Jet Grout Bottom Plug in Deep Excavations : Numerical Analysis of a Tunnel Project

Abstract: This master thesis has investigated the performance of a deep excavation with a jet grout bottom plug used to prevent bottom heave and hydraulic uplift failure. The concept of a jet grout plug as a structural and sealing component in a specific case was studied, namely one of the tunnel sections of The West Link tunnel project in Gothenburg. The section was to be built as a cut-and-cover tunnel with the excavation shaft consisting of secant pile walls, struts and a jet grout bottom plug. It is to be established where there are a lot of sensitive buildings and constructions close to the excavation site. This makes it important not to disturb the surroundings during construction with, for example, settlements.The study was carried out by doing a literature study, analytical calculations and numerical simulations in the finite element software PLAXIS 2D. More specifically, the checks evaluated were uplift equilibrium of the jet grout plug and of the whole structure, structural performance of the jet grouted plug and hydraulic conductivity of the jet grouted mass.The results show a design of a jet grout plug that together with the additional structures works sufficiently enough to make a secure construction for a deep excavation below the ground water table. The structural performance of the jet grouted plug to function as a strut level is fulfilled even if the jet grout is not perfectly performed. To work as a sealing component the jet grout column pattern and the centre-to-centre distance between the columns are important factors together with the alignment and diameter of the jet grout columns. To make a completely watertight construction is however almost impossible in this case since the jet grout plug is located at a large depth (around 20 m). To work properly against failure by uplift it does not need to be completely watertight. Instead, the use of relief wells makes sure that the pore water pressure is not larger than the vertical stress from the soil, otherwise failure by uplift is a real danger. Furthermore, to work properly against settlements it is not only the water tightness of the jet grout plug that decides whether it will work properly. Instead, it is also the pumping and infiltration system together with the, in this case, sensitivity of the lower aquifer that decides the risk of settlements.