Land that once formed the bottom of an ancient glacial lake provides a great fertile plain for growing crops. However, the same geology poses serious challenges for stable foundations of transmission structures. Such was the case for Xcel Energy’s Fargo–St. Cloud–Monticello 345-kV transmission line built as part of the CapX2020 transmission expansion initiative. This 240-mile (386-km) double-circuit capable transmission line was constructed on self-supporting tubular-steel poles. Known as the Fargo line, because it begins west of Fargo, North Dakota, U.S., the line generally runs south and east to Monticello, Minnesota, U.S., northwest of Minneapolis.
CapX2020 is a joint initiative of 11 utilities in Minnesota, North and South Dakota and Wisconsin. The utilities’ goal is to upgrade and expand the grid to ensure continued reliable service well into the future and expand access to new generation, including renewable energy. North and South Dakota as well as Minnesota rank among the top 10 states in the country for wind energy potential. One purpose of the CapX2020 initiative is to help harvest this potential energy by tapping into power generated by the region’s wind turbines.
Nevertheless, the same wind that turns the giant blades of a turbine can create problems for transmission lines, towers and foundations. Add in the region’s sometimes cold, harsh winters, and there are a lot of physical stressors on equipment and structures. The CapX2020 utilities considered the Fargo transmission line a major backbone system that needed to be highly reliable. As a result, Xcel Energy requested structures and foundations be designed to withstand extreme wind and ice storms of the intensity likely to occur once every 200 years.
Glacial Lake Agassiz
The northernmost 52 miles (84 km) of the Fargo transmission line contain about 284 structures located in the ancient lakebed of Glacial Lake Agassiz (GLA). GLA was an immense glacial lake. It was larger than all of the Great Lakes combined and held more water than all lakes in the world today. When GLA drained about 8500 years ago, it left deep, soft clay deposits.
Environmental desiccation has strengthened the surface clays to depths up to 20 ft (6 m) to form a stiff layer that produces exceptional cropland. However, the underlying soft clays up to 100 ft (30 m) deep create a challenge for design and construction of drilled-pier foundations commonly used for self-supporting tubular-steel structures. Some have likened the behavior of GLA clays to that of crème brûlée.
Prior to CapX2020, transmission lines in the GLA region were mostly single circuit using either wood H-frames or lattice-steel towers, neither of which require deep foundations. The Fargo project demanded double-circuit, self-supporting tubular-steel pole structures. They have a minimal footprint but require a deep foundation for stability.
Because of the geologic features and strong winds, Xcel Energy initially believed structure foundations would have to be driven piles with concrete pile caps and stems. The concern was GLA’s deeper soft clay deposits could introduce the potential for long-term creep in foundation deflection under typical wind conditions. That drove Xcel Energy’s initial cost estimates to assume the 284 foundations in the GLA region would be driven piles with concrete pile caps and stems.
Potential Cost Savings
Xcel Energy recognized huge cost savings could be achieved if drilled piers were used instead of driven piles. Xcel Energy foundation crews could install drilled piers and would avoid the need to employ specialty contractors. Drilled piers would save time because one foundation crew could install up to 10 drilled piers in a week, whereas specialty contractors would need about four weeks to install one pile foundation.
For these reasons, Xcel Energy studied intensively whether it could use drilled piers instead of driven piles. The utility tapped POWER Engineers Inc., Northern Technologies LLC (NTI) and its own construction staff to help. NTI’s geotechnical studies included comprehensive soil boring and pressure meter testing. POWER compared preliminary engineering and costs for drilled-pier and driven-pile foundations.
A three-phase geotechnical exploration assessed subsurface conditions and strength parameters of GLA soils supporting transmission structure foundations. The phase one geotechnical exploration involved 37 soil borings at readily accessible locations. The borings revealed the soils were highly variable, containing wet to water-bearing silt and fine-grained sand alluviums within the fat clays. They also showed artesian conditions were likely in the eastern portion of the GLA region.
For this reason, the project team decided phase two would involve soil boring at each structure location. Phase three involved NTI making 21 pressure meter tests at six locations of the soft to very soft fat clay as well as loose to very loose silt and fine-sand alluvium.
