Contents | About | Contact
article 18, issue 06
December 21, 2009 and March 21, 2011, (last update 7.11.2012)
Foundation laying usually means much back-breaking digging even for small buildings. Ground screws are a much easier method for supporting lightweight buildings and are very suitable for installation with human power. They are also easily removed and reused without using machinery. Historically called screw piles, they enabled the rapid construction of foundations for piers and bridges entirely by human power. They were popular for nearly a century and then somewhat forgotten until recently, reappearing for modern uses such as foundations for photovoltaic arrays.
[Editors' note: While we do cover stationary human power uses in HPeJ, this rather static application may seem "off-topic". It is however a good method for securing bicycle sheds and the like.]
for buildings or utilities are often made of poured concrete, sometimes reinforced with steel.
While these can be readily built using human power, there are
Figure 1: Archimedes: "Give me a place to stand on and I will move the Earth."
Ground screws may be loaded in compression, tension, and shear. While they may be installed using machinery, even large sizes can be screwed in with human power. As already recognized by Archimedes: "Give me a place to stand on and I will move the Earth." A great advantage is that they can also normally be screwed out again by human power - and reused elsewhere: a semi-mobile foundation. The screwing action also allows adjusting the vertical depth to millimeter-precision.
Figure 2: Earthscrews with flange (length of thread 0.5 m,
total lengths 0.8, 1.0 and 1.2 m, diameter of tube 75 mm)
Figure 3: Earthscrews (length of thread 0.4 and 0.5 m,
total lengths 0.8 and 1.0 m)
Several types and many sizes exist. I used three each for supporting a garden house, the flanged ones at the rear and the tubular ones at the front. I bought various sizes, unsure whether I might hit large rocks, but in the end was able to use the long ones everywhere. All six were screwed right in. The front three received extentions in the form of further steel tubes.
Figure 4: Garden house with 2.2 kW photovoltaic roof, total weight about one tonne
Inserting the earth screws is very easy. They screw with little effort into the top soil. A spirit level is used to keep the angle vertical. A short lever is sufficient intially.
Figure 5: Earthscrew being inserted. Note magnetic spirit level.
Figure 6: Earthscrew nearly in. Note spring scale and water level (plastic tube).
soil is relatively "ordinary", a farm pasture with topsoil/grass, then
compact soil resting on a layer of conglomerate, frequent rocks and
some clay. After a few turns the thread begins to bite and the torque
rises quickly. A magnetic spirit level is used to keep the screw
vertical. This is easy as long as both arms can be used with a
symetrical lever. Soon a longer lever is required and unless two people
are available, torque on a single lever produces a force which tends to
misalign the screw vertically. If the lever can be easily transferred
to the other side, it is however possible to apply force such that the
misalignment is immediately corrected. Careful attention to the spirit
level allows a near perfect vertical angle and a lateral precision to
better than ± 5 mm. The vertical position can be very precise
simply by stopping at exactly the right position. Multiple screws can
be be brought to the same height with a long spirit levels, a plastic
tube filled with water, or a laser-level.
Figure 7: Torque versus depth. The last peak could be a rock,
some of the screws didn't go over 300 Nm.
Table 3: Vertical loads at 5 and 10 mm settlement, several soil types and several screw lengths, from (Schulz 2004), Table A8-4
|Ground||Clay||Clay||Clay||Sand, loose||Sand, dense||Gravel, loose||Gravel, loose||Gravel, loose||Gravel, dense|
|Length||75 mm||100 mm||150 mm||100 mm||100 mm||80 mm||100 mm||150 mm||100 mm|
Schulz, H.; Schick, Peter; Schmid, Jürgen.
"Tragfähigkeit und Verschiebungen von Schraubfundamenten der Firma
Krinner" Institut für Bodenmechanik und Grundbau, Universität der
Bundeswehr München, Neubiberg, 05.03.2004;
Contents | About | Contact
Human Power eJournal