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December 1, 2020

Powerline Footing Design – It’s Geo-Technical

Powerline Structure Footing Design

Whilst a successful Overhead Powerline is the mainly thanks to the electrical engineer, we structural engineers play an important role to ensure it remains “overhead”!

Anatomy of an Overhead Powerline Structure

  1. The Conductors carry electric power from the sending end station to the receiving end station.
  2. Supports, which may be poles or towers, keep the conductors at a suitable level above the ground.
  3. Insulators are attached to the supports to insulate the conductors from the ground.
  4. Cross arms provide support to the insulators.
Anatomy of Powerlines Structures

Anatomy of an Overhead Powerline Structure

The electrical grid is dependent on the prudent design of the structural components by the structural engineer, who needs to understand all the forces at play during the design life, as well as the interaction between the ground and the footings.

When properly designed, overhead powerlines provide an invaluable service to industry and the public; however, when defective or deteriorated, power lines may yield devastating consequences – particularly during extreme weather and loading conditions.

In this week’s blog, we cover the geotechnical aspects important to structural engineers working on such projects.

You simply can’t design proper footings without geotechnical data, yet it’s often the least priority of a powerline project.

Before engaging the structural engineer, make sure you’ve got some data we can use to ensure the footing designs are appropriate and economical.

Geotechnical Information for Foundation Design

Site Investigations are carried out by Geotechnical Experts using bore holes, test pit excavations and penetrometer tests. These tests provide in-situ data or soil samples for testing in the laboratory.

In-situ tests such as Penetration tests provide relative density to sands and soils.

The soil and rock samples taken from bore holes and test pits are studied in the lab or sometimes on site to measure density, shear strength, chemical composition, moisture content, unconfined compressive strength (UCS), weathering and fracturing.

Soils:

Important information relating to the structural design of footings in soil include some of the following:

    • Classification
    • Unit weight (density)
    • Internal friction angle
    • Saturated unit weight
    • Elastic Modulus range
    • Lateral Subgrade Reaction Modulus (K)
    • Shear strength
    • Cohesion

Rock:

Important design figures for designing footings in rock include:

    • Unit weight
    • Internal Friction Angle
    • Saturated Unit Weight
    • UCS
    • GSI or RMR (Rock Mass Rating)
      • Weathering
      • Fracturing
    • Shear Strength
    • Elastic Modulus
    • Lateral Subgrade Reaction Modulus (K)

Geotechnical Testing

To obtain the crucial values for foundation design, testing is required. The following tests will provide the geotechnical parameters needed:

Geotechnical Tests

A Table of Geotechnical Tests Required

As there is so much variation in ground conditions and test methods, it’s best to consult a geotechnical engineer as to which tests will work best for the specific site in order to get the information we need.

Calculating Vertical Capacity of A Footing

The most common design properties used for calculating vertical capacity of a footing are the unit weight, internal friction angle for a cohesionless soil and the cohesion for a cohesive soil. For a rock, the UCS is used to determine the allowable bearing capacity. 

For pile footings friction between the side wall and pile face (known as skin friction or shaft resistance in soils, or adhesion in rock that is not highly weathered and fractured) contributes to the total vertical capacity of the footing, whether it is a compressive or tension load.

Information on the water table and water flow due to flooding is also required to determine the worst case scenario when calculating skin friction as it can have dramatic effects on the value, with cohesive soils behaving the worst in changing water conditions.

Skin Friction

Shaft Resistance in Soils

Calculating the Lateral Subgrade Modulus

For power poles, the vertical loads are not huge. Much more significant are the horizontal forces from conductor tension and wind loads. This calls for a footing suitably anchored to resist the shear force and bending moment so produced. We need to know the Lateral Subgrade Modulus.

The Lateral Subgrade Modulus is a calculated value of soil stiffness that uses the Elastic Modulus and Geological Strength Index (GSI) Value. This GSI is based on two factors, Level of Fracturing and Level of Weathering.

This determines how broken up and degraded the rock is, having a direct effect on the lateral subgrade modulus. GSI can be determined on site or in the lab by an expert geologist. An example of the table they grade with is presented below.

Geological Strength Index (GSI)

Geological Strength Index (GSI)

The Lateral Subgrade Modulus allows for calculation of rotation of the footing, soil stress and forces in the footing under service and ultimate loads.

This is important for multi-layer soil and/or rock profiles as it allows the designer to see the work done by each layer in the ground profile.

Rock usually has a higher Lateral Subgrade Modulus than soil, so when a rock layer is present amongst soil layers, the rock layer can be several times stiffer than the soil, attracting more of the load to that layer. It certainly helps, but it can cause stress concentrations in the footing which need to be designed for during footing design.

Stay tuned for next week’s blog which will cover how “Foundation Design Type is Determined” following the geotechnical study.

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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