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December 8, 2020

Powerline Footings – Got Geotech, Now What?

In our last post, you learned how important it was to get the right geotechnical data for footing design.

There’s a few different types, and the type of ground will help determine the most economical design.

Concrete pile footings are a favoured footing type for overhead powerline poles and towers. They usually use less concrete and reinforcement than a pad footing would require and (in soils) they can be easily dug with an auger.

They aren’t always suitable, however, especially when hard rock is found near the surface. It becomes difficult to drill, and who wants to remove good rock just to replace it with concrete?!

What then?

Well, there’s always the right solution, so long as the engineer is prepared to consider the geotechnical data, as well as the skills and equipment of the construction company. When the engineer works closely with the geotechnical engineer and the builder, significant savings in time and money can be had.

Brandon continues from his last post, this time outlining some of the other factors that help us to determine the best design solution.

Influencing Factors

A few factors that influence the choice of footing are:

1. Type of structure

The structure type for transmission lines will affect the foundation type as not all types are applicable for both. A pole will have a mono foundation while for lattice towers commonly uses a large foundation with plinths to each leg. It will also affect the construction process and transport as each structure has their differences to consider.

Powerline Structures - Tower & Pole

Pole & Tower Structures

2. Schedule

Several things can affect the project schedule, whether it’s a rebuild during a scheduled outage or a new line feeding power to a new mine. Certain types of foundations may be significantly quicker to construct than others, even if they’re a bit more expensive to supply.

3. Ground conditions

We addressed this in the last blog.

The ground in which the foundations are installed can vary from igneous, sedimentary or metamorphic rock, non-cohesive soils, sand or gravel to cohesive soil, usually clays.

Equally, soils with a high organic content, for example peat, can also prevail. Composite soils will also be found, and examples of these are sandy gravels and silty sand or sandy peat.

Fundamental to the proper design of foundations is an accurate series of soil tests to determine the range of soil types for which the foundation designs will be required.

It is good practice to carry out soil tests at a rate of 1 in 5 tower or pole sites. This is generally sufficient to enable an accurate forecast of the range of soil types to be established.

Soil Test

The risk in inadequate soil data will result in the need for conservative (costs more) design to balance the risk of under-designing footings in weaker soils encountered during construction. Conservative design generally leads to budget over-runs and late project delivery.

4. Construction equipment

Considerations for choice of footing type need to account for the availability of equipment suitable to the geotechnical findings.

Often an under-powered drilling rig is responsible for defining ‘rock’ at a site, which a decent machine would have no problems getting through.

But you can’t always get a huge rig to the footing site either, so access is another important consideration and depending on equipment options different footing solutions may become more desirable.

Drilling Equipment

Conclusion

The best outcome is achieved when the engineer works closely with the builder, with reliable and sufficient geotechnical data, to provide the optimum solution for all stakeholders.

Next week, in our final blog on this subject, we’ll share how we’ve collaborated with our client, the geotechnical engineer, and the builder to produce constructible designs “on the fly”.

Have a Project that You Want to Discuss?

Book a meeting with us and we will assist you with the structural design and analysis of your structure to help you produce a structure that is fit-for-purpose and capable of resisting all applied loads without failure during its intended life.

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