The scaling effect, or influence of bored pile (drilled shaft) radius on unit shear capacity at a given displacement, is examined via a review of the literature as well as data collected in the field. It is apparent that the scaling effect of pile-soil load transfer is a measurable phenomenon on both the small scale and large scale. Since results from smaller-radius bored pile load tests are often used for the design of larger radius production piles, it may be advisable to scale down the unit shear capacity obtained from test results by the ratio of the test to production pile radii to conservatively account for the scaling effect.

The scaling effect of bored pile radius on unit shear capacity

The Federal Highway Administration (FHWA) mandated utilizing the Load and Resistance Factor Design (LRFD) for all new bridges initiated in the United States after October 1, 2007. To achieve part of this goal, a database for Drilled SHAft Foundation Testing (DSHAFT) was developed and reported on by Garder, Ng, Sritharan, and Roling in 2012. Using the available data in DSHAFT, preliminary resistance factors were calibrated and proposed by Ng et al. (2014). Compared to the American Association of State Highway and Transportation Officials (AASHTO) LRFD Specifications, the preliminary locally-developed factors showed the potential for improved design efficiency. As additional load test data become available, resistance factors are expected to be recalibrated, thus the objective of this research was to utilize an expanded version of DSHAFT to refine and recommend final resistance factor values for implementation. To achieve this goal, the research examined recommendations given in AASHTO LRFD Bridge Design Specifications and the FHWA drilled shaft design guidelines and reviewed calibration studies conducted by Iowa and other states. Several challenges in the calibration process were identified, and it was found that a regional calibration can lead to resistance factors lower than code recommended values contrary to expectations. To overcome the main challenges in the resistance factor calibration associated with the lack of good quality load test data, the use of load test on small-scale drilled shafts as a costeffective approach to predict load-deformation behavior of larger diameter shafts was investigated. A total of five instrumented reduced-scale drilled shafts were constructed and load tested to investigate scale effects and develop appropriate scaling relations. Test data seem to indicate that skin friction decreases with increasing shaft diameter. Additional research is needed to confirm the findings and develop a suitable methodology to extrapolate test results on smaller shafts to larger diameter shafts. The research also investigated the accuracy of the finite element method to predict load-deformation response of drilled shafts so that displacement criteria can be integrated in design. Simulation results showed that the Mohr-Coulomb is simple to implement using CPT data, and it can provide adequate predictions. Resistance factors were calibrated in accordance with AASHTO LRFD framework for various drilled shaft design methods

https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=8233&context=etd

Improving efficiency of the load and resistance factor design of axially-loaded drilled shafts in bridge foundations

Performance evaluation of post-grouted drilled shafts: a review

In spite of the many recent advances in pile design and execution methods, the quantitative effects of grouted pile skin resistance and form on subsequent pile behavior remains an area for research. There are many parameters involved in the analysis of the bearing capacity of piles and descriptive method theory of the loading effect for each parameter is very complex. Many of these parameters are interrelated and investigation of the relationships leads to complex equations, which cannot be easily solved. The only reliable solution is to study the influence of each parameter by experimental model tests in equipped laboratories. This research presents the results of static compression tests on two model groups of pipe and grouted pile shafts (35mm, 50mm and 60 mm in diameters and 900 mm in length) installed into beds of Yazd siliceous sand (located in southeast Iran). The findings of the experimental research were to the average ultimate loads at failure for grouted piles were approximately 12% higher than for the pipe piles. The pile skin resistance is an effective factor on pile bearing capacity, the load transfer response appears to be more plastic with increasing pile diameter in siliceous sand and the skin resistance of the pile was not linearly proportional to the pile diameter and varied with increase in pile diameter.

M. A. Karimzadeh

Papers

Analysis of the Effect of Pile Skin Resistance Verses Pile Diameter Based on Experimental Research