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Ground source heat pumps (GSHPs) in urban environments can take advantage of surplus ground heat energy generated by subterranean infrastructures such as underground railways (URs). This paper describes seminal work in this area and a novel 3D numerical investigation into the heat energy recovery from URs with nearby vertical ground heat exchangers (GHEs). The investigation uses the London Underground (LU) as a case study but its results are generic worldwide. The investigation included a number of studies considering different geometrical configurations of the railway tunnels and the nearby GHE array, and the impact of that variation on the GHEs heat extraction rates. The results showed that heat extraction rates of GSHPs installed near UR tunnels can be significantly improved by up to ~ 43%. This will enhance overall GSHP system efficiencies, resulting in substantial savings in both operational costs and carbon emissions. In addition, the outcomes were used to develop a relationship which allows approximating the GHEs heat extraction improvements due to the nearby tunnel(s) heat load(s). This could give guidance to engineers working in fields where thermal interactions between URs and nearby GHEs arise.

Ground source heat pumps (GSHPs) can provide an efficient way of heating and cooling buildings due to their high operating efficiencies. The implementation of these systems in urban environments could have further benefits. In such locations the ground source heat is potentially more accessible through alternative sources such as through underground railways (URs). The heat from the ground surrounding an UR tunnel could be exploited to enhance the operation of GSHPs operating in heating mode. To achieve this, the interactions of GSHPs with neighbouring URs must be fully understood but there is little exploration of these in current literature. This paper focuses on the potential benefit of understanding such interactions. It starts with a summary of typical and alternative heat sources for heat pumps and then it highlights the reasons why URs can be regarded as one of the most attractive ones. Then the paper reviews the current approaches used to model GSHPs and URs. Based on that review the paper suggests a method for the combined analysis of GSHPs and URs. The reasons why London is a sensible choice for a case study are also described. Summary of results from a preliminary investigation are also presented.

This paper investigates the effect of suction and lime treatment on the volumetric and shear behaviour of a partially saturated high plasticity clay (London Clay). A series of triaxial tests were performed on statically compacted London Clay and lime-treated London Clay specimens. These were prepared at the same target void ratio and tested under partially saturated conditions. The tests concerned isotropic compression and two different types of shearing tests (a) shearing at constant suction and (b) shearing at constant water content, using the axis translation technique. Based on the results, the effects of suction and lime-induced bonding are evaluated and quantified within an elastoplastic partially saturated soil framework.

The paper presents a laboratory investigation, which quantified the beneficial effect of lime on the water retention, compressibility and overall volume changes of a high plasticity clay (London Clay). The study comprised oedometer testing (including suction controlled testing), shrinkage tests, and filter paper testing to determine the soil water retention curves of the drying soils. The latter tests showed that the shallow-mixed lime-treated soils are likely to be partially saturated in situ depending on the environmental conditions; thus, unsaturated soil mechanics concepts are applicable for the description of their behaviour. The oedometer results quantified the marked decrease in the swelling and compressibility of the lime-treated soil compared to the untreated clay and the increase in the vertical effective yield stress (due to suction and cementation bonding). The lime enabled the soil to maintain a more open structure with respect to the untreated soil throughout the compression process, and appears to be the main factor influencing the compressibility of the unsaturated lime-treated soil (rather than suction) for the ranges of suction tested.

Engineers need versatile tools in order to design successful dewatering schemes to counteract the effects of changing groundwater levels in cities. In the present paper, a finite-element program is used to perform pumping analyses, involving moving phreatic surfaces and possible desaturation of materials. First, a problem of local dewatering from a line of wells is analysed, involving desaturation of the soil. The results from an analytical solution as well as those of the numerical model are compared with field data. It is shown that the numerical results, accounting for soil desaturation, are closer to field measurements. Second, the ability of the finite-element program to model falling and subsequently rising groundwater levels in cities due to variable water abstraction (as in the case of London) is assessed. The techniques show promise and are sufficiently flexible to be applied to other geologies and boundary conditions.

The present paper deals with the numerical modelling of trench drainage in variably saturated porous media. In particular, the effect of the unsaturated storage coefficient form on the rate of transient seepage into the trench is investigated. Several numerical analyses are performed for two different soil types. The numerical results derived using a pressure head-dependent storage coefficient, are compared to those derived using constant storage coefficients. It is shown that simplifying assumptions using constant storage coefficients may predict too fast drainage rates.