A building was razed southwest of the intersection of Leavenworth Street and 14th Street for the construction of a new apartment building. Removal of the structure exposed the east exterior basement level wall of the Leavenworth Lofts, a five-story (plus basement) office/warehouse building constructed in 1915 and recently renovated to living space.
The two structures were built right up to the common property line such that the exterior walls were in contact. After removal of most of the foundation wall of the other building, a one and one-half inch wide separation was observed in the brick wall of the Lofts over a length of 60 feet. Any additional length of crack extending to the north of that exposed was covered by foundation wall remnants on the exterior and plywood panels and electric meters on the interior.
Based on the cracking pattern, it was estimated that approximately 80 feet of a middle section of the wall had settled. The crack extended completely through the two-foot two-inch wide multi-wythe brick wall. The settled section of the basement wall of the Lofts needed to be stabilized before the last section of foundation wall remnants could be removed and construction could begin on the new apartment building. The closest test boring completed for the new apartment building encountered fill soils to a depth of 13 feet, soft to stiff lean clay and loose sand to a depth of 38 feet, and stiff to hard sandy lean clay (glacial till) to the bottom of the boring at 65.5 feet.
The structural engineer recommended hydraulically-driven push piers to stabilize the settled section of wall. Push pier systems can achieve relatively high capacities with the versatility to be installed in limited access areas like the basement of the Lofts. The design included 36 Model 350 (3.5-inch OD by 0.165-inch wall) hydraulically driven push piers paired on opposite sides of the wall and spaced at four feet center to center. Each pier was to achieve a design working load of 32 kips. The crack in the brick wall was tuck-pointed and the mortar allowed to cure before installing piers.
The first phase of pier installation stabilized the southern length of the settled section of wall. With the first phase complete, the general contractor removed the remaining sections of foundation wall. The 18 pier locations on the exterior of the Lofts required pre-drilling to a depth of ten to 15 feet to allow the push piers to penetrate the old footing and construction debris in the fill soils. The 36 piers were installed to an average depth of 44 feet below the bottom of the footing to bear within the glacial till. The piers were driven to loads exceeding 1.5 times the design working load and then filled with concrete. At the end of the second phase, the 36 piers were fitted with hydraulic cylinders connected in series and reloaded to 32 kips to ensure uniform loading on each pier.
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