HAMPTON ROADS BRIDGE-TUNNEL EXPANSION PROJECT
TUNNELING PROGRESS
An overview of the complex planning, engineering and
construction progress behind the new HRBT twin tunnels.
norfolk
SOUTH ISLAND
NORTH ISLAND
HAMPTON
TUNNEL
New Tunnels
NORTH ISLAND
TBM TURNAROUNDIt took about 6 months to turn the TBM around to begin the second tunnel.
TBM RECEIVING PITThe TBM broke through the receiving pit after completing the first tunnel.
Spring 2022 – Fall 2023
JET GROUTINGThe ground was injected with a stabilization fluid to strengthen the soil for the tunnel approaches.
Winter 2022 – Fall 2023
Winter – Summer 2024
ISLAND EXPANSIONThe island was doubled in size to accommodate the new tunnels.
Fall 2020 – Summer 2021
BACK
NORTH ISLAND
ISLAND EXPANSION
Piece By Piece – How To Build An Island
A five-thousand-ton barge slowly cut through the waters near the City of Hampton, making its way toward the North Island of the Hampton Roads Bridge-Tunnel. On board were six-and-a-half-ton stones from a quarry in Maryland, destined for life in Virginia as part of the HRBT Expansion. As the stones arrived, crews measured them against “witness stones,” making sure they were the correct size and shape for placement. Finally, a grapple grabbed a stone and gently swung it into place before workers keyed and locked it in position like a 20-thousand-ton jigsaw puzzle. And so went the North Island Reclamation (NIR), a critical portion of the larger HRBT Expansion Project. The NIR expanded the island’s previous footprint by about 16 acres and laid the foundation for future elements of the project. The completed NIR provided space for a receiving pit for Mary, the Tunnel Boring Machine (TBM), as well as an approach roadway for the new, twin tunnels.
BACK
NORTH ISLAND
ISLAND EXPANSION
ISLAND EXPANSION
Piece By Piece – How To Build An Island
While it may seem like simple work, the job was extremely complex. Beyond what was visible from the surface, there was a larger rock footprint sloping further underwater, carefully designed to support and protect the island interior. The reclamation process included construction of the bund, or core perimeter layer, as well as an outer armor-stone layer, to protect the structure from the surrounding waves and currents. From here, several layers of sand were compacted to fill the expansion. It was a lot of work, only to then excavate a portion of this newly expanded island to accommodate the TBM receiving pit. Yet building the full island expansion with the precise soil mix and compaction was necessary in order to maintain its structural integrity as the walls of the receiving pit and new tunnel approaches were constructed. Wind and waves could batter or change artificial islands over time, so it was important to make sure each portion of the NIR was executed correctly, from design to materials. This will help Keep Virginia Moving for at least the next 100 years.
BACK
NORTH ISLAND
ISLAND EXPANSION
TBM Receiving Pit
Crews constructed a receiving pit on the North Island. Just as the name implies, this pit was created to “receive” Mary, the TBM, when she completed her journey from the South Island. At one-third the size of the TBM launch pit on the South Island, the TBM receiving pit was just one circle. Much like the process used for the TBM launch pit, slurry walls formed the perimeter of a circular receiving pit. After a capping beam was installed to stabilize the structure, crews excavated to a depth of 75 feet. A concrete slab then supported Mary's weight and formed the base for future roadway. When Mary “broke through” into the receiving pit, she had completed the first tunnel. In the receiving pit, Mary was turned around and begun her trek back to the South Island as she bore the second tunnel.
BACK
NORTH ISLAND
ISLAND EXPANSION
TBM Turnaround
The TBM broke through on the North Island and was then partially disassembled and placed on a turntable, section by section, to be turned around and start boring the second tunnel.
BACK
NORTH ISLAND
ISLAND EXPANSION
JET GROUTING
Jet grouting, or pressure grouting, is a method of soil stabilization that involves the high-pressure injection of a stabilizing fluid into the subsoil to stabilize ground conditions within the project site. Jet grouting was used to reinforce the surface, ensuring it was stable enough to withstand the weight and size of Mary and to facilitate the steering of the TBM.
