Face stabilization methods for excavating unconsolidated soil with high groundwater level in the Sambongihara Tunnel

Abstract

The Sambongihara Tunnel of the Tohoku Shinkansen is 4280 m long NATM tunnel under a tableland. The cover is 23 m on average, and 45 m at maximum. Geological features around the tunnel are sand, volcanic ashy silt and clay. Problems to excavate this tunnel are high groundwater level and an unconsolidated layer of poorly grained fine sand, so various pre-support systems and additional methods were adopted. The excavation method was short bench cut by machine. To lower the groundwater level, which was at around 25 m above the tunnel crown where ground around the tunnel was unconsolidated, the deep well method was adopted. Wells were arranged every 20 m alternately on either side, at a distance of 10 m from the tunnel centerline. However, since there was an impermeable clay layer in the upper section, it was impossible by the deep wells to draw off the groundwater completely between the clay layer and an upper layer of sand. Therefore, well point drainage and horizontal drain boring were adopted additionally. At the beginning, the ground around the tunnel crown was clay and forepoling was adopted. Then a sand layer, of low uniformity coefficient around the crown, and forepoling was replaced by AGF injecting urethane series chemical, since it was difficult to hold the face and crown. However, groundwater inflowed from the tunnel face drawing soil grain so that the face was partly washed out, and relatively dry sand avalanched from the crown between the forepoles. The cause of these phenomena was thought to be that the ground was loosened by the inflow of groundwater, drawing fine grain even as groundwater head had been lowered by the deep wells, and that sand layer of poor fine grain content, whose water content had been lowered by the deep wells, collapsed while loosened zone around the face became larger. To meet this situation, such as groundwater remaining on impermeable layer and easily avalanching sand layer of poor fine grain content, two measures were taken. One was inserting a drain material made of compound fiber of polypropylene and polyethylene into each well point and horizontal drain boring, to prevent soil grain from being drawn by inflowing groundwater. The other was adopting AGF injecting sodium silicate series chemical to strengthen cohesion, since shear strength of sand soil is weak in case with groundwater. According to experience, target for ground improvement to excavate without major trouble is 1 N/mm2 measured by Yamanaka's method for soil hardness. These additional methods and execution management made it possible to hold the face and crown, and to excavate the tunnel without troubles. (A). Reprinted with permission from Elsevier. For the covering abstract see ITRD E124500.