Screw piers, Pile caps and Houses

Transcript:
G’day. This is Matt Cornell from Cornell Engineers.

Today I wanted to go into a little bit about screw piers which are steel poles with the helix on the bottom that are screwed into the ground instead of bored piers. They have a few uses in residential construction. Some of these aspects I don’t like so much.

Let’s go ahead and we’re going to have a look at this video that has been put up by Ideal Foundations. “How a screw pier is Installed” – just so you get a feel for what a screw pier actually is and thank you to the guys from Ideal Foundations.

So they start with a steel pole with a screw on the bottom. An excavator pushes it into the ground and the screw turns into the ground and you can see it’s displacing the ground a little bit – push a little bit of dirt out of the way. Not a lot of dirt but still as it drives into the ground or is driven into the ground it creates this layer of loose soil as it goes in.

A little bit of that soil getting moved away by the worker on site and quite correctly they’re going to say in a second here that there isn’t that much soil comes out of the ground. No spoil to remove from site. A little bit of dirt comes out of the ground. The ground was firm and stiff in this location we’d assume and it’s being driven as the screw goes in it’s going to create a layer of loose soil in around the round steel shaft that you see getting driven into the ground there.

In this case, they’ve done a two meter. They’re about to put an extension on, they lift the auger out of the way the head drive head out of the way, lower it back in again and drive it in for another two meter length and then, in a second, once they get this down all the way down another two meters so that that spiral, that auger right down the down the bottom it’s being driven into the ground at the moment and say it’s about three metres into the ground creating this layer of loose soil in around the pier. They will pull the head out of the way again and install the last one meter section. The design here apparently says a five meter screw pier is required so the last meter gets driven into the ground. So now we have a five meter long steel column that goes into the ground.

At the bottom of it has that auger, that bit of steel plate that’s at the bottom, that’s what’s going to stop it pulling up and that’s what’s going to resist any downwards loads that are on these piers.

So that’s what a screw pier looks like and that’s how it’s installed.

Thanks once again to the guys from Ideal Foundations.

Now I just want to go through what I like and what I don’t like about this screw pier system, some of the problems and how we’re solving some of these issues.

All right so that’s what a screw pier is and that’s how it gets installed. So in our sketch we’re going to show screw piers like this they start off with the steel post and a helix which in some cases actually two half helixes which are attached to the post but while I’m doing this demonstrate, for this the sketch, for these sketches I’m just going to show a screw pier like this. A little bit different from the one we just saw on the screen, but it’ll get the picture.

So, these get drilled into the ground and the reason we’re putting a screw pier into the ground is that the ground surface, the designer, the engineer designing the building whatever it is that is being supported by these screw piers is concerned that this ground isn’t strong enough at the surface level. We have to go down a certain amount of depth to achieve a good, strong foundation. So an engineer’s to be the one specifying how much load is on these screw piers and the screw pier manufacturer has a set of rules about how deep then the screw pier has to go and often these are based on how much force or torque it takes to screw the screw pier down into the ground, so that the stiffer the soil the harder it is going to be to screw the screw pier and when they work out how hard is to turn the screw pier and that is registered on the excavator on the machine on the digger that’s when they stop.

Alright so now we’ve got screw pier, we’ve got soil, we’ve got a depth that they need to go which is to achieve a certain torque so let’s talk about what sort of forces are put into screw piers.

As engineers we design screw piers generally for a vertical load either for dead loads and live loads we have a download and for wind loads so that’s dead and live and for wind loads we might have uplift loads or and we’ll come into this later on we might have sideways bracing loads that need to be resisted by the screw piers or a combination maybe the house is on stumps into the ground here’s the house the stumps the wind blows sideways on the house that load is being transferred down through the cross-bracing into a screw pier that’s been put in the ground by an installer but it’s not pure vertical there’s a vertical component but because of this sideways load there’s also a resistance the sideways resistance there’s being put into the screw pier here and so I want to talk a little bit about how that is going to be resisted by screw piers. So, there are the three types of loads.

