Abstract:

In populated alpine regions infrastructure projects often compete with nature and the general public for open space or other natural resources. The project Wafer dab located near Sarans, Switzerland is a ground breaking infrastructure project utilizing underground space as a manufacturing facility for sensitive industrial processes requiring strict vibration and temperature controls. The "Wafer dab" is a multiple structure complex with the main offices situated on the surface connected to two main caverns, which serve as the production and storage facilities, by two access galleries with a length of about 100 m. Due to safety reason a shallow placement of the cavern was envisioned. During the cavern construction, steep open joints with apertures of more than 30 to 100 cm were associated with the main fracture set, while bedding parallel discontinuities with apertures up to 20 cm were locally encountered. The encountered ground characteristics, probably governed by the local tectonic and morphological conditions, caused large gravitational over breaks especially in the front of the production cavern.

1 INTRODUCTION

Wafer chip fabrication is a sensitive industrial process constrained by strict vibration and temperature controls. Utilizing underground space as a manufacturing facility provides a stable environment for these two factors. The decision to construct the facility underground was finally based on the economic advantage associated with decreased installation costs associated with the vibration sensitive equipment required for silicon wafer chip production [1]. Several locations near Sarans, AG were investigated in terms of the geological and seismic conditions, as well as for influences from far field noise and vibrations. The area in the vicinity of Sarans is geologically well known from historical iron mining in the Gone massive, from ongoing subsurface mining facilities for cement and gravel production, and the Gager bach Test Gallery, a 5 km underground research laboratory. Tunnelling and mining experiences gained from the known excavations reaffirmed good tunnelling conditions for the Quintet limestone formation. The cavern structures, ventilation, and associated access galleries were kept to a minimum length for safety purposes, and to limit local transport time and costs for staff and raw materials. The location of the facility, owned by ES PROS Phonics AG is on the western valley flank of the St. Galen Rhine valley at the foot of the Gone massif (Figure 1) Several other underground structures are located within the Quintin formation, a fine-grained to compact, thick-bedded limestone. The dimensions of the main production cavern are 100m*18m*18m (lash) while the "Lido cavern" (56m*18m*10m) is situated perpendicular to the main production cavern. Figure 2 shows a sketch of the cavern arrangement.

2 GEOTECHNICAL DESIGN OF THE UNDERGROUND STRUCTURE
2.1 Geological investigation

A geological investigation program was carried out with the emphasis to identify the transition from the valley sediments and in place rock in the access galleries and investigate the geological conditions and rock mass properties within the caverns. The geological investigation programme consisted of field mapping and 2 cored boreholes with a total length of 90 m. The alignment of each boreholes was situated approximately parallel to each cavern axis.

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