STRENGTH CHARACTERISTICS OF FIBER REINFORCED SUBGRADE SOIL
The purpose of this investigation was to study the variation in density, OMC, CBR and modulus of elasticity (E-value) of fine sand and clay due to randomly distributed fibers in different concentration and aspect ratio, and thereby to determine the optimum value of aspect ratio and fiber content. The effect of polypropylene fiber reinforcement on behavior of these soils under static and repeated triaxial conditions is also studied. The test results indicate that CBR value of reinforced sand and clay is 2.9 and 3.73 times its unreinforced value. The cohesion and angle of internal friction for both soils increase due to reinforcement. At a deviator stress of 194 kPa, and confining pressure 70 kPa, the resilient modulus of fine sand and clay at 100 load cycles were obtained as 125.16 MPa and 82.55 MPa, respectively. It increased to 179.00 MPa and 153.72 MPa respectively, due to reinforcement. The increases in resilient modulus of subgrade soils due to reinforcement with short fibers indicate that thickness of other layers in flexible pavement may be reduced.
Construction of a road involves substantial investment and therefore proper planning, construction and maintenance of these national assets is of paramount importance. The soil reinforcement is an effective and reliable technique for improving the strength and stability of the soil. In conventional method of reinforced soil construction, the inclusion of strip, fabric, bar, geogrid and geotextile are used and normally oriented in a predefined direction and are introduced sequentially in alternate layers. The discrete fibers on the other hand are simply added and mixed randomly with soil, much the same way as cement, lime or other additives. One of the main advantages of fiber reinforced soil is the maintenance of strength isotropy and absence of potential plane of weakness that can develop parallel to oriented reinforcement. Fiber reinforced soil exhibits greater extensibility and small losses of peak strength or greater ductility in composite material as compared to unreinforced soil or soil reinforced with high modulus of inclusive. Fiber reinforced soil can be used in embankments, subgrade, subbase, and other such problems.
2 LITERATURE REVIEW
Al-wahab and Al-Ourana (1995) studied effect of fiber reinforcement on properties of silty clay and found that fibers significantly increase the peak and post peak strengths, ductility, toughness and energy absorption capacity of soil. Maher and Gray (1990) observed that increase in fiber aspect ratio resulted higher contribution to increased shear strength in sand. Maher and Ho (1994) further extended their work on Kaolinite soil reinforced with polypropylene fiber at different water contents and reported that for a lower water content, reinforced soil behaves rigid and brittle whereas at higher content it behaves like a ductile material. Santoni and Webster (2001) studied the permanent deformation characteristics of fiber reinforced sand, and found that the higher fiber content of 1.5-2.4 % showed slow initial strength gain at lower deformation with rapid strength gains at higher deformation. Setty and Murthy (1990) observed reduction in the angle of internal friction and increase in value of the cohesion of black cotton soil due to reinforcement with fibers. Ranjan et al. (1999) found that soft clay sample and clay sample with granular core attained a peak value at about 10 % axial strain which then remained practically constant even up to 20 % axial strain. Consoli et al. (2002) carried out triaxial tests on mixture of cement (cement content 0 to 7 %) and sand reinforced with plastic waste fiber and studied the deformation characteristics of sand-cement-fiber composite. The greatest improvement in triaxial strength, ductility and energy absorption capacity was observed at 36 mm fiber length. Consoli et al. (1998) studied triaxial behavior of a thick homogeneous stratum of compacted sandy soil, reinforced with polypropylene fibers, as well as of some soil without reinforcement. One of the advantages of polypropylene fiber reinforcement was the strain hardening behavior induced even at large deformations. Consoli et al (2003) carried out plate load tests on a homogeneous residual soil stratum, as well as on a layered system formed by two different top layers: sand-cement and sand-cement-fiber overlaying the residual soil stratum for observing the load settlement and failure mechanism.
The soil reinforced with short fibers has been tried for many years and there is a considerable improvement in failure mechanism of soil fiber-matrix. Most of the studies have been conducted from geotechnical consideration with a view to use soil-fiber mix in foundations. The application of fiber in reinforcing the subgrade soil for pavement construction is still in primitive stage. The present study was undertaken to observe the effect of polypropylene fiber on properties of soil that are important in design and analysis of a flexible pavement.