We’ve got dead loads downwards loads, wind loads upward loads generally and also wind loads sideways loads. They go down into the ground into a screw pier which is really only designed for vertical loads. How we going to get around that? So the way we prefer to get around it is to get the builders to start with a bored pier in the ground, so a round excavation, whatever diameter to suit the size of the screw pier here they dig that into the ground first then they send the screw pier down and they screw pier into the bottom and then they go down to the required depth they need to get sufficient bearing and uplift because the soft soil at the surface is not good enough for this screw pier to be founded in the bored pier isn’t strong enough by itself to take the loads, the design loads so we specified a load for the screw pier designer in tonnes in working load limit and then we’ve got this concrete footing or excavation that sorry we’ve got this excavation that once the screw pier has been installed we fill up with concrete and then the house footing can sit on or into this footing. The problem though where we’ve got we just watched that video where the screw pier was sent straight into the ground down five metres and they’ve said that they’ve got sufficient bearing capacity but as you saw as the screw pier went into the ground it left a trail of loose saw as this group here was driven down that five meters it’s left this trail of soft saw as well as our friend on TV on the screen might have tried to compress the dirt at the surface we know because we saw the soil coming out we know that there’s this soft layer of soil all the way down following the screw pier and this is one of the problems that I have with screw pier designs being used in residential construction.

One we already know; designers already know that screw pier poles need to be designed as if there’s no support from this ground. At five meters for that pile that was just put in that we watched on YouTube I’d have serious concerns whether or not a five meter long steel pole in the ground with minimal support finishing at ground level – five metres overall – that’s a big column with no sideways restraint.

A big enough load is going to buckle that post so that really needs to be taken into account by the designer of the screw pier that’s the manufacturer, the design team, their engineers.

That’s one of the problems. The other one is, and I guess to help you picture it here’s what a buckle sorry a buckled screw pier might look like. Once it buckles and pushes sideways it’s not necessarily a defined point those connections may be fairly stiff but once it tilts sideways that load that was being placed on it at ground level is no longer being taken by the screw pile might end up touching the sides and might eventually stop but we’re going to experience vertical movement at the top of the screw pile. The purpose of the screw pile is obviously to not move to support our foundation, our building on something good and stiff but if it’s moving, if it flexes when it’s placed on the load then it’s not doing its job. So, for me that’s a fail. The way we get around that is obviously all the way the manufacturer gets around that is obviously to use a large pipe so the longer they think they might be going into the ground and it requires good soil testing to work out how far this distance is, the larger that pipe is going to need to be. Alternatively additional rather additional screw piers can be screws can be installed into the foundation and then these are going to be able to transfer a little bit sideways force into the ground, restrain that middle pole at whatever centres so I’d expect on a really long pole going ah a screw pier going into the ground that there’s going to be a couple more flights, a couple more auger screws that go down into the ground and they’re going to help stabilize these poles in that trail of soft soil. So don’t forget this is our preferred option at ground level that gives us the chance to put a, we call it a pile cap, we drill that first then the screw pier goes in, we fill that with concrete and either our house footings go down into that concrete or they sit on top on a baseplate. We’ve taken care of the uploads because the screw pier manufacturer designs the piers for that. We’ve taken care of the downloads because the screw pier manufacturers designs for that and we’ve taken care of those sideways forces because our bored pier that gets dug in the first, before the screw pier goes in, it’s got a little bit of sideways restraint and we also saw that sometimes we put cross-bracing underneath the house and that force is coming down so there’s a vertical component and a horizontal component so for steel posts for house posts that come down onto a bored pier or a screw pier foundation through a bored pier. We’re using the bored pier for the lateral support. We need to justify and prove that there’s enough bearing capacity there and there’s other ways of doing this obviously the bigger that concrete hole is the more sideways support we’re going to get and we’re getting the vertical support from the screw pier whether it be vertical up or vertical down.

