Section 6: Driven Piling

Overview

Conduct geotechnical design of driven pile foundations, and all related considerations, per AASHTO LRFD Bridge Design Specifications Article 10.7. Piling design should consider skin friction and may consider point bearing as well. Because piling has small tip areas and is generally placed in softer soil, the point bearing contribution is modest and is often disregarded in static design. Exception should be made when tipping a driven pile into bedrock or very competent, hard founding material. In these cases, end bearing is crucial, and piles could be designed exclusively from tip resistance or using a combination of tip and side (depending on pile type used).
Technical specifics of many common driven pile types can be found in FHWA GEC-12 Design and Construction of Driven Pile Foundations – Volume I (
FHWA-NHI-16-009
), Chapter 6, Section 6.1. These can be used when making preliminary pile type selection in design.
Driven piles are not designed nor accepted based solely on static analysis. The nominal bearing resistance of all driven piles must be calculated and accepted based on Hammer Formula (Item 404, Section 3.5, for dynamic bearing resistance), wave equation analysis (e.g., drivability analysis and final driving acceptance criteria by GRLWEAP), dynamic measurements with signal matching (PDA/CAPWAP), or full-scale load testing results.
On refusal, assume that the piling has developed the maximum allowable service load for the pile. If required, perform a drivability study to establish a hammer & driving system that can install the pile without overstressing. Include this study in Geotechnical Design Report.
Design based on process outlined in Section 1 with additional steps:
  • Specify resistance factors to use based on field methods to use dynamic formulas for pile acceptance (or refer to Item 404 Section 3.5 specification for Hammer Formula Method of Bearing Evaluation to determine the allowable dynamic bearing resistance).
  • Perform a pile drivability analysis to obtain required tip elevations (
    if required
    ). Pile acceptance based on the pile driving analyzer (PDA) is for projects where it is cost effective on large number of friction piles or where high pile driving stresses are predicted and require monitoring
Pile design resistance should meet or exceed the requirements specified for each limit state, both in static analysis and dynamically.
Design with caution when designing piling in areas with shallow hard or dense soils. If piling cannot be driven through these areas, the contractor will need to pilot hole or jet the piling to achieve the desired penetration. Jetting should avoid an area with existing foundations and utilities. Excessive pilot holes and jetting may affect the foundation capacity.

LRFD General Design

Use Resistance Factors for Driven Pile found in AASHTO 9th Ed. Table 10.5.5.2.3-1. For soils side resistance in static design, use the Norland/Thurman method in cohesionless soils in accordance with AASHTO LRFD Bridge Design Specifications Article 10.7.3.8.6f, and the a-method in cohesive soils in accordance with AASHTO LRFD Bridge Design Specifications Article 10.7.3.8.6b.
TxDOT utilizes static analysis for design and dynamic analysis for acceptance of driven pile foundations.

Resistance Factors

Use Resistance Factors for driven piles found in AASHTO 9th Ed. Table 10.5.5.2.3-1 for Strength and Extreme limit states.
Load testing and dynamic testing can be used to increase a resistance factor more than using Hammer Equations alone.

Pile Static Design

General components of static analyses to consider are:
  1. Nominal resistance in
    axial compression
     of a single pile or pile group. These calculation methods are used to determine the long-term resistance of the foundation and soil resistance subject to scour, downdrag, or events in the long term. Static analyses are used to establish minimum pile penetration requirements, lengths for bid quantities, and estimates of soil resistance at the time of driving (SRD) and the required nominal driving resistance (R
    ndr
    ).
  2. Nominal resistance in
    axial tension
     of a single pile or pile group. These calculations are performed to determine the soil resistance to uplift or tension loading.
  3. Nominal
    lateral
    resistance and lateral deformation of a single pile or pile group. These soilstructure interaction analysis methods consider the soil strength and deformation behavior as well as pile structural properties and are used in pile type selection.
  4. Settlement
    of a pile group. These calculations are performed to determine the vertical foundation deformation under the structure service loads.

Pile Dynamic Design

High pile stresses occur during pile driving operations. A pile drivability analysis is typically used to determine the nominal geotechnical resistance a pile can be driven to without pile structural damage.
The Hammer Equation found in Item 404 is used to determine acceptance criteria (for final embedment and length). Where piles are driven to higher resistances or where high pile driving stress is a concern (i.e., short, end bearing piles), the wave equation analysis (through GRLWEAP) should be used for drivability and final driving acceptance. In cases where high pile driving stress is predicted and require monitoring, consider using pile driving analyzer (PDA) with wave analysis (through program such as CAPWAP).

Dynamic Monitoring

Dynamic monitoring of a pile during driving can be accomplished using a Pile Driving Analyzer (PDA) testing system. PDA testing measures the strain and acceleration in the pile as a result of the impact of the hammer. PDA testing of a pile can help to determine the stresses in the pile during driving and monitor the pile for damage or integrity. The capacity of the pile and time dependent changes in capacity (if a restrike is undertaken) can be obtained when the PDA testing data is used with the Case Pile Wave Analysis Program (CAPWAP).
For critical structures, projects with a large number of piling, or in difficult soil conditions PDA testing should be considered for use. Consult with the Geotechnical Branch to determine if a specific project might be considered as a candidate for PDA testing.

