Surface-pipeline segments solve slide problems in B.C. gasline

Edward A. McClarty
Westcoast Energy Inc.
Fort St. John, B.C.
Drummond S. Cavers
AGRA Earth & Environmental Ltd.
Burnaby, B.C.

• This view looks south at the south approach slope to the tributary to Sikanni (the creek at bottom of the photo). At the crest of the slope, cracks extended across the ROW. This photo was taken before installation of the surface pipeline, and the original pipeline is daylighted on the upper part of the slope. Note the earthflows to the west (right of photo). On the lower part of the slope, the clay from the slides higher up the slopes was spilling over the underlying sands and gravels in thin sheet sliding (Fig. 2; photograph from AGRA Earth & Environmental Ltd., Burnaby, B.C. [46,892 bytes]).
• This north view across the valley floor of the tributary to Buckinghorse River shows the original pipeline after it had been daylighted. The pipeline had been moved laterally 9.5 m by ongoing slide movements before the sag bend pulled up through the soil. As further movement occurred, the left (uphill) side of the daylighting trench ran into the pipeline, once again increasing the pipeline-soil interaction (Fig. 3; photograph from AGRA Earth & Environmental Ltd., Burnaby, B.C.[35,001 bytes]) .
• This view looks north across the tributary to Sikanni (out of sight in the canyon), showing the surface pipeline on skids. The dashed lines show the locations of tension cracks. Later surface pipelines used double skids (two per set) at 3-m spacing because of breakage problems that were experienced. Note the cross berm on the far slope that is continued under the pipeline. The old pipeline was left in place and is visible on the lower part of the north slope. Continued slope movements have now buckled this abandoned pipe section. The side bend (SB in photo) was used to divert the surface pipeline away from a tension crack on a slide lobe out of view beyond the crest of the slope (Fig. 4; photograph from AGRA Earth & Environmental Ltd., Burnaby, B.C. [ 29,818 bytes]).
• Slide appears after construction in which corduroy was placed on a steep highly plastic clay slope south of Barker Creek. The corduroy concentrated the surface drainage that caused the failure. This area was reconstructed with geogrid-reinforced fill, French drains, and additional cross berms upslope of the failure (Fig. 5; photograph from AGRA Earth & Environmental Ltd., Burnaby, B.C. [37,097 bytes]).
• Geogrid-reinforced soil abutment with concrete block facing was installed at Barker Creek (right side of photo). Below the sight line, there is an additional three rows of blocks buried to allow for future erosion. On the left side of the photo, the sag bend was placed on several timbers with no separate abutment (Fig. 6; photograph from AGRA Earth & Environmental Ltd., Burnaby, B.C. [42,329 bytes]).
• Shown here is the crossing of the tributary to Sikanni. The new surface pipeline is on the right and the old daylighted pipe is on the left. No abutment was used for the new surface pipeline. Instead, the sag bend rests on several skids (Fig. 7; photograph from AGRA Earth & Environmental Ltd., Burnaby, B.C. [38,296 bytes]).

• South slope of the tributary to Sikanni shows the surface pipeline after large movements of an earthflow west of the pipeline and widening of the main scarp near the crest of the slope by 1.5 m. The scarp was filled-in prior to this photo. A buried geogrid reinforced rib ("rib" in photo) was constructed on the west side of the surface pipeline with the abandoned pipeline for additional support. Note the straight east edge of the earthflow that was constrained by the rib, ("straight edge"). The surface pipeline and the reinforced rib both functioned well and prevented failure of the pipeline that would certainly have occurred had it still been buried (Fig. 9; photograph from AGRA Earth & Environmental Ltd., Burnaby, B.C. [53,284 bytes]).

Skid-mounted surface pipelines have been used to cross four major creek valleys on Westcoast Energy's Fort Nelson main line where very large, deep-seated slides are progressively failing.

The surface segments range up to 2.2 km long and were used to cross very large slides on the creek approach slopes or, in one case, down-valley sliding of the valley floor that had resulted in major operational problems for the originally buried pipelines. The segments were used for slides where there were no other options for a conventional buried pipeline.

During the studies to support design of the surface pipelines, comprehensive geotechnical investigations included drilling, installation of slope indicators, and electric piezocone penetrometer testing to monitor subsurface pore water pressures and to detect deep-seated slide surfaces.

