NEWBURYPORT — With more than 70,000 passengers traveling across the I-95 John Greenleaf Whittier Bridge daily, replacing the historic structure is expected to present the largest, most complicated civil engineering challenge in the state since the commencement of Boston's Big Dig.

Now that the Massachusetts Highway Department has revealed preliminary plans for the $285 million project, expected to begin in 2011 and be completed by 2014, it's becoming clear that residents of Newburyport, Amesbury and Salisbury will be challenged to get around the construction zone in ways they hadn't imagined.

A total of 10 bridges or interchanges are being considered for replacement or renovation as part of the Whittier project so that eight lanes of traffic can be accommodated on I-95 and be built to current design standards "as much as practicable."

According to an Environmental Notification Form filed with the Executive Office of Environmental Affairs, MassHighway plans to replace, restructure or modify:

The Ferry Road (Pine Hill Road) I-95 overpass in Newburyport to accommodate a "wider" I-95 roadway,

the I-95 overpass at Evans Place in Amesbury,

I-95 north and south Route 110 (Elm Street) in Amesbury to accommodate an additional two lanes of traffic,

and three railroad bridges just north of Route 110 (Elm Street) in Amesbury.

"These bridges were constructed in 1967," according to the report. "Two of the bridges carry three lanes of traffic on I-95 over the railroad right of way and one carries Route 110 on ramp traffic to I-95 over the railroad right-of-way. The I-95 bridges will be widened to accommodate an additional lane of traffic on I-95."

Three additional overpass bridges at the north perimeter of the project will be improved, according to the document, including the I-495 north two-lane ramp over I-95, the Route 286 (Main Street) overpass over I-95 and the toll road over I-95.

MassHighway cites the Whittier Bridge as being listed in the inventory of Historic and Archaeological Assets of the Commonwealth, recognized also on the National Register of Historic Places as "one of three bridges in Massachusetts on the Federal Highway Administration's Final List of Nationally and Exceptionally Significant Features of the federal Interstate Highway System."

Despite its historic significance, however, MassHighway is not recommending it can be repaired for safe, continued use into the future. First built in 1954, it's been deemed structurally deficient and obsolete.

"The existing bridge is in need of extensive repairs to remain in service and cannot be rehabilitated to accommodate an eight-lane cross section," according to state engineers. "The bridge does not meet current traffic volume requirements, is the location of an elevated number of accidents and fails to meet current FHWA Interstate Highway Standards."

A variety of options for its future rebuild appear to be on the table, ranging from doing minor repairs to the recommended options of a full rebuild under a series of logistical scenarios. Though the expectation at this time is that no land will be permanently taken by eminent domain to complete the project, MassHighway cites a strong potential that land may be seized temporarily to complete the rebuild.

Among the building alternatives are the construction of a new bridge on the east side of the existing structure with six lanes, "enabling both northbound and southbound traffic to be relocated to the new bridge upon completion.

"The old bridge structure would then be demolished, and a new four-lane bridge for (southbound) traffic would be constructed," according to the report.

Another "West Side Option" would potentially begin at the opposite shore under the same schedule, and an "East and West Single Phase Option" would involve construction of new northbound and southbound four-lane bridge structures on each side of the existing bridge.

"Once all traffic is diverted to the new bridges, demolition of the existing Whittier Bridge would occur," according to MassHighway.

Before MassHighway can complete the work, they must get clearance from the Army Corps of Engineers, the Coast Guard, the Environmental Protection Agency, the Massachusetts Historical Commission and the Conservation Commissions of Newburyport, Amesbury and Salisbury. The report lays out that care must be taken to preserve the natural navigational channels flowing under the existing bridge, protect the surrounding wetlands and the habitat of the endangered short-nosed sturgeon and the bald eagle.

A public meeting has been scheduled at Newburyport's City Hall on Thursday, June 18 from 6-8 p.m.