TUNNEL
TUNNEL SEGMENTSReinforced pre-cast concrete tunnel segments were manufactured in Cape Charles and barged to the South Island for installation by the TBM.
Spring 2021 – Ongoing
TUNNEL BORINGThe TBM launched for the South Island excavating and building the first tunnel to the North Island, then turn around and created the second tunnel.
TUNNEL BORING MACHINE (TBM)Mary the TBM was manufactured in Germany and shipped to the US.
Fall 2020 – Fall 2021
Winter 2023 – Summer 2025
BACK
ISLAND EXPANSION
TBM (Tunnel boring machine)
TUNNEL
The Tunnel Boring Machine (TBM) was used to bore new twin tunnels next to the existing Hampton Roads Bridge-Tunnel. Hampton Roads Connector Partners (HRCP), the project's joint construction venture, awarded a contract to Herrenknecht of Germany for the design and construction of the TBM. It took approximately 14 months to build the 46-feet-tall machine, about four months to ship it from Germany, and another four to five months to assemble the TBM on the HRBT's South Island.
The TBM was longer than a football field. As you can see from the picture of the TBM's cutterhead (pictured to the right), the machine stood about the same height as the HRBT Project Office, a four-story building. The price tag for the TBM alone was $101 million, which included construction, shipping, and assembly costs.
BACK
ISLAND EXPANSION
TBM (Tunnel boring machine)
TUNNEL
The TBM launched from the South Island (Norfolk side), and bored at a rate of about 50 feet per day until it reached the layer of soil known as the Yorktown layer, approximately 50 feet below the current tunnels. The TBM excavated the tunnels with a circular cross section through the soil. The process for both tunnels took more than two years. Built specifically for the HRBT Expansion Project, the TBM served as a central component of the project.
BACK
TUNNEL SEGMENTS
ISLAND EXPANSION
TUNNEL
The existing ten tunnels in Hampton Roads are immersed cylinders that sink into holes in the waterway bottom. Advances in tunnel technology made the bored-tunnel approach possible for the expansion project. The method had fewer environmental impacts and did not disrupt Navy, marine, or commercial traffic in the busy federal channel.
BACK
TUNNEL SEGMENTS
ISLAND EXPANSION
TUNNEL
The finished concrete tunnel segments, over 150 at a time, were transported by barge down the Chesapeake Bay to the South Island. Once on the island, the segments were transported into the tunnel using specialized vehicles. The segments were fed onto a conveyor inside the TBM, where a vacuum erector lifted, turned, and fitted each segment into place. As Mary excavated the soil, she placed nine precast concrete segments into a ring that became the walls of the tunnel. Mary advanced by pushing 54 hydraulic thrust jacks against the newly completed ring of precast concrete segments, leaving a perfectly circular tunnel in her wake. Mary placed 21,492 tunnel-lining segments, each weighing 12 tons. She erected each segment one by one to form a total of 2,388 rings composed of nine segments each, leaving a perfectly circular tunnel behind her.
BACK
TUNNEL BORING
ISLAND EXPANSION
TUNNEL
The new twin tunnels were constructed using a massive tunnel-boring machine, or TBM, with a rotating cutting head that excavated soil along the tunnel’s path. As the cutter head moved through the soil, concrete liners were set in place by a rotating vacuum-powered lift, creating the outer shell of the tunnel. A significant advantage of this construction method was its reduced environmental impact on marine wildlife compared to the immersed-tube approach, which would have required dredging a deep, mile-long trench across the Hampton Roads waterway. Because the tunnel-boring machine excavated from beneath the riverbed, this technology essentially eliminated disruptions to commercial and military shipping in one of the nation’s most important navigation channels. The new tunnels were constructed approximately 50 feet deeper than the existing tunnels, each measuring about 8,000 feet in length.
BACK
TUNNEL BORING
ISLAND EXPANSION
TUNNEL
The HRBT Expansion Project was VDOT’s first bored tunnel and only the fourth and fifth bored tunnels in the United States. Although the immersed-tube method was used to construct all 10 of Hampton Roads’ existing crossings –from the original Downtown Tunnel in 1952 to the new Midtown Tunnel in 2016 – recent technology advances made bored tunnels feasible in the region’s soft soils. Preliminary construction work began in mid-2020 with a 65-foot pit excavated on the South Island to accommodate the TBM launch. The TBM tunneled toward the North Island (Norfolk to Hampton) completing the first tunnel in roughly one year. It took about four months to turn the TBM around and another year to bore a parallel tunnel back to the South Island.
