Pit/sag type mine subsidence has resulted in the formation of a void underneath the concrete slab floor resulting in cracking and continual sinking of the slab. The footprint of the void appears to mimic the boundary of the cracks in the slab (roughly elliptical) which span almost the entire length and width of the basement. Void is 12-18” deep (max) underneath the south-central portion of the basement where the slab shows the most amount of downward movement, and tapers toward the outer edges in all directions; in most areas the void appears to less than 6 inches. The work that is to be accomplished is described in the following text and associated figures: A) Furnish and Install Polyurethane Foam. The objective is to eliminate the hazard by filling the void with an expanding polyurethane foam (polyfill), thereby stabilizing the slab floor and preventing further downward movement. The polyfill is to be placed in the void by drilling small holes in the affected concrete slab and injecting the foam that expands, filling voids beneath the slab while creating a stable surface. The contractor shall make all reasonable attempts to place the material in such a manner that it flows into the voids completely. The injection pressure should be continually monitored to avoid any significant slab jacking/lifting of the existing sunken floor (to prevent potential destabilization of the load bearing elements of the structure). In addition to numerous existing cracks in the slab resulting from the sinking, two vertical holes currently exist within the slab (location of prior steel support jacks anchored to below grade footings) that should be monitored and/or accounted for during the injection process. All holes in the slab should be sealed with Portland cement after completion of remediation. A line item for 1,661 lbs of polyfill has been designated for bidding purposes (estimated for an average 6-inch void underneath the basement slab measuring approximately 39 ft x 23 ft). The actual quantity may vary. This item will be paid for per lb of polyfill placed in the void. B) Mobilization/Demobilization (LUMP SUM). This work item shall include all costs incurred by the Contractor during the project necessary to facilitate material and equipment transport to the project site and repair any damages caused by the contractor related to project implementation. This work item is to include all costs incurred by the Contractor for both access and egress. All disturbed areas shall be restored to pre-construction conditions. All injection holes in the slab should be sealed with Portland cement after completion of remediation. Deviation from this will be at the project manager's discretion. Debris generated in site restoration is to be disposed of offsite as per Section 84 requirements.
