In times when budgets are being slashed, communities continue to invest in the improvement of existing and creation of new bike trails. However, cost is a major consideration so officials need to be careful and creative when it comes to designing these projects and determining construction methods.
A recent trail project in Altoona, Iowa, required community services director Vern Willey to do some innovative thinking and called for impressive construction by a trenchless contractor.
According to Willey, the idea for the project goes back almost a decade. The city had adopted a plan for a bike path that looped through the city; the ultimate goal was to tie into another trail and eventually encompass a 110-mile trail system. The city received a $200,000 state enhancement grant, which designated an easement along the railroad right of way where the trail would run.
Finding the proper balance between existing areas and the new bike trail was an issue. There were tight locations along the right of way, an industrial area residents didn't want the trail near, and the plans had to account for a pedestrian bridge, multiple track crossings, and other challenges. The city and contractor knew they would need to go under the tracks. Willey and Snyder & Associates Engineering, Ankeny, Iowa, ultimately chose pneumatic pipe ramming to accomplish this crossing. Miller the Driller, Des Moines, Iowa, was contracted to install the 60-foot casing needed to facilitate the bike path tunnel under the rail line.
The project was the largest-diameter pipe ram ever completed. To create a tunnel large enough to accommodate bicyclists, the steel casing would need to be very large. The 147-inch outside-diameter (OD) casing used for the tunnel bested the previous record by 3 inches. To complete the job, Young chose a 24-inch-diameter Grundoram Taurus pneumatic pipe rammer from TT Technologies, Aurora, Ill.
Ramming Basics and Benefits
Trenchless pipe installation through ramming can have impressive results. A pneumatic hammer is attached to the rear of the casing or pipe. The ramming tool drives the pipe through the ground with repeated blows. A cutting shoe is often welded to the front of the lead casing to help reduce friction and cut through the soil. Bentonite or polymer lubrication can help reduce friction during ramming operations.
According to TT Technologies pipe ramming specialist Mike Schwager. “An entire length of pipe can be installed at once or, for longer runs, one section at a time. In that case the ramming tool is removed after each section is in place and a new section is welded on to the end of the newly installed section. The ramming tool is connected to the new section and ramming continues.”
Some casing installation methods are impaired by rock or boulder filled soils. Pipe ramming is different. Boulders and rocks as large as the casing itself can be “swallowed up” as the casing moves through the soil and can be removed after the installation is complete. Ramming requires minimal working depths and is proven effective for horizontal, vertical, and angled applications. Ramming is also ideal for installations under roads and rail lines because it displaces the soil without creating voids or slumps.
According to Willey, the contractor worked with town officials to address cost concerns. Initial bids came in higher than expected—thanks to skyrocketing steel costs—at around $3900 per foot. The teams reworked the design, raised the tunnel slightly, and cut approximately 10 feet off the total length, decreasing the project's price tag.
The ram took place under rail lines, which remained active throughout the project, owned by the Iowa Interstate Railroad Ltd. A launch pit was dug on the south side of the tracks to accommodate a 50× 16-foot concrete pad that would serve as a platform for operations. They built a concrete backstop and concrete launch pit on the south side of the tracks to minimize environmental impact where excavation and clearing would take place.
After completion of the pit construction, the crew assembled a driving stage for the Taurus from an auger track and used a hydraulic push sled to assist with ramming. The pipe chosen for the project was made by Permalok, St Louis. The casing was fabricated in 20-foot sections, each with a wall thickness of 1 ½ inches and weighing approximately 47,000 pounds. The casing incorporated a mechanical press fit design without an internal or external bell, which helped save time.