BACKGROUND

All state departments of transportation (DOTs) use the AASHTO LRFD Bridge Design Specifications as standards for design and construction of bridges in public infrastructure projects. However, various state DOTs have developed their own state-specific standards based on experience and differences in local/regional geology. For example, design of drilled shaft foundations varies widely across the United States, as do the construction methods associated therewith. This variation also extends to the various elements of the construction methods used for maintaining a stable drilled shaft excavation.

Some state DOTs have been conservative in their design approach and have discounted partially or completely the estimated friction resistance for shaft installations due to concerns about the effects of the various installation methods. Different types of installation methods, use of steel casing, and/or drilling support fluid may have significant effects on geotechnical load transfer characteristics and long-term performance.

During the past 40 years, design demands have increased with the magnitude of axial, lateral, and flexural loadings, in part due to larger superstructures, increased awareness to extreme events, and associated updated design codes. Concurrently, the diameter and depths to which drilled shafts can be constructed have grown substantially owing to advancements in equipment technology. As the methods and equipment have evolved, there remain concerns about how the various installation methods affect the design and long-term performance of drilled shafts.

Given the widespread use of drilled shaft foundations, research is needed to study the effect of construction installation methods on design and performance.

OBJECTIVE

The objective of this research is to quantify the effects of construction installation methods on the design and performance of drilled shaft foundations for different subsurface conditions and geologic materials. 

RESEARCH PLAN

The research plan should (1) include a kick-off teleconference with the research team and NCHRP convened within 1 month of the contract’s execution; (2) address how the proposer intends to satisfy the project objective; (3) be divided logically into detailed tasks necessary to fulfill the research objective and include appropriate milestones and interim deliverables; and (4) incorporate opportunities for the project panel to review, comment on, and approve milestone deliverables.

Accomplishment of the project objective will require at least the following tasks.

TASKS

PHASE I — Planning

Task 1. Conduct literature review and compile full scale drilled shaft load testing results into a searchable database.

Task 1a. Conduct a literature review of relevant research and current state-of-practice related to the effects of construction installation methods on the design and performance of drilled shaft foundations. The review shall include published and unpublished research conducted through the NCHRP; Federal Highway Administration (FHWA); and other national, international, state, and pooled-fund sponsored research. The review shall consider drilled shaft foundations in other industries such as commercial construction and utilities.

Task 1b. Compile load testing results for drilled shaft foundations with large diameters. It is desirable that a minimum of 10 drilled shaft load tests be included in the searchable database. Determine metrics and quality control procedures for the load testing procedures and their corresponding construction methods.

Task 2. Identify proposed areas of the AASHTO LRFD Bridge Specifications to develop draft language for consideration by AASHTO.  

Task 3. Develop a methodology to achieve the research objective. At a minimum, the research team shall:

·       Develop installation methods and subsurface conditions matrix and recommend which drilled shaft installation methods and accompanying subsurface conditions should be evaluated in detail in Phase II.

·       Develop matrix of installation methods and subsurface conditions. The matrix shall be developed from Tasks 1a and 1b findings, focus on installation methods and performance, and allow all major installation methods and subsurface conditions to be considered together.

·       Select the set of methods and conditions, and identify equipment and quality control measures for the detailed study. Using the installation methods and subsurface conditions in the matrix, the research team shall select a smaller set of methods and conditions for detailed study in Phase II.

Task 4. Prepare Interim Report No. 1 that documents Tasks 1 through 3 and provides an updated work plan for the remainder of the research. This report must be submitted to NCHRP no later than 6 months after contract execution. The updated work plan must describe the process and rationale for the work proposed for Phases II through IV.

PHASE II — Matrix Validation

Task 5.  Execute the methodology according to the approved Interim Report No.1.

Task 6. Conduct the study on a set of drilled shaft installation methods and subsurface conditions. This study shall quantify the friction resistance associated with each installation method and its corresponding subsurface conditions. This study may include full-scale tests or physical models. It may also consist of numerical modeling or other research tools.

Task 7. Prepare Interim Report No. 2 that documents Tasks 5 and 6 no later than 9 months after approval of Phase II. The updated work plan must describe the work proposed for Phases III through IV.

PHASE III — Development of the AASHTO Deliverable 

Task 8. Provide draft language for consideration by AASHTO in their next revision of the AASHTO LRFD Bridge Design Specifications for the geotechnical design of large diameter drilled shafts, accounting for the effects of construction installation methods thereon, hereafter called the AASHTO Deliverable, supported with examples.

Task 9. Prepare Interim Report No. 3 that documents Task 8 no later than 9 months after approval of Phase II. The updated work plan must describe the work proposed for Phase IV.

PHASE IV— Final Products

Task 10. Present AASHTO Deliverable to the AASHTO Committee on Bridges and Structures for comments and propose any revisions to NCHRP. Revise draft language considering the NCHRP’s review comments and prepare the draft final deliverables listed in Task 11 no later than 3 months after approval of Phase III.

Task 11. Prepare a final deliverable that documents the entire research effort. Final deliverables shall include, at a minimum (1) a research report documenting the work performed, (2) the AASHTO Deliverable, (3) presentation material, and (4) technical memorandum on implementation.