Parallel Thimble Shoal Tunnel, Virginia, US

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The Parallel Thimble Shoal Tunnel is expected to become operational in 2022. Credit: CBBT. The new two-lane tunnel will be built under the Thimble Shoal Channel. Credit: CBBT. The project will require approximately 42,000yd³ of concrete. Credit: CBBT. The Parallel Thimble Shoal Tunnel is a new bored tunnel under construction to connect two southbound lanes of the existing 28.3km-long Chesapeake Bay Bridge-Tunnel in the US. It is being constructed parallel to the existing Thimble Shoal Channel Tunnel, which was opened in April 1964. It will connect Norfolk/Virginia Beach areas to Virginia’s eastern shore. The Chesapeake Bay Bridge and Tunnel District (CBBT District) is responsible to manage the project. Expected to open for traffic in 2022, the new tunnel will provide two lanes for southbound traffic. It is expected to reduce the delays caused by lane closures due to accidents, disabled vehicles or other interruptions. The existing tunnel will carry two lanes of northbound traffic. The CBBT Commission approved the construction of the Parallel Thimble Shoal Tunnel in May 2013. A design-build contracting model has been preferred to reduce the project cost and delivery time. The project involves the construction of a 5,700ft bored tunnel with an outer diameter of 42ft. The maximum depth of the tunnel will be 134ft below the surface. Bored tunnel method was preferred for the tunnel construction in order to enable the passage of commercial and military ships through the Thimble Shoal Channel. An estimated 500,000yd³ of soil will be removed using a tunnel boring machine (TBM). The total amount of concrete required for making the tunnel sections is approximately 42,000yd³. The tunnel will be made of roughly 9,000 individual precast concrete pieces. The precast segments are made in a ground-based factory and will be transported to the site. The segments will be delivered to the rotating arm of the TBM and will be placed one by one to form a ring. The tunnel construction works started in October 2017 and are scheduled for completion in October 2022. The TBM used in the construction can be operated from the control room. It features a shield with an attached rotating cutting head and trailing support mechanisms. The machine will create a circular passage under Thimble Shoal Channel. The excavated material will be removed using a conveyor belt system. Hydraulic rams will push the TBM forward, while pressure will be applied to avoid uncontrolled penetration of soil and water. The $1.074bn project is being financed through a combination of district general fund investments, bond proceeds, and loans from Virginia Transportation Infrastructure Bank (VTIB) and TIFIA. The funding sources include senior bonds of $321.5m, a TIFIA loan of $338.5m, CBBT district contributions of $226.9m, a VTIB loan of $50m and $137.1m from other sources. A $50m loan from the VTIB was authorised for the project by the Commonwealth Transportation Board in December 2015, while the TIFIA signed the credit agreement for the direct loan to fund the project in November 2016. The TIFIA loan will be repaid through the revenue earned from the project’s toll charges. The Chesapeake Bay Bridge and Tunnel Commission awarded the design-build contract to Chesapeake Tunnel Joint Venture for the construction of the tunnel in July 2016. The joint venture (JV) partners include Dragados USA and Schiavone Construction Company. Mott MacDonald is the lead designer for the Parallel Thimble Shoal Tunnel.Herrenknecht was contracted to supply the TBM.

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