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I-70 Roadway Settlement Mitigation Project Spotlight

Location: Interstate 70 West of the Eisenhower/Johnson Tunnel - Summit County, CO
Application: Surcharge Reducing Caissons

Key Players:

Owner: Colorado Department of Transportation
Engineer: Shannon & Wilson
Drilling Contractor: Anderson Drilling
Cellular Concrete Applicator: Cellular Concrete Inc.

Background Information

Interstate 70 is built on a landslide mass 500-700ft wide and 250ft high that undergoes regular movement, causing significant settlement to the roadway, on the order of 3-6in a year. The settlement causes depressions in the roadway resulting in traffic problems for motorists. CDOT typically needs to overlay new asphalt when the settlement reaches more than 1-2in which results in overlay maintenance annually, sometimes multiple times a year.
landslide outlines

Map of I-70 showing location of landslide features and slide scars from original construction

Problem

CDOT was looking for a settlement mitigation plan that would reduce the need for annual/multi-annual overlays, minimizing maintenance costs and the inconvenience of lane closures and construction backups to motorists. Subsequently, the annual overlay operations had increased the thickness of the asphalt to more that 6ft in areas which added to the overburden on the slide materials. Milling for a 2007 overlay project exposed the presence of voids and cracks below the pavement. CDOT considered 3 options before choosing cellular concrete. These 3 options resulted in price tags of $10 million+ to implement. Additional challenges CDOT faced was the location of the project, a 3-4 month construction season with over 400 inches of snow per year, steep slopes and limited access, high traffic volumes and speeds, sensitive environmental considerations, plus a lack of funding.
voids

Milling exposed the presence of voids and cracking below the road surface.

Solution

For CDOT the performance goal was not to solve the slide issues but to minimize roadway life cycle maintenance costs and reduce driver distraction from the pavement distress. The critical goals were filling void spaces below the highway, reduction of dead load, and drainage; all items that could be beneficial for full slide stabilization in the future. These goals were met with a settlement mitigation plan utililzing drilled shafts filled with lightweight cellular concrete. This solution would reduce the dead load on the roadway, fit within the constraints of traffic control, expedite the schedule to fit the short construction window, and provide an economical solution to reduce long-term maintenance costs.
drilling

Drilling operations exposed areas of asphalt up to 6 ft thick.

Construction

Drilled shafts were selected because they could be built lane by lane within traffic constraints. Trenching or other excavation methods were considered but eliminated due to shoring requirements, difficulties protecting open trenches from traffic, and the increased time necessary for excavation and backfill. More than 300 5ft diameter caissons were drilled at 10 -20ft depths at 10ft on center. The caissons were filed with a 36-39pcf lightweight cellular concrete with a 2in slump and minimum strength of 80psi. The cellular concrete was foamed on site using Aerix Industries AERLITE-iX foaming agent and a self-contained trailer wet batch system. The WB lanes were completed in the summer of 2010 and the EB lanes were completed two years later in the summer of 2012
wb lanes 2010

Construction operations on the west bound lanes in 2010

Aerix Added Value

CDOT had some familiarity with cellular concrete going into this project, but the ability to design the material to produce a 2in slump was a technological break through. Traditional cellular concrete generally has a range of 8-11in slump. The Aerix team specially engineered the liquid foam concentrate to solve these specific project challenges. The stiffer material enabled the contractor to fill the 20ft deep caissons in one continuous lift instead of having to let smaller lifts cure before pouring another lift. Placing the material at up to 20ft lifts expedited the schedule in an already tight construction window. Even with a stiffer material, the cellular concrete was incredibly lightweight, with a density approximately 30% of existing soil. The mobility of the material also allowed it to flow into intersected void spaces, providing some additional ground improvement below the road surface that will be beneficial to future efforts of stabilizing the slide. The material will also not be cause for concern with disposal should excavation occur in the future. Initial performance suggests roadway settlement due to void spaces has decreased. More time is required to fully evaluate the performance.
value added

36-39 pcf cellular lightweight concrete with a 2" slump and minimum strength of 80 psi placed in single lifts up to 20'

Installation Animation

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