All 11 elements that comprise the new US$1.47 billion Midtown tunnel between Norfolk and Portsmouth in Virginia, USA, are layed and the bulkhead connections made between them.

Demolition of the bulkhead between Elements 10 and 11

The design-build JV of Skanska/Kiewit/Weeks Marine (SKW), which is delivering the Parsons Brinckerhoff-designed project as part of a fixed price contract with the concessionaire Elizabeth River Tunnels, is currently nearing completion of the joint locking process. Work is currently under way on the final joint between Elements 10 and 11. The temporary bulkheads between them have been removed, opening the tunnel up to the final bulkhead at the Norfolk end.

The final Omega seal – the secondary seal at each joint – is due to be installed in this joint this month (October). The final bulkhead at the Norfolk end of Element 11 will not be removed until the closure section is complete. Once it is removed the 1,700m underwater connection between the Norfolk and Portsmouth sides will be complete.

Remaining work inside the tunnel includes installation of fire protection boards on the roofs and walls, completing ballast concrete pours, topping slabs in the egress corridors and roadways and installation of permanent electrical and mechanical equipment.

Work is also under way in the evacuation corridor side of the tunnel, which also includes space that holds the electrical conduits and panels, communications equipment, fire protection systems and tunnel drainage equipment and piping.

Electrical contractors are installing conduits and panel boxes in the egress corridor while other contractors erect the walls for the emergency exit egress corridor.

The final, and critical part of the new Midtown Tunnel, the tunnel closure section, is nearing completion. The concrete invert has been poured and the wall and the roof sections are almost finished. The next critical part of the tunnel’s construction is the installation of the tide gate in the Norfolk cut-and-cover section.

New tide gate measures 24ft x 34ft

Weighing 16.5 tonne, and measuring 24ft high x 34ft wide, the new tidegate was delivered to the jobsite yesterday (Wednesday 21 October) from Oregon, and its arrival coincides with an annual hurricane season that often leads to tide surges and subsequent flooding.

The final bulkhead between Element 11 and the Norfolk shoreline has been deliberately kept in place to protect the new tunnel against potential tidal surges while the tide gate was being fabricated.

“The arrival of the new tide gate signifies significant progress on the new Midtown Tunnel construction,” said Gary Webb, a project manager for the Midtown Tunnel construction work. “The gate is just one of many new safety measures that will make the Midtown Tunnel safer and more accessible for drivers.”

Design of the new tunnel provides for enhanced emergency response and evacuation readiness, and features a higher elevation than the existing Midtown tunnel so as to take into account an expected future rise in sea levels, thereby making it less prone to flooding.

A section of the tunnel support building will house the gate and lower it into place in a slot inside the cut-and-cover section, as needed.

Meanwhile, the project team continues the backfill phase and in the next few months they will add the final fill layers on top of each of the 11 elements. The final layer, comprised of armor stone, will be added over the elements that fall within the Federal Channel to protect the tunnel from ship anchors.

Construction of the second immersed Midtown Tunnel between Norfolk and Portsmouth in Virginia, USA, passes the half-way point following the sinking of Element 7 of 11.

Doka formwork used for element construction

Fabrication of the second batch of 16,000 tonne elements in dry dock at Sparrows Point, Maryland, was completed late in March (2015). Formwork was supplied by Doka of Austria.

After an eight-hour operation to flood the dry dock Elements 7-11 were barged, one by one, the 200-mile journey down Chesapeake Bay to the Portsmouth project site. The sinking of Element 7 last month (April) completes placement work within the Federal Channel. The remaining elements are expected to be immersed at the rate of one every five weeks, with the final element scheduled to be moved into position in September-October.

The construction JV of Skanska/Kiewit/Weeks Marine team continues to outfit the interiors of each element as they are placed, working from the Portsmouth side of the Elizabeth River. One of the critical parts is the immersion joint, the connection between the elements. This requires several steps to complete the connection after the element is immersed and the joint is dewatered.

First, Omega seals (the secondary seal at each joint) must be placed. Omega seals have been installed and tested in each of the first five immersion joints. The second step is the removal of the bulkheads, the waterproof barriers at the end of each tunnel element that enable the structure to float and be towed into position.

Elements house two-lane roadway and escape chamber

So far 10 bulkheads at the end of the 46ft wide x 33ft high sections have been removed between the cut-and-cover approach and through Elements 1-5. Next, cover plates must be installed over the seals to protect them from the concrete that will be poured into each immersion joint. The final step includes the placement of structural steel and concrete in the void between the elements. The first five joints are in various stages of this work. Work on the walls and roof will follow in the process.

In the coming months, crews will continue placing and outfitting the elements through to the Norfolk side, install fire protection boards on the roofs and walls, complete ballast concrete pours, and install permanent electrical and mechanical equipment.

Meanwhile in Norfolk construction continues on the support walls for the tunnel control building atop the cut-and-cover approach section. In the slot, walls for the boat section and one additional invert are complete. Construction continues on two storm water pump stations. The first station will collect rainwater from the tunnel approach roadway, and the second station will collect drainage and prevent tidal and storm actions from flooding the tunnel approach area.

Trelleborg’s engineered products operation has broken into the American tunnel market with its range of high performance seals, for the construction of the second Midtown Tunnel between Portsmouth and Norfolk, in Virginia, USA.

Gina and Omega gasket sealing system

The immersed tunnel consists of 11 rectangular concrete tunnel elements of 48m circumference, each of which has been fitted with Trelleborg’s Gina and Omega gaskets to prevent water ingress. Trelleborg was specified by designer Parsons Brinckerhoff and the SKW construction joint venture due to the high stability, flexibility and low aging behavior of its seals.

Andre de Graaf, Sales Manager for Trelleborg’s engineered products, said: “Immersed tunnels are on the increase, especially in the US where investment for infrastructure is growing. Our immersed tunnel seals are renowned and used globally on major tunnel projects. We’ve had considerable success in Europe and China and were very pleased to be specified in this US project.”

Trelleborg’s Gina gasket is made from natural rubber combined from varying hardness and is supplied as a closed rectangular frame to seal each sectional element. The Omega seal connects each segment together and is made from multiple layers of styrene butadiene rubber and Nylon inlayers to improve durability. The combination of the two seals enables the transfer of hydrostatic loads and movements between the tunnel ends, caused by soil settlement, creep of concrete, temperature effects and earthquakes.