Overview of the Trans-Tokyo Bay Highway

The Trans-Tokyo Bay Highway, also known as the Tokyo Wan Aqua-line, is an approximately 15km highway linking Kawasaki City in Kanagawa Prefecture on the west side of Tokyo Bay with Kisarazu City in Chiba Prefecture on the east side of the bay. Construction of the highway is currently proceeding, and plans call for its completion on December 18, 1997. The Tokyo Wan Aqua-line will link the Bayside Route of the Tokyo Expressway with the Higashi-Kanto Highway. When it is completed, the length of a trip between the city halls of Kawasaki and Kisarazu will be shortened 70km--from 100km to 30km.

The volume of shipping traffic in Tokyo Bay is extremely high. In view of the need to allow for sufficient shipping lanes and other factors, it was deemed necessary to construct the first approximately 10km segment of the Tokyo Wan Aqua-line extending from Kawasaki within an undersea tunnel.

The remaining approximately 5km to Kisarazu is spanned by a bridge. Since this section of the bay is also traversed by numerous ships, the first 750m section of the bridge is elevated 27m above the water on 12 towers, allowing for the passage of 2,000t-class ships.

The Tokyo Wan Aqua-line encompasses two man-made islands--the Kawasaki Man-Made Island midway along the undersea tunnel and the Kisarazu Man-Made Island (also known as the Umihotaru, the Japanese name of a luminous, approximately 3mm-long crustacean found locally) where the tunnel joins the bridge. The Kawasaki Man-Made Island, which has a diameter of approximately 200m, incorporates ventilation towers. The above-water portion of the Kisarazu Man-Made Island is 650m long and 100m wide and includes ventilation towers as well as a parking area.

Kajima is responsible for the western portion of the Kawasaki Man-Made Island and the Kawajin North region of the tunnel extending from the island to Kawasaki. Construction of these sections was begun in May 1989.

The Kawasaki Man-Made Island

Kawasaki Man-Made Island, which weighs about 650,000t, has a 98m-wide core. The island is serving as the launching base for shield-driving machines that are boring four tunnels--the largest undersea holes ever created by mankind. The two tunnels stretching from the island to Kawasaki are together referred to as the Kawasaki Tunnel, and the two tunnels extending from the island toward Kisarazu are together referred to as the Central Tunnel. Located 20m beneath the seabed, which is about 28m below the water's surface, these tunnels have 14.14m external diameters, making them the world's largest tunnels created by the shield-tunneling method.

The Kawasaki Man-Made Island was created in the following manner.

･First, a firm foundation extending 32m below the soft seabed of Tokyo Bay was created through the use of the SCP and DMM methods. In the SCP method, steel pipes with diameters of approximately 1m were driven into the seabed. Sand was then injected through the pipes into the seabed under pressure to form stable, hard-packed sand columns. The DMM method involved the mixing of a stabilizer material similar to cement with the soft sea bottom material. The two materials underwent a chemical reaction when combined, thus improving the stability of the seabed.

･Next, the circumference of the island was encircled with a donut-like struc-ture, with the inner and outer walls clad in protective steel jackets, and a sand-based composite soil material was placed in the gap between the two layers, creating an artificial base for the island's construction.

･After the installation of interior guide walls, electromill excavating devices were used to dig spaces for the individual elements of an underground wall, following which a framework of steel reinforcement bars was set up and concrete pouring commenced. Using Kajima's precision management system (a weighted plumb-line system) to great advantage, the excavation for each of the 28 wall elements was performed to a depth of 119m while holding deviation to within 70mm, thus creating the world's largest cylindrical underground continuous wall.

･After completing the concrete pouring for the underground wall and water drainage, the interior guide walls and interior jacket were removed, using a 4,100t barge-mounted crane to lift each 1,100t jacket block.

･More than 10 backhoes of over 30t each were lowered to the bottom of the pit within the jacket. Soil excavated by the backhoes was winched out of the pit using a 1,500t barge-mounted crane with a 45 cubic meters clam-shell device. Excavation was continued to a depth of approximately 40m below the seabed, which is 75m below the island's surface. Kajima has considerable experience in large-scale excavation for building underground tanks, circular bases for bridge piers, and other structures, but the scale and difficulty of the excavation work for the Kawasaki Man-Made Island was unprecedented. In view of the huge water pressure on the unfinished island's exterior, the Company established a thorough safety system--installing approximately 800 measurement instruments around the circumference and monitoring real-time data provided by the instruments around the clock.

･When the excavation work was completed, prefabricated steel reinforced concrete blocks 27m tall and weighing approximately 2,200t each were lowered inside the island's circumference and more concrete was poured to create an integral unit, including starting-holes for shield-driving machines as well as the internal structure of the ventilation towers.

The Undersea Tunnels

After completing the Kawasaki Man-Made Island, tunnel excavation was begun. The Tokyo Wan Aqua-line is to be housed in two tunnels, one with two eastbound lanes and one with two westbound lanes. The underwater portion of the highway stretches 9.4km from Kawasaki to the Kisarazu Man-Made Island. The tunnels for this portion were created using eight shield-driving machines. Kajima is responsible for constructing the tunnel with eastbound lanes extending 1.8km from Kawasaki to the Kawasaki Man-Made Island, which is being carried out during the period from July 1992 through December 1997.

Because of such factors as those related to the durability of machinery, the maximum distance that can be tunneled with a single shield-driving machine based on current technology is approximately 3km. Accordingly, to create longer tunnels, shield-driving machines must tunnel from opposite directions with considerable precision so that the machines can meet at a specified position with less than 50mm deviation. In view of this stringent requirement, a relative position monitoring system for shields, developed with the participation of Kajima, was employed to ensure accurate meeting points for the shield-driving machines that created each of the Tokyo Wan Aqua-line's four tunnels.

With regard to the tunnel that Kajima was responsible for creating, when the two shield-driving machines were 50m from their meeting point, they were slightly out of alignment with each other. As a result of adjustments made over the remaining distance, the machines were ultimately less than 10mm out of alignment when they met.

After freezing the tunnel walls in the vicinity of the meeting point, the shield-driving machines were partially disassembled and removed from the tunnel, which was fundamentally complete at that time.

Aiming to complete the Tokyo Wan Aqua-line in December 1997, the lining of the tunnels with concrete is currently proceeding. When the highway is finished, following nine years of work, vehicles will be able to pass from one side of Tokyo Bay to the other in less than 15 minutes.