Typical peak movement rates of the slides varied to a maximum of several meters per year. The unstable soils were predominantly medium-to-high plastic silty clay tills and high plastic glaciolacustrine clays with residual internal angles of friction of 7.5-8.5°.

The pipelines were placed on pressure-treated timber skids on graded ROWs with comprehensive surface and subsurface water control. Aerial clear span crossings were used across the watercourses with geogrid-reinforced abutments on the unstable soil. Geogrids were also used to add stability to structural fills and to reinforce an area of the ROW subject to encroachment from a rapidly moving earth flow up to 6 m deep.

Over the past 5 years of operation of the pipelines, one of the slopes moved 1.5 m in a 24-hr period, and related total movements left a 5-m high head scarp on the ROW. Displacement of soil and areas of thrusting resulted in unsupported pipe spans of 35 m with negligible deleterious impact on the pipeline.

This magnitude of movement would have almost certainly caused a failure of the original buried line. All of the slopes have continued to move and would have caused a varying magnitude of operational problems had the pipelines still been buried.

Progressive instability

Westcoast Energy's 762-mm (30-in.) Fort Nelson main line was originally constructed in 1964 and runs from Fort Nelson, B.C., to 30 km west of Chetwynd, B.C.

Starting in late 1989, major slope instability on several northern sections of the main line created unmanageable operational problems. Pipeline integrity was severely threatened by the areas of instability.

Rerouting and directional drilling were rejected for economic reasons, technical unfeasibility, and, in some instances, an inability to get away from the major slides. It became apparent that the only long-term operationally manageable solution was to cross the unstable areas with skid-mounted pipelines on the ground.

Following are the four major creek valleys where surface-pipeline segments were constructed:

• Martin Creek, KP (kilometer point) 51.5: 2.1 km of 762-mm main line and 2.1 km of 914-mm loop
• Tributary to Buckinghorse River, KP 141.7: 1.1 km of rerouted 762-mm main line
• Tributary to Sikanni River, KP 159.6: 1.4 km of 762-mm main line
• Barker Creek, KP 165: 2.2 km of 762-mm main line.

Geotechnical investigations were performed at all sites, including aerial reconnaissance, air photo interpretation, surface-survey monitoring, test pits, slope indicator, and pneumatic piezometer installations. A more comprehensive program at Barker Creek and the tributary to Sikanni also included diamond drill coring and electric piezocone investigations.

Geology

The Fort Nelson main line is located close to the approximately north-south line where the Cordilleran ice sheet from the west met the Laurentide ice sheet from the east. ¹ As a result, the typical deposits along the pipeline route include a mixture of glaciolacustrine deposits and tills overlying older sands and gravels and, in some cases, tills and glaciolacustrine deposits that predate the last glaciation. ² While not all deposits are present in all areas, the deposits that have played a key part in the development of very large block slides in the four areas under discussion include (upper to lower):

• Glaciolacustrine clay: Thick glaciolacustrine clay deposits (for example, 30-48 m at tributary to Sikanni) partially fill several of the valleys. These deposits include medium-to-highly plastic clays interbedded with sands and silts formed in several glaciolacustrine lakes dammed between the two ice sheets as they melted. The residual shear strengths of the clay were very low (7.5-8.5).
• Glacial tills: Tills in the area consist of the same silty clays as above, with some cobble-to-sand inclusions. The clay mineralogy and residual shear strength of the tills are virtually identical to those of the glaciolacustrine clays.
• Slight variations in plasticity and the coarse fragment content are the differentiating characteristics between the two deposits that are frequently difficult to distinguish.
• Sands and gravels: All of the valleys have some deposits of sand and gravel underlying the clay deposits and overlying bedrock. In the case of the tributary to Sikanni (Fig. 1 [52,097 bytes]), these deposits are present along the preglacial valley of the Sikanni Chief River.

In general, the gravels act as an underdrain, but at the tributary to the Buckinghorse River, the underlying sands and gravels contain very high pore pressures, which increased the down-valley movement.

Bedrock in the area is typically Sikanni Formation that consists of weak-to-moderately strong platy or blocky sandstone interbedded with siltstone and shale. Within the siltstone and shale of the Sikanni Formation, there are thin but continuous bentonitic clay seams.

At Barker Creek, the shale bedrock on the north approach slope had artesian water pressures. Most of the slides are in the overlying surface materials, but the possibility of slide movement in the bedrock has not been ruled out.

