Abstract
The earthquakes that hit the region of Canterbury, New Zealand, in 2010-2011 resulted in severe damage to buildings and infrastructure due to widespread liquefaction of natural clean sand and silty sand deposits of fluvial origin. Despite the significant hazard posed by earthquake-induced liquefaction to New Zealand communities and economy, the undrained cyclic behaviour of silty sands remains poorly understood, with few laboratory data available to support developments in research and design methodologies.
For these reasons, a comprehensive laboratory testing programme on Christchurch soils has been undertaken at the University of Canterbury since 2006. After the 2010-2011 Canterbury Earthquake Sequence, these research efforts have been extended to include both broad field and experimental investigations, in collaboration with the University of California, Berkeley. Within this context, this paper describes the preliminary results from a series of undrained cyclic Direct Simple Shear (DSS) tests performed on specimens prepared with a natural clean sand retrieved from Christchurch, as part of a broader testing programme which will be extended to include also silty sands and stratified silt-sand specimens. Test specimens are reconstituted with the water sedimentation method. In comparison to other deposition methods, this technique allows the preparation of specimens with soil fabric and soil structural features, such as segregation and layering, which are more representative of the characteristics of natural fluvial deposits like those typically encountered in Christchurch. Analysis of experimental data provides the first evidence on how the DSS response of these soils is affected by the magnitude of the imposed cyclic loading and by soil density.
1 INTRODUCTION
1.1 Current research context on the liquefaction behaviour of sandy soils
Earthquake-triggered soil liquefaction is the temporary reduction in soil strength and stiffness accompanying the build-up of excess pore water pressures induced by seismic stress waves propagating through the ground. Early studies on soil liquefaction essentially focused on loose clean sands (i.e. sands with less than 5% fines or particles smaller than 75 µm), as the first well-documented case histories reported liquefaction in this type of soils (Seed, 1979). Later evidence, however, added a significant number of case histories of liquefaction, lateral spreading and flow failure in fines-containing soils (Cubrinovski & Ishihara, 2000), including the phenomena observed within the urban area of Christchurch in 2010-2011 (Cubrinovski et al., 2011). This leads to the necessity to establish a reliable basis for liquefaction evaluations of silty sands, rather than always referring to idealized clean sands.
Fabric, i.e. the arrangement of soil grains in the packing (skeleton), and structural features such as soil layering (micro- and macro-structure) are the outcome of the formation processes of natural soil deposits, and are unique to the depositional environment. The liquefaction strength of cohesionless soils has been shown to be strongly influenced by fabric (Ladd, 1977; Mulilis et al., 1977) and layered structure (Verdugo et al., 1995). In order to capture these effects, ideally one should test undisturbed specimens collected from the field. However, sampling of cohesionless soils without significant disturbance is a difficult task. This is the main reason why research in the past has often made use of reconstituted specimens prepared in the laboratory. Obviously, in order to get a better picture of the liquefaction behaviour of natural soil deposits, one has to produce in the laboratory a fabric similar to that encountered in the field. There exist several specimen reconstitution techniques, each one of them resulting in a different fabric. Among them, moist tamping has been widely used by researchers as it allows to easily prepare either loose or dense specimens. Fabric obtained with moist tamping, however, is not representative of the fabric of natural soils. Although it is more difficult to employ than moist tamping, the water sedimentation technique is considered to result in a fabric more closely resembling that of fluvial soil deposits (Vaid & Sivathayalan, 2000).
Previous research performed at the University of Canterbury includes undrained cyclic and monotonic triaxial tests of fines-containing sandy soils from Christchurch. Rees (2010) focused on specimens with various fines contents reconstituted by moist tamping, while Taylor (2015) presented comparisons between undisturbed and moist-tamped reconstituted specimens for another set of Christchurch soils. Additional data on undisturbed specimens of Christchurch soils are presented by Stringer et al. (2015) and Beyzaei et al. (2015).
This study is a continuation of these efforts. Its aim is to highlight how fabric and layered structure influence the undrained cyclic response of sandy soils from Christchurch in Direct Simple Shear (DSS) conditions. This will be achieved by performing comparative tests on undisturbed specimens, collected with the Gel-Push and Dames & Moore samplers, and on specimens of the same soils prepared in the laboratory using the technique of water sedimentation. In this paper, experimental results for DSS tests on specimens of a clean sand sourced from Christchurch reconstituted at different relative densities are presented. Given the extensive research performed in the past on the undrained cyclic response of clean sands, this test series represents the most relevant reference benchmark for the analysis of subsequent tests on fines-containing sands.
1.2 Features of cyclic DSS for liquefaction studies
Past laboratory studies on the undrained cyclic behaviour of cohesionless soils have made extensive use of the triaxial device because of its relative simplicity in use and its more common availability in research facilities compared to other testing apparatuses. However, level-ground free-field response induced by earthquake shaking involves a simple shear mode of deformation which is reproduced more rigorously in a Direct Simple Shear test. The conversion of triaxial test data to simple shear mode of deformation, as encountered in level-ground free-field conditions, has traditionally been expressed in terms of the cyclic stress ratio (CSR) using equation (1):
CSR = [τ/σ’v]field = (1 + 2·K0)/3·[|q|/(2·σ’c)]TX (1)
where the effective stresses terms herein employed are consistent with the total stresses shown in Figure 1. The actual relationship for the liquefaction resistances between triaxial and simple shear conditions is a complex function depending on factors such as tested soil, amplitude of imposed cyclic stresses, and soil fabric, among others, which are not captured by equation (1) (Tatsuoka et al., 1986). One of the main reasons for this discrepancy is that the stresses imposed on a triaxial test specimen are very different from the stresses induced by earthquakes to soils in level-ground free-field deposits. DSS testing was conceived as a means to overcome this shortcoming of triaxial testing.
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