By John Siwula
By now, more than 12,000 people have watched this extraordinary time-lapse video of a C.D. Smith Construction team easing an 85-foot-high wall 35 feet outward to expand Milwaukee’s Grand Theater and transform it into the new Milwaukee Symphony Center.
What few people may have thought about—and what GZA is proud to have played a key behind-the-scenes role successfully completing—is what needed to be done to ensure the soil could support the 1.25-million-pound weight of the historic wall being moved, including the precise installation of more than 45 truckloads of gravel to strengthen the soft, silty soils at the project location.
The Grand Theater is cherished in Milwaukee as one of the city’s most iconic buildings and an architectural gem of Wisconsin Avenue. As the Milwaukee Symphony Orchestra began developing its $90 million plan to convert the theater into a new 1,750-seat performance space, there was never a question that the historic 1932 steel-frame wall, with its cream city brick infill and terracotta cladding, would be preserved and restored rather than demolished.
Yet doing so would require some careful design. First, there was the wall move structure itself: Any system put in place needed to support the rails and jacks used to roll the wall outward, the 100 construction workers supervising the move, and the 625-ton weight of the wall itself. Secondly, the wall move had to be designed to avoid any impact on an existing, 70-year-old underground steam-heat tunnel, and had to be coordinated with eight separate utility relocations within Second Street. Furthermore, the structural engineers’ specifications were that the move could allow for no more than ½ inch of settlement.
Finally, the earth the building rested on presented its own challenges. The Grand Theater site is underlain with variable strength fill followed by compressible estuarine deposits which are subsequently underlain by high strength glacial till soil. The near surface estuarine soils consisted of very soft, highly organic, fine-grained soils (peat and OL- and OH-type soils) ranging from about 1 to 2 feet thick. Below the thin upper organic layer, soils were generally comprised of interbedded, elastic silt, and silty sand. The elastic silt (MH-type) soils were in a very soft to medium-stiff consistency. The estuarine soils extended to depths of approximately 50 to 60 feet below grade.
Instead of adding complexity to a job with many moving parts, GZA made use of what was already there. The company’s geotechnical engineers designed a ground improvement process that allowed the structural engineers and the wall movers to use the new floor slabs and underlying soil to support the cribbing for the move. The system consisted of a 12-inch thick layer of gravel placed on the over excavated subgrade followed by a single layer of Mirafi HP370 geotextile. Another 2-foot thick layer of gravel was placed over the geotextile and floor slabs cast on that material.
Over the course of six hours on August 13, International Chimney Corp., with support from Expert House Movers and structural engineering consultants Pierce Engineers Inc., completed the wall move flawlessly in a series of seven five-foot jack extensions in between which the systems were re-set to keep the wall moving to its ultimate destination. In case you’re wondering just how carefully the move was executed, that’s equal to 0.00114 miles per hour. At the end of the move, the total settlement was less than half of what the structural engineer specified.
With the new 3,500-square-foot space created by the wall’s move now being enclosed and finished, the MSO is looking forward to its first performance in the new space around this time next fall. For everyone at GZA, C.D. Smith Construction, International Chimney, Pierce Engineers, and Expert House Movers who worked on this one-of-a-kind project, that will be some especially sweet music to enjoy.
John Siwula is the Associate Principal of GZA.