Final Report Summary - NUMAGF (Numerical Modelling of Artificial Ground Freezing)
Artificial Ground Freezing (AGF) is a controllable method used profitably by civil and mining engineering alike to temporarily stabilize the ground, to provide structural support and/or exclude groundwater from an excavation until construction of the final lining provides permanent security. On the one hand, the main advantage of AGF as a temporary support system in comparison to other support methods, such as those based on injections of chemical or cement grout into the soil, is the low impact on the surrounding environment as the refrigerating medium required to obtain AGF is circulated in pipes and exhausted in the atmosphere or re-circulated without contamination of the ground water. On the other hand, the available methods may vary significantly in their sustainability and complexity, in terms of times and costs required for their installation and maintenance. The ability to predict the effects induced by AGF on granular materials is therefore crucial to assess construction time and cost, and to optimize the method.
AGF is one of the construction techniques that were extensively adopted, during construction of Napoli underground, to stabilise temporarily the ground and control the ground water during excavation of the station tunnels and the inclined passageways through loose granular soils and the fractured soft. In some instances the innovative techniques which were implemented had almost the character of full scale experiments; therefore, construction of the line was accompanied by an intense program of monitoring designed to measure and/or control the construction processes and their effects on adjacent structures. Monitoring included the measurement of the displacements of existing buildings, of the ground surface behind the excavations, and of the retaining structures, of any changes in the groundwater regime, of the temperature in the ground around freezing tubes, and of the forces in the anchors used to support excavations.
The main aim of the proposed research were to address some of the gaps of knowledge in the state of the art of constitutive modelling for frozen soils. A critical-state elasto-plastic mechanical soil model has been developed to consider problems involving frozen and unfrozen soil. Frozen soils are a four-phase system consisting of solid particles, ice, unfrozen water, and gas or air. The disappearance of ice on thawing yields the three-phase system typical of unsaturated soils. A fully coupled THM model has been developed to consider a variety of geotechnical processes involving freezing and thawing during construction of Napoli underground system. By employing a combination of ice pressure, liquid pressure and total stress as state variables, a mechanical model developed by Nishimura et al. (2009) that encompasses frozen and unfrozen behaviour within a unified effective-stress-based framework has been modified and validate. The model has been validated against experimental data and in situ monitoring during construction of Napoli underground.
The initial part of the project has been devoted to the collection of all monitoring data made available by the contractors (Icotekne SpA and Trevi SpA) who carried out AGF during constructions of the stations of Line 1 of Napoli Underground. These include the results of a field test in Piazza Municipio and the data collected during the construction works.
The second part of the project has been devoted to revise the constitutive model developed by Nishimura et al (2009) and implemented in CODE_BRIGHT. This period have been also used by the researcher to familiarize with the scientific background of the project before being able to contribute effectively to the proposed objectives. Owing to the strong non-linearity of the governing equations, and their reciprocal coupling validation of the implemented model has required a carefully verification of the different part.
In the third part of the project the revised model has validate against experimental data on natural samples of volcanic ash retrieved from two sites in Napoli, corresponding to Municipio and Toledo Stations of Line 1 of Napoli Underground. The tests were performed using a triaxial cell working under temperature controlled conditions, described in some detail by Casini et al. 2013a. The experimental programme consisted of several tests on natural samples of both volcanic ash and Neapolitan Yellow Tuff, carried out at different temperatures, confining stress and axial strain rates, as prescribed by the designers (see Casini et al 2013 a,b, Pelaez et al 2014, Casini et al 2014).
In the last part of the project the comparison of the laboratory and experimental field data have helped to identify the inconsistencies between in situ and laboratory as well as emphasizing the common trends of behaviour. After these stages, the final goal of the project has been the back analysis of some of the data collected during construction of Line 1. In this phase, still going on, has been devoted to the validation of the model through the finite element analysis of the enlargement of running tunnels to accommodate platforms and excavation of inclined access passageways of Municipio and Toledo Stations.
Besides protecting excavations, AGF has been used also to stabilize slopes, to retrieve undisturbed samples of coarse grained soils, to construct temporary access roads, and to maintain permafrost below overhead pipeline foundations and heated buildings (Harris, 1995). Recently, AGF has been considered as a possible solution to radioactive contamination of the water surrounding the compromised Fukushima nuclear power plant (www.groundfreezing.net/projects/ground-freezing-fukushima). The available methods to obtain AGF may vary significantly in their sustainability and complexity, in terms of times and costs required to install and maintain them. The ability to predict the effects induced by AGF on granular materials is therefore crucial to assess construction time and cost, and to optimize the method. This project aims to provide a deeper understanding of the process, its role as “temporary works” and the development of a constitutive model where the governing equations have been developed from fundamental physical requirements. This offer a powerful tool to predict a wide range of engineering soil frozen/unfrozen properties within the context of a single analytical framework.