3 EXPERIMENTAL PROGRAM
Two types of soils and one type of fiber were taken for present investigation. One soil was fine sand and another was cohesive soil. These soils are referred to Soil A and Soil B in subsequent discussion. The results of
physical tests on soils are presented in Table 1. The synthetic fiber used was polypropylene fiber. It is resistant to seawater, acid, alkalis, and chemicals. It has good breaking strength and abrasion resistance, as it is less, prone to wear and tear (Shetty and Rao, 1987). The fiber had a diameter of 0.3 mm cut into the required length of 15 mm, 25 mm, and 30 mm) giving an aspect ratio of 50, 84, and 100 respectively.
3.2 Laboratory Tests
This section describes in detail the plan of experimental work carried out to study the behavior of randomly distributed polypropylene fiber reinforced soil (RDPFS).
The influence of aspect ratio and fiber concentration on CBR value, E- value, c-Æ parameters and resilient modulus (Mr) of RDPFS were evaluated through California bearing ratio test, unconfined compression strength test, static triaxial tests and repeated triaxial tests. The fiber-reinforced specimens were prepared by hand mixing the dry soil, water and polypropylene fiber. The percentage of fiber used in samples was 0.75, 1.5, 2.25, and 3 percent by dry weight of the soil. The water was added prior to fiber to prevent floating problems. Fiber reinforced soil samples were prepared at the maximum dry density and the optimum moisture content obtained from standard Proctor tests on reinforced soil.
California bearing ratio (CBR) test is a penetration test developed by California State Highway Department for evaluating the strength of sub grade soil and base course material for flexible pavement. This test is conducted under controlled density and moisture conditions. The tests were conducted under soaked conditions. The preparation of samples and testing procedure was as per procedure laid down in IS-2720-XV1-1987.
Unconfined compression strength (UCS) tests were conducted on cylindrical specimen of 100 mm diameter and 200 mm height to determine the E-value. The moist soil- fiber mixture was transferred to the split mould in three equal layers and each layer was compacted by static compaction until uniform density was achieved. The optimum quantity of fiber and aspect ratio were determined based on CBR and E-value of reinforced soil.
Unconsolidated undrained triaxial tests were conducted on unreinforced and reinforced soil samples at confining pressures of 40 kPa, 70 kPa, 120 kPa at the optimum fiber content and aspect ratio, as per AASHTO T 180-90D and AASHTO T 226-90. For repeated triaxial tests, conventional cell was used. The loading device for applying a cyclic load is a fatigue-testing machine. The machine applies cyclic load through a loading lever and coil type spring load mechanism oscillating in a vertical plane and connected to an eccentricity. This can be adjusted to produce a required magnitude of the load. The load application frequency of fatigue testing machine is 70 cycles per minute. To measure the vertical or axial deformation in the specimen. Linear Variable Differential Transducer (LVDT) was used. The repeated compressive deviator stresses were applied at three different confining pressures. The test was conducted up to 10,000 cycles of load applications and permanent strain and resilient strain were measured after 1, 10, 100, 1000 and 10,000 cycles.
4 RESULTS AND DISCUSSION
4.1 Proctor’s Compaction Test
The OMC and dry density test results on fine sand (soil A) and cohesive soil (soil B) with different aspect ratio and fiber content are reported in Table 2. These data
indicate that the maximum dry density decreases gradually with increase in the fiber content, which is due to lower density of the fiber than the soil particles. The change in OMC was quite marginal for both types of soils.
4.2 CBR Test
The CBR tests were conducted on unreinforced soil and soil reinforced with fiber. The tests were carried out by static compaction after four days soaking in water. CBR values at different aspect ratio and varying fiber content are given in Table 3. The maximum limit of fiber content was kept 3.0 percent as mixing was difficult beyond this. Earlier studies have also indicated that increase in strength properties of a soil is very marginal and some times decreases after 3 percent fiber content. The maximum CBR value in the present study was obtained for an aspect ratio of 100 and 3 percent fiber content in case of soil A and aspect ratio of 100 and 2.25 percent fiber content in case of soil B. However, the mixing of fiber in soil beyond 1.5 percent was very difficult. Therefore, 1.5 percent fibers with aspect ratio
84 is considered as the optimum for soil A. Similarly, 1.5 percent fibers with aspect ratio 100 is considered as the optimum value for soil B. The CBR of soil A and soil B at their optimum fiber content increase by 190.80 percent and 273-27 percent, respectively.
