²= .53116798 F(2,57)=81.234 p<.00000 Std. Error of estimate: 5.11 St. Err. Soil I BETA of BETA Intercept Penetration depth 0.85976 0.06750 Geotextile weight 0.03306 0.07563 B 287.290 2838.620 0.12545 St. Err. of B 116.739 222.866 0.25616 t(57) 2.461 12.737 0.48973 p-level 0.01691 0.00000 0.62620 Fig. 6: Deformation and cause penetration failure in the geotextiles 250 g/m2 through CBR testing From the penetration stress curve of figure 5 and table 4, geotextiles weight in final strand has a p-level of 0.6262 which is not statistically significant effective in penetration stress. This is due to the fact that soil I is very hard and the light weight of geotextiles (250 and 300 g/m2) are deformed and cause penetration failure in the material through CBR testing as shown in figure 6. Furthermore, increasing geotextile weight (at high weight levels of 500 and 600 g/m2) has slightly increasing on penetration stress but these have not significant effective. In this case, the geotextiles material is used as a separation between sub-soil and aggregate but the soil particles and geotextiles equivalent opening size is taken into consideration. Soil particles must be bigger than geotextiles material opening size. Soil II: Table 5 shows the regression analysis for penetration stress. It is clear from table 4 that the multiple correlation factors are about 0.862 at a high significant level which is a good correlation. 2383 J. Appl. Sci. Res., 9(3): 2378-2385, 2013 Fig. 7: Penetration stress of soil II -Aggregate with different geotextiles weight Table 5: Multiple regression analysis of soil II for penetration stress R= .86234543 R²= .74363959 Adjusted R²= .73489242 F(2,57)=59.531 p<.00000 Std. Error of estimate: 4.36 St. Err. Soil II BETA of BETA Intercept Penetration depth 0.78987 0.07536 Geotextile weight 0.22882 0.05344 B 100.741 1850.919 0.61631 St. Err. of B 92.506 176.603 0.20299 t(57) 1.089 10.481 3.036 p-level 0.28073 0.00000 0.00361 From the penetration stress curve of figure 7 and table 5, geotextiles weight in final strand has a p-level of 0.00361 which is statistically significant effective in penetration resistance. It is clear that increasing geotextiles weight leads to increase penetration resistance. Maximum penetration stress is obtained at higher geotextiles weight. 3. 3. Soil III: Table 6 shows the regression analysis for penetration stress. It is clear from table 6 that the multiple correlation factors are about 0.884 at a high significant level which is a good correlation. It is clearly observed that there is a significance effect for geotextiles weight in resistance of penetration stress at different levels of penetration depth as shown in figure 8. Geotextiles weight in final strand has a p-level of 0.00008 which is statistically significant effective in penetration resistance. Fig. 8: Penetration stress of soil III -Aggregate with different geotextiles weight 2384 J. Appl. Sci. Res., 9(3): 2378-2385, 2013 Table 6: Multiple regression analysis of soil III for penetration stress R= .88466840 R²= .782638177 Adjusted R²= .77222045 F(2,57)=45.662 p<.00000 Std. Error of estimate: 4.68 St. Err. Soil III BETA of BETA Intercept Penetration depth 0.70255 0.09843 Geotextile weight 0.34947 0.08211 St. Err. of B 49.158 93.848 0.10787 B 39.824 802.983 0.45910 t(57) 0.81011 8.556 4.256 p-level 0.42124 0.00000 0.00008 Further, in order to quantify the amount of increase in the penetration resistance, the reinforcement ratio is taken into consideration. The reinforcement ratio at a particular penetration is: Reinforcem ent ratio (1 - ( reinforcem ent withou t geotextile s )) x100 reinforcem ent with geotextile s Table 7: Reinforcement ratio for different Geotextiles weight and soil samples Geotextiles weight g/m2 Soil I 250 0.43 300 0.51 400 1.01 500 1.35 600 2.87 Soil II 2.55 4.92 9.61 15.99 26.92 Soil III 1.07 8.55 14.80 25.61 33.38 Table 7 shows the reinforcement ratio for different geotextiles material weight and soil samples at the high levels of penetration depth. It is clear that increasing geotextiles weight leads to an increase of reinforcement ratio for all soil samples expect soil I (hard soil). It is clear for soft soil III sample; there is a more effective for geotextiles material weight in CBR strength in general. This is evident from Table 7. The small size of the test samples creates concerns about the difference of dimensions of the aggregate particles, mould diameter, and plunger (loading head). Although the smaller load head allows for the development of a non-constant stress profile in the bounds of the mould, but it is given some indication about the relation between laboratory scale and full-scale pavement. For example, soil I did not need reinforcement geotextiles, but needed only separation layer between aggregate and sub-soil. Furthermore, contact area of the truck tire has a nominal radius of 6 inch approximately, and the stress profile is distributed over a much greater side level for the full-scale pavement. These are considered limitations of the study along with the rigid boundary; calibration against full scale testing is required to determine the actual behavior. Conclusion: A series of CBR tests have been conducted to investigate the effect of geotextile weight on the bearing ratio of soils. The conclusions from this study are summarized as follows: The improvement of bearing ratio of soils with geotextile materials depends on the soil type. 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