The United States’ National Park Service was recently faced with a challenge at Mesa Verde National Park – how to transport snowmelt water safely from the foothills of SW Colorado to a water treatment plant inside the park boundaries without disturbing the natural beauty of the area. Digging a trench 11 km long for a water pipeline through the national park was out of the question. Instead, horizontal drilling was used to cause minimal land disturbance. In addition to this drilling technique, RTW Engineers, the project engineers, specified stainless steel S31600, containing 12% nickel. Stainless steel was supplied in 16-metre lengths compared with 6-metre lengths for the other materials, and welded to final length, resulting in fewer joints. Also, the fully welded line offered less opportunity for water leakage. High-density polyethylene (HDPE) could not handle the high water pressures involved. With an elevation difference of 760 metres, pressures were in the order of 4.1 megapascals. And ductile cast iron was too bulky and heavy for use in combination with directional drilling.
Other advantages of stainless steel in this project include the following: stainless steel pipe could be welded on-site in lengths of up to 500 metres. It can be joined to itself, pumps and valves using a wide variety of joining techniques, including mechanical, welded and flanged. Tapping sleeves, couplings and restrained clamps are also available in stainless steel (AWWA standards C223 and C606). Its ductility and flexibility allow it to withstand soil movement. It does not embrittle in cold weather. Water quality is maintained, so no water treatment other than biocides (chlorination) is required. It is expected to have a long service life with minimal maintenance. It was not necessary to coat the external surfaces of the stainless, although sacrificial anodes were installed to ensure long life in variable soil types.
Stainless steel pipe, 168 millimetres (mm) in diameter and 7 mm in wall thickness (6-inch Schedule 40) was supplied by Marcigaglia U.S.A. These lengths were fully welded (four weld passes) into 150-to-500-metre lengths, ready for directionally drilled insertion.
