helical screw piles 1

[rittz]

Who can tell us the present status of industry acceptance of helical screw piles as a deep foundation system for small commercial buildings as well as acceptable design procedures for vertical and lateral loads?

 

[dik]

I've only used these for two buildings on the same site and only within the last year or so. Installation, other than alignment, went without a hitch, and they were quickly installed. They were screwed into the ground, the end cut off to the proper elevation and a new cap plate with headed studs welded in place.

I had originally spec'd friction piles, but the contractor was concerned about ground water. It was basically a clay with some gravel/sand seams. None of the geotech reports indicated water issues. I used a larger shaft than normal to engage shear (not having used them before, I wanted some added capacity).

The shaft was checked for torsion during installation and buckling and the most difficult part of the exercise was to get information for the design of the helices... finally, used a centre loaded circular plate and increased the weld a tad.

There were several different lengths with 2'dia helices welded at 6' centres. The top 10' was galvanised and there is a corrosion allowance. Skin friction and bearing values on the helices were provided by the geotekkie. They seem to work, so far.

Dik

 

[ron9876]

They are fairly common where I am. My real concern is corrosion. Attended a Chance seminar not too long ago and they saved the corrosion discussion to the very last thing before the seminar ended. Gave me the impression that they didn't want to have much discussion about it. I think that in a tidal area that the rate of corrosion would be prohibitive but I don't know about in other regions.

 

[bdlc2k]

I recently spec'd and used helical piers on a renovation project. They worked out great since they had high capacity and are quick to install.
With our application, we needed to submit to the installer/engineer loads, a geotech report and the pier cap design.

ron9876-As far as corrosion is concerned, our helical pier engineer says they last over 100 years!

Check it out.

 

[hokie66]

"...our helical pier engineer says they last over 100 years!" Easy for him to say.

 

[demayeng]

We have been involved with them a fair bit, but we always make sure that we design the helix diameter for bearing capacity based on normal design principles.

The screw pile contractors are always trying to provide the lowest quotes by providing the smallest piles - often it works, however sometimes they can't get the torque reading they're aiming for and have to keep welding on extensions and drilling down deeper and deeper.. at extra cost to the client! and the piles get more dodgy the longer they are. Even their method of determining appropriate bearing from torque reading is completely unreliable. So it is best to determine length and helix size from predetermined geotechnical information, so you know exactly what you're getting.
You can look up corrosion rates and allow appropriate extra wall thickness, and possibly provide galvanizing, to get you to 50 years design life pretty easily.. but they still don't seem like a very good solution in aggressive environments to me. Seems a shame to design a building that could potentially last more than 100 years on piles that clearly have a limited life.

So to try and answer the question - I haven't designed them for lateral loads - I know you can install them on an angle/install pairs etc to deal with lateral and/or eccentric loads.
For vertical loads, the design process for the shaft is the same as for a steel column, just have to allow for corrosion and for eccentricity, and also decide on an appropriate effective length based on the soil (which is the tricky bit! I think it's just a matter of staying conservative)... Contractors I have dealt with seem to have reasonably good design spreadsheets for shaft design and the helix thickness, slope and weld.. I always get them to supply their proposed parameters to have a look at.

 

[dcarr82775]

I have used them, but always require a load test.

 

[Brad805]

We have used them quite a lot. Agricultural bins, bldg foundations, residential decks and bollards.

We have the same issue as Ron pointed out. We have spec'd heavier wall thickness piles and even field galvanizing the top portion of the piles.

The other issue we have with these piles is the accuracy of the site installation. These piles are very good for vertical loads, but their stiffness does not lead to significant lateral load capacity. Our commercial contractors are good, but the residential guys do not always plan their jobs well enough. I recall one job where 6 of the 8 piles were greater than 3" out of alignment. The screw pile guys think they can just weld on a stiffener and larger plate and all is good, but that is not always the case. I generally spec a larger pile to account for this.

Considering the cost of steel, field welding, and rust protection measures, we are not convinced they are cost effective in all situations. We generally specify these mainly in locations where concrete delivery is problematic or for lightly loaded piles subjected to greater frost uplift forces.

Brad



Brad

 

[EBSEngineering]

Helical piers are a very reliable product. We use them all the time. There is considerable research that correlates the torque to capacity ratio. Torque is a very accurate way to predict capacity. If you are unsure, specify a test to prove it. They can be designed for lateral loads. They are a very acceptable way to complete a foundation. As far as corrosion, AB Chance has a helical pier with a grouted shaft that provides extra resistance to corrosion if necessary.

Just like every other product, the installation is only as good as the installation. Get a good contractor. Get a reputable helical pier company (AB Chance is very good). Watch out for the contractors that "weld them up in their shop". No QA/QC program.

 

[0431945785]

I did a lot of researches for my master degree in screw piles.I found there are a lot of researches about screw piles from ASCE.


