Geotechnical Soundings Research Articles

Field measurements, laboratory tests and empirical relations for investigating the solid-to-fluid transition of a rapid earthflow

Earthflows are widespread phenomena in the Mediterranean area. These landslides involve fine-grained soils and clay-bearing rocks, and despite their low speed are responsible for significant economic losses in vast areas. The dynamics of these landslides is still relatively obscure thus reducing our ability to forecast and mitigate their effects. In this study, we present a methodological approach for the characterization of active earthflows based on the combination of geophysical surveys, laboratory tests, and empirical formulas. Geophysical surveys consist of periodic measurements of Rayleigh wave velocities repeated over time to evaluate the change of stiffness with time of the landslide material. Laboratory tests combine Atterberg limits, fall cone and oedometric tests and allow to constrain the empirical correlations between geophysical and geotechnical properties. The method is designed to obtain relevant data when direct methods like boreholes or geotechnical soundings are not possible for safety reasons, with the aim of investigating the solid-to-fluid transition that can occur in rapid earthflows. We applied this approach to study the Montevecchio landslide (Northern Apennines of Italy), an active earthflow in young marine clays which was affected by multiple reactivations in the recent years. Results show that after a surge the earthflow material is very soft (shear wave velocity in the order of 50–60 m/s) and characterized by a high water content. However, the exact value of the Liquidity Index remains unknown due to the lack of direct measurements and to the uncertainty dictated by the empirical relationships.

Improvement of Shallow Seismic Characterization Using the Singular Value Decomposition (SVD) Method in Seismic Data Inversion: A Case Study of a Site in Northeast Mexico

Several approaches can be taken to conduct seismic data inversion. However, usually, these approaches are unable to distinguish vertical and horizontal heterogeneities. Seismic inversion through the singular value decomposition (SVD) method offers an adequate and simple way to improve these traditional inversion models. For this study P and S wave data were acquired at a site located in northeastern Mexico, obtaining their travel times. An inversion algorithm involving the SVD analysis was then developed to establish the seismic velocities of the lithological units. Further, images of compressional and shear-wave velocities ( Vpand Vs, respectively), Vp/ Vsratio, and elastic moduli (bulk, shear and Young's moduli, Lamé's constant, and Poisson's ratio) were obtained. These were compared with two geotechnical soundings positioned over a geophysical profile line. The geological features of the exposed units were recognized on some trenches. Further, seismic images demonstrated correlations with the thickness and distribution of the geological units. Unconsolidated sediments and fine-grain clastic rocks (in the Méndez formation) were clearly distinguished by the high velocity contrast. SVD seismic inversion has shown the ability to distinguish small physical heterogeneities of shallow geological units. Its application in civil engineering, hydrogeology, and to solve soil pollution problems can be relevant.

Probabilistic analysis of offshore geotechnical site investigation in a homogeneous stiff clay deposit

Unlike most onshore geotechnical projects, conducting offshore site investigations is often difficult and technically challenging due to the harsh ocean environment. Hence, an important cost consideration is minimizing the number of soundings for an offshore project while ensuring reliable and safe designs. This study examines the possibility of reducing the number of soundings at site by using practical numerical tools to predict cone tip resistance (qt ) at unsampled locations. Prediction errors were quantified within a probabilistic framework. Two prediction approaches, i.e. 2D linear regression and Kriging, were adopted. Site investigation data from an existing offshore wind farm with homogeneous stiff clay deposit were collected and analyzed. Result shows that in a homogeneous soil deposit, it is possible to use as little as 20% of the available data to predict the remaining 80% qt profiles with small error. Monopile deformation under typical design load was also calculated using shear strength derived from the qt profile at each wind turbine location. The small spatial variation of monopile deformation suggests that a reduction in the number of geotechnical soundings for sites with uniform soil conditions would not significantly impact the foundation design.

