Geologic hazards evaluation boosts risk-management program for Western U.S. pipeline

Jill Braun
Northwest Pipeline Corp.
Salt Lake City

Graeme Major
Golder Associates Inc.
Reno, Nev.

Donald O. West
Golder Associates Inc.
Redmond, Wash.

Michal Bukovansky
Bukovansky Associates Ltd.
Lakewood, Colo.

• An operator reads a strain gauge below a landslide head scarp. This landslide is a small, localized slide that is part of a large one at Douglas Pass, Colo. (Photograph courtesy of Northwest Pipeline)[18,317 bytes]
• This segment of pipe at Douglas Pass, Colo., has been stress relieved. (Photograph courtesy of Northwest Pipeline)[29,699 bytes]

The experience of Northwest Pipeline Corp., Salt Lake City, indicates that systematic management of geologic hazards can be part of an overall risk-management program.

Northwest, a Williams Co., operates its 3,900-mile-plus, natural-gas main line (from the San Juan basin in northwestern New Mexico to the U.S.-Canadian border at Sumas, Wash.) and numerous laterals that provide service to customers in six states.

The original main line was built in the mid-1950s when concern for, and the ability to identify geologic hazards as part of alignment selection was much more limited than today.

As a result, the pipeline traverses extensive areas with exposure to numerous potential geologic hazards. Several of these (particularly landsliding) have resulted in significant damage to or rupture of the pipeline.

Northwest has typically addressed these situations by detailed investigations to characterize and mitigate the specific hazard. The company has developed and implemented methods over more than 15 years to identify and characterize the location, nature, and magnitude of the geologic hazards.

More recently, Northwest has formalized these methods into a comprehensive program for identifying, evaluating, monitoring, and mitigating these hazards.

Limited knowledge

Northwest's system, a 26-in. OD main line and more than 50 laterals of various sizes, traverses Colorado, Utah, Wyoming, Idaho, Oregon, and Washington ( Fig. 1 [32,597 bytes]).

The main line and many of the laterals were built in the mid-to-late 1950s and have subsequently been looped. Additional laterals have been added to meet customer demand. With few exceptions, the line was buried.

At the time of original construction, knowledge and experience of geologic-hazard identification and mitigation were limited. The few geotechnical engineers or engineering geologists proficient in these fields were rarely used in pipeline construction.

Pipeline companies relied largely on the experience of their own pipeline engineers and contractors. With this approach and the most current methods available, Northwest's original alignment was selected to avoid areas of unfavorable topography and known geologic hazards.

Still, sections of the right-of-way (ROW) traverse rugged topography and areas of significant active and potentially active geologic hazards.

Two of the longer sections include the Columbia River Gorge in southwestern Washington and Douglas Pass in northwest Colorado where the ROW traverses wide ancient and active landslides. At that time, with the exception of visual observation, no methods were available to monitor these and similar areas.

Pipeline construction practices in the 1950s differed significantly from current ones, and there were few environmental regulations. Although Northwest abided by industry construction techniques current at the time, sidehill construction (in which the ROW is located partly in a cut and partly in fill) was common in steep terrain.

Sidehill cuts were usually not backfilled to the preconstruction terrain configuration and fills were not generally compacted. The ROW was and is still generally maintained free of shrubs or trees to facilitate access to the pipeline for maintenance. Except for water bars, surface-water control was rare.

As a result of a limited understanding of geologic hazards and standard construction practices at that time, the pipeline was installed in rugged topography and unfavorable geological and/or climatic conditions. Slope-stability problems, erosion, and scour have been the most common geotechnical problems encountered during pipeline operation.

Original construction practices or unfavorable geotechnical conditions do not account for all geotechnical problems that influence Northwest's system.

During the 40 years since original construction, residential and industrial developments have encroached upon areas adjacent Northwest's ROW. When such development replaces dense forests, the invariable result is increased runoff and often-dramatic increases in peak surface-water flows.

Clear cutting has also affected hydrology at many locations leading to erosion and slope instability.

With the rapidly developing areas that the system serves, Northwest has become increasingly aware of both naturally occurring and human-triggered geologic hazards. Northwest has recognized the need to devote more people and money not only to identifying and mitigating potential geologic hazards that threaten the pipeline, but also possibly to influencing human activity along the ROW.

