Earth pressure balance shield driven tunneling for super-shallow earth covering (Jobando Tunnel)

Abstract

The Jobando Tunnel is located midway between Miraidaira Station and Midorino Station on the Tsukuba Express Railway that runs between Akihabara and Tsukuba. This tunnel crosses the Joban Expressway between Yawara and Yatabe Interchanges at a crossing angle of about 21.5 degree on a level surface. Parallel single tracks were constructed by means of U-turn shield-driven tunneling with the outer diameter of concrete segments of 7.3 m. Tunnel length is 303 m with 2 single tracks. The separation between 2 single tracks is 2 m in length. The soil conditions encountered during tunneling are as follows. Most of the tunnel passes through fine sand whose N-value is less than 10 plus ground water (Narita sand layer: Ds3). Soil at the starting shaft area is mixed with tuff clay (tc) and clay (Dc3) at the arriving shaft. Earth covering conditions: in the portion passing under the Joban Expressway, covering height is 7.1 m, about 1D (D, outer diameter of shield machine) but some other portions are only around 2 or 3 m. An embankment was therefore constructed to secure a covering height of 0.6D. Prior to shield-driven tunneling, there was concern about possible subsidence of the road surface of the Joban Expressway and interference with traffic flow due to the shallowness of the earth covering of the tunnel. To monitor subsidence of the road surface, a total of six automatic 24-h survey system detectors were installed on survey towers on the slope of the Joban Expressway embankment to detect any subsidence of the road surface above the tunnel crossing area. Concerning ground behavior, sensors to detect settlement were installed in a line 2 m above the tunnel at 5 m intervals. Their output was fed directly back to allow adjustments to be made in real time to the shield-driven tunneling process. During a trial shield-driven tunneling carried out prior to crossing the Joban Expressway, the results showed that four control items for shield-driven tunneling (facing earth pressure, volume of addition to excavated materials, discharging volume and backfill for the tail void) markedly affect ground settlement. After the experiment, these control items were reset to take ground settlement into consideration. While tunneling under the road, these survey control and tunneling control are related and achieved. As a result, the subsidence of road surface was limited to a maximum value of approximately 4 mm. (A). Reprinted with permission from Elsevier. For the covering abstract see ITRD E124500.