Multiple Grade-Sensitive Pipe Rams in Soft Soils

North American Society for Trenchless Technology (NASTT)
NASTT’s 2015 No-Dig Show
Denver, Colorado
March 15-19, 2015
TA-T4-01
Multiple Grade-Sensitive Pipe Rams in Soft Soils
Michelle L. Macauley, GeoEngineers, Redmond, WA
Anthony Sanich, Washington State Department of Fish and Wildlife, Olympia, WA
1.
ABSTRACT
This fish hatchery project involved three casing installations under Highway 162 in Orting, Washington. The 12foot-high roadway embankment required 120-foot long casings for three highway embankment crossings, two of
which were grade-sensitive. The casing diameters were 30-inch, 36-inch and 48-inch. The 36-inch diameter casing
was about 28 feet below the pavement while the 30-inch casing and the 48-inch casings were both about 23 feet
below the highway.
Voights Creek is about 80 feet away from the toe of the roadway embankment and 4 feet above the bottom of the
excavations for the launch and receiving pit. Site soils consisted of soft to stiff sandy silt. Auger boring and pipe
ramming were 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 had concerns about roadway settlement over the pipe crossing locations. Since pipe
ramming forms a soil plug that retards groundwater/soil inflow, the project was designed/bid as a pipe ram.
Twelve inches of grade was lost when the 36-inch diameter casing was installed. This was a pressure pipeline;
however, concerns were raised about grade control for the two 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-inch casing lost 12-inches of grade (mostly in the last 40-feet of installation). For the 48-inch casing,
the contractor set the pipe at a reverse grade (i.e. inclined to account for grade loss). Within the first 13 feet 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-inch casing, there was grade loss but within the acceptable
tolerances.
2.
INTRODUCTION
The trenchless crossings were part of a construction project for a new fish hatchery near Orting, Washington for the
Washington State Department of Fish and Wildlife. This paper only addresses the trenchless portion of the project.
The trenchless portion involved installation of three new pipelines under State Highway 162 (SR 162). The new
pipelines were 24-inch diameter, 30-inch diameter and 48-inch diameter and each were about 120 feet long. The 24inch pipeline is a pressurized water intake pipeline to provide fresh water to the hatchery. The 30-inch pipeline is a
gravity return water pipeline and the 48-inch is a fish ladder for returning adult fish. Ultimately the 24-inch and the
30-inch diameter pipelines were upsized by the contractor to 30-inch and 36-inch diameter casings, respectively.
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The Washington State Department of Transportation (WSDOT) was the governing permit agency for the highway
and had a strong preference for pipe ramming due to the perception that a soil plug would form within the casing
and minimize the risk of settlement of their highway. The project is located southeast of Orting, Washington.
Figure 1 shows the project area relative to nearby cities and relative to Mount Rainier (bottom right of the primary
image).
Project Site
Figure 1. Vicinity Map of Orting, Washington
The geology of Orting is strongly influenced by Mount Rainer. The Orting vicinity is in the path of mudflows
(lahars) emanating from Mount Rainier and is underlain by a series of mudflow deposits. The most recent of these
mudflow deposits is the Electron Lahar, which occurred about 600 years ago. The Electron Lahar overlies the
Osceola Lahar, which has been dated as being about 5,700 years old. Figure 2 shows the paths of previous
mudflows from Mount Rainer. Mudflow deposits are characterized by undifferentiated silt, clay and sand deposits
with inclusions of gravel, cobbles and boulders. The mudflow deposits resulted from rapid mass flowage (and
deposition) that was mobilized by water. Because of the rapid deposition, relatively recent mudflow deposits are
typically very loose or soft.
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Project Site
Figure 2. Lahar Map (courtesy of USGS website,
http://volcanoes.usgs.gov/volcanoes/mount_rainier/mount_rainier_hazard_50.html) accessed December 2014)
Subsurface soil conditions at the site were evaluated based on two previous geotechnical borings near a nearby
WSDOT bridge abutment and four explorations in support of this project. The four project-related explorations
consisted of two geotechnical borings and two backhoe test-pits. The approximate locations of the previous and
current explorations are show on Figure 3. Based on the geotechnical explorations advanced at the site, the
subsurface soils at the trenchless crossing locations were expected to consist of loose to very loose silty sand and
sandy silt. The test pits gave an indication of stand-up time of the soil based on the sidewalls of the test pits
standing vertical during the test excavation and an indication of groundwater seepage based on observations of very
small volume of water seeping into the test pit during excavation.
