**2. Materials and Methods**

#### *2.1. In-Site Clay*

Sampling the in situ CCCs was based on ASTM D3441 [30], The *Geotechnical Engineering Investigation Handbook* [31] and other references [32,33]. The specific sampling procedure was displayed in the authors' previous study [18]. A total of 40 sampling points were set up. Sampling depth was one-half of the CCC thickness. The GXY-1 engineering driller was adopted for rotary drilling sampling. The methods of the drilling operation were employed as per the *Geotechnical Engineering Investigation Handbook* [34], Standard practice for Classification of Soils for Engineering Purposes [35], and Geotechnical design, Part 2: Ground investigation and testing [36]. Samples should be wrapped in an impermeable material (plastic sealing bag) and stored in a shockproof box (filled with foam buffer) to prevent transport disturbance. The soil's fundamental physical properties were tested based on the standards of GB/T 50145–2007 [37] and ASTM D2487 [38]. Test results were shown in Table 1. Figure 1 is the plasticity chart to identify the soil classification. As the organic content of the CCC sample is only 4.2%, the soil of the contaminated site can be classified as clay with low LL, based on Figure 1 and ASTM D2487.

**Table 1.** Basic physical indicators of the clay used in the test.


**Figure 1.** Plasticity chart for classification of fine-grained soils.

#### *2.2. Polyacrylamide*

There are four kinds of PAM in the market, cationic PAM (CP), anionic PAM (AP), nonionic PAM (NP), and zwitterion PAM (ZP). According to the authors' preliminary test

results (see Supplementary Materials), the zwitterionic PAM worked best in inhibiting the cracking of CCCs. The modified clay had a much lower gas permeability (Kp) and gas diffusion coefficient (Dp) with good gas barrier performance than that modified by cationic, anionic, and nonionic polyacrylamide. Hence, zwitterionic polyacrylamide was selected as the tested modifier in this research, which was provided by Henan Zhengzhou Lvjie Environmental Protection Material Co., Ltd. (Zhengzhou, China), with a molecular weight of 12 million and solid content no less than 90%. The parameters of PAM were shown in Table 2.


