The Dundalk Marine Terminal located just south of Baltimore’s Inner Harbor is one of Maryland Port Authority’s busiest ports. It is a beehive of activity – importing and exporting vehicles of all types from construction and farming equipment to economy and luxury grade automobiles. The brackish water that surrounds the port has wreaked havoc on the buried infrastructure and led the port to invest in a new stormwater discharge system.
A force main system including a pump station, three manholes and more than 2,000 lf of 36-in. HDPE discharge pipe was bid in summer 2016. The final 750 lf of discharge pipe was to be installed 6.9 ft below sea level to avoid a long list of existing utilities that include potable water, gas and of course, the existing storm water system.
Much of the deep section of pipe was designed to be open trenched. The use of the jack-and-bore method was included at three locations to avoid excavation of rail lines. These crossings ranged from 50 to 140 ft each. The general contractor, Marine Technologies, reviewed the design in spring 2017 and partnered with Aaron Enterprises Inc. to evaluate the option of boring the entire deep section. Marine Technologies weighed the bore and jack option to a sheeted excavation and decided that a trenchless solution was the most cost-effective option.
The Jack-and-Bore Solution
Aaron Enterprises suggested a single 56-ft long pit in the middle of the deep section to install two crossings 330 lf in one direction and 310 lf in the opposite direction. The location was chosen because a manhole was to be placed in that area regardless of the pipe installation method.
Soil borings in the area indicated sands, silts and some gravel. Using the empirical formula for total jack force in Najafi and Gokhale’s book “Trenchless Technology,” a theoretical force of just more than 600,000 lbs was anticipated for the longest crossing. AEI elected to use an Akkerman Guided Pilot Machine and an American Augers 48-900 auger boring machine to complete the work. With the boring machine capable of 900,000 lbs of jacking force, ability to push the casing was not a concern.
Installation of 330 lf of pilot tube was completed in less than 10 hours. The team anticipated a 42-in. steel casing installation rate of 60 lf per 12-hour working shift. “As the first 60 lf of 42-in. casing was installed, we observed a rise in jacking pressure and wood fragments being ejected from the bore machine hopper,” said AEI foreman Andrew Holcomb.
Some settlement was also observed along the bore path. At the time, little could be done to investigate the cause of the pressure change because the leading edge of the casing was under the influence line of a rail siding and in extreme proximity to an existing 16-in. ductile iron water main.
The bore-and-jack operation continued with pressure increasing dramatically over the next 50 lf. With 90 lf of casing installed, water was observed at the ground surface. It became apparent that the construction disrupted the 16-in. ductile line.
The crews worked through the night to expose the main and repair the leak. Again, AEI was not permitted to dig down to the leading edge of the casing pipe to investigate what may have been causing the settlement and disturbance to the existing infrastructure. AEI had no choice but to continue with the proposed method of installation.
A Change in Methods
With 116 lf of 42-in. casing installed, the crew was unable to push using the bore and jack method. A concrete thrust block used to counter the jacking forces cracked as the machine reached its capacity. Again, the location did not allow for excavation at the pilot to 42-in. transitions, so Aaron Enterprises had to find an alternate solution to the difficult and unexplained condition. The team elected to pull the head and auger and change to a pipe ram method. Using a TT Technologies Taurus pipe rammer, the crew’s first step in setting up for this method included reinforcement of the pipe welds.
“If the ramming method was to be used, the welds would defiantly need to be reinforced. We felt the 42-in. x 0.625-in. wall casing had the structural ability to withstand the stress, but the welds are always a concern. The crew elected to weld the inside circumference of the pipe to insure failure would not occur at the joints,” said AEI superintendent Glenn Grove.
With the proper air flow, a Taurus rammer can generate more than 4.4 million lbs of impact force. AEI was able to install another 162 lf of 42-in. casing pipe using the ramming method, but with a total of 278 lf installed, the rammer slowed to a crawl. Fortunately, the pipe was at a location that could be excavated. What was discovered confirmed the belief that unforeseen wood shoring left in place from a previous pipe installation was encountered. The wood was wedged around the leading edge of the 42-in. transition section. The pilot to 20-in. step up transition was completely blocked, creating a 20-in. plug that led to the refusal. The wood dragging along the casing fully explained the rise in pressure and disruption to the water main.
Retrieval and Completion
To limit the amount of ground disturbance and mitigate dewatering costs, a 26-ft long by 12-ft wide retrieval pit was excavated over the transitions. The crew cut the transitions out and removed the pilot tubes. With the transitions out of the way, another attempt to ram from the original launch pit was made. Unfortunately, while the retrieving pit was being excavated, the surrounding earth adhered to the circumference of the pipe and the crew was unable to break the 278 ft of casing free from the ground. The remaining casing pipe was then cut to 10-ft lengths and the retrieval pit was used to ram the remaining casing into place.
The second crossing of 310 lf was installed without issue. The crew experienced a maximum thrust force of 640,000 lbs, which strongly correlated to the theoretical jacking force derived from Najafi and Gokhale’s empirical formula. Some small wood fragments were also observed throughout the installation, but they did not have a significant impact on the jacking force. The second crossing validated the use of the jack and bore method for this type of installation and it is believed that had obstructions not been encountered on the first crossing, jack and bore would have been successful at both locations.
Unforeseen jacking obstructions are part of the trenchless industry. Obstructions force the contractor to think of alternate solutions and they often must change methodologies to accomplish the end goal. The key to a successful installation is having an experienced crew that can recognize the changes and a support staff that can dispatch the proper tools to get the work done.