Posted by Jonathan Maguire on Sat, Jul 31, 2010
Failing foundations can happen for a variety of reasons. We have discussed several issues in previous posts and will continue to do so in future posts. But when you have a failure, what should you do? One historically bad idea is to bring in bags or trucks of concrete to pour under the existing home. The thought behind this is – Concrete is a solid base, it is used for footings, it spreads the load out, and it will fill all the voids. And while these are all true of concrete, the one aspect you do not see listed is the weight of concrete. Concrete weighs around 145 lbs./cubic ft, although depending on the mix it could be more or less. If you multiply by 27 you will get the weight by cubic yard (which is a standard for concrete trucks) = 3915 lbs/cubic yard. (as a side note: soil can weigh between 60-120 lbs/cubic foot).
We will use an example of a corner of a home that is showing settlement on a 10×10 corner of the house. If a contractor decided to pour concrete under this corner, he may want to pour it one foot thick and may go 2-3 feet into the existing home. If that’s the case then he would have poured 40-60 cubic ft of concrete (or 2-3 yards). This equates to adding between 7,830 and 11,745 lbs to the corner of your home. Stay with me – because an average single story brick house on a slab weighs around 1,141 lbs per foot (or for this 10×10 area – 21,679 lbs). Basically, the contractor just increased the weight of your home by 50% – and this is using conservative numbers of what the contractor poured, and a very modest home (a 2 story brick house on a basement would have a weight of 3,656 lbs per foot). We have heard stories and seen jobs where the amount of concrete poured under a corner of house is 6-7 yards or around 25,000 lbs.
All of this to say that if your home is settling due to poorly compacted soils, loose soils, organic debris, builder debris, water issues, etc then the soil does not have the adequate bearing capacity to support your home. If the soil was not good enough to support the existing structure, why would anyone add a 10-30,000 lb anchor to an already sinking foundation? Once more, what do you think this will do when you have to repair the house after the concrete is poured? This repair – which would have cost a few thousand dollars to begin with, will ultimately cost several thousand dollars – not to mention you may not be able to lift a structure once you have added all the additional weight of concrete.
Before you decide to go with a contractor that wants to pour concrete under a corner of your home, or if you are thinking of pouring concrete under your home, do some research about helical piers and resistance piers. If you are a homeowner, then your largest investment depends on how you decide to repair a sinking foundation.
Background: Helical and Resistance Piers are considered deep foundations, which mean they both penetrate areas of poor soils to reach soils of adequate bearing capacity. They do not add weight to your structure, and require little disturbance around your home to install. They last for years and years and typically come with long warranties to protect against any future issues. Atlas Piers provides services in repairing foundations. We lift and stabilize houses all across Atlanta and the State of Georgia. We have been in business for over 20 years and have seen other companies come and go. If we can’t lift a house – it can’t be lifted. Visit our home page or call us @ 770.740.0400 for more information.
Posted by Jonathan Maguire on Wed, Jul 21, 2010
Garage Column failure is a very common issue around the Atlanta metro area. In fact, Garage Column settlement is one of the top 3 issues we see in foundation failures. So what is it and how do you fix it?
Garage Columns are the center columns between two or more garage doors into a garage. The Column is generally a structural element that carries either a brick load, beam load or header load. What this means is that the column generally carries a concentrated load where other parts of the home carry a line load across a continuous footing (think of the house over the garage door openings, instead of sitting on the ground they area passed to either side of the opening).
When many houses were built, the soils for the garage were moved into the area (especially when there is a basement) and then compacted. In many cases, the builder or grader failed to compact the soils enough to carry a concentrated load. These poorly compacted soils can cause the garage column to settle over time – if you have a brick house then you will have a crack above the garage column as this happens. As a result of the failure, you may also notice sheet rock cracks in any rooms above the garage. You may see a crack on either side of the garage floor where the floor slopes towards the middle.
