## 4 Retaining Structure and Instrumentation  
Figures 2-5 show the plane views of the enclosure structure of foundation pits A-D, respectively. Sheet pile-braced cuts with two internal support beams were selected as the retaining structure. The diameter of the bored cast-in-place pile was 800 mm, and the clear spacing was 200 mm. The length of the bored cast-in-place pile varied for different foundation pits, ranging from 19 to 22 m. Triaxial mixing piles were used as the waterproof curtain. To reduce the mutual influence during foundation pit construction, the construction sequence of the four foundation pits was optimized. The principle of the construction sequence was that adjacent foundation pits should not be excavated at the same time. In general, the construction sequence was to construct foundation pits A, C, and D on the outside first and then foundation pit B in the middle. The zoned excavation technique was adopted for the excavation of the second and third layers of the foundation pits. The zoned excavation technique refers to dividing a foundation pit into different areas for excavation to ensure the safety of the foundation pit itself and the surrounding environment. After the excavation of each layer of the foundation pits, the strut (i.e., lateral support inside the foundation pit) or the base slab should be cast in time. A detailed construction schedule is presented in Table 3. The excavation processes is shown in Figures 6 and 7.  
Figure 4: Site plan of the project site and instrumentation layout (foundation pit C).  
Figure 3: Site plan of the project site and instrumentation layout (foundation pit B).  
Figure 5: Site plan of the project site and instrumentation layout (foundation pit D).  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Stage** & **Event** & **Date** & \\ \hline
1 & Excavation of soil layer 1 of foundation pits A, C, and D & 3 March 2019 & 17 March 2019 \\
2a & Excavation of soil layer 2 of foundation pits A, C, and D & 18 March 2019 & 29 March 2019 \\
2b & Casting the strut (level 2) of foundation pits A, C, and D & 18 March 2019 & 24 March 2019 \\
3 & Excavation of soil layer 1 of foundation pit B & 18 March 2019 & 24 March 2019 \\
4a & Excavation of soil layer 3 of foundation pits A, C, and D & 30 March 2019 & 13 May 2019 \\
4b & Casting the base slab of foundation pits A, C, and D & 7 May 2019 & 13 May 2019 \\
5 & Casting the strut (level 1) of foundation pit B & 19 May 2019 & 23 May 2019 \\
6a & Excavation of soil layer 2 of foundation pit B & 19 May 2019 & 23 May 2019 \\
6b & Casting the strut (level 2) of foundation pit B & 16 June 2019 & 30 July 2019 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Detailed construction schedule.  
Figure 6: Excavation of soil layer 2 and casting the strut (level 2).  
Monitoring the deformation of the retaining system and surrounding strata during excavation is of great significance to ensure the safety of the foundation pit. The monitoring scheme was formulated according to the _Specification for engineering measurement_ (GB50026-2007), _Specification for the first and second order leveling_ (GB12897-2006), _Building foundation pit monitoring technical specifications_ (GB50497-2009) and _Monitoring rules for construction of foundation pit engineering in Shanghai_ (DG/TJ08-2001-2006). The main instruments of this project included two parts. One was the surrounding environmental monitoring: the settlement behind the enclosure piles surveyed by a level instrument (designated as JS1.1-JS1.5 to JS28.1-JS28.5). The other one was the enclosure structure monitoring: lateral deflections of the enclosure pile surveyed by inclinometer casings (designated as CX1-CX34) and vertical column movements surveyed by a level instrument (designated as L1-L49). The layout of the instruments is shown in Figures 2-5.