SOUTH ISLAND
QUAY DOCKA temporary dock was built so that tunnel segments could be unloaded from barges and lowered into the launch pit to the TBM using a gantry crane.
CONVEYOR BELTA conveyor belt system was installed to move excavated soil from the STP to waiting barges for removal to approved disposal sites.
Summer 2021 – Fall 2022
TBM REASSEMBLY170 pieces of Mary the TBM were reassembled on the South Island in the launch pit.
Fall 2020 – Summer 2022
TBM LAUNCH PITSlurry walls were placed around the perimeter of a tri-cell, which was then excavated to a depth of roughly 65 feet. A concrete base slab and headwall were installed to support the TBM operations.
SLURRY TREATMENT PLANT (STP)The STP, manufactured and shipped from
France, was used to separate the slurry from
the soil that was excavated by the TBM.
Spring 2022 – Fall 2022
Spring 2021 – Fall 2022
Fall 2020 – Fall 2022
BACK
SOUTHISLAND
ISLAND EXPANSION
TBM Launch Pit
Preparing the TBM launch pit
In order to get Mary, the TBM, tunneling, she needed a good start. The HRBT Expansion Project team worked for months to prepare a home for Mary’s launch from the South Island. The launch pit was constructed in three “cells,” or overlapping circles, that formed its peanut shape. The first step in preparing the TBM shaft was installing slurry walls, 4-feet-thick, 185-feet-deep concrete panels that formed the perimeter of the launch pit. Next, crews added struts and a capping beam around the top of the shaft to provide support and stability to the structure as the launch pit was excavated. Crews worked around the clock to excavate the launch pit for Mary. The team used heavy equipment to dig, including a specialized machine called a teledipper, which featured a telescoping arm to remove excavated soils from the pit into awaiting dump trucks for removal. The goal was to excavate to a depth of over 80 feet at the front of the launch pit to accommodate Mary’s cutterhead and the trailing gantries that supported the tunnel boring operations. This equated to roughly 120,000 cubic yards of soil excavated in preparation for Mary’s journey.
BACK
SOUTHISLAND
ISLAND EXPANSION
TBM Launch Pit
Preparing the TBM launch pit
Once the soil was removed, the slurry walls that formed the perimeter of the launch pit were smoothed, and crews installed a specially engineered waterproofing system to prevent groundwater from seeping into the pit. This system, comprised of geotextile material, waterproof PVC membrane, waterstop barriers, and seals, was installed on the walls and floor of the shaft. After the waterproofing was complete, crews placed a 7- to 9-foot thick concrete base slab that supported Mary’s weight while also providing added weight to counteract the buoyancy of the launch pit. While it’s hard to imagine, at nearly 4,700 tons, that Mary could float, the water pressure underground pushed upward with a force of up to 5,000 pounds per square foot.
Finally, a concrete headwall was installed at what would become the opening of the new tunnel. The TBM cut directly through the headwall to start her journey. The headwall provided a perpendicular surface ensuring equal pressure across the face of the cutterhead to provide traction as Mary started tunneling. A lot of preparation had been needed to get Mary ready, and the reward was watching her work in the coming year as she constructed VDOT’s first bored tunnel.
FUN FACT: The amount of soil excavated for the launch pit could fill up Harbor Park Stadium to a depth of 8.25’
BACK
SOUTHISLAND
ISLAND EXPANSION
QUAY DOCK
A quay dock was a specially designed dock where barges delivered the tunnel segments. It was outfitted with a gantry crane that took the segments off the barge, moved them across into Cell 1 of the launch pit, and then lowered the segments into the pit for the TBM.
