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Foundation Engineering,  

Concrete Press Piles Foundation Repair: Building Code non-compliance Part 1

c) Why this type of foundation repair doesn’t meet the code

             c.1) No Connection between Cylinders

 As we have seen, the first requirement of the code is that the piles or piers that go into expansive soil need to be able to resist uplift. From the description above, almost all of the foundation repair systems which use concrete press piles do not connect each of the cylinders to each other. This is a major problem, because uplift in the piers is caused through surface area, and the rate at which there is uplift varies with depth. Therefore, as the soil pushes up at different rates through out the depth of the active zone, each of the pieces pushes up at different rates and start to separate from each other because there is nothing anchoring or holding them together. The result is that the piles push up on the existing foundation creating damage to the finishes and possibly the structure. This type of foundation repair is by no mean a reliable foundation repair. Not only it doesn’t meet the code, but it just doesn’t work unless you want a temporary solution.

                         c.2) Poor Connection between Cylinders

 There are some companies that run a cable through the middle of the cylinders because they realize the problem with meeting the code and resisting uplift, and trying to make a pile that actually is a viable foundation repair system. However, although the system has good intentions, it’s efficacy it’s questionable. There’s the very wrong assumption that when these concrete cylinders are pushed into the ground that they are pushed perfectly vertically straight; we know this is not true because all other types of piers or excavations done into the ground are never perfectly vertically straight. When pieces of concrete are pushed into the ground, they will tend to go sideway in one direction or the other. Also, soil is not homogenous through out the depth and so as the cylinders are pushed the cylinder will just slide to the pocket of soil that provides least resistance (i.e. it will follow the path of least resistance). In addition, the force applied against the top of each cylinder has some eccentricity (that is, the load on the pile is not perfectly and exactly centered on the cylinder), so that eccentric force will tend to tilt the cylinder as it is pushed into the ground.

Therefore, the cylinders are slightly or completely tilted, and each of the cylinders will not be complete and fully on top of each other, allowing for the uplift on the pile to be more than the uplift exerted on a monolithic pier system because the soil will not only push up on the pile through skin friction (as it usually is the case for piers) but also push up at the bottom of each cylinder. But there’s even a worst reason why a concrete press pile, regardless if it has a cable or not, is a poor foundation repair choice, and the reason is the depth of the pile is not deep enough.   

By the way, if you’re inquisitive enough and feel like doing a homemade experiment, you can repeat the procedure on a very small scale using toy cylinders and pushing them with your finger against the soil in a vase. Make sure each of the cylinders have the same dimensions and you know the exact depth of the soil in the vase (ideally, you want to put enough soil so that the depth of the soil is equal to a whole number of cylinders). The vase would need to be big and deep enough and the cylinders small enough so that you can recreate at a smaller scale the same conditions (if the vase is too narrow or not deep enough, or if the cylinders are not proportional to the vase size you have, then the vase might break). If you know the depth of the soil, then you know how many cylinders are needed to reach the bottom of the vase (the “rock” stratum”). As you start pushing down on those cylinders you will realize one of the two (depending on the type of soil and strength at which you push, and the size of the vase): a) either the amount of cylinders you push is more than you expected (which will prove that the cylinders were not driven straight into the ground), or b) the amount of cylinders is less than you expected (which means you didn’t push hard enough into the ground).

                        c.3) Depth of pile not deep enough

The depth of the pile or pier or any type of deep foundation system is probably the most important factor for a foundation system. You may have the best type of pier, a fully reinforced cast in place concrete pier, which is the pier of choice in the commercial construction (extremely reliable), but if this pier is not deep enough, it is just as useless of a foundation repair system (or new foundation system) as a concrete press pile foundation repair system.

As we said previously, the active zone is about 15ft from the surface of the ground (the active zone is a little different in San Antonio, Austin, Dallas, Houston, etc.). This is the depth at which the soil will swell and shrink with fluctuation in moisture content from changes in the weather. So, the active zone (those first roughly 15ft of soils from the surface) will expand when the soil is wetter and shrink when the soil is drier. Here are different scenarios based on depth of the pile or pier IF it is fully connected:

  • Bottom of the pile is within the active zone (less than about 15ft depth): When the soil shrinks, the pile settles (and everything else on top of it, your walls, slabs, roof, etc.). When the soil expands, the pile pushes up (and pushes everything up, you walls, roof, etc.).
  • Bottom of the Pile is AT or just below the active zone: When the soil shrinks, the piles don’t settle (it’s never zero, but close to it). When the soil expands, the piles push (cracking your walls, ceiling, etc.).
  • Bottom of the Pile is sufficiently below the active zone: When the soil shrinks or expands, there is not movement (it’s never zero, but close to it).

So, even if you would, somehow, be able to connect all the cylinders together, you still have the problem that the pile depth is not deep enough. Many times, even with a system that attempts to make all the cylinders act as one (which still doesn’t work too well, as explained before), the pile depth is not deep enough, in fact, it will usually stop within the active zone, which means the pile will move and the foundation repair system is, hence, temporary.

By the way, if you live in San Antonio or Dallas, most of those towns will not have rock stratum for many feet (some areas at 80ft no rock was found). In Austin most of the city has rock no deeper than 20ft, with a few areas deeper. No Rock is usually found in Houston.

 

Next Page: Foundation repair concrete press pile Building code non-compliance Part 1: d) Conclusion: Don't use concrete press piles foundation repair


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