Engineering geotechnical evaluation of soil for foundation purposes using shallow seismic refraction and MASW in 15th Mayo, Egypt

Abstract

Despite the modernity of the 15th Mayo city, buildings suffer progressively from engineering problems which are increased over the last 10 years like cracks, fissures, cavities and tilting of building. The main target of this work is to delineate the dynamic engineering characteristics of subsurface rocks and soils through interpretation of P- and S-wave velocities, knowledge of probable causes of structural construction damages and calculate Vs30 for determining site class. Therefore, field work of shallow seismic refraction (SSR) and multichannel analysis of surface waves (MASW) were undertaken in this work to evaluate causes of these effects. Nineteen profiles for both SSR and one dimension (1D) MASW were performed to assess the geotechnical characteristics of subsurface soil for building purposes and calculating Vs30 for site class evaluation.SSR survey was implemented using a seismograph of 48-channel with vertical geophones of 40 Hz. The first breaks were analyzed employing SIPT2 program. S-waves were generated using 1D MASW technique with seismograph of 24-channel model Strata View with 2 m offset distance and two shooting. Field-work was undertaken employing vertical geophone of low-frequency (4.5 Hz). SeisImager/SW program package was employed for processing seismic results. Some important elastic moduli, engineering and petrophysical characteristics were evaluated. Findings of VP and VS revealed that the proposed site composed mainly of three geoseismic zones; the first zone is the surface layer of weathered limestone, fragments of gravels and fractured marl which characterizes by incompetent to slightly competent rock quality. The second zone is underlain the previous layer, its lithology is clayey marl of fairly to moderately quality of competent rock, which causes serious problems for the buildings due to presence of clay. The third zone is the hardest layer of limestone content and was employed as a bedrock layer which characterizes by good quality of competent rock.VS30 (averaging VS for the uppermost of 30 m) was calculated for near-surface geologic units applying NEHRP's rules. The proposed site was categorized into two classes: class C (very dense soil and soft rock), that characterizes majority of the sites, and class D (stiff soil). These parameters are principally used as key inputs for building design.