Joining industry peers at the NZTA Bridge and Geotechnical Conference 2024, brings together the leading minds in the structural and geotechnical engineering segments of the NZ infrastructure sector, to showcase projects and to demonstrate the application of best practice in infrastructure.
Raathiv Shanmuganathan Associate Engineer presented a technical paper ‘Recommendation of design parameters for driven steel piles in ignimbrite from dynamic pile load testing’. Here he shares insights into his paper which was coauthored by Senior Consultant, Harry Poulos.
The Waikato Expressway is among the seven Roads of National Significance designated by the New Zealand Transport Agency (NZTA). Tetra Tech Coffey, as part of the City Edge Alliance (CEA) contracted by Waka Kotahi, was tasked with designing and constructing the Hamilton Section of the expressway. This involved building several bridges across streams and local roads to connect the 22 km motorway. These bridges were predominantly supported by bottom-driven steel tube piles founded into the Walton subgroup layer. The Hamilton Basin is divided into two geological regions: the Hamilton Hills and the Lowlands. The pile founding layer in the Hamilton Hills is composed of non-welded ignimbrite, described as silty sand with minor gravels, whereas, in the Lowlands, it consists of silty sand with interbedded alluvial deposits.
What were the primary design and construction challenges?
One significant challenge was estimating the pile termination depth in old ignimbrite layer which consists of interbedded alluvial layers and heterogeneous geology. In addition, the crushable pumice in the ignimbrite layers cause significant reduction in the bearing capacity of the piles.
Tell us about the solution which was used in the construction.
To verify the axial vertical capacity of the driven steel piles, several high-strain dynamic load tests were conducted at each bridge abutment and pier location using the Pile Driving Analyzer (PDA) method. Sensors installed at the top of the piles estimated the base bearing and shaft friction of the test piles. Hiley’s formula was used during the construction to correlate the pile capacity derived from PDA testing and establish the termination criteria for the remaining piles. Additionally, multiple deep boreholes were drilled near the abutment and pier locations to study the geology of the founding layers during the design stage. It was noted that the geological layers at the pile founding sites in Hamilton Hills and Hamilton Lowlands differ. The required pile length to achieve the minimum necessary axial capacity also varies significantly between these two regions.
How was this study conducted?
Two common methods, the standard penetration test (SPT) and the effective stress method, are used to correlate the pile end bearing and shaft friction in this study. We analyzed the correlation between the end bearing and shaft friction of piles derived from PDA testing with nearby SPT N values and effective stress estimates. Nearby deep investigation data and as-built construction drawings/ details were used to calculate the parameters. The correlation for non-welded ignimbrite revealed unique results, deviating slightly from standard industry practices. This presentation provides recommendations for the founding layer and depth for heavily loaded bridge piles in both terranes to achieve maximum axial capacity. It also suggests the most suitable correlations for end bearing and shaft friction of driven piles using effective stress and SPT N values for piles founded in the non-welded ignimbrite, Walton subgroup layer.
What were the outcomes of this study?
The study shows that the standard formulas and charts are overestimating the end bearing and shaft friction of the piles in ignimbrite layers. Ignimbrite has pumice and is crushable, hence standard available formulas/ methods can’t apply directly. New correlations and recommendations derived from this study may be useful to design a driven pile in ignimbrite layers in the future projects.
Presented at The NZTA Bridge and Geotechnical Conference 2024, 8-9 July 2024 Claudelands, Hamilton, New Zealand.
Paper title: “Recommendation of design parameters for driven steel piles in ignimbrite from dynamic pile load testing”
Authors: Raathiv Shanmuganathan Tetra Tech Coffey, Associate Geotechnical Engineer, Auckland, New Zealand. Email – [email protected]
Harry Poulos Tetra Tech Coffey, Senior Consultant, Sydney, Australia. Email – [email protected]