These efforts include development and application of a formal geologic-hazards policy and procedure. In fact, Northwest is implementing a more comprehensive program that addresses identification, evaluation, monitoring, and mitigation of geologic hazards.

Geologic hazards

A buried natural-gas pipeline inevitably is exposed to active and/or potentially active geologic processes and hazards. The type, nature, magnitude, extent, and rate of geologic processes and hazards can influence pipeline alignment selection, construction, and long-term performance.

The extreme effect of geologic hazards is pipeline rupture and resulting associated hazards to public health and safety and to the environment.

A full understanding of geologic hazards is therefore important for pipeline construction, as well as for long-term operation and maintenance.

Northwest's pipeline traverses diverse topographies in a region characterized by numerous and varied active geologic and tectonic processes. The topographic conditions range from broad, relatively flat valleys, basins, and plateaus (the Colorado Plateau, Snake River Plain of Idaho, Puget Lowland of Washington) to mountains with significant relief (Middle Rocky Mountains of Colorado, Utah, Wyoming; Blue Mountains of northeastern Oregon; Columbia River Gorge of Washington).

In addition, the climatic regimes range from humid environments of high annual precipitation and runoff (western Washington) to semiarid and arid environments of low precipitation but intense rainfall (eastern Washington and Oregon, Snake River Plain, Colorado Plateau).

Generally, geologic hazards (particularly landslides) in humid mountainous regions with significant precipitation are of more concern than those in flat terrain. The climatic, topographic, and geologic conditions along Northwest's main line in western Washington and the Columbia River Gorge are, for example, particularly unfavorable.

There is significant annual precipitation, concentrated during the winter, locally significant relief, and weak geologic materials susceptible to slope movements and landsliding. Exceptionally high precipitation in the winters of 1995-1996 and 1996-1997 contributed to numerous landslides and extensive erosion in western Washington.

Landslide movement along Northwest's pipeline in this area contributed to three pipeline ruptures at separate locations. In regions traversed by Northwest's main line, the following geologic hazards, listed approximately in order of importance, are particularly important for pipeline alignment selection, construction, and operation:

• Slope stability hazards
• Erosion-related hazards
• Earthquake hazards
• Ground subsidence hazards
• Collapsible and expansive soil hazards
• Shallow groundwater hazards.

This list derives from a series of geologic-hazard reviews conducted by Northwest along the main line and selected laterals in 1995, 1996, and 1997.

Slope stability, erosion-related hazards, and earthquake hazards either have significantly influenced the pipeline or have the most potential significantly to impact future construction, maintenance, and operation.

These three hazards are discussed in detail in the following sections.

Slope instability

Landslides are likely the most important geologic hazard affecting Northwest's system; landslides have caused most of the ruptures in modern pipeline engineering.

Because people often may live nearby, ruptures become a concern for both regulatory agencies and the general public. Following landslide-caused ruptures accompanied by gas ignition on Northwest's main line (at Douglas Pass in Colorado, and the Columbia River Gorge, Castle Rock, Kalama River, and Everson, Wash.), investigations and mitigation of future landslide hazards have been closely watched.

Landslide slope movements generally include falls, topples, slides, spreads, and flows. Ground deformations associated with slides, flows and spreads, are most likely to result in pipeline rupture.

Landslide movement is accompanied by the translation downslope of a rock or soil mass. In addition, distinct shear planes define the lateral limits of the landslide.

If the pipeline crosses the landslide shear, movement of the landslide can result in bending and subsequent rupture of the pipe. Additionally, if the pipeline lies within the moving mass of the landslide, it will be subjected to compression and tension associated with downslope of the moving landslide mass.

In most cases, pipe failure occurs when the pipe is subjected to either compressive or bending stresses. In both cases, the pipe may buckle; only rarely will it break in tension.

Where landslides have been a major contributing factor, Northwest's ruptures have almost exclusively been in localized reactivations of larger, ancient landslide masses.

Where documented, the reactivation of the landslides has been primarily associated with periods of increased annual precipitation and may have been exacerbated by adjacent human development.

One of the more interesting examples of Northwest's installation across a large, ancient landslide is in the Columbia River Gorge where approximately 5 miles of the main line cross the 14-sq-mile Bonneville Landslide (Fig. 2 [95,282 bytes] ).