Let’s talk about some of the problems that we have with screw piers and how we’re going to overcome them. Houses on reactive clay soils. Invariably the designer of perhaps a waffle slab because this where I’ve seen it more often used, put screw piers in around the outside and that’s well and good. We’ve got a foundation, a fairly strong stiff matrix of waffle pad beams running across the building, the slightly thicker one around the outside with support the outside load where the load from the house main load of the house is supported which is the walls. the roof and the load from the walls coming down on the perimeter footings so the screw piers are located appropriately for those vertical loads, the weight of the house. Let’s talk about when this ground starts to move because the idea of these screw piles is to support in, and they’re more often used in highly H1 /H2, so super highly reactive and E extremely reactive sites and class P sites. We’re likely to see screw piers in a waffle pod design on the H1, H2, Class E, Class P sites and the designer’s using them to get the weight or the support of the building down into a soil that, or in a layer of soil that is more stable and stronger than the surface supporting soil. First issue that causes is especially when it’s class P soil and this is soft soil, the soil eventually consolidates and it’s not a major problem but there’ll be times when you see a house supported on a screw pile but the ground has actually dropped away. Standing over here it can be quite alarming for someone because they can see in underneath the house. They may not see that there’s these steel posts in underneath the ground but they can see in underneath the house and that’s why this is more of a problem on waffle slabs because they’re built entirely on top of the ground so someone’s standing outside looking down can see underneath the house. Not such a big problem if the house is supported on piles whether they be screw piles or conventional bored piers. That’s fine. A little bit of digging down around the outside here we’re going to identify whether these piers are here and if the house is performing okay not really a problem. I mean this ground around the outside out here can be filled in.

I don’t recommend, depending on the soil class, filling in underneath the building by placing in extra fill underneath the building just in case this ground movement reverses – in case this settling isn’t consolidation but is actually the soil drying out (which is the opposite of slab heave where the ground surface lifts up). There’s ground dropping because dry conditions and one day the ground might get wet again and start to lift back up again. We don’t recommend filling in this space between the underside of the waffle pod and the ground surface. We don’t want the ground to lift back up and lift the house back up.

What about if we have reactive ground movement? If the house is supported only around the perimeter on screw piers down to the firm layer with loads going into the screw piers (as they’re designed for) everything’s going pretty well until the ground in the middle of the house either drops or lifts.

These footing systems are designed for a certain amount of lifting and swelling. If there are screw piers around the outside and the house is well supported around the outside we’ve got a portion of the house which is supported really, really well at a depth where the ground isn’t moving at all and we’ve got a portion of the house that’s potentially fully being affected by ground movement and ground heave or subsidence. Invariably these footing systems aren’t fully designed for that. So my recommendation would be if your engineer is going to specify screw piers around the outside we really would recommend putting screw piers in the internal footings as well and depending on the width of the house that’s really going to be up to your engineer.

There’s a few more screw piers but it means if that ground does drop the house is all still fully supported at that firm layer down at depth.

Unfortunately it also means that because we don’t have pile caps we don’t have any lateral support because we’ve got a layer of soft soil surrounding the posts we also end up with a house that doesn’t really have a lot of lateral support.

So recommendation for waffle slabs is if you’re going to have screw piers around the outside at whatever centres that the engineer specifies you really should be looking for screw piers at intermediate points in the middle of the slab so everything is founded on that same firm level down at depth where the screw piers are embedded into.

What if instead the ground lifts? We start with our flat ground and now the waffle pod system hasn’t got a lot of choice but to follow the ground – the heaving ground conditions – particularly if we’re talking about screw piers on the outside.

Of course if there’s a special system where these pile caps can be isolated from the waffle pod system or the foundation system can be isolated from the screw pier. There’s a special cap that’s been patented and trademarked that can be used to isolate the screw pier from the slab to allow this movement. Our only concern now is if the ground moves up so far that the screw post disengages and move sideways because we’ve got this layer of soft soil again down around the screw piers that these screw poles may move sideways and not re-engage when the house comes to sit back down again.

So that’s primary concern with these pile caps. If there’s screw piers in the middle and the grounds lifting, so it was centre heave then the screw piers aren’t going to provide any support especially if there’s a pile cap and the waffle slab lifts up off the screw pier, off the shaft. Now we’re relying once again on the waffle slab having enough strength to do its spanning. So between this case where the ground subsides and in this case where the ground is lifting there is no justification for designing this waffle slab for a lower site classification. Remember we’re talking about remember the H1, H2, E and P classes – there’s really no justification for designing a waffle slab lower than what the soil classification is simply because you’re supporting it on screw piers. I’d caution any designers out there that are saying we’re using screw piers, we’re going to design for a H1, H2 whether its uplift and heave or consolidation one way or the other – the foundation system still needs to stand on its merits.

