Construction of a new Con Edison 345 kV underground transmission line between Yonkers and Manhattan, NY included installation of a secant pile wall-supported access shaft under 60 feet of water head and immediately adjacent to the Hudson River. Although it is common in the New York City area for the Inwood Marble that underlays the entire site to be highly weathered, geotechnical borings had, in fact, indicated competent bedrock. Close to the river, the rock was overlain by outwash sands. However, during shaft excavation, considerable inflow of these sands into the excavation, and corresponding ground loss outside of the excavation, signaled an unanticipated rock condition of some significance. Further investigation revealed many high-angle, sand-filled features in the rock that communicated directly with the outwash sands. Proactive remediation of this condition was of particular importance since a super-elevated curve of the North Metro rail line ran between the shaft and the river. Further ground loss as excavation progressed could result in uncontrollable ground inflow leading to unacceptable and dangerous track settlement.
Initially, the general contractor, Kiewit Constructors, attempted a permeation grouting solution to stabilize these sand-filled features. When this approach met with very limited success, Kiewit called in Moretrench to evaluate the problem and provide an effective solution that could be implemented quickly and minimize the impact on the overall contract schedule and costs.
Jet Grouting Solution
Drawing on previous rock grouting experience, Moretrench determined that to properly seal off the rock features, the infill material had first to be displaced to the greatest extent possible before the voids could be grouted. This was not achievable by permeation grouting methods. A jet grouting solution was therefore developed that would take into account the sensitivity of the adjacent rail tracks and be compatible with the restricted site conditions while effectively meeting the subsurface project objectives.
Jetting was accomplished using small diameter tools through pre-installed sleeves in the perimeter secant pile wall, an innovative approach that proved to be very successful. First, water jetting was used to ‘cleanse’ the infill from the upper 25 feet of rock, closely followed by jetting with grouting. Given the tidal groundwater conditions, sodium silicate was introduced into the grout to accelerate set times. This also was advantageous in speeding up the work schedule.
As in every jet grouting program, quality control and quality assurance was integral to a successful resolution. To verify performance, Moretrench installed an open borehole well inside the shaft which permitted pumping down the interior water levels and monitoring recovery as well as measurement of any sand inflow following completion of the grouting program. A real-time automated data system was used to record grout pressures, jetting rates and grout takes. Moretrench affiliate Ground/Water Treatment & Technology (GWTT) handled treatment of the wastewater generated during the work.
The grouting program was successful in eliminating further inflow into the excavation, allowing Kiewit to safely complete the shaft excavation and construction.