Geosynthetics October/November 2019 - 35

Case Study

Repairing an oil well
platform with a geogrid
reinforced soil slope
By Jeff A. Segar

T

he Mormon Butte site is in the Little Missouri National Grassland in the
Badlands region of western North Dakota (Figure 1). Constructed in 1985 and
leased through the U.S. Forest Service, the pad elevation was created by cutting into
the side of the butte and depositing the spoils at a 1H:1V slope on the west side of
the cut line to fill up to the desired grade. From the east side of the platform, grade
rises 100 feet (30 m) to the top of the butte, and on the west side, grade drops 75
feet (23 m) to a small creek.
In February 2014, the site operator noticed some deflection in the elevated line that
transports waste gas to the flare. Melting snow was also ponding and refreezing in a
depression at the top of the pad in this area. By April the operator was able to photograph cracks in the top of the platform. In May, there were seven days with rain events
each producing more than 0.25 inch (6 mm) of rain, including three days-from May
24 to 27-when a total of 4.5 inches (115 mm) of rain fell. Upon returning to the site in
the first week of June, the site operator found that 12,000 square feet (1,115 m2) of the
west edge of the platform had failed. Less than 50 feet (15.2 m) from the wellhead, the
platform had dropped 18 feet (5.5 m) in elevation (Figure 2 on page 36). The failure
impacted 75 feet (22.9 m) vertically down the slope and pushed material into the creek.

Geotechnical exploration
Braun Intertec performed nine standard penetration borings to depths of 50 to 70 feet
(15.2 to 21.3 m). Due to safety concerns, the borings were taken on the platform elevation through the stable soils. The borings show that the soil underneath the Mormon
Butte site consists of the Sentinel Butte Formation characterized by weak sedimentary
mudstones. The mudstones present in the soil exploration are siltstone, sandstone and
claystone, as well as layers of lignite. Blow counts within all these rock layers ranged
from 5 to 75 blows per foot, indicating rather soft to hard soils present. The variation
in the blow counts with respect to depth for the soils and rocks at the Mormon Butte
site is shown in Figure 3 on page 36. The trend line in the figure indicates a linearly
increasing trend of blow counts with depth.

PROJECT HIGHLIGHTS
OIL WELL PLATFORM REPAIR
LOCATION

Little Missouri National
Grassland, N.D.
DESIGN ENGINEER

Braun Intertec Corp.
GEOSYNTHETICS PRODUCT

Uniaxial geogrid
Needlepunched
nonwoven geotextile
GEOSYNTHETICS MANUFACTURER

Strata Systems

Jeff A. Segar, P.E., S.E., is principal,
temporary structures/geostructures,
with Braun Intertec Corp. in
Minneapolis, Minn.
All figures courtesy of the author.

www.GeosyntheticsMagazine.com

35


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Geosynthetics October/November 2019

Table of Contents for the Digital Edition of Geosynthetics October/November 2019

Geosynthetics October/November 2019 - Cover1
Geosynthetics October/November 2019 - Cover2
Geosynthetics October/November 2019 - 1
Geosynthetics October/November 2019 - 2
Geosynthetics October/November 2019 - 3
Geosynthetics October/November 2019 - 4
Geosynthetics October/November 2019 - 5
Geosynthetics October/November 2019 - 6
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Geosynthetics October/November 2019 - Cover3
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Geosynthetics October/November 2019 - Blank1
Geosynthetics October/November 2019 - GeoConf20_1
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