Screw in Piles

[Jacko2]

Does any body have experience with a screw in pile system.
These systems are being marketed in Australia for replacement of bored piers. I have had a bad experience with one company and wondered if was a one off or common.

 

[VAD]

Jacko2

Probably a one off. What was the nature of your problem.

Regards

 

[eric1037]

Are you referring to the screw piles as they are known in European terminology (basically a drilled displacement pile) or are you referring to helical piers?

 

[Jacko2]

Answer to eric1037. They are drilled displacement piles.
Answer to VAD. The problems are numerous.
Approx 50 % of piles failed to perform. The builder had to underpin a three storey unit project.
Be carefull.

 

[jdonville]

Jacko2,

Sounds like it was a case of poor modeling of the subsurface conditions, poor installation, or poor information supplied by the geotech. Possibly all three.

I am surprised that that many piles failed. What was the predominant problem/symptom? Did you perform a dedicated pile load test on the initial pile to verify the design?

Jeff


Jeffrey T. Donville, PE
TTL Associates, Inc.

http://www.ttlassoc.com

 

[Jacko2]

All three is correct. What happened on this project was beyond Belief.
The supervising Engineer used it as a case study for screw in piles in a University Lecture.
It is not the normal standard of construction in Australia but a series of events came together and not detected by all members of the construction process.
The soil structure was sandy loam/clay in water charged strata. It was low lying near the ocean shoreline with floating boulders of sandstone.
The demand for Contractors was high due to the Olympic
Games construction.
The installation contractor’s supervisor I found out had little experience in the construction Industry.
The builder’s site foreman was young and inexperienced.
The following were observation made during the underpinning process. Who was responsible is unknown.
1. A pile cannot be found in the position indicated on the plan and entered into the bore log with the torque pressure.
2. A 20-ton pile was proof loaded with a hydraulic 20-ton bottle jack. A measuring gauge fitted to the footing and to the structure was installed. Each day the pile was loaded and sank 15-20 mm. This occurred for 6 consecutive days. The pile never set. My opinion is the shape of the helical thread and the gap between the leading edge of the flight and the trailing edge allowed the moist material to bleed into the 20 mm gap left on top of the flight. This released the pressure from the bottle jack and the pile stopped with the surface bearing on the flight only.
3. The pile flight design was made from a 500mm square plate with 4 x 75mm corners snipped off. This pile was 1200mm into the ground when the corner jammed into the side of a floating sandstone rock. Possibly giving a torque reading high enough to comply. There was no support under or beside the pile. A 4000mm bored pier was installed in place of the pile.
4. A pile was placed 1000mm sideways from the footing beam due to a floating sandstone rock close to the surface. The pile was not incorporated into the footing system. We latter canter levered back to another pad footing with a steel beam and concert encased the beam under the footing.
5. The top of the piles is trimmed to height after installation. This was done with an oxy acetylene torch. The cutting was very rough leaving sharp high points where a metal cap was to bring the load into the pile. The load would have crushed these high points. Where they could be they were welded on site. The top of the piles was accessible in the footing because 200mm of mud was lying in the bottom of the footings when they were poured. In 3 cases these were left off.
6. A group of piles were extracted and 2 x 1200mm caissons were sunk to 5000mm to firm ground
7. Several piles were proof loaded and failed then a series of 10-ton piles were installed in a group around the pile with a pile cap. A hole was blown through the top of the pile and reo bar placed horizontally thorough the pile to transfer the loads. This occurred on at least 20% of the piles.
From my experience with this project and others in Australia I am advising anyone using a drilled displacement pile to be careful very careful.

 

[jdonville]

Jacko2,

My understanding of your piles, from your description, sounds like what we call helical piers in the US - essentially discontinuous metal helical flights connected to a relatively thin shaft. The element is basically screwed into the ground until a target installation torque (indicative of soil frictional resistance) is achieved.

http://www.ottawastructural.com/Imagesjpgs/pierint.jpg

Drilled displacement piles, in US parlance, are essentially auger-cast-in-place piles where the sides of the boring are displaced laterally to create the shape of the pile. Concrete/grout is placed under pressure as the auger/displacement tool is withdrawn.

http://www.berkelapg.com/APGDa.html

Do you have any information on the paper that was presented based on this construction?

Jeff


Jeffrey T. Donville, PE
TTL Associates, Inc.

http://www.ttlassoc.com

 

[VAD]

Jacko2

First, it isalways trick to use torque to decide on pile capacity although this is wwhat is often promoted by helical/screw pile installers. Some of them have a rough factor to convert torque to capacity and is based on fora few on field testing. One has to be careful which type of strata this factor applies to.

It seems that there was only one helix used. Generally for the soil descibed one would expect perhaps three helixes. The number etc should be based on calculations as a first case procedure for given the soil conditions.

It appears that there was no geotechnical investigation done. The concept might have been reliance on torque. If so this is a mistake that should not be applied to discredit or condemn the pile type.

Your advise is well taken re being careful, but sometimes I see it very often that salemanship often wins despite that we have the ability to check from an engineering point of view. This is similar to a variety of products and their use such as geosynthetics etc. The problem is that many of these have gone ahead so fast that many engineers cannot cathch up with the technology and have to rely on salesmen many of whom do not possess technical background but are good talkers. Unfortunately, our business is full of this type of marketing.

I trust that my interpretation is not out to lunch re what I gleaned from your post.

Regards

 

[Jacko2]

Thank you for your response.
They are the helical type.
Vad is close when saying salesmanship won on the day. I believe cost saving was a deciding factor. They paid dearly in the end.
I will see if the Geo Engineer produced a paper.

 

[PSlem]

Rule here is to average three torque readings over the last three feet (1 meter) so you don't get a false reading. Helix size here is generally 12" (160 mm) due to helix strength, 3/8" plate (9.5mm). I'm surprised to see a 500mm plate. What's the thickness? How large was the shaft? Did any of the extracted piles show coning of the helices? Also pitch here is 3" (75 mm). A 20mm gap seems too small to allow dirt to pass while advancing the helix as even a 3/4" stone would clog the opening. Please post a paper source if poossible.

 

[PSlem]

I see I copied the wrong number off my scratch sheet. 12" is 300 mm and the corners were clipped so it is only 14"(350 mm. Still it would have to follow the Standard Bearing Equation so that soil would not give enough resistance for an 80,000# pile (Safety Factor of 2)

 

[PSlem]

OK I got my calculator. I wish we had edit. The area on the 500mm square helix with 75 mm clipped corners equates to a 21.7" circle. That's huge.