# Advanced Scanning Technology for Volume Change Measurement of Residual Soil
## 1 Introduction  
Singapore has a tropical climate accompanied by heavy rainfall, high temperature, and high humidity during the year due to its location close to the equator [1]. Depending on the mineralogy of the parent rock, the degree of the weathering, the amount of rainfall, and the temperature, such environments may produce distinct engineering and index properties [2]. Therefore, residual soil is formed in thick layers and typically placed in four major geological formations: Bukit Timah Granite, Kallang Formation, sedimentary Juong Formation, and Old Alluvium Formation in Singapore [3]. Such soils shrink both horizontally and vertically as a result of moisture evaporation. Residual soils, a kind of problematic soils, are prone to shrink by a significant amount in periods of intense drying following heavy wet seasons [4]. The porous soil structure that was brought about by the leaching of specific types of soil minerals, during which water and air replaced the soluble minerals, along with the geochemical transformations associated with weathering, such as the formation of new minerals like clay, leading to a very high void ratio, are affected by a deep groundwater table on many residual soil slopes [5]. The zone between the groundwater table and the soil surface is called an unsaturated zone, which is very sensitive to variations in climatic conditions [3]. One of the key properties of unsaturated soil is the soil-water characteristiccurve (SWCC), which essentially illustrates the relationship between soil suction and water content [6]. SWCC can be presented in the form of gravimetric water content, volumetric water content, and degree of saturation. These three forms of SWCC are differentiated based on the dependency on soil volume change [7]. Since the shrink-swell phenomenon in the soil only affects a set depth known as the active zone, measuring volumetric water content and degree of saturation requires a shrinkage curve [4]. As water content in soil decreases from full saturation to a dry state, the shrinkage curve describes the relationship between void ratio and water content [8; 9]. This relationship is critical because as unsaturated soils lose water, their volume reduces due to the shrinkage of the soil matrix. Neglecting this volume change can lead to inaccuracies in measuring the volumetric water content and degree of saturation [10]. Many researchers concluded that the SWCC is associated with the degree of saturation, which is very important in the determination of air-entry value and the permeability function of soil [11]. In the geotechnical design, with respect to the effect of rainfall, the SWCC corresponding to volumetric water content is one of the main parameters in running transient seepage finite element analyses [12; 13; 14]. In numerical analyses, there is a lot of demand for SWCCs with respect to volumetric water content and degree of saturation, indicating the significance of the shrinkage curve in unsaturated soil analysis [15; 16; 17]. Hence, there is a need to have an appropriate measurement of soil volume change for the development of a shrinkage curve. Past research has faced plenty of challenges in obtaining accurate measurements of volume change during SWCC tests [18; 19]. The challenges in determining the shrinkage curve were mostly due to the presence of cracks and non-homogeneity in the reduction in soil dimensions during the drying process. Other challenges were linked with the methodologies and technologies utilized to measure soil volume change. Manual measurement using Vernier calipers is the most common method for determining the shrinkage curve. It is easy, cheap, and non-destructive; however, it is limited to homogeneous specimens [20; 21; 22]. This method does not produce high-quality results for an irregular shape of a specimen. The volume displacement method could produce a better measurement of soil volume. This method requires the specimen to be wrapped with wax or equivalent materials prior to submergence into water [23; 24].