For decades, New England's most populous city has been troubled by bridges, tunnels, and highways that have kept Bostonians in a perpetual traffic jam. So in 1973, local interest groups began to lobby the Massachusetts state government for a new traffic system. It wasn't until 1981, however, that the administration of then Gov. Michael Dukakis made it a priority not only to repair the problem, but to fix it once and for all.

In 1990, after years of lobbying Congress, the state of Massachusetts received $8 billion to build what is now known as the Boston Central Artery/Tunnel project. The project began in late 1991 and has since become the single largest highway project in the history of the United States. The state expects the project to be completed by year-end 2004.

The Central Artery/Tunnel project is using some of the industry's most modern construction techniques to date. Two such techniques are the soil stabilization process and the foundation jet grouting process. Both these techniques assist in stabilizing the ground before actual grouting begins. The Fort Point Channel Grout Plant of South Boston will produce 1,000 gpm of mother grout for this stage of the project.

Soil stabilization process The soil stabilization is a process of mixing grout and a cement/flyash/water mixture with soil in place. A series of rows will be drilled 125 ft into the ground on both sides of the future Ted William Third Harbor Tunnel Extension. The rig has three augers. On the way down, water is used for drilling. On the way up, grout is injected and then mixed with the soil. At the deepest level, the auger rising rate is slowest as a greater amount of grout mixes with the soil to give the highest strength. This level is below the road level. When the roadway level has reached an intermediate rate, it can produce the required soil strength. Above the road/tunnel, strength requirements are less. After the soil has been stabilized, the tunnel construction will begin.

In the Fort Point Channel area, a trench will be dredged for precast tunnel tubes that will be formed in the adjacent casting basin. In the area between the channel and the Massachusetts Turnpike (I-90), the highway leading into Boston, precast tunnel tubes will be jacketed through the area under the commuter rails and above the Massachusetts Transit Authority's red line. During the construction, traffic is not expected to be affected.

The first rig pass will be a primary hole with the left and right augers injecting 70% grout flow into holes one and three, respectively. The center auger will inject 100% grout flow into hole two. The next set will also be a primary hole with augers making holes number five, six, and seven. Next, the rig is repositioned over holes three, four, and five for a secondary hole. Now the left and right augers inject 30% flow into hole three and five respectively and the center auger will inject 100% into hole four. The process continues alternating primary and secondary holes.

Jet grouting process In areas where existing structures and foundations have to be supported and reinforced, the jet grouting process will be used. In this process, a machine drills to the area that needs reinforcing and injects grout under high pressure. Modules 13 through 16 will supply grout to three jet grout machines.

The Fort Point Grout Plant will consist of two complete continuous grout mixing lines, with one operating while the other is in standby mode. The grout consists of cement, flyash, and water only. The powders ratio varies between 75% to 100% cement and 0% to 25% flyash. The mixers produce up to 1,000 gpm of mother grout. The grout may later be diluted in one of 16 modular grout stations. As many as 12 of the modules could be operating at any given time.

The grout mixer system comes with its own density controls. The cement/flyash, water and recycling grout mix fall out of the bottom opening into the premix chamber. The premix chamber has a working capacity of approximately 200 gal. The premix chamber is a partitioned section of the main vessel. The rest of the vessel is called the averaging chamber, which has a working volume of approximately 700 gal. Both chambers have mixers. The recycle pump takes suction out of the premix chamber and pumps back up to the premix inlet. The averaging chamber has an open top. The transfer pump empties this section into the grout holding tank, which has a working volume of approximately 1,300 gal.

The deep soil stabilization grout modules operate in groups of three, each taking as much as 150 gpm of grout. There are 12 of these modules. The other four modules transfer grout to the jet grouting operation. The jet grout modules pump 75 gpm. The cement feed rate to the mixer is set by the main plant operative based on the field requirements. The feed rate of flyash is ratioed off the cement feeder using a Ramsey DE-10 Impact Weigher. The weigher measures the mass flow rate and total mass of free-flowing dry solid and powders that go into the averaging chamber and the holding tank. The combined volume of the averaging chamber and the holding tank is 2,000 gal. Figure 1 (below) illustrates this process.

For more information on the Boston Central Artery/Tunnel project, consult www. bigdig.com on the World Wide Web.