Mud rotary investigation holes were drilled at the tributary to Buckinghorse River and Barker Creek soon after the identification of stability problems in the late 1970s. Slope Indicator casings and pneumatic piezometers were installed at both sites.

Comprehensive geotechnical investigations were undertaken from 1989 to 1992 at the tributary to Sikanni and Barker Creek to increase the understanding of the slope failures. The work included mud rotary drilling, diamond coring, and electric piezocone testing.

Conventional Shelby tube and split spoon sampling provided undisturbed and disturbed samples of the in situ soils.

History of movements

All four locations have a long history of slope movements with large deep-seated block slides up to 1 km or more from crest to toe extending for several kilometers along the valleys.

Typically, atop the large slides are numerous smaller, faster-moving slides, also in high plastic clays. Most of the large deep-seated slides on the slopes discussed in this report are very old slides, probably originally formed by valley downcutting through the valley fills of glaciolacustrine clay and clay tills.

As a result of the low angles of residual friction, the slides tend continually to creep, but faster movements may occur in response to increases in pore water pressures and/or the buildup of water in tension cracks.

Originally ice-rich permafrost was present on several of the north-facing slopes. But over the last 30 years, the permafrost has degraded allowing increased infiltration and introducing additional moisture into the slides. In recent years, deep snow has fallen early in autumn, resulting in less ground frost, leading to increased infiltration during spring melt.

Also, annual precipitation has increased up to 20-30% since 1987, when several of the slides became more active, and even more since the pipeline was originally constructed in 1964.

Shallow earthflow slide movements were recognized on the south slope of the tributary to Sikanni soon after construction. At that time, test pitting showed that shallow permafrost was present.

In 1989, deep-seated movements occurred on the north side of the valley, and the pipeline was "daylighted" (uncovered and left exposed) to reduce soil-traction forces. The pipeline was daylighted on the south side in 1991 due to accelerated slide activity.

Eventually, movement on the south side resulted in the pipeline being off the ground over a span of approximately 35 m. An earthflow was also present on the south approach slope a short distance west of the pipeline.

The earthflow was approximately 3-6 m deep and became more active in the late 1980s due to degradation of permafrost and increased water infiltration. Fig. 1 and Fig. 2 [46,892 bytes] show some of the main slide features at the tributary to Sikanni, south approach slope.

At Barker Creek, the pipeline failed by buckling on the lower part of the north approach slope in October 1990 due to large deep-seated slide movements. This was an interesting failure since scratches on the pipeline coating showed that the movement of the soil relative to the pipeline had been at least 3.6 m since installation.

In spite of relatively large movements, only very small tension cracks and scarps were visible on the ground surface as a result of the high plastic clay remolding and filling in the tension cracks and scarps created by the slide. Mosses and surface vegetation further masked the full extent of soil movement.

Martin Creek has also had long-term slide problems that were first identified in the 1970s. From 1987 to 1993, there were large movements which displaced the pipelines laterally by up to 10 m. The two pipelines (the 762-mm main line and a 914-mm loop) were daylighted, and slope clearing, grubbing, and surface compaction were used to reduce surface-water infiltration.

A large drainage ditch was excavated at the crest of the slope to divert surface water from the headscarp. This reduced the movement rate for a few years, but gradually the slope-movement rates increased, tension cracks opened up again, and increasing movements occurred farther to the north requiring the pipelines to be realigned twice. In 1991, other areas of slides across the pipelines were found farther to the north and on the south approach slope.

At the tributary to Buckinghorse River, down-valley slides were identified during routine geotechnical patrols. The pipeline was found to be deflected 9.5 m laterally and pulled up through the soil at the sag bend at the lower portion of the north approach slope.

The overall pattern of movement was translational movement down the valley with the slide being in the order of 16.5 m thick. High pore pressures in the sands underlying the sliding clays increased the rate of sliding.

After the original daylighting in 1989, the movement increased, and an area of much more rapid movements (several meters per year) started to return up-valley toward the pipeline. Significant problems were also encountered with the uphill edge of the daylighting trench moving into the pipeline and increasing the lateral loading (Fig. 3 [35,001 bytes]).

As indicated, all the pipelines were daylighted to some extent for a few months up to a few years before installation of the surface pipelines. In the short term, daylighting the pipelines was a successful strategy to reduce the soil traction forces on the pipeline. From an operational perspective this was not acceptable long-term.

