Metropolitan Area Outer Underground Discharge Channel

Protecting Society from Flooding with Water Tunnels

Over 30 km north of the city center, around Kasukabe City in southeastern Saitama Prefecture.
The tract is lowland at a height of 20-30 meters, and forms a dish-like topology that makes it easy for water to pool. Despite this, three rivers, the Nakagawa, Kuramatsugawa and Otoshi-furutonegawa rivers, flow through a narrow region, and for that reason once heavy rain starts to fall, inundation occurs across a wide area, and the floods take a while to subside. This was the difficult flood control problem the area faced. In fact, over the past two decades, the area was hit by severe flooding as many as five times. To protect the land from flooding, a large-scale project was advanced: the Metropolitan Area Outer Underground Discharge Channel. As a drastic flood control measure, the plan involved constructing an underground discharge tunnel under a national road running through the area to capture the overflow from the three rivers and discharge the flow into the Edogawa River, with its greater capacity. The comprehensive technologies of Pacific Consultants were fully demonstrated in the construction of this vast underground discharge channel, which ran to a total length of more than 6.3 km.

Technologies to Analyze Flooding

The project was started in 1993. Pacific Consultants was in virtually all aspects, from overall conceptualization to hydraulic analysis, as well as the planning and design of structures and facilities. The work began with analyzing regional flooding.
In the event heavy rain has caused flooding, the water needs to be directed into the underground tunnel before overflowing the banks. The question was where it would be most effective to set up the intake facilities. The local topography was recreated with a computer and hydraulic phenomena were simulated while estimating the functioning of the flood control facilities currently operating.

The River Department, one of the flagship departments at Pacific Consultants, was given this task. Takeshi Maeda commented: “The plan took a team of five people three years to make. If the plan was wrong in any way, flooding could occur at a different place. In our estimation, the safety of many people was on the line. We came up with several water discharge routes and after making various comparisons that also took economic effects into account, we refined the final plan.”

An Unprecedented Scale

With the route plan set, the project entered the tunnel planning phase. The first step was to create vertical shafts to drain the water from the overflowing rivers. The vertical shafts would be constructed at five locations. However, the scale of the shafts was staggering. Each had a diameter of 30 meters and a depth of 70 meters. These vast structures beyond one’s imagination had to be installed underground. Moreover, at a point fifty meters below the ground, a tunnel ten meters in diameter with a length of more than 6 km would be built.

Actually, in the initial plans, the diameter of the vertical shafts didn’t need to be 30 meters. But it was determined that this size was needed in order to set up the tunnel excavation equipment underground. Naturally, the greater size of the vertical shafts also impacted the overall hydraulic plan. This was another area where the Tsukuba Research Center played a big role.

Water has some incomprehensible fluid dynamic properties that can’t be measured at a purely theoretical level. Paper theories and actual water flows don’t always match. To test whether the plan would really function as intended, we created a 1:20 scale model at the Tsukuba Research Center, and performed a bevy of tests while adjusted all manner of data. Through that process, we verified whether the simulated results were correct.

Conceptual diagram of the No. 3 shaft structure

Technologies to Analyze Flooding

The project was started in 1993. Pacific Consultants was involved in virtually all aspects, from overall conceptualization to hydraulic analysis, as well as the planning and design of structures and facilities. The work began with analyzing regional flooding.
In the event heavy rain has caused flooding, the water needs to be directed into the underground tunnel before overflowing the banks. The question was where it would be most effective to set up the intake facilities. The local topography was digitally recreated with a computer and hydraulic phenomena were simulated while estimating the functioning of the flood control facilities currently operating.

The River Department, one of the flagship departments at Pacific Consultants, was given this task. Takeshi Maeda commented: “The plan took a team of five people three years to make. If the plan was wrong in any way, flooding could occur at a different place. In our estimation, the safety of many people was on the line. We came up with several water discharge routes and after making various comparisons that also took economic effects into account, we refined the final plan.”

Design Methods Were Also Built from Scratch

Both the vertical shafts and the tunnel were structures of an unprecedented scale. Masao Masuno of the Tunnel Department, which was involved with the planning and design, recounted the difficulties they faced. “To give one example, to dig the vertical shafts we had to make earth retaining structures deep underground to hold off the earth and water pressure, but nothing of that length had ever been done in Japan. We had to start to examine whether the design methods we had used to date would translate across. The same went for the tunnel. Design methods for road tunnels have been well established, but water discharge tunnels are an unknown quantity. It’s fine if you only consider the earth pressure from outside a road tunnel, but a water discharge tunnel has to contend with pressure from the inside as well. The design requires non-conventional expertise. We had to develop the design methods by ourselves, by consulting university experts and so on.”

Mobilizing All Forces Towards Completion

During the actual construction, an underground continuous wall with a depth of 140 meters was constructed as an earth retaining structure for the 70 meters of excavation. A trench 140 meters underground was dug, reinforcing bars were placed inside and concrete was poured in. This was the first construction work of its kind in the world. For excavation of the tunnel, the slurry-type shield method (a method to prevent the entry of groundwater by supplying water with granular soil) was employed. A dedicated shield machine was independently fabricated for this purpose. Specialized expertise was utilized during every process. In addition to the Tunnel Department, other departments were also involved with the project. The Construction Department worked on the design of the shed above the shafts, while the Facility Engineering Department designed the intake facilities to capture flood waters and the equipment to discharge the water at the discharge station. Pacific Consultants truly mobilized its combined forces to see the Metropolitan Area Outer Underground Discharge Channel through to fruition. The knowledge to protect people’s lives from disaster is yet another social asset that has accumulated within Pacific Consultants.

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