Abstract

Traditional geotechnical investigation consisting of drilling many of boreholes, soil sampling and lab testing becomes uneconomical and time consuming for large and geologically intricate areas. In such cases, non-destructive geophysical surveys offer a time-efficient and economical solution, simultaneously providing an extensive areal coverage of subsurface profiling of geotechnical and geophysical properties. For the estimation of subsurface properties, three geophysical methods were employed in this study, which consisted of Ground Penetration Radar (GPR), Electrical Resistivity Tomography (ERT), and Multi-Channel Analysis of Surface Waves (MASW). Thus, the sub surface’s shear wave velocity profiles and apparent resistivity profiles were obtained. In the shallow depths, discontinuities and fractures were studied using electromagnetic radargrams from GPR. These tests were complemented with borehole exploration at the site. Results obtained from surface-based methods were validated against bore log data and visual observation of surface terrain. Subsequently, bore logs were integrated with the geophysical survey results to construct an integrated subsurface profile. The integrated subsurface profiles presented a three-layer subsurface structure consisting of dense gravelly sand in the top 1m, followed by strong rock formations and, ultimately, very strong fractured rocks. The extent of fracture in the rocks was studied using samples obtained from boreholes and available trenches at the test location. These findings helped compare the three methods and their applicability in delineating different subsurface layers in this study. GPR proves to be effective at shallow depths, while ERT and MASW help investigate deeper layers better. Further, this study offers critical insights for site characterization and engineering decisions in complex geological environments, improving the knowledge base of efficient and reliable subsurface evaluation techniques.

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

Abstract

Six structural domains were defined using data from acoustic televiewers and oriented boreholes as part of the reopening of the old Aznalcóllar mining site in Sevilla, southern Spain, by Minera Los Frailes (MLF). Additionally, geotechnical subdomains were defined within each structural domain, classifying the subdomains based on geotechnical parameters obtained from the geotechnical logging of boreholes and through correlations. The methodology used to define the domains allows mine design and planning to be done with enough accuracy to determine the underground excavation needs and the systematic methodology. The main source of this new structural information was acoustic borehole image televiewers (ABI) and, on a much minor scale and as a complement for ABI data, oriented boreholes. This paper describes the methodology used for this definition, some of the challenges encountered during the investigation, the main results obtained and the utility of these defined structural domains in the development of the MLF mining site reopening. Additionally, a preliminary approach to an updated methodology in the definition of structural domains with the combination of televiewer data, both acoustic and optical, with data obtained from other downhole probes is also included.

Abstract

Offshore wind plays a pivotal role in enhancing Europe's energy security and achieving energy decarbonization goals. However, expediting offshore wind deployment necessitates efficient and economical site investigation surveys. To address this challenge, we introduce a novel approach utilising a deep neural network (DNN) to establish correlations between geotechnical cone penetrometer test (CPT) data and shear wave velocity (𝑉) from seismic CPT. Subsequently, porosity and P-wave velocity (𝑉) are derived using a 𝑉 to bulk density correlation and a dynamic poroelastic model. The DNN is trained and tested on a dataset comprising 5284 instances of public-domain geotechnical CPT test data, including depth, tip resistance, sleeve friction, and 𝑉 from seismic CPT. During testing, the DNN model demonstrates a mean absolute error of 55 m s-1 between predicted and measured 𝑉 values. The uncertainty in 𝑉 predictions is attributed to factors such as (i) limited training data for some soil types such as gravelly sands, (ii) intricate relationship between geotechnical CPT features and seismic properties influencing 𝑉, (iii) the presence of CPT features and 𝑉 combinations that lie well outside the region from most combinations (i.e. outliers), and (iv) CPT features and 𝑉 measurements that are averaged over different depth ranges. The derived porosity and 𝑉

Abstract

Understanding subsurface conditions is critical to creating and maintaining resilient infrastructure systems, such as dams and levees. Seismic geophysical tools can be very effective for site characterization of these structures as they directly measure the elastic moduli and can provide insight into both the soil properties and groundwater conditions. Full waveform inversion (FWI) is one processing option for seismic geophysics that seeks to overcome some of the limitations in the traditional approaches by using the full time-domain recording of the wavefield to develop 2D or 3D profiles of shear wave velocity. In addition to providing characterization data, FWI can also potentially be used as a monitoring tool for dams and levees to assess how elastic moduli are changing with time and to infer how these changes might relate to changes in the hydromechanical properties of the soil. This study seeks to explore the use of seismic FWI as both a characterization and monitoring tool through numerical simulations of seismic surveys on a hypothetical levee with a low velocity anomaly in the foundation. The simulations are used to assess both the spatial resolution and the ability of the simulations to detect changes in properties that might be related to softening of the foundation or development of internal erosion failure modes. The findings from the study will be used to highlight potential benefits and challenges to using seismic FWI for characterization and monitoring of dams and levees.