The fix for this failure is typically to install 1-2 helical piers. In years past, when helical piers were not used as much, we would install resistance or push piers. It was a dangerous process, that put unnecessary stress on the column and resulted in many companies having warranty issues. Overall, it’s not a good idea to install a push pier under a garage column.
The typical installation for garage columns is to remove the driveway slab around the column, install 1 helical pier on either side of the column and then lift the column back in to place. Many of the cracks in the brick veneer will close, but you may still need to repoint (fill in) the mortar for a better look.
Some of the unknowns in repairing garage columns are 1) footing or grade beam under the garage slab, 2) depth of the helical piers, 3) lift results. Give us a call to determine what solution is best for you.
Posted by Jonathan Maguire on Tue, May 11, 2010
In part one of this discussion, we talked about what a helical flight is. This post will hopefully add some insight into how torque equates to loading in helical piers. We will also discuss one of the HUGE blunders of a local manufacturer.
Helical piers are screwed in by a hydraulic gear motor. This motor generates a hydraulic pressure on the gear that equates to a torque (force) on the pier being installed. This torque is then multiplied by a soil factor (when soil testing is done there is an actual number defined for the soil and for the product, when soil testing is not performed a generic ‘average’ is used). For the purposes of this post lets use the following equation (used from a technical manual written by Don Clayton, PE) -
Ultimate Capacity = Torque x “k”
where “k” is equal to the soil multiplication factor.
This is a simple equation, and to give an example for a light duty pier – we generally install our helical piers to 4,500 ft/lbs and use a standard average soil factor of 8.
Capacity = 4,500 x 8 = 36,000lbs ultimate capacity
since the ultimate capacity on a single flight is between 40,000-50,000 lbs (steel and weld capacity) – a contractor could use a single flight configuration for this loading.
Now for the HUGE blunder by a local manufacturer. Let’s review some details of helical piers:
- Torque is one of the main factors of the ultimate capacity of piers
- Helical pier types can handle different torques (thicker pipe = stronger, etc)
- Flights have an ultimate capacity of 40,000 lbs for the flights – but the pier torque is what generates the ultimate capacity.
I have run across the situation where a standard residential pier (.203 wall) is sold as a high capacity pier (60,000-70,000 lb ultimate capacity) by the salesman telling the contractor that he’ll just add another flight to the pier. If you double the flights you do add additional capacity, but the problem is – You simply cannot install a standard residential pier to the torque required to generate 60,000-70,000 lbs. The pier will fail during installation – remember the equation
Capacity = Torque x “k” where “k” is the soil factor.
60,000 = Torque x 8
60,000/8 = Torque = 7,500 ft/lbs
Typical residential piers can be installed to around 5,500 ft/lbs – max. You cannot get there from here no matter how many flights you add to the pier shaft (in fact you will notice the equation doesn’t even factor in the number of flights). But I’ve seen this, and I’ve seen general contractors buy this lie from the local company “engineer”.
Posted by Jonathan Maguire on Thu, Apr 08, 2010
This post will help describe how a helical pier works. Helical piers are used for foundation repair projects as well as new construction projects in poor soils. A helical pier is a round or square metal shaft with flights or helixes. What is a flight? It’s a steel plate welded onto the main shaft of a helical anchor (see picture). These plates are generally 3/8-1/2″ in thickness and have a pitch of 3″. This pitch allows the helical anchor to advance into the soil 3″ per revolution or 4 revolutions per foot.
How many flights or helixes are needed? Great question and there is no definitive answer for all applications. On a standard residential project where the loading is 10-30,000 lbs, one-two plates will work (each plate can handle between 20-25,000 lbs/ea depending on the manufacturer). Adding flights can increase the surface area that is made with soil and multiple flights decrease the loading required on each flight. We have found that adding flights can also help us install the product better as you have two or more flights pulling a helical into a the ground instead of just one single.
In part two of this blog, I’ll give some insight into how adding flights helps, and what are the limiting factors of helical pier capacities.