Settlement of fine-grained soils
Preloading or surcharge loading is a common technique used in construction to obtain adequate bearing capacity and acceptable post-construction settlement. For fine-grained soils however, the time required to allow the dissipation of the excess pore-water pressure is often unacceptably long due to the very low permeability of these types of soils. For these cases, it is now common to combine preloading with the installation of vertical drains. Nevertheless, in some specific cases, the time needed for soil settlement is still considered too long and consequently too expensive. In order to develop a method to reduce this time, we used our expertise in the field of thermo-mechanical behavior of fine soils.
Several works in the field show that a temperature increase induces firstly an increase of the permeability and secondly, for normally consolidated states, an irreversible contraction and a modification of the yield limit. These results point to the fact that a temperature increase could lead to an increase of the settlement rate and an increase of the final value of settlement. In-situ the required thermal loading could be carried out by means of thermal vertical drains.
Large oedometer with vertical drain and temperature control
General scheme of the experimental device
In order to simulate in our lab the effect of temperature on the consolidation process in the case of soils fit out with vertical drains, a new device has been developed.
This device consists of three main parts: a stainless steel tube, a central thermal vertical drain and a piston. This device allows to apply a thermal loading and a mechanical loading through the central drain and the piston respectively. The vertical displacements, the quantity of water exchanged by the drain and the variation of pore pressure and temperature in the soil are monitored. The whole device is 1.5m high and 0.5m wide.
Some results: Temperature effect on soil’s consolidation process
Comparison of the kinetics of soil settlement process.
This figure shows two kinetics of consolidation process obtained at 25 and at 60°C. These curves highlight that an increase in temperature accelerates the kinetics of consolidation. From those results, the mean velocity during the first 10 minutes is about:
- at ambient temperature: 3.10-5m s-1
- at high temperature: 5.10-5m s-1
and during the first 20 minutes:
- at ambient temperature: 1.5 10-5m s-1
- at high temperature: 1.9 10-5m s-1
In conclusion, the rate of settlement increases by more than 50% for an increase of 20°C. The equilibriation time is strongly reduced by the increase in temperature.