City Commissioners will be asked during their March 19 meeting to approve a $111,600 professional services agreement with Terracon Inc. for the Giant Springs Road slide repair.
Staff is recommending approval of the agreement for Terracon to conduct a topographic survey, complete the project design, develop the plans, assemble bid packages, assist with bidding, and complete as-built drawings of the slide repair improvements associated with this project.
A fill-slope failure of the western slope of the Giant Springs Road embankment was first reported by Montana Fish, Wildlife and Parks in January 2021 to the city.
City staff and Terracon confirmed the slide during a site visit, observing a shallow failure on the down slope side of the section, which is the west side, towards the river.
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The existing road embankment is sloughing, which can cause a potential hazard to the public, according to the staff report.
In May 2022, the city retained Terracon to conduct subsurface evaluation of the slide area to develop conceptual strategies to select a final repair design option.
The subsurface investigation and monitoring of the landslide included three geotechnical borings used to determine subsurface conditions and failure mechanisms of the slope. Along with the borings, inclinometers were placed across the site to monitor and determine depths and rates of movement over the course of a year, according to staff.
In conjunction with the monitoring, laboratory testing was conducted to determine soil parameters to facilitate slope modeling and analysis of the failed section to design conceptual repair strategies.
The Giant Springs Road site comprises of an existing roadway fill section.
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An upper walking trail parallels and abuts the east/southeast shoulder of the road and the River’s Edge Trail is located downslope and to the west/northwest of the failed area, according to city staff.
Near the north end of the project area is a pedestrian tunnel that goes under Giant Springs Road.
A culvert collects water from east of the road fill and exits the slope near the south end of the pedestrian tunnel on the west side of the embankment, which consists of a shallow fill-slope failure along the northwestern slope of the Giant Springs Road overlooking the River’s Edge Trail to the west, according to the staff report.
The failure is a typical circular failure plane in which the destabilized plane rotates downslope from its original position with a pronounced drop at the top of the failure and a bulge of material moving near the base of the failure.
The adjacent drainage appears to be the primary cause of the slide as the drainage between the natural hillslope and the embankment fill has resulted in saturation and undercutting of the embankment fill along the drainage.
The project will implement the design phase of the improvements recommended in the geotechnical report from March 2023.
Given the surficial nature of the fill-slope failure, two embankment slope repair alternatives are proposed. The first alternative includes grading the slope to a flatter slope condition. The other alternative is installing a rip-rap shear buttress keyed into the bedrock at the embankment toe.
Drainage repair alternatives will need to transport surface water down the drainage to the drainage basin without allowing saturation and undercutting of the embankment fill, according to the staff report.
Once the project construction is bid, the commission will be asked to approve that contract.
City staff are planning to fund the Terracon contract with $106,020 in state funds and $5,580 in city street funds. The project has been selected and prioritized within the city’s public works capital improvement program.
