Helical Piles

This was from a job I did a few years ago in the Everett area.

FOUNDATION:

Soils report Available: Yes
Firm: , Inc.
Number/Date: / 4 Nov 2002
/ 22 Jan 2003
Test Pile Installation Memo / 18 April 2003

Screw pile foundation of type and installation procedure as developed and provided by PTI (Pile Driving Technologies, Inc.) utilizing both 3.5” and 5.5” diameter steel pipe with one 360 degree helical blade welded to the bottom of each individual pile. Pile grades and sizes per PTI specifications as determined from on-site testing and past experience.

Should any pile break or be rendered unusable during the installation process so that it cannot carry the required design load, that pile shall be abandoned and a new pile installed three feet from the original location and in line with one of the grade beams, subject to the approval of the foundation engineer of record and the geotechnical engineer. Adjust the grade beam steel reinforcing and size, if necessary, in accordance with the foundation engineer of record. A pile driving report of all successful and abandoned piles shall be furnished the foundation engineer of record upon completion of the installation process.


Mike McCann
McCann Engineering

Check on line. Check out the web site for A.B. Chance or Ram Jack Helical anchors and piers. There is much info available on line if you look.

easier... yes.. cheaper? I dont know about that. Maybe if you only have 1 or 2 piers.

Finally, a subject that falls within my range of experience! lol, check below...

Helical (screw) piles are always proprietary products from different companies, ie there is no standard screw pile design spec but many companies make their own. I work for a screw piling company here in Australia and we work off AS2159 Piling Code (check Standards Australia os ISO).

The main things to worry about are load capacity and corrosion allowance for a fixed service life. Nominate the loads on your drawings and note on them that pile selection should be undertaken by an approved contractor with reference to the subsurface geotechnical reoprt/ bore logs.

So you guys are saying that the pile size, pile depth, and material size should be determined by the contractor and all I do is indicate the loads on the plans and refer the contractor to the soil report?

What kind of material do they usually use? I would think galvanizing is not enough for prolonged exposure to the soil. Is stainless steel common or even feasible? What grade of stainless steel would normally be used?

Relsting to yourfirst question, not exactly. I would state that the soils engineer will have an experienced geotech on site during the driving of the pile to make a pile log and supervise the installation. If there were any structural concerns such as a broken pile, he would consult you for pile relocation.

A structural grid system here is very helpful and necessary for over the phone communication and paperwork.

Mike McCann
McCann Engineering

A few general notes on screw piles:

-uplift capacity is usually almost as much as downward capacity.
- they do not work on rock.
- do not work well for lateral loads - use skewed piles if there are any significant lateral loads.
- shaft can be as small as 1.5x1.5 inches solid bar.
- sizes larger than about 200kips capacity will usually need to be pre-ordered.

Check out:

How would you analyze a skewed pile?

yahoo123,

Typically, a skew or battered pile is analyzed for detailed design using specialty software - GROUP is one package.

Jeff

It is pretty simple to do it by hand as far as the required capacities are concerned.

Analyse as a vertical pile group ignoring horizontal reaction then batter the piles to an angle so that the horizontal component of all piles is equal to the lateral load.

Final pile load is the resultant of these 2 components.

Usually you will batter the piles either side in opposite directions so that the dead load components cancel each other out.

csd

For corrosion, the galvanised and stainless steel options you listed would be rather expensive (rare manufacturing too).

Our screw piles are not corrosion proofed, the reason being:

1. The mild steel tube has a thin coating on it from the steel manufacturer (not much but it's something).
2. During installation, the constant abrasive action of the soil against the pile shaft would compromise ANY coating, including hot dip galvanising.
3. As the pile corrodes in the ground, a layer of rust will form thus slowing corrosion after a period of time.
4. The hollow circular shaft is always filled with concrete, and this concrete is protected for the entire design life. Assuming that the steel gets completely corroded, the concrete will be intact and still load-supporting even after total section loss.

