33 CONGRESS SQUARE - A CASE STUDY BOSTON, MA-Paper

Paper

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The 33 Congress Square project in Boston involved adding a 14-story structure within four existing buildings. Its foundations used high-capacity, rock-socketed micropiles installed in a confined, partially demolished building, including areas with very limited headroom.<br /><br />Two load tests informed the design. The compression test reached 1,010 tons (8.9 MN), with modest creep and only 0.372 inches of residual settlement after unloading. The tension test reached 458 tons (4.1 MN); the reinforcing bar remained essentially elastic to about 400 tons, and movement at the pile tip was small.<br /><br />Load-distribution measurements showed that much of the compression load was carried at or just below the casing tip: 54% of the maximum test load reached the casing tip, but only 30% remained one foot below it. In tension, about 75% of the maximum load reached the top of the rock, while most of the load transferred within the upper five feet of the socket. Based on the test results, the specified rock socket was shortened from 34 feet to 17 feet.<br /><br />The paper argues that current micropile design methods may be overly conservative because they treat socket resistance as friction alone and design the full socket for the entire load. It recommends reviewing allowable stresses for steel and grout, accounting for load transfer at the casing tip and near the top of the socket, and reconsidering the structural design of the socket. The authors call for industry organizations to evaluate these changes, particularly for permanently cased micropiles in dense soils and rock.

Keywords

33 Congress Square

Boston construction

micropiles

rock-socketed foundations

load testing

compression test

tension test

load transfer

socket design

micropile design methods