Search

Underground construction is "special" because the work may be performed at great depths and under high groundwater pressures. All too often, it seems to take place in urban areas where there is lim...

The East Side Access Project will connect the Long Island Railroad to a new terminal beneath Grand Central Station in Manhattan, NY. Contract CQ026 in Queens, NY involved construction of a bathtub work excavation, approximately 30.5 x 91 m in plan, excavated through the granular overburden to rock at depths between 18.3 and 24.4 m below working grade. Design of the excavation was complicated by several factors related to construction in an urban setting, notably that one area of the excavation would extend down and uncover an existing subway tunnel that would remain active during the work, excavation would proceed to 15 m below the groundwater table, and that plumes of contamination existed nearby that could not be disturbed by off-site drawdown. The primary means of support of excavation were a structural slurry wall extending to rock and connecting to an existing slurry wall (installed during construction of the active subway), and steel sheetpiles which had been installed over the existing subway. In order to ensure that the joints between the different means of excavation support did not allow water infiltration, zones of soil immediately outside of the joints were permeation grouted prior to the start of excavation to complete the watertight bathtub. Additionally, a groundwater cutoff bulkhead was required around all four sides of the existing active subway structure. Permeation grouting was used to close the gaps and joints and create the bulkhead around the existing structure to limit offsite drawdown and the movement of contamination. The permeation grouting was re-designed after bid time to allow all of the grouting to be performed from outside of the excavation. This reduced the risks associated with the performance of the grouting and allowed the contractor to accelerate his schedule. This paper discusses the design, re-design and execution of the permeation grouting program, including significant quality assurance and quality control measures.

Effective in all soil and ground conditions, artificial ground freezing has proven many times to be a viable and versatile construction alternative for both open excavation support and groundwater control. The technique is cost-effective where complete groundwater cut-off and in-built ground support are required, where the work is deep, occurs in difficult ground conditions, or where access restrictions limit the use of other displacement-type ground improvement techniques. Ground stabilization at great depths also lends itself well to the use of ground freezing. The technique also works well in ground conditions and situations that are sensitive to vibration or settlement with minor ground losses associated with conventional support methods. In some cases, the protection afforded to adjacent buildings by a monolithic, impermeable structural wall is a significant factor favoring the use of ground freezing. Liquid nitrogen ground freezing has been used successfully for emergency situations in disturbed, displaced, and loose ground conditions, where detailed soil delineation cannot practically be performed. This paper demonstrates the viability and versatility of ground freezing for construction purposes through the presentation of various case histories. In addition, given that the current interest in artificial ground freezing is becoming increasingly focused on environmental remediation, this important - and growing - application is discussed in more detail and illustrated with case histories.