Pile Tip Elevations

To ensure constructed foundation meets the design requirements, pile tip elevations or pile lengths are required on the contract plans. As noted in section 1: Design Process, the final length and tip elevation may be controlled by any or all of the following criteria:
  • Pile tip to reach designated bearing layer
  • Scour
  • Downdrag
  • Uplift
  • Lateral Loads

Difficult Driving and Drivability

If it is necessary to advance the piling through a strong or stiff layer where refusal is possible, an additional pile penetration note as follows may be required, "The contractor’s attention is drawn to the hard material in the soil profile, jetting and/or pilot holes may be necessary to advance the piling to the required penetration depth.”
Be aware that under these conditions of potentially high driving stresses, a wave equation drivability analysis is necessary to ensure piles can be driven to required embedment depth. Higher grade steel can be specified if needed to meet drivability criteria. Coordinate any changes in the pile size, section, or tip elevations with the structural engineer. The geotechnical foundation engineer is responsible for reevaluating pile drivability during this iterative process.
Candidate pile types that cannot be driven to the required nominal resistance and/or minimum pile penetration without exceeding material stress limits and within a reasonable blow count of 30 to 120 blows per foot with appropriately sized driving systems should be eliminated from consideration. 120 blows per foot or 10 blows per inch is often considered refusal driving conditions by many hammer manufacturers.

Pile Setup and Restrike

Using a waiting period and restrike after initial pile diving may be advantageous in certain soil conditions to optimize pile foundation design. Setup for a specified waiting period allows pore water pressures to dissipate and soil strength to increase. Restriking then confirms if higher nominal resistance is achieved. The length of the waiting period depends on the strength and drainage characteristics of the subsurface soils, and the required nominal resistance. Refer to Standard Specification 404 for additional pile driving construction criteria.

Wing Wall Piling

Found and tip wing wall piling in similar founding material as abutment cap piles to minimize the potential for differential settlement.

Steel Piling Special Considerations

  • Corrosion:
    Steel piles driven through contaminated soil and groundwater conditions may be subject to high corrosion rates and should be designed appropriately through the use of larger section, galvanization or concrete cover. Corrosion may occur if piles are driven into disturbed ground, landfills or cinder fills, or low pH soils. Corrosion should also be evaluated for piles located in marine environment, or if piles are subject to alternate wetting and drying from tidal action. Rates are a function of the ambient temperature, pH, access to oxygen, and chemistry of the aqueous environment in contact with the steel member(s).
  • Grade Separations:
    Foundation elements for grade separations are subject to potential vehicular impact. The use of steel sections in a trestle configuration in those potential impact zones is highly discouraged. Instead, steel H piling can potentially be used under pile footings for interior bents or abutments at grade separations.
  • Water Crossings:
    Foundation elements for crossings over waterways are subject to scour, drift impact and have a higher propensity for corrosion. Steel piling needs to be analyzed for potential corrosion over the life span of the structure and need to be evaluated for both axial and lateral loadings under the scoured condition. Steel piling that have been evaluated for the above conditions and found to be acceptable could be used for trestle bents. However, the steel piling must be coated to a minimum depth of 15 feet below the maximum predicted scour elevation. Steel piling can be used to support pile footings as long as the footing is embedded at a depth below the maximum predicted scour depth thus minimizing the risk of exposure. Piling used in a footing configuration must be coated a minimum distance of 15' below the bottom of footing. Piling can be used for foundation elements for abutments.

Service Loads

See the following table for maximum piling length and structural loads recommended without conducting a detailed structural analysis. Many soils are not capable of developing these maximum loads. Before final structural design, conduct foundation design using site specific soil information to verify the ability of subsurface to provide resistance to the loading.
Table 5-2: Maximum Allowable Precast Concrete Pile Service Loads
Size
Maximum Length
Abutments and Trestle Bents
Footings (per Pile)
16 in. Square
85 ft.
75 ton
125 tons
18 in. Square
95 ft.
90 tons
175 tons
20 in. Square
105 ft.
110 tons
225 tons
24 in. Square
125 ft.
140 tons
300 tons

Pile Lateral Resistance

Pile foundations are subjected to horizontal loads due to wind, traffic loads, bridge curvature, and vessel or traffic impact. Evaluate the nominal resistance of pile foundation to horizontal loads based on both subsurface strata and structural properties.
Refer to AASHTO LRFD Bridge Design Specifications Article 10.7.2.4 for detailed requirements regarding determination of lateral resistance. Use a minimum spacing of 3 pile diameters (3D) to the extent possible. Should closer spacing be required due to geometric constraints, the following P
m
 values may be used at spacing 3D to 2D in accordance with Article 10.7.2.4:
  • For Row 1, P
    m
     = 0.45
  • For Row 2, P
    m
     = 0.33
  • For Row 3 and higher, P
    m
     = 0.25

Pile Foundation Design Reporting

Include the following in Geotechnical Design Reports for driven pile foundation design:
  • Geotechnical Data Report and Borings (see Chapter 4)
  • Summary of proposed construction, factored foundation loads, applicable limit states, performance criteria (settlements, lateral deformation)
  • Scour and hydraulic assumptions
  • Applicable site constraints including any suspected environmental restrictions, utility conflicts, adjacent structures, or limitations on construction (ROW, headroom, etc.)
  • Summary of soil and bedrock and intermediate geomaterial parameters and design analysis
  • Recommendations for ground improvement to increase bearing resistance and reduce settlement
  • Description of design procedures with summary of results and explanation of interpretation, particularly:
    • Pile tip elevations or estimated pile lengths
    • Minimum pile penetration (see AASHTO LRFD Bridge Design Specifications Article 10.7.7)
    • Pile driving requirements (hammer size, sequence, etc)
    • Nominal driving resistance and resistance determination method (driving criteria) (see AASHTO LRFD Bridge Design Specifications Table 10.5.5.2.3-1)
    • Corrosion effects or chemical/biological attack susceptibility
    • Specified load testing requirements or test piles
    • Expanded analysis preformed such as drawdown with neutral plane axis, settlement estimates, and lateral load resistance and deformation.
  • Construction recommendations and recommendations for notes required on contract drawings/plans
  • Signed and sealed by the Engineer