Every time a space shuttle is about to blast off into space, Cape Canaveral stages a dramatic scene. A giant clock, located about a mile from the launchpad, counts down the time prior to the event, which is notoriously delayed as NASA rechecks every nook and cranny of the craft.
But, really, what is the big deal? Boston can resist the thrusting power of four space shuttle launchings simultaneously on the word "go."
That?s the word out of the Central Artery/Tunnel Project?a.k.a. "The Big Dig"?as crews work to complete a vital section on the city?s south side. In order to prevent disruption of train service at South Station, which handles approximately 400 trains daily, tunnel jacking has been used to place three sections into the deepest part of the project. It?s been said that the tunnel jacking pit walls can handle the thrust of Columbia, Discovery, Atlantis and Endeavour.
"That?s at the biggest push of the tunnel," Mike Walker, project manager for GEI Consultants Inc., Winchester, Mass., told ROADS & BRIDGES. "It?s really a rough calculation."
It is by far the fuzziest of the entire project, which has demanded precise formulas and anecdotes throughout the design and construction process.
Through the latter part of November two of the tunnels, ramp D and westbound, were in place while the eastbound section was 55% complete. Ramp D connects I-90 westbound to the northbound underground portion of I-93. The west and eastbound tunnels form part of I-90.
The entire concept of sliding a tunnel underground using a huge pit and powerful jacks was relatively new to the U.S. Alternatives, however, were scarce.
Steve Taylor is a tunnel project engineer for Hatch Mott MacDonald, Pleasanton, Calif., which served as a subconsultant to section designer Maguire/Harris. Digging for underground transportation is a specialty of Taylor, who was surrounded by related work in his native city of London.
He recalled one of the biggest challenges was installing confidence during the design phase, not necessarily hallow blocks of concrete during construction.
Tunnel jacking has been practiced extensively abroad, particulary in Japan where it?s called the "endless self-advancing method."
"It?s a very unusual technique, a very specialist technique," Taylor told ROADS & BRIDGES. "During design it was really getting the whole thing moving forward by convincing everybody that tunnel jacking was the way to go. Systematically we addressed the concerns."
But not before other options were addressed. The traditional "cut-and-cover" was discussed, but it wasn?t physically feasible in relation to railroad track movements.
Using a series of drifts was another idea. Drifts are smaller tunnels systematically tied and locked to form one large unit. A grouted arch was also examined, but nothing compared to the tunnel jacking approach.
"Sometimes (tunnel jacking) is appropriate, sometimes it isn?t," said Taylor. "If you want to get something beneath a road or a railway, or something you don?t want to move, then tunnel jacking is worthy of taking its place in the alternatives that are available. It should be considered where it is appropriate. It should not be considered where it isn?t appropriate."
These are the pits
Boston?s pasty historic soil created a mess of problems throughout the tunnel line, and the complexities magnified when the design team was dealing with tunnel jacking forces in need of a solid foundation.
Jet grouting was used exclusively to counteract the soft soil and improve base stability during the construction of the jacking pits, which are roughly 100 ft wide and 200 ft long. A 23.1-ft-thick jet grout layer was installed in the soft clay immediately below the pit subgrade. Jet grouting is the high pressure injection of a water/cement fluid, using air and water to cut through the ground.
Crews drilled down 60-80 ft from ground level in an attempt to install cylinders of improved ground 6 ft in diam. A nozzle actually spins in a circle while releasing the high pressure liquid that carves the clay. Then at some point a cementitious grout is injected and mixes with the clay to form a solid mass.
"Once you had the jet grout in place you had the added benefit of having this really stiff subgrade strut," said Walker. "Using that subgrade strut we were able to design for long spans so we didn?t have to put bracing within the profile of the tunnel."
Providing ample support for the pit walls without filling the pits with bracing was the only way construction of concrete tunnels 40 ft high and 80 ft wide was going to happen.
The excavations required non-routine support system elements?cantilevered T-panel slurry walls, post-tensioned slurry walls with a single level of bracing and soldier pile tremie concrete headwalls. Supporting the latter were post-tensioned concrete deck beams and a steel truss-shaped rigid frame fabricated from welded box sections and rolled beams.
The use of jet grouting and subgrade struts eliminated several layers of bracing. Jet grout also provided support to the cantilevered T-panels adjacent to the rail lines.
In the eastbound pit the first 59.4 ft of soil next to the headwall was treated with jet grout. Beyond 59.4 ft the design called for subgrade struts to support the T-walls. The struts, which were unreinforced concrete panels set at subgrade levels and installed with slurry wall techniques, also helped transfer load from the base slab into the clay below during tunnel jacking.
In the southwest corner of the eastbound pit the typical length of the T-wall stems?19.8 ft?would have interfered with the railroad lines. The necessary adjustment, which used shorter stem beams reinforced with wide flange sections or post-tensioning, resisted the large loads in the cantilever walls.