The studies proved successful. They showed it was possible to use drilled-pier instead of driven-pile foundations if certain precautions were taken. A finite element analysis by an independent foundation design consulting firm validated the results.
The project team also conducted some full-scale test installations geared toward confirming drilled piers indeed could be constructed in GLA by Xcel Energy crews. The team selected four test locations in the area with the deepest and softest clay and silt deposits. Two of these were considered production piers because they were eventually used to support a project structure. The other two were temporary installations discarded after testing.
Each of these was located near one of the two production piers to enable a comparison of construction techniques. Xcel Energy’s construction team first built one production pier and one temporary pier in soft clay. The team successfully drilled and poured both. The team then installed the second production pier and second temporary pier in silty soil, where water seepage was expected but did not occur, and the need for drilling fluids was expected but not needed.
When installing the test piers, the project team also installed access tubes to enable cross-hole sonic logging, which confirmed concrete integrity. The test installations verified drilling methods, tooling requirements and concrete placement steps. These tests also measured productivity to set a baseline for the project schedule. This enabled Xcel Energy crews to begin installing drilled piers in GLA.
Of the 284 structure locations in the GLA region, 280 were installed successfully as drilled piers; attempts to install drilled piers proved unsuccessful in only five locations. Two holes collapsed during drilling. They had temporary casing but only near the ground surface to stabilize the top of the hole. The others suffered necking, which became apparent when a less-than-expected concrete volume filled the hole. They had temporary casing to a depth near the bottom of the hole.
These unsuccessful attempts occurred early in the construction schedule and provided additional insight into precautions that should be taken during construction. At one of those five locations, the project team made a minor structure move, and a second attempt at a drilled pier was successful. For the other four locations, the team eventually installed a pile-type foundation.
For the holes that failed during drilling, the project team concluded that lateral soil pressures caused the unsupported portion of the hole to collapse. The team also believed lateral soil pressures caused the necking in the drilled piers poured with casing — either in the uncased region near the bottom of the hole or because of maintaining insufficient concrete-head elevation during casing removal.
To minimize the risk of losing additional holes, the foundation designer, geotechnical engineer and construction foreman collaborated to agree on the appropriate method to be used at each site. After this approach was adopted, no further holes were lost, no further drilled piers were rejected and no further pile foundations were required in the GLA area.
Cost Savings Realized
The soft clay deposits in the GLA region presented a major challenge for foundation design and construction of the CapX2020 Fargo 345-kV transmission line project. By identifying the challenge early and implementing a comprehensive program to evaluate soil behavior, install test foundations and explore drilling options, Xcel Energy could install drilled-pier foundations that were reliable and met performance criteria.
This avoided the need for expensive pile foundations, saving more than $11 million on the preconstruction estimate of the foundation cost. Even more important was the positive effect on the construction schedule because one foundation crew could install seven to 10 drilled piers a week, whereas it took several specialty contractors roughly three to four weeks to install a single pile foundation.
The key to this success was the implementation of a collaborative approach among geotechnical, engineering and construction personnel to assess the risks and determine appropriate construction methodology for each foundation. ♦
Gerald Chezik is a retired project manager from Xcel Energy. He was responsible for the overall design and construction phase of the CapX2020 Fargo–Monticello project. His experience included transmission line design and transmission project management, transmission engineering and construction, and quality project management. He is a professional engineer.
Bret Anderson is a principal for Northern Technologies LLC. He has more than 35 years of experience in geotechnical engineering, providing consulting on foundations for infrastructure, buildings and other civil construction projects. As the geotechnical engineer of record, he provided region-specific consultation in the design of drilled-pier and drilled-pile foundation support for the transmission line crossing extremely soft, deep fat clays of the Glacial Lake Agassiz formation. He is a professional engineer.
Dave Wedell is a senior project manager for POWER Engineers Inc. He has extensive experience in design and management of transmission line projects. He served as project engineer on the Fargo–Alexandria portion of the Fargo–Monticello transmission line project. He is a professional engineer.