Trenchless Rehabilitation in the Tidal zone Rehabilitation of Sewer Systems in Coastal Communities

With an ever increasing number of communities being developed within the coastal zone, the further development and use of trenchless rehabilitation technologies is vital to avoiding the environmental impacts posed by our aging infrastructures. In order to illustrate some of the unique advantages and challenges of trenchless rehabilitation in a tidal environment, this technical paper and presentation will provide an overview of the technical issues overcome by HDR for its clients on several rehabilitation projects within the tidal zone.Due to it's proximity to the tidal waters of the Chesapeake Bay, The Hampton Roads region of Virginia presents unique challenges to local communities and utility districts. With some sewer systems approaching 200 years of age, a fast growing population, very high water tables and vast development within the flood plain, collection system construction and rehabilitation is often very difficult. Two recent projects highlight the challenges presented to sewer rehabilitation by the coastal environment.In Norfolk Virginia, The Hampton Roads Sanitation District (HRSD) was experiencing frequent and chronic leakage from an aging 8” cast iron force main. The main ran along a City park that was completely submerged by tidal waters on a regular basis. The salt water saturation of the area coupled with high levels of organic material and chlorides in the soil, promoted deterioration of the pipes and turned what would be ordinary leaks into potentially serious environmental impacts. Traditional rehabilitation or replacement of this pipe had been impractical due to the daily submergence of the site and the presence of landmark Oak trees, so HDR and HRSD devised a unique plan. Rehabilitation plans were created considering multiple methods and materials, in order to gain the most cost effective solutions from the contractors. Weighted direct-bury HDPE, HDPE slip-lining, direct-bury ductile iron and directional drilling were all specified within the plans. Each method had its pros and cons. The direct-bury solutions would involve submerged trench construction within 10 feet of an aged submerged bulkhead, but would reduce the possibility of damage to the dense existing utilities. HDPE sliplining would also reduce the risks associated with existing utilities but requires numerous insertion and reception pits. The selected contractor chose the option of directional drilling with HDPE. This solution reduced the problems associated with both site flooding and high density existing utilities. Working carefully, at times of low tide and strategically using silt fencing, the contractor was able to avoid the usual environmental impacts caused by this type of construction. Through creatively arranged design documents, HRSD was able to efficiently upgrade their force main while protecting the popular park and the fragile coastal environment.In Portsmouth, Virginia, the City was concerned with an aging 12” sewer vacuum main that was suspended beneath a pedestrian bridge that spans a tidal marsh land. The pipe was 70 years old and showed its age. Due to environmental regulations, the proximity of the pipe to the tidal marsh and various permitting issues, conventional construction and replacement of this pipe were impossible. The City and HDR worked together to produce a rehabilitation plan for this unique situation. Using an advanced computer model, developed by HDR and a local university, the sewer system was modeled to determine the most viable rehabilitation methods. Slip-lining with solid wall HDPE emerged as the preferred solution, but the exposed and suspended nature of the host pipe presented challenges to the design. The temperature variation the HDPE liner might experience approached 90 degrees. This level of variation caused a tremendous amount of predicted thrust within the liner pipes walls. To restrain the liner against the forces of expansion and contraction, large thrust blocks were poured in place at each end of the bridge. The pipe was secured within the blocks with “electro-fusion flex restraints". The soil conditions and the proximity of the wetlands to the excavation posed severe challenges with respect to spoil, pipe cleaning, erosion and sedimentation control. By using cutting edge equipment and techniques the City was able to prevent any potentially serious environmental damages while avoiding prohibitively high costs. As coastal communities continue to grow larger and older, the lessons learned during these projects could be a valuable tool to help other communities maintain their systems while also protecting their fragile tidal waters.