# Advanced Scanning Technology for Volume Change Measurement of Residual Soil
## 2 Residual Soil and Geological Formation  
Residual soils are developed because in situ physical, chemical, and biological weathering affect parent rock formation [39]. Physical weathering occurs when the earth or surrounding environment moves, leading to the rock formation breaking. Chemical weathering occurs when chemicals dissolved in rainwater react with the minerals found in rocks. Furthermore, the underground rock formation is influenced by the strains and tensions generated by the plant roots that grow on it. Thus, biological weathering can lead rock formations to break [19; 40]. Weathering processes cause residual soil layers to accumulate on the parent rock formation over time. This is especially true in tropical climate countries with year-round high temperatures, high humidity, and excessive rains [3]. As shown in Figure 1, Singapore has four major geological formations with thick residual soil layers: Bukit Timah Granite, Kallang Formation, sedimentary Jurong Formation, and Old Alluvium Formation. Bukit Timah Granite is mostly found in the country's northern and central regions. The majority of the rocks in Bukit Timah Granite are granite and granodiorite [7]. Reddish to yellowish brown silty soils occur in the top layers of Bukit Timah Granite.  
Old Alluvium Formations is the oldest among the drift deposits. Old Alluvium is an extension of southern Johore deposits, and they form a continued plate in east Singapore [41]. The area of the Old Alluvium is around 12 km\({}^{2}\) and is located in the northwestern part of Singapore, as displayed in Figure 1. However, the formation is situated mostly in the island's eastern part and is formed as a near-successive plate at the top and bottom of younger deposits [41]. The soil of the Old Alluvium Formation is mainly reddish muddy dense sand with clay and silt [40]. The soil's density varies from loose sand closer to the surface to stiff clay at larger depths.  
Residual soils can be divided into six categories (Grade I-VI) depending on the weathering extent of geological formation [42]. Grade I is the fresh rock (parent rock formation), whereas Grade VI is entirely weathered soil (residual soil). The grade rises with the increasing degree of weathering and atmospheric exposure. Higher-grading residual soils mainly have an increased ratio of fine particles as clay and decreased density due to continuous weathering processes [3]. Consequently, the shear strength of higher-grade residual soils has been reduced compared to that of their parent rock. Due to their larger pores, lower-grade residual soils have a higher potential to leach, whereas higher-grade residual soils have a higher capacity to hold water [43]. Residual soils in Jurong Formation tend to be finer and less uniform than Old Alluvium and Bukit Timah Granite residual soils [44; 45]. When residual soils are developed on a slope, it may lead to slope instability and failure, specifically when water enters the soil. Therefore, it is essential to comprehend unsaturated soil behavior and to define the soil-water characteristic curve, including the required volume change measurement.  
With the increasing depth, the effective cohesion (c\({}^{\prime}\)) value decreases because of the decrease of fine particles. Bukit Timah Granite residual soils have lower effective cohesion than Jurong Formation and Old Alluvium residual soils. The proportion of sand, on the other hand, increases with increasing depth, which corresponds to a rising effective friction angle (\(\phi^{\prime}\)) [3]. The saturated permeability of residual soils of Bukit Timah Granite ranges between \(10^{-10}\) and \(10^{-5}\) m/s [46]. This was later confirmed by [PERSON] et al. [47] with an extensive empirical study of Bukit Timah Granite, which indicates a great extent of saturated permeability for residual soils of Bukit Timah Granite.