The original failure of the Bonneville Landslide was more than 10,000 years ago. It has undergone at least three episodes of significant reactivation since then.^{1 2}

The most recent major movement was about 700 years ago when a 5-sq-mile lobe of the Bonneville Landslide moved rapidly into the Columbia River, briefly damming it and permanently changing its course.¹

Current monitoring by Northwest of the pipeline and area through the Bonneville Landslide indicates there has been continuous landslide movement and pipe deformation in some sections of the landslide over at least the past 18 years.

Destructive debris flow and mudflow landslides typically involve moderate-to-large volumes of loose, saturated soil that have been transported rapidly down steep slopes in a semifluid state.³

Failure is generally triggered by the sudden and large infiltration of water.

Debris flows and mudflows can form in natural materials or in the placer fills that are common along the main line.

If the pipeline is in the source area of the debris flow, movement of the soil mass can rupture it. If the pipeline is in the debris runout area, enhanced erosion by the debris flow may affect the pipe buried at typical construction depths.

Large numbers of destructive debris flows developed in the Pacific Northwest during the winters of 1995-1996 and 1996-1997 and in Utah during the winter of 1983-1984. However, none appear to have damaged the pipeline.

The width of the pipeline construction pad and the nature of excavation, grading, and fill placement can affect the stability of slopes traversed by the pipeline. The excavation of a wide construction pad in rugged and steep terrain, as was common in the 1950s in side-hill construction, may decrease the stability of the slopes and increase the potential exposure to landslides.

In addition, the ROW was typically not backfilled and graded to the original slope contours.

The overall effect of such construction practices reduces slope stability. The original Northwest main line was largely constructed in this fashion, and this has resulted in a number of slope movements with potential pipeline damage.

Erosion

Erosion hazards affecting Northwest include stream-bed scour, surface erosion, subsurface erosion, and sinkhole development. These erosion hazards are significant because they can leave the pipe exposed and/or unsupported.

Lateral erosion and stream migration can expose pipelines adjacent to streams or at stream crossings.

An example of such lateral erosion along Northwest's system occurred in the 1960s along the Camas-to-Eugene lateral where the Clakamas River eroded at least 20 ft of the stream bank, exposing the pipeline.

Vertical streambed erosion and scour occurs in many streams and is accelerated when a stream is in flood stage. Pipeline exposure typically occurs where the pipeline has not been buried at a sufficient depth below the streambed.

ROW surface erosion, with potential for pipeline exposure, is more pronounced in sections where the slope is steep, the underlying soil is loose, precipitation occurs in intense events, surface drainage is uncontrolled, and vegetation has been removed.

Subsurface erosion and sinkhole development are more common where loose, erodible, fine-grained soil occurs in combination with a relatively steep groundwater. Such conditions are common in eastern Washington, northeastern Oregon, and the Colorado Plateau.

Earthquake

Typical potential earthquake hazards include strong ground shaking, surface fault rupture, soil liquefaction, earthquake-induced landslides, ground deformation, ground settlement, and tsunami.

For Northwest's system, earthquake strong shaking, surface fault rupture, and soil liquefaction have the most potential to affect the pipeline. Although earthquake strong shaking may damage pipelines, the underground installation of modern (that is, since 1950), well-constructed, natural-gas pipelines generally resists strong ground shaking from even large-magnitude earthquakes.⁴

Soil liquefaction represents the sudden transformation of a loose, saturated, granular soil to a liquefied state under strong earthquake cyclic shaking. Areas susceptible to liquefaction include alluvium-filled valleys with shallow groundwater, located in seismically active regions.

The surface displacement of active (in the past 10,000 years) faults may be on the order of a few inches to a few tens of feet and may occur as lateral shearing or vertical offset of the ground surface along rupture traces that may be several miles long and tens of feet wide.

Northwest's system is in the tectonically active western U.S. The area is characterized by moderate-to-great magnitude (6.0 to 8.0+ on the Richter scale) and frequent (recurrence intervals less than 100-500 yr) seismicity.