Abstract

The presence of a non-liquefiable crust overlying a liquefiable layer plays a significant role in determining the occurrence of liquefaction damage, as originally formulated by Ishihara in 1985. Following the 2010-2011 Canterbury seismic sequence (New Zealand), almost no foundation deformation occurred in areas characterized by soils susceptible to liquefaction overlaid by at least 3 m-thick intact crust. In contrast, the 2012 Emilia-Romagna earthquake (Italy) provided evidence of liquefaction in silty-sandy layers below 3 to 9 m-thick crusts. Therefore, Ishihara’s approach and the variety of liquefaction severity indices need to be further tested to assess to what extent they can be considered reliable predictors of performance. This study aims at better understanding the role of non-liquefiable crusts in preventing damage to buildings and infrastructures. In this respect, in situ and laboratory tests were conducted at selected sites in EmiliaRomagna. The results of two case studies in Mirandola (Modena, Italy), which share similar soil profiles but exhibited different liquefaction evidences following the 2012 seismic sequence, are presented. Comprehensive geotechnical and geophysical surveys were performed at both the sites, by means of piezocone tests, seismic dilatometer tests, boreholes, laboratory tests, electrical resistivity tomography and multichannel analysis of surface waves. These surveys document the geotechnical and geophysical properties of the 5 m-thick non-liquefied (or potentially non-liquefiable) crust and of the liquefied (or potentially liquefiable) silty-sandy deposits. This effort is aimed at understanding how the surface layer properties contributed to the different behavior observed at the two sites during the earthquake events.

Abstract

The Cañaveral shopping center is located on a low slope formed by recent alluvial deposits of low consistency, supported on sedimentary rock of the Girón formation, which is found at depths that change abruptly between one and twelve meters in short distances, influenced by the presence of a geological fault line and processes of erosion and weathering by subway water currents. Considering that the loads on the foundation are of great magnitude, it is necessary to build about 100 deep caisson foundations to reach the rock. However, performing a borehole on each one of them represented a very high cost and long execution times that would generate discomfort in the operation of the warehouse. Therefore, an alternative was proposed to determine the depth levels of the rock through different geophysical tests of seismic lines and electrical tomography and try to calibrate the results through the correlation of some boreholes. During the execution and data processing phase of the geophysical tests, difficulties were encountered due to the level of environmental noise and the obtaining of the wave records, given the variability of the rock levels and even the effect that in some sectors of the study area the rock is very sub-surface. In conclusion, it was possible to identify the strengths and weaknesses of each of the exploration methods and generate a 3D map of the depths of the rock levels that served as input for the design and budget of the foundations, which were verified by constant supervision during construction and thus be able to determine the error percentages, as well as to present recommendations for future works where it is intended to use this type of techniques

Abstract

The Metropolitan Area of Bucaramanga is located within one of the three seismic nests in the world and therefore presents a high seismic and geotechnical threat (INGEOMINAS, 2001), therefore certain zones, especially those with alluvial and colluvial deposits, have limitations for the development of infrastructure projects, unless a threat assessment is carried out that indicates otherwise or, in its absence, it is reduced to low levels with stabilization works. The thicknesses of these deposits vary from a few meters to two hundred meters and their definition and dynamic behavior (damping coefficient and terrain degradation curves), would allow analysis with special non-linear constitutive models to better understand their response to a possible earthquake and to ensure the stability of the structures and even an optimization based on a possible lower spectrum. A new two-tower housing project of up to 36 stories high is a perfect scenario to carry out a local seismic response study and perhaps transform the way future projects are designed. It should be noted that, given the acceptable stiffness and granular composition of the soils, it is difficult to take unaltered samples and the Colombian NSR-10 standard does not require local behavior studies and therefore its study has not been properly established. It is sought through geophysical tests and laboratory tests such as triaxial, resonant column and bender element to perform such characterization and to perform soil modeling that allows the calculation engineer to have the appropriate inputs for their analysis.