*2.1. Preparation of Amended Compacted Clay*

Three types of amended compacted clay were prepared in this study: (1) the compacted clay amended by attapulgite with the dosage of 0%, 1%, 3%, 5%, and 10% (dry weight of attapulgite to dry weight of mixture powder); (2) the compacted clay amended by diatomite with the dosage of 0%, 1%, 3%, 5%, and 10% (dry weight of attapulgite to dry weight of mixture powder); and (3) the compacted clay amended by attapulgite and diatomite, here referred to as dual-additives, with the ratio of 4, 2, 1, 0.5, and 0.25 (dry weight of attapulgite to dry weight of diatomite); the dosage of dual-additives was attapulgite 4% and diatomite 1%, attapulgite 3.3% and diatomite 1.7%, attapulgite 2.5% and diatomite 2.5%, attapulgite 1.7% and diatomite 3.3%, and attapulgite 1% and diatomite 4% (dry weight of dual-additives to dry weight of mixture powder). The powdered clay used was obtained from Jining (Shandong, China), the powdered attapulgite and diatomite were manufactured by Zhengzhou (Henan, China). The optimum moisture content (*w*op) and maximum dry density (*p*dmax) were obtained by compaction test as per JTG E40 T0131-2007. This laboratory compaction method was used to determine the compaction curve compacted in a 152 mm diameter mold with a 45 N rammer dropped from a height of 450 mm, producing a compactive effort of 2677 kN-m/m3 with 3 layers and 98 blows per layer. While, the ASTM D1557 compaction method was used to determine the compaction curve compacted in a 152.4 mm diameter mold with a 44.48 N rammer dropped from a height of 457.2 mm, producing a compactive effort of 2700 kN-m/m3 with 5 layers and 56 blows per layer. The compaction curve of clay and dual-additives (attapulgite 4% and diatomite 1%)-amended clay are shown in Figure 2. The physical properties and main oxide content of these constituent materials used to prepare amended compacted clay are shown in Tables 1 and 2, respectively. The powdered materials used in this study were air dried and passed through a No. 200 (0.075 mm) sieve as specified in ASTM D 698 [32]. The initial moisture content of the specimens was determined to be 30% (weight of water to dry weight of solid). The liquid–plastic limit test and specific gravity test were conducted as per ASTM D 4318 [33] and ASTM D 854 [34], respectively, using distilled water.


**Table 1.** The physical properties of constituent materials used for preparing amended compacted clay.

**Figure 2.** Compaction curve of the clay and dual-additives-amended clay.



The amended compacted clay in this study was prepared by mixing predetermined amounts of powdered clay, attapulgite, and diatomite directly. The dosage and ratio (dry weight basis) of additives are shown in Table 3. Then, the specimens were prepared by (1) mixing the air-dried clay, attapulgite, and diatomite thoroughly to prepare the dry mixture (Table 3); (2) adding predetermined amounts of tap water incrementally to the dry mixture, then mixing thoroughly and sealing for 24 h to assure homogeneity; (3) compacting by hydraulic pressure and sealing for 14 d until the specimens reached moisture balance. The number of parallel specimens was three. The coefficient of compaction was targeted to be 85%. The diameter of the specimens in the moisture retention and flexible-wall hydraulic conductivity test was set as 50 mm and the height was 50 mm. While, the diameter of the specimens in the gas diffusion test was set as 61.88 mm and the height was 20 mm.

**Table 3.** The experimental scheme of the moisture retention, liquid limit, and gas diffusion.


## *2.2. Moisture Retention Test*

In the moisture retention test, the effects of attapulgite dosage, diatomite dosage, and dual-additives ratio on the moisture retention capacity of amended compacted clay were studied respectively. It was characterized by its moisture retention percent, as shown in Equation (1).

$$\text{W}\_{\text{t}} = \frac{\text{m}\_{\text{wt}}}{\text{m}\_{\text{w}0}} \cdot 100\% = \frac{\text{m}\_{\text{s}} \cdot \text{w}\_{0} - \text{m}\_{0} - \text{m}\_{\text{t}}}{\text{m}\_{\text{s}} \cdot \text{w}\_{0}} \cdot 100\% \tag{1}$$

where, *W*<sup>t</sup> is the moisture retention percent of the specimens at time *t*; *m*wt is the mass of moisture in the samples at time *t*; mw0 is the mass of moisture in the specimens at the initial time; ms is the total mass of soil during specimens preparation; w0 is the initial moisture content during specimens preparation; m0 is the total mass of the specimens at the initial time; *m*t is the total mass of the specimens at time *t*.

The detailed test scheme of the moisture retention test is shown in Table 3. The dosage and ratio of dual-additives were determined through the orthogonal test. The test was conducted according to the following procedure: (1) placing the specimens in a 70 × 55 mm perforated aluminum box after sealing for 14 d; (2) placing the specimens in the oven, the temperature was targeted to be 60 ◦C, recording the mass change per three hours, and calculating the moisture retention percent of the specimens according to Equation (1) [35–38].
