## 3 Buried Pipe Tests
### Experimental Setup and Tests  
Other artificially induced leaks (further 20 mm longitudinal cuts) were generated and examined. The new tests considered five leaks at different locations along the pipe (Figure 7). The distance of each leak from the terminal valve is reported in Table 1.  
A static electromagnetic flow meter, namely a SENSUS iPERL with low starting flow rate (about 4 L/h) was installed at the pipe T-joint to monitor the leaking flow (Figure 7). This kind \begin{table}
\begin{tabular}{c c c} \hline \hline
**Leak (Buried Pipe)** & **Distance (m)** & **Flow Rate (L/h)** \\ \hline \# _1b_ & 3 & 200 \\ \# _2b_ & 8 & 295 \\ \# _3b_ & 13 & 200 \\ \# _4b_ & 18 & 180 \\ \# _5b_ & 26 & 4000 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Main features of the induced leaks for the buried pipe tests. Figure 6: Diagram of AF vs. RA, comparison between conditions _NL_ and _Lk-3_. Figure 7: Schematics of the test bed with buried pipe. of flow meter was not expected to affect measurements with additional noise, since it does not include mechanical moving parts. The flow rate measured for each active leak is shown in Table 1. Leak # 5b, larger than the other ones, had been generated for other experiments and its flow rate was notably higher. Nonetheless, Leak # 5b was taken into account as well to assess the effects of a large leak located relatively distant from the transducer. All the pipelines were buried under about 0.5 m of backfill soil (Figure 8a). Manholes were installed to allow access to the leak locations and to the valves (Figure 8b). The same sensor setup and acquisition parameter of the previous tests were adopted to better compare the results, except for the threshold level. Indeed, it was necessary to reduce this parameter, as the first trial acquisitions revealed extremely low signal levels. In particular, two different sets of measurements were carried out with threshold levels of 26 and 27 db, respectively. It is worth noting that a further tuning of all the acquisition parameters may be required for measurements on the real network, due to a certain variability of the boundary conditions. Nonetheless, the refined values would not be expected to differ significantly from the ones adopted in the experiments, since the test bed replicates the main features of actual HDPE service pipes. Consistent with previous tests, the following measurements were performed, where the letter '\(b\)' designates the _buried_ condition:  
1. acquisitions in non-leaking state (referred to as \(N\!Lb\) hereafter), set by deactivating all the leaks with the repair clamps, to assess the environmental noise;
2. water leaking against backfill soil from the \(j\)-th active leak (\(L\!k\)-\(j\!b\)), a condition set by temporarily removing the corresponding repair clamp (Figure 8b). As for the second condition, the presence of soil over the leaking flow was simulated by covering the section of pipe inside the manhole with a plastic sack filled with soil, which could be easily installed and removed, thus permitting rapid operation of the repair clamps. The sack was arranged to match a large portion of the pipe's external surface near the leak, so that it was expected to stay basically in contact with the pipe during tests. This setup was meant to replicate the conditions characterizing leaks at an early stage (i.e., without cavities being yet generated by the leaking flow). The other test procedures were not modified. In particular, it is worth recalling that the terminal valve remained closed in all tests, hence ensuring that the flow rate inside the pipe was entirely ascribable to the leaking flow of an active leak and that the water flow at the sensor location was null. Moreover, repeatability was achieved by repeating measurements three times for each condition. Experimental data were examined consistently with the protocols defined in the preliminary tests (cf. Section 2.1).