“The empirical relationship between installation torque and capacity is considered to be the greatest attribute of helical foundations.” – Helical Foundations… What an Engineer Needs to Know, Structure Magazine (June 2004), Gary Seider, P.E.
In studying the general approach to screw piles across many jurisdictions in Manitoba one could be mistaken that not much has changed or improved with regards to screw pile technology in 185 years.
A trip outside the province or across the border to the US or even overseas would demonstrate that in fact, many advances have occurred and that screw piles are considered a reliable, predictable and suitable foundation solution for residential, cottage, commercial or heavy/light industrial applications.
Based on the sample engineering below, Manitoba building authorities have approved the full capacity rated use of screw piles to support decks, three season sunrooms, ramps, additions, etc.
Notes 3 and 4 clearly indicate that the engineer has limited the stated allowable loads to soil with a 1,500psf bearing capacity. If soil is other, the engineer is not accepting liability unless retained to re-engineer the pile capacity; the common term used by engineers for these notes is weasel clause.
The ground anchors above can be bought and installed by anyone in Manitoba either by hand (walking in a circle with a bar for leverage) or machine (generally with a posthole auger drive), there is no method for accurately determining if the soil is in fact 1,500psf; commonly described as “blind installation”.
Today, across Canada and beyond, engineers and building authorities have moved away from blind installation methods towards the best practice of torque monitored installations.
“The torque required to advance a helical pile can be correlated to soil shear strength. Torque should be measured on all projects since it provides such an important verification of capacity.” – Helical Piles – A Practical Guide to Design and Installation (2009) by Howard A. Perko, PhD, PE
The National Research Council has extensively reviewed testing performed on screw piles which validates torque monitored installations and has proven screw piles installed in this manner to meet National Building Code (NBC 2010).
Hoyt and Clemence, in conducting 91 full-scale tests, concluded the standard deviation of the predictions using capacity to torque ratio was less than the standard deviation of calculated capacity using various accepted formulas. (Installation Torque as a Predictor of Helical Pile Axial Capacity (2007) – helicalpileworld.com)
There are several accepted methods for measuring torque which include but are not limited to shear pins, mechanical dial indicator, digital device such as Digga Align, Intelli-Tork or Torque-Pin, anchor drive refusal, hydraulic pressure differential, etc. Methods for determining torque do vary in levels of accuracy and allowable loads charts may be adjusted by engineers for safety.
A company equipped with torque monitoring abilities generally works with brand specific engineered allowable loads charts which provide allowable loads for the piles based on torque values achieved. The engineer providing the stamp on the allowable loads charts does not limit their liability to one specific soil value assumption but rather the entire range of torque to capacity values included on the chart; such values might range from 2,000lbs to 50,000lbs or greater.
Comparing two scenarios below, it is possible to see how the two approaches to screw pile installation differ based on liability for failures.
Scenario 1: A home owner buys 5,000lbs ground anchors at the local building supply store; the brand is pre-approved by the building authority for use in the jurisdiction. The approval is based on engineering for 1,500psf soil. The structure, which has loading upwards of 5,000lbs fails. The engineer points to notes 3 and 4. An expensive geotechnical investigation concludes the soil capacity was less than 1,500psf; the engineer is not liable. Looking to hold someone responsible, the home owner looks to the building authority asking how they approved the capacity when soil values in their jurisdiction sometimes are less than the assumed 1,500psf and where it is not possible for most DIY installers to verify their soil’s bearing capacity?
Scenario 2: A home owner engages the services of a trained professional screw pile installation company. Using equipment capable of monitoring torque and referencing engineered allowable loads charts, the installer confirms the required capacities are achieved. Although the torque monitored installation greatly reduced the probability of pile failure, in this extremely rare circumstance a failure occurs. The engineer references the piling installation report, the torque values achieved indicate the required loading was met, for reasons unknown or that are extremely rare, a failure occurred for which the engineer assumes liability.
Entering jurisdictions where screw piles are accepted for residential, commercial or even industrial use, engineers work with torque values and building officials verify that piling reports with torque value are provided. If a geotechnical investigation has been completed and soil bearing capacity indicated, design engineers may use such information to pre-engineer screw piles, however they will still reference the torque achieved during installation to verify each pile achieved the desired capacity.
Torque monitored installation of screw piles has become a norm and is considered a best practice which greatly improves the probability that screw piles installed will perform as required.
Building authorities insisting on torque monitored installations using engineered allowable loads charts endorse a best practice, greatly reduce occurrences of pile failure in their jurisdiction and limit their liability.
Customers insisting on torque monitored installation of screw piles greatly reduce the probability of a pile failure and best protect the money invested in piling.
For more information, please contact our team of screw pile obsessed nerds at 204.793.0653 or toll-free 1-855-4-PILING or visit www.screwpiling.ca