The Haruna and the Nakayama tunnels on the Joetsu Shinkansen Line linking Tokyo and Niigata pass through one of the typical volcanic range actived in the Quaternary period, therefore they have had to struggle for their work against collapse of face due to extremely loose volcanic products saturated with water which has high pressure head.
To cope with the geology, several auxiliary measures have been adopted such as chemical grouting, pneumatic and dewatering method. Mentioning about dewatering method, impermeability and holding of much water required long term of lowering the table of groundwater, and moreover caused collapse at face. Therefore another dewatering method which remove water from face without lowering the water table was in urgent need. t Based on studies including laboratory best, so called Vacuumed Drain Method have been adopted and excavation have carried out rapidly.
This article is to be reported the background of introduction of the method and out line of the excavation work with it at the Haruna and Nakayama tunnels. Additionally the limit conditions for the method are to be mentioned.
The Haruna tunnel is a 15.35 km in length the running through under 2 to 150 m from the ground at the gentle skirts of a volcano, Mt. Haruna. To shorten the construction period, it was divided into five sections with two vertical and four inclined shafts. Excavation commenced in 1972 and have already completed in last October.
This tunnel is geologically featured by the volcanic products of Mt. Haruna actived in the Quaternary period. The geology intersected by this tunnel was classified by activity period into the old and new volcanic mud flow strata. The former are generally composed of semi-solidified or solidified sand and gravel, and have capacity to hold cutting face self standing despite existence of groundwater. The latter which encountered at Kaneko section consist of unsolidified andesite breccias gravels and pumice with volcanic sand and ash as matrix. The Jimba strata which have almost similar character stratified over the new mud flow strata.
The feature of these strata are described as below.
Coefficient of permeability is about 10−3 to 10−4 cm/sec.
Uniformity coefficient is about 6 at new mud flow strata and 40 to 90 at Jimba strata. They are sandy and unsolidified and moreover saturated.
They are easily fractured and cause sand flow with relatively little amount of seepage water.
(Figure in full paper)
The Ashikado vertical shaft provided for the Kaneko section is 9 m in diameter and 32.5 m in depth and took one year to be excavated including additional service facilities at the bottom.
For its excavation, deep well, well point and chemical grouting have been adopted as auxiliary method. However, these dewatering method could not stabilize the face, and then it was considered that these methods were not suitable to the above mentioned geology.