Pumice soil grains are characterized by their vesicular nature, which leads to lightweight, crushable grains with an extremely rough and angular surface texture. These characteristics give pumiceous soils particular engineering properties that are distinct from more commonly encountered hard-grained materials, making them problematic for engineers interested in assessing the risk and potential consequences of liquefaction. Natural pumice-rich soils are found with varying amounts of pumice; however, it remains unclear how the quantity of pumice present in a soil mixture alters the behaviour. This paper investigates the effect of pumice content on cyclic resistance using blends of a hard-grained sand and a pumice sand through a series of triaxial tests. Overall, the cyclic resistance was found to reduce with increasing pumice content. Furthermore, the cyclic resistances appeared to fall into three bands: (a) little apparent reduction in cyclic resistance for pumice contents up to 40%, (b) a reduction in cyclic resistance of approximately 20% at pumice contents of 80% and higher, and (c) a transitional zone. However, despite the lower cyclic resistance, the patterns of pore pressure generation and strain development did not appear to be affected by the amount of pumice in the soil mixture.

Volcanic ash air-fall or tephra deposits comprise nearly 31% of the North Island of New Zealand. For those belonging to slightly weathered or negligible to little cohesionless nature (silty sands to sandy silts), the compressibility and collapsibility features of compacted tephras have not been largely investigated. Correspondingly, the compressibility and collapse potential (CP) of compacted airfall tephras (at 90% and 100% degrees of compaction) were evaluated. At vertical stresses up to 200 kPa, the consolidation coefficient cv and permeability coefficient k were in the range of 10-5-10-8 m2/s and 10-6-10-8 m/s. Interestingly, for feldspar-silica type cohesionless tephras, it was possible to correlate the weathering degree and mineralogy to the compressibility. The results of the CP tests showed that the CP increased with the decrease in degree of compaction and increase in vertical stress; with values ranging from 0% to 5% indicating low-to-moderate collapsibility of the tephras upon properly compacted placement condition. The compacted tephras, as such, could be considered suitable structural fills for typical geotechnical applications owing to features such as low compressibility, low permeability and low-to-moderate CP.

The North Island of New Zealand is a region of high volcanic activity, with significant eruptions over the past. Analogous to past events, future volcanic eruptions would produce a considerable volume of ash and granular soils, covering widespread areas and raising concerns for their disposal and storage. Such deposits, primarily airfall tephra, could be potentially used in geotechnical engineering applications such as foundations, roadway embankments and land reclamations. However, before their use as structural fills can be recommended, detailed laboratory investigations of their physical, chemical, compaction, and geotechnical engineering properties (strength, compressibility, collapsibility, liquefaction potential, etc.) must be conducted. Different tephra deposits can be products of different eruptions, so chemical composition analyses can be combined with the physical, compaction, and engineering properties to characterize such deposits. Accordingly, this paper provides useful insights from physical (grain size, specific gravity, and morphology), chemical (elemental and mineralogy using X-ray fluorescence and X-ray diffraction), and compaction tests (maximum dry density, optimum water content, and particle breakage) for eleven selected volcanic tephra samples sourced from the North Island of New Zealand in the Rotorua, Taupo, and Auckland regions.

The use of weathered airfall tephra deposits for geotechnical applications such as backfilling in embankments or foundations requires investigation. As a part of an experimental laboratory investigation addressing this issue, this paper focuses on the monotonic shear strength evaluation under drained and undrained shearing conditions of three airfall tephra deposits—namely Kaharoa (white–grey and golden brown) and Maungataketake (black-grey) ashes belonging to New Zealand. The shear strength results include tephra samples compacted at 90

This paper reports and discusses the results of a series of monotonic compression drained and undrained triaxial tests performed on three compacted, slightly weathered silty sand tephras. In total, 18 drained and 18 undrained tests were performed on compacted specimens (at Dc ≈ 90 and 100%) isotropically consolidated at confining pressures of 50–200 kPa. It was observed that particle size distribution, weathering state, and mineralogy of the tephra deposits had significant effects on the stress–strain responses, friction angles, stress–dilatancy relations, and critical state characteristics. For instance, the coarser tephra (namely white–grey Kaharoa, that was less affected by weathering processes) showed a primarily dilative response. The effects of chemical composition, namely weathering degree and mineralogy, on geotechnical properties such as friction angle were investigated with an attempt to interlink the two characteristics for heterogeneous tephras. The measured friction angles (ϕ = 32.7°–42.8°), combined with the results of weathering degrees and mineralogical investigations, indicated that silty sand tephras, if properly compacted, are suitable fills for use in typical geotechnical applications.

