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DEEP EXCAVATION CASE STUDY:
State Transportation Building, Boston MA

.The Transportation Building is a nine-floor structure located at the corner of Tremont and Stuart Streets in Boston MA. Excavation construction started in early summer 1981 and was completed in spring 1982. Perimeter slurry walls were used to form the two levels of underground parking that had to be provided.

Soil conditions in the project site are typical of the Boston area. Two levels of temporary tiebacks were used to hold back the slurry walls during the excavation phase. Braces were used for additional support in corner panels. Tiebacks and braces were removed once the garage floor slabs were installed and cured.

Tremont and Stuart Streets bound the eastern and southern sides of the project. Many buildings are adjacent to the north slurry wall. Starting from east to west these buildings were: Saxon Theatre, Walker Building, Tavern Club, Mansfield Beauty Academy, Hound Stooth, Fisher Music, Steinhert Hall, and Park Building. Most of these structures were supported on shallow foundations. Settlement control was important due to the proximity of these structures to the excavation.

Wall movements were generally small through out the project site, especially in the northern project side, since most panels deflected less than 0.6". The majority of these wall panels had maximum deflections around 0.4". In the southern wall a similar pattern in wall movements was observed. In the eastern corner of the project wall deflections were the greatest (I-83: Max 1.2", I-1: Max 0.7").

The slurry wall at panel 83 was embedded basically in clay. This probably explains the mostly translative and slightly rotational movement of that panel. Panel 83 in the southeastern corner had a complex construction history but the final deflections were much smaller than then maximum deflections. Panel 83 and panels at the east wall (I-1) were supported by braces. Deflections at I-83 decreased when the deflections at I-1 increased and vise versa (for recorded data after 10-Dec-1981). Apparently stiffening of the local corner bracing was effective in reducing wall movements in I-83. Some panels showed deep seated movements (Panel I-24) but non-the less overall movements were generally small. Some movement could be attributed to mat shrinkage. It is interesting to note that inclinometers extending beneath the slurry wall showed that wall deflections at the base of slurry wall were in the order of 0.3" to 0.4". This kind of movement corresponded for close to half of the total wall deflections in some panels.

Building settlement control was crucial in the northern side of the project. Building settlements were generally small to insignificant in that area. Reference points in the northwestern corner settled a little less than 0.4" whereas most other points in that area settled less than 0.2". Reference points in the southern side showed the largest settlements during the excavation progression. Near inclinometer I-70 a Boros anchor reference point settled a little more than 1.0". The increase in settlement in that area coincided with the maximum movements (towards the excavation 0.5") recorded by I-70.

A gas main located near panel 75 showed significant settlement, with max settlement of 1.3". Some small settlement occurred during the slurry wall installation in that area 0.2" but there was a small slope failure at that area due to improper trenching procedures (trenching without slurry). However, it is not clear if these settlements should be considered as representative of surface settlements because steel hangers supported the gas main.

Recorder settlements in the vicinity of panel I-83 were small, despite the fact that panel 83 had the largest wall deflections in the project. However, this may not be realistic since settlement monitoring in that area begun after the excavation had progressed to some significant level (Midst Nov-1981). Slurry wall settlements were recorded in panels 83 and 82a after that time (Nov-1981) and showed a 0.2" max slurry wall settlement.
During slurry wall trenching problems arose with the guide walls since in many instances guide walls pieces cracked and fell within the excavated trench. A slope failure took place during trenching of a panel (panel 75) without slurry.

The slurry wall contractor had some difficulty maintaining the required slurry levels that period in time. Slurry pockets were removed from completed panels in two instances. Minor leaks through the panel jointing were observed and easily patched with concrete. The slurry wall finish required occasionally that the slurry wall contractor return and cut excess concrete pieces. Leaks were also recorded in some tieback sleeve holes. At the final stages of the project (near total completion) there were some cracks recorded in the upper garage floor. Stress transfer from the slurry wall could have caused these cracks in the garage floor.

Water levels outside the excavation were not significantly affected either by the excavation or by the dewatering process. Overall, the slurry wall earth support system performance was satisfactory. Settlement control was successful in crucial areas and wall movements were typically small to moderate.