Pumice materials, which are problematic from engineering viewpoint, are widely spread in many parts of North Island. Following the 2010-2011 Christchurch earthquakes, a clear understanding of their properties under earthquake loading is necessary. For example, the 1987 Edgecumbe Earthquake showed occurrences of localised liquefaction in sands of volcanic origin. The aim of this QuakeCoRE-funded research is to investigate the liquefaction resistance of in-situ pumice deposits through field testing, especially at sites where liquefaction was observed following the Edgecumbe earthquake. Using field-obtained data (CPT, Vs-profile), attempts will be made to explain the occurrence/non-occurrence of liquefaction at the target sites following the earthquake using available empirical chart-based approaches. The applicability of the current field-based empirical approaches in assessing the liquefaction potential of pumiceous deposits will be scrutinized vis-à-vis the observed liquefaction characteristics of pumice sands.
Pumice deposits are found in several areas of the North Island. They originated from a series of volcanic eruptions centred in the Taupo and Rotorua regions, called the “Taupo Volcanic Zone”. The pumice material was initially deposited by eruptions; followed by erosion and river transport. Presently, pumice deposits exist mainly as deep sand layers in river valleys and flood plains, but are also found as coarse gravel deposits in hilly areas. Although they do not cover wide areas, their concentration in river valleys and flood plains means they tend to coincide with areas of considerable human activity and development. Thus, they are frequently encountered in engineering projects and their evaluation is a matter of considerable geotechnical interest.
Previous research at the University of Auckland showed that the penetration resistance (qc) values obtained from cone penetration tests (CPT) on pumice sand were only marginally influenced by the density of the material. The reason for this behaviour is possibly because the stresses imposed by the penetrometer are so severe that particle breakage forms a new material and that the properties of this are nearly independent of the initial state of the sand (Wesley et al. 1999). Thus, conventional relationships between the qc value and relative density, which in turn is correlated with liquefaction resistance, appear to be not valid for these soils.
Figure 1: Comparison between laboratory-based and field-based cyclic resistance ratio (Orense et al. 2012; Orense & Pender, 2013).
Recent research showed that penetration-based approaches, such as CPT and seismic dilatometer tests, underestimated the liquefaction resistance of the pumice deposits (shown in Figure 1), confirming that any procedure where the liquefaction resistance is correlated with density will not work on pumiceous deposits (Orense et al. 2012; Orenseand Pender 2013). The same research showed that empirical method based on shear wave velocity seemed to produce good correlation with liquefaction. A possible reason is that the shear stresses during penetration were so severe that particle breakage formed new finer grained materials, the mechanical properties of which were very different from the original pumice sand. Admittedly, the above conclusions were obtained from limited number of test data and such conclusions have not been well-validated. With many consultants and practitioners constantly asking for advice on how to evaluate the liquefaction susceptibility of pumice deposits, there is indeed a need to clarify and address this issue.
Research Objectives
The key objectives of this research are as follows:
[1] Characterise various sites in the Rangitaki Plains by performing field testing at designated sites through cone penetration tests, shear wave velocity profiling and screw driving sounding tests.
[2] Assess the liquefaction potential at the said sites using available field-based empirical methods, considering estimated peak ground accelerations associated with the 1987 Edgecumbe earthquake.
[3] By validating the factor of safety against liquefaction vis-à-vis the observed performance (as evidenced by available literature and reconnaissance reports), provide recommendations on the best possible field-based method(s) to be used in evaluating liquefaction potential of pumiceous deposits.
It is envisioned that the field test results would be used in the next stage of research which would attempt to correlate the field parameters with the liquefaction resistance of pumiceous deposits derived from laboratory testing, in order to aid in the development of guidelines for evaluating liquefaction potential of pumiceous soils.
Figure 2: Target sites for the study.
Selected Test Sites
Two sites have been identified where liquefaction had been observed during the 1987 Edgecumbe Earthquake. Test site #1 is located adjacent to the Whakatane Sewage Pump station, while Test site #2 is opposite the Edgecumbe substation (see Figure 2). Manifestations of soil liquefaction, such as sand boils and ejected materials, have been reported at both sites (Pender et al. 1989).
To date, borehole sampling, cone penetration tests (CPT), seismic cone penetration tests (sCPT), and screw driving sounding (SDS) have been performed at these sites. Surface wave profiling is being planned. The results of the study will be published by the end of the year.
Expected impact
One of the issues that have been constantly brought to attention by the engineering community is the lack of guidance for geotechnical characterisation and evaluation of pumiceous soils. Existing empirical correlations based on hard-grained sands may not work for their characterisation and could mislead engineering assessment. In this context, the profession is facing serious issues on many large engineering projects especially in the Waikato – Bay of Plenty region in relation to liquefaction evaluation of pumiceous soils and associated significant costs for mitigating potentially non-existent liquefaction hazards.
It is envisioned that, in the short-term, the research output will assist the local profession through guidelines on how to evaluate liquefaction potential of pumiceous deposits based on field parameter(s). In the long-term, the research outputs will be combined with previous, on-going and future work to develop design guidelines for evaluating liquefaction potential of pumiceous deposits.
Acknowledgments
Funding of the research project was through QuakeCoRE (Project # 16025). The assistance of Whakatane District Council, AECOM and GNS Science in determining the target test sites is gratefully acknowledged.
References
Orense, R.P., Pender, M.J. and O’Sullivan, A. (2012). “Liquefaction Characteristics of Pumice Sands,” Final Report of EQC Project 10/589, 131pp.
Orense, R.P. Pender, M.J. (2013). “Liquefaction characteristics of crushable pumice sand,” Proc. 18th International Conference on Soil Mechanics and Geotechnical Engineering, Paris, 4pp.
Pender, M.J., Robertson, T.W. et al. (1987). “Edgecumbe Earthquake: Reconnaissance Report.” Earthquake Spectra, Vol. 3, No. 4, 659-745.
Wesley, L. D., Meyer, V. M., Pronjoto, S., Pender, M. J., Larkin, T. J. & Duske, G. C. (1999). “Engineering properties of pumice sand,” Proc. 8th Australia-NZ Conference on Geomechanics, Hobart, Vol. 2, 901 – 908.