Assessment of the seismic site effects based on earthquake recordings and in situ borehole measurements in Bucharest, RomaniaAndrei Bala(1), Alexandru Aldea(2), Stefan Florin Balan(1), Cristian Arion(2) Within the NATO Science for Peace Project 981882 “Site-effect analyses for the earthquake-endangered metropolis Bucharest, Romania” we obtain a unique, homogeneous dataset of seismic, soil-mechanic and elasto-dynamic parameters. Ten 50 m deep boreholes are drilled in the metropolitan area of Bucharest in order to obtain cores for dynamic tests and vertical seismic profiles for an updated microzonation map related to earthquake wave amplification. The boreholes are placed near former or existing seismic station sites to allow a direct comparison and calibration of the borehole data with actual seismological measurements. A database is assembled which contains P- and S-wave velocity, density, geotechnical parameters measured at rock samples and geological characteristics for each sedimentary layer. All the VS-30 computed values belong to type C of soil after this classification (Romanian Code for the seismic design for buildings - P100-1/2006). Approximately 250 samples were gathered from the 10 drill sites. These samples were mostly not disturbed (samples as they were recovered from the tube of the drilling rig) and partly disturbed (those which had no proper consistency). The geotechnical laboratory analysis consists in the following parts: geological identification of the sample, identification of the sample after the ternary diagram, percentage of clay – dust - fine sand - medium sand - big sand - gravel, density mineral skeleton, particle percent with diameter d<2µm, plastic limit determination, tests of compression - settling, triaxial (dynamic) test and resonant column tests. Results obtained by the down-hole method in the 10 boreholes drilled in Bucharest City as well as from laboratory measurements are used as input data in the program SHAKE2000. The mean weighted seismic velocities for the first 6 (of 7 types) of Quaternary layers present in Bucharest underground are computed for all the sites, in order to be compared with seismic velocity values obtained from previous seismic measurements and to be used as input for modeling with program SHAKE2000. Using SHAKE2000 we compute spectral acceleration response and transfer functions for every site in which in situ measurements were performed. The acceleration response spectra correspond to the shear-wave amplifications due to the models of sedimentary layers down to: a) 50 m depth; b) 70 m depth. A comparison with a real signal recorded at surface is made in order to calibrate the spectral acceleration response. |
