Effect of temperature induced excess porewater pressures on the shaft bearing capacity of geothermal piles

- Geothermal piles in low permeability (k<1E−11m/s) and low compressibility clays (KS>2E10Pa) can develop excess porewater pressures comparable to shaft friction.
- A shaft friction reduction ratio is presented to account for this.
- The solution presented provides an explanation to the difference between back-calculated and observed shaft frictions for a test pile.

Changes in temperature in clays of low permeability typically induce excess porewater pressures. In the context of geothermal piles this effect has typically been overlooked since most installations have occurred in soils with higher values of permeability. A parametric study is presented that solves the governing differential equations one dimensionally in a pile to study the influence of the various parameters: temperature of the fluid, permeability and soil compressibility. A new shaft resistance reduction ratio has been also defined to illustrate the loss of bearing capacity. The study shows that when the value of permeability is 1E−11 m/s or lower, combined with a soil compressibility in excess of 20,000 MPa, the developed excess porewater pressures can potentially reduce the effective stress locally to very low values. The solution applied to the case of the Lambeth College, London, also provides a plausible explanation to the observed loss of shaft friction of the tested pile.