Site effect estimation for shallow engineering bedrock using microtremor HVSR method and geotechnical borehole data, Riyadh city, Saudi Arabia

The estimation of local site response and geotechnical properties of near-surface deposits becomes essential for improving the building codes especially in big cities as Riyadh. This study aims to estimate the site response parameters in the northern expansion zone of Riyadh city. Microtremor measurements were recorded at 45 sites at the study area, north of Riyadh, and the resonance frequency and amplification factor were calculated using the H/V spectral ratio approach. The microtremor data has been processed using geopsy software to get the H/V curve and get the value of the resonance frequency (fo) and amplitude (Ao). The values of the resonance frequency span from 0.79 to 45 Hz, while the amplification factor ranges from 1.22 to 8.5, indicating the presence of foundation bedrocks at various levels with varying soil thickness. Moreover, the N60 parameter was calculated to correct the SPT-N uncorrected field readings in order to estimate the shear wave velocity parameter down to 30 m depth (Vs30). The computed shear wave velocity for a depth of 30 m (Vs30) ranges from 210 to 630 m/s. Accordingly, the soil at the studied area can be divided into two site soil classes C and D according to the National Earthquake Hazard Reduction Program (NEHRP). The findings of this study should be considered as part of the development of the Saudi Building Code for Riyadh City.