NH ROUTE 123/124 BRIDGE REPLACEMENT OVER THE SOUHEGAN RIVER AND 1829 STONE ARCH BRIDGE IN NEW IPSWICH, NEW HAMPSHIRE

Unusual site conditions at the NH Route 123/124 crossing over the Souhegan River in New Ipswich, New Hampshire have resulted in a number of design challenges for the replacement bridge design. A 1957 era bridge will be replaced with a new structure spanning the entire width of the Souhegan River gorge, obtaining all support from an abutment on each side. The 1957 bridge is a paved concrete deck, supported by steel girders, which are supported by multiple concrete pier footings resting directly on an 1829 stone arch bridge. When the piers were built, the top 4 feet of the stone bridge was reconstructed. No additional work was performed to reinforce the stone bridge. Since its construction, the 1957 bridge has deteriorated significantly, most in the steel components from roadway salt. The 1957 bridge has deteriorated and has been placed on the NHDOT’s red list, which means plans for its replacement are given priority and funding.

The new bridge configuration requires an evaluation of the condition of the historic stone bridge at the site, a determination of the stability the bedrock on the sides of the river gorge, and abutment foundation designs that don’t impact the stone arch bridge. The presence of the stone arch bridge, as well as a 6 foot diameter pipe penstock on the western side of the gorge which once fed water to a nearby mill, were complicating factors in the bridge foundation design. A two phase subsurface investigation was completed, which included collection of rock cores for compressive strength testing from each boring to characterize the subsurface materials. Due to difficult site access a surficial bedrock mapping program was not performed. To obtain the bedrock structural orientation data for foundation design a borehole geophysical survey was completed. This included both acoustic and optical televiewer methods to assess the elevation, dip, and dip direction of the bedrock structures including fractures, foliations, and veins. The bedrock data collected from the explorations, geophysical logging, and laboratory testing of bedrock cores were used to evaluate the impacts of rock fractures on foundation design.