MODELLING OF TUNNEL BEHAVIOUR UNDER STATIC LOAD

Abstract

The continuous development of cities caused a deficiency of land in metro regions which arises various transportation problems. The construction of underground structures is extremely effective in solving these problems, as they have solved various traffic issues in significant urban areas. In the transportation system, underground structures play a vital part. Various civil structures essentially depend on underground frameworks in metro areas, where there is an absence of ground space for moving vehicles. Underground structures are utilized for different purposes in civil engineering. The most remarkable of these structural design uses is the utilization of the underground framework of the metro tunnels. Therefore in the improvement of traffic problems, the development of underground constructions plays a vital role. Tunnel deformation mainly depends upon various types of loading, like static loading, dynamic loading or impact loading. It is hence very important to keep these underground structures safe from the burdens of following up on them in field conditions. When a tunnel is constructed in weak rock, the primary motive should be to keep it protected from the stresses that act on it. The main objective of the present study is to evaluate the extent of deformation experienced in single and Twin tunnels under the effect of static loading. Numerous investigations have been done in the past to determine the deformation of tunnels under static loading conditions using numerical and analytical solutions. Whereas limited experimental study has been done to determine the deformation of single and twin tunnels. Due to a variety of adverse conditions, performing the in-situ tests in the field is extremely difficult. As a result, an advanced laboratory compressing testing facility is essential for determining the degree of deformation that occurs in tunnels. For the safe and cost-effective construction of underground openings in rocks, accurate estimation of deformation behaviour is very essential. vi Hence there is a need to design and fabricate an equipment where these difficulties and limitations can easily be overcome. As a result, there is a need to build a compression testing device that can easily control such difficulties and limitations. A digital compression testing machine is designed and fabricated in the present study that can perform experimental tests on physical rock tunnel models. This automated compression testing unit calculates the deformation behaviour of single and twin tunnels under static loading conditions. In the present study, the extent of deformation is measured with the help of experimental investigation and numerical analysis. For the experimental investigation, the selection of the model material is done according to the feasibility and easy accessibility of the material. For single tunnel models, Plaster of Paris, Sand and Kaolinite clay are mixed in different proportions. Three types of model materials are selected according to their strength characteristics. The basic engineering properties of all three model materials are determined in the laboratory. In the single tunnel samples, the cover depth of the tunnel is varied as 3cm and 5cm. Lined and unlined samples of single tunnels are prepared in the laboratory. For lined tunnels, PVC pipe is used as the liner material. After the preparation of the samples, the tunnel models are subjected to static loading conditions in a compression testing machine. Then the deformation is measured with the help of LVDTs which are fixed in the tunnel sample from the bottom side. The reading obtained from the LVDTs is recorded in the CPU data. After the testing of single tunnel samples, Twin tunnel samples are also casted in the laboratory. For the Twin tunnel sample, the model material is kept the same for all the models. All the samples of Twin tunnels are prepared with the help of Plaster of Paris by adding the prescribed amount of water into it. In Twin Tunnel samples, the spacing between the tunnels is varied as 1.5D, 2D and 2.5D where “D” is denoted as the diameter of the tunnel. The diameter of the vii tunnel is kept the same in both single and Twin Tunnels sample i.e. 5cm. The cover depth of the tunnel in the Twin Tunnel sample is varied as 3cm and 5cm similar to the case of a single tunnel model. After the preparation of Twin Tunnel samples, these samples are subjected to static loading conditions in a compression testing machine after fixing LVDTs in the sample. Then the load is applied with the help of a compression testing machine on the Twin Tunnel sample and the reading is recorded. The present study is basically conducted in two steps. Firstly, the deformation behaviour of small scale tunnels model is determined in the laboratory with the help of experimental investigation and then the results obtained from experimental modelling are validated with numerical modelling with the help of ANSYS software. For single tunnel numerical models, the cover depth of the models is varied as 3cm and 5cm. Similarly, for the Twin tunnel sample, the spacing between the tunnels is varied as 1.5D, 2D and 2.5D same as in the case of experimental investigation. Then the meshing and boundary conditions are provided to the tunnel model and subjected to static loading. The results obtained from numerical modelling are recorded. After the experimental and numerical analysis of single and Twin tunnels, the results are then compared with each other for validation. From the results it can be concluded that both experimental and numerical modelling results are in close agreement with each other. From the study, it can also be concluded that there are various factors such as cover depth of tunnels, strength properties of rock, spacing between the tunnels and presence of liner material which affect the deformation of single and Twin tunnels under the effect of static loading conditions. Hence, it can be concluded from the present study, the tunnel structure can remain safe if the design parameters are well selected.