Michele Cercato

Shear-wave velocity profiling at sites with high stiffness contrasts: a comparison between invasive and non-invasive methods

Three sites of the Italian Strong Motion Network (RAN) have been selected for detailed S-wave profiling, using both borehole and surface wave seismic methods. At these sites, the presence of stiffness contrasts within the soil column is found to influence the surface wave propagation profoundly. Advanced aspects in surface wave inversion such as resolution, accuracy and higher-mode interpretation must be properly taken into account to obtain realistic results from the surface wave dispersion observations. The possibility of mode misidentification and the loss of resolution with depth in surface wave interpretation are explored using synthetic modelling together with active and passive seismic data sets. With high stiffness contrasts, the possibility of mode jumps and higher mode dominance over specific frequency ranges is very probable. This is true also for normally dispersive sites, where the shear velocity increases with depth, though higher mode dominance is recognized as more common in the case of a shear-wave velocity inversion within the soil column and the sensitivity of the dispersion curves with respect to those layers beneath the low-velocity zone may be significantly reduced. Pitfalls in the inversion resulting from mode misidentification can be avoided by investigating the effective phase velocity distribution, using active data sets and full waveform seismic modelling. When an unambiguous modal identification is achieved, the results obtained by surface wave inversion are very satisfactorily consistent with borehole data.

Detection and imaging of piping sinkholes by integrated geophysical methods

Piping sinkholes may naturally develop in the case of a thick overburden overlying calcareous bedrock. Their detection and imaging is a challenging task for geophysical methods, not only because of the required resolution and depth of penetration, but also because major pitfalls may arise, in such geologically complex areas, from the speculative interpretation of geophysical anomalies as geological features. Data integration from different geophysical methods is essential to remove these interpretation ambiguities, caused by large near-surface gradients and heterogeneities in the soil properties, as well as by oscillations of the water table and anomalous water circulation. We present an investigation procedure consisting of the sequential application and integrated interpretation of Electrical Resistivity Tomography (ERT), Seismic Refraction Tomography (SRT) and Self Potential (SP) measurements for locating and monitoring piping sinkholes with application to a site in Central Italy. This approach is a compromise between resolution and cost-effectiveness, and it is designed to be economically affordable by the private end user. In complex geological scenarios, it is usually not possible to rate a single geophysical technique as superior to all the others in terms of resolution, cost-effectiveness and diagnostic capability. The independent information coming from the different geophysical methods is the key to removing interpretation ambiguity when evaluating the position and the development over time of the piping sinkholes. The application of the proposed investigation procedure allowed us to individuate a small area subject to the formation of a piping sinkhole. The geophysical results were confirmed about one year after the execution of the geophysical measurements, as the site exhibited surface evidence of a piping sinkhole, with the formation of a small pond filled with sulphurous water and gases coming from below.

Assessing foundation stability and soil-structure interaction through integrated geophysical techniques: A case history in Rome (Italy)

We present a case history in which several geophysical methods were used to investigate a five-floor residential building, constructed from reinforced concrete, which has been seriously damaged by differential settlement. The results from various geophysical methods (electrical resistivity tomography, seismic refraction, multichannel analysis of surface waves, cross-hole seismic surveys) were integrated and correlated to give an overall picture of the subsoil geometry and characteristics in the area under investigation, with particular regard to the stiffness properties and the degree of saturation. The ability of geophysics to investigate non-invasively has proven successful for dealing with buildings affected by foundation instability. The resolution of the results obtained from experimental data provides additional insight into the geological scenario and the causes of the settlement. Moreover, geophysical data may be used as a guide to optimize borehole locations for coring and soil specimen collection, which are necessary steps in designing the restoration intervention.

Geophysical investigation for the rehabilitation of a flood control embankment

To comply with recently published seismic regulations and environmental standards, existing dams and embankments have to be evaluated for safety control, in addition to standard maintenance and repair, which is common practice for aging structures. In either case, engineering geophysics is almost the only viable option for investigating these structures and the underlying soil as a whole. In this contribution, electrical and seismic surveys are performed on an outdated flood control embankment that is expected to be put into service again. Integration of DC resistivity results with those of various seismic prospecting methods (seismic refraction, cross-hole S-wave and P-wave tomography and surface wave analysis) is found to be successful for defining a clear physical representation of the entire structure. The low-strain elastic properties (from seismic speeds of propagation) as well as the geometrical characteristics of the internal core of the dam and of the foundation soil serve as guidance for the rehabilitation intervention.

The Importance of Geological Models in Understanding and Predicting the Life Span of Rockslide Dams: The Case of Scanno Lake, Central Italy

In order to explain the long life span of the Scanno rockslide-avalanche dammed lake (Central Italy), whose age of impoundment is older than 2,300 years, and to get indications about its present and future stability conditions, it was assumed of fundamental importance to build a representative geological model aimed at defining (i) the geometry of the boundary surface between the rockslide debris and the bedrock, (ii) the characteristics of the debris with special reference to hydraulic behaviour, (iii) the flownet within the landslide deposit taking into account the complex geological and hydraulic boundary conditions imposed by the palaeovalley morphology and by the numerous springs downstream of the dam which are fed by different aquifers. The reconstruction of the natural dam geometry (volume, heigth and width) was possible by means of integrated geological (site surveys and boreholes) and geophysical (electric tomography and seismic refraction) investigations: the so obtained data were processed in order to assess the stability conditions according to existing geomorphological indexes. As a result, good stability conditions of the analysed natural dam are highlighted on the basis of its geometric features. A campaign of pumping tests, hydrogeological measurements, chemical and isotope analyses was carried out with the aim of defining the flownet within the rockslide-avalanche deposit and its hydraulic properties. The so depicted hydrogeologic characteristics indicate that dam failures by piping processes are very unlikely, thus supporting the dam stability conditions.