Long-term problems included sloughing of the trenches, lateral movement across the pipeline, the need for additional lengths of pipeline to be daylighted, the lack of adequate surface drainage control, and the fact that the steel was 1960s vintage with low notch toughness and poor low-temperature properties.

As a result, it was decided to proceed with a long-term solution to the problem.

Alternatives

Several alternatives were considered prior to deciding to use surface pipelines:

• Conventional buried pipelines were not feasible on the original routes because of excessive soil-traction forces which would result in failure of the pipelines, such as at Barker Creek in 1990.
• Low-friction coatings and/or low-density backfills were examined, but detailed stress analysis showed that the length of the slope would result in traction forces which would still be too high, potentially resulting in buckling.
• Ongoing mitigative attempts to reduce the slide movements showed that it was not economically or technically practical to stop these very large slides or to reduce movements to operationally manageable rates.
• Reroutes were either not available or would have been 10-15 km long. This amount of new ROW and the cost of construction and maintenance of the longer pipeline were prohibitive.
• Absent of other solutions, the preferred one at these four locations was installation of permanent replacement segments of surface pipelines. It is noted that in many other areas with slide problems, traditional mitigative methods have been successfully employed.

Routing considerations

There were several routing considerations during design and construction of the pipelines:

During construction, the original pipelines had to be left in place and in service. Therefore, in most areas, the new surface pipelines could not be constructed directly on top of the hotlines to avoid traffic loading.

Access for construction and future maintenance was required. For example, this meant that a route beside the pipeline for equipment was required.

It was desirable to keep the new surface pipeline as close as possible to the original route to reduce the environmental impact of new ROW and to reduce clearing requirements.

Particularly where movement would occur across the surface pipeline, cuts on the up-movement side were avoided as much as possible to reduce the risk of the cut slope running into the pipeline in the future.

Rapidly moving small slides were avoided wherever possible, particularly where the movement was across the pipeline on steep sideslopes. As much as possible, the surface pipelines were routed on the larger slide blocks.

Pipeline sidebends and overbends were made as gradually as possible because the stress analyses showed that these would be areas of stress concentration as further movement of the soil occurred.

Steep sideslopes were avoided because of the risk of support loss for the surface line.

Surface-water control relative to tension cracks and large grabens was a major consideration in order to try to reduce future slope movements as much as possible. The ground surface was graded and compacted to enhance surface runoff and reduce infiltration.

Stress analysis

Detailed stress analysis was carried out with a finite-element program that was capable of modeling the pipeline into the plastic range. This was necessary because ongoing slope movements would plastically bend the pipeline. Attempting to operate the pipeline completely in the elastic range would therefore result in unnecessarily conservative design and operating parameters.

The soil parameters used in the program were based on ASCE guidelines.³ The modeling included various combinations of operating pressure, thermal movement, and ongoing slide movement. The aims and results included the following:

• Verification that the pipelines would operate within the relevant acts and codes. At the start of the design, this was of particular interest because the pipelines would be tending to slide down high plastic remolded clays at slope angles greater than the residual angle of friction. The results of the analyses were used to finalize the design and grading requirements at key locations such as bridge abutments and sidebends/overbends.
• Development of operational guidelines to allow field personnel to determine when movements of the pipeline or spans were becoming excessive and correction was required.

Basic design

The basic design was a surface pipeline on skids to allow soil movement under the pipeline.

The main reason for using the skids was to allow the pipeline to remain on the surface of the slope, rather than sinking into the highly plastic clay and therefore being subjected to higher soil traction forces. The skids were 3 m x 150 mm x 150 mm pressure-treated timbers set on rectangles of erosion control mat.

There were two objectives in using the mat:

• To facilitate revegetation around the skids and to avoid their being undermined by erosion
• To allow the skid to move laterally at the side bends during initial pressurization. The skids were laid on the ground across the slope so that they assist in reducing water erosion parallel to the pipeline that might otherwise undermine the skids.

The grading and pipeline designs are very important in terms of reducing the in-operation pipeline stress levels as slope movement occurs. Grading, geotechnical considerations, and pipeline design are all linked. It is desirable to avoid abrupt bends in both the horizontal and vertical planes because these act as stress and strain risers during pipeline/slope movements.

