We were referred by a deck construction company to a job where a deck was literally sinking. The end of the deck where it was being supported by posts had dropped more than 5” in less than 5 years. This home is built on a steeply sloping lot, and the deck is over 18 feet off the ground. Not a good situation.
The homeowner also wanted to remove 2 of the 6 posts and evenly space the remaining 4 posts. There was a concrete pad under the deck that the landscape company removed prior to our arrival. They had also rebuilt a retaining wall under the deck area, as the original wall was bowing outward due to poor construction techniques and the pressure of the soil behind it.
We had four main tasks for our work order: 1) shore up the deck with temporary supports, 2) remove 4 of the original posts so that we could 3) dig 4 footings that were 30” x 30”x 4’ deep and, 4) drive a helical pier in each footing to the engineer’s specifications.
Why were helical piers needed? The soil under the deck was fill dirt and much too soft to carry the load of the deck. When we used a probe to check the soil structure, it went four feet into the ground with hardly any resistance. In order for footers to be able to carry the weight of the deck, the soil under it must be solid, or compacted, enough so that it isn’t going to sink or shift. Because the lot is extremely steep, there was easily 6 feet of fill dirt below where the footers needed to go. Footers must be as wide as they are deep, so if you had to dig down 6 feet to reach stable soil, the footers would be 6’ x 6’. That takes a lot of concrete to fill holes that size. Not to mention the fact that soil even that far down may not be stable. Helical piers to the rescue!
What are helical piers and how do they work?
Helical piers are essentially steel shafts with a helix plate at the end that resembles a large screw. The helical pier is rotated into the ground using a hydraulic head on a large piece of construction machinery, in our case, a Bobcat mini-excavator. The pier is screwed into the ground until it reaches the correct depth and torque rating, usually specified by an engineer. A plate is then attached to the top, which will act as a support for the footer. Helicals are pretty fast to install and footings can be poured over them the same day.
However, sometimes the job site has challenges that can make what should be an easy job more difficult. For this job, the area where we were installing helicals had more than just soil. There was a ton of construction debris dumped at the bottom of the fill dirt. We hit a huge slab of concrete on the first helical we attempted to drill. The slab was about five feet below the surface and about 8-10 inches thick. We had to dig a larger hole to access the concrete, and then chip away at it with drill bits and a jackhammer until we could get through it. We found more concrete blocks, slabs, wood, and even a huge tree stump under that soil.
So what did that mean for the job? We had more time involved than expected. What should have been a day-and-a-half job turned into a 3-day job. That meant more labor costs for the homeowner. Debris dumping on the job site is a common practice in the construction industry, but it can cause major issues if the waste is dumped under an area that needs support. In this case, it no doubt contributed to the sinking soil under the deck. Wood of any kind will eventually rot, so if there is wood under the ground, once it rots it will leave empty spaces that will settle and compact. If there is any type of structure on that ground, it will drop along with the soil. As with all foundation or structural issues, addressing the issue as soon as you notice it is key. In this case, the deck would have continued to sink and eventually would have pulled away from the house, creating a dangerous situation.
We were able to get all of the helicals in place in the footers and they will be filled with concrete. We will return to the job site to drive 5 more helicals to support footers for a new hot tub. We will update this article with photos of the job once it’s finished.