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December 16, 2020

Powerline Footing Design

Powerline Footing Design

In our last post, we discussed the various factors that influence the choice of a particular footing type.

Though the soil properties are important, there’s more to it than that. There’s also the schedule, budget constraints, the type of equipment, access etc.

Consideration of all the parameters and consultation with all the stakeholders will likely result in the optimum or at least near-optimum solution. Doing this before the design is complete is always even better. Ain’t nobody like rework!

Now what? It’s time to design the right footing. In part 3 of our blog on powerline footing design, Brandon shares the types he considered on FMG’s various powerlines currently being constructed to provide power to their flagship projects.

Footing Solutions

Pile footings are commonly used for powerline monopoles as they are economical and easy to install. There is often minimal formwork required and it suits a wide range of geotechnical properties.

Pile footings may be used in rock conditions provided the drilling equipment is capable of reaching the depth required.

What about rock?

Our design optimised the rock socketed pile for use in the highly weathered rock where the length needs to be maintained for stability and a second design case where medium to hard rock is found and the footing simply needs to have the required capacity to transfer the load and reaction forces into the rock strata.

 

Soil Pile Footing

This is ya’ typical pole footing. It looks like this where the pole has a baseplate, but could also be a bored hole in which the pole itself is buried to sufficient depth.

Soil Pile Footing

Soil Pile Footing

Rock Socketed Pile Footing

Pile footings may be used in rock condition provided the drilling equipment is capable of reaching the depth required.

Our design optimised the rock socketed pile for use in the highly weathered rock where the length needs to be maintained for stability.

And a second design case where medium to hard rock is found and the pile is shorter as it simply needs to have the required capacity to transfer the load and reaction forces into the rock strata.

Rock socketed pile footing

Rock socketed pile footing

Alternative Footings

Where hard rock is found at the surface, alternate footing designs were made as the drilling would be slow-going with such large pile diameters. (Pile diameter ranged from 1800-2400mm to accommodate pole diameters at the base of up to 1800mm).

Two alternative footings were designed where hard surface intact rock is encountered.

1. Micropile Pad footing

The first is a micropile group footing with a cap.

This allows for smaller 900mm diameter piles to provide the embedment depth needed for stability while also allowing for the PCD size of the bolt cage to be adequately supported in the footing.

The smaller 900mm pile is easier to drill due to less pressure on the face of the rock drill bit. The downside to the footing is it has a larger concrete volume and more formwork and reinforcement setup onsite.

Micropile pad footing

Micropile pad footing

2. Rock Anchor Pad Footing

The second alternative footing is a rock anchor pad footing. This footing utilises rock anchor tendons to stabilize the pad footing. As the anchors are small in diameter compared to the piles, drilling should not be an issue in the hard rock.

The anchors are grouted into the holes providing tensile support to the footing to aid in stabilising the moment forces experienced by the Monopole.

Rock anchor pad footing

Rock anchor pad footing

The alternative footings would provide options onsite to allow for construction to continue with minimal delays if a pile footing is not constructible.

It was found onsite that the pile option is preferred by the contractor and so far fits all of the conditions found on site. The rock socket was redesigned 3 times to optimise the design for the conditions being found on site.

Designing for our Clients “on the fly”

We can provide construction support on large projects to help optimise designs. Specifically when the onsite construction team reaches problematic areas with Poor soil conditions or rock that is getting refusal from the drill. 

By following the onsite team along with the drilling program we can provide location-specific designs when they are having difficulties on site. This often results in saving time and money onsite. 

We keep regular contact with our client to aid in location-specific design through clear communication and conducting regular meetings to discuss technical queries, Borehole data, and where the issue is being discovered in multiple locations, consider design optimisation to further reduce costs or simplify construction.

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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