The soil conditions were similar in the borings advanced on each side of the roadway embankment; however, the
groundwater level on the north side (the side nearest the creek) was about 4 feet lower than the groundwater noted
on the south side.
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N
Scale is approximately
1 inch = 80 feet
Figure 3. Site and Exploration Map (base map courtesy of MWH Americas)
3.
DESIGN CONSIDERATIONS
Based on the relatively high groundwater levels, the loose to very loose soil, and the presence of a state highway, the
design evaluated several trenchless risks before selecting the preferred method of installation (pipe ram). In
particular, the face stability of the loose, saturated soils was a concern. While the test pits indicated good stand-up
time and firm soil above the groundwater table, the borings indicated that the groundwater level changed across the
site. Depending on the actual silt and clay content of the soil and the actual groundwater level during construction,
the soil could behave as a flowing or squeezing soil. This condition would increase the risk of settlement of the
highway and loss of grade for the casings.
Figure 4 shows a plan view of the proposed pipelines. The blue line (top line) indicates the 36-inch diameter
pressurized intake pipeline, the green line (middle line) indicates the 48-inch diameter gravity fish ladder pipeline
and the yellow line (bottom line) indicates the 30-inch gravity juvenile release pipeline. The three pipelines were
spatially separated in both plan view and in profile view.
Figure 5 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. As shown in Figure 5, the 36-inch diameter pipe is below the lower boundary of
the assumed groundwater level; however, the 48-inch and the 30-inch diameter pipelines could be above or below
the groundwater level. Since the 36-inch 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 re-designed
for on the south side.
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N
36-inch
48-inch
30-inch
Scale is approximately
1 inch = 40 feet
Figure 4. Plan View (base map courtesy of MWH Americas)
48-inch
gravity
36-inch
pressure
30-inch
gravity
Scale is approximately
1 inch = 40 feet
Figure 5. Profile View (profile courtesy of MWH Americas)
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. 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.
2.
3.
4.
The drawings indicated that the launch pit be on the north side where the grade control of the gravity lines
was most important.
The specifications required daily settlement monitoring of the highway.
The specifications included language stating that the grade of the two gravity lines is “critical to the
hydraulic performance” of the system. 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
Construction started with the contractor requesting to construct the launch pit and install all pipelines from the south
side of the embankment. After some discussions, the contractor was permitted to install the 36-inch pressure
pipeline from the south side; however, the gravity lines were required to be installed from the north side as per the
contract drawings.
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The launch pit on the south side was excavated and pipe ramming started by pushing the pipe casing into the ground
using the jacking frame of an auger bore machine (no-augers). Figure 6 shows the auger bore machine set-up. The
contractor wanted to get a portion of the casing in the soil so that there would be some friction on the outside of the
casing to hold it in the soil and it would not “bounce” back when pipe ramming started. After the casing had been
pushed about 4 feet in to the soil at the face of the launch pit, the contractor indicated that the guide rails had started
to move. In other words, the auger machine wouldn’t be pushing the casing squarely if movement of the rails
continued. At this point the contractor switched over to the pipe ramming set-up. Unfortunately, the pipe ram was
not able to be set up until the next morning. Overnight the soil was able to slough into the open end of the pipe
casing and resulted in a sinkhole outside the launch pit. Figure 7 shows the sinkhole that formed above the end of
the casing pipe.
Figure 6. Auger Bore Jacking Frame Set-up
Figure 7. Sink Hole Outside the Launch Pit
Possible continued settlement of the ground outside the south launch pit was a concern. Fortunately, settlement
monitoring points had been placed and measured pre-construction along the crossing alignment. These points were
re-measured after each 20-foot section of pipe, or at a minimum at the end of each day of construction. After
discussions with WSDOT and review of the settlement data for the nearest monitoring point (which had not
experienced measureable movement), pipe ramming commenced for the 36-inch pressure pipe crossing.
As the 36-inch pipe was being rammed, soil was splashing out of the spaces between the collar and the hammer.
