Case Study: Millenium Place, Boston, MA - 55' Excavation

Millenium Place, Boston, MA

The Millennium Place project includes a 55'-deep excavation with five underground parking levels constructed by the up/down method. The excavation has just been recently completed. Numerous buildings are right next to the excavation along the northern, western, and southern sides of the project. The MBTA Orange Line runs under Washington Street to the east of the excavation. Thus, slurry walls were selected to protect adjacent structures from excavation induced damage. The figure on the left displays a typical excavation section while the following figure shows the site plan.

Soils at the site are representative of profile B as classified by Johnson 1989. The soil profile consists of fill, organics, outwash deposits, marine deposits, outwash deposits, glacial till, and bedrock.

The diaphragm walls are 3.0'-thick and have their toe embedded 40' beneath the base of the excavation, into the marine and glacial till deposits. Load bearing elements and caissons were constructed to carry the vertical loads from the basement floors and the superstructure.

Measured wall deflections are in the order of 0.3" to 0.8" towards the excavation, with some bending evident. Measured settlements are in the same order as wall deflections but at some localized points settlements were larger probably because of other construction activities.