By Jeff A. Segar
The 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.
The moisture content of the soils ranged from 15% to 36%, except in the lignite where moisture contents of 57% and 93% were measured. Proctor tests for the fine-grained soils determined the optimum moisture contents to be between 19% and 22%. Three inclinometers and three piezometers were installed during the geotechnical investigation. The results of the inclinometer measurements indicated the failure plane was at 38 feet (11.6 m) in boring S07 and at 36 feet (11.0 m) in boring S08 (Figures 4a and 4b).
Slope stability failure analysis
The piezometer measurements and the elevation of the creek were used in a seepage model to establish the groundwater flow conditions in the slope. Using this groundwater condition along with the information from the inclinometers and triaxial testing, a limit equilibrium slope stability analysis was performed for the prefailure geometry to determine the in situ strength parameters of the backfill slope. The parameters of the soil in the failed slope were varied until a factor of safety of 1.0 was achieved, indicating an imminent failure condition.
The specified search limits required the failure plane to exit the ground surface at the existing crack location. The depth of the failure plane correlated well with the depth and location of the failure surfaces found from the inclinometers, providing further confidence in the parameters used in the analysis.
Design
One of the constraints on the design of the repair was that the head scarp was only 50 feet (15 m) from the well. The well was shut down and did not present signs of damage; however, the construction of the repair needed to maintain the integrity of the well. The integrity of the well limited how close construction and excavation activities could encroach upon the well in addition to limiting how much failed soil could be removed at elevations lower down the slope. The design was performed assuming all fill placed below the reinforced soil slope (RSS) would have the properties determined for the failed soil from the failure analysis to allow for selective removal of the failed soils, if necessary, to maintain stability.
The site was also in a remote location; therefore, to avoid considerable cost associated with removal of existing soils, the design would need to reuse as much soil as possible. This required moisture conditioning of the soils to get them to a moisture content for proper compaction. Adjacent platforms were used for laying out and drying soils, which required handling the soil multiple times.
Retaining wall systems, such as sheet piles and secant walls, were initially investigated but were determined to be inappropriate given the site and design constraints. The alignment of the wall would place the system into the existing failed soils, requiring considerable tiebacks and excessive embedment depths to develop the resistance necessary to restrain the soils. Additionally, a system that could manage subsurface water flows was desired to prevent future slope stability problems due to pore water pressure increases. The retaining wall systems did not meet these criteria.
An RSS combined with riprap at the base of the failure was the preferred solution to meet the site and design constraints. Some of the positive features were that it could be designed with drain tile to manage water, it would be flexible to adapt to long-term consolidation of the fine-grain soils, it could be designed to reuse some of the site material at the upper elevation, and it could be vegetated to fit better into the surroundings.
An analysis was performed to determine the amount of material that could be removed while maintaining the stability of the wellhead. It was determined that a 3H:1V slope starting 35 feet (10.6 m) from the wellhead could be cut down to provide a stable working bench. At this stage, a drain trench on the slope was installed to capture water and reduce pore water pressures in the soils. Also, another drain trench was installed on the opposite side of the platform to capture water affecting the site from the upslope side of the butte.
From this bench, test pits would be performed to determine the extent of the failed mass and the necessary soil removals for the slope below the RSS.
Next, riprap was placed at the toe to buttress the system and provide erosion protection from the stream that the slope abuts. To complete the platform, the soils under the RSS needed to be removed and compacted in select locations to bridge over the failed slope. Global stability controlled the design of the RSS, requiring grid lengths of 14 feet (4.3 m) at the top and increasing in length to 39 feet (11.9 m) at the bottom. Because the existing material was all fine-grained plastic soils, a well-graded aggregate had to be imported for the reinforced fill of the RSS wall. The available Class 5 aggregate locally used for road bedding was used. A slope face of 1.5H:1V was used on the RSS to allow for the face to be easily vegetated.
Construction
Construction began in the fall by creating the temporary excavation and trench drains. Once access to the bench was established, test pits were performed in the failed slope. The test pits discovered a slickenside condition at the contact between a claystone and siltstone layer. A keyway was placed across the contact to reinforce this zone. Construction was halted for the winter to avoid difficulties with thawing and compacting frozen soil. The final construction of the RSS was completed in the spring.
During construction, it was discovered that the lignite was situated in veins rather than layers, as could be seen in the exposed excavation. Rainfall events during construction showed the lignite acted as a conduit for water flow, discharging the water only a few days after the rain events. One of the veins of lignite that transported water discharged water at the back of the reinforced fill of the RSS. To prevent excess buildup of water behind the RSS, a chimney drain was added to capture the water from this vein.
For construction of the RSS two types of uniaxial geogrids were used, one with an ultimate strength of 6,400 pounds/foot (8.7 kN/m) for the primary reinforcement, and three levels of secondary reinforcement with an ultimate strength of 5,000 pounds/foot (6.8 kN/m). The wall was created using galvanized welded wire reinforcement bent at a right angle to create a form (Figure 5b). The two legs of the form were supported by regularly spaced diagonal tie wires. Each lift was 18 inches (0.46 m) tall. The geogrid was placed to extend up the face of the form and wrap back to be buried under the next lift (Figure 5a). A face wrap of needlepunched nonwoven geotextile was placed inside the geogrid to protect against the migration of fines through the face.
Figure 6 shows the final stage of construction as the excavator is placing the soil on the RSS. The RSS is approximately 280 feet (85.3 m) long at the pad elevation. The riprap buttress is seen at the bottom of the photograph. Above the high waterline, the riprap was changed to a well-cemented sandstone to save cost. Also shown in the photograph are the drain lines exiting from the trenches.
Conclusion
This case study describes how a failed slope adjacent to an oil well was designed and repaired. Failed soils were selectively removed to provide a factor of safety for the existing well. Soils that were not able to be removed were surface compacted and accounted for in the design. Drainage trenches were installed on the east side of the platform to capture surface water from higher elevations and on the west side of the platform below the RSS. The drainage will mitigate the buildup of pore water pressure that created the driving force that caused the failure. The final construction used riprap to stabilize the base of the remaining failed soils and protect against scour of the toe and used an RSS to re-create the working surface.
References
Berg, R. R, Christopher, B. R., and Samtani, N. C. (2009). Design of mechanically stabilized earth walls and reinforced soil slopes, U.S. Department of Transportation, Federal Highway Administration, Washington, D.C., FHWA NHI-09-083 and FHWA GEC 011.
Jeff A. Segar, P.E., S.E., is principal, temporary structures/geostructures, with Braun Intertec Corp. in Minneapolis, Minn.