Mitigation of Seismic Accelerations by Soft Caissons

Introduction

Seismically-induced accelerations create many problems for structures on the ground surface. Excessive shaking can cause many types of failure, as summarised by, e.g., Wiegel (1970). Ordinarily, seismic structural design is performed by generating a surface motion for the site soil profile, using a site response code such as SHAKE (e.g. Schnabel et al., 1972), and then using this as an input motion for further seismic analysis. Soil would normally be expected to amplify the accelerations as they are transmitted from bedrock to surface, depending on its natural period (e.g. Iwasaki et al., 1978). Response spectra are commonly used to quantify the effects of soil on the response of single degree-of-freedom oscillators at the surface, for particular earthquake motions. Whether this motion is affected by interaction with the soil would not normally be considered, although recent evidence suggests that such soil-structure interaction can significantly affect structure response, either positively or negatively (Gazetas & Mylonakis, 1998; Ghosh & Madabhushi, 2005; Mylonakis & Gazetas, 2000).

Earthquake loading in general contains pressure wave, shear wave and surface wave components, of which the shear wave component is usually the most damaging for structures. Softened liquefied soil is seen to reduce shear accelerations felt by surface structures (e.g. Tokimatsu et al., 1996). The effect of underground liners that slip to prevent shear wave transmission has also been found to reduce seismic shear stresses experienced at the ground surface (Yegian & Catan, 2005; Yegian & Kadakal, 2005). An idea suggested has been to incorporate a soft layer horizontally in the ground beneath a structure such as in Figure 1a, but unless this is very wide then shear waves still diffract around the ends, and there is no protection from surface waves such as Rayleigh or Stonley waves. Open trenches have been studied as a barrier to surface wave propagation whether seismically-induced (May & Bolt, 1982; Wang et al., 2009), rail/traffic-induced (Ahmed & Al Hussaini, 1991; Beskos et al., 1986; Yang & Hung, 1997), machine-induced (Celebi et al., 2009) or construction induced (Ye & Tan, 2011). The present research work investigates a combination method, utilizing the filtering effect of a soft layer in the ground, akin to liquefied soil and related to a sliding surface, with soft trenches to prevent diffraction of shear waves around the horizontal layer. Such a configuration is shown in Figure 1b with a basic surface structure. The soft material can be seen to resemble a caisson, impeding wave travel in all directions. For this study, only unidirectional shear loading is considered.

Figure 1.

Schematic representation of soft layers impeding seismic motion

Modelling Techniques

In this paper, both physical and numerical models of the system are presented. This section presents the basis behind these analysis techniques, firstly the physical modelling, including the physical modelling of soft layers, and secondly the numerical analysis.