Design of Sewage Lagoon Discharge Pipeline in Landslide Area Near Fort Smith, NWT

The 2006 Annual General Conference of the Canadian Society for Civil Engineering 2006 Congrès général annuel de la Société canadienne de génie civil

Calgary, Alberta, Canada May 23-26, 2006 / 23-26 Mai 2006

Design and Winter Construction of Sewage Lagoon Discharge Pipeline in Landslide Area Near Fort Smith, NWT Kenneth R. Johnson Earth Tech Canada Inc., Edmonton, Alberta, Canada

Abstract: The Town of Fort Smith, Northwest is situated on the bank of the Slave River on the 60th parallel. This community of 2500 people serves as the administrative centre for the Fort Smith Region of the Northwest Territories. The banks of the Slave River are prone to landslides, the most serious of which occurred in 1968, and caused the death of one individual, and the abandonment of a portion of the community. Another significant landslide occurred in August 2004, covering about 500 metres along the river bank, and destroying the community’s sewage discharge pipeline. Resources were immediately mobilized to start the process of replacing the discharge pipeline. These resources included heavy machinery to stabilize the river bank, and engineering resources to investigate the landslide, and design a new discharge pipeline. The design of a new discharge pipeline included several unique characteristics to absorb future movement in the river bank, recognizing that the slide area would continue moving in the future. Construction of the new discharge pipeline occurred over the winter of 2004/05 and, included a number of cold region construction techniques. The new discharge pipeline was commissioned in June of 2005.

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Introduction o

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Fort Smith is located at 60 00'N latitude and 111 53'W longitude, and is the southernmost community in the Northwest Territories. The town is situated on the shore of the Slave River, south of the "Rapids of the Drowned" and immediately north of the NWT/Alberta border. Fort Smith is 322 air km southwest of Yellowknife. The banks of the Slave River are prone to landslides, the most serious of which occurred in 1968, and caused the death of an individual.

Another significant landslide occurred in August 2004, covering about 500 metres along the river bank, and destroying the community’s sewage discharge pipeline. The slide mass is located along the west bank of the Slave River immediately north of the Town, and close to the town's sewage lagoons (See Figure 1).

The discharge line from the sewage lagoons was located within the slide mass, and was severed when movement occurred. This catastrophic event caused no injuries; however it destroyed the sewage lagoon discharge pipeline, which is a significant element of the Town’s waste management system (See Figure 2). It was immediately recognized that the uncontrolled sewage discharge created by the landslide would require immediate attention in the form of stabilizing the slide area, and retaining the resources to design and complete construction of a new pipeline discharge system.

Resources were immediately mobilized to start the process of replacing the discharge pipeline. These resources included heavy machinery to stabilize the river bank, and engineering resources to investigate the landslide, and design a new discharge pipeline.

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Limit of Landslide

Figure 1. Fort Smith landslide area.

Figure 2. Fort Smith sewage discharge pipeline after landslide.

The slide mass was about 650m wide, at the edge of the river and about 350 m wide in the crest area.

The length of the slide, from the scarp crest to river edge, was about 190 m. It was estimated that about 15 to 20 m (horizontal) of failed slope crest material was deposited on the upper portion of the lower slope area, and that about 15 to 25 m of material was pushed out into the river at the slope toe.

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Within days of the slide the Town of Fort Smith completed a temporary gravity discharge pipe (flexible hose) attached to the severed end of the original pipeline. The Town also began grading of the slide area to a depth of about 10 m below original grade, and the excavated material was pushed down slope into the slide zone. The purpose was to provide local stability in the escarpment, and to construct a uniformly graded access to the edge of the river. The excavated area was about 100m wide and was sloped down at about a 6 percent grade. The base of the excavation intersected the original ground surface about 200 3 m behind the slide escarpment. This activity represented about 100,000 m of material that was excavated and replaced on the slide area.

2.

Geotechnical Assessment

Based on the historical information, the riverbank prior to the landslide was considered to be zone of marginally stable colluvium; the slope was about 35 m high, with an overall angle of about 13.0 degrees.

The ground surface was extremely rough, which indicates ongoing movement and localized soil exposures. The riverbank surface also contained numerous springs, surface erosion features, and ponds at various locations.

