Trenchless Applications for New Wastewater Collection and Force Main Pipelines Located in Public Right-of-Way

As metropolitan areas in the United States grow in population, population density and by land communities are challenges with unique and significant new pipeline engineering and construction. Additionally, utilities are challenged by providing reliable and redundant water supplies to expanding populations, increased density with new vertical construction of residential areas and declining population areas in different areas. Thus, the demand for installation of new underground water infrastructure in congested areas has increased the necessity for innovative and economical systems to go underneath and alongside in-place facilities. Trenchless technology, particularly horizontal directional drilling, microtunneling and pipe jacking have become increasingly popular solutions as they minimize disruption of the public and reduce the environmental impact. These facts have guided permitting authorities to gain interest in these methodologies. Permitting often dictates planning, engineering, and construction of underground water pipelines. This is particular for trenchless pipeline installations beneath roads, railways, environmentally sensitive areas, water bodies, urban areas, and existing utilities. Different governing authorities may have unique standards with requirements of pipe diameter, material, and thickness as well as depth of cover. Thus, engineers, owners, manufacturers, and contractors are challenged to develop solutions that are economic, timely and safe. The purpose of this presentation is to highlight trenchless pipeline construction as it is inherently the most challenging and risky activity involved with pipeline projects. Trenchless engineering and construction require operational expertise, utilizes sophisticated and advanced technological equipment, and faces challenging and non-visible geotechnical conditions. Owners, contractors, and engineers are more challenged by trenchless applications as they need to use best judgment and experience to evaluate numerous project risks and costs with sometimes little available information. This presentation will provide a benefit to our industry by discussing all aspects of trenchless construction with respect to feasibility, conceptual design and detailed engineering. It will further discuss construction considerations as follows that are often overlooked by engineers and owners not familiar with the newest applications and implementations of trenchless. We risks associated with these methods and mitigation strategies to reduce overall project risk. Safety risks, for example, include moving parts and turning equipment, uneven ground, potential Injuries, man-entry operations, fatigue, and mental health. Jobsite challenges include availability of water, site access, site constraints, elevation change, underground utilities, site considerations, long crossing distances and dewatering requirements. Economic considerations include supply chain disruptions, skilled labor availability, increased lead times, increased freight costs, unpredictability of imported equipment and material, material price escalation and potential protesting. Environmental considerations include acts of God, extreme heat, cold, dry and wet conditions. Geotechnical considerations include alluvial soils with very low blow-counts, hard rock with high unconfined compressive strength, millennial boulders, manmade and unknown obstacles, corrosivity, abrasively, potential contamination, groundwater level and artesian pressures, ground settlement, differing soil conditions, gravels, cobbles and boulders, hydraulic fracturing, and inadvertent returns. Technology considerations include ability to steer, accuracy limits / line of grade, equipment breakdown, mislead of lasers and insufficient torque. Direct costs include direct labor costs for both engineering, construction and inspection, easement acquisition, scheduling considerations, permitting fees, project contingency, upfront studies including geotechnical investigations and environmental impact studies. Indirect costs include, but are not limited to, traffic control, job site support, insurance, bonding, and potential litigation fees. Social costs are related to road damage and restoration, noise mitigation, loss of business and business effectiveness, vehicular delays, and extended commutes due to traffic control and road shutdowns, and general community impacts to everyday operations. Bottom line there is a lot to consider. Three trenchless installation methodologies are being evaluated for this presentation: Horizontal Directional Drilling (HDD), Microtunneling (MT), and Direct Steerable Pipe Thrusting (DSPT). Each of these options presents benefits and challenges to critical crossings. For the HDD process, a small diameter pilot hole is first created, then the reaming process begins to enlarge the hole for the pull. This method requires the pipe to be installed at the deepest of all trenchless methods to avoid hydraulic fracture of the in-situ soil. For microtunneling, large launching and receiving shafts are required to launch and breakdown microtunnel boring machines (MTBMs). The cutter head is selected according to the face of the tunnel in this case would be a mixed face condition. Slurry will be necessary to maintain the face of the tunnel to avoid unraveling. Direct Steerable Pipe Thrusting (DSPT) is a modified microtunneling methodology that is launched from surface along a path like HDD in geometry. Unlike HDD, the pipeline is thrusted from the entry side and requires limited space on the exit side. These three options will be discussed as unique yet complimentary alternative methodologies for collection system construction. Final methodology selection will be determined by contractor means and methodologies and qualifications, overall pricing and risk profile for the benefit of the overall project program. The takeaway message will address the objective to understand and quantify the risk levels of trenchless crossings project to facilitate, enhance and model risk assessment. The steps to this process includes analysis and identification of the qualitative risk level factors of each trenchless crossing as well as they contribute to overall project scope. Next, we will determine the significant factors that contribute to the risk level prediction and rank these factors using an Analytic Hierarchy Process. Using this information, we will build an assessment based on both known and assumed factors. Finally, we will make a professional recommendation of method based on criteria above.