Project Overview

This summer, the Montana Department of Transportation (MDT) and Sletten Construction, in cooperation with Lewis and Clark County, will replace the Dalton Mountain Road Bridge that runs over the Blackfoot River, located three miles west of Lincoln. Sletten Construction was awarded the contract to build the bridge for $3.9 million.

Because the bridge is owned by Lewis and Clark County, the Dalton Mountain Road Bridge Reconstruction project is being funded through MDT’s Off-System Bridge Program and Montana Coal Endowment Program funds. Lewis and Clark County nominated the bridge for replacement after the 68-year-old timber bridge had several foundation piles fail in 2017. Emergency repairs conducted in the fall of 2017 temporarily stabilized the bridge so it could be reopened to the public as a one-lane bridge with loads restricted to 8 tons.

The new bridge will be a single-span steel pony truss with a cast-in-place concrete deck. A single-span bridge was chosen because it eliminates the need for in-stream piers, which minimizes debris snags and reduces the time that construction is being conducted in the river.

The bridge will span 150 feet, accommodating two 12-foot travel lanes and 1-foot shoulders. Safety enhancements include adding bridge rail. Additionally, the bridge alignment was shifted slightly to maximize sight distance and safety, and ditch and irrigation work will be done to improve drainage.

Dalton Mountain Road Bridge before construction

Timeline

Sletten Construction will begin work on the Dalton Mountain Road Bridge Reconstruction project in April. The initial activity will include tree trimming and removal with minimal impact. The traveling public should expect flaggers and minimal delays.

Once tree work is complete, crews will suspend activities until July. This is because this stretch of the Blackfoot River is designated as critical habitat for bull trout, and due to regulations from the United States Fish and Wildlife Services, construction work in the river cannot occur until July to protect the bull trout.

In July, construction will begin on the bridge. The bridge will be closed, and a detour will go into effect. Construction is anticipated to last through the fall.

Impacts to the Public

During the closure of the Dalton Mountain Road Bridge, traffic will be detoured around the project site on Herrin Lake Road via the detour shown on the project map. The detour length is approximately 11 miles, bridge end to bridge end.

Detour directions from the north side of the Dalton Mountain Bridge:

• Head north on Dalton Mountain Road.
• Turn right, heading east on Montana Highway 200 (MT 200).
• Turn right, heading south on Stemple Pass Road.
• Continue on Stemple Pass Road as it bears right/south.
• Turn right on Herrin Lake Road, heading northwest.
• Turn right on Dalton Mountain Road, heading northwest.

Detour directions from the south side of the Dalton Mountain Bridge:

• Head south on Dalton Mountain Road.
• Turn left on Herrin Lake Road, heading east.
• Turn left on Stemple Pass Road, heading north.
• Stay on Stemple Pass Road as it bears left/east.
• Turn left on Main Street/MT 200.
• Turn left on Dalton Mountain Road, heading south.

When traveling during construction season, watch for the “cone zone” and workers on the highway. These workers spend their days working a short distance from fast-moving vehicles, and while they make every effort to work safely, they count on you to pay attention, slow down, and be careful as you pass through a work zone.

Engineer to English

Pile or Pier: a concrete or wooden post that is driven into the ground to act as a support for a bridge. Piles and piers are the vertical support structures of bridges.

Single-span bridge: a single-span bridge stretches across the entire water body and has no piles or piers in the water for support.

Truss bridge: a bridge whose load-bearing superstructure, which is the structure built upon the foundation, is composed of a truss, a structure of connected elements, usually forming triangular units.

Steel pony truss bridge: a type of truss bridge where the traffic deck sits between two trusses, but the trusses are not connected at the top. These types of bridges are designed for lighter loads.

Deck: the surface of the bridge that is driven upon.

Cast-in-place: cast-in-place concrete is used as the method of placing a deck on a short-span bridge; in this method the concrete is poured for the deck at the construction site. These bridges use reinforced steel embedded in the bottom of a thick concrete slab to carry the bridge load.

This is an example of what a pony truss bridge looks like.

MDT’s Partner in Progress

UPN 9776000