We have heard many comments related to leaning piers, whether we installed them or they were done by others. In Texas we have some pretty active clay in the ground. This soil can have some pretty good movement to it, and when you put something in that band it moves right along with it, including piers. With that said, it is tough to accept criticism for building piers in this soil, particularly if it was an accepted risk and beyond your control.
On pretty much every job, we ask to see the soils report for the project. This report has been conducted, usually, by a third-party inspection group that has done bores into the soil and had conducted various tests on it. In these reports it may have mentioned the use of drilled piers and would have contained recommendations for the typed of piers for this situation and to what depth. Typically you will have three options, including 'belled' piers, straight shaft, and extended straight shaft piers. Each type of pier comes with pros and cons and different price points. What we have found is that the Owner/GC may not have taken the due diligence to read the appropriate sections regarding the recommendations. We at Hawk utilise an Engineer to design our piers to pass the associated building codes and for the type of soil contained in the soils report. Most of the time, like 99.99% of the time, our piers will be a design where we are deep enough to pass the building code, which may keep the pier in the so called 'active' zone of the clay. Here lies the rub, because our choice will pass the building code and is economical, that choice usually comes with the risks of heaving. We can explain this to the customer the best we can, but most of the time, because the other options are far more expensive, our standard pier outweighs the accepted risk. Our job is to clearly explain those risks and let the customer decide their tolerance of those risks.
The standard pier option is the straight shaft pier, built to the local building code. This means that depending on the area, the pier may or may not be in the so called 'active zone' of movement. This is the design you will get pretty much on all our jobs.
In the image above, you notice that pier is extends down below the 'active zone'. As we stated, this may or may not happen depending the area and the code by which the pier was designed. If the standard pier is in the 'active zone', this pier should be expected to move vertically; it may not happen but it is a risk nonetheless. If you had a soils report, this would be clearly stated in that document. Now, if Hawk designs a pier, you can expect the pier to probably be of this type, which is cheaper, passes the building code, but comes with risks the customer should know, if they read the document.
So what are the advantages and disadvantages of this pier? Quality control is pretty high with this pier. Pier gives good lateral resistance and good load capacity. Its real easy to inspect and has been used for over a hundred years. It is very to easy to build, typically. It does have a bad side to it, like it really needs to be built deep. Most reports do not provide for skin friction capacity values, which add to holding power. You have to move the pier spoils all the time. These piers may require casing due to sandy conditions and may require slurry during water intrusion. Soil has a tendency to slough off into the hole, making cleaning and clearing a problem. With water intrusion, inspection and pier integrity cannot be easily verified. Deep piers may require heavier and larger equipment, increasing your costs. Did I mention encountering water down there?
Another option in soils reports is to have these straight shaft piers extend down below the 'active zone' to some specified depth. This allegedly will help mitigate any vertical movement of the pier due to the soil because it is below any soil that may move it; simple. This type of pier will be more expensive the option above due to pier depth; no way around it. If your project cannot tolerate to have any movement of the soil, this option will be your best bet, including being cheaper than 'belled' piers in many cases. The advantages and disadvantages to this pier are the same as our standard pier above; its just deeper. The image above clearly shows how the pier is built below the 'active zone'.
The next option is the drilled and underreamed pier, otherwise known as the 'Belled' pier. This pier, if you imagine, is like an upside nail. The pier has a large belled end on it at the bottom, which is formed to be larger than the pier shaft. This design resists vertical movement of the pier and can literally minimize pier movement down to 1/2 inch in many cases.
These 'belled' piers should be extended to a depth below the 'active zone'. This depth should mitigate this vertical movement and prevent excessive depth of other straight shaft piers. To install this pier, it really involves 4-steps where a) drill the shaft, b) you start the underreaming c) build the steel cages, and d) pour your concrete. This pier serves its purpose but can come with serious difficulties.
Let's visit the pros and cons of this pier. Its been used for a long time; good there. It is real good with lateral resistance, like the others. The 'belled' portion of the pier provides a large bearing capacity; a plus. It is relatively easy build if no water or sand is encountered. The cons are many. Its a more complicated process to build it. You still have to remove the pier spoils somewhere. Soil sloughing can be a problem, especially if its occurring at the bell section. Like other piers, if water is encountered, you will have difficulty proving integrity and doing inspections. This is a specialised type of construction and thus reduces the amount of choices of who can do it. This is expensive, relatively speaking , so the cost benefit ratio should be considered. Will this pier prevent movement? It will come pretty close.
There is a bit of debate as to where your 'active zone' is located. It could vary with the area. It could depend on whether you consider the top of the soil as contributing or not. There so many variables, that just giving a depth of the' active zone' may not be completely accurate. Many measurements are based on conditions in the lab, or equations based on dry soils. These values are sometimes based on just assumptions. Is it helpful? It can be, but so can experience.
The biggest thing to remember, is that the Hawk standard build, is not wrong by any means. In the years this company has been in business, the number of pier failures is small compared to how many we have built. Most of the time, simple forensic analysis and site investigation will reveal that the complete pier was somehow interfered with. This interference can include, but not limited to, mechanical disturbance nearby, utilities nearby, irrigation, water to one side, bad water drainage, excavation that has taken place nearby. This list could continue.
What do we do then? We go forward. We will always ask for the soils report and when there is none, we ask our engineer to use experience and knowledge of the area. We need to explain risks of building in expansive soils and what it means for our product. If we clearly explain our limitations, clearly explain the soils report and give all the information to the customer, we have done our job. The customer , when presented with all the facts, can decide what to do at that point. They can accept the risks that are known. They can opt to build a 'belled' pier is movement is unacceptable. They may choose to just go to a deeper depth where it can be safely assumed that soil movement is heavily mitigated. The customer really needs to to tell us to level of comfort their budget exists at and the long term goals.
The next time you see something odd about a screen wall pier, remember that there usually is more to the problem then assuming it was built wrong. Like the counting 1 2 3 4 5 6 7 9 10. Folks will immediately zoom in on the fact that 8 is missing, without realising that 90% of it is correct.