2397 FACTORS AFFECTING THE SWELLING PRESSURE MEASURED BY THE OEDEMETER METHOD C.P.G. Jayalath 1 , C. Gallage 2 and N.S. Miguntanna 3 1,3 Faculty of Engineering, University of Ruhuna, Sri Lanka; 2 Science and Engineering Faculty, Queensland University of Technology, Australia ABSTRACT: Expansive soils are common in arid and semi-arid climate regions of the world and cause severe problems on civil engineering structures. The Swelling potential of the expansive soil mainly depends upon the properties of soil and environmental factors, and stress conditions. Swelling pressure is a key parameter used in designing structures in and on expansive soil. The swelling pressure of soil is measured in the laboratory using a representative soil samples. The size and the surface friction of the sample ring used in the swelling pressure test have effects on the measured swelling pressure and they have not properly been investigated. In this study, a series of constant volume swelling tests were conducted using an automated consolidation-swell apparatus to evaluate the effect of sample ring size, ring friction, initial dry density, and initial moisture content (IMC). Test results indicate an exponential growing trend of swelling pressure when the dry density is increased. Similarly, high swell pressures are achieved when the IMC is increased for the same dry density. A higher swelling pressure was measured when the friction of the specimen ring was reduced. The measured swelling pressure increases with increasing the height of the sampling ring and it decreases when the ring diameter is increased. Therefore, it is recommended to use a standard sample ring reducing inside wall friction using lubricants when measuring the swelling pressure in the laboratory. Further, the sample ring size, initial density and initial moisture content of soil should be given when reporting swelling pressure of soil. Keywords: Expansive Soil, Swelling Pressure, Unsaturated Soil, Consolidation/Swell Apparatus, 1. INTRODUCTION Expansive soils which are common in arid and semi-arid climate regions of the world contain clay minerals such as montmorillonite, smectite. These clay minerals absorb water and expand eventually and shrink when soil is dried out. A cyclic wetting and drying triggers excessive volume changes in expansive soils [1] causing significant distresses in lightweight structures such as pavements, residential slabs, driveways, sidewalks, shallow depth pipelines. The losses due to widespread damage to structures constructed on expansive soils are estimated to be in billions of dollars all over the world. In the United States alone, annual losses due to expansive soils are estimated to be approximately $1000 million [2]. Similarly, it has been estimated that the damage due to expansive soils is greater than twice as the combined damage from floods, tornadoes, hurricanes, and earthquakes [1]. Swelling pressure which is defined as the pressure required holding the soil at constant volume when water is added can simultaneously cause lifting, or heaving of structures whereas differential settlement can be caused by shrinkage [3]. Failures result when the volume changes are irregularly distributed underneath the foundation. For example, swelling pressure beneath the perimeter of the building can be caused by changes of the water content in the soil around the edge of a building while remaining the water content of the soil constant beneath the center. This resulting failure is known as end lift. The center lift is the opposite scenario where shrinkage is focused under the edges or where swelling takes place beneath the center of the structure [4]. Therefore, it is essential to estimate/measure the swelling pressure of expansive soil when designing structures to be constructed on or in expansive soils [5]. The swelling pressure is evaluated in the laboratory by a number of testing methods [6]. An oedometer testing method is extensively used to determine the swelling pressure due to its simplicity and operational ease. There are three different oedometer methods for the determination of swelling pressure [7]. They are: Swell- Consolidation Method, Different Pressure Method, and Constant Volume Method. Each of the methods is equally sensible, but provides different values of swelling pressure for the similar placement conditions of the soil [8]. As a practice, different sizes of specimens are used in testing for swelling pressure of expansive soils. According to ASTM D4546 [9], the minimum specimen diameter or inside diameter of the specimen ring used in determination of one- International Journal of GEOMATE, Aug., 2016, Vol. 11, Issue 24, pp. 2397-2402 Geotec., Const. Mat. & Env., ISSN: 2186-2982(Print), 2186-2990(Online), Japan