The project is located on the Sacramento County/San Joaquin County line. The project roadway/bridge (Franklin Boulevard in Sacramento County and Thornton Road in San Joaquin County) runs roughly parallel to and approximately 1 mile east of Interstate 5.

The Franklin Boulevard/Thornton Road Bridge at the Mokelumne River is actually two separate structures: a timber trestle structure across the overflow floodplain north of the Mokelumne River and a steel truss swing bridge over the Mokelumne River. Within the project area, the center of the Mokelumne River delineates the Sacramento-San Joaquin County line. While the majority of the project lies within Sacramento County, the steel truss swing bridge extends into San Joaquin County. The steel truss bridge is jointly owned by both jurisdictions.

Together Franklin Boulevard and Thornton Road form an important farm-to-market route between San Joaquin and Sacramento Counties and serve as an alternate route for Interstate 5 (I-5). Thornton Road serves as a major collector to the rural areas east of I-5 between Sacramento and the City of Thornton. The Franklin Boulevard/Thornton Road Bridge project is actually a replacement for two separate structures: Bridge No. 24C-0048 (a timber trestle structure that crosses the overflow floodplain of the Mokelumne River (owned by Sacramento County), and Bridge No. 29C-0392, a steel truss swing bridge over the Mokelumne River flanked by reinforced concrete approaches (jointly owned by both Counties). In total, the existing structures consist of a 230-foot long steel truss swing bridge, north and south reinforced concrete slab, and a timber trestle, approximately 1700-feet long.

The timber trestle bridge is currently closed to traffic due to extensive scour and sub-structure damage resulting from the flooding of 1997.

The bridges over the Mokelumne River and surrounding floodway consist of (starting from north) approximately 1700 feet of timber approach trestle, a concrete slab bridge on concrete piles on the north approach (8-25 foot span) to the 230 foot swing span steel truss bridge crossing the river channel, and finally 2-25 foot spans of concrete slab bridge on concrete piles on the south approach. The existing bridges on their current alignment were built in 1950.

The Mokelumne River is considered navigable for recreational traffic. Shortly after the construction of the structures in 1950, the dredging of the Mokelumne River was discontinued and the swing span was no longer required. The trestle bridge crosses an environmentally sensitive area and is currently home to one of the largest maternal bat colonies in Northern California.

The project proposes replacement of the timber trestle and steel truss swing bridges with a new structure at Franklin Boulevard and Thornton Road in southern Sacramento County and northern San Joaquin County.

The existing horizontal channel clearance under the swing span is 80 feet and the existing vertical clearance is 21 feet above MLLW (Mean Low Low Water). The superstructure proposed over the Mokelumne River is a three-span (120-140'-120) continuous, spliced bulb tee girder with a composite reinforced concrete cast-in-place deck. The substructure consists of two column bents (4'-0" diameter) supported on 5'-0" diameter cast-in-drilled-hole (CIDH) concrete piles.

The structure type proposed for the replacement of the existing timber trestle consists of a 55-foot multiple-span PC/PS voided slab system. This design provides both suitable and substantial bat habitat to replace that which will be lost due to demolition of the existing trestle structure by including roosting spaces on the underside of the bridge similar to the crevices on the existing bridge.

The Bean Ranch Road intersection with Franklin Boulevard bridge will be improved over the existing condition. The proposed voided slab structure will widen at the intersection to accept truck turning movements safely. The trees along the west side of the new structure will be removed for a sufficient distance, from the edge of bridge deck, to provide proper sight distance at the intersection.

Bridge construction can be effectively broken into two segments. The main channel crossing, which extends from the south bank of the Mokelumne River to just south of Bean Ranch Road and the trestle replacement which extend from the end of the main channel structure to the north abutment approximately 1700 feet away.

Demolition of the existing bridges could take from 2 months to 6 months depending on the restriction placed on the activities. Restrictions may include measures that will protect other sensitive species at the site. Trestle replacement construction may take place concurrently with the construction of the main channel structure.