|Title of thesis||Viscoplastic behaviour and design of tunnels|
|PhD course||Geotechnical Engineering of Politecnico di Torino|
|Adviser||Prof. Giovanni Barla|
|Date||May 20, 2008|
In the framework of Geotechnical Engineering and Rock Mechanics many tunnels are known where, even during construction, large deformations and high stresses in the lining are observed.
This is often the result of squeezing behaviour. Tunnel construction in such a condition is very demanding and difficulties are met in making reliable predictions at the design stage. The selection of the most appropriate excavation-construction method to be adopted (i.e. mechanized tunnelling versus conventional tunnelling) is highly problematic and uncertain.
The present thesis is to contribute to the understanding of the squeezing behaviour of tunnels with a major interest on the time dependent response. The research is focused on the experimental investigation of time dependent characteristics of weak rock at the laboratory scale and on the formulation of a new constitutive model (SHELVIP) to be used in design practice. The considered case of study is the Saint Martin La Porte access adit, along the Torino-Lyon Base Tunnel, which experienced very important squeezing problems during excavation.
Following the introduction of the various time dependent phenomena that are known to exist for soils/rocks and that are unanimously accepted by the geotechnical community, the thesis examines the constitutive models that have been proposed so far to capture the time dependent behaviour. The bibliographic study highlights that only few models can reproduce satisfactorily all the features of the time dependent behaviour of weak rocks involved in tunnel excavation, with a simple formulation to be used in design practice. This observation led to the study of the peculiar characteristics of weak rocks and to the formulation of a novel viscoplastic constitutive model.
The availability of rock samples obtained from the Saint Martin La Porte tunnel and the peculiar characteristics of the material has determined the choice of coal as the rock material of interest for the present study. An experimental program has been performed on this material, involving the determination of the physical properties and mineralogical composition, in conjunction with oedometer tests, direct shear tests and triaxial tests, performed by using two advanced laboratory equipments: the High Pressure Triaxial Apparatus (HPTA) and the High Pressure Back Pressure Shear Apparatus (HPBPSA). Particular attention has been posed on the deformability, strength characteristics, and time dependent behaviour.
The results obtained highlight that most of the time dependent characteristics of coal follow in line the experimental evidences reported in literature. Only few aspects differ significantly. These observations form the background for the formulation of the new constitutive model proposed.
The SHELVIP (Stress Hardening ELastic VIscous Plastic) model, a new viscoplastic constitutive law, has been developed to incorporate the most important features of behaviour observed in tunnels excavated in severe to very severe squeezing conditions in a simple, but complete, manner. This model couples the elastoplastic and time dependent behaviour by using a plastic yield surface, as frequently adopted in tunnel design analysis, and the definition of a state of overstress referred to a viscoplastic yield surface. The model has been formulated in all its detailed aspects. The related analytical closed-form solution for representing triaxial creep deformations has been developed. By observing the behaviour of the model with reference to classical time dependent tests, is should be noted that the SHELVIP model can reproduce almost all the important aspects of time dependency of soils/rocks. Finally, the new model has been implemented into the finite difference code FLAC, in order to allow to perform numerical analyses of geotechnical problems. The SHELVIP model has been calibrated by using the laboratory tests performed on coal specimens. The model is shown to fit very satisfactorily the experimental results of creep and stress relaxation triaxial tests.
In the final part of the thesis a series of numerical analyses, which have been carried out using the newly developed SHELVIP model on a representative section of the Saint Martin La Porte tunnel with the intent to evaluate the ability of the model to describe the squeezing conditions with reference to a real case study, are described. If the results of the numerical analyses are compared with the monitoring convergence data, radial and longitudinal displacements, it is possible to state that the agreement of the numerical results with the mean monitored data is excellent, notwithstanding the scattering of the monitoring data due to the heterogeneity and anisotropy of the rock mass. Also the results of stress distribution around the tunnel perimeter are satisfactory and reliable. It is possible to conclude that the SHELVIP model can be used with confidence in order to reproduce by numerical analysis the behaviour of tunnels under severe squeezing conditions.
Further developments are needed and some open questions remain to be addressed for the study of the time dependent behaviour of weak rocks in relation to tunnel excavation, in particular for the assessment of the stability conditions of both the face and the heading, the timely installation of the tunnel support, and the excavation method to be adopted.