A pile caps usefulness for a single pile falls under a similar category as your appendix. Experts don’t fully understand what its for and while it may seem to be useful, it is something we can apparently live without… or can we? So do you need a pile cap for a single pile foundation on your project?
Some seismically sensitive regions do not allow a single pile foundation. However in regions where this is allowed, there is no strict structural requirement to specify a pile cap. This is provided that the sizing of the column, the pile and the detailing of the connection allows tolerance to prevent clashing of reinforcement and adequate transition of the loading.
You may also find great additional support reading via THIS article. There I cover what pile foundations are, the different types of pile foundations and their benefits.
To determine if you need a pile cap on your single pile, we fist need to understand what a pile cap actually does…
What does a Pile cap Do?
A pile cap is a spanning structural element that transfers loading from a column or wall to one or more piles in a pile group foundation. The span to depth ratio of a pile cap is often proportioned such that the structural behaviour does not satisfy Euler-Bernoulli bending behaviour. Rather, the pile cap is often designed as a rationalised strut-and-tie system with its behaviour resembling that of a truss.
The following list outlines what a pile cap does:
- Allows tolerance: The tolerance requirements for a pile (in plan location, top of pile and pile inclination) is more than that of a column. The pile cap can provide a buffer to allow this increased tolerance to exist between the pile and column above.
- Facilitates transition of loading: If the column is constructed of high strength concrete and the pile is two grades lower in strength or more, the pile cap can add a transitionary zone for both load spread and a step down in concrete grade.
- Load Sharing: If the column is heavily loaded and requires several piles to support the load, the pile cap can effectively share the load between several piles through strut-and-tie action.
Lets take a look at a pile cap in its simplest spanning form; a pile cap above two piles in a single pile group. The image below illustrates the construction arrangement of a simple two pile system as well as the strut-and-tie rationalisation overlaid on top…
The strut-and-tie arrangement for two piles resembles that of a simple “A” frame or a single bay of a truss. For an in-depth look at how to design a truss from start to finish, refer to THIS link. Note that for a strut-and-tie analysis, there are additional checks you need to perform compared to a simple truss analysis, the main one being adequate checking of the nodes where compression struts and tension ties meet.
Solving the Issue of Tolerance for a single Pile with No Pile Cap.
One issue with taking away a pile cap is how to handle the issue of tolerance. Depending on the piling contractor, the top of pile cut off level may not be at the exact required location.
Also most piling codes around the world allow an out of location tolerance of around 75mm (3 inch) for pile construction. Both of these possibilities need to be taken into consideration when detailing your single pile with no pile cap.
Lets take a look first at the implications of an out of position pile. Say for example a pile is constructed out of position by the full allowable tolerance (75mm). The column location above the pile usually cannot change as this will effect room layouts and planning within the building. If the pile and the column are a similar size, this may mean you will have a clash of reinforcement occur, or worse, the column won’t fully overlap with the pile…
To prevent this from occurring, it is best to provide an oversized pile to allow this tolerance to exist. This is where it starts to become a question of cost and having a pile cap may actually be a cheaper alternative. Here are a couple of things to think about:
- Money and labour can be saved if the pile cap is removed, however if your single pile is 30m long (98.4 ft) or more, the money saved can be quickly offset by the additional concrete and reinforcement in a larger diameter pile at that length.
- If you are in good soil conditions (shallow rock) and your pile socket is only a couple of metres then a larger diameter pile at that shorter length may still be much more cost efficient than a smaller diameter pile with a pile cap included.
To summarise, your decision to include a pile cap or not for a single pile should be treated on a case by case basis. Column size, pile size and soil conditions should all be taken into account while making your decision.
Here is a schematic on what a larger diameter pile would look like constructed 75mm (3inch) out of position). Note that it is generally good practice to add a construction joint some distance below top of pile to allow accurate placemat of the column starter bar reinforcement. This allows the column reinforcement to be accurately placed, especially with the assistance of a concrete surface to chair the reinforcement from.
The introduction of the construction joint also allows the final top of pile location to be more accurately poured. It does however introduce an additional pour of concrete which may not be feasible for the construction schedule on your project.
Removal of this construction joint is possible if the column starter bar reinforcement is accurately placed before the pile concrete is poured. However the issue of top of pile cut off accuracy would still remain. Lets take a look at some further detailing to help allow for this tolerance.
The main issue occurs when the pile is poured too high. This may require your slab on grade to be reduced in thickness. Reducing your thickness locally however could encourage cracking. The alternative would be to jack-hammer the top of your pile back down to the required pile cut-off level. But this would defeat the purpose of trying to come up with an efficient detail to remove your pile cap!!
The best way to address the height cut-off tolerance is to specify the cut off to be around 300mm (11.8 inch) below top of slab. Assuming you have a 150–180mm (5.9–7.08 inch) thick slab this gives you 130–150mm of tolerance for your pile cut off. You simply pour some additional concrete thickness on your slab to make-up the difference (whatever the difference is depending on final pile cut-off height)
Transition of Loading for a Pile with no Pile Cap
The other purpose of a pile cap is to facilitate load transfer. A single piled cap may be required if your column is high strength concrete and your pile is comparatively low strength concrete. Load transfer is made harder if the pile is considerably larger in size compared to the column as well.
Here is a schematic elevation of how this arrangement might look like…
If you’ve completed all your checks and this transition zone is not required then from a load transfer perspective you may not need the pile cap afterall.
If you are going from a small column to a large pile, it is a good idea to sanity check the interface where the two surfaces meet. I would usually complete an un-confirmed bearing check at this face to make sure the stresses aren’t too high. In Australia, this is found in AS3600 Clause 12.6. In the United States, you would use ACI318-08 clause 10.4.1. Both equations are near identical with the main difference being that the Australian equation gives results in stress and the US equation gives the result in force.
So given the right project and right circumstances, it is indeed possible to detail a single pile without a pile cap. I have used the detailing outlined in this article a number of times on past projects. Most of the time it works on low rise buildings which lie above soil conditions that have quite competent shallow rock.
Have you used something similar on your projects? Let us know by leaving a reply below.
Have you ever wondered how Structural Engineers verify that the final constructed pile is strong enough to support the design loading? Take a look at THIS article which covers all the different types of pile testing.