Paper ID: GE-7 International Conference on Recent Innovation in Civil Engineering for Sustainable Development (IICSD-2) Department of Civil Engineering DUET - Gazipur, Bangladesh 48 Shear Strength Characteristics of Reinforced Clay Soil M. R. Islam 1*, M.A. Hossen 2, M. A.Alam 2, and M. K. Hossian 3 Abstract Geotechnical engineers often encounter problems in designing foundations of structures on highly compressible clayey soil due to its poor bearing capacity, low shearing strength, etc. Soil reinforcement is an effective and reliable technique for improving strength and stability of soils. Several studies have been conducted to investigate the influence of randomly oriented discrete inclusions (fibers, mesh elements, waste material e.g. plastic strips, tire chips, etc.) on the highly compressible clayey soils. Most of these studies were conducted on a single type of reinforcing element. In the present study, different fibers were used to strengthening the soft clay soil. In this study three different types of fibers (Nylon Polymer, Jute and Coir) are used to determine the optimum fiber content for maximum shear strength and the result is compared with unreinforced soil and with each other. In this study amount of 12 mm Nylon, Jute, Coir fibers mixed with highly compressible clayey soil varies from to 1 %. The results of unconfined compression test reinforced with various amount of fibers and for different types of fibers were recorded [2]. The optimum fiber content for maximum shearing strength is also calculated. The soil used for this study was CL (Clay with low plasticity) type according to Unified Soil Classification. The optimum fiber content for Nylon Polymer, Jute and Coir is.78%,.77% and.8% corresponding to the increment of shear strength about 23%, 33% and % respectively. The results indicated that reinforcement of highly compressible clayey soil with randomly distributed fibers caused an increase in the shearing strength of soil hence increase the bearing capacity of foundation. Keywords: Soil reinforcement, shear strength, fiber content, bearing capacity, soft clay. 1. Introduction The improvement in strength properties of soil has become one of the important tasks of geotechnical engineers due to scarcity of good sites, dramatic rise in land prices and increase in infrastructure growth. There are different improvement techniques to stabilize the poor ground in which soil reinforcement is an effective and reliable technique. The concept of soil reinforcement was first developed by Vidal (1969) [1]. He demonstrated that the introduction of reinforcement elements in a soil mass increases the shear resistance of the soil matrix. In the case of geotechnical engineering the idea of inserting fibrous materials in a soil mass in order to improve its mechanical behavior has become very popular. The concept of earth reinforcement is an ancient technique and demonstrated abundantly in nature by animals, birds and the action of tree roots. This reinforcement resists tensile stress developed within the soil mass thereby restricting shear failure. Reinforcement interacts with the soil through friction and adhesion. The practicing engineers are employing this technique for stabilization of thin soil layers, repairing failed slopes, soil strengthen around the footings and earth retaining structures. The inclusion of randomly distributed discrete fiber increases strength parameters of the soil as in case of reinforced concrete construction. Foundation is a part of structure which transmits load of super structure to the sub soil. Geotechnical engineers face various problems while 1* Post Graduate Student, BUET, ranaduet@gmail.com 2 Former student of DUET, akbar9duet@hotmail.com 2 Former student of DUET, ashraful18duet@gmail.com 3 Professor, Dept. of Civil Engineering, DUET, drkamalhossain@yahoo.co.in IICSD-2
49 designing the foundations on highly compressible clayey soil due to poor bearing capacity and excessive settlement. Thus bearing capacity is one of the important aspects of soil engineering. Now, stability of any structure depends on the properties of soil. Using land having soft soil for construction leads to various ground improvement techniques such as soil stabilization and reinforcement. Most of the soil available is such that they have good compressive strength adequate shear strength but weak in tension/poor tensile strength [3]. To overcome the same, many researchers have concentrated their studies on the development of new such materials, through the elaboration of composites. The investigations indicate that strength properties of fiber reinforced soils are the function of fiber content, fiber surface friction along with the soil mass and fiber strength characterizes. Increasing the bearing capacity of the soil and the stability of soil in slopes are only two applications of reinforcing the soil with fibers. The main effect of this reinforcement is the increase of shear strength of the soil. Previous fiber soil reinforcement studies indicate that the fibers significantly increase the shear strength of different types of soils in optimum conditions (Wayne, 1988). According to Tezarghi s and Vesic s soil bearing capacity studies, the bearing capacity of the soil has a direct relationship with the shear strength of the soil. In foundation engineering the bearing capacity of the soil is defined as the maximum homogeneously distributed pressure in direct contact, a soil can withstand before suffering shear failure. Therefore an increase in the bearing capacity of the soil would allow the soil to hold a larger load in the same area, which can prove useful in many aspects of housing development. Some of these aspects are the ability to build larger structures, reduce the size of footings, and easily stabilize soil for roads []. 