Effects of Construction Installation Methods on the Design and Performance of Drilled Shaft Foundations

Some state DOTs have been conservative in their design approach and have discounted partially or completely either the estimated side or end resistance due to concerns with the effects of the various installation methods. For example, AASHTO (2014), Section 10.8, provides minimal guidance or commentary on using casing reduction factors to estimate the side friction of a cased drilled shaft. The predicted values of axial resistance for drilled shafts should be reduced accordingly when permanent steel casing is to be used along the partial or entire length of the element. AASHTO LRFD Bridge Design Specifications (2014), Section 10.8.3.5.2b (side resistance), provides the following: “Steel casing will generally reduce the side resistance of a shaft. No specific data is available regarding the reduction in skin friction resulting from the use of permanent casing relative concrete placed directly against the soil…Casing reduction factors of 0.6 to 0.75 are commonly used. Greater reduction in the side resistance may be needed if oversized cutting shoes or splicing rings are used.” For some state DOTs, the recommended factors vary from the range presented in the above-referenced section of AASHTO; for others, the factors are considerably lower or even zero, thereby eliminating any contribution from side resistance. During the past three to four decades, design demands have increased as the magnitude of axial, lateral, and flexural loadings have considerably increased due to larger superstructures, updated design codes, etc. As such, the diameter and depths to which drilled shafts can be constructed have increased substantially during this same time as a result of advancements in equipment and tooling technology and capabilities. As the methods, equipment, and tooling have evolved, there are remaining concerns regarding how the various installation methods affect the design (i.e., load transfer characteristics via side and end resistance) and the long-term performance of drilled shafts. Experience has demonstrated that different types of installation methods, use of steel casing, and/or the drilling support fluid (i.e., slurry) may have significant effects on geotechnical load transfer characteristics (e.g., side and end resistance) and long-term performance. Given the widespread use and dependency on drilled shaft foundations, ongoing concerns regarding the effect of construction installation methods warrant a focused research investigation. Moreover, there are other construction effects that are just being recognized in the industry, such as the effect of a natural-synthetic (i.e., bentonite-polymer) blend of drilling support fluid (slurry) and the influence of rotator/oscillator-installed steel casing. FHWA GEC-10 provides emphasis on the importance of understanding the various construction methods used to install drilled shaft foundations, noting the effective use and design of drilled shafts requires knowledge of the construction methods used for these foundation elements. Drilled shaft construction is sensitive to the ground conditions encountered at the site, and the costs and magnitude of effort involved are closely tied to the ground conditions and the construction techniques that must be used for a particular circumstance. Performance is related to the effectiveness of the construction technique in preserving the integrity of the bearing materials and ensuring the structural integrity of the cast-in-place reinforced concrete drilled shaft foundation. AASHTO LRFD Bridge Design Specifications (2014), Section C10.5.3.4, further reinforces the position statement in FHWA’s GEC-10, noting the design of drilled shafts … should include the effects of the method of construction, including construction sequencing, whether the shaft will be excavated in the dry or if wet methods must be used, as well as the need for temporary or permanent casing to control caving ground conditions. The design assumptions regarding construction methods must carry through to the contract documents to provide assurance that the geotechnical and structural resistance used for design will be provided by the constructed product. The main objectives of the research program are to quantify the installation effects on side and end resistances for different types of drilled shaft foundations (e.g., uncased, cast in drilled hole [CIDH], and cast in steel shell [CISS]) in different soil conditions due to various installation methods (e.g., dry method vs. wet method; natural vs. synthetic vs. blend drilling support fluids; temporary vs. permanent steel casing; driven vs. drilled vs. spun into the ground; rotary drilling methods vs. vibratory/impact hammer vs. grab methods; etc.). Ultimately, a report will be developed that will present how the various construction techniques affect design, which will form the basis for design guidance for state DOTs regarding technique-dependent installation effects.

Language

  • English

Project

  • Status: Proposed
  • Funding: $600000
  • Contract Numbers:

    Project 24-51

  • Sponsor Organizations:

    National Cooperative Highway Research Program

    Transportation Research Board
    500 Fifth Street, NW
    Washington, DC  United States  20001

    American Association of State Highway and Transportation Officials (AASHTO)

    444 North Capitol Street, NW
    Washington, DC  United States  20001

    Federal Highway Administration

    1200 New Jersey Avenue, SE
    Washington, DC  United States  20590
  • Project Managers:

    McKenney, Christopher

  • Start Date: 20210511
  • Expected Completion Date: 0
  • Actual Completion Date: 0

Subject/Index Terms

Filing Info

  • Accession Number: 01739658
  • Record Type: Research project
  • Source Agency: Transportation Research Board
  • Contract Numbers: Project 24-51
  • Files: TRB, RIP
  • Created Date: May 18 2020 3:05PM