The depth to bedrock in the vicinity of Fort Smith is about 50 m below the ground surface. The materials immediately above the bedrock generally comprise clay layers, which are overlain by varying thickness of sand, silt and clay layers. The uppermost unit is comprised of sand layers with a thickness of 10 to 20 m; the thickness of the silt and clay layers is between about 15 and 40 m.

The slide originated in unconsolidated clay sediments above the bedrock surface near, or slightly below the level of the river. The slide is probably the result of both groundwater level fluctuations in the slope and on-going erosion at the toe in the river. The on-going erosion of the toe takes place over several years or decades, and gradually reduces the bank stability to a state of eventual failure. The slide essentially adjusted the internal slope stability by creating a flatter profile. The process then repeats itself with alternating periods of marginal stability (on-going creep movements) and large slides.

In addition to continuing slide movement in the slope, the upper fill area of the slide, which was created by the Town immediately after the slide, will be prone to fill settlement. This may result in eventual total settlements at ground surface in the range of 450 to 750 mm in this area, however, the settlement is expected to occur over several years. Part of the settlement could occur quickly or suddenly if the loose fill becomes saturated. Settlement will be less in areas with thinner fill and will likely be almost zero at the edges of the fill mass, near the escarpment excavation and the middle of the lower slide mass.

In the lower slide mass, the ongoing slide movement will be primarily horizontal at probable rates of about 50 to 200 mm per year. As with the upper part of the slide, mass the rate of movement over the short term is likely greater than the long-term rate. Table 1 presents a summary of expected order-of-magnitude displacements in these two general slide areas.

Table 1. Summary of Expected Displacements.

2004 (mm) Slide Zone Upper Slide Zone Lower Slide Zone

Vertical 350 Near 0

After 2004 (mm/year)

Horizontal 200 200

Vertical 350* Near 0

Horizontal 50 - 200 50 - 200

* Assumes maximum settlement from fill prorated over a period of about 5 years. When fill settlement slows the expected ongoing vertical movement rate will be similar to the horizontal rates for this zone after 2004.

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3.

Pipe Replacement Options and Related Issues

Given the magnitude and complexity of the ground movements in the slide area it was obvious that a buried replacement pipeline would not be an appropriate solution the problem. Two above ground solutions, based upon different pipe materials, (High Density Polyethylene (HDPE) and steel pipe) were developed and considered, along with an appropriate anchoring system, outfall structure and erosion control features.

3.1

HDPE Pipe on Ground Surface

The main advantage of installing of a flexible, insulated HDPE pipe directly on the ground surface is that it may accommodate the potentially complex movement patterns and magnitudes expected from the slide mass. Overall, the slide movement will tend to apply tensile forces to the pipe through lateral displacement toward the river and ground surface settlement. This could result in the pipe losing contact with the ground in some locations in the upper slide area and/or increased stresses in over bend areas in the lower slide areas. Periodic soil excavation and/or placement beneath the pipe will be required to maintain relatively uniform contact areas between the pipe and the ground surface, and to manage pipe stresses and strains due to pipe curvature.

3.2

Steel Pipe Supported On Sleepers

The main advantage of installing of rigid, insulated steel pipe supported on adjustable sleepers or cribs is that the need for periodic earthworks is generally eliminated. However, the locations and elevations of the sleepers will have to be adjusted periodically to maintain appropriate span lengths between support points and curvature of the pipe for proper stress conditions in the pipe. The support points should be placed directly on the ground surface to reduce the potential for induced stresses points due to twisting, leaning, or bending of elevated supports.

3.3

Outfall Structure

Movement at the toe of the slide mass could be significantly greater than the movement in the fill area.

Any outfall structure will move relative to the pipe and apply tensile stresses to the pipe unless a flexible connection is designed. An appropriate solution is comprised of a rock filled trench about 3.0 m deep with a sloped base to promote drainage toward the river, to reduce the potential for freezing in the winter, and to allow for annual fluctuations in the river elevation. The trench should be lined with a layer of woven geotextile to prevent migration of fines into the large void spaces of the rock trench.