BACK
SOUTHISLAND
ISLAND EXPANSION
SLURRY TREATMENT PLANT (STP)
Another important component of the HRBT Expansion Project was Mary’s partner in progress, Katherine, the Slurry Treatment Plant (STP). The STP was an equally important part of the construction process, ensuring that the sand, clay, and other by-products of the tunneling process were removed from the construction area in an efficient, environmentally responsible manner. As you can imagine, Mary excavated a tremendous amount of soil as she dug her way under the seabed. The estimated total volume was around one and a half million cubic yards of material. This output was often called “spoil” and included coarser sands, clay, and other fine silt materials. Slurry, an engineered mixture of bentonite clay and water, was added to the excavated material to help facilitate pumping and removal. As the TBM tunneled forward, spoils were pumped to the surface through a 22-inch steel pipe to the STP on the South Island.
Katherine
BACK
SOUTHISLAND
ISLAND EXPANSION
SLURRY TREATMENT PLANT (STP)
At this plant, the excavated soil mixture passed through a series of rotating screen filters (called “trommels” and “cyclones”) with progressively smaller mesh sizes to separate particles and finer sands from the slurry. The recovered slurry was reused to feed the TBM, and the remaining spoils were processed through a filter press to squeeze out the residual water.
Katherine, the STP, was named in tribute to the pioneering African American mathematician Katherine Johnson, whose pivotal contributions to NASA’s space flight program were depicted in the motion picture “Hidden Figures.” Just like their real-life namesakes, Katherine, the STP, and Mary, the TBM, formed a perfect team.
Katherine JohnsonMATHEMATICIAN
LEARN MORE
BACK
SOUTHISLAND
ISLAND EXPANSION
CONVEYOR BELT
FUN FACT: It was estimated that the TBM excavated 1.5 million cubic yards of soil, or enough soil to make more than 840 Mt. Trashmores.
The conveyor belt was built to remove the soils excavated by the TBM. After they were processed at the slurry treatment plant, the remaining soil was placed on the conveyor belt, which carried it to the barge for removal to the designated disposal sites.
BACK
SOUTHISLAND
ISLAND EXPANSION
TBM REASSEMBLY
Mary, the Tunnel Boring Machine, was manufactured in Germany before being disassembled for shipment to the United States. Mary arrived in the U.S. in November 2021 in 140 bulk pieces and 30 containers of parts. Before Mary could be reassembled, special accommodations needed be prepared. Crews worked for months to excavate the launch pit on the South Island where she started her tunneling journey. At 4,700 tons, it wasn't feasible to move Mary around once fully assembled. While some components could be pre-assembled, the majority of Mary’s assembly happened inside the launch pit. As crews made the final touches on the launch pit, pieces of the TBM were welded together to expedite reassembly there.
At Mary’s face was the cutterhead with a striking design reflecting the Hampton Roads regional flag. The cutterhead featured numerous tools called disc cutters, bits, and scrapers that broke up the soil. Behind the cutterhead and housed within the shield, Mary’s main drive contained the motor and bearings that moved and rotated the cutterhead. Behind the shield came the tailskin, which served as a seal designed to prevent water from seeping in as the TBM worked to install the tunnel segments. These three components together formed the TBM shield. Behind the shield were four trailing gantries. These movable frameworks provided supporting equipment and services needed for tunnel operations.
BACK
SOUTHISLAND
ISLAND EXPANSION
TBM REASSEMBLY
The first gantry housed the control room for the TBM operator, as well as the oil for the hydraulic station. The second gantry contained the substation that distributed power across all components of the TBM. The third gantry contained the conduits and pipes for the electrical, air, and water utilities needed to operate the TBM. The fourth and final gantry supported the extension of all cables, wires, and pipes as the TBM advanced.
As Mary journeyed to the North Island to form the first tunnel and then back to the South Island for the second tunnel, crews extended connections up to 16,000 feet. These extensions carried power, air, water, and slurry from the South Island to the TBM. Mary was a complex piece of industrial machinery specifically designed for the HRBT Expansion Project. As she tunneled, crews performed preventive maintenance, including changing out the cutterhead tools as needed, to ensure that Mary remained in shape for peak performance.
