Trenchless Technology

NOV 2018

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Page 48 of 68

48 T R E N C H L E S S T EC H N O LO GY N OV E M B E R 2018 T H I S P R OJ EC T WA S I N S U P P O R T O F A N E W F I S H hatchery and involved three casing installations under Highway 162 in Orting , Washington. Highway 162 is supported on a 12-ft-high roadway embankment that required 120-ft long casings for three trenchless crossings, two of which were grade- sensitive. There was a 36-in. diameter casing about 28 ft below the pavement and 30-in. and 48-in. casings both about 23 ft below the highway. Adding to the overall project com- plexity, Voights Creek is located about 80 ft away from the toe of the em- bankment and 4 ft above the bottom of the excavations for the launch and receiving pit . Fortunately, site soils consisted of soft to stiff sandy silt , which impeded groundwater seepage. Auger boring and pipe ramming were both considered for the crossings dur- ing design. It was acknowledged that grade control was difficult for pipe ramming , but an on-grade set-up was expected to achieve specified grade tolerances. The Washington State Department of Transportation (WS - DOT) had concerns about roadway settlement over the pipe crossing lo - cations and precluded auger boring from being carried further into de- sign. Because WSDOT had previously had good experiences with pipe ram- ming forming a soil plug that retards groundwater/soil inflow, the project was designed/bid as a pipe ram. Design Considerations Figure 1 shows a plan view of the pro- posed pipelines. The blue line (top line) indicates the 36-in. diameter pressur- ized intake pipeline, the green line (mid- dle line) indicates the 48-in. diameter gravity fish ladder pipeline and the yel- low line (bottom line) indicates the 30- in. gravity juvenile release pipeline. The three pipelines were spatially separated in both plan view and in profile view. Figure 2 shows a profile view of the pipelines. The color scheme for the profile view is the same as for the plan view. Added to the profile view are two blue lines representing the variation in groundwater level across the site based on the geotechnical borings. The 36-in. diameter pipe is below the lower boundary of the assumed groundwater level; however, the 48- in. and the 30-in. diameter pipelines could be above or below the ground- water level . Since the 36-in. diameter pipe is a pressure pipe and grade con- trol was not as much of a concern, the possibility of excessive grade loss was less critical for that crossing . For the two gravity pipelines, the primary el - evation control point for the tie-in to the rest of the system was on the north side of the crossing . Grade differenc- Multiple Grade- Sensitive Pipe Rams in Soft Soils By Michelle L. Macauley, P.E. Figure 1. Plan View (base map courtesy of MWH Americas)

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