Horizontal directional drilling was the trenchless method selected to upgradean aging cast iron sewer force main that served the popular coastal town ofOgunquit, Maine — a project that also necessitated some innovative thinking bythe contractor handling the job.
The concern with the cast-iron main was that it was handling approximatelyfour times the amount of sewage during the spring and summer (considered inseason time) than it does the rest of the year. Town officials wanted the pipeeither replaced or upgraded to handle the additional flows.
“A lot of thought went into the project prior to [HDD] being decided on,”said Bruce Hubbard, project manager for Enterprise Trenchless Technologies Inc.(ETTI), the contractor that handled the project. “Pipe bursting was looked at,as well as pipe relining the existing pipe… But both [methods] were moreexpensive in this case due to the older homes involved and [those options] wouldnot provide the required flow capacity.”
In the end, ETTI would install a new 8-in. HDPE pipe using directionaldrilling and, subsequently the existing cast-iron pipe would be relined — thethinking being that having both pipes operating together would be able to handlethe heavy summer flows.
The Ogunquit Sewer District in 2005 was faced with replacing the originalsewer line that served as the major mainline cross-country system for the town.Problem was several multi-million dollar structures had been built atop the100-year-old sewer line, which was experiencing full capacity with summertimeflows and was in such desperate condition that it required immediatereplacement. The line’s location left few economical options, as traditionalopen-trenching away from the shoreline, through the downtown business districtand back to the shore would have taken months to complete and it might crippledlocal commerce.
To complete the project, ETTI employed horizontal directional drilling, aprocess that causes so little disruption, it often goes unnoticed by the public.To drill the 1,100-ft bore, ETTI utilized a Ditch Witch 8020 drill with80,000-lb pullback capacity. ETTI also used a Ditch Witch FM50 Mud Mixing Systemand a Subsite Tracking System.
The initial scope of the work had the HDD process installing a new 8-in.,HDPE line parallel to the existing sewer line directly below a hotel, across aroadway, under many utilities and staying clear of the Atlantic Ocean. Since theETTI team had completed a number of other crossings nearby with favorable soils,test boring was deemed unnecessary. However, 400 ft into the pilot bore, thedrill encountered solid rock. Two additional attempts were made at varyingelevations, but ledge was struck there as well. After several days ofinvestigating options — including costly choices such as advancing through thesolid rock — the contractor decided to try to avoid the ledge outcrops by testboring to explore depths in several alternate locations approximately 200 ftoffshore.
The decision to test bore paid huge dividends for ETTI as the crew found arock-free path to the exit location. The new design wasn’t without itsdifficulties, though, as most of the run now would be located under the oceanwith changing tides and swift currents. The new design also involved workingfrom a small boat. None of these issues had been anticipated when the job wasinitially bid. During the drilling, a crew was positioned in the company’s boatto check the location of the drill head underneath the inlet bottom, using theSubsite system.
Remobilization of all needed equipment was completed without delay anddrilling commenced. It was going according to plan when one last obstaclepresented itself. On the exit side of the bore — just before the pumping station— a large, three-quarter-inch steel seawall spanned the shoreline as protectionfor the homes and hotels, preventing the drill from reaching its goal.Approximately 45 ft before drilling out into the exit pit at 12 ft deep, thedrill head was stalled and couldn’t advance. A small excavation pit was made oneach side of the seawall to enable a crew access with a torch set, which it usedto remove a segment of steel, thus allowing the drill to continue and pull backthe 8-in. product line without further delay.
It took ETTI about two-and-a-half days to finish its portion of the project,which took place in March.
What made this project so unique was that — to the knowledge of everyoneinvolved — it marked the first time HDD was utilized to drill from shore, outunder the Atlantic Ocean floor and back to the shore in a 180-degree radius.This route was necessitated by the ledge outcroppings in a direct path from thepumping station to outfall structure.
High-density polyethylene pipe was used for the drilling application due toits rugged durability, the flexibility needed to make the radius bends and itsability to withstand the corrosiveness of saltwater. ETTI also had to produce acustomized bentonite slurry mixture that would withstand saltwater chlorides, asthey can be a harsh enemy of the drilling process if not handled properly. Thebentonite slurry allowed the drilled hole to remain open and stable while thepullback of the product line was completed.
The community benefited greatly due to ETTI’s ingenuity. Not a singlebusiness, residence, motorist or even a pedestrian was impacted by the drillingoperations. In fact, the scope of the HDD work was completed without many peopleeven aware that a 1,100-ft run of 8-in. sewer force line was being installedbetween the pumping station and outfall manhole. Had traditional open-cutmethods been employed, a significant stretch of Route 1 would have experiencedtraffic delays and local businesses would have been disrupted.
The environment also benefited thanks to HDD’s impact being limited to 8-ftby 8-ft entry and exit pits and the seawall excavation. And since the processutilized just one drill rig, three support trucks and a small excavator, totaldiesel emissions were limited to 36 hours. By comparison, other methods wouldhave used three times the support equipment and an estimated 450 hours of dieselemissions.
The drilling also saved 8,500 yds of excavated material, 320 yds of pavementand 600 ft of granite curb and sidewalk replacement. Best of all, at a cost ofjust $120,000, the HDD process saved the Town of Ogunquit more than $1 millionin construction costs.
For this project, ETTI was recognized for construction excellence with a 2006Build Maine Award from the Associated Constructors of Maine Inc. The LisbonFalls-based specialty contractor was honored in the competition’sMunicipal/Utility category for its innovative approach to replacing the Town ofOgunquit’s sewer main.
This article was assembled with material from the Associated Constructors ofMaine Inc. and ETTI.