4.3 Unconfined Compression Strength
The results obtained from UCS test are reported in Table 4. This test was performed to determine the modulus of elasticity (E-value) of two soils. The maximum E value of soil A is obtained at the aspect ratio of 50 and fiber content 2.25 percent. Similarly, the maximum E value for soil B is obtained at an aspect ratio of 50 and fiber content 2.25 percent. The maximum values of E for soil A and B are 12250 kPa and 6500 kPa respectively. It was observed that initial stiffness of reinforced soil decreases whereas failure stress and corresponding strain increase with increase in the percentage of fiber. The loss in initial stiffness may be due to the change in fabric of subgrade soil produced by fiber. The fibers produce nonuniform voids distribution preventing the dense packing. It results in lower value of stiffness at initial portion of the loading. Hence initial correction of 0.5 percent strain was applied while calculating the E-value of reinforced soil.
By studying the variation in CBR value and E value of subgrade soil with aspect ratio and fiber content, the optimum aspect ratio and fiber concentration were determined. The aspect ratio of 84 and 100 with 1.5 percent fiber were considered optimum for Soil A and Soil B, respectively, as mixing and compaction were extremely difficult beyond these limits.
4.4 Static Triaxial Test
Static Triaxial tests were conducted to know the change in strength characteristics of subgrade soil at optimum aspect ratio and fiber content. This test was conducted on both unreinforced and reinforced soils at confining pressures of 40 kPa, 70 kPa, and 120 kPa to study the stress strain behavior, modulus of elasticity, and shear strength parameters of subgrade soil. Figure 1 and 2 show typical stress-strain curves for soil A and soil B at confining pressure of 40 kPa. Similar curves were drawn for other test conditions also and the results are presented in Table 5.
The shear strength parameters c-s-a‾ based on static triaxial tests on reinforced and unreinforced soils are presented in Table 6. It is observed that for soil A, cohesion increased from 25 kPa to 80 kPa and angle of shear
resistance increased from 26.55 to 37° due to reinforcement. Similarly, for soil B cohesion increased from 37.5 kPa to 62 kPa and angle of shear resistance increased from 21.5 to 25.5° due to reinforcement. Increase in cohesion for both soils may be due to apparent bond developed between soil particles and the fiber.
The behavior of soil over wide range of stress is nonlinear, inelastic and depending on magnitude of confining pressure. Jambu (1963) reported that the initial tangent modulus for any soil can be conveniently expressed in terms of confining pressure.
4.5 Cyclic Triaxial Test
The cyclic triaxial tests were conducted on unreinforced and reinforced soil samples at optimum quantity of fiber and aspect ratio. The recoverable and permanent strains were measured and resilient modulus calculated.
4.5.1 Recoverable or Resilient Strain
The cyclic triaxial tests were conducted at different deviator stress of 124 kPa and 194 kPa and two confining pressures of 40 kPa and 70 kPa on both soils. It was observed that resilient strain increases with deviator stress and with number of load cycles in all the samples. The resilient strain at a given deviator stress is higher for unreinforced sample than that for reinforced sample. The variation of resilient strain in unreinforced and reinforced sample of soils at 124 kPa and confining pressure of 70 kPa for different load cycles are shown in Figure 4. The values of resilient strain at 124 kPa and 194 kPa with confining pressure 70 kPa are presented in Table 8.
4.5.2 Permanent Strain
The permanent strain in reinforced and unreinforced samples of soils at different cycles is given in Table 9 Permanent strain increases with the number of load cycles. It is observed that the reinforced soil has lower permanent strain than unreinforced soil for the same number of cycles and deviator stress. The permanent strain
These are much more than the values obtained for unreinforced soils.
1. AASHTO T 180-90D, AASHTO T 226-90, Guide for design of pavement structures. Washington, D.C.