Emergency Jacking Tunnel in Compressible Soil

The Jersey City Municipal Utilities Authority (JCMUA), discovered that an existing sewer pipe had collapsed under the New Jersey Transit Conrail Railroad between Pine Street and Ash Street in Jersey City New Jersey. Immediate action was taken to abandon the pipe in place and install a new sanitary sewer. The borings showed 10-20 ft (3.04 m.- 6.09 m) of competent fill overlaying approximately 8 ft. (2.44 m.) of compressible peat and organics. Groundwater was at 8 ft (2.44 m) below ground level. The poor condition of the soils at subgrade could not withstand the extreme weight of the jacking machine or the thrust force required to push the casing from the launching pit to receiving pit. The support of excavation (SOE) would therefore not only need to retain the soil to allow the jacking equipment to reach subgrade at 14 ft (4.27m) below working grade, but also the back wall of the SOE would need to be sufficiently robust to function as the reaction block for jacking operations. The alignment and allowable tolerance for the jacked casing was minimal, therefore the performance of the SOE was extremely critical to the success of jacking operations. This paper will discuss in detail the design concepts that were developed and implemented by the team to overcome these obstacles. HISTORY The discovery of a collapsed section of an existing sanitary sewer led the Jersey City Municipal Utilities Authority to act on an emergency contract. The project design team was tasked with developing an abandonment plan for the broken pipe and installation of the new pipe through challenging geology. The new 180 ft.(54.96 m) long, 54-in. (1.37 m) . diameter sewer pipe was designed to be contained within a 72-in. (1.83 m) protective steel casing. The installation of the new sewer pipe would require a detailed understanding of the geology. The geotechnical subcontractor assembled a subsurface investigation report on November 2nd, 2017. It was quickly realized that the only way to replace the pipe was with a jacked pipe system since the large grade change and the Conrail Right of Way created no other alternative. Delaying or closing the railway was absolutely not an option as that particular railway served as the main heavy-haul route to and from Jersey City. It became apparent that a major obstacle to pipe jacking was the challenging geology present across and beneath the proposed sanitary sewer. The following challenges faced the design team prior to the installation of the replacement pipe: