The Economic Benefits of Testing Driven Piles Sponsored

Read an article by Patrick Hannigan, President of GRL Engineers Inc., and Brent Robinson, Vice President of Pile Dynamics Inc., to learn how using reliable testing methods can save money for pile driving projects.

We have all heard grumbles that the only thing testing does is increase driven pile foundation cost. In actuality, several modern codes reward the increased reliability and reduced uncertainty obtained through more reliable testing methods with higher resistance factors in LRFD (Load and Resistance Factor Design) codes or lower factors of safety in ASD (Allowable Stress Design) based codes. Two good examples of this philosophy are incorporated into the AASHTO LRFD Bridge Design Specifications (2020) which govern highway design practice in the USA, and the Australian Standard, AS 2159-2009: Piling - Design and Installation (2009). The codified testing philosophy in these documents saves money by lower pile capacity and thus shorter piles, or higher loads per pile resulting in fewer piles for the project.

The AASHTO LRFD Bridge Design Specifications state that the uncertainty in “nominal resistance” (e.g., ultimate capacity) is strictly due to the reliability of the resistance determination method used in the field during installation. AASHTO Table 10.5.5.2.3-1 (Figure 1) lists resistance factors for different driven pile resistance determination methods. Piles could also be driven to a predetermined depth based on the AASHTO resistance factor for the static analysis method. However, the AASHTO resistance factors for static analysis methods range from 0.35 to 0.50 depending on the soil conditions and analysis method which are substantially less reliable than most field methods.

*Dynamic testing requires signal matching, and best estimates of nominal resistance are made from a restrike. Dynamic tests are calibrated to the static load test, when available

In the Australian Piling Standard, a geotechnical strength reduction factor, Ø_g , is determined based on a site’s average risk rating, foundation redundancy, the percentage of piles tested, and the selected test type. As shown in the Figure 2, the geotechnical strength reduction factor for a moderate risk site ranges from 0.62 to 0.90 depending on test type, percentage of piles tested and group redundancy. Hence, the higher the percentage of piles tested (up to a limiting value), the greater portion of the geotechnical strength that can be used for the driven pile foundation.

Using the AASHTO code as an example, consider one site condition that covers five substructure locations each containing 18 piles. The soil conditions for these five substructures are similar. Figure 3 presents the soil conditions along with the nominal and factored resistance versus penetration depth for a 16 inch (406 mm) diameter closed-end pipe pile. The foundation piles have a required factored resistance of 600 kips (2,667 kN) per pile. The estimated pile penetration depth for the required nominal resistance varies from 110 ft (33.5 m) if a static load test and 2% PDA testing (minimum 2 piles) (Ø_dyn = 0.80) is performed, to 137 ft (41.8 m), if only the Gates dynamic formula (Ø_dyn = 0.40) is used.

The total foundation cost of a given field testing method can be quantified based on the total pile length costs, testing method costs, and the schedule impact cost associated with the testing method. These costs will vary depending upon a project’s geotechnical conditions and associated design construction, and contract delivery costs.

However, these estimated costs can be compiled to determine the economic impact of a given field testing method on a driven pile project. As observed in Figure 4, testing, when rationally selected, will usually reduce, rather than increase, the total foundation cost.

For additional information about Dynamic Testing Driven Piles, visit www.grlengineers.com. 

Find out more about Dynamic Testing Driven Piles