Geotechnical Engineering Circular No. 9 Design, Analysis, and Testing of Laterally Loaded Deep Foundations that Support Transportation Facilities
Considerations for the Design of the Test Pile/Shaft
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| 12.1.5 Considerations for the Design of the Test Pile/Shaft Pile/shaft length maybe governed by axial capacity or lateral capacity. Fora lateral load test, the length of the test pile/shaft should beat least the minimum length estimated for the production foundations. It is recommended that an additional length be considered for driven test piles, beyond the minimum indicated by analysis, in the event the piles need to be driven deeper to achieve the required resistance to axial loads. If the pile is driven to the minimum design length, and that length is determined to be inadequate for axial loading considerations, the data from the lateral load test may not be representative of production pile installations. The additional length will be a matter of judgment and may vary based on the overall pile length and size, as well as subsurface conditions (depth to harder strata, cost of additional length, etc) It is important that the foundation element belong enough to exhibit bending and not fail through rotation. The rigid body rotation response of short stiff piles or shafts (say length to diameter ratios less than 10) is quite different to the bending and deflection exhibited by long flexible foundation elements the test pile length should be sufficient to produce bending and deflection rather than rotation. If the axial capacity of the foundation requires a greater length than required for lateral resistance, it maybe sufficient to design the test pile/shaft based on the lateral resistance length and not necessarily the full length that is required for axial resistance, provided the response to lateral loading of the shorter element will be the same as for the full-length element. The preferred approach is to construct the test pile/shaft to the same diameter as the production piles/shafts to provide consistency in the test results, interpretation of p-y curves, and calibration of soil models with the production piles. Smaller diameter or prototype piles or shafts will be more strongly influenced by soils at shallower depths compared to larger diameter production piles/shafts. Also, a smaller diameter pile or shaft may not have enough stiffness to transfer stresses to the depths that would be influenced by a larger pile/shaft because the flexural stiffness of a pile or shaft is proportional to the fourth power of the shaft diameter (Brown et al. 2010). 179 If lateral load tests are performed during the design phase with a view to optimizing the design of the foundations, or if the test foundations are for research purposes, it is recommended that the foundation element be designed with a high shear and moment capacity, possibly higher than what would be used fora production pile/shaft. In this way, the foundation element will be able to deflect significantly under loads that are likely to be high enough to mobilize the soil resistance, but without failing the structural element. This maybe particularly applicable for CFA piles, drilled shafts, or micropiles, where additional reinforcement can be included in order to increase the shear and moment capacity of the test pile compared to a production pile. However, using a stiffer element may lead to smaller deflections at the service load. Any other aspects of the design that will be included on production piles/shafts should also be included for the test piles/shafts. For example, coatings on piles for corrosion protection, which may also impact the frictional and shearing resistance between the pile and the adjacent ground, should be included on the test pile. If it is not possible to do so, the potential differences and risks of different behavior between the test and production piles should be assessed and possibly addressed through the use of a resistance factor. Download 6.03 Mb. Share with your friends:
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