## 4 Quality Detection of Waterproof Curtain
### Resistivity CT  
The waterproof curtain is formed by mixing and solidifying the in situ soil with cement, and its resistivity should be higher than that of surrounding undisturbed soil. When there are quality problems in the waterproof curtain, such as low cement content, thin curtain thickness, defects or discontinuities in the curtain, the resistivity in these potential weak areas will be reduced. Resistivity CT uses this principle to detect the quality of waterproof curtain.  
Figure 10 shows the equipment of resistivity CT and the arrangement of detection holes. Four boreholes are symmetrically arranged on the two sides of the waterproof curtain in the form of plane rectangle, wherein the short side is 2 m, the long side is 20 m, and the depth of the boreholes is 2 m deeper than that of the waterproof curtain to ensure that the transmitting and receiving paths of electric signals can cover the wall. Based on the data obtained by the equipment, the resistivity distribution between the four boreholes is obtained through 3D inversion technology, and then the resistivity distribution at the position of the waterproof curtain is obtained.  
Figure 10: Equipment and detection holes arrangement of resistivity CT: (**a**) Resistivity CT equipment. (**b**) Arrangement of detection holes.  
Figure 9: Borehole television imaging equipment and results: (**a**) Borehole television imaging equipment. (**b**) Images in the borehole.  
In reference to the resistivity of the qualified section detected by coring and borehole television imaging, the 2-level resistivity thresholds of 10 \(\Omega\cdot\)m and 20 \(\Omega\cdot\)m were adopted to evaluate the quality of the waterproof curtain. The specific evaluation criteria are as follows: if the resistivity distribution is uniform and \(\rho>\) 20 \(\Omega\cdot\)m, the quality of the waterproof curtain is very good; if the resistivity distribution is relatively uniform and 10 \(\Omega\cdot\)m \(<\rho<\) 20 \(\Omega\cdot\)m for most of regions, the quality of the waterproof curtain is good; if the resistivity distribution is relatively uniform and \(\rho<\) 10 \(\Omega\cdot\)m for a local region, there may be discontinuous defects in the region with low resistivity; and if \(\rho<\) 10 \(\Omega\cdot\)m for most of regions, the quality of the waterproof curtain is bad.  
After the waterproof curtain was built, a total of 25 sections of the waterproof curtain were detected using resistivity CT along the edge of the foundation pit. Figure 11 shows two representative detection results of the waterproof curtain corresponding to the underground 2-floor and 3-floor basement areas. It was found that the resistivity in the detection area was larger than 10 \(\Omega\cdot\)m and even exceeded 50 \(\Omega\cdot\)m. In the vertical direction, the resistivity distribution along the depth of the waterproof curtain was uniform without a low resistivity region, and in the horizontal direction, the joint area showed a high resistivity, thus inferring that this may be due to the overlap of two pieces of the waterproof curtain during construction. The detection results show that the quality of the waterproof curtain constructed by the trenching-and-replacing-style TRD technology is good, thus overcoming the adverse effects of the large-particle gravel strata.