Having said the above, if your site has problem soils/ agressive ground conditions, screw piles would not really be suitable unless the design life is short, say 25 - 50 years max.

When designing for a job I predict the corrosion (mild corrosion is 10 to 20 microns per year, severe is 40 to 300 microns per year) over the design life and make sure I load the piles so they can still carry load at the end of their design life.

You can multiply the corrosion rate by your intended design life and arrive at a section loss. This section loss is proportional to the reduced load carrying capacity of the pile. It is this reduced capacity that is used for design purposes.

The above would be done by the contractor's engineer, ask if they can be present whilst installing the piles. Nowithstanding, a marked up plan and a log sheet needs to be produced. Gove the contractor you footing layout plan and they will mark on there where they need the piles to go (and number them). Then, during installation, they can log the depths and torque values for each pile - this is how the piling gets certified, by correlating these depths against the geotech report (or vise versa during tendering).

If youre really worried, ask the installers to log the torque/ hydraulic oil pressure (from the drive head) readings at every 500mm (20 inches) of pile depth. This only needs to be done to say 5% of piles installed, it geenrates a torque profile of the ground and can be used as supplimentary info for the geotech/ structural eng to sign off on it. Again, the piling contractor should be taking care of this part (as we do).

With regards to skewed piles, if they're skewed at 5 or 10 degrees, gfenerally this doesnt pose too many problems. It does introduce additional bending moments in the pile shaft though so make sure that the shaft has thick sidewalls and will be concrete filled. If piles are raked at extreme angles then comprehensive analysis is required.

Hope this helps.

Galvanizing for helical piers is very common, if not predominate in the US. Talk to the suppliers. Chance has galvanized material. Ram Jack, I believe, has epoxy-coated material (which I am not too excited about), may have galvanized. They can give you much information on corrosion protection.

how you determine bearing capacity of compacted recycled material, material was placed in 12 inch lifts and compacted to 95% of maximum dry density.

kidcivil,

There has been some good Australian research that confirms that the galvanising for screw piles stays sufficiently intact to provide reasonable protection.

You obviously must do bridges if you refer to 50 years as short duration.

csd

wow very informative posts.
Another question. How is the torque factor obtained from testing? Is a compressive capacity test performed at a given torque value to determine their relationship?

There is a direct relationship between the amount of torque required and the bearing capacity. I cant tell you more than that.

The factor relating torque to anchor capacity varies for different soild. Chance uses an average value of 10 lbf per ft-lb of torque. For very small projects in good soils, this MAY be sufficient for confirming capacity. For larger projects or projects with with less tham optimal soils, you should do load tests to confirm the capacity.

this is a great thread

I havent heard of anyone in Australia galvanising screw piles but I sure would like to learn more about how they behave/ costs associated with. Most of the time corrosion isnt a major issue for us here, but sometimes it is the predominant requirement and needs to be dealt with (see above).

csd72 did you say 'Australian Research" on Galvanised piles? Can you point me in a good direction on this please? Yes 50 years is considered short in the sense of design life that we would warrant in the event of fast corosion. Typically with mild or no corrosion, we can warrant our piles for in excess of 100 years, only when it's a probnlem site we reduce the certified service life.

The torque applied whilst installing the pile is directly propotional to the load carrying capacity of the pile. One of the main concepts is that the pile undergoes it's highest stresses during installation. The actual torque versus load capacity relationship is dependent on which hydraulic motor is used to install the pile, wether the installer was in high or low gear and the (general expectation) of material that is underfoot. The installer can note the oil pressure PSI during installation (at founding depth) and the engineer can correlate this to a torque value (if the equipment is setup nicely, the gauge reads in torque Nm or ftlb).

5125er at 95% dry density the load bearing capacity of the ground is pretty high.

In my xperience, installing screw piles into engineered fill is more difficult then into natural material because it's been compacted so much.

if it's shale at 95% youre talking about, it'll be so hard you;ll get full capacity within 2 - 3 metres (thats about nine feet in American).