UPDATED - HRBT TIMELINE
Melissa Williams
Created on November 12, 2025
Start designing with a free template
Discover more than 1500 professional designs like these:
View
Akihabara Connectors Infographic
View
Essential Infographic
View
Practical Infographic
View
Akihabara Infographic
View
Vision Board
View
The Power of Roadmap
View
Artificial Intelligence in Corporate Environments
Explore all templates
Transcript
HAMPTON ROADS BRIDGE-TUNNEL EXPANSION PROJECT
TUNNELING PROGRESS
An overview of the complex planning, engineering and construction progress behind the new HRBT twin tunnels.
norfolk
SOUTH ISLAND
NORTH ISLAND
HAMPTON
TUNNEL
New Tunnels
NORTH ISLAND
TBM TURNAROUNDIt took about 6 months to turn the TBM around to begin the second tunnel.
TBM RECEIVING PITThe TBM broke through the receiving pit after completing the first tunnel.
Spring 2022 – Fall 2023
JET GROUTINGThe ground was injected with a stabilization fluid to strengthen the soil for the tunnel approaches.
Winter 2022 – Fall 2023
Winter – Summer 2024
ISLAND EXPANSIONThe island was doubled in size to accommodate the new tunnels.
Fall 2020 – Summer 2021
BACK
NORTH ISLAND
ISLAND EXPANSION
Piece By Piece – How To Build An Island
A five-thousand-ton barge slowly cut through the waters near the City of Hampton, making its way toward the North Island of the Hampton Roads Bridge-Tunnel. On board were six-and-a-half-ton stones from a quarry in Maryland, destined for life in Virginia as part of the HRBT Expansion. As the stones arrived, crews measured them against “witness stones,” making sure they were the correct size and shape for placement. Finally, a grapple grabbed a stone and gently swung it into place before workers keyed and locked it in position like a 20-thousand-ton jigsaw puzzle. And so went the North Island Reclamation (NIR), a critical portion of the larger HRBT Expansion Project. The NIR expanded the island’s previous footprint by about 16 acres and laid the foundation for future elements of the project. The completed NIR provided space for a receiving pit for Mary, the Tunnel Boring Machine (TBM), as well as an approach roadway for the new, twin tunnels.
BACK
NORTH ISLAND
ISLAND EXPANSION
ISLAND EXPANSION
Piece By Piece – How To Build An Island
While it may seem like simple work, the job was extremely complex. Beyond what was visible from the surface, there was a larger rock footprint sloping further underwater, carefully designed to support and protect the island interior. The reclamation process included construction of the bund, or core perimeter layer, as well as an outer armor-stone layer, to protect the structure from the surrounding waves and currents. From here, several layers of sand were compacted to fill the expansion. It was a lot of work, only to then excavate a portion of this newly expanded island to accommodate the TBM receiving pit. Yet building the full island expansion with the precise soil mix and compaction was necessary in order to maintain its structural integrity as the walls of the receiving pit and new tunnel approaches were constructed. Wind and waves could batter or change artificial islands over time, so it was important to make sure each portion of the NIR was executed correctly, from design to materials. This will help Keep Virginia Moving for at least the next 100 years.
BACK
NORTH ISLAND
ISLAND EXPANSION
TBM Receiving Pit
Crews constructed a receiving pit on the North Island. Just as the name implies, this pit was created to “receive” Mary, the TBM, when she completed her journey from the South Island. At one-third the size of the TBM launch pit on the South Island, the TBM receiving pit was just one circle. Much like the process used for the TBM launch pit, slurry walls formed the perimeter of a circular receiving pit. After a capping beam was installed to stabilize the structure, crews excavated to a depth of 75 feet. A concrete slab then supported Mary's weight and formed the base for future roadway. When Mary “broke through” into the receiving pit, she had completed the first tunnel. In the receiving pit, Mary was turned around and begun her trek back to the South Island as she bore the second tunnel.
BACK
NORTH ISLAND
ISLAND EXPANSION
TBM Turnaround
The TBM broke through on the North Island and was then partially disassembled and placed on a turntable, section by section, to be turned around and start boring the second tunnel.
BACK
NORTH ISLAND
ISLAND EXPANSION
JET GROUTING
Jet grouting, or pressure grouting, is a method of soil stabilization that involves the high-pressure injection of a stabilizing fluid into the subsoil to stabilize ground conditions within the project site. Jet grouting was used to reinforce the surface, ensuring it was stable enough to withstand the weight and size of Mary and to facilitate the steering of the TBM.