Applications of findings from this project span a number of sectors in engineering from the analyses of frost heave to the effect of freezing-thawing cycles in cold region. Global climate change in cryogenic regions has dominated the research agenda recently, as investigators seek ways of identifying the hazards to infrastructure in cold regions to establish distinct uncertainties through a risk based consideration of sensitivity and consequences and thereby mitigate the risk of permafrost degradation.
AGF is one of the construction techniques that were extensively adopted, during construction of Napoli underground, to stabilise temporarily the ground and control the ground water during excavation of the station tunnels and the inclined passageways through loose granular soils and the fractured soft. In some instances the innovative techniques which were implemented had almost the character of full scale experiments; therefore, construction of the line was accompanied by an intense program of monitoring designed to measure and/or control the construction processes and their effects on adjacent structures. Monitoring included the measurement of the displacements of existing buildings, of the ground surface behind the excavations, and of the retaining structures, of any changes in the groundwater regime, of the temperature in the ground around freezing tubes, and of the forces in the anchors used to support excavations.
The main aim of the proposed research were to address some of the gaps of knowledge in the state of the art of constitutive modelling for frozen soils. A critical-state elasto-plastic mechanical soil model has been developed to consider problems involving frozen and unfrozen soil. Frozen soils are a four-phase system consisting of solid particles, ice, unfrozen water, and gas or air. The disappearance of ice on thawing yields the three-phase system typical of unsaturated soils. A fully coupled THM model has been developed to consider a variety of geotechnical processes involving freezing and thawing during construction of Napoli underground system. By employing a combination of ice pressure, liquid pressure and total stress as state variables, a mechanical model developed by Nishimura et al. (2009) that encompasses frozen and unfrozen behaviour within a unified effective-stress-based framework has been modified and validate. The model has been validated against experimental data and in situ monitoring during construction of Napoli underground.
The initial part of the project has been devoted to the collection of all monitoring data made available by the contractors (Icotekne SpA and Trevi SpA) who carried out AGF during constructions of the stations of Line 1 of Napoli Underground. These include the results of a field test in Piazza Municipio and the data collected during the construction works.
The second part of the project has been devoted to revise the constitutive model developed by Nishimura et al (2009) and implemented in CODE_BRIGHT. This period have been also used by the researcher to familiarize with the scientific background of the project before being able to contribute effectively to the proposed objectives. Owing to the strong non-linearity of the governing equations, and their reciprocal coupling validation of the implemented model has required a carefully verification of the different part.
In the third part of the project the revised model has validate against experimental data on natural samples of volcanic ash retrieved from two sites in Napoli, corresponding to Municipio and Toledo Stations of Line 1 of Napoli Underground. The tests were performed using a triaxial cell working under temperature controlled conditions, described in some detail by Casini et al. 2013a. The experimental programme consisted of several tests on natural samples of both volcanic ash and Neapolitan Yellow Tuff, carried out at different temperatures, confining stress and axial strain rates, as prescribed by the designers (see Casini et al 2013 a,b, Pelaez et al 2014, Casini et al 2014).
In the last part of the project the comparison of the laboratory and experimental field data have helped to identify the inconsistencies between in situ and laboratory as well as emphasizing the common trends of behaviour. After these stages, the final goal of the project has been the back analysis of some of the data collected during construction of Line 1. In this phase, still going on, has been devoted to the validation of the model through the finite element analysis of the enlargement of running tunnels to accommodate platforms and excavation of inclined access passageways of Municipio and Toledo Stations.
Besides protecting excavations, AGF has been used also to stabilize slopes, to retrieve undisturbed samples of coarse grained soils, to construct temporary access roads, and to maintain permafrost below overhead pipeline foundations and heated buildings (Harris, 1995). Recently, AGF has been considered as a possible solution to radioactive contamination of the water surrounding the compromised Fukushima nuclear power plant (www.groundfreezing.net/projects/ground-freezing-fukushima). The available methods to obtain AGF may vary significantly in their sustainability and complexity, in terms of times and costs required to install and maintain them. The ability to predict the effects induced by AGF on granular materials is therefore crucial to assess construction time and cost, and to optimize the method. This project aims to provide a deeper understanding of the process, its role as “temporary works” and the development of a constitutive model where the governing equations have been developed from fundamental physical requirements. This offer a powerful tool to predict a wide range of engineering soil frozen/unfrozen properties within the context of a single analytical framework.
Applications of findings from this project span a number of sectors in engineering from the analyses of frost heave to the effect of freezing-thawing cycles in cold region. Global climate change in cryogenic regions has dominated the research agenda recently, as investigators seek ways of identifying the hazards to infrastructure in cold regions to establish distinct uncertainties through a risk based consideration of sensitivity and consequences and thereby mitigate the risk of permafrost degradation.