BACKGROUND
The American Furniture Warehouse in Colorado Springs is a large footprint retail furniture store and warehouse constructed in the late 1990’s. The store was built with precast panels and interior columns bearing on a friction pier and grade beam foundation system. The structure has experienced intermittent differential settlement causing the precast panels to develop unacceptable joint separation and distress. Several attempts to mitigate the settlement, including additional shallow piles and compaction grouting, have been attempted over the past twenty years with results not meeting the owner’s expectations.
OBJECTIVE
The owner requested a value engineered solution to stabilize the foundation while keeping the retail store, warehouse, and loading docks unobstructed and fully operational.
CHALLENGES
The forensic structural and geotechnical engineers decided a deeper foundation underpinning system, bearing in bedrock, would be preferred over another attempt to use supplemental support in the overburden material. The interior columns would then be addressed through structural adjustment methods. Site conditions were very challenging with steep grades, lateral space constraints, and bedrock which was 50 – 70 feet below grade with overburden consisting of collapsible colluvium and fill with water flowing over the surface of the deep bedrock.
SOLUTION
It was quickly determined that creating access for a traditional cased piling rig within the topographical constraints of the site was not going to meet the owner’s budget – not to mention their request to keep the entire site operational.
DRS installed and tested a variety of limited access micropiles and resistance piles in attempts to identify a cost acceptable approach to the deep foundation request. Ultimately pre-drilled resistance piles proved to be the best value approach to meet the loading parameters set by the structural engineer. Although there was a tendency for the pre-drill holes to partially collapse prior to installation of the resistance piles, we found that the resistance piles were usually capable of penetrating through the collapsed zone with the proper amount of patience and persuasion. During production, we developed a successful formula for a bentonite drill slurry which reduced the hole collapse rate to less than twenty percent – greatly improving the rate of pile installation and yet not impeding the advancement of the piles.
DRS successfully underpinned the foundation into bedrock with more than 100 pre-drilled resistance piles with an average depth approaching 70 feet and an average capacity in excess of 70 kips.
