Simplified equivalent soil profiles based on liquefaction performance for shallow-founded structures

Earthquake-induced liquefaction is responsible for considerable structural damage. However, conventional liquefaction assessment focuses only on triggering. In this paper, the evaluation of liquefaction is refocused to consider the performance of buildings. A new hazard-independent liquefaction classification is proposed where the soil profile is defined as an equivalent 3-layered soil profile. The classification consists of only three features, highly influential to the performance of buildings: the depth of the non-liquefying crust, and the thickness and liquefaction resistance of the potentially liquefiable layer. The influence of these parameters is explored with reference to changes of the ground surface acceleration and foundation bearing capacity. A procedure to obtain the 3-layered soil profile from CPT data is developed and set of soil profile classes are developed for rapid loss assessment purposes. The procedure and classes are demonstrated on a case study site considering 100 CPT from Christchurch and a comparison is made regarding the computed LSN value for the equivalent and CPT profiles. liquefaction triggering assessments that use different assumptions can provide considerably different results. Recent investigations of the performance of soil deposits in Christchurch during the 2011 earthquake by Cubrinovski et al. (2017) identified the role of pore water flow and seismic isolation as key differences between the CPT-based simplified triggering procedure from Boulanger and Idriss (2016) and nonlinear effective stress analyses. In turn, soil layers in terms of the normalised cone tip resistance and the information criterion were readily identified and consistent across both assessment procedures. This paper proposes a simple three criteria, hazard-independent liquefaction classification system for performance and loss assessment of buildings on shallow foundations, using the height and depth to the critical liquefiable layer, as well as the average cyclic resistance of the liquefiable layer for 15 cycles of uniform load. The influence of these properties on ground surface shaking and bearing capacity are explored. The classification system is implemented as an algorithm and used to identify classification difficulties (e.g. in highly stratified soil). A set of criteria for classification of soil profiles for regional scale loss assessment are also presented and demonstrated on a case study site in Christchurch. The main advantages of this approach are: • Can be determined from CPT, DMT, SPT or borehole data • Captures the soil profile performance across the full hazard range using just three values • Information is directly related to building performance • Intuitive, physically represented, easy-to-communicate parameters are used • Can provide a definition of the profile without knowing the seismic hazard at the site 2 INFLUENCE OF PARAMETERS ON THE DEFINITION OF AN EQUIVALENT SOIL PROFILE 2.1 Bearing capacity The bearing capacity of a foundation on a soil deposit in its liquefied state (degraded bearing capacity) is a key indicator of expected settlement and tilt (e.g. Karamitros et al., 2013; Bray and Macedo, 2017; Bullock et al., 2018). According to Karamitros et al. (2013) and Bray and Macedo (2017), the degraded bearing capacity can be computed according to Meyerhof and Hanna (1978), for a strong soil crust underlain by a weak soil layer. The degraded bearing capacity depends on the shear strength of the crust and the residual shear strength of the liquefied sand (Karamitros et al., 2013), at least for cases where the stress bulb of the foundation does not reach deeper more resistant and non-liquefiable soils. To demonstrate the importance of the crust height, Figure 2 shows a series of calculations performed using Meyerhof and Hanna (1978) for different crust heights (Hcrust), different crust undrained or total stress resistances – “cohesive” strengths (ccrust), different liquefied/degraded layer angle of shearing resistance – “equivalent” angle (deg) and different foundation widths (Bf). What is apparent is that for a strong crust and weak liquefiable layer strength, the thickness of the crust has a great influence on the degradation of the bearing capacity (qult,deg/qult,i). The influence of the crust becomes less important with increased strength of the liquefiable layer with respect to the crust. Also shown in Figure 2 are the lines corresponding to 2.5 times the foundation width. This is approximately equal to the point where the thickness of the crust no longer has an influence on the bearing capacity. The influence is less than this limit for wide foundations, if the liquefied soil is modelled with an equivalent friction angle, since the strength of the layer increases with depth, whereas assuming an equivalent cohesive strength (undrained or other derived in total stresses) would mean constant strength with depth. While the liquefied shear strength is a key parameter in the estimation of bearing capacity, it could also be expected to be highly correlated to the liquefaction resistance, as increased density typically results in increased liquefaction resistance and increased dilative behaviour. Figure 1. Influence of crust properties on bearing capacity