Pile stiffness and soil stiffness are crucial parameters for pile design. Soil stiffness cannot be calculated, and rough estimates can deviate significantly from reality, which has a negative impact on pile design. Thus Pile load tests (“POT”) are a must in both terms of safety and economics. In regard to the later: If a pile is deemed to be to short to suscept the lateral structural loads arising from wind load for example it won’t help and will mean a waste of material cost, if the pile is elongated, but is not stiff enough to transfer the loads into deeper, firmer soil layers… In this case, it may be necessary, for example, to choose a stiffer pile cross-section so that the pile embedment depth can be reduced in order to optimize both pile safety and material consumption. This way to restore a good balance between the technical and economic perspectives—and satisfy both: Engineers and business administrators.
Three different pile cross sections (example)
In the image below, the left rod and the right rod symbolize piles with different degrees of rigidity; these piles could be driven piles or screw foundations, for example.
Pile stiffness, length, and subgrade modulus: Illustrative model explaining the interaction between these three design parameters
While the left pile is a brand-new pile installed in the ground, the pile on the right shows that if the pile cross-section is too thin, i.e., not rigid enough, or has been weakened by corrosive processes over the years, the deflections can increase dramatically in comparison to each other under identical horizontal loads. This can become a problem, for example, when piles with different stiffnesses occur alternately: If, for example, foundations with different stiffnesses are located under a solar table, this can even lead to load shifting resulting in a zipper effect with overload of the stiffer pile foundations. Furthermore, it shows, that (a) the steel at the tip of the pile is wasted material if it doesn’t contribute to the load bearing and (b) in the event of load reversal, weakened pile stiffness can also lead to dynamic effects if the weakened stiffness is a matter of corrosion in the joints of a screw foundation which might lead to reswinging due to increased play and thus a change from static to dynamic loading when load direction changes from South to North direction for instance.
For this reason, AquaSoli uses pile load tests to precisely examine the bending stiffness of the pile-soil system and determines the corrosiveness and thus the risk of rust through on-site soil samples and subsequent laboratory tests.