TUNNEL
TUNNEL SEGMENTSReinforced pre-cast concrete tunnel segments were manufactured in Cape Charles and barged to the South Island for installation by the TBM.
Spring 2021 – Ongoing
TUNNEL BORINGThe TBM launched for the South Island excavating and building the first tunnel to the North Island, then turn around and created the second tunnel.
TUNNEL BORING MACHINE (TBM)Mary the TBM was manufactured in Germany and shipped to the US.
Fall 2020 – Fall 2021
Winter 2023 – Summer 2025
BACK
ISLAND EXPANSION
TBM (Tunnel boring machine)
TUNNEL
The Tunnel Boring Machine (TBM) was used to bore new twin tunnels next to the existing Hampton Roads Bridge-Tunnel. Hampton Roads Connector Partners (HRCP), the project's joint construction venture, awarded a contract to Herrenknecht of Germany for the design and construction of the TBM. It took approximately 14 months to build the 46-feet-tall machine, about four months to ship it from Germany, and another four to five months to assemble the TBM on the HRBT's South Island. The TBM was longer than a football field. As you can see from the picture of the TBM's cutterhead (pictured to the right), the machine stood about the same height as the HRBT Project Office, a four-story building. The price tag for the TBM alone was $101 million, which included construction, shipping, and assembly costs.
BACK
ISLAND EXPANSION
TBM (Tunnel boring machine)
TUNNEL
The TBM launched from the South Island (Norfolk side), and bored at a rate of about 50 feet per day until it reached the layer of soil known as the Yorktown layer, approximately 50 feet below the current tunnels. The TBM excavated the tunnels with a circular cross section through the soil. The process for both tunnels took more than two years. Built specifically for the HRBT Expansion Project, the TBM served as a central component of the project.
BACK
TUNNEL SEGMENTS
ISLAND EXPANSION
TUNNEL
The existing ten tunnels in Hampton Roads are immersed cylinders that sink into holes in the waterway bottom. Advances in tunnel technology made the bored-tunnel approach possible for the expansion project. The method had fewer environmental impacts and did not disrupt Navy, marine, or commercial traffic in the busy federal channel.
BACK
TUNNEL SEGMENTS
ISLAND EXPANSION
TUNNEL
The finished concrete tunnel segments, over 150 at a time, were transported by barge down the Chesapeake Bay to the South Island. Once on the island, the segments were transported into the tunnel using specialized vehicles. The segments were fed onto a conveyor inside the TBM, where a vacuum erector lifted, turned, and fitted each segment into place. As Mary excavated the soil, she placed nine precast concrete segments into a ring that became the walls of the tunnel. Mary advanced by pushing 54 hydraulic thrust jacks against the newly completed ring of precast concrete segments, leaving a perfectly circular tunnel in her wake. Mary placed 21,492 tunnel-lining segments, each weighing 12 tons. She erected each segment one by one to form a total of 2,388 rings composed of nine segments each, leaving a perfectly circular tunnel behind her.
BACK
TUNNEL BORING
ISLAND EXPANSION
TUNNEL
The new twin tunnels were constructed using a massive tunnel-boring machine, or TBM, with a rotating cutting head that excavated soil along the tunnel’s path. As the cutter head moved through the soil, concrete liners were set in place by a rotating vacuum-powered lift, creating the outer shell of the tunnel. A significant advantage of this construction method was its reduced environmental impact on marine wildlife compared to the immersed-tube approach, which would have required dredging a deep, mile-long trench across the Hampton Roads waterway. Because the tunnel-boring machine excavated from beneath the riverbed, this technology essentially eliminated disruptions to commercial and military shipping in one of the nation’s most important navigation channels. The new tunnels were constructed approximately 50 feet deeper than the existing tunnels, each measuring about 8,000 feet in length.
