DEEP EXCAVATION CASE STUDY - Review Options in DeepEX Software

One Memorial Drive, Cambridge MA

The project site is triangular in shape and is located between Memorial Drive and Main Street near Kendall Square in Cambridge. An asphalt lot separates the project from adjacent buildings on the west side. Approximately 35' from the north side is the MBTA Red Line subway tunnel. The project plan and a typical cross section can be seen in Figure 4.20. The ground surface is at approximately El. 21' CCD (City of Cambridge Datum). Three basement/parking levels (P-1B, P-2B, P-3B) extend beneath the entire building footprint.

A 2'-thick perimeter slurry wall provided lateral earth support for the excavation of this project. Most of the slurry wall was constructed in 20'-long panels, and was supported by one level of tiebacks. The slurry walls were typically 36' deep with the toe embedment depth below the excavation base ranging from 11' to 16'.

The subsurface exploration for this project indicated the presence of six major strata namely: fill, organic silt, sand, BBC, glacial till and bedrock. Soil conditions are similar to those in Back Bay corresponding to a soil profile A according to Johnson [1989].

Two levels of tiebacks supported central panels on long faces. Tieback design loads ranged from 85 to 130 kips with average of 124 kips. The design line loads ranged from 10.90 kips/ft to 16.74 kips/ft with the latter being most common. Loads of around 50 kips were used for second level tiebacks. Estimated total tieback lengths averaged 40' to 45' with some tiebacks having total lengths of 35'. All tiebacks picked up load as the excavation progressed and wall movements increased, until equilibrium was reached.

The overall performance of the lateral earth support system used in this project was satisfactory. Lateral movements of the slurry wall were moderate to insignificant (maximum 1.3"). Most of the wall movement occurred prior to tieback installation, when the wall cantilevered. After the excavation was completed an additional movement of up to 0.3" occurred when loads were transferred from the tiebacks to the base mat. Such movements should be expected since stress transfer from the tiebacks to the floor bracing does take place.

Wall movements were noticeable despite the fact that the excavation was relatively shallow. The floating slurry wall of this project translated up to 0.15" near its base as offset surveys indicated. Such translation at the toe of the slurry wall is to be expected when the wall is floating. One of the interesting aspects that offset surveys indicated was that wall movements can vary significantly even between adjacent panels. For example, at one stage one panel moved back into the soil by as much as an adjacent panel moved towards the excavation.

Excavation for this project did not affect any adjacent structures. Settlements increased as the excavation progressed deeper and reached dV=1.05" near the excavation, while building settlements further away were practically insignificant.
One panel did cave-in during trenching, due to fill "sliding" into the trench under the guide walls. This was probably caused by the difficulties in stabilizing the loose granular fill. Slurry wall water leakage was very small and temporary (i.e. detensioning and removal of tiebacks, covering of the sleeve hole).

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