This is associated with tectonic plate subduction along the Cascadia subduction zone off Washington and Oregon, and moderate-to-large magnitude (6.0-8.0) and relatively frequent (recurrence intervals on the order of 300-1,000 yr) earthquakes of the basin and range of southeastern Idaho and southwestern Wyoming.

Active and potentially active faults have been mapped in western and eastern Washington, southeastern Idaho, and southwestern Wyoming that cross the main line or are adjacent to it. In addition, some segments of the pipeline alignment in western Washington and southeastern Idaho are in areas susceptible to liquefaction.

Potential hazards

Potential geologic hazards are documented and investigated with traditional geotechnical investigation techniques of surface mapping and subsurface investigation to delineate the surface extent, character, and depth of the hazard.

Once hazards have been identified and documented, available engineering solutions include:

• Monitoring to determine if the hazard is or becomes a threat to the integrity of the pipeline
• Mitigation of the hazard to reduce the risk to the pipeline.

Monitoring

Monitoring can include monitoring of the geologic process or hazard, and/or direct monitoring of the effect of the hazard on the pipeline. This is the traditional geotechnical approach.

Hazard

Unfortunately, monitoring the hazard gives no direct information on how the hazard is affecting the pipeline. It is of limited use, therefore, in deciding when hazard mitigation is necessary.

Because of this limitation, Northwest developed a system to provide information on how the pipeline was being affected.

The following sections describe monitoring options:

• Observational. Visual observations determine initiation of a hazard such as a landslide or rockfall but are not very sensitive, reliable, or quantitative for assessing changes in conditions. Using a pipeline locator and lath to assess deflection of pipe is insensitive for small deflections but can be very useful for large ones.

• Surface survey. The traditional engineering approach to landslide monitoring, surface survey is implemented routinely as one element of monitoring of most active landslides. On-ground monuments and on-pipe cad welds have been used for this purpose. Survey costs can be high, and the implications of measured ground movements for pipeline integrity are difficult to assess.

• Subsurface displacement. Inclinometers are sensitive instruments that can accurately identify a landslide failure surface and detect small ongoing movements. Although inclinometer installations remain operational only through small displacements (typically inches), they can be inexpensively converted to extensometers to enable continued measurement of large ground deformations of any magnitude (OGJ, Feb. 17, 1986, p. 68).

• Groundwater. Standpipe piezometers are easily monitored with water-level indicators or vibrating wire transducers. Although groundwater levels do not give a direct measure of effects of hazards on the pipeline, they can be convenient indicators of when landslide movements are likely to be initiated and of the effectiveness of drainage measures.

Monitoring the pipeline

• Strain gauges. Mounted on the pipeline, strain gauges provide a direct measure of the changes in strain as a result of ground movements. Northwest has used strain gauges to monitor strain changes on pipelines for 15 years.

Vibrating-wire strain gauges have been used exclusively because of their ease of installation, sensitivity, direct reading of strain change in microstrains (1x10-6 strains), and lack of drift. They have proven reliable and durable with few gauge failures.

Three axial strain gauges are installed 120° apart. This enables interpretation of direction of pipeline movement and calculation of complete longitudinal stress state; provides redundancy in case one gauge is damaged or malfunctions; and is nominally unaffected by internal pressure changes.

Trigger levels for stress relief or other mitigation for a stressed pipeline are set based on allowable longitudinal stress. This provides some safety margin to account for such uncertainties as initial stress condition, future deformation of the pipe, corrosion effects, and the fact that the strain measurements may not be at the point of maximum strain.

Because absolute stress cannot be measured, strain-change measurements provide a measure of absolute stress only if the initial strain reading corresponds to a stress-free condition. For pipeline strain monitoring, it is preferable to install strain gauges on new pipe or pipe that has been effectively stress relieved by excavation or cutting.

In general terms, the nature of pipeline deformation and bending can be interpreted from the distribution of tensile and compressive strains around the pipeline (Fig. 3 [369,577 bytes] ).

The maximum longitudinal strain measured at any of three equally spaced monitoring points around a pipeline is not generally the maximum longitudinal strain acting on the pipeline. The maximum strain, however, can be calculated from three longitudinal strain measurements.

Fig. 4 [37,254 bytes]) shows a sample output, for actual field data, from a program developed for this calculation and illustrates how the maximum strain can be substantially higher than measured strains.

