One question I get asked frequently is why we don’t carry screw piles with the double helix design. Surely 2 helicals are better than 1 right? The answer is not always my friends and in most cases, certainly not in Winnipeg. Let me explain…
Winnipeg is known for it’s “gumbo” like soil, we live on what is considered “expansive clay”. This means that the moisture in the soil combined with fluctuating temperatures creates soil movement; the area prone to movement is known as the “active zone”. A reason helical screw piles are so effective against ground movement is that so long as the helical is anchored below the active zone, it will secure the slender pile shaft firmly in place.
If you use a double helix screw pile in expansive clay, it is imperative that BOTH helices be anchored below the active zone, otherwise the pressure created by expansive clay on the second helix could result in heaving.
Some might be quick to say, “no big deal, we will just get both helices below the active zone.” This may be possible but most likely only through the use of extensions and sometimes even extensions won’t work.
The frost line in Winnipeg is as deep as 8 feet and the “active zone” should be considered at the same depth. The minimum engineered design requirements of a double helix screw pile dictate that the distance between the helices be a minimum of 3 times the diameter of smaller bottom helix. Assuming the bottom helix is small and only 8 inches in diameter, you just added 2 feet to your pile. Since the bottom helix generally ends at or around 6 inches from the bottom of the pile and you need at least 6 inches of pile above grade to allow for heaving, you now require a pile at least 11 feet in length to guarantee that both helices are anchored below the active zone.
Most Winnipeg piling contractors carry a 7 to 8 foot helical screw pile (I don’t agree with this length0 but the odd contractor can order 10 foot piles; as an aside Postech Winnipeg only carries 10 foot piles, click this link to learn why: https://screwpiling.wordpress.com/2012/01/18/8-foot-ground-anchors-in-winnipeg-why-one-piling-contractor-is-going-with-10-foot-screw-piles/. I don’t know of any piling contractors in the area who carry 11 foot double helix screw piles.
Here’s more on why I indicated that only sometimes is it possible to anchor both helices on a double helix screw pile below the active zone. Often very dense soil is found below the active zone. If the soil is dense enough, the pile will reach “refusal” a short distance below the active zone; at refusal the pile has reached its ultimate depth and can not be screwed down any further. If the pile reaches refusal before the second helix has passed the active zone, extensions won’t work and your foundation will be at great risk of heaving.
There are additional reasons we do not generally carry double helix screw piles, however under certain circumstances where soil tests have been preformed and the engineers are certain a double helix design will not jeopardize a foundation’s stability, we can custom order double helix screw piles. Note that load tests have proven that a double helix screw pile and a single helix screw pile both carry equivalent capacity should they reach equivalent torque.
If you are looking for a high quality heave proof foundation, call Postech Winnipeg toll free at 1 (855) 4-PILING (474-5464) or locally (204) 793-0653 or email firstname.lastname@example.org.