Face stabilizing method for shallow tunnels in loose sandy ground

Abstract

The overburden, stiffness of the ground, underground water level and the size of a tunnel are influential factors for tunnel face stability. The Ushikagi double-track tunnel was constructed by the shotcrete tunneling method, or the so-call New Austrian Tunneling Method (NATM) on the Touhoku Shinkansen line (between Hachinohe and Shin-Aomori). The tunnel is 2,070 m long drilled in a loose sandy ground at a high groundwater level. The overburden of the tunnel is 2-10 m thick throughout the whole section. For this reason, we worried about the face stability of the tunnel and carried out soil stabilization test as a means to keep the face stability. The soil near the crown was improved by a mixing method from the ground surface for 90 m from the entrance of the tunnel. When the tunnel was bored after the ground improvement, we measured some convergences, stresses of the primary lining, displacement and earth pressure of the ground, etc. to investigate the effect of the measure on the face stability. We evaluated the face stability with and without the measure applied by the elastic FEA, based on the plastic theory (so-call rigid-plastic FEA or RPFEA), such as the one to calculate the balance of the mass in front of a face. (1) This paper describes an outline of the construction, results of measurements and analyses, and a study on the mechanism of stabilization. Following things are contained: The ground improvement for a loose sandy ground. (2) The results of some convergences, displacement and earth pressure of the ground. (3) Two-dimensional finite element method (elastic analysis). (4) Two-dimensional rigid plastic finite element method (ultimate analysis). (5) About the stabilization based on the results of measurements and evaluation. (A). Reprinted with permission from Elsevier. For the covering abstract see ITRD E124500.