Modelling soil as it freezes
Commonly, excavation and underground construction of subway systems or mining infrastructures must be done in difficult ground in close vicinity of buildings. AGF of saturated or partially saturated soil is a promising green alternative to conventional methods to stabilise the soil with injection of chemical or cement grout. Like other techniques, AGF stabilises the ground and provides structural support. It also prevents influx of water to an excavation site. The chemical freezing medium is circulated in closed pipes and exhausted into the atmosphere or recirculated such that groundwater is not contaminated. The EU-funded project 'Numerical modelling of artificial ground freezing' (NUMAGF) improved the ability of scientists and engineers to predict the effects of AGF on granular materials. This will enable better assessment of construction time and cost as well as optimisation of techniques. Team members amassed extensive data assimilated while monitoring AGF during construction of Line 1 of the Napoli underground. This included information on building displacements, ground temperature changes, and changes in the groundwater system and in forces on the anchors supporting excavation. Scientists revised a highly non-linear elasto-plastic-mechanical model of soil freezing and thawing, and developed a fully coupled thermo-hydro-mechanical (THM) model to consider the engineering behaviours of the earth materials. Models were validated against experimental data obtained during tests of volcanic ash from two sites in Napoli. AGF is becoming a very promising and important technology with many applications. In addition to its use in underground excavation, it has been used to stabilise slopes, retrieve undisturbed soil samples and maintain permafrost below heated buildings. It has even been suggested as a way to contain radioactive contamination of groundwater near the Fukushima nuclear power plant. The THM models of granular soil behaviour during freezing and thawing delivered by the NUMAGF project will ensure that AGF is done with prior knowledge-based assessment of stability and safety. Applications in civil and mining engineering abound. In the face of global warming and permafrost degradation, the descriptions of freezing and thawing will also be welcomed by investigators of climate change in cold regions.
Keywords
Soil, tunnelling, underground excavation, artificial ground freezing, stabilisation technique, saturated soil, chemical freezing, groundwater, granular materials, building displacements, ground temperature, soil freezing, thermo-hydro-mechanical