Characteristics of Sites Response of a Series of Monitored Slopes in Sichuan Mountain Area

Abstract

Slope site seismic response is different from one of the flat sites, it has been known for many years; and this phenomenon has also attracted great attention in China since the “5.12” Wenchuan earthquake. From 2008, the State Key Laboratory of Geohazard prevention and Geoenvironment protection (SKLGP) has started to complete long term and systematic monitoring of the site amplification for different slopes. This chapter presents monitoring characteristics of slope site response of seven typical sites. A total of four main shocks were monitored by the Zigong (recorded Wenchuan earthquake), Luding (recorded Lushan and Kangding earthquakes) and Shimian (recorded Changning earthquake) monitoring points. Hundreds of aftershocks were observed by our monitoring stations. The Peak ground acceleration (PGA), horizontal to vertical spectral ratio (HVSR), normalized Arias intensity (Ia), standard spectral ratio (SSR) and azimuth information has been extracted from the monitoring data. The monitoring characteristics of the slope seismic response show that the soil site is always affected by stronger amplification than the bedrock site in a small amplitude range (Reference site < 0.076 g). This is due to the high wave impedance between soil and bedrock promoting site ground motion effects. Meanwhile, significant polarization effects have also been observed on the mountain top (hilltop), the normalized Ia and polarization diagram of the HVSR show the same directivity characteristics of ground motion near the mountain tops, marked by strongest amplitudes transverse to the ridge, where the extension direction of the ridge and orientation of geological structures influence the seismic energy redistribution. In addition, the PGA amplification shows nonlinear increasing with the topographic relief and changing geological conditions. Amplification of PGA of the slope in the upper part can be quite high (>10) with respect to a reference site (for weaker motion of an intensity less than V), but is less marked (<4.5) for strong motion conditions (Intensity ≥ VI). This means that the PGA amplification effect has significant variable characteristics under low intensity of earthquake. The HVSR analysis shows that the amplified resonance frequency of most monitoring sites is in the range of 2–4 Hz, and part are in the range of 6–8 Hz, and that multi peak frequency values can be observed in most slope monitoring sites. The complexity of the slope morphology and of the geology conditions has an influence on related results. This paper presents some analyses of the seismic response of a series of monitored slope sites, which show that predicting the slope response is still very difficult, due to the variability of site effects. An even greater challenge is the prediction of the seismic slope failure potential as there is a great lack of recorded strong motion data on sites affected by permanent deformation after an earthquake.