Wednesday, November 25, 2009

How Do You Know How Deep to Push a Push Pier?

by C. Sherman Henes and Cassi Henes

Gwen V. from Aurora, CO writes:

Dear Foundation Guy,

How do you know how deep to push a push pier? What is this "load-bearing strata" you keep talking about? How do you know where it is?

Thanks,
Gwen

Gwen, those are great questions! Many of our clients have asked us the very same things.
First, I want to answer your question about "load bearing strata". Load bearing strata is a technical term for soil that can bear significant weight without compressing, or shifting. It is commonly referred to as "bedrock", "claystone", or "sandstone". These soils all have the characteristic that they do not sink, shift, settle, or compress. They are called "non-active", which means that they are in an inert state and don't suffer from the expansion and contraction like the soil directly below your foundation. A good example of an active soil would be the silty sand, or clay that your home probably rests on given the soil conditions in Aurora. Aurora's load bearing strats is generally claystone. Push Piers are hydraulically driven through the soil under your foundation until they reach bedrock, or other load bearing strata. The home is then lifted off the active soils and the weight of the home is transferred onto the piers and in turn, onto the load bearing strata. See the picture below:




Without a test boring or at least a general knowledge of the anticipated local soil conditions, estimating pier depth may be a shot in the dark. However, proper installation procedures can identify a consistent pier depth and suitable load bearing strata.

In the case of the FSI push pier that we install, one pier is driven at a time, using the maximum weight of the house and the soil around it as counter weight. Hydraulic pressure readings are monitored and recorded as each pier is installed. A simple mathematical equation is used to correlate hydraulic pressure to capacity. An engineer does these calculations based on the soil conditions of property, the specific home's weight, and other environmental factors. The pier is load-tested as it is installed. After all the piers are driven individually, lifting devices are attached to all the piers in a series to allow the entire structure to be lifted at the same time. When pressure is applied to all the piers at the same time, it take much less pressure to lift the structure than it took to drive the piers individually.
Here are some step by step photos of the installation process:
1. The footing is notched out and prepped, so the bracket attaches to the house firmly and the piers sits under the foundation wall.







2. The bracket is put in place and the external sleeve is driven into the ground using protractors to insure the pier will be driven in straight.
3. The pier sections are also driven into the ground under proper resistance is reached.
4. The home is lifted using hydraulic pressure on all piers.
For example, an average pier is load tested to approximately 38,000 lbs. If an average home puts 3000 lbs of weight per linear foot on the footing and the piers are spaced 6 feet apart, then each pier is asked to hold 18,000 lbs. This means that each pier only has to hold less than half the weight that it would be capable of bearing. In engineering terms, this gives us a factor of safety of 2.1.
Thanks for the great questions! Keep them coming!
Your Foundation Guy














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