The liquefaction resistance of partially saturated soil was experimentally investigated for one clean sand and one silty sand collected from a site in Christchurch, in an area severely affected by liquefaction in the 2010–2011 Canterbury earthquakes. A series of cyclic undrained tests were performed on fully and partially saturated sand and silty sand specimens, in conjunction with evaluation of saturation conditions in situ based on comprehensive field measurements of P-wave velocity (Vp) in Christchurch deposits. The Skempton’s B-value and P-wave velocity were comparatively used as measures for partial saturation in the laboratory. B-value - Vp relationships from the test results indicate that Vp steadily increases with the B-value until a threshold B-value is reached beyond which Vp remains unchanged at values indicating full saturation, i.e. Vp ;≥ ;1600 ;m/s. In general, the liquefaction resistance of tested sand and silty sand increases with a decrease in the B-value or Vp, i.e. with a reduction in the degree of saturation. Furthermore, test results suggest existence of threshold B-values and Vp for tested soils beyond which no significant increase in the liquefaction resistance was observed. This threshold B-values and Vp were found to be dependent on soil type and applied confining stress. The effects of partial saturation on liquefaction strength are different for the sand and silty sand when using Vp as a measure for the degree of saturation. While a gradual rate of increase in liquefaction strength with decreasing Vp is observed for the tested sand, the liquefaction strength of silty sand shows similar gradual increase with a decrease in Vp up to about 800 ;m/s, which is then followed by an abrupt increase in the liquefaction strength for Vp ;< ;800 ;m/s. Generally good agreement between liquefaction strength of tested soils and published data was observed, with a clear distinctive feature in the behaviour of the silty sand as compared to clean sands.

Soils containing pumice are frequently encountered on engineering projects in the North Island of New Zealand. The presence of pumice is known to result in different material behaviours, including the resistance to cyclic loading. In this paper, results of triaxial testing on undisturbed specimens of dense, pumice-rich soils are presented, and examined to identify the apparent effects that differing pumice content has on the observed behaviours. It is shown that significant reductions in the cyclic resistance were observed in these soils compared with expectations for hard-grained materials, but that this effect appears to be fully developed with limited amounts of pumice in the soil. It is further shown that the undrained strength is significantly reduced by increasing amounts of pumice and that typical predictions of post-cyclic reconsolidation strains are unconservative in pumice bearing materials.

The small-strain shear modulus (Gmax) of soils is an essential dynamic parameter for any seismic design. This paper aims to investigate the Gmax of two reconstituted sands—beach and volcanic sands of variable shape and grain size distribution. A series of bender elements tests were carried out on dry samples at different frequencies and confining pressures. The interpretation of results is carried out through first-time arrival method. The results indicate lower Gmax values of volcanic sand in comparison with beach sand, which can be attributed to lower particle density and higher void ratios for volcanic sand. Using the available relationships in the literature, the Gmax values were estimated, and a brief comparison was made between measured and estimated values.

A series of cyclic direct simple shear tests are performed to investigate the liquefaction resistance of sandy soils from Christchurch, New Zealand. The study focuses on the combined effects of soil density and fines content on liquefaction resistance of sandy soils. Two sands, a non-plastic silt, and their mixtures prepared at different fines contents are tested at two sets of relative densities. Test specimens are reconstituted using a procedure for water sedimentation, yielding soil fabric and soil structure resembling those of fluvial soil deposits. Differences are observed between the two host sands in the sensitivity of the cyclic liquefaction resistance to changes in relative density. Differences are also seen in the effects of the addition of fines to the liquefaction resistances of the two sands. Monotonic undrained direct simple shear tests are employed to explore an interpretation of the liquefaction resistance of the tested soils within the critical state framework. The state-concept interpretation provides more consistent quantification of the effects of initial state on the liquefaction resistance; however, the relationship between the state parameter and liquefaction resistance is soil dependent. The results of cyclic direct simple shear tests for soils containing up to 30% fines show reasonably consistent liquefaction resistances relative to estimates from empirical CPT-based liquefaction triggering relationships.

