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Section 3: Highway construction |
1 James Williamson and Partners (Mott MacDonald Group), Melrose House, 15–23 Cadogan Street, Glasgow G2 6NW, UK
2 Tayside Regional Council, Department of Roads and Transport, Tayside House, Crichton Street, Dundee DD1 3RE, UK
Stage XII of Dundee Inner Ring Road runs through a built-up part of the city of Dundee and includes a 145 m long cut and cover tunnel and a 355 m length of road in vertical open cut with retaining walls. Excavations extended 14 m below the level of adjacent ground. During construction, disturbance to nearby buildings and services had to be minimized; close attention was paid to the stability of the excavations and the selection of excavation techniques. This paper presents an outline of the site geology, describes the approach to geotechnical investigations and highlights certain aspects of design and construction, particularly the stabilization of the sidewalls and the role of detailed geotechnical mapping.
The route runs within a Devonian age sandstone-mudstone-shale-basic lava sequence dipping at 20 to 30 degrees. Historically, stability problems have occurred in these formations in the Dundee area. Site investigations included inclined coring to clarify the disposition of faults, exploratory trenching, cross-hole seismic surveys and rock anchor trials. Particular attention was paid to elucidating the discontinuity characteristics with reference to the presence of low shear strength planes. Rock classification techniques were used to assist in excavatibility studies and to provide guidelines for rock reinforcement selection.
Excavations were mapped in detail during construction, and supplementary rock testing was carried out. Updated interpretations were prepared, enabling the design for the temporary and permanent works to be kept under review and adjusted as work progressed.
