Complex drill+blast challenges in Norway 21 July 2015

Delicate drill+blast operations on the Eiganes E39 traffic tunnel in Norway are proceeding as the twin tube alignment passes just 300mm from the base slab of an existing road tunnel above.

The work follows earlier modifications to the 638m long Byhaug Tunnel in Stavanger at its offset-from-perpendicular intersection with the new alignment below. These complex works involved installation of two protective roof slabs atop a pair of “bridges” consisting of piles sunk into the rock mass either side of each of the new tunnel’s headings.

Construction of the bridge deck in the Byhaug Tunnel

The roof/base slabs, installed in advance of the new headings reaching the intersection, afford a measure of protection that is now enabling careful drill+blast work on the lower tunnel to proceed.

The new 3.7km long twin tubes of the Eiganes Tunnel have large, T9.5 profiles, which gives them excavated heights and base widths of approximately 7.43m and 10.5m, respectively.

The tunnels follow a straight alignment from a portal in the northwest of the city, before dropping towards – but not passing sufficiently deep below – an existing loop of the Byhaug Tunnel (Byhaugtunnelen), a single tube tunnel with T8.5 profile.

To overcome the clash the Norwegian roads authority, Statens Vegvesen (NPRA) worked with consultant Norconsult to develop a “bridge” solution. The design called for two sections of the Byhaug Tunnel floor to be excavated where it crossed the future paths of the Eiganes tubes. Concrete slab decks would then be installed insitu, in readiness for the blasting below.

The bridge decks were completed last year by the Implenia/Stangeland JV. Over recent weeks the JV has been gradually blasting below the bridge decks that keep the new and old tunnels safely apart.

Gunnar Eiterjord, Project Manager for Statens Vegvesen, told TunnelTalk the gap between the Byhaug Tunnel’s bridge slab and the theoretical top of the southbound Eiganes Tunnel is little more than 300mm.

“The theoretically expected blast profile intersects with the bridge construction, which has required careful blasting when entering the junction zone,” explained Eiterjord.

Crossing alignments of the Eiganes and Byhaug tunnels

The new tunnels pass diagonally below the Byhaug Tunnel, creating skewed junctions. The layout requires the 1.2m thick concrete bridge decks to have rhombus-type geometry in the floor of the existing tunnel. The sides of the “bridges” are approximately 21m long to ensure each passes with sufficient clearance over its respective Eiganes tube and also have space for foundations to be anchored deep into the rock mass. The widths of the bridges are approximately 18m, parallel to the axes of the Eiganes tubes.

To facilitate construction of the bridge deck the Byhaug Tunnel was closed to all traffic for 10 weeks from June 2014. Two pits, each up to 3m deep, were excavated in the Byhaug Tunnel floor, though these were not deep enough to open up the full depth of the Eiganes profile below.

At the ends of each pit the bridge deck design called for installation of foundations of steel piles sunk deep into the rock mass. The string of piles were tied into position with rock anchors. The tops of the piles were then encased in two-part, insitu cast concrete beams to act as support abutments for the bridge decks.

Eiterjord told TunnelTalk that after the foundations were completed, the contractor backfilled the excavated pits, then cast the bridge decks. Once cast, the two “bridges” were covered as the road was resurfaced and the tunnel walls relined to enable the Byhaug Tunnel to reopen for normal service in August 2014.

Traffic stopped in Byhaug Tunnel for Eiganes drill+blast rounds immediately below

This year, when the Eiganes faces approached and were preparing to pass below the Byhaug Tunnel, the task was to remove the gravel infill and the bench of rock below each deck. The contractor adjusted their drill+blast method, which has involved dividing the profile excavation in the bench area into an excavation plan involving three separate blasts, and using reduced charge hole lengths. During each blasting round the Byhaug Tunnel above is closed to traffic for 10–15 minutes.

Eiterjord said: “We’re finishing the work beneath both bridges almost at the same time. We’re almost finished excavating at this location – just a few metres remain. Careful blasting will be continued until we’ve reached some distance from the junction.”

The E39 Eiganes Tunnel is part of the major road network improvements in southwest Norway, focused on Stavanger. Challenges here include relatively shallow cover in the urban setting. Overall, excavation is still at an early stage but the Eiganes project is due for completion by late 2019, said Eiterjord.

In addition to Eiganes, Statens Vegvesen’s other road tunnel projects under construction in the area include the Ryfast scheme – focused on the 14.3km long Ryfylke project, which will be the country’s longest undersea road tunnel crossing. Ryfast also includes the 5.5km long Hundvåg Tunnel, which links the Rv13 road into the E39 Eiganes.