3.4

Erosion Control

The sand in both cut and fill sections is prone to erosion from moisture and wind, and requires protection to limit the formation of ruts and gullies, and loss of material. Small drainage diversion berms/swales should be constructed at about 40 m spacing on the slope surfaces to direct runoff to areas away from the pipe and access road. In addition to diversion berms and grading for erosion protection, surface vegetation should be encouraged to regenerate in areas with exposed mineral soils to provide additional protection against erosion due to surface runoff and wind effects.

4.

Detailed Design

Based upon the geotechnical information and consultation with the Town, a replacement pipeline design was developed to address four specific design zones which included: stable ground; the upper slide zone; the lower slide zone, and the pipeline discharge zone (See Figure 3). The design criteria for the replacement pipeline included:

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Figure 3. Slide zones and anticipated slide movement

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Pipe vertical stability in each zone; Pipe horizontal stability in each zone; Pipe Anchoring in the stable ground; Freeze protection of the entire pipeline; and Suitable discharge configuration.

Applying the experience that the Town has accumulated with a water supply pipeline in similarly unstable ground along the Slave River, two support configurations were developed for the pipeline replacement.

The upper support system consists of an I-beam “on edge� structure that cradles the pipe, and anchors it to the upper stable portion of the slide area; the I-beam is in turn supported by wood sleepers resting on the ground (See Figures 4 and 5). The lower support system consists of wooden sleepers with restraints bolted to each side (See Figure 6).

Figure 4. Pipeline support in upper slide zone.

Figure 5. Detail of pipeline support in upper slide zone.

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Figure 6. Pipeline support in lower slide zone.

The anchoring of the entire pipeline was accomplished by tying the I-beam into the base of a new manhole at the top of the slope, and tying the pipe itself to the I-beam. The tie to the new manhole used reinforcing steel laid into the concrete base of the manhole, and extending through holes drilled into the I-beam.

Overall freeze protection was accomplished with a urethane foam insulating layer protected by a metal jacket (See Figure 7).

Figure 7. Detail of pipeline insulation

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5.

Construction and Commissioning of Pipeline

The design, and regulatory review of the late summer slide necessitated that the project be scheduled for winter construction in order to replace the pipeline as soon as possible. Consideration was given to postponing the construction until the following summer; however this was ultimately rejected because of the unknown performance of the temporary discharge during the very cold midwinter temperatures, and the availability of the emergency funding from the territorial government. It was recognized that the above ground configuration of the replacement pipeline would accommodate winter construction.

The insulated HDPE pipe was pre-purchased by the Town in order to advance the construction schedule as much as possible. The construction work was undertaken by a local contractor, and proceeded slowly, but steadily over the course of the winter. The most significant construction issue was the connection of the HDPE pipe on site. HDPE pipe is manufactured in discrete lengths and connected on site using butt fusions technology (heating and connecting of pipe ends). Problems were encountered with the butt fusion machinery, therefore the contractor elected to use electro fusion technology (couplers that adhere to pipe end using internal heat coils).

The strength and flexibility of the pipe system was put to the test in the spring of 2005 with an exceptionally high spring runoff in the Slave River, which pushed the pipe 20 to 30 metres downstream.

This occurred before the placement of the outlet anchoring system for the pipeline. The pipe length remained intact without any serious damage, and was pulled back into position and anchored.

The pipeline was commissioned in June of 2005. The total cost of the pipeline replacement (engineering and construction) was approximately $400,000.

6.

Conclusions

The design of the project was based upon the experience of the geotechnical consultant, the design engineers, and the operating staff of the Town of Fort Smith. This combination of experience has provided the Town with a very robust and timely solution to this catastrophic event.

Winter construction of civil related projects in the far north is generally expensive because of extreme temperatures and darkness that will significantly reduce working efficiencies. Frozen ground may also make excavation expensive and time consuming. This particular project was successfully executed during the winter because the majority of the construction was above ground, and the limited excavation was completed in very dry sandy soil.

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References

AMEC Earth and Environmental. 2004. Geotechnical Evaluation of Fort Smith Sewage Discharge Landslide.

AMEC Earth and Environmental. 2004. Preliminary Engineering Evaluation of Fort Smith Sewage Discharge Landslide.

Earth Tech Canada. 2004. Detailed Design Submission for Replacement of Sewage Discharge Pipeline.

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