Liquefaction characteristics of pumice sands
Authors: R P Orense, M J Pender (University of Auckland), A S O'Sullivan (Hiway Geotechnical)
Paper number: 375 (EQC 10/589)
Abstract
Pumice materials are frequently encountered in many engineering projects in the North Island of New Zealand. They originated from a series of volcanic eruptions centred in the Taupo and Rotorua regions, called the “Taupo Volcanic Zone”. Because of their lightweight, highly crushable and compressible nature, they are problematic from engineering and construction viewpoint. Most existing engineering correlations originally developed for ordinary sands are not applicable to this material. In terms of evaluating liquefaction potential, empirical procedures currently available for sands were derived primarily from hard-grained (quartz) sands. No information is available whether these procedures are applicable to pumice deposits because there has been very little research done to examine the liquefaction characteristics of pumice.
Thus, a research programme was undertaken to understand the cyclic/dynamic properties of pumice. Several series of laboratory tests, where the behaviour of soils when subjected to earthquake loading can be simulated, were performed on pumice samples. The results of the tests were compared with those of hard-grained sands. Particle crushing, both at the end and at various stages of the tests, was examined.
The laboratory tests showed that although dense pumice specimens have higher liquefaction strength than loose ones, the difference was not as remarkable as that observed on hard-grained sands, where relative density affects the response significantly. Moreover, pumice is generally more resistant against liquefaction when compared with hard-grained sands. This is because pumice particles crush during the cyclic load application, resulting in gradual stabilisation of the soil structure which leads to higher liquefaction strength.
A comparison of laboratory-obtained liquefaction strength and those estimated from empirical procedures conventionally adopted in geotechnical investigation showed that penetration-based approaches, such as cone penetration tests and seismic dilatometer tests, underestimated the strength of pumice. It is hypothesized that the shear stresses during penetration in pumice layers were so severe that particle breakage formed new finer grained materials, the mechanical properties of which were very different from the original material. On the other hand, empirical method based on shear wave velocity measurement produced better correlation. It follows that non-destructive methods of measuring shear wave velocity are more appropriate to estimate the in-situ liquefaction strength of pumiceous deposits.
Technical Abstract
Because of their lightweight, highly crushable and compressible nature, pumiceous sands are problematic from engineering and construction viewpoint. There has been very little information on their liquefaction characteristics and most empirical procedures available in evaluating the liquefaction potential of sands are derived primarily from hard-grained sands.
To understand the liquefaction characteristics of pumice sands, several series of undrained cyclic triaxial tests were performed on two sets of pumiceous soils: “undisturbed” specimens taken from pumiceous deposits, and reconstituted specimens consisting of commercially-available pumice sands. The tests showed that as expected, undisturbed soil specimens have higher liquefaction resistance than specimens reconstituted to the same density because their soil structure (fabric, stress history, cementation, etc.) was still intact. Although dense reconstituted pumice specimens have higher liquefaction resistance than loose ones, the difference was not as remarkable as that observed on hard-grained Toyoura sand, where relative density affected the response significantly. Pumice sands have higher liquefaction resistance when compared with hard-grained sands because of particles crushed during loading; as cyclic shearing and particle crushing occurred, the soil structure was gradually stabilized, resulting in higher cyclic shear resistance.
The cyclic tests were supplemented by monotonic undrained triaxial tests. Specimens reconstituted under loose and dense states practically showed similar response, confirming that relative density did not have significant effect on the behaviour of pumice. The stress-strain relations showed a stiffer response at small strain level, followed by development of large strains and greater dilatancy when the phase transformation state (i.e., from contractive to dilative) was reached. Pumice sands have angle of internal friction at failure of about 42-44o, which is much greater than those of natural hard-grained sands. Even under large strain level, they did not reach steady state of deformation, possibly due to continuous breakage of particles during shearing, which resulted in more resistant soil structure that did not allow deformation at constant shear stress to occur.
Finally, comparison of laboratory-obtained liquefaction resistances and those estimated from conventional empirical procedures showed that penetration-based approaches underestimated the cyclic resistance of pumiceous soil. It was hypothesized that the shear stresses during penetration are so severe that particle breakage forms new finer-grained materials, the mechanical properties of which are very different from the original pumice sand. Thus, any empirical procedure where the liquefaction resistance is correlated with density will not work on pumiceous deposits. On the other hand, empirical methods based on shear wave velocity measurement seemed to produce good correlation with liquefaction resistance. Thus, non-destructive methods of measuring shear wave velocity are more appropriate to estimate the in-situ liquefaction resistance of pumiceous deposits.
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