to the density of the glacial till and the existence of 

 boulders in the glacial till soils, it would be difficult, 

 if not impossible, to drive sheeting to the required 

 depths without damaging the sheets and adversely impacting 

 the integrity of the wall and therefore its effectiveness 

 as a groundwater cut-off. 



Due to the depth of excavation for the proposed garage, 

 the excavation support wall will have to be temporarily 

 braced during construction. Temporary, high capacity 

 tiebacks anchored into the glacial till soils are 

 considered both economical and feasible for use on the 

 north and west perimeter garage walls which are not 

 adjacent to portions of Boston Harbor. The use of corner 

 bracing, rakers and possibly cross lot bracing may have to 

 be considered for use in providing temporary support to 

 portions of the south and east perimeter walls where use 

 of tiebacks is not feasible. Past experience indicates 

 that excavation between individual members of an 

 internally braced system (corner and cross lot bracing) is 

 somewhat slower than excavation in an open, externally 

 braced excavation (tiebacks) . 



The southernmost wall (closest to the waterfront) of the 

 proposed underground parking area poses significant 

 constructibility problems. In the vicinity of boring 

 B102, the alignment of the proposed southern perimeter 

 wall of the garage appears to pass through sections of the 

 existing granite block seawall and within or directly 

 adjacent to the open water of Boston Harbor. Installation 

 of the recommended cast-in-place concrete diaphragm wall 

 (slurry wall) would be impossible at the current proposed 

 location without filling of this portion of the site. 

 Filling within the tidewaters at Boston Harbor raises 

 significant environmental and permit problems which may 

 best be avoided. 



Therefore, it is recommended that the alignment of the 

 southern wall of the proposed garage be shifted to the 

 north to avoid excavation of the granite block seawall and 

 to remain beyond the tidal zone. In areas where the 

 proposed garage is adjacent to existing seawalls, along 

 the southern and eastern perimeter walls, a setback of 

 approximately 15 ft. is suggested to minimize construction 

 difficulties. This setback distance should be verified by 

 test pit excavations which would permit direct 

 observations of the seawall structures. 



The weight of the proposed building is insufficient to 

 resist the hydrostatic pressure that would develop below a 



-14- 



GO 85 



