Shear Strength Behaviour of Indian Fly Ashes 18 SHEAR STRENGTH BEHAVIOUR OF INDIAN FLY ASHES Sujit Kumar Pal Lecturer (Selection Grade), Civil Engineering Department, National Institute of Technology, Agartala–799 001, India. E-mail: skrpal@yahoo.co.in Ambarish Ghosh Professor, Civil Engineering Department, Bengal Engineering & Science University, Shibpur–700 103, India. E-mail: ghoshambarish@civil.becs.ac.in ABSTRACT: The potential fields of application of fly ash in large volumes are mainly construction of roads, embankments, and filling of low lands. Shear strength of fly ash is one of the important parameters required for the design of such projects. To study the variation of shear strength parameters of fly ash with varying composition, unconsolidated undrained triaxial tests have been conducted on nine fly ash samples collected from different thermal power plants of the Eastern part of India. This paper presents shear strength parameters of all the nine fly ash samples, compacted at optimum moisture content and maximum dry density obtained from standard Proctor (AASHO) compaction tests. All the fly ash specimens were tested under three different confining pressures of 100, 200 and 300 kPa. Effects of confining pressure, MDD and OMC on shear strength parameters of fly ash samples are discussed herein. The values of the shear strength parameters i.e., angle of internal friction, φuu and apparent cohesion, c uu of all the fly ash samples vary within the range of 29.91°–36.93° and 14.31–59.59 kPa respectively. As confining pressure (s 3 ) increases the shear strength (S) of fly ash increases, irrespective of type of fly ash. Effect of confining pressure (s 3 ) on deviator stress at failure (q ƒ ) is more prominent for the fly ash samples of low strength than for fly ash samples of high strength. The angle of internal friction of the strength of fly ash samples is more significant than apparent cohesion, irrespective of type of fly ash. The friction angle φ uu of the fly ash was found to be a function of the type of fly ash. The post- peak behaviour of all the fly ashes indicated brittle rupture in which axial strain values of almost all the fly ash samples are less than 15%. 1. INTRODUCTION A basic knowledge of shear strength characteristics of the media is necessary to solve any geotechnical engineering problem. The maximum shearing resistance which the materials are capable of developing is called the shearing strength. Depending on the field problem the total strength parameters or the effective strength parameters are to be used. Triaxial testing is very much important since different field conditions can be simulated in this test. Shear strength of fly ash is one of the important para- meters required for the design of engineering structures where fly ash is used as construction material. Unconsolidated un- drained triaxial shear strength test takes less time and the total strength parameters obtained from this test may be used for the corresponding problem without much error. Previous re- searchers studied unconsolidated undrained triaxial shear strength characteristics of fly ash (Raymond 1961; DiGioia & Nuzzo 1972; Gray & Lin 1972; and Kaniraj & Gayathri 2003, 2004). Porbaha et al. (2000) and Pandian et al. (2001) have studied consolidated direct shear tests of fly ash and shear strength characteristics of coal ash respectively. Mixtures of fly ash have been also tested for shear strength (Kim et al. 2005). Unconsolidated undrained triaxial tests have been conducted to study the shear strength characteristics of fly ash stabilized with additives (Ghosh & Subbarao 2007) and fly ash-soil mixtures (Kaniraj & Havanagi 2001). From the previous studies of literature on shear strength, it is revealed that there is paucity of data on strength characteristics of varieties of fly ash which may find potential application as embankment material, subgrade material and fill material. To study the suitability of fly ash as materials for embankment, subgrade and land filling nine fly ash samples collected from different thermal power plants have been used in the present study. This study presents the unconsolidated undrained triaxial shear strength characteristics of the fly ash samples. Effects of confining pressure and MDD on shear strength parameters of fly ash samples are discussed herein. 2. MATERIALS Nine numbers of class F fly ash samples have been used in this investigation to study the suitability of using these materials for construction of embankment, subgrade and land filling. The test results on physical properties (Table 1) and chemical composition (Table 2) of all the fly ash samples discussed are presented in this paper. The nine fly ash samples used in this study are designated as follows: KF: Kolaghat fly ash, from Kolaghat thermal power plant KP-1: Kolaghat pond ash, from Ash pond 1 of Kolaghat thermal power plant KP-2: Kolaghat pond ash, from Ash pond 2 of Kolaghat thermal power plant IGC 2009, Guntur, INDIA