Thrasher Foundation Repair Case Studies: Elkhorn River Bridge Approach

Challenge

Douglas County Engineers commissioned the repair of a 22-foot-wide, 21-foot-long stretch of Q Street at the west side bridge approach to the Elkhorn River. It was determined that the area under this slab had lost significant amounts of fill causing an approximate two-foot void under much of this area.

Complicating this project is the fact that the bridge abutment and wing wall were both allowing fill materials and soil to escape from under the slab. Repair options included removal and replacement of the slab panels, traditional flowable, utilizing high density polyurethane to fill the void or using cellular concrete as a flowable fill material. Replacing the slabs was deemed too costly and disruptive. Utilizing traditional flowable fill material was strongly considered but the additional weight being added to the soil was considered excessive and the cost was higher than other options. A high density polyurethane material was also considered but due to the size of the void, this too was considered too costly.

A more viable option ultimately chosen by the Douglas County Engineers was to use a StableFILL™ cellular concrete as the flowable fill material to fill this void. Since the road slab was still in place and no slab lifting was required, this option was perfect because it is light weight, extremely flowable and made the most economic sense.

Solution

Thrasher's cellular concrete product, StableFILL™, provided a safe, quick, effective and economical means to fill the void beneath the Q Street bridge approach while minimizing the additional weight being added to the supporting soil. StableFILL™ is a combination of concrete slurry and a specially designed foam which blends with the concrete slurry to create a lightweight (typically between 20 and 65 PCF) yet extremely durable flowable fill material with compressive strengths typically ranging between 50 and 1000 PSI. Because the bridge abutment and wing wall were both allowing soil materials to escape, these areas needed to be sealed to prevent the free flowing StableFILL™ cellular concrete from escaping.  

To remedy this situation,  Thrasher utilized a high density polyurethane spray foam called PolyLEVEL™ to seal the cracks on both the abutment and the wing wall and then began the StableFILL™ injection process. For this application, StableFILL™ was installed using a concrete pump and an inline injector that blended the foam with the concrete slurry. From there, the blended product is carried through a concrete hose to a 4” hole cored in the road above the void.

Prior to installing, the mixture is weighed to determine the appropriate wet cast density and adjusted until the mix design meets the predetermined mix criteria for this application. Four additional dime sized air escape holes (burp holes) were drilled above the void 20 feet away to prevent air locks which would prevent StableFILL™ from filling the entire void. Visual inspections of the drilled and cored holes showed the void filling and pumping ceased as the void was completely filled. All were patched with concrete.

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