## 4 The Lambeth College Case  
### Basic Information  
Lambeth College is situated in South London and uses all 143 of its bored pile foundations as energy piles. Prior to its construction a trial was carried out by Cementation Skanska, Geothermal International Ltd., and Cambridge University, which involved the installation of a test pile that was subjected to extreme heating and cooling cycles. The intention of the trial was to improve the understanding of what impacts heating and cooling cycles have on the structural and/or geotechnical performance of a pile and to analyze the thermodynamic response of the system [17].  
The TRT carried out on the Lambeth College test pile was analyzed. The TRT was selected from a part of the data monitored during the project. The object of the project was a load test with heating and cooling. The test pile was initially subjected to a period of loading alone between the dates of the 14 th and 15 th June 2007, before being subjected to a cooling mode cycle on 18 th June, lasting for four weeks, in which the input fluid temperature in the ground loop was \(-6\)\({}^{\circ}\)C. On 19 th July the pile was then heated, using an input fluid temperature of 40 \({}^{\circ}\)C, until 31 st July.  
The test pile was fabricated with a nominal diameter of 610 mm through the made ground and terrace deposits, and 550 mm through the London clay. Its length was designed to be 23 m in order to resist the required working load of 1200 kN and to include a safety factor of 2.5. The ground loops and instrumentation required for the test were attached to the pile reinforcement cage, which consisted of six 32 mm diameter bars.  
The details of the layout of the components used in the test can be found in [17]. In addition to the construction of the test pile, four anchor piles were installed 2 m away in order to simulate the load of the building. A heat sink pile was positioned 20 m away from the test pile so that heat could be transferred to and from it via an 8 kW heat pump which provided the heating and cooling outputs. In addition, a borehole containing instrumentation was constructed 0.5 m away from the test pile to record the nearby temperature profile. Two data loggers were also installed to record and analyze results from the instrumentation installed in each component.  
Optical Fiber Sensors (OFS) were installed in the test energy pile and in the borehole in order to collect temperature and strain data. The details of how this instrumentation was embedded within the test pile and borehole can be found in [17]. Optical fibers enabled continuous distributed strain and temperature profiles to be measured, instead of the point measurements taken by more well-known equipment such as strain gauges. This technology is, therefore, ideal for monitoring the performance of piles. Thermistors (in the vibrating wire strain gauges) were also installed in the test energy pile to compare to the data measured by the OFS. VWSG thermistors that were located at the depth shown in Table 1, which shows how the temperature of the test pile changed over time at 9 m below ground level (bgl), and highlights the main activities carried out during the trial. The strain and mechanical results were analyzed by [17].  
In this study, the temperature data of the project are used to discuss the change of heat transfer co-efficient across the pile-soil interface. The temperature was collected by both OFS and thermistors. There was a borehole 0.5 m away from the test pile. Thermistors and OFS were also installed into the borehole to monitor the ground temperature. These data are used to evaluate the heat transfer coefficient values in cooling and heating modes separately.