Determination of optimum umbrella arch size by numerical modelling in tunnel excavations

Tunnelling has developed fastly depending demands of fast growing world in the last century. Public transportation with underground railways have become a necessity especially for urban transportation within big cities began to become a big problem. Metro demands of our big cities have been understood and works has been accelerated in our country in recent years.One of the big problems of metro tunnel constructions except distinctive problems of tunnel engineering is, tunnelling under urbanized area. The safety of the buildings above the tunnel route are incredibly important. Generally metro tunnels are shallow tunnels so buildings over the tunnel can be easily affected by ground settlements. Therefore tunnelling philosophies which allow settlements are not available for this kind of tunnels.Generally, works get difficult especially in weak grounds and making excavation becomes impossible. Ground improvement methods are developed for conditions like ground can?t support itself or tunnel face can?t be stabilized. Umbrella Arch Method is a ground improvement method developed for tunnels. In this method, hollow steel pipes with a certain length are embedded through the tunnel direction around the tunnel face. An artificial shell is created around the tunnel by pumping injection from inside of these pipes. This method mostly prevents ground settlements.However safety with least cost is very important in this application as in all engineering applications. The optimization of the pipes? sizes is extremely important because it directly affects the quantity of steels used and choice of machine for drilling and embedding both.The objective of this research is, choosing the optimum steel pipe diameters by numerical modelling to prevent face and surface deformations. Thus, safety can be ensured with minimum cost. PLAXIS 3D Tunnel Version is used for Finite Element Model in this research.