Length may or may not matter depending on the topic of conversation but when it comes to screw piles here in Manitoba length matters a whole lot and some companies offer products that come up short.
First, if you live in Manitoba, you should know that the frost can get as deep as 8 feet and that a frost map of the region puts us just over 7 feet. The article below by CBC confirms how deep frost has gotten.
Without referencing any graphics or a calculator you can assume that a minimum length of 8 feet for a screw pile is required to ensure frost won’t be problematic. However, there are a couple of things that are often overlooked but that are important considerations.
1: You have to measure from the top of the helical blade, not the bottom.
Too many people assume an 8 foot screw pile is good in an 8 foot frost line but I took a measuring tape to a popular DIY ground anchor / screw pile sold at big box stores in Winnipeg and there is a total of 26 inches from the bottom of the screw pile to the top of the helical blade (I drew a quick picture below so I didn’t feature the name brand product). When you deduct 26 inches from the total 8 foot length, that leaves you with 5 foot – 10 inches. Interestingly, 5 foot – 10 inches doesn’t meet code anywhere in Canada for screw piles.
2: You will require 6 inches of screw pile above grade to allow for ground heave.
Typically a screw pile is installed in thawed ground so it is important to leave approximately 6 inches above grade to allow for ground swell during the winter months. If you build to grade, the frost can heave the ground and your structure regardless of having been built on piles. A good rule of thumb is to deduct 6 inches from a screw pile’s total length.
Now, it is very easy to see how screw piles have received mixed reviews in Manitoba regarding their ability to resist frost. When choosing a screw pile for your project, length matters, ensure the screw piles under your investment have helical blades that are below 8 feet and that there is pile shaft above grade to account for season ground swell.