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?


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

Tuesday, November 10, 2009

The Service Company You Choose Matters

by C. Sherman Henes and Cassi Henes

You care about the environment. So should your service company.

You’ve made changes in your life—whether it’s recycling, composting, or carrying eco-friendly bags to the grocery store. You’re doing your part—shouldn’t your service company be doing it’s part, too?

In the office:
We believe in taking care of our precious natural resources. That’s why we’re committing to sending electronic documents whenever possible—including this email. It’s reduced the waste we send to landfills by over 1 case of paper a month.

When we do send out paper documents, rest assured that we’ve used recycled paper and low-impact, high efficiency solid ink—never hazardous, powdered toners. This reduces the need for harsh bleaching when our paper is recycled.

We’ve even created an e-book, rather than a printed book, called Foundation Technology. We’ve saved 96 trees so far this year and kept over 740 different chemicals out of the groundwater supply.

The CDs on which we distribute our ebook is laser-engraved, cutting out the need for harsh chemical labeling and the use of adhesives.

On the job:
We recycle both our steel and our concrete, which reduces the amount of long-term waste present in our landfills.

We buy our rock and roadbase from recycling facilities.

We use washable uniforms and rugs, rather than disposable dropcloths to protect your home.

Our crew members work four 10 hour days, rather than a 5 day work week, to insure that less fuel is consumed moving our crews and longer time is spent on a jobsite.

All of our products are shipped from within the US, meaning less fossil fuels are consumed transporting equipment and supplies over shorter distances.

It’s our responsibility to color the world around us.
We choose the color green.

Wednesday, November 4, 2009

How Do I Know if I Have a Foundation Problem?

Dear My Foundation Guy,

My home is on the market and the buyers are requesting that I have someone evaluate the foundation for possible structural damage. How do I know if there's structural damage or if the cracks in my foundation are no big deal?

~excerpt from email inquiry from seller courtesy of Jack McGuire, Peak Structural

Cracking in concrete is a natural phenomenon because it is strong in compression, but very weak in tension. When a concrete foundation wall encounters stress forces from outside elements like expansive soil, the portions of the wall that are experiencing tension will crack. Some of the forces that can act against your foundation on a regular basis include: seasonal temperature changes, seasonal soil swelling and shrinking due to weather and watering, and changes to grading due to landscaping or soil erosion over time. Occasionally, the addition or removal of plants, shrubs, or trees can also affect your concrete foundation.

Most residential foundation walls are designed to withstand the forces present in standard/normal soil conditions. In some cases, foundation walls have been reinforced, or otherwise altered due to high ground water or soil concerns. You will usually see cracking occur in three ways: vertically, horizontally, or diagonally from a stress concentration.

Just because a wall has cracked doesn’t mean that it has failed as long as the crack is small and unchanging. If the crack is 1/8 inch or less, is nearly vertical, has no lateral separation (spreading), is not very deep, and does not have moisture present, then it should be monitored, but action is not usually required. This type of crack is a shrinkage crack and occurs as moisture in the wall evaporates, causing the concrete to shrink into voids created by the escaping water.

Horizontal cracks require more attention. A horizontal crack is concrete’s way of ripping or splitting due to excess tension in that area. According to Residential Concrete Magazine, all horizontal cracking should be checked out by foundation repair industry professional as soon as possible because these cracks tend to grow rapidly.

You may notice cracks spreading out from a corner diagonally. Whenever concrete forms a sharp angle, there is a stress concentration that almost always results in tiny, surface crack. These cracks are called “reentrant cracks”. If these cracks deepen, thicken, or grow, it is time to consult an expert about reinforcing the area since the stress concentration is breaking down the concrete.
There is one type of crack that is always a concern: tapered cracks. If a crack is larger at one end, it indicates shifting and other movement in the foundation . If you’d like more information about tapered cracks, please visit our website by clicking here.

The easiest way to figure out if the cracks are real issues or natural cracking is to have a Structural Evaluator take a look at the property. Virtually every reputable foundation repair company in Denver will offer you a FREE estimate. Once the evaluator determines if there is damage or not, you will know whether or not to get an engineer involved. Using the free estimate to determine damage will save you money in the long term.
~Reply from Aaron Henes, General Manager, Peak Structural located in Lakewood, CO 80228