One of the most challenging elements of the MTA Capital Construction Project’s East Side Access Project was the Northern Boulevard Crossing, which required an SEM tunnel between two 85-foot deep access shafts that extended as deep as 55 feet below the water table. Moretrench’s approach of horizontal ground freezing in conjunction with an innovative grouting program for settlement and heave control was key to problem-free tunneling without impact top overlying structures.
The tunnel alignment crossed beneath an active subway tunnel, an elevated rail structure, and the heavily-traveled Northern Boulevard. Mining was to be accomplished by sequential excavation (NATM) methods, 55 feet below the groundwater table through highly complex glacial deposits, including Bull’s Liver-like silts, sands, boulders, till and bedrock. While SEM tunneling is the most economic method for short tunnel reaches through soft ground, a relatively long stand-up time is required. Earth support and ground- water control were therefore critical to safe tunneling operations. Of equal importance was that the overlying structures should not be impacted during tunneling.
Vertical drilling, both from ground level and from within the existing subway structure itself, was prohibited, as was lowering of the groundwater table because of contaminate plumes in the vicinity. These restrictions severely limited ground water control and ground support options. The method ultimately selected was the creation of a horizontal protective frozen arch above the tunnel alignment, extending to bedrock for complete groundwater cut-off. Since the top of the frozen arch would be just a few feet beneath the bottom of the subway structure, a program was developed to control ground heave during the active freeze period and settlement during thawing.
Ahead of freeze pipe installation, pre-grouting with horizontal compaction grouting was performed in the soils beneath the subway box to improve the ground and fill any open, water-filled zones. Compensation grout pipes were also installed to mitigate any settle- ment of the overlying structures during freeze pipe installation. A specially formulated non-cementitious grout was developed that essentially mimicked the strength and consistency of the in situ soils but would not leave cemented obstructions for any subsequent heave control that might be required. If heave occurred during freezing, soil extraction would be performed through casings installed between the frozen arch and the base of the subway box. Provision was also made for compensation grouting to be performed through the pre-installed grout pipes to mitigate structural settlement during thawing of the frozen ground following tunnel completion.
Heat pipes were pre-installed above the crown of the arch to limit growth of the freeze should excessive movement of the subway structure occur during freezing. Accurate horizontal drilling for freeze pipe installation was critical. Significant measures were therefore put into place by Moretrench to install the pipes within tolerances while preventing ground loss by means of blow-out preventers. Each hole was surveyed with a gyroscope. Drilling of the bottom row of grout pipes through competent rock to allow an impenetrable froze seal to bedrock was facilitated by pits excavated into the rock.
Once closure of the freeze had been confirmed, mining was successfully completed with complete crown stability. Rigorous controls instituted by Moretrench resulted in only minimal heave during freeze, and activation of the heat pipes and soil extraction were not required. Compensation grouting was performed during the thaw period to correct minor differential settlements of the subway box.