I have been designing screw piles for many years for many projects including high rise buildings, mining jobs, huge retaining walls or even bridges. AS2159-2009 requires four parts for screw piles design including structural design, geotechnical design, durability and settlement. Everyday I finished about 10 projects for screw pile design in clay, sand or rock for quoting. My boss gets about 80% of the quote or even more. We have 3 installation scews.

There are some jobs where structural engineers required a specific limitation for settlementes of the structure under the design serviceability loads and load tests to confirm. Settlement is more difficult to estimate than the capacity of screw piles. In Clay or Sand, we now can estimate the settlement within 0.25mm tolerence that allows us to pick up the helix and shaft size and length depending on the soil report to limit the settlement under 5.0mm or 10.0mmm (some structural engineers did asked for).

If there are SPT boreholes or especially CPT, the design will be perfect. Static load tests can be carried out to prove the functions of the designed screw piles.

Structural design for screw piles are just like columns in soil but not in the air. There are a lot of researches about the buckling of piles (shaft) in different types of soil. We tested a lot until the piles are fail in bearing or buckling

Corrosion is not a big concern as we can do some tests to check the exposure conditions. This part is durability design.

 

[dik]

Settlement within 0.25mm tolerance?

Dik

 

[0431945785]

Hi Dik,

Pretty good isn't it? Lol

To be perfectly honest, most of the jobs in clay (very stiff or hard), my calculation for settlement are ok.

The job I have just done the load test in Townsville (Annandale Christian School) in hard Clay. This job we have 3 CPT borelogs. Only one hole showed weathered rock at about 6.0m. Others not encountered rock up to 10.0m. To save money for the client, I stopped the pile at 5.0m. Even my friends questioned why don't try to put in rock. The builder also doubted. But as I had done a thousand static load tests so I am confident with my settlement calculations. My boss said we will load to prove it. AS 2159 allows up to 7.0mm settlement. My estimation was 4.25mm under SWL and the load test show it was 4.5mm.

Or the test I have just done in 11 11 Cristobel Crt Broadbeach Waters in medium dense sand to dense sand. Again the maximum allowable settlement from AS2159 is still about 5.95mm. My estimation is 4.5mm and the test showed 4.0mm. This job we just have DCP borelogs.

Piles in rock is a different story. Don't need to check the settlement at all. It is very small.

For a big jobs, if the load up to 200tonnage (2000kN), the maximum allowable settlement from AS2159-2009 is a lot, even more than 20.0mm, while the structural engineers required only 10.0mm. Settlement estimation will be very helpful in this case.

Geotechnical and structural capacities of screw piles are easy, just follow Professor Nathan and Peter in Membourne or Howard Peko or other researchers. They are almost the same and not much different from the traditional method from Broms or even Polous H.G.

The settlement of the piles depends on the tube size and length. While tube size and length depends on not only the loads and founding materials but also the torque capacity of the section of the tube. Australian Steel Institute gives the equations to get the torque but it is still conservative in comparison with other codes.

I have checked the settlement with reference to Polous H.G. and others. They are ok. They give me a better figures than AS2159 but still conservative for screw piles I reckon (LoL).

There are a lot of great geotechnical engineers everywhere. They would give you better answers. I think RymarkEngineers and Russell Heale Engineering are having a group of talented engineers. I learned a lot from the senior semi retired engineers.



Steve

 

[rowingengineer]

Rittz,
helical screw piles are not known for having large lateral load capacities, in the instance of high lateral loads a secondary system should be consider or alternative system.

sorry I just have to ask this side note of Steve.

Why would you install pile on a school building? the area you are discussing Annandale Christian School has what I would considered to be reasonable bearing conditions. The only interesting part of the soils would be expansion and I fail to see how pile would help here unless you fully suspended the ground floor.

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[0431945785]

Hi Rittz,

Are you a boss looking for a structural engineer?

for high rise buildigns, when you design for seismic, you will have a huge lateral load. All the lateral loads will be solved by a group of piles. I have designed for a preheter tower 123.0m high for a 6000tons per day cement plant. The bored piles were 1500 diameter.

For a house in Australia, we often ignore the seismic load in Queensland because it is even less than wind load

It does not matter, a house or a school building, whenever the site is classified as P class, you shall not apply AS2870 anymore. Yes, sometimes, it is a suspended concrete slabs with several levels. The load is huge, you can rest the footing on the 100kPa working soil. You need to go deeper to find stronger founding material. That is why piling system is used.

Other things, if it is E class with file, it would be safer to put piles in to prevent differential settlements in 30 or 50 years.

Please note the suction depth is mentioned for different parts in Australia. Deeper than that, ys = 0. That is why piles or piers need to go deeper than that level.

Cheers

 

[0431945785]

Hi Rittz,

If you want to know more about screw piles. Give me a job with SPT borelogs and loading for screw piles (axial loads, lateral loads, bending moments at bottom of reinforced concrete columns or steel column)

I will give you a brief calculation to see if you need a group of piles with a pile cap or not.

Each pile has its own lateral capacity, depending on its diameter of the tube and the depth.

Regards

Steve