Wednesday, January 30, 2013

The Truth and Rumors behind Drilled/Bell Bottomed Piers

        There is a war in the foundation repair industry that has been waged for as long as they have been buildings have been built in Texas.  The basics of the war is this: Bell Bottomed/Drilled Piers vs. Pressed Pilings.  I would first like to state that as a contractor that installs every type of pier, I believe Pressed Pilings, Drilled Style Piers and Helical Piers are all great piers.  As I've said in every one of my posts the pier that is most appropriate for that home will depend on a number of variables including soil type and condition of your home.  What I have tried to do with all of my posts is point out that both all pier types are great when installed properly, and when the application is appropriate. My blogs are never to sway you towards one type of pier, but to inform you in order to help make the right decision with your home.

First, let's get into the basics....


Drilled and Bell Bottomed Piers fall under the same category.  They are poured piers.  First, a small box is dug next to the home in order to get the drill bit under the foundation.  Then a hole is drilled next to a home.  The hole is drilled until stable soil is reached.  In the case of bell bottomed piers, an attachment that is then put onto the drill in order to bell the pier shaft.  Once the hole has been dug, concrete is poured into the hole.  After the piers are drilled and poured, time has to pass in order for the concrete to set or dry.  Once the concrete is dried appropriately, caps are placed on top of the piers and the home is lifted.

Proper Methods:
a) Machinery.

There are many different devices in which to drill the holes.  The most common include: 1) Hand Held Augers 2) Stand Beside Augers 3) Attachments to front end loaders (Bobcat type machines).   The differences between these would be achievable depth.  The bigger the engine behind it, the greater depth can be achieved.  Really, the front end loader is the best route to go for installing these piers, however it's not always the best method possible with every home.  In order to use these, you need enough room for them to fit beside your home, and a little patience as they will tear up your yard a little bit.  All of these methods are acceptable in terms of residential foundation repair.  But like I said, the bigger the machine behind it, the bigger the pier you can drill.

b) Steel Reinforcement - In order to have lasting power, these piers need steel reinforcement.  Most commonly, rebar cages are built and set in place before the concrete is poured.  Size and amount of rebar varies, depending on the size of the hole.  A rebar cage is basically what it sounds like.  They are square or triangular in shape, and should be tall enough to fill the depth of the pier.  In special cases (for example, if these are pre-build piers) the rebar will actually extend out of the pier shaft so they can be tied in to the foundation.  This method is not possible with repair, only pre-build. Rebar adds tensile strength to to the pier column.

c) Concrete Type - Structural grade concrete (3,000 P.S.I. or better) should be used without exception. Concrete should also be vibrated after the pour to remove any air pockets, therefore making it stronger.

d) Diameter of Pier - This is variable.  Most homes (single story/two story) require a minimum of an 6" shaft.  8", 12" and anywhere in between or greater are also available.  The diameter of the pier should be sufficient to hold the load of your home, and larger homes require a larger diameter pier.  Larger diameter piers also create more skin friction on the pier shaft as they have greater surface area in contact with the soil.  This factor is important, because more friction = more stability long term.  More stability = a lesser chance that these piers will fail.

Benefits for you:
Less Stressful on the Home:  The biggest benefit with drilled piers is that they do not stress the home during instillation,  as no contact with the home is required.  This means that the only stress placed on the home is during the lifting process.
Depth is observable:  As opposed to piers being in the ground, depth of these piers can be seen and measured.

Drawbacks for you:
 Repair Process is more lengthy:  As these piers have to be fabricated on site, and concrete has to have time to dry and harden, the process for drilled piers can take up to a week or longer.
Heavier Machinery to install:  The biggest drawback for this can mean that their is a bigger mess to clean up after the work is done.  Especially if internal piers are needed. 
Internal Piers:  In order to install internal piers with this method, the machinery will need to be inside your home, a large amount of dirt will need to be removed and discarded from inside the home, and wet concrete will need to be poured inside your home.  Also, the large equipment (stand besides, front end loader attachments) will not be able to go inside the home.  This limits the machinery you can work with to hand held augers.
More Expensive:  Because these are not pre-fabricated prior to being on site, they do require a larger amount of labor to install.  They also require some testing.  Testing on site usually requires drilling a "test hole" in order to determine how deep stable clay actually is.  They also require hauling a large amount of soil away from the home.  Pressed pilings are pressed into the ground, where with drilled piers you are removing the soil and filling it with concrete.  I can tell you, as an installer of these piers, that most of the other differences are greatly over-exaggerated.

