A half-mile bridge over the Neponset River separating Boston and Quincy, Mass., is being earthquake-proofed as part of a major rehabilitation that includes new tops for hammerhead piers and hundreds of new seismic bearings.
J.F. White Contracting Co. of Framingham, Mass., has a $20-million contract with the state's Department of Conservation and Recreation (DCR) to overhaul 55 piers of the bridge, which carries four lanes of Route 3A between the two cities. Inspections by DCR personnel revealed that the tops of the existing piers were crumbling, exposing rebar. DCR's consultants, Dewberry-Goodkind, recommended not only that the tops of the piers be demolished and replaced, but new isolation bearings be installed at the same time to bring the bridge up to modern seismic code requirements.
"The original bearings met the requirements of the code in 1969, when the bridge was built, but the code has changed," noted Mark Griffin, project engineer for DCR.
Dynamic Isolation Systems of Sparks, Nevada, is supplying the new bearings, some weighing 1,800 pounds, which are being installed by Local 7 Ironworkers for subcontractor Atlantic Bridge & Engineering, a Salisbury, Mass., company.
J.F. White began work on the bridge in August 2006, and has until August 2009 to complete the project. Crews are jacking beams at one pier at a time, due to the complexity of the operation and safety requirements.
The rehab process includes the following steps:
- jack beams up 1/4-inch, taking loads off pier
- remove existing bearings under each beam
- demolish the top of the hammerhead pier
- form and place concrete for a new pier top
- install the new seismic isolation bearings
- transfer bridge loads to the new bearings
- remove falsework and temporary footings.
It is a complicated, labor-intensive project, according to Craig Bateman, J.F. White's job superintendent. "We had to fabricate about 400 tons of falsework for the jacking, and dig down to the footings of the existing piers," he said. He pointed out that crews excavate next to each pier to locate the existing footing, then set falsework, jacking post and jacking box on the footing. In a few cases they've had to build temporary concrete footings to support falsework and jacking posts.
Each jacking post and section of falsework placed directly beneath a beam is designed to support 200 kips, according to White's project engineer, Chad Sowersby. He added that jacking is restricted to the hours between 11 p.m. and 5 a.m.
"We shift traffic from one side of the road to the other before we jack. Then we might have anywhere from 10 to 20 hydraulic jacks going — all synchronized — to lift the dead load off the pier a quarter-inch, and insert shims to hold that elevation."
With the load removed from the pier, crews take out the old bearings, and begin demolishing the top of the pier. This is difficult, since there is little clearance for the workers who have to duck under beams as they move along temporary scaffolding.
Two kinds of equipment are employed to demolish pier tops. For the "wings" of the hammerhead piers, J.F. White uses a Gradall equipped with a conventional breaker. The Gradall's unique configuration allows it to work in the limited clearance beneath the beams. But the middle section of the top — that portion in line with the stem of the pier — is left to subcontractor Fisher Contracting Co. The Worcester company uses a Brokk Robotic 330 demolition machine for this application.
Sowersby explained that the more precise breaking action of the compact Brokk prevents damage to existing #11 rebars in the piers' central portion that have to be incorporated in the casting of the new top.
Once demolition is complete and forms have been set up, subcontractor L. Guerini Inc. of Mattapan, Mass., moves in under the bridge with a Putzmeister 20Z-Meter Boom Pump to convey the 5,000-psi, 3/4-inch stone ready mix from transit mixers to the pier forms. Designed with a low unfolding height and compact outrigger spread, the Putzmeister's four-section boom is able to work in the limited overhead height and narrow staging area under the bridge.
In the even lower areas under bridge approaches, Guerini uses a Putzmeister VS 2070 Truck Mounted Concrete Line Pump with a 3-inch "slickline" to pour the bridge supports.
After the concrete cures, Atlantic Bridge & Engineering workers install the seismic isolation bearings, attaching them to the top of the piers using anchor bolts.
According to George Kober of Dewberry-Goodkind, the bearings conform to the recommendations of the American Association of State Highway and Transportation Officials (AASHTO).
"The bearings should be good for any earthquake likely to happen in New England," said Kober.
Retrofitting the Neponset River Bridge for an earthquake may seem unnecessary since earthquakes are usually associated with California, not the Northeast. However, some of the largest seismic events in the United States have occurred east of the Rocky Mountains.
The recorded history of earthquakes in the New World began shortly after the Pilgrims landed, when in 1638, a damaging earthquake rocked Plymouth Colony, according to The Northeast States Emergency Consortium (NESEC), a not-for-profit all-hazards emergency management organization located in Wakefield, Mass. NESEC, funded by the Department of Homeland Security (DHS) and Federal Emergency Management Agency (FEMA), is governed by a board of directors comprised of the directors of the State Emergency Management Agencies from the six New England states, New York and New Jersey.
The consortium says new information shows that French colonists on the St. Lawrence River and the English along the Massachusetts coast felt the 1638 quake with equal intensity. In order for this to happen the most likely epicenter would be central New Hampshire, and the magnitude of the event was likely between 6.5 and 7. Another major New England quake, this one well documented, occurred October 29, 1727, with its epicenter located off the New Hampshire and Massachusetts coast, and caused damage from Boston, Massachusetts, to Portland, Maine.
And on November 18, 1755, the Cape Ann Earthquake occurred, with an estimated magnitude of 6.0 that caused widespread damage along coastal New England. It was felt from Nova Scotia to New York and Chesapeake Bay, and even by the crew of a sailing vessel more than 100 miles off the coast. In Boston, the shaking, which lasted more than a minute, reportedly rang church bells, toppled 1,200 to 1,500 chimneys and broke off the gable ends of buildings, blocking the streets with fallen bricks.
If this magnitude 6.0 quake were to hit Boston today, experts say it would cause more than $40 billion in damage with many thousands injured or killed. One of the reasons for so much damage: much of the city is built on artificially created landfill and thus is particularly vulnerable to the effects of a strong earthquake.
Not widely known is the fact that New England as a whole experiences between 30 and 40 earthquakes per year, although most are too small or remote to be felt. But due to the bedrock geology of this region, a large earthquake will affect a much wider area — say from four to 40 times greater than that of California because of intra-plate forces.
Exacerbating the impact of a quake here, most major cities in New England have a greater population density than the major cities in California, and much of this region's structures are old, non-seismically designed and brittle.
Data on earthquakes recorded at Weston Observatory, a geophysical research laboratory of the Department of Geology and Geophysics at Boston College, show that earthquakes occur in widespread locations of New England. For example, quakes were recorded in Plymouth, Mass., (12/17/05, 2.3 Magnitude); Middlebury, Vermont (10/06/06, 2.3 Mn); and Bar Harbor, Maine (10/03/06, Mn 4.2) — the latter caused a rockslide in Acadia National Park and was felt widely throughout coastal Maine.
Another fact about earthquakes not commonly known is the significance of the magnitude given in Richter Scale units. In technical parlance, the magnitude of an earthquake is determined from the logarithm of the amplitude of waves recorded by seismographs. What this means is, due to the logarithmic basis of the scale, each whole number increase in magnitude represents a tenfold increase in measured amplitude. For example, the wave amplitude of the Bar Harbor 4.2 quake mentioned above was 100 times that of the Middlebury 2.3 quake.
Another way to define an earthquake is by the amount of energy released. In line with this, each whole number step in the Richter Scale corresponds to the release of more than 30 times the energy of a quake one whole number less in magnitude. The Bar Harbor quake, then, had 900 times more energy than the Middlebury event.
Carrying the math one step further, the Cape Ann quake that shook New England in 1755 had 810,000 times as much energy as the 2.3 Middlebury quake.