PERMFILE64444

• Permit No: M1998058
• IBM Index Class Name: Permit File
• Doc Date: 9/15/1998
• Doc Name: MEMO-STABILITY OF HIGH PRESSURE GAS PIPELINE TELLIER GRAVEL PIT JOHNSON EXCAVATION INC FN M-98-058
• From: ALLEN SORENSON
• To: ERICA CROSBY
• Media Type: D
• Archive: No

,.
West Gas in investigating the area and
in designing and implementating miti-
gating measures at individual critical
locations.
Strain gauges installed. Monitoring
methods using vibrating-wire strain
gauges have systematically been used
along the lateral. Such monitorinp~
methods are described elsewhere.'
They are based on installations of
strategically located vibrating-wire
strain gauges in sets of three at select-
ed locations where stability problems
are suspected.
Continuous monitoring of strains in
the pipe provides reliable information
on slope or landslide deformations
and enables the line operator to insti-
gate early mitigating measures to pre-
vent breakage of the line.
Because of the importance of the
line, West Gas has been systematical-
ly improving its safety. The improve-
ments have included stabilization of
some critical areas, relocation of cer-
tain sections of the line, and special
installations. Monitoring with vibrat-
ing-wire strain gauges has enabled the
line operator continuously to evaluate
the safety of the line. During 1986,
the installed monitoring system pro-
vided West Gas with an early warning
of landslide deformations that would
be difficult to detect by any other
method. The experience gained in this
case, referred to as Landslide 7, is
presented here.
Landslide 7. During the geotechni-
cal studies performed in 1984-85, a
geologic feature was identified as a
landslide along the alignment in the
vicinity of historic Camp Hale, site of
the 10th Mountain Division.
The landslide did not indicate any
deformations during the time of the
geotechnical studies. The landslide is
shown on Figs. 1 and 2.
The landslide is approximately 600
ft long and 220 to 380 ft wide. Its toe
extends into the Eagle River.
As no deformations have been iden-
tified on the opposite side of the river,
it can be reasonably assumed that the
landslide slip plane outcrops at ap-
proximately the elevation of the river
bottom. It is probable that river ero-
sion contributed to the development
of the landslide and possibly to the
renewal of deformations at the present
lime. The course of the landslide slip
plane has been interpreted based on
topographic features and without any
exploratory drilling. The inteprelation
is Shown in Section A-A' on Fig. ?.
Based on the assumed course of the
slip plane, the total yardage of the
landslide was estimated to he approxi-
mately 200,000 cu yd.
The pipeline was installed across
the landslide in its slower third. In
order to monitor strains in the Ilne,

Landslide 7 orofile of A-A'
Fig 2
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one set of vibrating-wire strain gauges
was installed in the central portion of
the landslide in 1985.
This set of gauges comprises one
part of a complex monitoring system
where strain gauges were installed on
the line at numerous other locations
that are either landslides or areas with
potential slope stability hazards.
Three strain gauges were installed
at this location. strain Gauge I was
installed at the top of the line, and
Gauges 2 and 3 al 120° intervals.
Fig. 3 shows strain increases from
installation omit the present.
Actual strains In the pipe at the lime
of installation were not known. For
this reason, the basic reading at the
time of installation is assumed to rep-
resent 0 in the plot of the strains.
Steady increase. The strain plot be-
tween October 1984 and April 1986
indicated a slow but steady increase
of strains. While [he upper gauge
(Gauge 11 indicated the slowest in-
crease, both the lower gauges (2 and
3) Indicated a more significant strain
increase. From April [o May 1986,
strains began to increase rapidly, and
West Gas decided to inspect the area;
the site inspection revealed numerous
new tension cracks in the upper por-
tion of the landslide.
It was evident that old landslide
deformations had been renewed and
the safety of the line endangered. The
longitudinal, relative strains in the line
prior to mitigation are shown on fig.
4. The line showed significant in-
creases in longitudinal tensile strains
al all three gauge locations.
Maximum and minimum tensile
strains are within 2° of a vertical
plane. This indicates that a vertical
component of landslide deformations
has an important influence on the
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