If it’s consolidation of loose soil then screw piers really come into their own because the ground can settle away from the underside of the foundations, the house the building will be supported on screw piers which are appropriately designed by the manufacturer. The house is supported and a gap forms up. That’s fine, especially if it’s consolidation of loose soil then we’re in the position where we can fill that gap around the outside so that nobody’s too worried about it.

That is my comments on screw piers their relationship with foundation systems.

When they’re being used in foundation systems I’d really would prefer that whether it’s a raft foundation which is dug into the ground or a waffle slab system and it’s a kind of a merged system where I really recommend that the waffle system end up having a strip footing around the outside around the perimeter that actually goes in below ground level to give us, give that building a little bit of lateral stability by all means have a screw pier that takes the support of the building all the way down into firm soil at depth but by having this combined system a little bit of footing down into the ground I think we can achieve a lot better result, the internal footings can still be on polystyrene void formers if you so wish as a designer that’s fine.

I really think that having a merged system would be a great idea and solve a few of the other issues we have with waffle pod systems with drainage and water getting in underneath waffle pod systems as well.

There you have it. These are screw piers.

We’ve seen how they’re installed. We’ve seen how they’re being used.

The only other thing really that I’d like to comment on is rectification for when a builder manages not to be able to put a post centrally down on the screw pile.

Remember we said that with a pile cap and a post load and a screw pier in underneath that these are all fairly vertical loads the sideways loads were taken care of by that pile cap.

You’ll find that when you have a look at the engineer’s drawings that they’re relying on the house post that sits down on this pile cap to be pretty much directly in line with the centre of the screw pier. The effect of that not being in line if the post is somehow being placed offset on the post, on the bored pier with the screw pier directly in the middle is that that distance, that eccentricity between the distance between the middle of the bored pier footing or the centreline of the screw pier and where the load is actually being applied is creating a tipping force which we call a moment which this pad footing, pile cap, is not designed for. This steel is not designed for, this auger in the ground is not designed for. None of the system is designed for this offset footing – this offset load on top of the screw, on top of a pile cap so if the situation arises where offset footings are identified on site or the builder finds himself in a position where his posts don’t align perfectly with the pile caps then the builder has got some more work to do. Our preferred way to take care of these eccentricity issues is with a series of ground beams.

So here’s our pile cap with the offset post load offset from the centre of the pile because the screw pier is on the centre of the pile cap offset load with a series of ground beams that go back and join into the other pile caps. In this way, in two directions mind you, in this way we can take care of the eccentricity by turning it into a series of beams in this direction for example would be a beam with a load maybe just on the inside between supports so a vertical load a vertical reaction going into the screw piers and the beams doing a little bit of work transferring some load into this screw pier and some load into this screw pier and for this eccentricity this because the post is out on the outside away from the centre of the, the centre of the screw pile, in this case we have a load on the outside of this group this being becoming integral with the pile cap we have a support there which is a vertical load vertical reaction into the screw pier and another vertical reaction into the screw pier which is this screw pile and pier cap. So that’s how we’re going to take care of post being offset from the centre of a screw pier. We’re going to insist that the builder installs a series of ground beams to help tie and transfer loads into adjacent screw piers. So there you have it. We’ve been through a few things. We’ve talked about screw piers, we’ve talked about pile caps and their purpose, talked about how good screw piers are with reactive soils, talked about direction of load whether it’s sideways, vertical up and down and sideways and we’ve talked about how we can do some rectification of these things.

I’m Matt Cornell, I’m from Cornell Engineers.

Thank you very much for joining us. If you like this video please like it. If you’d like to know more about some other topics please leave a comment underneath and check out our website, I’ve got a heap of details on there about house rectification, waffle slabs, ground movement, concrete masonry, concrete slabs, the whole works. Come and check out our website and we’re looking forward to seeing you and helping you out. Thanks very much for your time. Have a great day.

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