Denver MicrotunnelingIn Denver, a new light rail system is being constructed — a spider web of rails spurring from downtown Union Station. Nearly every city in the metro area is somehow involved, but perhaps none more than the City and County of Denver.

High on the list of infrastructure improvements associated with the light rail construction was storm water management along the alignment of the new rails. Significant research went into the drainage along the rail’s routes, measuring anticipated design flows for five-, 10- and 100-year storm events. At the intersection of 40th Avenue and Blake Street, Denver was faced with a large obstacle in getting the excess storm water to the Platte River: The Union Pacific Rail Yard.

Design for the storm sewer began several years ago, with a variety of routes being evaluated. Denver’s design team looked at several options including box culverts, open channels, tunnels and split systems. Working with a budget well below preliminary estimates and a looming deadline for light rail construction, the design team began to think outside the box. Ultimately, it was decided that the only feasible route was to go under the rail yard.

In July 2013, BT Construction Inc. (BTC) of Henderson, Colo., was awarded the High Street Outfall Project for the City and County of Denver. The project consisted of side-by-side 96-in. diameter storm tunnels under the Union Pacific Rail Yard, each 400 ft long. On both sides, a Y-shaped concrete transition structure was to be constructed, tying the twin tunnels into a single box culvert. Crossing under 17 sets of rails with the tunnel inverts 27 ft below grade in a sandy material, BTC elected to construct the tunnels using an Akkerman microtunnel machine. That being decided, the first major hurdle was receiving the railroad’s approval.

During design, the City had filed for preliminary permits with the railroad. The design included permeation grouting along the alignment of the tunnel under all 17 tracks. However, the final construction method was to be determined by the contractor and approved by Union Pacific. BT Construction analyzed the design and proposed method of installation and felt there was a better, safer and more economical way to complete the project. In a submittal nearly 700 pages long, BTC outlined every detail of the machines, materials, and methods of the proposed tunnel. Contingency plans for various scenarios were submitted, as well as case studies of past projects. The submittal outlined the use of a closed-faced slurry microtunnel system. With the alignment under such valuable real estate to the railroad, any settlement on the rails was of major concern. Using the closed-face slurry system vastly minimized the risks associated with the landscape of the project.

The second part of the submittal, while consisting of fewer pages, required more extensive efforts. To meet the requirements of constructing both the tunnels, the jacking pit excavation had to have minimum inside dimensions of 40-ft wide x 48-ft long x 27-ft deep. Furthermore, the bottom 16 ft of the excavation had to be free of any cross supports to facilitate the construction of the cast in place structure. Such an excavation, in the best of situations, was challenging but reasonable. However, this excavation had one additional complexity: the front side of the pit had to be placed only 27 ft away from the nearest railroad track, thus dramatically increasing the load rating requirements of the shoring system.

BT Construction turned to Wylaco Inc., the local representative for the GME sliderail system. Griswold Machine and Engineering (GME), Union City, Mich., offers a full line of shoring equipment and had partnered with BTC on many past projects. In addition to the load ratings of an excavation that close to rails, one requirement of the railroad is the constant support of soil during excavation. The sliderail system chosen was able to accomplish that. A sliderail system is a dig-and-push style system. With its modular, flexible design, the system can comply with a wide variety of shapes and sizes. Installed from the top down and removed from the bottom up, the system minimizes excavation size, soil disturbances, restoration time and cost. Installation is done with low vibration, providing soil support for excavations, adjacent structures and existing utilities.

Prior to excavation, four corner posts and four additional linear posts were installed. The first of three strut carts was then installed, followed by the panels. As the excavation progressed, additional panels and strut carts were added, ensuring that the adjacent soil was supported at all times.

Excavating a pit of this size had another obstacle: Strut carts spanning 40 and 48 ft are not readily available. In addition, the depth of the pit required a total of three carts to be installed during excavation. GME’s engineers were able come up with a design that met all the criteria of this project. Design in hand, BTC contracted with GBC Inc. of Lakewood, Colo., to fabricate the carts per GME’s design.

Denver MicrotunnelingOne last obstacle remained in the preliminary design of the shoring. The concrete transition structures were 16 ft tall and no cross supports could conflict with the structure. In normal instances, sacrificial beams would be placed in an X on the floor of the pit prior to raising the strut carts. However, this design did not meet the required load ratings. BTC hired a structural engineer to collaborate with GME’s engineers to find a solution to the problem. In the end, a variety of unique systems were integrated together meet all the requirements.
Before being submitted to Union Pacific, the designs for each pit were reviewed at length by BTC and the City. Once approved at the field level, the shoring plan drawings and calculations were submitted to the City of Denver’s design engineer. Passing his review, the design was sent on to the Union Pacific for their final acceptance. With construction being so close to the railroad’s yard, Union Pacific was careful to scrutinize the shoring design, ensuring that it met all the requirements of excavation. GME’s final design was approved without comments.

The submittal process — beginning to end — took only 72 days.

The final step, before construction could begin, was executing a Right of Entry Agreement with Union Pacific. Generally, this process takes 30 to 45 days. For the High Street Outfall Project, the process was not so easy. After 108 days of paperwork and phone calls, BTC received the green light from the UPRR to proceed with construction.

Both pits were excavated with ease, with the shoring system performing perfectly. Tunnels were completed ahead of schedule and, currently, the concrete structures are under construction. The project is expected to be completed by June 2014.

Kevin Juliano is project manager with BT Construction.

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