Good applications for strain-gauge monitoring include:

- Slow-moving landslides
- Monitoring following stabilization and remediation to confirm effectiveness
- Areas of uncertain ground movement.

Poor applications for strain-gauge monitoring include:

- Fast-moving hazards such as mudflows or rockfalls
- Poorly located gauges, that is, positioned without adequate understanding of the extent and character of the hazard.

• Remote sensing. Viable for some types of monitoring, remote sensing uses telephone, microwave, or satellite links. Vibrating-wire instrumentation (strain gauges and piezometers) is very suitable.

Monitoring data can be relayed at selected intervals to a central location and tied into an alarm system with preset trigger levels and can be interrogated at any time from the central location. Remote sensing is currently being implemented by Northwest at selected locations in southern Washington.

Overall, however, monitoring the hazard (or geologic process) alone is inadequate because it tells nothing of what is happening to the pipe. Monitoring the pipe strains alone is inadequate because it tells nothing about the hazard or how to mitigate it.

The best approach to monitoring includes identifying the hazard; evaluating the character of hazard and risks; designing the monitoring of the hazard and pipeline as an element of mitigation based on the hazard and risks; implementing the monitoring program with trigger levels and contingency plans; and reviewing the monitoring data regularly.

Mitigation

Mitigation requires an engineering evaluation of the hazard and an evaluation of the costs, benefits, and advantages of alternative mitigation methods. Available mitigation methods are the following.

Avoidance

Avoidance often minimizes operating and maintenance costs, but capital costs can be high. Various avoidance options include:

• Alignment selection to avoid geologic hazards is often the cheapest method of mitigation. This is now routine for new pipeline alignments but was not implemented to the same degree for the initial construction of older pipelines such as the Northwest main line. It is not always possible to avoid all hazards.
• Rerouting of an existing pipeline is much more expensive than avoiding a hazard during initial alignment selection but may be the only option when hazards become a problem after construction.
• Deep burial is feasible where the failure surface is well defined by subsurface investigations.
  For this option to be viable, the failure surface has to be relatively shallow, up to 20-30 ft, for example, and excavation conditions favorable so that shoring costs are low.
• Tunneling is an expensive method of very deep burial that may be viable for avoiding very extensive and active hazards.
• Directional drilling is widely used for small-diameter pipe up to several thousand feet and is becoming more feasible and competitive for large diameter pipe.

Aboveground installation

Constructing the pipe on top of a landslide affords a simple pipe-support system designed to allow any future movements of a landslide to simply pass beneath the pipeline without affecting it.

Backfill, aggregate

Compressible backfill or lightweight aggregate may be used within wide, shallow trenches to mitigate the effects of landslide lateral scarps or fault crossings.

Stress relief

When pipelines have undergone unacceptable deformations and stress changes, stress relief can provide effective mitigation. It is accomplished by either or both of the following:

• Excavating the pipe trench upslope of the pipe is often effective, particularly for displacements across the pipeline axis, if sufficient length of pipe can be exposed.
• Cutting the pipe, which may be necessary if it is impractical to expose sufficient length of pipe for effective stress relief, particularly where deformation is predominantly axial compression or tension.

Hazard stabilization, containment

Hazard stabilization may be feasible for landslide and rockfall following an engineering evaluation. Rockfall fences, ditches, and berms can provide effective containment.

Mitigation cases

Landslides in the Douglas pass area caused disruption and relocation of the pipeline in 1962, 1963, and 1979. Exceptionally unfavorable climatic conditions during the early 1980s resulted in large deformations of several landslides in which the pipeline was located.

Northwest's systematic geotechnical evaluation of the area led to implementation of a landslide and pipeline-monitoring program. The program was successful in preventing pipeline failures under exceptionally unfavorable climatic conditions, including a case of landslide movements in excess of 10 ft perpendicular to the pipe axis.

Fig. 5a [220,692 bytes]) presents an example of strain- monitoring data that resulted in multiple stress-relief excavations of the pipeline. The data show seasonal strain changes from temperature changes but none related to ground deformations at Columbia Gorge, Wash.

In Fig. 5b (also for Columbia Gorge), the pipeline appears to be climbing up the slope of the stress-relief excavation trench as the landslide moves in down slope beneath the pipe. The data show initially stable conditions followed by annual strain changes from landslide deformations during four successive late winter and early spring periods.

