The influence of installing UTES boreholes on the bearing capacity of piles

Abstract

The transition to renewable energy has lead to many new ways of energy production, such as wind and solar energy. For solar energy power, there is an excess of energy during summer and an deficiency of it during winter, which is reversed for the energy consumption. In order to amend this problem, there is an introduction of new ways of energy storage in the form of heat for domestic and industrial use. One of these is the use of Underground Thermal Energy Storage (UTES). These systems make use of the underground to store heat during summer and extract heat during winter by using a fluid as the heat transferring agent. There are many different ways of UTES installation, however there are two which are widely used in the Netherlands: 1) Aquifer Thermal Energy Storage (open systems) which are based on heat-transfer by convection and 2) Borehole Heat Exchangers (closed systems) which are based on heat-transfer by conduction. While the way of transferring heat differs, they both need vertically drilled boreholes to reach a certain storage depth. These boreholes are drilled close to, or underneath, pile-founded buildings. The drilling of boreholes influence in-situ soil stresses around them, which might influence the bearing capacity of the piles. The aim of this thesis is to investigate the influence of installing UTES systems on the bearing capacity of piles. This is done by: 1) investigating the stress changes in sand due to the drilling of boreholes (stress analysis) and 2) the influence of these stress changes on the bearing capacity of piles (bearing capacity analysis). All of the modelling was done in PLAXIS, by using the 2D-axisymmetric model for the first part and the 3D plane strain model for the second part. An advanced soil model for sand was used, with soil mechanical behavior according to the Hardening Soil small strain model. The stress analysis consists out of a parameter sensitivity analysis, including the key parameters: back-fill grout shrinkage and expansion, Over-Consolidation Ratio and Relative Density. After this, the influence of the borehole fluid pressure and borehole diameter on the stress states were investigated. As not all boreholes are drilled perfectly, the last part of the stress-analysis takes into account drilling complications. The bearing capacity analysis includes simulations of static pile load tests of non-displacement piles. Load-displacements curves are produced for several scenario's: 1) perfectly drilled boreholes, 2) boreholes with drilling complications, 3) influence of a soft soil layer and 4) displacement piles and the influence of a varying installation phasing. The results indicate no large effects of perfectly drilled boreholes on bearing capacity of piles located outside a zone of 1.5 times the borehole diameter. For boreholes with drilling complications, the bearing capacity is influenced for piles located in a zone of influence of only several meters (2-6m), depending on the severity of the complication. At last, loaded piles of existing buildings are extremely sensitive for stress changes due to the drilling process, which will cause severe loss of bearing capacity.