## 1 Introduction  
The southwestern foothills of Taiwan contain an abundance of mudstone. The local mudstone softens or slakes easier when it is soaked with water, so mudstone material is rarely developed. However, Taiwan is a densely populated small island, where the usage of its plain area is extremely limited. In recent years, oil storage tanks had to be moved to or constructed in mudstone areas, while reservoirs, landfills, industrial areas, and universities were also established in mudstone areas. Thus, the engineering properties of crushed mudstone need to be investigated beforehand. Studies have shown that mudstone is silt/clay sedimentary rock that lacks lamination or fissility, while durability may be its most crucial engineering property. When discussing a major cause of landslides in mudstone, [PERSON] [1] discussed environments with severe weathering effects. In Taiwan, mudstone is mainly distributed across the southwestern foothills, specifically the southern section of the Coastal Mountain Range of Eastern Taiwan and in Kenting, which is located in the Hengchun Peninsula. Exposed mudstone is most abundant in the southwestern foothills that span the area from Chiayi to Kaohsiumg (Figure 1a). Figure 1b shows the mudstone environment along the sides of the northern exit of Chungliao Tunnel in the Second National Expressway of Taiwan. In total, the mudstone stratum area in Taiwan exceeds 1000 km\({}^{2}\). When it comes to a major cause of landslides in mudstone-distributed areas, environments with severe weathering effects are a key factor according to [PERSON] [1]. Under dry conditions, boring tunnels through mudstone is extremely difficult. When moisture content increases, mudstone strength decreases, thereby resulting in engineering disasters [2]. [PERSON] et al. stated [3; 4; 5] that dry mudstone exhibits high strength; however, the strength of mudstone that has absorbed an excessive volume of water is substantially lower. This is because when water infiltrates mudstone, the diagenetic bonds between mudstone grains are gradually destroyed. When these bonds are destroyed, the recoverable strain energy stored in the compressed and deformed mudstone grains is released. Partial mudstone swells [3; 4; 5]. A study explored the bedrock characteristics of Plio-Pleistocene mudstone slopes in southwestern Taiwan and their effects on hydraulic and geophysical processes; the researchers used pore-water chemistry to demonstrate that mudstone causes high rates of erosion during heavy precipitation periods in Taiwan and, consequently, the formation of badlands [6]. Numerous methods have been utilized to strengthen soil bearing capacity, including piling or soil improvement. For instance, [PERSON] and [PERSON] [7] applied construction waste in the reinforcement of soft soil foundation with cementation materials in coastal cities. Cementation was also used in some on-site soil improvement projects [8; 9; 10; 11; 12], in which the effect of waste or waste liquid on clay engineering properties was tested [13; 14]. In laboratory research, Portland cement was added to improve clay and the remolded specimens were created for unconfined compression tests. The specimen curing duration can be 1 day, 7 days, 28 days, 56 days, or 90 days. The test results revealed that an increase in the curing period is associated with an increase in unconfined compression strength. Similar tests were conducted to achieve soil improvement by using pozzolanic reaction materials such as fly ash, lime, and furnace slag [15; 16; 17; 18; 19; 20; 21; 22]. Epoxy was also added in soil specimens to facilitate the performance of triaxial compression tests; the test results revealed that the compression strength, tensile strength, and elastic modulus increased with curing period and decreased with an increase in moisture content based on the study of [PERSON] et al. [23]. Research on low cement use involves the addition of cement and sewage sludge ash to reinforce the weak soil.