SHERIDAN — Those who utilize Sheridan’s pathway system along Goose Creek have likely noticed earth movers and construction workers tasked with adding stability to the hill that borders the Sheridan Junior High School parking lot.
Crew members that look small in comparison to the large trucks, walls and mounds of earth started work on the hill stabilization project in June and aim to have the project substantially completed by Sept. 29.
While to the casual observer, it may look like crews have just moved dirt around, the planning for the project has taken months of preparation and cooperation among various entities including the city of Sheridan, Sheridan County School District 2 and U.S. Army Corps of Engineers, among others.
The problem with the landslides in that area date back decades. Mathers Heuck, facilities director for SCSD2, said the creek’s alignment was changed somewhere around the 1950s. Maps from the late ‘40s show the stream looping away from the slope and south through the area that is now the YMCA. Changing waterways along with complicated water tables in the area may have contributed to the landslides over time.
In most cases along the slope, the lower bedrock is low-permeability claystone with sandy soil above it. This means water moves through the dirt and collects on top of bedrock, creating a slip plane. As the upper portion of the slope slides across the bedrock, moist layers of sandstone and coal are exposed above. These seams act like conduits and contribute even more water to the already failing soil.
Because the top layer cannot drain efficiently, the weight on the top part of the hill has proven too much for the bottom of the hill to support, causing the hill to slide.
The crews from Hayward Baker, a Colorado-based company rated No. 1 for 17 consecutive years by Engineering News-Record for excavation and foundations, will remove approximately 40,000 yards of earth from the hillside with the aim of lightening the weight supported by the bottom part of the hill. The Colorado company has also utilized local companies to help with the work and has sought to purchase as many materials locally as possible.
In addition, the work crews will construct a system of horizontal drains that go into the hillside, helping to dissipate the water that collects on the hill.
The most visible portion of the project, once it’s completed, will be walls the company has started to erect. Retaining walls constructed from the top down will help apply pressure to the hillside and provide additional support for the hill along the creek. The walls will include a finish that makes them look like natural cascading boulders.
While it may look like crews are mostly moving dirt from one place to another, Heuck noted that a geotechnical engineer has to ensure dirt being moved and roads being created can support the weight of trucks needed to complete the project.
“This kind of work is very specialized in nature,” said Hanns Mercer, city engineer for Sheridan. “There are not a plethora of contractors that do this.”
Construction on the second wall, which is nearest the creek at the bottom of the hill, will begin in the next couple weeks.
The project, expected to cost $7,819,243.78, will be paid for by the city of Sheridan with various grants, loans and a contribution from SCSD2.