2. Properties of Materials Used for Investigation The Engineering properties of clay soil are given in Table 2.1. The natural moisture content was 23.98 %. The specific gravity of the soil was 2.46 which are out of range 2.6 to 2.6 which indicate that the soil has certain amount of clay particle. Table 2.1 Engineering properties of clay soil used No Parameters Symbol Value 1 Specific gravity G s 2.46 2 Natural moisture content w 23.98% 3 Grain Size Analysis a) Gravel, % d>2. mm % b) Sand, %.7 mm<d<2. mm 12% c)clay, %. mm<d<.7 mm 43% d) Silt, % d<. mm 4% 4 Consistency Limit a) Liquid limit LL 48.7% b) Plastic limit PL 27.% c) Plasticity index PI 21.7% USC Classification CL 6 Compaction Study (Standard Compaction Test) a) Maximum Dry Density, gm/cc ϒ d(max) 1.48 gm/cc b) Optimum Moisture Content, % W OMC 17.9% Compaction characteristics of soils depend on the test procedure such as compacting methods, maximum diameter of the soil, compacting energy and repeated and unrepeated use of sample etc. Soft clay soil are commonly in moist conditions are quite influence by test procedure. However we followed the ordinary compaction test procedure.[7]the results from the test are shown in Figure 2.1. Optimum moisture content and maximum dry density were found to be 17.8% and 1.48 gm/cc, respectively. IICSD-2
UC Stress, q (KN/m2) UC Stress, q (KN/m2) Maximum Dry Density, gm/cc 41 1. 1.4 1.4 1.3..1..2.2.3 Moisture Content, w% Fig. 2.1: Dry density vs. moisture content 3. Unconfined Compression Test Result of Remold Natural and Reinforced Clay Soil Figure 3.1, 3.2 and 3.3 represent the unconfined compressive test result of remolded natural and fiber reinforced clay soil reinforced with different percentage of fiber. We used three types of fiber like polymer (Nylon), Jute, Coir and prepared four set of fiber reinforced sample for each fiber in different percentage of fiber. 18 16 14 12 1 8 6 4 2..1..2.2.3.3 Strain, ϵ Clay+. % Clay+.7 % Clay+1. % Clay+.2 % Unreinforced Clay 2 2 1 Fig 3.1: Stress strain curve for unreinforced and Polymer fiber reinforced soil. Clay+. % Clay+.7 % Clay+1. % Clay+.2 % Unreinforced Clay..1..2.2.3.3 Strain, ϵ IICSD-2
Maximum Stress, qu kn/m2 Maximum Stress, qu KN/m2 UC Stress, q (KN/m2) 411 Fig 3.2: Stress strain curve for unreinforced and Jute fiber reinforced soil. 3 3 2 2 1..1..2.2.3 Strain, ϵ Clay+. % Clay+.7 % Clay+1. % Clay+.2 % Unreinforced Clay Fig 3.3: Stress strain curve for unreinforced and Coir fiber reinforced soil. 4. Optimum Content Figure 4.1, 4.2 and 4.3 represent maximum unconfined compressive test result of fiber reinforced clay soil reinforced with different percentage of fiber. Initially the UC strength increases with the increment of fiber content up to.7% then the UC strength decreases with the increment of fiber content. Therefore we found the optimum fiber content for maximum UC strength. 2 1.2..7 1 1.2 % of Fig 4.1: Optimum fiber content for Polymer fiber 2 2 1.2..7 1 1.2 % of Fig 4.2: Optimum fiber content for Jute fiber IICSD-2
Maximum Stress, qu KN/m2 412 4 3 2 1.2..7 1 1.2 % of Fig4.3: Optimum fiber content for Coir 4.1 Comparison of unconfined compressive strength between different fibers reinforced soil The table 4.1.1 represents the maximum unconfined compressive strength for each fiber reinforced soil reinforced with different percentage of fiber. The coir fiber has maximum strength therefore it shows the maximum strength. Table 4.1.1: Maximum Unconfined Compressive Strength of Different Reinforced Soil Natural Soil.4 % of Polymer Jute Coir.2 6.69 7.76 1.21. 8.61 1.28 14.27.7 16.9 2.3 31. 1. 12.9 16.18 27.. Conclusion This study was concern with the investigation of unconfined compressive strength properties of different fiber reinforced soil. In this study 12 mm long Nylon, Jute, Coir fibers is mixed with highly compressible clayey soil varies from to 1 %. At first the strength was increased with increase in fiber content but after.7 % fiber content, the strength tends to decrease. Therefore the optimum fiber content for maximum strength was determined. The optimum fiber content for Polymer, Jute and Coir fiber reinforced soil was.78%,.77% and.8% respectively. The unconfined compressive strength for Coir fiber shows the higher strength, Jute fiber shows medium and Polymer (Nylon) fiber shows lower strength comparin among the three types of fiber. 6. References [1] Masheshwari,K.V, Desai, A.K. and Solanki, C.H. (21),Model Footing Test on Reinforced Soil, Indian Geotechnical Conference-21, GEOtrendz (December 16-18, 21) IGS Mumbai Chapter &IITBombay. [2] Masheshwari,K.V, Desai, A.K. and Solanki, C.H. (211), Performance of Reinforced Clayey Soil, EJGE,Vol. 16[211], Bund. J [3] Parag, M.Chaple and Dhatrak, A I. Performance of Coir Reinforced Clayey Soil(213), The International Journal of Engineering and Science(IJES), Volume 2 Issue 4 Pages 4-64 213 ISSN(e): 2319-1813 ISSN(p): 2319-18. IICSD-2
413 [4] Raihan T, Syed A. &Hossain Kamal. (1999). Behavior of Geo-reinforced Residual Soil In Triaxial Test, World Engineering Congress 99- Towards the Engineering Vision: Global Challenging Issue (19 th -22 th July 1999, Kuala Lumpur). [] Praveenkumar, G.Swami S. and Ravikant M. (28).Behavior of fiber reinforced sand in different test conditions. Indian Geotechnical Journal, 272-282. [6] Andersland, O. B. and Khattak, A. S. (1979). Shear strength of kaolinite/fiber soil mixtures. Proc. International Conference on Soil Reinforcement, Paris, France. Vol 1,11-76. [7] Das,B.M. principles of geotechnical engineering, PWS Kent publishing company. JOSEPH E. BOWLES. (1978), Engineering properties of soil and their Measurement, McGraw-Hill Book co. New York. IICSD-2