Tools for characterizing thin layering and groundwater table conditions are evaluated at silty soil sites being assessed for liquefaction. Thin interlayered stratigraphy and groundwater table fluctuation are two potential causes for inconsistencies observed during the Canterbury earthquake sequence, wherein liquefaction did not manifest at several silty soil sites, despite simplified liquefaction assessment procedures indicating severe manifestations would be expected. Site investigations should capture these features to allow for improved assessment of liquefaction potential at silty soil sites. Cone penetration tests (CPTs), mini-CPTs, and sonic borings do not adequately capture thin layering. However, detailed logging of high-quality samples captures the actual in situ layering that may help explain the limitations of simplified liquefaction assessment procedures at these sites, revealing the need to understand underlying limitations in current site investigation techniques. Piezometers, sonic borings, high-quality sampling, crosshole testing, and regional groundwater maps are evaluated to assess their ability to capture complex groundwater conditions. Multiple groundwater measurement methods are typically required to characterize groundwater fluctuations. An approach to using enhanced site characterization tools is recommended for liquefaction assessments at silty soil sites with thin layering and groundwater fluctuations.

Pumice materials, which are problematic from an engineering viewpoint, are widespread in the central part of the North Island. Considering the impacts of the 2010-2011 Christchurch earthquakes, a clear understanding of their properties under earthquake loading is necessary. For example, the 1987 Edgecumbe earthquake showed evidence of localised liquefaction of sands of volcanic origin. To elucidate on this, research was undertaken to investigate whether existing empirical field-based methods to evaluate the liquefaction potential of sands, which were originally developed for hard-grained soils, are applicable to crushable pumice-rich deposits. For this purpose, two sites, one in Whakatane and another in Edgecumbe, were selected where the occurrence of liquefaction was reported following the Edgecumbe earthquake. Manifestations of soil liquefaction, such as sand boils and ejected materials, have been reported at both sites. Field tests, including cone penetration tests (CPT), shear-wave velocity profiling, and screw driving sounding (SDS) tests were performed at the sites. Then, considering estimated peak ground accelerations (PGAs) at the sites based on recorded motions and possible range of ground water table locations, liquefaction analysis was conducted at the sites using available empirical approaches. To clarify the results of the analysis, undisturbed soil samples were obtained at both sites to investigate the laboratory-derived cyclic resistance ratios and to compare with the field-estimated values. Research results clearly showed that these pumice-rich soils do not fit existing liquefaction assessment frameworks and alternate methods are necessary to characterise them.

Relative density has been shown to be useful for interpreting soil behaviour and for estimating the strength and other soil characteristics in geotechnical engineering. Previous studies have shown that the index void ratios (maximum and minimum void ratios) used to determine relative density depend on the particle shape and the shape of the grain size distribution. However, the effect of the median grain size on the index void ratios has not been resolved. In this study, the effects of the particle weight and the particle size relative to the inter-particle attractive forces are examined by conducting maximum void ratio (emax) tests in elevated gravity fields through the use of a centrifuge. The results indicate that both an increasing particle size and increasing gravity affect emax, providing evidence that inter-particle forces affect the packing of materials in this limit state. The change in emax, occurring in an elevated gravity field, implies that the relative densities estimated for centrifuge model experiments may not be accurate.

Pumice-rich deposits are found in a number of locations around the world, and in particular across large areas of the North Island of New Zealand. Pumice grains are commonly described as being lightweight, highly crushable, and vesicular in nature. These characteristics give rise to a unique set of behaviours under loading, and pumice-rich soils are highly problematic in terms of in situ characterisation in large part due to their crushability. The presence of pumice within a soil mixture has the potential to completely alter the stress–strain behaviour of these soils as well as require a different interpretation of results from commonly used site characterisation technique. It is therefore important to be able to determine quantitatively the percentage of pumice within a given soil deposit. This paper proposes a methodology based on a gravity separation of pumice-bearing mixtures with a heavy fluid. The application of the method to artificial mixtures of fine pumice and non-pumiceous sands is shown to be sufficiently accurate for engineering purposes.

State-of-practice liquefaction assessment procedures captured the post-earthquake liquefaction observations made in most areas of Christchurch, New Zealand after the 2010-2011 Canterbury earthquake sequence. However, there were also cases where state-of-practice procedures indicated that significant liquefaction would occur, yet no surface manifestations were observed. These no-liquefaction case histories are concentrated primarily in southwest Christchurch, an area characterized predominantly by alluvial silty soils. Cyclic triaxial testing was performed on high-quality ("undisturbed") specimens retrieved from several sites to evaluate the cyclic response of these soils. Steady state testing was then conducted on reconstituted specimens prepared using soil from the cyclic test specimens. This paper presents laboratory data and steady state lines for three silty soil units. Steady state response findings are compared with cyclic response observations. The in-situ states of the tested soils are estimated, and the use of reconstituted specimens to represent layered alluvial soils is critiqued.