Tub platform pier footing settlement

by Ken
(Michigan, USA)

Question:

I built a hot tub platform 10' x 10' with nine pier footings (8-inch diameter holes) using 4x6 treated posts. The tub retailer placed and filled the tub about six months ago without any problems. I recently drained and turned the tub one quarter turn to have it face a different direction. After the move was completed, I noticed a gap of 1/4 inch on one corner of the tub and 3/16 - inch at the opposite corner. Checking the level of the platform, I believe one of the piers is low or has settled.

My thought to correct this problem would be to jack-up one and possibly both corners and running water into the soil around the "low" posts. The intent is to have the sandy soil settle between the top of the concrete footing (42" below the surface) and the bottom of the treated post (that will be lifted from the footing as a result of the jacking process.

Will this work?

Answer:
Hi Ken,
I think the best solution is to fill gaps at the two low corners with non shrink grout or similar material. I would check with your tub supplier for advice first, this appear to be common occurrence that they might already have a fix for.

If you go the "fill the gap" route, you need to brace the tub first and hold it to level position while the grout is applied and allow time to cure before releasing the brace.

Go to non-shrink grout for possible products that may fit your project.


Since the 9 piers are in so close proximity of each other but only two settled led me to believe that the settlement was caused by either outside force or the posts were installed too low.

Outside force means may be during installation there were too much force/load applied between these two posts causing them to settle or may be the two settled points are the favorite spots of the tub users.

To jack-up the posts and let them stay in place require a lot of pressure. Flooding the piers will have the opposite effect, instead of pushing upwards as intended the piers will likely sink further down. Instead of problems with 2 piers, the rest of the piers will likely affected because water cannot be controlled to just two posts due to their closeness to each other.

If the settlements were caused by the usage of the tub, rotating it will likely cause two different posts to settle. I would leave the piers alone and fix the gaps with non-shrink grout, your best and least expensive option.

Hope this helps
Tonga