## 2 Materials and Methods
Brillouin frequency shift \(v_{B}\) is proportional to the local velocity of the acoustic waves. \[v_{B}=\frac{2 nv_{a}}{\lambda}, \tag{1}\]  
where \(n\) is the refractive index of the fiber core, \(v_{a}\) is the velocity of sound in glass and \(\lambda\) is the free-space wavelength. Figure 4: Construction site: (**a**) survey plan; (**b**) engineering-geological profile. Changes in strain and temperature are obtained by calculating the difference between the backscattered Brillouin frequency shift measured at different times, so Equation (1) can be extended into the following form using the temperature and strain conversional coefficients (\(C_{\varepsilon}\) and \(C_{T}\)). \[v_{B}(z)=v_{B0}(z)+C_{\varepsilon}\varepsilon(z)+C_{T}T(z), \tag{2}\]  
where \(v_{B}(z)\) is the Brillouin frequency shift at location \(z\), \(v_{B0}(z)\) is the original Brillouin frequency shift, \(\varepsilon(z)\) is the strain change, and \(T(z)\) is the temperature change. The location \(z\) where Brillouin backscattering happens is determined as a function of time \(t\) with known light velocity \(c_{0}\) in a vacuum:  
\[z=\frac{c_{0}t}{2n}, \tag{3}\]  
The proposed layout for the fiber optic cable on the base raft as illustrated in Figure 6 was made according to the results of the preliminary simulation to cover the areas expected to have the greatest settlement and based upon an eight-meter grid spacing. However, there are areas, especially around the core of block R, where the distance is well over eight meters. For any point on the raft within the core, the greatest distance between that point and the nearest fiber optic cable is approximately six meters, corresponding to an equivalent grid spacing of 12 m. The routing generally maintains a half-meter separation between the channel and vertical reinforced concrete elements, such as walls and columns. The cores  
Figure 5: Part plan of cable routing in Block R monitoring area on B4 raft. at Blocks Y and Z are constructed adjacent to ramps connecting to the B3 level, and the fiber optic cable routing avoids these ramped areas. The routing also avoids penetrating reinforced concrete walls, but in places the cable passes beneath blockwork walls. In these places, the blocks immediately above the channel used to be temporarily removed to enable access for both cutting the channel and passing the cable reel through the gap in the wall. To reduce the number of additional connections, the reel used to be passed through the blockwork wall wherever possible, and no additional connections were made. Between levels B1 and B4, as well as between the Block R observation zone and the riser, there is a conduit containing two cables: one cable routes the light from the analyzer into the monitoring zone, whilst the other routes the light back to the analyzer. The total length of the fiber optic cable on the tested sections was 2153 m, on which 2261 m of reels were used to avoid kinks when turning (Figure 6). The distance range is the maximum distance along the cable from the analyzer where a measurement is required and is equal to 2040 m. A light tension was applied to the cable by pulling the cable taut before being locked down at the fixing points so that it could register compressive strains along the gauge lengths. The total optical loss is estimated in Figure 6 to be 11.0 dB and assumes the following:  
* cable lengths and connectivity;
* a total fiber length of 4.4 km, including both the strain and temperature-measuring fibers running in parallel within the same cable;
* unit attenuations;
* the use of connectors rather than splices to join two fiber ends on site. To maintain the structural integrity of the basement raft, no part of the cable installation interfered with the top reinforcement of the raft, the cover of which is 40 mm, and 25 mm-deep channels are shallow enough to clear the reinforcement (Figure 7). The minimum bending radius of the fiber optic cable during installation was 400 mm.