Group thermal response testing for energy piles

Thermal response testing is an in situ technique for characterising the thermal conductivity of the ground around a borehole heat exchanger. The test has seen renewed interest in recent years as an increasing number of ground heat exchangers are being constructed to provide renewable heating and cooling energy as part of ground source heat pump systems. The thermal response test involves applying a constant heating power to the ground via a circulating heat transfer fluid. Most test rigs are set up to cater for deep boreholes, with available heat transfer lengths typically more than 100m, and therefore have electrical heater capacities of a corresponding size. Pile heat exchangers are generally much shorter and the heat exchange length can be a little as 10m. This means that many standard thermal response test rigs cannot provide a low enough heating power and there is a risk of excessive temperature changes developing, especially during longer duration tests which can be recommended for larger diameter piles. One solution is to carry out the thermal response test on a group of piles, thereby increasing the effective heated length. This has the added advantage of testing a larger volume of soil. This paper examines the principles behind group thermal response testing for energy piles and considers the advantages and limitations of the approach with reference to a case study

Stress transfer from rocking shallow foundations on soil-cement reinforced clay

Equivalent-static pushover analyses with a three-dimensional (3D), nonlinear, finite-difference model are used to investigate the static and seismic stresses imposed on soil-cement grid reinforcements in soft clay profiles by overlying structures supported by shallow footings. The goal is to evaluate the potential stress concentrations in the soil-cement grid during foundation rocking and the potential for large foundation settlements associated with the local crushing of the soil-cement. The numerical analyses are first validated using data from dynamic centrifuge experiments that included cases with and without large foundation settlements and localized crushing of the soil-cement grids. The experimental and numerical results indicate that the stresses imposed on the soil-cement grid by the overlying structures must account for foundation rocking during strong shaking and stress concentrations at the soil-cement grid intersections. The numerical analyses provide reasonable predictions of the structural rocking loads and the zone of the expected crushing or lack of crushing, but underestimate the accumulation of foundation settlements when the seismic demands repeatedly exceed the soil-cement strength. The simulated moment-rotation and uplift behavior of the footings under monotonic lateral loading are reasonably consistent with the dynamic centrifuge test results. Parametric analyses using the validated numerical model illustrate how the stress transfer varies with the area replacement ratio, the thickness of the top sand layer, the properties of the bearing sand layer, and the relative stiffness of the soil-cement and the surrounding soil. A design model for estimating the stresses imposed on a soil-cement grid by rocking foundations was developed and shown to provide a reasonable basis for assessing whether or not local damage to the soil-cement grid is expected

A simple method for detecting cracks in soil-cement reinforcement for centrifuge modelling

This paper presents the development, implementation and experimental evaluation of a new crack detection mechanism for centrifuge modelling. The proposed mechanism is a brittle conductor bonded to cement providing a binary indication of if, and when, a sensor is cracked. The results of a pair of large centrifuge tests were used to evaluate the effectiveness of the proposed crack detection mechanism. Each test model included a soil profile consisting of a 23 m thick layer of lightly over-consolidated clay, underlain and overlain by thin layers of dense sand. The centrifuge models had two separate zones, a zone without reinforcement and a zone with an 'embedded' soil-cement grid, which had a unit cell area replacement ratio Ar=24%. Models were subjected to 13 different shaking events with peak base accelerations ranging from 0·01 to 0·55g. The performance of the proposed crack detection mechanism was examined using (i) post-test crack mapping in the soil-cement grids, (ii) results of the crack detection system and (iii) time series of accelerations, displacements and footing rotation. The results from the centrifuge test showed that the proposed crack detection method accurately captured if, and when, cracking occurred in the soil-cement grid at the locations of the sensors