Details of the monitoring program and instrumentation have been presented (OGJ, Feb. 17, 1986, p. 68).

The Northwest main line on the Washington side of the Columbia Gorge traverses several known large landslides (Fig. 2). Because of the slow rates of movement that currently characterize active landslides in this area, however, there is generally little or no surface indication of recent movements.

Landslides are identified and delineated primarily based on general morphological characteristics. Pipeline ruptures due to landslide movements occurred within the Bonneville Landslide at two locations in 1972 and 1978.

Northwest installed a surface-displacement monitoring system through the area in 1979, but it was of limited value in the short term because of the slow rate of landslide movements. This system was subsequently superseded by pipeline strain-monitoring installations beginning in 1985.

Review of strain-gauge monitoring data for 1985-1997 revealed that many locations within known landslides showed no significant strain changes (Fig. 5c). One location, however, showed significant strain changes in the late winters and early springs of the past several years. These changes resulted from landslide deformations during wet periods (Fig. 6 [347,368 bytes]).

Another site showed a gradual but systematic change in measured strains over the 13 years since installation of the strain gauges. The pipeline had ruptured here in 1978. The measured strain changes of tension at the top of the pipe and equal compression at the two gauges mounted at the 4 o-clock and 8 o-clock positions, indicate that the pipe was being bent vertically upward.

Review of historic records indicated that the 1978 rupture followed the pipeline rising out of the ground at the rupture location. This is consistent with the monitored strain changes and indicates that this section of the pipeline lies within a compressional zone of the landslide. This site was excavated for stress relief in 1997.

Other sites also showed systematic strain changes. Northwest resurveyed surface survey stations to determine whether detectable landslide movements had occurred 1979-1997. This survey defined movements of as much as 6.9 ft downslope in the direction of the Columbia River.

Significant strain changes generally corresponded to areas of documented landslide movement. This enabled identification of active and inactive landslides and landslide lobes, which facilitated planning of future monitoring and mitigation.

Geologic-hazards program

At present, Northwest has developed a policy and is implementing a more comprehensive program, which addresses the identification, monitoring, mitigation, and documentation of geologic hazards.

Because there is currently no prescriptive code, which describes how geologic hazards are to be addressed, this program has been developed from knowledge and experience derived from internal and external sources.

The policy and developing program are briefly described in the following sections.

Identification

Regularly scheduled visual monitoring, via regular aerial and ground patrols, is essential in the initial detection and identification of slowly developing hazards.

As needed, unscheduled, low-level aerial patrols are performed to monitor the progress of known geologic hazards and identify potential geologic hazard areas following the occurrence of significant events, such as abnormally high precipitation, flooding, and seismic activity.

In addition, Northwest recognized that field personnel who patrol and maintain the ROW represent one of the first lines of defense against geologic hazards. Golder Associates assisted Northwest by developing a training course on identification and mitigation of geologic hazards to be presented to a variety of operating and engineering personnel.

Monitoring, mitigation

Once a geologic hazard has been confirmed, a site-specific plan is developed for monitoring that hazard. This plan addresses the particular monitoring methods employed and the frequency of monitoring activities.

Once a geologic hazard has been confirmed and been determined to threaten pipeline integrity, a site-specific plan is developed for mitigation of that hazard.

A database of confirmed geologic hazards and related information will be maintained.

References

1. Palmer, L., "Large Landslides of the Columbia River Gorge, Oregon and Washington," Geological Society of America, Reviews in Engineering Geology, Vol. 3 (1977), pp. 69-83. 2. Allen, J.E., The Magnificent Gateway, A Layman's Guide to the Geology of the Columbia River Gorge, Forest Grove, Ore., Timber Press, 1975. 3. Turner, A.K., and Schuster, R.L., Eds. "Landslides, Investigation and Mitigation," Special Report 247, Transportation Research Board, National Research Council, National Academy Press, Washington, 1996. 4. O'Rourke, T.D., and Palmer, M.C., "Earthquake Performance of Gas Transmission Pipelines," Earthquake Spectra, Vol. 12 (1996), No. 3, pp. 493-527.

The Authors