DEEP EXCAVATION CASE STUDY - Review Options in DeepEX Software

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.

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