Pile driveability analysis is typically about assessing the feasibility of driving a contemplated pile design with a given piling hammer through a soil profile with more or less specified characteristics.
Pile Driveability Analysis Provides Information About:
Whether a given pile driving scenario given a certain combination of piling hammer, pile makeup and soil characteristics is feasible in terms of e.g., blow-counts (or penetration speed), pile material stresses and total number of blows (or installation time).
The implications of altering hammer capacity or type
The implications of altering pile design, e.g., pile material, -cross section, open- vs. close end.
Potential causes of pile driving problems concerned with e.g., piling hammer capacity vs. observed driving resistance, restart of driving after pile setup and pile material/-joint stresses during driving.
The magnitude of pile driving induced fatigue by applying the calculated pile material stress ranges and number of blows for installation to fatigue design parameters and formulas.
Principles of the Analysis Model
Pile drivability analyses can be performed with different dynamic analysis tools, but it is a prerequisite that they shall be able to model the stress wave propagation (including reflections) throughout the pile.
Two leading providers of pile driveability analysis software are:
GRLWEAP for which the development was initially supported by the US highway authorities and later converted into commercial software by GRL/PDI and
Allwave (and similar preceding software) which has been driven by the development of the Dutch offshore fields.
Both software packages are widely used and recognized in the engineering, pile testing and pile driving industry and facilitate both driveability prognosis and efficient extraction of stress range and -cycle data for fatigue analysis.
A pile driveability analysis is based on a dynamic model of the hammer, the pile, and the soil profile.
The hammer and the pile are represented by masses and springs constants. Specific hammer data are typically taken from the software databases and should be reconciled with equipment manufacturer’s specifications whenever possible.
Pile data are typically taken from specific project documentation and assigned the appropriate pile material elastic modulus, specific weights, length, and cross section properties.
The soil profile is represented by forces (resistance) acting along the pile axis and at the pile toe. For each of the soil units the following parameters shall be specified, which are rarely stated in geotechnical reports and must be assigned by means of particular guidelines or from experience:
Ultimate shear- and toe resistance,
Required axial movement of the pile relative to the surrounding soil where the (ideal) linear elastic behaviour changes to (ideal) plastic behaviour for respectively the pile shaft- and toe resistance. In the context of pile driveability this value is termed as “quake”,
An appropriate soil damping factor for respectively the pile shaft- and toe,
An inverse soil setup factor to account for the remoulding effects during pile driving for the pile shaft resistance.
Typically, three types of analyses are run, which serve different purposes:
The driveability analysis where blow counts and pile material stresses are calculated for a simulated pile driving scenario from the level of pile and hammer self-weight penetration (if applicable) to target depth with ascribed hammer energy settings to emulate a realistic pile driving scenario where the hammer energy is adapted to the actual pile driving resistance. This method is feasible if there are detailed design soil profiles available and the hammer and pile specifics are known.
Where the pile driving resistance can only be estimated with significant uncertainty or for the purposes of assessing the sensitivity of the driveability analysis, a simplified analysis can be carried out, where the corresponding blow counts are calculated for a pre-defined range of pile driving resistances ranging from (lower bound) below the most likely value to (upper bound) beyond the most likely value.
Where the pile driveability model has been calibrated to site specific pile test data, another analysis variant can be prepared for aiding assessment of pile driving by means of a driving criterion to reach a required permanent pile set per blow (or series of blows with a certain piling hammer energy setting). For the required end-of-driving pile driving resistance and a pre-defined range of hammer energy settings, the corresponding blow counts can be calculated, hence provide and efficient verification of whether the driving criterion is fulfilled or not.
Analysis and reporting
As indicated above, the pile driveability analyses can be adapted the individual project in terms of the available site data available and the actual needs.
Typically, an analysis workflow will involve an appraisal of the available site data and on that basis define a number of relevant analysis scenarios to assess the pile driveability issue at hand. While working through scenarios interim findings may render some of the contemplated scenarios irrelevant and call for definitions of alternative scenarios in cooperation with the involved parties of the project.
The requirement for reporting typically varies with the pile driveability analysis assignment at hand and range from a simple feasible/unfeasible e-mail statement for a decision making during e.g., a bidding process to elaborate reporting prepared for third party verification of large energy or traffic infrastructure projects.
It should always be considered good practice to assess the sensitivity of the pile driveability analysis results in particular concerning the assumptions about pile driving resistance, as these are prone to uncertainty even with detailed soil investigation data available. After all, pile driveability analysis is a prognosis, not a measurement performed during a test – but it can be augmented significantly by including site specific test results or experience in the model preparation and verification.
DS/EN 1997-1:2007 Eurocode 7: Geotechnical design – Part 1: General rules.