500 Bolyston street (Boston, MA)
The 500 Boylston Street building is located in the Back-Bay Boston area and includes two medium-rise towers, numerous low-rise structures and plaza areas. Boylston Street bounds the site to the north, with the MBTA Green Line subway running underneath. Clarendon Street bounds the site to the west, with historic Trinity Church opposite of the site. St. James Street bounds the southern side of the project. Subsurface conditions at the site are typical of the Back Bay area in Boston (Johnson, 1989).
The 2.0'-thick slurry walls were typically 40' to 45' deep with a final toe embedment, 15' below the base of excavation (BOE). Four levels of tiebacks provided temporary lateral support along the western wall. Tieback inclinations of 12°, 5°, 30°, and 30° were used for the 1st, 2nd, 3rd, and 4th levels respectively. Floor bracing provided permanent lateral support. Tiebacks were spaced at 5 ft intervals horizontally, and a minimum of 10 ft vertically. Numerous additional tiebacks were used due to the poor performance of the original tiebacks so that the final tieback spacing was even smaller. Rakers were used together with tiebacks along St. James Street for the same reasons. The top figure shows a typical north-south excavation profile.
In this project, settlement control was very important along Clarendon and Boylston Streets. Lateral wall movements were the greatest along Clarendon Street where there was significant lateral soil straining as well near the Church. The maximum horizontal slurry wall deflection was dW=3.3" along Clarendon (I-10) whereas other walls deflected to a much smaller extend. Inclinometer I-2 in front of Trinity Church and opposite of I-10 across Clarendon had a maximum deflection of close to dH=2". Ties were not successful in reducing wall movements since their tensioning was not able to reduce inward wall deflections. Wall movements were smaller along St. James Street, where rakers were used in place of the three lower levels of tiebacks. Walls translated up to 1" at their bases even though they all extended into stiff overconsolidated clay. Such translation movements should be expected under such conditions where the wall is floating and soft base material is present.
Considerably larger than expected surface settlements along Clarendon and St. James Streets were observed during the excavation. Maximum settlement was dV = 5.2" along Clarendon Street and 3" along St. James whereas the Boylston Street settlements were insignificant. Settlements along St. James are puzzling since rakers were used in place of the three lower levels of tiebacks. Possibly the groundwater lowering below minimum levels that went on for three or more weeks might be responsible for these settlements. Wall deflections were smaller than surface settlements along Clarendon Street (same was true for other project sides). This indicates that there were possible soil losses through the tiebacks or soil disturbance caused by the tieback installation procedure. Tieback load deficiencies are also to blame for the excessive movements. Major parts of the effective length of tiebacks were in the organic silt, resulting in load shedding and excessive creep. Furthermore when additional tiebacks were installed adjacent previously locked tiebacks could not sustain their initial lock-off loads. Wooden pile extraction in the preliminary excavation construction phase may be responsible for part of the induced movements. Building settlements were small, with Trinity Church settling up to dV=0.62". The excavation did not affect other buildings, supported by pile foundations.
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