Nelson River HVDC Transmission Line Foundations

The Nelson River HVDC Transmission Line (Bipole I & II) is a 895 km long double circuit transmission line (two parallel power lines) from Gillam to Winnipeg. It has 3,918 guyed towers and 96 self-supporting major angle towers. Foundation conditions varied along the alignment of the transmission line. Some of the foundation conditions encountered included rock, deep muskeg, discontinuous permafrost, soft clays, boulder tills and ground subject to seasonal frost heave. The discontinuous permafrost and adfreeze forces were the greatest challenge for the tower foundations.

Where

The southern 530 km of transmission line through the Interlake are of Manitoba has level terrain containing few large swamps. Limestone bedrock varies from surface to in excess of 15 m. The predominant soil is glacial till with boulders, and silts and clays. Some sands and artesian flow conditions occur.

The northern 365 km of the transmission line is in the Precambrian Shield characterized by widely varying granite rock, deep muskegs, and extensive peat lands. The soils vary from hard glacial till to very soft silty clays and intermittent submerged sand deposits. This section lies within the intermittent permafrost zone: with sporadic permafrost in the south starting near the north end of Lake Winnipeg; to widespread in the north near Kettle Rapids and Gillam.

The ground conditions within a tower vary considerably and it is not unusual to have four anchors of the guyed towers in different sub-surface conditions.

thumb|center|upright=2.0|Figure 1: BiPole I & II Routing

When

• Concept, terrain analysis & route selection - 1966
• Anchor tests – 1966 - 1968
• Surveys and geotechnical – 1967 to 1969
• Design – 1967 to 1969
• Construction - 1968 to 1970

Research

The size and importance of the project justified more than the usual amount of research principally related to permafrost and adfreeze forces.

Extensive laboratory testing, analytical studies, and literature reviews were carried out to obtain basic data on the engineering properties of thawed permafrost soils, rates of regression of the permafrost table and the nature and extent of settlements resulting from thawing. Close liaison was maintained with the National Research Council of Canada and the U. S. Army Material Command’s Cold Regions Research and Engineering Laboratory. Pioneering work in the U.S.S.R. was also used.

Extensive anchor test programs were carried out by Manitoba Hydro at several sites representing various soil conditions. The anchors tested by Manitoba Hydro included grouted, belled, screw-in, plate and Malone types. Teshmont Consultants conducted, for Atomic Energy of Canada Limited, additional anchor test programs including tests on overburden drilled anchors, driven pile anchors and a series of detailed field tests to determine the behavior of straight shaft and belled cast-in-place concrete anchors in thawed permafrost affected ground. Twenty anchors were installed in permafrost and the ground was thawed by means of rod type electric heaters. The anchors were then tested to failure. The behavior of the anchors under anticipated thawing conditions was predicted within an acceptable degree of accuracy.

thumb|center|upright=2.0|Figure 2: Tower Cross Sections

Permafrost

The permafrost occurs in areas as small as a few square metres to as large as several acres. The depth of the permafrost varied from less than 3 m to 30 m or more, in no set pattern. The frozen soils are silty clays containing ice inclusions and lenses up to 300 mm thick. Ground temperature in the upper layers is -1C to 0C. Studies indicated the ground will not remain frozen. In the frozen state the soils have substantial pullout resistance. In the thawed state the soil is transformed into a fluid structure with a drastic reduction in strength and consolidation results in large settlements.

The rate of regression of the permafrost table was estimated based on one dimensional heat transfer. Five idealized soil profiles were analyzed.

Frost Forces

Frost heave and adfreeze forces were computed by an empirical approach described by N. A. Tsytovich and field and laboratory testing by Dalmatov and Vyalov. The average force of 96 kPa over 1.8 m was compared to observations by Crory in Alaska and the U.S.S.R. permafrost design manual. Reducing the forces by granular backfill or providing a slip surface was rejected due to high water table and difficult maintenance.

Designs

The legs of self supporting towers and the masts of the guyed towers are supported on spread type footings consisting of a galvanized steel framework on a treated timber pad. For the 190 km section south from Gypsumville the masts of the guyed towers are supported on 1.07 m by 3.35 m deep concrete caissons.

The 150 mm overburden drilled type of anchor was chosen as the most versatile and economic system. The maximum drilling depth was initially 15 m. In the soft clays and permafrost 400 – 450 mm diameter anchors were installed in 1969. This system was slow and costly and drilling equipment was improved to advance the 150mm system to 24.4m.

Approximately 8000 anchors were tested and 150 required corrective measures consisting of an anchor of greater length or a second anchor with a yolk.

thumb|center|upright=2.0|Figure 3: Foundation Cross Section

thumb|center|upright=2.0|Figure 4: Anchor Cross Section

thumb|center|upright=2.0|Photo 1: OB Drill

thumb|center|upright=2.0|Photo 2: Testing Guy Anchors at Structures

Stories

In the 1970’s ID Engineering Company, one of the founding members of Teshmont, supported research into adfreeze forces at the University of Manitoba. A 2.4 m steel tank was fitting with refrigeration equipment to freeze soils with embedded structural members and measure forces.

Shortly after construction a number of mast support quadruped legs were bent and the top cap was torn at the corner weld by the block of frozen soil rising within the sloping legs. The soil was replaced by a Styrofoam block.

Also within a few years a number of spread footings experienced excessive settlement that was corrected during maintenance.

Influence

The Nelson River Transmission Line is an innovative, successful facility that brings 75% of Manitoba power to southern Manitoba and for export.

Key players

Client – AECL/Manitoba Hydro:

• Harry Smith P. Eng., AECL
• Gordon E. McLure P. Eng, Manitoba Hydro

Designers/Engineers/Geoscientists:

• Ilmar Reinart P. Eng., Teshmont Consultants Ltd.
• A. Staudz P. Eng., Teshmont Consultants. Ltd.
• Larry Hurwitz P. Eng., Independent Testlabs Ltd.
• Jack Mollard P. Eng., J. D. Mollard & Associates

References

Nelson River Transmission Line - Foundation Design Aspects. I. Reinart P. Eng. Project Engineer – Foundations, Teshmont Consultants Ltd. Manitoba Power Conference EHV – DC Winnipeg. Canada. June 7-10, 1971