How to design a bridge over the Rio Grande?

The proposed I-25 interchange Los Lunas Boulevard Corridor will extend eastward as an extension from Morris Road and will include a bridge over the Rio Grande.

How do engineers and geologists select the best design for a bridge over a sandy river with significant seasonal water level variations? In addition to geological and engineering concerns, the proposed bridge construction must also satisfy multiple agencies, maintain the drainage channels and associated agricultural water demands, and ensure environmental protection of natural resources.

The last new bridge across the Rio Grande in central New Mexico was the Montano bridge opened in 1997.

Five bridge type options were considered, each with an overall span of about 2,265 feet and various pier and structural configurations. Placing about 500 feet of fill in the river channel would result in too much of a water rise at the bridge and farther upstream, threatening overtopping of the non-engineered levees that protect the surrounding areas from flooding.

As a result of assessing these effects on river flows, functional requirements, economics, future maintenance requirements, construction feasibility, and aesthetics, a prestressed-concrete girder bridge was recommended as the preferred bridge type.

The recommended design is an 18-span bridge with abutments located at the existing levees and 17 piers in the river channel. The proposed bridge would be 2,265-feet long and 80-feet wide with two driving lanes in each direction, raised median, shoulders, and a multi-use trail on the north side.

To maintain the river’s hydraulic function and protect biological resources during construction, the bridge will be accessed and constructed by placing fill in half the active channel and diverting the river flow, installing the piers and the bridge girders, then removing the fill and repeating the same operation on the other side of the river. Located in a moderate seismic region and within the perennial riverbed of the Rio Grande, each bridge pier will be built on 18 steel driven pipe piles 24-inch diameter and approximately 100-feet deep.

Above ground, each pier will consist of continuous reinforced concrete pier walls that ultimately support the bridge girders and deck. Throughout the bridge’s construction, mitigation measures and monitoring will be conducted to ensure the protection of natural resources.

The construction of the bridge is expected to take about two years but will only be conducted within low water level winter seasons within the active river channel and year-round on the dry bosque areas.

Further information can be found in the April 2024 Environmental Assessment Report on the Los Lunas Boulevard Corridor prepared by Ecosphere/Barr Engineering. The final bridge design is being prepared by Bohannan Huston for Molzen Corbin and the village of Los Lunas in coordination with the New Mexico Department of Transportation and the Federal Highway Administration.

(Paul Parmentier, a certified professional geologist retired from California and living in Los Lunas, shares the rich geologic features in Valencia County. He earned a Bachelor of Science degree in geology from Belgium and a master’s degree in geochemistry from Japan. The Geology Landscapes of Valencia County are featured monthly.)