Abstract
This paper presents a summary of an example of application of pseudo-static analysis in the design of a bridge on a site prone to liquefaction. The example is one of the outcomes from Stage 2 of a research project commissioned by the NZ Transport Agency (NZTA) to develop a comprehensive design guidance for piled bridges for liquefaction and lateral spreading effects. The Stage 1 report, published on the NZTA website in 2014, provides the key design recommendations for bridges located on sites prone to liquefaction and lateral spreading. The Stage 2 report gives detailed procedures for geotechnical field and laboratory testing, liquefaction evaluation methods and examples for two bridge sites, detailed description of recommended procedures for pseudo-static analysis (including flow charts for the design process), overview of dynamic analysis methods and detailed design examples for two bridges. The design requirements and guidelines given in Stage 1 report are to be incorporated in the NZ Transport Agency’s Bridge Manual and disseminated to the wider New Zealand engineering community. A summary of the Stage 2 report is given and the design procedure for the design of two bridges for liquefaction and lateral spreading effects is described in detail.
1 INTRODUCTION
There are many case histories worldwide where extensive damage to piled bridges has been observed due to excessive lateral ground displacements and subsidence associated with liquefaction. A variety of methods is available in the literature for the evaluation of the performance of piled bridges on sites susceptible to liquefaction. With a goal of developing a unified approach consistent with the concept of Performance Based Earthquake Engineering (PBEE), the NZ Transport Agency commissioned a research project towards the development of guidelines for the design and assessment of piled bridges at sites prone to liquefaction and lateral spreading in New Zealand. The first stage of this work has culminated in the NZTA Research Report 553 (2014). This report provides recommendations on pseudo-static analysis procedures to assess the effects of liquefaction and lateral spreading in the design of pile foundations for bridges. These recommendations have recently been incorporated in the 3rd edition of the New Zealand Transport Agency’s Bridge Manual (2016).
The objective of the second stage is to provide practical examples of the use of the recommended procedures for the investigation and assessment of liquefaction and the assessment and design of piled bridges on sites with liquefiable soils. The project team comprised Opus Consultants, University of Canterbury (Prof. Misko Cubrinovski and Dr Jennifer Haskell) and Dr John Wood. Two practical examples are presented in Stage 2 report to demonstrate the Pseudo-Static Analysis (PSA) procedure. The first example illustrates performance evaluation of an existing bridge, ANZAC bridge, a 4-span reinforced concrete bridge crossing the Avon River in Christchurch. The bridge was severely affected by liquefaction and lateral spreading in the September 2010 M7.1 Darfield Earthquake and the February 2011 M6.2 Christchurch Earthquake. The second example demonstrates design of a new 2-span bridge at a grade separated intersection over the four-lane expressway north of Christchurch. This example is summarised in this paper.
2 ANALYSIS FRAMEWORK
The PSA procedures described in NZTA research report 553 (Murashev, 2014) are intended to provide a simplified yet accurate enough analysis tool for routine design or performance evaluation of pile foundations of a bridge. Among various PSA methods described, the displacement based approach developed by Cubrinovski is recommended in the Stage 2 report. In this approach, ground displacements are applied to the piles through bi-linear soil springs. This is consistent with the PBEE framework.
Being a simplified static procedure, three separate sets of analyses are required to capture the peak responses of piles during an earthquake, as recommended in NZTA research report 553 (Murashev et. al., 2014) and in the Bridge Manual (2013). The phases considered in the PSA are:
- Pre-liquefaction.
- Cyclic liquefaction.
- Lateral spreading.
The design is an iterative process that starts with an assumed layout and sizing of the load resisting elements followed by analysis of the piled system using the procedure outlined in Murashev et al. (2014) and then adjustments to the structure and re-analysis until the design meets the performance requirements. The Stage 2 report provides detailed guidance on the analysis procedure and how different parameters are estimated.
3 SITE AND STRUCTURE DESCRIPTION
This example demonstrates the use of the recommended analysis procedure in the design of a new bridge constructed as part of a new expressway between Christchurch and Kaiapoi. The bridge is part of a grade separated intersection taking a two lane local road over a new four lane expressway. The site is a relatively flat site. Approaches to the 53 m long bridge are formed with earth embankments up to 8 m high with spill through slopes at the abutments. The site geology consists of deep alluvial and marine deposits with loose sands and soft organic silt deposits in the upper 9.5 m of the ground profile. The site is within 20 km of the faults that caused the M7.1 Darfield and the M6.3 Christchurch Earthquakes in 2010 and 2011 and experienced peak ground accelerations estimated to be between 0.16 g and 0.22 g in these two events.
The superstructure, as shown in Figure 1, comprises two equal spans (2×26.50m) 16 no, 900mm deep single hollow core units seated on elastomeric bearing strips at the pier and connected integrally to the abutment cap. The foundation system comprises 3 no of 1200mm diameter concrete piles at the pier and 5 no of 900mm diameter concrete piles at each abutment. The pier piles are 24m long and the piles at each abutment are 30m long.
