## 2 Materials and Methods  
### Physical and Mechanical Properties of Waste Soil  
The waste soil samples were collected from the coastal industrial area in Keqiao District, Shaoxing, as shown in Figure 1. This location is situated in the middle latitude with a relatively flat terrain, positioned on the south coast of Hangzhou Bay, China. According to the engineering geological survey, this area is influenced by a subtropical monsoon climate and has plentiful rainfall and well-developed river systems. The buried depth of the groundwater level in this area is generally from 0.1 m to 1.4 m. The underground space in this area contains various types of geotechnical soil layers with an uneven distribution. In this work, all the waste soils were obtained from excavations related to pile foundation construction, often reaching depths of up to 50 m below ground level. The wasted soils tested in the field have a high natural moisture content, more than 40% on average. After removing plastic garbage, plant roots and other junk within the soil, the waste soil was put into a large package by operating a large construction machinery and was then transported to the laboratory. To examine the physical and mechanical parameters of the waste soils, geotechnical tests were performed, adhering to the guidelines specified in the \"Test Methods of Soils for Highway Engineering\" (JTG 3430-2020) [40]. Waste soil, in appearance, is grayish yellow or greenish gray with plastic, medium toughness and strength. The mineral composition of the waste soil was assessed through X-ray quantitative phase analysis and consisted of approximately 13.6% clay minerals and 86.4% non-clay minerals, including quartz (55.8%), plagioclase (10.5%), hornblende (8.2%) and other minerals, as depicted in Figure 2. The gradation curve obtained from sieve tests is shown in Figure 3, with a \(d_{10}\), \(d_{30}\) and \(d_{60}\) of 0.007, 0.02 and 0.12 mm, respectively. The  
Figure 1: Field Sampling. coefficient of uniformity \(C_{\rm u}\) and coefficient of curvature \(C_{\rm c}\) were determined to be 17.1 and 0.476, respectively, which are calculated using the following formula:  
\[C_{\rm u}=\frac{d_{60}}{d_{10}} \tag{1}\]  
\[C_{\rm c}=\frac{d_{30}^{2}}{d_{10}.d_{60}} \tag{2}\]  
The fineness modulus MX of the waste soil was calculated to be 0.7 using the following formula:  
\[\rm MX=[(A0.15+A0.3+A0.6+A1.18+A2.36)-5A4.75]/(100-A4.75), \tag{3}\]  
where A0.15 is accumulated sieve residue percentage of a particle of the size of 0.15 mm, and others are in sequence. The specific gravity was obtained to be 2.603. Based on the results of \(\rm w_{L}=35.3\) (liquid limit), \(\rm w_{P}=21.1\) (plastic limit) and the corresponding \(\rm I_{P}=14.2\) (plasticity index), the soil was classified as silty clay. This classification adheres to the criteria outlined in \"Specification for Design of Highway Subgrade\" (JTG D30-2015), which states that the liquid limit should be less than 50% and the plasticity index should be less than 26 ([PERSON] and [PERSON], 2016) for embankment filling materials. The direct shear test showed that cohesion c and internal friction angle \(\varphi\) were 10.47 kPa and 35.88\({}^{\circ}\), respectively. The key parameters and properties of the untreated soil are listed in Table 1.  
Figure 3: Particle size distribution. Figure 2: Mineral content of engineering waste soil. To ensure sufficient stability and durability for the subgrade, the waste soil cannot be directly used as subgrade filling materials due to the following pending problems. Firstly, the high natural moisture content of the waste soil makes it difficult to meet the required high compactness. Secondly, according to \"Specification for Design of Highway Subgrade\" (JTG D30-2015) [41], the California bearing ratio (CBR\({}_{2.5}\)) of the subgrade filler needs to be greater than 8 for high-speed and first-class highways.