BACK
TUNNEL BORING
ISLAND EXPANSION
TUNNEL
The HRBT Expansion Project was VDOT’s first bored tunnel and only the fourth and fifth bored tunnels in the United States. Although the immersed-tube method was used to construct all 10 of Hampton Roads’ existing crossings –from the original Downtown Tunnel in 1952 to the new Midtown Tunnel in 2016 – recent technology advances made bored tunnels feasible in the region’s soft soils. Preliminary construction work began in mid-2020 with a 65-foot pit excavated on the South Island to accommodate the TBM launch. The TBM tunneled toward the North Island (Norfolk to Hampton) completing the first tunnel in roughly one year. It took about four months to turn the TBM around and another year to bore a parallel tunnel back to the South Island.
SOUTH ISLAND
QUAY DOCKA temporary dock was built so that tunnel segments could be unloaded from barges and lowered into the launch pit to the TBM using a gantry crane.
CONVEYOR BELTA conveyor belt system was installed to move excavated soil from the STP to waiting barges for removal to approved disposal sites.
Summer 2021 – Fall 2022
TBM REASSEMBLY170 pieces of Mary the TBM were reassembled on the South Island in the launch pit.
Fall 2020 – Summer 2022
TBM LAUNCH PITSlurry walls were placed around the perimeter of a tri-cell, which was then excavated to a depth of roughly 65 feet. A concrete base slab and headwall were installed to support the TBM operations.
SLURRY TREATMENT PLANT (STP)The STP, manufactured and shipped from France, was used to separate the slurry from the soil that was excavated by the TBM.
Spring 2022 – Fall 2022
Spring 2021 – Fall 2022
Fall 2020 – Fall 2022
BACK
SOUTHISLAND
ISLAND EXPANSION
TBM Launch Pit
Preparing the TBM launch pit
In order to get Mary, the TBM, tunneling, she needed a good start. The HRBT Expansion Project team worked for months to prepare a home for Mary’s launch from the South Island. The launch pit was constructed in three “cells,” or overlapping circles, that formed its peanut shape. The first step in preparing the TBM shaft was installing slurry walls, 4-feet-thick, 185-feet-deep concrete panels that formed the perimeter of the launch pit. Next, crews added struts and a capping beam around the top of the shaft to provide support and stability to the structure as the launch pit was excavated. Crews worked around the clock to excavate the launch pit for Mary. The team used heavy equipment to dig, including a specialized machine called a teledipper, which featured a telescoping arm to remove excavated soils from the pit into awaiting dump trucks for removal. The goal was to excavate to a depth of over 80 feet at the front of the launch pit to accommodate Mary’s cutterhead and the trailing gantries that supported the tunnel boring operations. This equated to roughly 120,000 cubic yards of soil excavated in preparation for Mary’s journey.
BACK
SOUTHISLAND
ISLAND EXPANSION
TBM Launch Pit
Preparing the TBM launch pit
Once the soil was removed, the slurry walls that formed the perimeter of the launch pit were smoothed, and crews installed a specially engineered waterproofing system to prevent groundwater from seeping into the pit. This system, comprised of geotextile material, waterproof PVC membrane, waterstop barriers, and seals, was installed on the walls and floor of the shaft. After the waterproofing was complete, crews placed a 7- to 9-foot thick concrete base slab that supported Mary’s weight while also providing added weight to counteract the buoyancy of the launch pit. While it’s hard to imagine, at nearly 4,700 tons, that Mary could float, the water pressure underground pushed upward with a force of up to 5,000 pounds per square foot.
Finally, a concrete headwall was installed at what would become the opening of the new tunnel. The TBM cut directly through the headwall to start her journey. The headwall provided a perpendicular surface ensuring equal pressure across the face of the cutterhead to provide traction as Mary started tunneling. A lot of preparation had been needed to get Mary ready, and the reward was watching her work in the coming year as she constructed VDOT’s first bored tunnel.
FUN FACT: The amount of soil excavated for the launch pit could fill up Harbor Park Stadium to a depth of 8.25’
BACK
SOUTHISLAND
ISLAND EXPANSION
QUAY DOCK
A quay dock was a specially designed dock where barges delivered the tunnel segments. It was outfitted with a gantry crane that took the segments off the barge, moved them across into Cell 1 of the launch pit, and then lowered the segments into the pit for the TBM.