Ground freezing was another technique used to combat the weak clay and organic layers during tunneling. However, because ground freezing could not begin until completion of the jacking pits another problem lurked?soil heave.
The heave associated with the freezing created lateral pressures with more than twice the loading developed during downstage excavation before freezing. The relief effort came in two parts: using a pressure-relief hole system and a hydraulic-jack/floating-headwall system.
The pressure-relief hole system was installed in an unfrozen strip of ground between the frozen ground and a 181.5-ft-long portion of the south sidewall of the westbound pit. In this area the contractor drilled 49.5-ft-deep, 8-in.-diam. relief holes spaced at about 3 ft.
As the ground froze and expanded, the relief holes squeezed closed and limited the load applied to the wall. Collapsing relief holes were redrilled to accommodate the large lateral movements in the soil.
At the headwalls the design called for a combination of relief holes, hydraulic rams and a floating headwall. Because the tunnels would eventually be jacked through the headwalls, the ground behind them had to be frozen solid. A series of 1300 kN to 3100 kN hydraulic jacks were installed between the walls and the bracing systems. In this application, the jacks were at a constant load and the headwalls were allowed to move into the pits as the frozen ground expanded. The jacks allowed controlled movement of the wall while keeping stresses in the wall and bracing to acceptable levels and supporting the soil and railroad behind the wall.
There were two stages of ground freezing at the headwall. Initially, relief holes were drilled in a 9.9-ft-wide unfrozen strip in front of the headwall. Jacks were used to control wall movement when the strip was frozen to allow demolition of the headwall.
The installation of jet grout also produced minimal ground heave beyond the walls of the pits.
"What we found was that the jet grout, where it was being installed, was significantly stronger than we required it to be," said Taylor.
The ground was supposed to have a compressive strength of 150 psi. After re-evaluating the amount of grout necessary to accomplish the required strength a method was developed where small areas of the ground were left untreated.
"That gave softer areas of the ground into which the pressure could relieve itself rather than finding its way to the surface or pushing the walls out. We effectively treated about 80-85% of the ground," said Taylor.
Upon completion of the tunnels, a portion of the pit headwall was demolished and the process of pushing the sections underground began. The bottom, top and walls of each tunnel are 7 ft thick.
The tunnel at ramp D is the shortest (165 ft) and was installed 25 ft below the tracks. The westbound tunnel measures in at 260 ft and is 20 ft below the tracks. The giant of the project is the eastbound structure, which was originally 360 ft long and had to be placed in two sections. The tunnel, however, was extended to 380 ft to aid an adjacent contractor who was looking to use 100 ft of slurry wall next to an electrified rail.
Crews are currently jacking the second section of the eastbound tunnel, where granite obstructions in the ground have created complications. The granite is being broken down into pieces and shipped out of the tunnel.
Road headers are being used to cut through the frozen soil. There are six work bays on two levels of the tunnel conducting the mining. Debris is placed in a 5 1/2-yd bucket and delivered to the surface.
There are 25 tunnel jacks applying force at the rear, and at least two intermediate stations containing 28-32 jacks were used with the westbound and eastbound tunnels. Each jack has a 535-ton working force and extends a maximum of 3 ft. After the pushing mechanisms reach their limit a spacer is installed. A 10-hour working shift can produce one full push, and as many as 6-9 pushes can be completed a week barring any obstruction.
In order to prevent soil movement an anti-drag system is being utilized. This system consists of a blanket of steel ropes 3/4 in. in diam. between the top and bottom of the tunnel. The ropes are greased and tied into a brace beam located above the section. Rope is pulled from reels located inside the tunnel as the section moves forward. Slightly over a million linear feet of cable is being used.
Prior to mining, the soft ground was frozen using steel pipes 41/2 in. in diam. installed vertically into the ground 5 ft above what was supposed to be the base of the tunnel. Inside each steel pipe is a plastic tube, which injects a calcium chloride and water solution chilled at the site. Freezing created a 1,000-1,500 psi strength.
Those involved with the tunnel jacking pits were recently awarded the American Consulting Engineering Council?s Grand Conceptor for Engineering and Innovation in Massachusetts for jet grout and slurry wall techniques used in the project. Honored were Hatch Mott MacDonald Inc., GEI Consultants Inc. and Weidlinger Associates Inc.
"The project is going very successfully, but they are still very, very big tunnels in very complex ground conditions . . . so what we musn?t do is sit back and say it was all very successful," said Taylor. "We have 55% of the (eastbound) tunnel in, and what we?re trying to do is maintain the focus on making sure it continues to go successfully and does what we intend it to do."
Information for parts of this story was provided by Walker and Marco Boscardin of GEI Consultants Inc. ROADS & BRIDGES also would like to acknowledge the assistance of Slattery, Interbeton, J. F. White, Perini, Joint Venture (contractor), the Massachusetts Turnpike Authority (owner) and Bechtel/Parsons Brinckerhoff (construction manager).