Multiple Grade-Sensitive Pipe Rams in Soft Soils

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This project was in support of a new fish 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 complexity, Voights Creek is located about 80 ft away from the toe of the embankment 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 during 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 (WSDOT) had concerns about roadway settlement over the pipe crossing locations and precluded auger boring from being carried further into design. Because WSDOT had previously had good experiences with pipe ramming forming a soil plug that retards groundwater/soil inflow, the project was designed/bid as a pipe ram.

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Design Considerations

Figure 1 shows a plan view of the proposed pipelines. The blue line (top line) indicates the 36-in. diameter pressurized intake pipeline, the green line (middle line) indicates the 48-in. diameter gravity fish ladder pipeline and the yellow 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 1.

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 groundwater level. Since the 36-in. diameter pipe is a pressure pipe and grade control 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 elevation control point for the tie-in to the rest of the system was on the north side of the crossing. Grade differences between design elevation and installed elevation could more easily be accommodated (and deviations could be re-designed) on the south side because the open-cut portion could be re-aligned to accommodate.

Figure 2.

Once the risks associated with the soil and groundwater conditions were identified, design drawings and specifications were written to reduce the potential adverse effects of identified risks.

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There were three primary ways the drawings and specifications attempted to mitigate risks and emphasize the importance of grade control and careful construction practices:

  1. The drawings indicated that the launch pit be on the north side where the grade control of the gravity lines was most important.
  2. The specifications required daily settlement monitoring of the highway.
  3. The specifications included language stating that the grade of the two gravity lines is “critical to the hydraulic performance” of the system.
  4. The specification required the contractor to either meet the line and grade requirements (within tolerances) or pay for re-design of the piping system (including bearing all schedule and cost impacts of the re-design).

Construction

At the start of construction, the contractor requested launch pit placement and pipelines installation from the south side of the embankment. After some discussions, the contractor was permitted to install the 36-in. pressure pipeline from the south side; however, the gravity lines were required to be installed from the north side as per the contract drawings.

The first casing installed was the 36-in. pressurized intake pipe. Over the course of the crossing, 12 in. of grade was lost. While the lost of grade for the pressure pipeline was concerning, it did not impact functionality. However, concerns were raised about grade control for the next two installations of gravity pipelines. The contractor elected to move launch operations to the north side of the highway because soils on the south side appeared wetter/looser than the soils on the north side. It was hoped that more competent soils in the launch vicinity would provide better grade control. The 30-in. casing lost 12 in. of grade (mostly in the last 40 ft of installation). For the 48-in. casing, the contractor set the pipe at a reverse grade (i.e. inclined to account for grade loss). Within the first 13 ft after launch, the reverse grade was lost.

At this point the contractor requested to switch to auger boring. Vibrations associated with pipe ramming combined with the combined weight of the casing and soft soil within the casing may have caused the grade loss. Auger boring eliminates vibrations and removes the weight of soil from within the casing. There were still over-excavation concerns during auger boring, but the contractor’s request was allowed. At completion of the auger bore installation for the 48-in. casing, there was grade loss but within the acceptable tolerances.

Conclusion

A number of lessons were learned from this project. In particular, soil plugs do not always form at the leading edge of a pipe ram. This is especially true in loose, saturated soils that can liquefy and flow under cyclic dynamic loading. Design considerations should be made to account for this possibility and specifications should require the contractor to have a contingency plan. This contingency plan may be as simple as placing sand bags in the pipe prior to ramming.

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Other lessons learned were to have flexibility during construction, to listen to the contractor, and to be prepared to make changes based on the actual conditions encountered in the field. A design is based on the information available at the time of design. With three pipelines being installed within essentially the same footprint, the best information we get is from the previous installations. The contractor anticipated possible challenges during construction due to the very soft soils and pro-actively set-up the launch pit with the flexibility to switch between ramming and boring. With a good contractor, it is very helpful to be able to discuss options and changes to construction based on soil information and soil behavior learned during construction.

Michelle L. Macauley, P.E., is principal engineer and owner at Macauley Trenchless.

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