BACK
SOUTHISLAND
ISLAND EXPANSION
SLURRY TREATMENT PLANT (STP)
Another important component of the HRBT Expansion Project was Mary’s partner in progress, Katherine, the Slurry Treatment Plant (STP). The STP was an equally important part of the construction process, ensuring that the sand, clay, and other by-products of the tunneling process were removed from the construction area in an efficient, environmentally responsible manner. As you can imagine, Mary excavated a tremendous amount of soil as she dug her way under the seabed. The estimated total volume was around one and a half million cubic yards of material. This output was often called “spoil” and included coarser sands, clay, and other fine silt materials. Slurry, an engineered mixture of bentonite clay and water, was added to the excavated material to help facilitate pumping and removal. As the TBM tunneled forward, spoils were pumped to the surface through a 22-inch steel pipe to the STP on the South Island.
Katherine
BACK
SOUTHISLAND
ISLAND EXPANSION
SLURRY TREATMENT PLANT (STP)
At this plant, the excavated soil mixture passed through a series of rotating screen filters (called “trommels” and “cyclones”) with progressively smaller mesh sizes to separate particles and finer sands from the slurry. The recovered slurry was reused to feed the TBM, and the remaining spoils were processed through a filter press to squeeze out the residual water. Katherine, the STP, was named in tribute to the pioneering African American mathematician Katherine Johnson, whose pivotal contributions to NASA’s space flight program were depicted in the motion picture “Hidden Figures.” Just like their real-life namesakes, Katherine, the STP, and Mary, the TBM, formed a perfect team.
Katherine JohnsonMATHEMATICIAN
LEARN MORE
BACK
SOUTHISLAND
ISLAND EXPANSION
CONVEYOR BELT
FUN FACT: It was estimated that the TBM excavated 1.5 million cubic yards of soil, or enough soil to make more than 840 Mt. Trashmores.
The conveyor belt was built to remove the soils excavated by the TBM. After they were processed at the slurry treatment plant, the remaining soil was placed on the conveyor belt, which carried it to the barge for removal to the designated disposal sites.
BACK
SOUTHISLAND
ISLAND EXPANSION
TBM REASSEMBLY
Mary, the Tunnel Boring Machine, was manufactured in Germany before being disassembled for shipment to the United States. Mary arrived in the U.S. in November 2021 in 140 bulk pieces and 30 containers of parts. Before Mary could be reassembled, special accommodations needed be prepared. Crews worked for months to excavate the launch pit on the South Island where she started her tunneling journey. At 4,700 tons, it wasn't feasible to move Mary around once fully assembled. While some components could be pre-assembled, the majority of Mary’s assembly happened inside the launch pit. As crews made the final touches on the launch pit, pieces of the TBM were welded together to expedite reassembly there. At Mary’s face was the cutterhead with a striking design reflecting the Hampton Roads regional flag. The cutterhead featured numerous tools called disc cutters, bits, and scrapers that broke up the soil. Behind the cutterhead and housed within the shield, Mary’s main drive contained the motor and bearings that moved and rotated the cutterhead. Behind the shield came the tailskin, which served as a seal designed to prevent water from seeping in as the TBM worked to install the tunnel segments. These three components together formed the TBM shield. Behind the shield were four trailing gantries. These movable frameworks provided supporting equipment and services needed for tunnel operations.
BACK
SOUTHISLAND
ISLAND EXPANSION
TBM REASSEMBLY
The first gantry housed the control room for the TBM operator, as well as the oil for the hydraulic station. The second gantry contained the substation that distributed power across all components of the TBM. The third gantry contained the conduits and pipes for the electrical, air, and water utilities needed to operate the TBM. The fourth and final gantry supported the extension of all cables, wires, and pipes as the TBM advanced. As Mary journeyed to the North Island to form the first tunnel and then back to the South Island for the second tunnel, crews extended connections up to 16,000 feet. These extensions carried power, air, water, and slurry from the South Island to the TBM. Mary was a complex piece of industrial machinery specifically designed for the HRBT Expansion Project. As she tunneled, crews performed preventive maintenance, including changing out the cutterhead tools as needed, to ensure that Mary remained in shape for peak performance.