With the new AS/NZS 7000 standards coming into effect in 2016, many utilities saw their pole foundation requirements increasing in depth and diameter. As a result, designers and site crew lost faith in the software being used.
The foundation calculator (PY-Pole) is used to evaluate and specify the footing requirements for an individual power pole. This is done by entering the minimum pole size, clearance requirements, backfill type, excavation diameter and soil properties. The calculator then iterates to find the best pole size and embedment depth. It can even take into consideration sloping sites and breast/toe blocks.
The software employs the P-Y method for calculating the required footing depth, while staying in-line with the AS/NZS 7000 and international standards.
This blog will look at why the P-Y method was chosen, how the calculator works, the results from the new foundation calculator and a real-life example of the software in the field.
Current Standards and Methods
Traditionally, power poles for use in overhead line networks were installed at 10% of the pole length plus 600-1000mm depending on location. Often this was done regardless of the tip load capacity of the pole or the soil conditions.
However, a change to this method is required in Australia and New Zealand due to the introduction of AS/NZS 7000 standards. The goal of the AS/NZS 7000 is to provide a more technically sound method for pole foundation design. While there are no specific method requirements in the standards, the informative Appendix L recommends the Brinch Hansen method when calculating the required embedment depth and references HB331 for other alternatives.
The Brinch Hansen method was historically used for more rigid engineering purposes, e.g. building piles, that have low tolerance for foundation movement. When we began researching and specifically testing foundation design methods, we discovered data that showed an alarming lack of consistent correlation between the Brinch Hansen design embedment depth and test results, which explains why it is not commonly used globally.
Overhead line pole structures, have a much higher tolerance for foundation movement than provided by the Brinch Hansen method (despite what is written in AS/NZS 7000 Appendix L). The latest version of AS/NZS 7000-2016 allows for a strength reduction factor of up to 1 to compensate for the overdesign inherent in the Brinch Hansen method. However, this fails to consider the situations where the method significantly under-estimates the required embedment depth. In our research, the other issue discovered with the Brinch Hansen method was the disproportionate rise in strength as diameter increased. This doesn’t match with real world results.
This led us to assessing a range of different methods to compare their suitability to the overhead line design field. See Table 1 below for our assessment conclusions on the different methods, on a scale of 1-5 with 5 being suitable and 1 being unsuitable.
Table 1 – Comparison of Different Methods
After extensive research, we chose the P-Y method as the most appropriate for the foundation calculator that we originally developed for Ausgrid. The P-Y method is widely used in the design of laterally loaded piles. It also has the added benefit of producing ground line deflection and rotation values as a standard output, which can easily be selected as criteria to match with inspection and design criteria for pole lean, etc.
Even though it is widely used in laterally loaded piles, the P-Y method is rarely used in the overhead line design field. This is due to the relative complexity of the calculations when compared to the alternatives. Our foundation calculator is designed with the goal of keeping the user interface and input requirements as simple as possible while providing a high level of efficiency in the resulting design. By automating most of the process, the complexity is reduced and manual iteration is avoided to further improve the ease of use.
What is PY-Pole?
The foundation calculator software is developed with the goal of helping utilities to accurately and quickly specify the footing required for a utility pole. It is accessed through our Structural Lines app and can be run on Windows, iOS and Android but requires an active internet connection.
PY Pole covers a range of common pole types and foundation conditions, and we can easily add more to the library to suit your needs.
A few benefits of the calculator are:
- Avoids backfilling timber poles with concrete. Concrete backfill reduces long-term durability.
- More consistent results compared to real-world tests.
- Greater confidence that the results will only give larger embedment depths in poor soils.
- Gives similar results to old rule-of-thumb methods in regular strength soils for regular strength poles.
- Minimal soil properties are required. All can be identified from standard Geotech reports or field assessments.
- User manual also covers interpretation of Geotech information and a field guide for soil strength estimation.
- Saves significant costs for foundation constructions compared with alternatives.
- Calibrated to give allowable rotations under design, serviceability and sustained load combinations.
- Takes into consideration the non-Newtonian behaviour of soil by limiting deflections under sustained loads.
- Checked against real-world pull-over testing of distribution poles.
Development of Ausgrid’s WebPEC Foundation Design Software
We recently assisted Ausgrid in updating and improving the foundation design for power poles on their network. Ausgrid contacted us due to all the above reasons with the new standards, combined with our expertise in this field.
Working alongside Ausgrid, we were able to create the new software, based on the research conducted, as a checking tool in Microsoft Excel. The excel tool iterates through different pole sizes from the Ausgrid catalogue if longer poles are required to maintain the clearances. The user can choose between steel, concrete, wood and fibreglass pole catalogues, and has control over the backfill type and soil information.
Ausgrid then programmed the tool into a web version, which was checked by Revo Group before technical documentation was drafted. The trick was to build the new software into a format that was easy for a range of people to use, while also linking the soil classification from Ausgrid’s soil database system, and to allow changes based on information relayed from site. This new software is used by Ausgrid and ASP3 designers on their network.
According to one Ausgrid employee: “It has been really helpful in the planning of designs, or for quick ad-hoc use on an emergency basis.”
For more information, you can find our project blog here, or you can watch the video below.
Open Version of the Software Now Available!
At Revo Group, we are excited to let you know that we have recently launched our own pole embedment calculator, PY-Pole. PY-Pole is an open version of the software that is available for use on non-Ausgrid installations/networks.
We developed PY-Pole to increase designer confidence when determining the most effective pole foundation depth, meeting the requirements of the standards and the needs of individual networks. It is specifically designed for use on streetlights, power poles, sports lighting, netting poles, and any other free-standing shallow foundation pole.
To find out more about our new foundation design software or to purchase, click the button below.