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HDD and Microtunneling: Trenchless Methods Meet Today's Needs

Trenchless construction methods, including horizontal directional drilling (HDD) and microtunneling, have been used in the United States since the mid 80s, and are likely to grow in popularity due to the advantages they offer. To construct utilities within busy urban areas, underneath waterways, or other natural or manmade barriers presents environmental concerns, safety hazards, and community pressures to minimize disruptions to businesses, transportation, and residences. Government authorities and corporations that develop or rehabilitate utilities are finding trenchless methods provide an array of construction alternatives that can address these concerns. Trenchless methods can improve worker safety because the workers do not work in trenches like they do on conventional cut-and-cover projects. Environmentally sensitive areas such as wetlands and streams can be bypassed underground with trenchless methods. For areas impacted by soil or groundwater contamination, reduced excavation, compared to cut-andcover trenching methods, also reduces public and worker exposure to potential contaminants. In addition, in congested urban areas, trenchless construction methods can save time compared to traditional cut-and-cover construction, and can also facilitate construction where cut-and-cover trenching may not be practical. All of these advantages can combine to reduce project costs and risks. HDD and Microtunneling Methods Horizontal directional drilling (HDD) and microtunneling are two of the various "trenchless" construction methods used to install new pipelines and other facilities. HDD refers to a steerable system for the installation of pipes, conduits, and cables in a shallow arc using a drill rig at the ground surface. Microtunneling is a procedure that uses a remotely-controlled, relatively small diameter tunnel boring machine. One major advantage of trenchless pipeline construction is the freedom to follow the most direct route between two points if the right-ofway allows. Open-cut trench construction normally has to follow existing pipe utility easements, roadways, or other unobstructed surface alignments. Because the HDD and microtunneling remain underground, these methods allow the flexibility to go straight between two points, even if the alignment traverses directly beneath utilities, structures, or other surface features. Recent technological advances in HDD and microtunneling equipment allow longer drives with greater accuracies, resulting in a reduction in both project costs and completion times. Equipment guidance systems that use gyros for steering can be used for curved drives to tunnel between inaccessible obstructions.

The applicability of HDD or microtunneling for pipeline construction is primarily a function of required pipe size and material, alignment tolerances, ground conditions, and available staging areas. Selection of an appropriate method depends on site conditions and project priorities. Recent trenchless projects undertaken by Haley & Aldrich illustrate the ways these construction alternatives contribute to meeting project goals of minimizing surface disruption and excavation hazards. Vista/Carlsbad Sewer/Storm Drain Replacement The Vista/Carlsbad Interceptor Sewer Replacement in California was well suited for microtunneling due to the groundwater and soil conditions, the need to avoid existing underground utilities, and the need to keep traffic flowing in the community business district. The Carlsbad Village Business Association, a group of local business owners, influenced the decision to use microtunneling in order to minimize construction impacts on the community. Vertical shafts were at jacking locations to launch the microtunneling boring machine (MTBM) and at receiving locations to recover the MTBM. Along the 5,200-ft microtunnel alignment, surface impacts were minimized by keeping the jacking shafts and staging areas limited to one lane wide by 200-ft long, and spaced at 2,000-ft intervals. Noise impacts were limited to the 65-decibel range. The receiving shaft impacts were even less disruptive, where two retrievals of the MTBM, construction of the manhole riser, and backfill were the only activities. Traffic was maintained and businesses were accessible throughout construction. In addition to reduced disruption, another advantage of microtunneling over traditional cutand-cover methods is that it produces less material that requires disposal. This turned out to be a significant advantage for the Vista/Carlsbad project when soil and groundwater contaminated by diesel fuel was encountered approximately 200 ft into the second microtunneling drive. The contaminated soil and groundwater was readily contained and successfully managed with little impact to the work. The impacts would have been much greater had the contamination been encountered in a trench excavation. As the trenchless design and construction engineer, Haley & Aldrich prepared drawings and specifications for microtunneling and associated geotechnical instrumentation. Haley & Aldrich reviewed trenchless technology submittals as the Engineer-of-Record, and inspected trenchless construction portions of the work as part of the construction management team. Horizontal Directional Drilling Used to Install Electrical Cable To increase service capacity, National Grid needed to install two 115-KV underground cables through congested portions of Quincy and Dorchester in the greater Boston area. Connecting two electrical substations, the 3-mi-long transmission lines include two 8-in.-dia pipe-type cables, welded joint steel conduits, and two 4-in.-dia communication conduits. The transmission lines cross beneath the Neponset River, a 1,200-ft-wide tidal river that separates Dorchester and Quincy, at the mouth of the river where it empties into Boston Harbor. The lines also cross beneath a portion of an Interstate I-93 embankment, an I-93 on-ramp, and twice underneath the Massachusetts Bay Transit Authority's Red Line rail system. In the planning phase, an extensive geotechnical exploration program was undertaken to assist in evaluating trenchless construction alternatives in an area where cut-and-cover trenching could be problematic. The use of HDD for construction of the cables underneath the river was recommended, because it offered significant benefits. For example, HDD did not require

mobilization of any marine equipment; and, therefore, did not impact shipping traffic or sensitive marine life. Also, a lengthy permitting process was avoided. A 1,600-ft-long bore was used at this river crossing with approximately a 26-in.-hole diameter. The cables were bundled and pulled back directly through the bore in one single operation. HDD was also used to construct a 600-ft-long portion of the alignment that is adjacent to the toe of the 20-ft-high, I-93 highway embankment, and beneath an I-93 on-ramp embankment. A curved horizontal alignment was used for this section. Using HDD for this section eliminated impacts on vehicular traffic on the highway and on-ramp, and avoided the need to pass under a structure owned by an adjacent property owner. These two applications of HDD avoided the need for permitting and right-of-way acquisitions processes, which enabled construction to be completed in significantly less time than would have been required by trenching methods. Interceptor Relief Project Constructed by Microtunneling Microtunneling methods were used to install 3,000 lf of new 66-in.-dia sewer and 4,700 lf of 36-in.-dia sewer in Chelsea, MA. The project is one component of the Massachusetts Water Resources Authority's Combined Sewer Overflow Facilities Plan, and will improve overall water quality in Chelsea Creek and Boston Harbor. Haley & Aldrich completed a combined geotechnical and hazardous materials site investigation that identified the presence of contaminated soil and groundwater along the sewer alignment. Alternative construction methods were evaluated including cut-and-cover, soft ground tunneling, pipe jacking, and microtunneling. Microtunneling was selected primarily to avoid surface disruption and impacts to adjacent businesses that would have occurred with trench construction. The sewer alignments pass through industrialized reclaimed land along Chelsea Creek. Soil types encountered included organic soils, estuarine fine sands and silts, marine deposits, and glacial soils. Inverts are below groundwater level and range from 15 to 28 ft below ground surface. In addition to the trenchless design, Haley & Aldrich provided construction management and hazardous materials management services. As new population growth necessitates expansion of transportation, energy, and water-related infrastructure; and the demands for new telecommunications technologies drive the expansion of underground cable and fiber optic networks, construction technologies must keep pace. HDD and microtunneling are moving trenchless engineering and construction options forward.

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