Moreover, a series of vertical bends on rolling topography can either lock the pipeline into the terrain movements, or result in much larger distortions of the pipeline during movement. Therefore, it is necessary carefully to balance the geotechnical issues associated with grading and routing on the slope with the pipe strains that will occur during service.

During installation, careful bending is required because over-excavation to compensate for a "sloppy" bend is not an option.

The skid design is critical. Initial designs had close spacing to reduce the loading on an individual skid in case the pipeline was not touching all the skids. The spacing was subsequently increased for economic reasons.

This resulted in excessive skid breakage as a result of high loading on some of the skids where slope movements caused the pipeline to touch only some skids or where skid penetration resulted from very low bearing capacity in the soft highly plastic clay.

The final design uses double skids every 3 m and functions adequately.

As was expected, it has been necessary to reposition the skids where ongoing slope movement results in their being displaced from under the pipe. Three-meter long skids have generally been adequate, although on sidebends, a longer skid length is sometimes used in anticipation of the large (1-2 m) thermal and pressure-induced movements that may occur in these areas.

The aims of the surface-water control program were to reduce water infiltration into tension cracks and grabens (thus reducing slide-movement rates) and to control surface erosion and sediment generation. Surface water control was a consideration in the choice of routing, as discussed previously.

Standard methods such as surface berms, diversion ditches, weirs within ditches, and careful choice of culvert locations were used. Surface berms were extended across the surface pipeline with shallow swales and berms between the pipeline skids.

While the methods were fairly typical, there was a lot of effort put into fine-tuning the design since major benefits could be obtained in terms of reduction of operational problems. Fig. 4 [29,818 bytes] shows surface drainage-control measures at the tributary to Sikanni.

Subsoil drainage was used on most of the slopes to control local slides. For example, at Barker Creek, corduroy placed during construction concentrated drainage into a sensitive area of slope that was constrained by topography to steep angles. The placement of the corduroy resulted in a slide after construction (Fig. 5 [37,097]). The corduroy was removed and the slope reinstated with compacted native clays reinforced with biaxial geogrid. French drains were placed to alleviate groundwater pressures, and surface cross berms were installed. At the tributary to Sikanni, drilled "horizontal" drains have been considered to relieve water pressures below shallow clay slides overlying sand and gravel at the toe of the south approach slope.

To date, however, seepage appears to be relieving these pore pressures and it has not been considered justified to install the relatively high cost drains (Fig. 1).

The pipelines discussed in this article are capable self spanning to lengths exceeding 50 m (providing that the pipeline is not hydrostatically tested) and therefore no additional truss is required at the aerial stream crossings.

For smaller pipelines, either a larger-diameter length of pipe or a steel bridge has been used to support the pipe. Two types of bridge abutments have typically been used:

1. Soil abutments using compacted, geogrid reinforced soil. The aim is to construct a pad which will move with the slope and which is simple to repair if it is disturbed by ongoing slope deformation. This type of abutment may be required to attain sufficient pipeline elevation above the creek. A variation on this type of structure was used at Barker Creek where geogrid reinforced concrete blocks made up a nearly vertical front face that was under more than 3 m of water during high flow (Fig. 6 [42,329 bytes]).
2. Closely spaced skids on steep approach slopes. This avoids the construction of an abutment in an area where there may already be stability problems. The pipeline sagbends rest only on skids (Fig. 7 [38,296 bytes]).

Revegetation of the grade and ROW play an important part in reducing erosion that could undermine the surface pipeline. Typical local northern forestry or pasture seed mixes containing appreciable legume contents have been successful in the revegetation of the inorganic clays exposed on the surface.

Geotechnical challenges

Each surface pipeline usually has some special and unique geotechnical aspects. For example, on the south approach slope to the tributary to Sikanni, there was an extensive earthflow just west of the surface pipeline that was a result of degrading ice-rich permafrost. Retrogression of the earthflow under the surface pipeline was a concern during design.

To mitigate the earthflow, the daylighted abandoned pipeline trench was backfilled with compacted native soils reinforced with geogrid (Fig. 8 [22,050 bytes]). The aim was to reduce the possibility of the earthflows retrogressing laterally into the new surface pipeline. The soil was reinforced with grids placed in a U-shaped pattern around the old pipeline on each horizontal soil lift.

