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Bridge building from the ground up

To the average commuter traveling on I-25 past Lincoln Avenue and RidgeGate Parkway, it may appear that besides a lot of dirt being moved (close to 160,000 cubic yards to be exact), not much else is being constructed on the new light rail extension project.

However, if you look beyond and under the dirt, the building of foundations for two of the line’s bridges is officially underway. And, when the Southeast Rail Extension project is complete, there will be a total of three new bridges:

rendering-of-I25-Bridge
I-25 bridge drawing

rendering-of-Lincoln-Bridge
Lincoln Avenue Bridge drawing

rendering-of-RidgeGate-Bridge
RidgeGate Bridge drawing

To understand the process of bridge building, let’s start at ground zero. The first step, of this multiple phase process, begins below ground to build the foundation.

Below Ground Construction

Using a drill rig (shown below), holes are drilled deep into the ground to place reinforced steel cages, which are filled with concrete. This forms the below-ground caisson foundations for the bridge.

Drill-rig-and-rebar-cage-placement-images

Once the foundation is formed below ground the work above ground begins to complete the substructure. This is the construction phase for the bridge's primary support.

Caissons-for-RidgeGate-Bridge
Caissons for RidgeGate Bridge

Substructure Construction

The next construction phase builds the pier columns and abutments.

The pier columns are the intermediate supports that allow the bridge to span obstructions like buildings, roadways, highways and waterways.

The pier columns are also built with rebar cages that are filled with structural concrete.

Once the concrete has cured to full strength, forms are then placed on top of the pier columns with more steel and concrete to create the pier caps. On each end of the bridge is an abutment, where the structure's girders rest and connect.

Bridge-abutment-construction
Bridge abutment construction

Superstructure Construction

After the substructure work is completed, construction begins on the superstructure.

Pre-cast girders, which are made at an off-site facility, are delivered by truck and placed on the bridge supports by large cranes.

Girders typically are 120-150 feet long and can weigh between 100,000-150,000 pounds, depending on the bridge site requirements.

Typically, girders are lifted into place overnight and for safety reasons, lanes and highways are closed. Careful consideration is made to protect the traveling public and construction crews during this critical activity.

After the girders are in place, the concrete bridge deck—which provides the surface for the track or roadway—is poured and allowed to cure.

Since all three bridges are rail bridges on this project, track will be placed on the bridge deck and a guardrail, which is a third rail located in between the two running rails, will be installed to insure that the train stays on the bridge.

Additionally, fencing is installed on the bridge curb along the walkways as a precaution against falls of the maintenance crews and for the traveling public during emergency events.

Bridges built on overlapping schedules

Each of the three bridges under construction on the project will be built with overlapping schedules.

Each phase of bridge construction is scheduled to maximize the contractor’s crews and resources.

While the total construction duration for each bridge overlaps, the majority of the work will take place in late 2016 and 2017.

When all three bridges are completed, a great amount of work remains before the project opens for revenue service in spring 2019.

Other activities on the project include:will include:

  • Track work
  • Train systems elements
  • Station construction
  • Parking structure construction
  • Testing