Figure 8 shows the trail of saturated silty sand on the floor of the pit from the continued splashing of soil and water
out of the pipe. There was concern that during hammering this soil was essentially flowing out between the collar
and the hammer and that the plug that usually forms in a pipe rammed pipe was not present. Pipe ramming
continued with a close eye on volume of material splashing out and the settlement monitoring points. The pipe was
successfully rammed the 120 feet. After installation the contractor surveyed the grade of the pipe. The north end of
the pipe was 12-inches lower than design; however, it was within the tolerances set in the specifications for the
pressure pipe. Additionally, since the contractor had elected to install a larger diameter casing than required, there
was additional latitude within the casing to place the casing closer to the design grade. Ultimately casing spacers
where used to align the 30-inch diameter HDPE pipe within the 36-inch steel casing and to minimize the grade loss.
Paper TA-T4-01 - 6
Figure 8. Wet soil on the floor of the launch pit
Figure 9. 24-inch Casing Spacer in 30-inch Pipe
The next pipe installed was the 30-inch diameter casing pipe. While the 36-inch casing had lost significant grade
during installation, it was hoped that grade would be maintained more effectively for the 30-inch installation.
Because the groundwater level was assumed to be lower on the north side and the pipe installations were higher than
the 36-inch installation, it was also hoped that pipe ramming starting from the north side would be in drier soils and
would maintain the starting grade throughout the ram. Unfortunately, the 30-inch installation lost about the same
amount of grade as the 36-inch over the length of the installation. The grade loss was still within the tolerances of
the project specifications and some of the grade loss was able to be recovered due to strategic use of casing spacers.
Figure 9 shows an example of the 24-inch casing spacer in the 30-inch casing.
Thus far the pipelines installed were either a pressure pipeline (where grade was less of an issue) or a gravity
pipeline within an over-sized conductor casing. The 48-inch diameter fish ladder pipe was neither a pressure pipe
nor in an over-sized casing. As such, maintaining grade within the specified tolerances was particularly critical for
the 48-inch pipe. In anticipation of eventual grade loss, the contractor began installation of the 48-inch pipe by pipe
ramming with a reverse grade. Additionally, the contractor set up the pipe ram in conjunction with the auger bore
jacking frame to apply both static push and pneumatic hammering. An I-beam placed between the bottom portion of
the collar and the bottom portion of the jacking frame transferred the jacking force to the pipe. Figure 10 shows the
pipe ram/jacking frame combination set-up.
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Figure 10. Pipe ram/ jacking frame combination for 48-inch installation
Within the first 13-feet of ramming/pushing, the 48-inch pipe lost all the reverse grade and was essentially flat.
Extrapolating to the end of the installation, the contractor believed he would be off grade by a few feet. The
contractor proposed an alternative solution. He believed that the pipe was losing grade due to a combination of
three things happening together:
 The pipe/soil combination was heavier than the original soil.
 The pipe ram was creating significant vibrations at the face of the pipe.
 The vibrations were “liquefying” the soil under the pipe which allowed the pipe to lose grade.
The contractor proposed completing the 48-inch crossing by switching to auger boring. By changing the
construction method, he believed that the vibrations would go away, the weight of the soil in the pipe would be
reduced and the soil would not liquefy. The design team was concerned that if auger boring was used there would
be over-excavation at the face of the casing and may cause excessive settlement or sink holes at the overlying
roadway surface. After discussions with the Contractor and WSDOT, the contractor was allowed to change
methods, provided that he was still held to the grade and
settlement requirements in the specifications. Additionally, he
was required to maintain the cutting head of the augers at least 3
feet inside the lead casing.
Figure 11.Auger bore set-up for 48-inch installation
Figure 12. Soil from auger bore
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The contractor switched to auger boring and resumed installation of the 48-inch pipe.
The soil coming out of the auger bore was silty sand with the consistency of low slump concrete. The contractor
was able to install the 48-inch casing the entire length. At break through, the pipe had lost about 8 inches of grade
from the design grade. This was within the tolerances in the specifications.
Figure 13. Break through for 48-inch pipe
5.
CONCLUSIONS
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 or using air adjustable pigs to restrict soil movement out of the pipe.
The other lessons learned are 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.
6.
REFERENCES
GeoEngineers (2013) – Trenchless Design Report for the Voights Creek Hatchery Fish Ladder project, Orting,
Washington
Voight’s Creek Hatchery, Site Plans by MWH Americas for WDFW, May 2013.
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