Chapter 5: Foundation Design

Section 1: Design Methodology

Overview

Current TxDOT practice is to use the Load and Resistance Factor Design (LRFD) methodology for foundation design whenever practical and in accordance with AASHTO LRFD Bridge Design Specifications (current edition) and applicable Federal Highway Administration (FHWA) reference materials. This reliability-based design methodology (compared to the former TCP driven design correlations) creates greater utility for the state by accounting for a uniform level of reliability due to multiple factors and allowing for local calibration of factors depending on level of confidence through research. The basic equation for this method is:
∑ηiγiQi ≤ ϕR
n
 = R
r
Where: η
i
 = a factor that includes the effects of ductility, redundancy, and importance
γ
i
 = the load factor for a particular load
Q
i
 = a service level load
ϕ = the resistance factor
R
n
 = the nominal (i.e., ultimate) resistance
R
r
 = the factored resistance
Proper foundation design requires communication between the geotechnical engineer and the structural engineer with consideration of data collected to address what information is needed along with when and how information will be exchanged.

Loading and Resistance

Substructure elements are designed to carry all the loads specified in AASHTO LRFD Bridge Design Specifications and the TxDOT Bridge Design Manual-LRFD. Selecting the controlling load conditions requires good judgment and coordination with the bridge engineer.
In accordance with AASHTO LRFD Bridge Design Specifications 3.6.2, neglect the dynamic load allowance on foundation components completely buried.
Evaluate structural resistance in accordance with the TxDOT Bridge Design Manual-LRFD. Evaluate geotechnical resistance according to criteria in this chapter.

Service Limit States

Include the following service limit states for foundation design:
  • Settlement,
  • Horizontal movements,
  • Overall stability, and
  • Scour at the design flood.
Foundation movement criteria shall be consistent with the function and type of structure, anticipated service life, and consequences of unacceptable movements on structure performance. Consideration of foundation movements shall be based upon structure tolerance to total and differential movements, rideability and economy. Foundation movements shall include all movement from settlement, horizontal movement, and rotation. The tolerable movement criteria shall be established by empirical procedures and/or structural analyses.
Evaluate foundation settlement, horizontal movement, and rotation using applicable loads in the Service I Load Combination specified in AASHTO LRFD Bridge Design Specifications Table 3.4.1-1. Transient loads may be omitted from settlement analyses for foundations bearing on or in cohesive soils that are subject to time-dependent consolidation settlement.

Strength Limit States

Evaluate the nominal foundation geotechnical resistance at the strength limit state considering the following:
  • Axial compression resistance,
  • Axial uplift resistance,
  • Punching of shafts or piles through strong soil into a weak layer,
  • Lateral geotechnical resistance of soil and rock strata,
  • Resistance when scour occurs,
  • Axial resistance of the structural element when downdrag may occur, and
  • Pile drivability and driving stresses (for driven piles only).

Extreme Event Limit States

Structures must remain stable for an Extreme Event II limit state that considers scour due to the check flood required by the TxDOT Hydraulic Design Manual. This limit state need not include ice loads, vehicle collision loads, and vessel collision loads simultaneously. See Section 8 of this Chapter for additional information regarding scour analysis.
See the TxDOT Bridge Design Manual-LRFD for structures requiring consideration of earthquake effects.

Constructability

Design of foundations must consider the effects of the anticipated method of construction, including the construction sequencing. Such considerations shall consist of, but not be limited to: the need for shoring, the use of cofferdams, tremie seals, dewatering, excavation stability, downdrag considerations, and the need for permanent or temporary casing for drilled shafts or micropiles.

Design Process

Typical design steps are as follows:
  1. . Establish design requirements for layout/geometry, loading, scour depths, tolerance to settlement (see recommendations above) and other service deformation/deflection
  2. Determine depth of scour and hydraulic requirements of the structure in coordination with the hydraulic engineer
  3. . Conduct geotechnical investigation (see Chapters 2, 3, and 4)
  4. Select most appropriate foundation type and shaft/pile diameter(s) in coordination with structure designer
  5. Evaluate need for permanent casing at individual foundations
  6. Calculate nominal (unfactored) resistance of single drilled shafts or static compressive resistance (for piles) as a function of depth
  7. Apply resistance factors to nominal axial resistance for strength and extreme limit states. Driven piles require additional resistance factors to be used during dynamic analysis based on field method to be used for pile acceptance (e.g., Hammer Formulas, wave equation, high strain dynamic load testing, etc.)
  8. Conduct more extensive, nonstandard design required if deemed from subsurface conditions, bridge geometry, lateral loading, or service level criteria:
    1. Estimate downdrag potential and downdrag loads
    2. Check service level loads for shaft/pile single vs. group settlement as a function of depth (to maximum permissible settlement criteria)
    3. Check for uplift resistance as a function of depth
    4. Use P-Y curve parameters and horizontal movements in strength/extreme limit states to check for pushover/global/fixity. P-multipliers are not required for shaft/pile groups installed in rock sockets and lateral displacements are minimal (i.e., < 0.5 inches, or < 10% of shaft diameter)
    5. Structural engineer evaluates applied lateral loads at the strength limit state using soil parameters determined by the geotechnical engineer.
    6. Service level checks using unfactored service loading for top of shaft/pile deflection, including influence or downdrag loads if present, and effect of lateral squeeze and lateral deformations
  9. Enter final parameters coordinated with structural analysis into plans and contract (with construction notes).
    1. Pile driving foundations can contain notes to perform pile drivability analysis and testing to obtain final required tip elevation or details for pile tip reinforcement
    2. Field control methods (such as integrity testing) can be included in notes and quantities

Lateral

Lateral depth checks and resistance should be considered depending on the height of the column, proposed substructure elements, and span configuration. Typical first checks on section involve the maximum moment and shear at top of shaft to determine the depth to the 2nd zero in the service load case, or the depth at which lateral defection at the top of shaft or pile is not affected by increased foundation depth in the strength load case.

Group Effects

Design close drilled shaft or closely spaced driven piles using group effect factors per AASHTO LRFD Bridge Design Specifications Article 10.8.3.6.