Sunday, October 23, 2011

The Truth and Rumors behind Pressed Pilings.

I hear and read a lot of comments about pressed pilings.  In my opinion, they have a bad name and there is a lot of false information out there about them.  Poor instillation techniques, unskilled people installing them, and excellent marketing by people who don't use them all lead the bad name that they've taken on.  In my opinion, they are a highly effective repair method.  I see some of the top engineers recommend this system every day.  It is my goal to clear the cloud around them, and address both their strengths and weaknesses.

 Working for a company that uses every type of piering system available, I need to know both the strengths and weaknesses of all them.  We install drilled piers, we install bell bottomed piers, we install multiple types of both pressed concrete and pressed steel, and we install helical piers as well.  My job as an evaluator, is to make sure that I have all of the data available to make sure that a pressed piling system is right for your home.  To be honest, if it qualifies for a pressed piling system I generally have all of these piering systems available at my disposal. From that point it really just depends on how much you want to spend and what you feel the most comfortable with. 

These are the statements about pressed pilings that I hear/read most often:

#1) Cylinders tilt, rotate, and are misaligned during the instillation process.
#2) Because they are not secured together, they can move horizontally or laterally.
#3) Because you use the home as a driving force, there is no way to know how deep you need to go, and can not tell if you reach stable soil.  If you try to drive past the refusal point, then you can put too much pressure on the slab and and break it.
#4) Piling depth is limited by the weight of your home.
#5) There is no significant safety factor for these pilings (this will be explained further in a later segment.)
#6) Pressed pilings installed during a drought may shrink (or settle) when the rainy season hits.

Now, of the above listed statements against pressed pilings, I would like to offer a break down of each.  Some of these are absolutely true, some of them are sort of true, and some of them are simply marketing ploys.

#1) "Cylinders tilt, rotate and are misaligned during the instillation process."  - When you press a pier into soil, the dynamics of that soil act very much like the dynamics of a fluid.  Since soil is not truly a solid, and also not a fluid, but a combination of the two soil mechanics have both properties of solids and fluids.  One of the properties that is shared between fluids and soils is when pressure is put into a system, it is equally distributed to all sides of the object putting pressure on it.  In this case, the pier.  If pressure is equally distributed to all sides, how can they be misaligned?  I hear tree roots, rocks, and boulders will cause them to rotate.  While this may be true in some areas, native Houston geology (with the exception of the Willis and Lissie sandstone formations on the far reaches of the greater Houston Area) has anywhere from 150-600 ft of clay deposits between the soil horizon and the nearest rock layer.  You are pressing into clay, and leaning of soil friction, and soil pressure to encase them and hold them in place.

#2) "Because they are not secured together, they can move horizontally or laterally."  - My question is, how are they going to move laterally?  Where is this soil moving laterally?  In order for soil to move laterally it needs a place to go.  If you are in an area with a large amount of downward slope away from the house, this can be true, if you are in a flat landscape there is no place for it to go.  Soils move in a vertical plane.  They move up and down, not left and right (unless you are on a fault line and even then a bell bottom pier is absolutely no help (:)

#3)  " Because you use the home as a driving force, there is no way to know how deep you need to go, and can not tell if you reach stable soil.  If you try to drive past the refusal point, then you can put too much pressure on the slab and and break it." - If they are using your home as a driving force, they definitely don't know how to install this type of pier.  Your home is not part of the driving force.  It is the area from which they are driven off of.  Those are two completely different things.  In order to exert force in one direction, you need an object to keep it from moving the opposite direction (I believe we all know what Mr. Newton had to say about this.).  Your home is the "normal force" in the equation for a certain point.  When the object used to drive the pier downward exerts that downward force, your home exerts force on the object driving it into the ground keeping it in place and allowing downward movement.  There will be a point however, when friction builds up on the pier, and the pier becomes harder to drive, that the downward force will begin to be greater than the weight of your house.  This is when you should stop driving the pier.  If the house is coming up, it's time to either make it easier to drive the pier (water infused into the hole) or stop driving the pier.  Now as far as how deep you need to go to be into stable soil is a great point.  There is no way to prove that you are in it.  If you are only drive 3-8 feet, you probably aren't in it.  This is why it's important to use a machine that is capable of reaching significant depths on a structure that has the ability to let you drive that deep.  If neither of those factors are there, don't use a pressed piling system.  Also, the final part of this statement drives me insane.  The point of refusal is a relative term.  You can't drive past the point of refusal, that's why it's called the point of refusal.  If you drive to it, that's as deep as you can go. Refusal is defined "as the point when the structure is lifted 1/4 inch to 1/2 inch above its elevation at the location of the pile during the driving process. The Pile is said to reach refusal and the driving operation is stopped before the occurrence of significant vertical movement of the building that may cause damage to the superstructure." (FPA-SC-08-0) 

#4) "Piling depth is limited by the weight of your home." - This is partially true.  What determines soil depth is really three things, they type of soil you are driving it into, what you are driving with, how much your house weighs.  As stated in the above line, when the frictional forces on the pier are greater than the weight of your home, the house will begin moving upward.  So house weight does come into play, however it's not the only thing in play.  If you are driving with a 12,000 P.S.I. hydraulic ram, you can reach greater depths than you can reach with a 10 ton bottle nose jack (which I have seen companies use to press into the ground first hand).  WHAT you are pressing with is extremely important, and actually more important for depth than what you are pressing off of.

#5) "There is no significant safety factor for these pilings"- Absolutely true. What does this mean?  It basically says that since when the point of refusal is reached, forces on the pier are balanced, or zero'd out.  That gives it a safety factor of 1:1.  If things change, and weight is added to the structure, it can effect the piers and cause them to settle.  This does not mean that if you put a grand piano in your dining room the piers will sink.  It does however mean that if you put another story on top of your house, or change your roof shingles to a heavy composite type they can.  If you are planning on doing either of these things as after your pier work is done, you should probably use a bell bottom.  This is something that should be considered in the repair process

#6)."Pressed pilings installed during a drought may shrink (or settle) when the rainy season hits." - This can absolutely be true as well.  That's why drainage, proper grading, gutters and all other factors should considered and recommended when the piers are installed.  Once the piers are installed, proper maintenance and drainage should be in place to ensure that they don't fail.  However, it is also true that improper depth and  improper spacing, can cause piers to heave as the soils swell.  Pier heave is a problem that I see fairly often in the field.  When Piers heave, they can be very difficult to fix.  Almost all companies don't warrant this type of movement.

So, how should they be installed? I think my friends at the Foundation Performance Association say it best with their guidelines outlying Segmented Pile Instillation.:

Design, Manufacture and Instillation Guides of Precast Segmented Concrete Piles for Foundation Underpinning

Note how this diagrams not only how they should be designed, but all the "ins and outs" of how they should be installed.  Be leery of companies that don't have use proper methods, or mention things like engineering reports or plumbing tests in their cost proposal.  Make sure they are pulling permits, using proper materials, pushing to proper depths etc.

Also, beware of estimates that are astronomically low.  As I tell customers all the time, I can level a house for that cost, but I can't FIX a house for that price.  Astronomically low cost estimates usually mean that within 5 years you'll have to pay to level your house again, or if the piers heave that you have a situation that's very difficult to ever fix.

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