Asian Journal of Applied Sciences (ISSN: 2321 – 0893) Volume 04 – Issue 06, December 2016 Asian Online Journals (www.ajouronline.com ) 1319 A Review of Clayey Soils Ali Akbar Firoozi 1,* , Ali Asghar Firoozi 2 and Mojtaba Shojaei Baghini 3 1 Department of Civil & Structural Engineering, Universiti Kebangsaan Malaysia (UKM) Selangor, Bangi, Malaysia 2 Department of Civil & Structural Engineering, Universiti Kebangsaan Malaysia (UKM) Selangor, Bangi, Malaysia 3 Department of Civil & Structural Engineering, Universiti Kebangsaan Malaysia (UKM) Selangor, Bangi, Malaysia * Corresponding author’s email: firoozi.aa [AT] gmail.com _________________________________________________________________________________ ABSTRACT— Clays in general and expansive soils in particular have been a major concern to geotechnical engineers for many years. Moisture variations produce big volume changes in these types of soils. Several factors like amount and type of clay minerals, soil structure, dry density, confining pressure, moisture content and climate changes influence the amount of swell and shrinkage. These volume changes finally result in serious damage to the various structures including pavements. In addition, clayey soils are usually stiff when they are dry and give up their stiffness as they become saturated. Soft clays (i.e. illite/kaolinite/ montmorillonite) are associated with low compressive strength and excessive settlement. This reduction in strength due to moisture leads to severe damages to buildings and foundations. Keywords— Clay structure; Cation Exchange Capacity; Atterberg limits _________________________________________________________________________________ 1. INTRODUCTION Naturally occurring expansive soils have been found in various regions across the globe [1]. Soils with higher percentage of clay minerals like montmorillonite, expandable illite and vermiculite, are susceptible to swelling and shrinkage. They cause numerous costly damages to the roadways, buildings, bridges and other civil engineering infrastructures. Furthermore, clay soils are generally stiff in dry state but when become saturated, they lose their stiffness. Soft clays are characterized by low compressive strength and excessive compressibility. The reduction in bearing capacity of soft clays results in compressive failure and excessive settlement, leading to severe damage to buildings and foundations [2-4]. Maintenance and rehabilitation costs for the infrastructure on these soils reach billions of dollars annually. These problems primarily stem from the presence of montmorillonite clay minerals which are derived from basic and ultrabasic igneous rocks; essentially the minerals area by product of the decomposition of these rocks [5, 6]. These minerals swell when moisture is introduced and shrink when the same moisture is retracted. In the case where the soil undergoes excessive heat, i.e. drought, expansive soils tend to contract and shrink excessively [7-12]. Al-Rawas and Goosen [10] noted that clay minerals and cations come in various forms and that it is the relative quantities of each type of these minerals that are important factors contributing to the swell/shrink behavior along with the dry density, soil structure, and loading conditions present. Other researchers added that the arid climate, alkaline environment, and local geology are accountable for the expansive nature of soils [11, 12]. The main objective of this paper is a review on properties clay soils which have more effects on geotechnical properties. 2. CLAY STRUCTURE Clays are generally composed of micro-crystalline particles of a group of minerals. Since clay science has been the interest of people from different backgrounds, a specific definition of this material is not available [12]. Generally, clays are naturally occurring material primarily composed of fine-grained minerals, show plasticity when mixed with appropriate amount of moisture and become hard when dried or fired [13, 14]. Das [14] indicated the characteristics of clay, which included: a) Small particle size (usually smaller than 0.002 mm) b) Net negative charge c) Show plasticity when mixed with moisture