 Here is a great breakdown of the differences. Please note that reliable contractors offer most of the services on this breakdown as part of their pressed piling method as well.
Dawson foundation repair's cost analysis.

Problems with instillation:
Ah yes, the part no one talks about.  Despite what most people say about drilled piers, there can be huge drawbacks in instillation, and all of these will effect the cost.  Some of these can be accounted for during the testing phase, but not all of them.  The problems that can pop up are:
  • Groundwater
  • Soil Collapsing into the hole
 Groundwater filling up the pier shaft can cause huge problems long term.  Constant water penetration can deteriorate these piers over time.  If this is run into during the time of instillation, water should be pumped out of the holes, the piers should be "cased" - lined with a water proofing material - before the concrete is placed in the hole.

If the concrete is not poured immediately after the piers are drilled, problems with soil collapsing around the pier shaft can arise.  The soil can collapse to where the bell of the pier is no longer existing, the piers lose their dept, and they are no longer shaped correctly.  If weather does not permit pouring to take place immediately after drilling, you can in turn have to pump, and then redrill all of the shafts.

One last note I would like to make is that companies that specialize in drilled piers will market that "pressed pilings are not straight". Drilled piers aren't straight either.   In order to install a drilled pier, you have to drill at an angle.  There is an acceptable, allowable angle to these (usually 12 degrees off the vertical). 

The process of getting this drill bit in the ground, and getting it another 12 feet in the ground means one of two things: a) the piers are installed at an angle, and the load of the home is distributed in an angled direction in the ground.  b) the piers are drilled vertically, and the load of your home is distributed on a "notch" on top of the pier.  Both of these forms are acceptable, and engineer approved.  I just find it funny that the marketing behind drilled piers forgets to mention that their piers aren't straight either.

Why would a company that installs both piers, need to use a more expensive drilled pier over a pressed piling?  Here are a few reasons.
  1. When soil conditions aren't conducive to pressing a pier.  Some soils are soo stiff that pressed pilings will only go 3-4 feet in the ground.  If the active zones of these soils extend beyond that depth, a drilled pier or a narrower steel piling would be necessary in order to achieve proper depths.
  2. Your foundation won't support a pressing method.  If the concrete in the foundation has deteriorated to a certain point pressing a pier off of it would actually create more damage to the foundation.  Pressing a piling in this situation, and then trying to lift it can be like trying to lift a stack of books and very hard to do. It could actually break the foundation trying to repair it.
When are drilled piers not recommended?
  1. When the home is built on fill dirt that exceeds the depth of the pier.
  2. When the soil has an active zone that exceeds the depth of the pier.
  3. When access around the home limits the equipment that can be used for instillation.
  4. When large amounts of groundwater or known water tables are in the area.
I want to reinforce that I'm a huge fan of drilled piers.  They are effective, they are proven, and they are a tremendous form of foundation repair.  However, they aren't the only great pier on the market.  Every type of foundation repair is effective, it just all comes down to application.  The question always goes back to these two concepts: 1) Does this style of pier fit the needs of your home? 2) Does this style of pier fit your needs/expectations?  If you have a question over the different methods, or are confused over which style of pier is best for your home, give us a call.  We'll be happy to help.

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Brian Gilchriest
Sales Manager-Houston
Office: (877) 940-9993

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  1. Thanks for the interesting article! Question, what happens to the pre drill pilings? Are they used again somehow or are they trashed? Thanks.

  2. Do you know of any one who could help me pre drill pilings. THat would be really helpful.

  3. Jake you could go and try out because they know a lot about pre drill pilings. So they might be worth checking out.

  4. Nothing was said about not being able to get all of the dirt out of bell, or that the drilled piers not being deep enough, or that drill piers do not hold in sand...

    one must ask yourself, why do most companies use press pilings?

  5. I plan to build build on piers but I just dont know how deep to go . The property is new caney tx . Its in the back woods and I plan to do it myself with rental equipment.