|
DEEP EXCAVATION CASE STUDY:
World Bank, Washington D.C.
This project involves the construction of a 12-story tower with
a 5-level deep basement, in the World Bank complex at the southwestern
corner of 18th St. N.W and H St. N.W in Washington DC. The new
structure replaced a pre-existing 12-story tower built in the
1940s. Excavation support was provided by a permanent 30"-thick
perimeter slurry wall, keyed into decomposed bedrock, and braced
with 4 to 5 levels of permanent tiebacks. Slurry walls were selected
in this project as the preferred system in order to minimize settlements
and water seepage. Settlement control was crucial along the western
and southern sides of the site due to the protection of other
existing World Bank buildings. Typical soil conditions at the
site consist of sandy clay, sand & gravel, sandy clay, decomposed
rock, and bedrock. Excavation for the project took place from
late 1990 through 1991.
The lateral earth support system of this project provided very good control of
wall movements and surface settlements. However, the variation of the measured
deflections of slurry wall panels was almost double the actual maximum
deflections. Maximum slurry wall deflections towards the excavation reached up
to 0.45" at the final readings. Most of the slurry wall deflections
occurred after the excavation progressed below the 3rd level of tiebacks (25 to
30ft deep).
The wall bulged in locations were the sandy clay (Stratum III) and the
decomposed bedrock was present and moved back in a location where these layers
were absent. In addition, the base of the wall appears to have translated by as
much as 0.2" when the panels were embedded in decomposed rock. Walls
adjacent to buildings moved towards the excavation.
Most slurry wall panels did not show significant settlement or heave but a few
panels settled more than 1.0". It is very interesting that almost adjacent
panels displayed large differences in the magnitudes of settlement. A panel
that was embedded in decomposed rock and moved towards the excavation settled
by as much as 1.9" whereas a panel that was embedded in rock and moved
back into soil moved upwards by 0.6". Surprisingly there were no surface
settlements observed in locations were the slurry wall settled measurably along
the 18th Street.
|
