## 3 Design and Construction Technology of Waterproof Curtain
### Trenching-and-Replacing-Style TRD Technology for Waterproof Curtain  
In view of the problems in the traditional TRD technology under these complicated engineering geological and hydrogeological conditions, the trenching-and-replacing-style TRD technology, which combines the trenching technology of the diaphragm wall with the traditional TRD technology, has been proposed. This technology is generally implemented in three steps: (1) Excavating the trench using a trenching machine for the diaphragm wall; (2) replacing the large-particle pebble gravel and backfilling the homogeneous cohesive soil in the trench; and (3) cutting and mixing the cement-soil to build a continuous and homogeneous waterproof curtain wall using TRD technology. Figure 6 shows the schematic diagram of the construction process. The technical points of each step are described as follows:  
Figure 6: Construction process of trenching-and-replacing-style TRD technology: (**a**) Step 1. (**b**) Step 2. (**c**) Step 3. Step 1: excavating the trench  
To ensure that the bottom of the waterproof curtain wall enters the (c)-2 layer of strongly-moderately weathered argillaceous siltstone, the high-power trenching machine of BAUER BG80S was adopted to excavate the trench. The width and depth of the trench came out to 0.8 m and 63 m, respectively. In the process of trenching, the trench wall is prone to collapse, so the following measures were taken:  
Before trenching, the trench wall was reinforced using biaxial mixing columns, and the guide wall was constructed, as shown in Figure 6a. The reinforcement depth came out to 11 m, thus reaching the bottom of the (d)-1 layer of silty clay. The cement used is ordinary Portland cement with strength grade of 42.5, the cement content is 15%, and the water-cement ratio is 0.5. High-quality bentonite was used, CMC tackifier was added to prepare mud to protect the trench wall, and barite powder was used to appropriately increase the mud proportion, thus making it in the range of 1.15-1.18. The mud level in the trench was kept at least 0.5-1.0 m higher than the groundwater level. In the process of trenching, the grab bucket of the trenching machine enters and exits the trench slowly and steadily. Especially in the pebble gravel layer, the trenching speed keeps slowly. The interval trenching mode was used to control the trenching width. Stacking heavy objects and the walking of large machinery around the trench were avoided to prevent instability and collapse induced by overloading on the surrounding ground surface. Step 2: replacing large-particle gravel  
After the trenching depth of each trench section reached the design depth confirmed by geological exploration, we backfilled the silty clay in time to replace the large-particle gravel, and the backfilling amount was about 2/3 of the excavated soil volume, as shown in Figure 6b. The next trench section could be built only after the previous trench section was backfilled. Step 3: building the waterproof curtain wall  
After the large-particle pebble gravel in the trench was replaced by backfilling the silty clay, TRD technology was used to build the waterproof curtain wall according to the following steps, as shown in Figure 7:  
Primary cutting of soil: After the cutter box was placed to the expected depth, the bentonite fluid was injected, and the cutter box moved horizontally to cut a section of backfilled soil, as shown in Figure 7a. Retreat cutting of soil: After cutting a section of backfilled soil was completed, the cutter box retreated horizontally to the starting point to cut the soil again, and it overlapped the wall formed in the previous section by 30-50 cm, as shown in Figure 7b. Mixing to form wall: After the cutter box was retreated to the starting point, the cement curing liquid was injected, and the cutter box moved forward horizontally to mix with the soil to form a section of cement-soil wall with equal thickness, as shown in Figure 7c. Figure 7: Construction process of waterproof curtain using TRD technology: (**a**) Primary cutting of soil.