Abstract

The seismic forces must be properly defined as required by European geotechnical design Codes. They are dependent from the morphological and stratigraphic characteristics that determine the local seismic response. The local seismic response can be evaluated using a simplified approach which is based on the classification of the subsoil according to the values of the shear wave propagation speed Vs. In the present case study, to complete the quay of the Rovigo Interporto (facing the River Canal Bianco), where a not negligible load is planned to be applied (50 kPa), has been designed a foundation with a large slab founded with not pre-casted reinforced concrete piles diameter 350 mm, drilled without removing soil, with vibrating casing, for settlement reduction (rafted piles as settlement reducers), on an area of approximately 6000 m2. The subsoil in this area, from geological and geotechnical point of view, is very variable and with layers of soft clay and loose sand or silt, with no presence of a dense and uniform sand layer where to transfer loads to. Moreover, the geotechnical model was complicated because of the presence of an old diaphragm wall with anchors (whose projection in plant covers around half of area) and was designed a large underground basin for hydraulic invariance. For this reason, an extensive campaign of CPTU and DMT tests, together with boreholes, sampling and laboratory tests was carried out in this area, which allowed a detailed characterization of the subsoil. Through direct measurement of seismic micro-tremors, carried out before and after the installation of the piles, has been evaluated the modification of the resonance frequency of the site, and modification of the propagation speed of the shear waves Vs induced by the slab over rafted piles over the entire area.

Abstract

This study estimates the fundamental frequency of an 85 m height sand tailings dam in Central Chile using the singlestation Horizontal-to-Vertical H/V Spectral Ratio Method (HVSR), calculated from seismic ambient noise and earthquake records. The dam was constructed by compacting hydraulically deposited sands classified from integral copper tailings, following the downstream construction method. A proper estimate of the dam fundamental frequency is critical for evaluating its seismic response and secure the safe tailings containment when subjected to intense ground motions generated by interface and intraslab earthquakes. Results from the HVSR method are compared with those of the Standard Spectral Ratio method (SSR) calculated as the ratio of the response of the dam crest and that recorded at the dam toe. The fundamental frequency of the dam is about 0.9 Hz in the dam central part away from the abutments, but differences in the vibration frequency are identified along the downstream slope and along the crest axis from the north to south abutment. An analytical relationship between the dam fundamental frequency, the dam height, and the shear wave velocity (Vs) profile is validated.

Abstract

The horizontal to vertical spectral ratio (HVSR) passive seismic method consists in processing measurements of ambient noise performed in three perpendicular directions. The objective is to define the primary resonant frequency of the site and estimate its amplification characteristics, which can be deduced from visible peaks or troughs in the HVSR curves. The shape of HVSR curves depend on the interaction of waves with the interface between sediments and the formation bedrock and, hence, can be related to the bedrock depth through derived relationships. This work presents 4 study cases in the Paraguayan oriental region where HVSR of microtremors were obtained. Furthermore, the obtained HVSR curves are inverted to get approximate shear wave velocity profiles under the assumption of sub-vertically propagating P and S body waves. HVSR curves were calculated through Geopsy (Wathelet et al. 2020) and the inversion process was made through OpenHVSR (Bignardi et al. 2016), both open-source tools for ambient vibration processing. In general, the method proves to give sufficient accurate estimates of sediment thickness and stratigraphy in typical Paraguayan formations of the oriental region and presents the advantage of being an economical and fast survey option, especially for early stages in a geotechnical campaign