Load and Resistance Factor Design (LRFD) Resistance Factors for Augercast In Place Piles

Data from 78 load tests from 21 sites in the State of Florida were collected to develop Load and Resistance Factor Design (LRFD) resistance factors for Auger Cast In Place (ACIP) pile design. Forty-four of the piles were embedded in layers of soil and rock, 4 were only clay, 7 were only sand, 15 were sand and clay layers, and 8 had no borings. Inspection of the load test data revealed the majority (90%) of pile top displacements ranged from 0.1” to 0.3” and only a few reached 1”. Because of the relatively small displacements recorded during testing, the majority of the ACIP’s top load was carried by skin friction, where the maximum recorded tip resistance was 30% of top load, and occurred in the instance where the pile underwent 1.2” of top displacement. Current practice in the State is to design ACIP to carry the load in side friction only. The procedure has been adopted to address the load transfer characteristics of the relatively thin elements (when compared to drilled shafts for example), avoid issues with punching shear when tipping near the bottom of a calcareous bearing layer (e.g., Limestone), and to avoid uncertainty associated with the condition of the bottom of excavation, since neither manual (e.g., weighted tape, or Ding inspection device) nor visual (Shaft Inspection Device) monitoring can be performed. Consequently, the focus of this research effort concentrated on the development of LRFD resistance factors for side shear. Using the results of instrumented piles, and a segmental numerical analysis for un-instrumented load tests (i.e., no strain gauges embedded in the piles), the nominal side friction for soils (Cohesionless and Cohesive) and rock (Florida Limestone) was assessed for each site. For the calibration, various methods that make use of Standard Penetration Test (SPT) or unconfined compressive strength and splitting tension were analyzed and compared against the load test database. The bias, λ, and coefficient of Variation, CV of the measured/predicted side resistances were assessed, and the First Order Second Moment (FOSM) approach was followed to generate resistance factors for each design method. In the case of soils, the Federal Highway Administration (FHWA) (1999) approaches gave the higher Φ, and Φ/λ values vs. newer methods. In the case of ACIPs in rock, design methods based on laboratory strength data (Ramos, Herrera, and Florida Department of Transportation (FDOT)) gave the higher Φ, and Φ/λ values vs. SPT N design methods. Given the limited number of fully instrumented piles, vertical movement of the piles, and distance of boring data from load tests, the report recommends that FDOT perform further static load tests on sites using ACIPs. It is expected that with the addition of more instrumented load tests and nearby boring data, that the LRFD Φ for ACIPs could be further evaluated with Bayesian updating (Kwak, 2010).

Language

  • English

Media Info

  • Media Type: Digital/other
  • Edition: Final Report
  • Features: Figures; References; Tables;
  • Pagination: 164p

Subject/Index Terms

Filing Info

  • Accession Number: 01594584
  • Record Type: Publication
  • Report/Paper Numbers: UF Project No: 113180
  • Contract Numbers: BDV31-977-12
  • Files: TRIS, ATRI, USDOT, STATEDOT
  • Created Date: Mar 17 2016 7:55AM