Constructed in the mid-1940s, the existing raw water transmission main was at risk of failure due to river bed scour, and in urgent need of replacement. To ensure adequate water supply for 60,000 domestic and commercial customers during the peak season, the existing transmission main needed to be replaced by 1 May 2009. In September 2008, Cape Fear Public Utility Authority (CFPUA) secured the engineering services of Black & Veatch, declaring the project an emergency to expedite the construction process.

The replacement crossing site lies within the Tidewater physiographic subdivision of the Coastal Plain Province in the Cape Fear River Valley. Compared to other rivers in North Carolina, the Cape Fear is a relatively fast flowing river originating from headwaters in the Blue Ridge, thus carrying greater discharge than rivers that drain only the Coastal Plain. The three geological units encountered at the project site are the River Valley Alluvium (tidal marsh peat, organic silts, gravelly sand, and clay), Coastal Plain Deposits (clean sands), and the Pee Dee Formation (silty sands, sandy silts, and sandstone). The Pee Dee is a productive aquifer supplying much of the groundwater supply to the Brunswick and New Hanover counties. Artesian conditions are known to exist in these strata.

Design of the horizontal directional drilling (HDD) bore profile was completed using Drill Path software and a secondary check performed in accordance with the guidelines presented in ASTM F 1962-05 ‘Use of maxi-horizontal directional drilling for placement of polyethylene pipe or conduit under obstacles, including river crossings’. Calculations were performed by the design team to evaluate the HDD vertical alignment including maximum pullback force, maximum pullback stress, allowable external pressure on pipe, and limiting mud pressures.

The installed carrier pipe was HDPE, with a DR11 (160 psi) pressure rating, inside diameter of 30.919 inches, outside diameter of 38.30 inches, and a wall thickness of 3.482 inches. The HDD bore, as designed, had a entry angle of 11 degrees (pipe entry), exit angle of 8 degrees (pipe exit), radius of curvature of 1850 feet, maximum depth of cover of 91 feet, total horizontal distance of 2,415 feet, and a minimum depth beneath the Cape Fear River of 54 feet.

The project design started in mid-September 2008 with a geotechnical investigation taking place between 23 October and 6 November 2008. The Request for Proposal was issued on 20 November 2008, a Pre-Bid Meeting on 24 November 2008, and bids received on 9 December 2008. The project was awarded to Paul Howard Construction Company, Inc. (Greensboro, North Carolina) with Mears Group, Inc. (Houston, Texas) as the HDD subcontractor with a bid price of $US3,276,477.

Mears Group Inc., the HDD contractor, mobilised on 5 February 2009 and the carrier pipe pullback was completed on 15 April 2009. The entire project from design through to HDD completion took approximately seven months. The project was substantially complete before the peak demand season.

The scope of work included installation of 2,415 feet of HDPE pipeline and connections to the existing raw water transmission main using ductile iron pipe. The entry hole was located on the west side of the river on land, with the exit hole located on the east side of the river, in an environmentally sensitive tidal marsh consisting of soft yielding saturated sands that was peat inundated twice a day at high tide.

Major equipment onsite during construction included a Mears 140 HDD drill rig with a pulling capacity of 140,000 lbs, a Mears 330 HDD drill rig with pulling capacity 330,000 lbs; and a MCS 4310 mud mixing, cleaning, and pumping unit with a flow rate 555 gpm, holding capacity 1,000 gallons. Prime Horizontal’s ParaTrack steering system was used for guidance control during construction of the pilot hole. Other equipment onsite included drill pipe, downhole tools, centrifugal pumps, hydraulic pumps, frac tanks, guidance trailers, and tool vans.

Due to Coastal Area Management Act (CAMA) permit requirements, no mud processing could be performed in the tidal area at the east end of the alignment. Drilling fluids were pumped through a drill pipe inside the carrier pipe back to the west side for recycling via the MCS during pullback. All equipment and materials required for construction on the east side were delivered via barges.

Teamwork ensures success

This project was different from many other HDD projects. Strict environmental controls and regulatory agency oversight influenced all aspects of the design and construction processes. Innovative construction techniques were applied including the operation of two rigs working in tandem, state-of-the-art mud mixing and mud recovery operations including a mud recovery system inside the 38 inch HDPE carrier pipe string, and performance of all carrier pipe layout and fusion, HDD drilling operations, and mud operations confined to the west side of the alignment.

Site access was also an issue, with construction on tidal and soft marsh land requiring subgrade preparation to meet stringent environmental and engineering requirements that included the use of geotextile, geogrid, and multiple layers of wooden mats to facilitate mobilisation and operation of the HDD drill rig, crane and other specialised equipment for cofferdam and HDD construction on ground with little or no engineering value.

The timing of the project meant that from the preliminary engineering/feasibility study to construction the job was completed within seven months.

A key component in achieving success on the fast track project was the informed knowledge of the owner. CFPUA had a good understanding of the project constraints, challenges, and construction techniques, which resulted in rapid decision making.

The full-time involvement of designer Black & Veatch as onsite observers during construction, helped ensure all schedule, technical and regulatory requirements were followed, and that information was quickly communicated among the contractor, designer and owner.

The tight schedule and environmental constraints mandated close co-operation at all times between the owner, the designer, the contractor, and the HDD subcontractor. Ultimately the project was successful and CFPUA customers can again rely on a secure water supply system.