Screw piles: international standards and real experience

Features of screw piles, standard indicators, personal experience of FORUMHOUSE participants

Screw piles are considered a universal type of foundation, which is made in one day, suitable for any soils, serves quietly for more than a hundred years (but this depends on the quality of the steel) and does not harm the soil cover plot. It is beneficial for builders to use screw piles - it takes not a couple of months, but a couple of days to build a foundation, but what can this turn into for future home owners? Especially in Russian conditions, when there is no clarity with the official building codes and recommendations for the use of screw piles in low-rise construction. This article tells about what standards and recommendations exist in the global construction community, and what guides the members of our portal when installing screw piles.

Benefits of screw piles

Almost 200 years ago, the first screw-pile structure appeared in the United States. It was the Stonington Harbor Lighthouse in Connecticut: a ten-meter high stone tower connected to the caretaker's stone house. This lighthouse is still intact.

The screw piles were designed by a young Irishman, Alexander Mitchell. Lighthouses and piers, railway bridges and overpasses were built on the "Mitchell screw piles". The benefits of screw piles were obvious:

• speed (ready to use immediately after installation);
• manufacturability (it is understood that piles for industrial use are not screwed in manually);
• low cost (they are always cheaper than reinforced concrete foundations);
• independence from climatic conditions (they can be installed even in frost, even in heat from -35 to +35 degrees Celsius).
• independence from groundwater level: no need to worry about swelling soils and drain the site.

Take a look video on how easy it is to drill piles even in winter:

International Building Code Limitations for Screw Piles

But all of the above applies only to foundations made of piles, made by an industrial method and in all respects corresponding International Building Code ICC AC358 Helical Foundation Acceptance Criteria screw piles). And even for such piles, a number of restrictions have been introduced.

• Screw piles are used only for structures with seismic resistance classes A to C, from “structures for seismic safe areas” to “structures for areas with moderate seismic activity”.
• The specific electrical resistance of the soil should be more than 1000 Ohm-cm (this is due to the high rate of electrochemical corrosion of steel);
• The pH of the soil must be more than 5.5.

• The soil should not contain a large amount of organic matter;
• Sulfation of the soil should be less than 1000 ppm (otherwise, there is a threat to the unprotected concrete of reinforced concrete piles);
• The soil should not contain soil dumps, debris, etc.
• The soil should provide lateral support for the piles. Loose dusty sands, peat, soft sedimentary rocks are unacceptable here.

To understand what kind of soil one has to deal with, they usually do a test screwing or drilling. It will not be superfluous to talk with neighbors who have already put the house on screw piles: they have already identified all the advantages and disadvantages empirically.

What soil is not suitable for screw piles

Features of the soil, for which screw piles will be irrational and even dangerous, was formulated by FORUMHOUSE user with nickname Igor3.

Weak soil - the pile can be easily screwed in to its full depth or rotates freely without twisting. Faced with this phenomenon, usually the pile is welded all the way to solid soil, but:

Igor3

FORUMHOUSE user

The depth to solid ground can be more than 7 meters, which makes this type of foundation already irrational.

Solid ground. This is a fairly common phenomenon: the soil is so hard that the metal cannot withstand, a regular screw pile with blades simply cannot be screwed in.

Stony ground. Even small stones in the path of the blades make screwing difficult or impossible.

As with all types of buried foundations, lateral soil support is mandatory for screw pile foundations. In p. 1810.2.1 of the IBC International Building Code for screw piles that are installed in fluid soils or water and are not tied together in groups, the requirement is mandatory:

The depth of pile burial in dense soils is at least 1542 mm, and in soft soils - at least 3048 mm.

Requirements for structural elements of a screw pile

The thickness of the metal of the walls of the screw pile depends on two parameters:

• load on it;
• operating conditions.

Under the operating conditions, we mean to a greater extent how the metal will become thinner due to corrosion - it depends on this whether the pile will last 400 years or it will rust in 40 years and the house will collapse.

Igor 3

FORUMHOUSE user

Let's say you have an ordinary loam on your site (not saline and not acidic soils) and a pipe GOST 10704-91 "longitudinal electric-welded steel pipes" with a wall thickness of 3.8 mm (the most common size). The pile is not painted, or painted with cheap, but not special paint. According to my observations, such a pile can last about 50 years. And if the steel is protected with a special paint, or better with zinc, then the life of the pile is much longer.

The international standard ICC AC358 Helical Foundation Acceptance Criteria is required to use zinc coatings on piles in highly corrosive soils. Below in the table we have collected the requirements of this document for the design of a screw pile:

Most of the Russian-made piles do not meet these parameters. They are made from welded pipes (corrosion resistance of seamless hot-rolled pipes is much higher, and it is from such pipes had to be made a pile shaft according to the Soviet reference book "Piles and pile foundations", issued in 1977 year). As already mentioned, the "most common size" of the wall thickness is 3.8 mm (but almost always up to 4 mm). And the corrosion coating is so thin that it often does not even survive transportation.

Igor3

FORUMHOUSE user

It is very important that a high-quality, wear-resistant coating is applied to the pile. You can test the quality of the paint by scratching it near the drilled hole at the end of the pile. The quality of welding can be seen from the seam: it should be solid, even, beautiful, without snot and shells. And the metal should be smooth. You need to be sure that there is an unpainted rusty pipe in front of you.

If you turn the pile over and knock on it, rust should not fall out of it.

The popular in our country "scalding" of steel screw piles with ties made of steel channels, I-beams and angles accelerates corrosion. In soils with poor lateral support, this technology is a harsh necessity and cannot be dispensed with. But in this case, a galvanic connection may arise between the piles and other steel construction elements and reinforcement of reinforced concrete structures: this leads to the appearance of stray currents, accelerating corrosion. Clause 3.9 of ICC AC358 states that there should not be that link. Also, this paragraph prohibits the combination of galvanized and non-galvanized elements in the pile foundation.

In general, if a person does not know the degree of corrosive aggressiveness of the soil in his area, it is better for him not to take risks and use piles with a trunk:

• hot-dip galvanized;
• wall thickness 8-9.5 mm or more.

conclusions

On our portal you can often come across the opinion that screw piles are not suitable for critical structures.

dimitrii37

FORUMHOUSE member

Screw piles are the lot of sheds and change houses, well, in no case for the house!

But in reality, even large houses stand reliably and for a long time on screw piles. The most important thing here is the high quality of the piles and the correct calculation. It is necessary to take into account both the characteristics of the soil and the degree of its corrosive aggressiveness. The foundation is the foundation of the house, and lightheadedness in this matter is unacceptable.

To learn more about screw piles and determine their admissibility in low-rise construction, the topics on FORUMHOUSE will help "Foundation on screw piles - 6" and "20 myths about screw piles". Read our article - it proves that there are only three reasons to build pile-screw foundation. Watch our video about insulated Finnish foundation, which tells the nuances of installation, design features, differences from similar types of slab foundations.