Children's Hospital of Minneapolis
City: Minneapolis
State: MN
Owner: Children's Hospitals & Clinics
General Contractor: Glenn Rehbein Excavating
Project Description:
This job used jet grout columns as underpinning and excavation support for the construction of an addition to the hospital. The new addition required a 21 ft deep excavation next to an existing parking garage. The parking garage was founded on spread footings bearing on clean sands. In order to minimize movement of the garage, it was very important to provide underpinning support prior to excavation. Jet grouting was selected as the best way to accomplish these goals.
The sequence of construction was as follows. First, steel soldier beams were installed along the edge of the footings. Next, the jet grout columns were installed through core holes in the footings. The jet grout columns extended from below subgrade to the bottom of the existing footings. The columns were 4 ft diameter and were spaced 3 ft apart. Only one column per footing per day was installed. Finally, each jet grout column was carefully topped with cement grout at the end of each day to ensure good contact with the bottom of footing.
During the subsequent excavation, tiebacks were installed through the soldier beams to provide lateral support. The jet grout columns spanned between soldier beams, eliminating the need for timber lagging and preventing loss of sand during excavation. The jet grout columns contributed axial support to the footings and settlement of the parking garage was limited to less than 1/8”.
