Tough trenchless crossing effort.

The City of Franklin, population 8,712, is located in central New Hampshire, at the junction of the Pemigewasset and Winnipesaukee Rivers. In 2008, the City of Franklin was obtaining the majority of its municipal water supply from four ground water supply sources.

Three of these wells, designated Acme Wells #1 and #2, and the Franklin Falls well, historically have demonstrated high levels of iron and manganese. High levels of iron and manganese in drinking water do not present a health concern, but can cause the water to have a bad taste and odor, and result in staining of fixtures and laundry.

Acme Wells #1 and #2 and the Franklin Falls well are located on the southwest and northeast banks of the Pemigewasset River, respectively. These wells also are located to the southeast and downstream of the Franklin Falls Dam. At this location, the river includes two separate channels, each about 300 feet to 500 feet wide, separated by an approximately 560-foot-wide wooded peninsula, known as Piney Point. For the purpose of this project, these channels are referred to as the southwest and northeast channels. The Franklin Falls Dam, an earthfill flood-control dam, was constructed in 1939. The dam and Piney Point currently are maintained by the U.S. Army Corps of Engineers (USACE).

In 2009, the City of Franklin Water Department received funding from U. S. Department of Agriculture Rural Development and the New Hampshire Environmental Services Drinking Water State Revolving Fund for the construction of a ground water treatment facility to reduce elevated iron and manganese levels from both the Acme and Franklin Falls well fields. At this stage, Tata & Howard Inc., Marlborough, Mass., was retained as the city’s design engineer for the treatment facility and related infrastructure.

The treatment facility would be situated at the Acme Well site, on the southwest bank of the Pemigewasset River. Also proposed were two 12-inch water mains – one raw water and one finish water – between the Franklin Falls and Acme site, a distance of approximately 2,000 feet. To minimize the impact of construction to the Pemigewasset River, the water mains connecting the Franklin Falls and Acme sites were planned to be installed through horizontal directional drilling. An easement across (and below) Pine Island was granted by the USACE for this purpose.

Subsurface Investigations

The test boring program conducted by the project geotechnical consultant consisted of 11 sampled borings, drilled to depths of 42 feet to 62 feet below site grades. Additional test borings drilled for the water supply wells were obtained to supplement this information. Subsurface conditions below the Pemigewasset River channels were investigated by means of ground penetrating radar (GPR). The test borings suggested that the north and south sides of the river were underlain by granular deposits resulting from fluvial and glaciofluvial deposition. The geophysical survey suggested that the channel and the island also were underlain by these materials, which appeared to contain occasional cobbles and boulders, believed to represent coarse river channel lag deposits.

Based on the GPR survey, the bedrock surface was interpreted at depths of 20 feet to 30 feet below the bottom of the southwest river channel, and to 85 feet to 100 feet below the bottom of the northeast river channel. Outcrops located to the northwest of the site suggested the bedrock would consist of schist and meta-sandstone of the Rangely Formation.

The geotechnical consultant also obtained and reviewed copies of the original (1938) design plans for the Franklin Falls Dam. This review suggested that the southwest channel of the Pemigewasset River was excavated by the USACE to provide a discharge channel, and that the excavated spoils were placed on Piney Island. These plans suggest that the southwest discharge channel was excavated to depths of 19 feet to 27 feet below original grade, and that the vicinity of the channel previously consisted of low-lying wetlands. As-built plans for the dam and discharge were not available during project design.

Trenchless Design

In 2010, following the completion of the test borings and the geophysical survey, Brierley Associates, Bedford, N.H., was retained to assist in the trenchless design. Following analyses of the site and subsurface conditions, draft pilot bore paths were produced, which favored an installation below the bedrock surface. It was believed this approach would minimize risk of drill-fluid loss to the river, and encountering cobbles/boulders associated with the aforementioned interpreted lag deposits. However, the cost of a bedrock installation was considered prohibitive relative to budget constraints with available project funding. As such, four parallel, soil-based bore paths were designed to skirt the interpreted bedrock surface. Each of the bore paths were designed to exit on Pine Island.

To account for potential obstructions, the contract documents prepared for the project required the bidding contractor to include costs for up to 16 pilot-hole redirects. The contract also required the horizontal directional drilling contractor provide and implement the means of a drill fluid contingency release and containment program, intended to control and clean up a drill fluid release, should one occur.

The project owner required that the pipe installed below the river be 12-inch plastic pipe. Following analysis of the anticipated buckling pressures and pull forces required by the bore geometry, an SDR-9 HDPE pipe was recommended and selected for the trenchless installations.

The project was bid during August 2010, and in September, R.D. Edmunds, Franklin. N.H., was selected as the general contractor. The trenchless subcontractor to R.D. Edmunds was Henniker Directional Drilling of Henniker, N.H. Henniker is a small, family-owned directional drilling firm, with an 11-year history of successful utility installations throughout northern New England.

The Construction Phase

Henniker mobilized to the southwest side of the site in late October 2011 to begin the first crossing of the southwest channel. Henniker selected a 50x100 Vermeer rig to complete the bores. Given the limited depth of the drill, a walkover tracking system was used, conducted with a plastic dinghy. During drilling of the first bore, multiple obstructions were encountered, requiring numerous redirections and changing of the drill bits due to excessive wear. In some instances, the driller described a series or layer of obstructions above the bore path, which prohibited upward movement, and in other instances, obstructions below the bore path, preventing downward movement. In addition to the obstructions, variable drill-fluid return was noted in the coarse soils.

Henniker spent approximately one month attempting to complete the first pilot bore beneath the southwest channel using soil bits (eagle claw-type bits), without success. At this stage, the southwest channel bores temporarily were abandoned to allow Henniker time to obtain a larger drill rig, and a mud motor suitable for penetrating the obstructions.

In the meantime, Henniker mobilized the drill rig to the opposite side of the project to attempt the bores below the northeast channel. Both of these bores were completed in December 2010 without incident, followed by reaming and pipe installation. While granular soils were encountered, the obstructions encountered below the southwest channel were not present along the bore paths below the northeast channel.

During drilling below the northeast channel, project personnel conducted additional site research within the files of the local USACE office in an effort to determine the nature of the obstructions encountered below the southwest channel. At this time, numerous photos of the 1940s-era construction of the Franklin Falls dam were recovered, revealing the methods involved with

excavation and preparation of the southwest (discharge) channel of the Pemigiwasset River. Photos indicate that channel excavation was accompanied by placement of a layer of closely spaced boulders over the channel surface, presumably to protect against channel erosion and downcutting. This armoring, which was not shown on the design drawings for the dam, apparently was the source of the obstructions encountered below the southwest channel, and likely was the source of the shallow bedrock detected by the GPR. Following the revelation that a boulder armoring was present, revised bore paths were developed that would allow drilling below the armoring.

Henniker elected to use the larger drill rig (Vermeer D100X120) and the mud motor to complete the bores below the southwest channel, at depths 20 feet to 40 feet deeper than planned. Ultimately, the bores were drilled within a wide range of geologic conditions, including weathered bedrock, relatively unweathered bedrock, cobbles and boulders, and sand. Henniker commenced drilling the rock bores in March of 2011, and they were completed by September.

The Pemigewasset River crossing encountered significant unanticipated man-made obstructions, requiring temporary abandonment of two bore paths. Fortunately, historic records were obtained; they provided an explanation for these obstructions, and allowed development of a plan to complete the bores. Ultimately, the success of the project became a function of the contractor’s perseverance, and the patience of the project team.