Shear-wave velocity profiling at sites with high stiffness contrasts: A comparison between invasive and non-invasive methods

Three sites of the Italian Strong Motion Network (RAN) have been selected for detailed S-wave profiling, using both borehole and surface wave seismic methods. At these sites, the presence of stiffness contrasts within the soil column is found to influence the surface wave propagation profoundly. Advanced aspects in surface wave inversion such as resolution, accuracy and higher-mode interpretation must be properly taken into account to obtain realistic results from the surface wave dispersion observations. The possibility of mode misidentification and the loss of resolution with depth in surface wave interpretation are explored using synthetic modelling together with active and passive seismic data sets. With high stiffness contrasts, the possibility of mode jumps and higher mode dominance over specific frequency ranges is very probable. This is true also for normally dispersive sites, where the shear velocity increases with depth, though higher mode dominance is recognized as more common in the case of a shear-wave velocity inversion within the soil column and the sensitivity of the dispersion curves with respect to those layers beneath the low-velocity zone may be significantly reduced. Pitfalls in the inversion resulting from mode misidentification can be avoided by investigating the effective phase velocity distribution, using active data sets and full waveform seismic modelling. When an unambiguous modal identification is achieved, the results obtained by surface wave inversion are very satisfactorily consistent with borehole data.

Geophysical Investigation for the Rehabilitation of a Flood Control Embankment

To comply with recently published seismic regulations and environmental standards, existing dams and embankments are now being examined for maintenance, repair or rehabilitation. Engineering geophysics is almost the only viable option for investigating these structures and the underlying soil as a whole system. In this contribution, electrical and seismic investigations are performed on an outdated flood control embankment, that has to be put again into service. Geophysical investigation has proven successful to determine the relevant properties of the embankment and the main geometrical features of the underlying subsoil, serving as an important guidance for the rehabilitation intervention.

Focusing on soil-foundation heterogeneity through highresolution electrical and seismic tomography

The reconstruction of the current status of a historic building is essential for seismic safety assessment and for designing the retrofitting interventions since different safety and confidence factors have to be assumed, depending on the level of information about the subsoil structure. In this work, we present an investigation of the shallow subsurface below and around a historic building affected by differential settlements in order to define its geometry and to characterise its stiffness at low strain. To this end, we employed high-resolution electrical resistivity and seismic (both P-wave and S-wave) tomographies. A three-dimensional electrical resistivity tomography survey was performed to obtain more information about the type and the maximum depth of the building foundation. Electrical resistivity and seismic tomographies were carried out alongside the building, aimed at imaging the top soils and the near-surface geometry. The corresponding inverted models pointed out a remarkable heterogeneity of the shallow subsoil below the building, which is partly founded on a weathered layer and partly on a more rigid lithotype. This heterogeneity is probably a concurrent cause of the building’s instability under both static and seismic loading. Our results demonstrate that the man-made fillings and the top soils have to be thoroughly investigated to fully understand the soil-structure behaviour. In this light, the integration of non-invasive high-resolution geophysical techniques, especially tomographic methods, has been proved to properly address the problem of imaging the shallow subsoil.

Focusing on Soil Foundation Heterogeneity through High-resolution Tomography

An historical building affected by differential settlements, which were triggered by an earthquake, is investigated by means of high-resolution tomography, both electrical and seismic. The objective is to image the geometric structure of the shallow soil below the building and to characterize its stiffness at low strain. A preliminary reconstruction of the geological units has been recovered through the combined use of electrical and seismic data, where the depth of the travertine bedrock varies significantly within the study site. The range of variation of the main geophysical parameters (resistivity, P- and S-wave velocities) inferred from these models has been set as reference point for tuning the results obtained from the geophysical survey performed near the building. The inverted tomographic models obtained from data acquired alongside the building exhibit heterogeneity of the shallow subsoil, which is partly founded on a weathered layer and partly on a more rigid lithotype, probably a fractured travertine or a gravel layer. Therefore the fill anthropic soils can play a relevant role for the structural stability in case of shallow foundations built on a heterogeneous subsoil.

Shear-wave velocity profiling at sites with high stiffness

Three sites of the Italian Strong Motion Network (RAN) have been selected for detailed S-wave profiling, using both borehole and surface wave seismic methods. At these sites, the presence of stiffness contrasts within the soil column is found to influence the surface wave propagation profoundly. Advanced aspects in surface wave inversion such as resolution, accuracy and higher-mode interpretation must be properly taken into account to obtain realistic results from the surface wave dispersion observations. The possibility of mode misidentification and the loss of resolution with depth in surface wave interpretation are explored using synthetic modelling together with active and passive seismic data sets. With high stiffness contrasts, the possibility of mode jumps and higher mode dominance over specific frequency ranges is very probable. This is true also for normally dispersive sites, where the shear velocity increases with depth, though higher mode dominance is recognized as more common in the case of a shear-wave velocity inversion within the soil column and the sensitivity of the dispersion curves with respect to those layers beneath the low-velocity zone may be significantly reduced. Pitfalls in the inversion resulting from mode misidentification can be avoided by investigating the effective phase velocity distribution, using active data sets and full waveform seismic modelling. When an unambiguous modal identification is achieved, the results obtained by surface wave inversion are very satisfactorily consistent with borehole data.