Scaled pile testing generally refers to load testing a dedicated pile or drilled shaft which has a smaller diameter than the production design diameter. The design and ultimate load of the scaled test pile are correspondingly smaller. There is a penalty, however, in the uncertainty created by constructing and testing a foundation element geometrically different from the full scale design elements, possibly using different equipment and techniques.
The FHWA Drilled Shaft Design Methods Manual calls these scale piles “prototype” shafts and addresses them in Section 17.2. The manual essentially admits (and many of its authors and other experts agree) that the engineering is not fully developed and more research is needed. What we present below is a brief summary or the reasons for the scale pile testing, current practice and some of the pros and cons.
The pros of scaled pile testing basically all boil down to cost savings, and include:
- Direct cost savings – less material and less time to construct, less expense for expendable jack (if bi-directional testing is utilized), and/or making a top-load test feasible where a full-scale test load would be too expensive or technically challenging.
- Indirect cost savings – especially on pre-construction testing programs, mobilizing a different (smaller) rig and ancillary equipment to site ahead of main project start may be very advantageous to the contractor.
The cons of scaled pile testing include but are not limited to:
- Non-representative technique – if the shaft is excavated using a different set of tools, by a rig which is able to deliver different amounts of crowd and torque, the resulting foundation element may be different in quality, not just scale. In harder materials, side wall grooving and roughness plays an important role in developing friction capacity for example.
- Non-representative timing – Volume is a function of diameter squared. A contractor who can easily construct a 4-foot diameter shaft in one day may struggle to complete an 8-foot diameter equivalent in a work week. The amount of time the shaft wall is exposed to slurry plays a role in the resulting shear capacity.
- Scaling factor – Even if the scaled pile is a fair representation of the full scale pile, there are geometric factors still at play, colloquially known as a “scaling factor”, which serve to decrease unit capacities. The contributors to this factor include radial shear attenuation (see pdf here for a discussion) and the depth of the Boussinesq stress bulb which forms at the shaft base (see pdf here).
Generally, load test providers discourage scaled pile testing, because it is a conservative way to eliminate the cons. LTC believes that proper engineering analysis can mitigate some of the drawbacks while rationally addressing the differences between a scaled-down test pile and full-scale production pile. If for example the test is being carried out primarily to gut-check design assumptions and calculations, then there may be no good justification to charge the owner the extra money for a full-scale test. For now we are agnostic on this topic, and will work with clients to achieve the optimal solution for their particular project. But we do look forward to contributing to the knowledge base this subject so badly needs.
UPDATE (May 2019) – It looks like FHWA is planning to specify only full-scale test shafts for production-phase testing in the upcoming revision of their drilled shaft design manual. Scaled shafts should still be acceptable for design-phase testing.