Empirical Assessment of Liquefaction Resistance of Crushable Pumiceous Sand Using Shear Wave Velocity

Sands consisting of pumice particles are found in the northern part of the North Island of New Zealand. These pumice sands are highly crushable, compressible, and lightweight due to the vesicular nature of the particles, making engineering assessment of their properties problematic. When performing geotechnical assessments of these deposits, questions have been asked whether existing empirical correlations derived primarily from normal (hard-grained) soils apply to the crushable natural pumiceous (NP) soils. Such lack of guidance for the geotechnical characterization and liquefaction assessment of NP soils has long been highlighted by the local geotechnical community. In this research, laboratory experiments, such as cyclic triaxial and bender element tests, were performed on reconstituted and high-quality undisturbed NP samples. These were supplemented by field-based shear wave velocity ($V_{\mathrm{s}}$) profiling conducted at the same sites where the undisturbed samples were collected. For comparison purposes, similar laboratory tests were also performed on the hard-grained Toyoura sand. The laboratory results showed that NP sands have considerably different behavior when compared to Toyoura sand. For instance, NP sands have lower small strain shear modulus ($G_{\max }$) and $V_{\mathrm{s}}$ and higher liquefaction resistance under the same level of packing. Next, results from the laboratory studies and field characterization were synthesized considering the effect of various parameters, such as the degree of packing, overburden pressure, and pumice contents, on the liquefaction assessment of NP sands. Based on the results, attempts were made to develop a $V_{\mathrm{s}}\text{-}\mathrm{based}$ chart to estimate the liquefaction resistance of such problematic soils for use by the geotechnical profession. Finally, the applicability of the proposed $V_{\mathrm{s}}\text{-}\mathrm{based}$ chart was examined using available case studies from the 1987 Edgecumbe earthquake and then compared with an existing empirical $V_{\mathrm{s}}\text{-}\mathrm{based}$ method developed for normal sands. The results showed that the proposed approach better reflected the reported liquefaction manifestation at the sites.