Discovering undocumented underground conditions is always a nerve-wracking part of construction. When the underground conditions are under a 6 story brick building, the stakes are higher, and the whole project team has to pull together to keep the project from slipping into delay, or worse, slipping into the mud!
When excavations began under the Howard Brubeck Theater at Palomar College in San Marcos, the team knew there could be issues. The design called for threading a new tunnel between the existing caissons that hold up the 65′ tall building, in order to provide elevator access the new orchestra pit. The caisson and grade beam system had done an excellent job of holding up the building since 1975, but as the excavation reached 7′ below grade, the caissons suddenly flared out and stopped… far shallower than documented, and shallower than the floor of the new tunnel!
As the dewatering pumps worked, the design team and contractors worked together to brainstorm how to hold up the massive building, while passing the tunnel under its foundations. Many site visits were made where the team had the pleasure of looking at the UNDERSIDE of massive foundations while standing in the soupy mud that passed for soil on the project.
After considering micro piles, post tensioned tie backs, special miniature drilling machines, soil grouting, and many other exotic solutions, the team developed a system of shoring, cantilevered grade beams, stepping footings, and specialized waterproofing materials that fit the tighter constraints , and allowed the construction to move forward with only a minimum of delay.