Regardless of their type, all piles are exposed to lateral, compression and tension forces. Depending on their purpose, they are tested for one or more of these to establish whether their actual capacity corresponds with the design requirements.
Since a designed pile capacity is essentially a set of calculations based on assumptions coming from a soil investigation report that was conducted in the vicinity of the pile, as well as on theoretical modelling of forces acting on an ideally installed pile, it is necessary to conduct tests in order to confirm those calculations.
Lateral Load Test
Marine piles are subject to lateral loads generated by current, wind, waves, berthing, mooring, etc. These are not always complementary forces, as for example, berthing impact can act in the opposite direction of a mooring rope force and never at the same time. However, the ones that can occur simultaneously and that are compatible, need to be taken into account and multiplied by an appropriate safety factor to determine the required pile capacity. Lateral load tests are very rarely conducted until failure. It is sufficient that the test proves that piles can sustain the load as per specification (often 150 or 200% of the designed working load).
The easiest way of laterally testing marine piles is by using Tirfor between two piles and pulling them together, thus applying the test loads. Pile displacement is then measured and results calculated. Another way is to push a pile from a stable reaction platform or from another pile, by using hydraulic jacks. In case of a major repetitive character of forces at work on piles, both pulling and pushing is applied.
Measurements are most commonly taken at two points – at the top of the pile and at the water level (preferably at low tide). That way a double-check of designed deflections in relation to bending moment is achieved.
Compression/Tension Load Test
Testing of marine piles for compression or tension (uplift) capacity is a bit more complex than testing them on the solid ground. For a start, the testing set-up must tackle the problems inherent to the marine environment, in particular the restrictions in size of the operational platform and the logistical problems arising from it.
The most common set-up for static compression and tension load tests is the use of reaction piles, on top of which heavy test beams are placed. Between the reaction piles, the actual test pile is located. Hydraulic jacks are used to apply the load on the pile, while the reading gauges measure the pile’s movement.
Especially with the uplift test, which represents the force of waves on a marine structure, it is important to cyclically load pile, either 25 times at the maximum test load, or 10 times per increment.
Results obtained by tests require proper interpretation. It is a common mistake to believe that there is any kind of universal standard for pile acceptance criteria. There are various specific guidelines as to what is deemed a failure load (load that exceeds a pile’s capacity). For example, it could be a settlement of more than 0.25mm per 10kN (1t), or a load at which net settlement exceeds 6mm, or a load at which settlement continues without a further increase in load, and so on. But, at the end of the day, it is the project’s specification documents that should determine the acceptance criteria, because the piles are, after all, project specific.
Written by Darko Matijas, Division Manager at Ecocoast. To get in touch with our marine piling division, email Darko at email@example.com or call +971 4 885 3944.