In September 1994, soon after completion of the reinforced rib, the tension crack at the head scarp of the deep-seated main slide started to widen at measured movements of up to 25 mm/hr. The earthflow to the west of the ROW also moved continuously for 2 weeks. The east edge of the earthflow impinged on the ROW, but the reinforced earth rib was prevented retrogression across the ROW (Fig. 9 [53,284 bytes]).

A head scarp approximately 5 m high was present on the west side of the ROW when the movement rates decreased. Displacement of soil and areas of thrusting resulted in unsupported free spans on the pipe up to 35 m long.

Pipe design

The decision to construct permanent surface-pipeline segments presented challenges to the pipeline designers. Following are some of the major considerations:

• Existing pipeline maximum operating pressure had to be maintained so that the main line capacity was not reduced.
• The pipeline would be exposed to a greater ambient temperature range without the insulation effect of the soil backfill.
• The potential for external damage to the pipeline would be higher in its exposed state on the ground surface.
• Ductile-fracture arrest and rupture control had to be addressed to prevent brittle fracture propagation.
• Deterioration of the external corrosion coating would be higher than normal due to exposure to ultraviolet rays and precipitation.

The surface pipeline segments were designed in accordance with Canadian Standards Association's pipeline design and operating code. Maximum design operating pressure was calculated from the specified minimum yield strength of the pipe steel and pipe geometry.

A minimum design factor of 0.8 is specified by CSA. A design factor of 0.5 was utilized in the surface-pipeline design to add an additional safety allowance. To maintain the existing pipeline operating pressure, the designers chose Grade 448 steel with an 11-mm (0.433-in.) W.T. for the 762-mm pipe.

The lack of soil cover meant that the pipeline could potentially be subject to a temperature range of ?45° C. CSA Z245 notch-toughness Category II requirements were met to ensure that the pipe operated at greater than its fracture-propagation transition temperature at all times.

This would ensure ductile behavior of the steel and resist brittle fractures that could propagate at greater than the acoustical velocity of the decompression wave. In the event of severe external damage, the resistance to fracture propagation would result in a leak, not a catastrophic rupture.

Pipe coating is an integral component to maintaining long-term pipeline integrity. A fusion-bonded epoxy (FBE) coating provides the main corrosion protection.

An exterior coating of light colored, impact-resistant urethane protects the FBE from ultraviolet deterioration and also reduces thermal expansion of the pipeline in the sun. An epoxy-based, heat-shrink polyethylene sleeve provides corrosion protection at the weld joints. Annual inspections monitor the coating integrity.

Regulatory, social issues

The construction of permanent segments of surface pipeline presented several issues that are not a concern for conventional pipeline techniques. These issues had to be addressed to satisfy federal and provincial regulators and to prove due diligence with the design.

Some factions considered large-diameter surface lines to impede wildlife movements. As a result, earth-fill animal crossings were constructed over the pipe at various intervals.

Monitoring of tracks has indicated that the pipes pose no restriction to the wildlife movement, and bears and coyotes have even used the aerial crossings as preferred routes across the creeks. Hoofed animals regularly use the ROW for movement corridors and feeding, routinely stepping over the pipe instead of using the crossings.

Although all surface segments were in areas of low population density, public safety was an issue, as the pipeline could potentially be exposed to external damage.

As previously discussed, the design of the pipe incorporated allowances for external damage to the extent that the pipeline should withstand a rifle shot, with the exception of a high-powered, hard-tipped shot perpendicular to the pipe from close range. The properties of the steel would preclude fracture propagation even if the pipeline were punctured.

Signs provide public warnings at access corridors, at the animal crossings, and at the transition points out of the ground.

The warmth of the flowing gas keeps the surface pipeline segments free of snow and therefore visible throughout all seasons. The remote location of the surface pipeline segments also reduces public proximity to the pipelines.

References

1. Mathews, W.H., "Retreat of the Last Ice Sheets in Northeastern British Columbia and Adjacent Alberta," Geological Survey of Canada, Bulletin 331, 1980.
2. Mathews, W.H., "Quaternary Stratigraphy and Geo-morphology of Charlie Lake (94A) Map-Area, British Columbia," Geological Survey of Canada, Paper 76-20 with accompanying surficial geology map, 1976.
3. American Society of Civil Engineers, Guide for the Seismic Design of Oil and Gas Pipeline Systems, the Committee on Gas and Liquid Fuel Lifelines of the ASCE Technical Council on Lifeline Earthquake Engineering, 1984.

The Authors