Mapping bedrock topography and moraine deposits by transient electromagnetic sounding: Oslo graben, Norway

ABSTRACTIn this research, we have studied the capabilities and limitations of the transient electromagnetic sounding applied to investigate the structure and composition of the moraine deposits covering a crystalline basement. It has been experimentally demonstrated that using the ungrounded antennas one can not only determine the electrical resistivity of different layers in the alluvium deposits and map the surface topography of the crystalline basement down to a depth of 70–100 m, but can also minimize the costs and the damage done to the environment. Additionally, transient electromagnetics helps to reveal the layers within the moraine strata that often have different resistivities and depths. The use of single antennas of size 12 m × 12 m–25 m × 25 m for field excitation and registration of transient responses allows for maximum localization of the targets and provides a high horizontal resolution in strongly heterogeneous media. The limitations of the method due to the induced polarization effect have also been investigated. Overall, despite some limitations, the transient electromagnetic method appears to be effective in the mapping, assessment and analysis of hazards in landslide‐prone areas, prior to geotechnical soundings in wells.

Combined analysis of electrical and electromagnetic methods with geotechnical soundings and soil characterization as applied to a landslide study in Campos do Jordão City, Brazil

Every year, especially in summer, Brazil suffers from floods and landslides triggered by intense rainfall. These landslides are responsible for many fatalities and material losses that greatly affect the local population. The municipality of Campos do Jordão, in São Paulo state, presents such a history of mass movements that have resulted in fatalities and financial losses. A map of the municipality's risk assessment shows that about 32% of the municipality's population lives in risky areas. For this reason, Campos do Jordão is currently one of the pilot areas being used in studies for understanding landslides and mitigation techniques. In this context, this paper presents the results of an integration of geophysical surveys of, and geotechnical information on, the area. The geophysical methods applied were electrical resistivity, induced polarization, and frequency-domain electromagnetics. The geotechnical data included standard penetration test soundings and soil characterization (using particle density and X-ray fluorescence methods). The main contribution of this work is to present results from the joint application of these methodologies. Results from one of the critical areas, where a school had suffered the impact of a mass movement, indicated the displacement and burial of the retaining wall, which was built in 2002, as well as the shape of the movement, which affected the foundations of the school. It was also possible to determine the critical geological structures involved in the slope dislocation, and the potential problems with the school's foundations. The combined application of geophysical and geotechnical methods to landslide research is not very common, and so this study aimed to provide a field-based example to demonstrate the potential utility of such an integrated approach. The aim was also to foster collaboration with studies of mass movements in Brazil and help create methodologies for investigating and understanding the phenomenon.

Geophysical pre‐investigation for a Stockholm tunnel project: joint inversion and interpretation of geoelectric and seismic refraction data in an urban environment

ABSTRACTUnderground constructions for public traffic purposes are becoming increasingly important for urban areas in order to use the limited space more efficiently. Several electric resistivity tomography and seismic refraction tomography measurements were performed crossing a water passage near Stockholm during the pre‐investigation phase of a tunnel building project. The objective was to determine the bedrock interface and qualitatively assess the rock quality. The scope of this study is to present a field case in an urban environment and show improvements of geophysical results due to additional model constraints by a joint inversion. Results of individual inversions show a large transition zone below the seabed from electric resistivity tomography. Some parts of the seismic refraction tomography have a low model resolution, due to gas‐bearing sediments with a low velocity together with a high noise level, which leads to insufficient investigation depth that makes it difficult to determine the bedrock interface. However, the bedrock interface could be reconstructed in the resistivity model by performing a joint inversion, using the seismic velocity model to constrain the electric resistivity tomography model and vice versa. Adjacent geotechnical soundings support the joint inversion results. A vertical low resistive zone could be identified as a dominant fracture zone in the southern part of the investigated area. In general, the joint inversion approach significantly improved the electric resistivity tomography results and provided a more reliable bedrock estimation.

Combined geophysical surveys for the characterization of a reconstructed river embankment

The managing and monitoring of natural and artificial river levees are crucial in order to reduce the hydrological risk. As these hydraulic structures are very extensive, the typically applied techniques (e.g. geotechnical soundings and visual inspections) provide only punctual information that are generally focused on areas already recognized as troublesome. To overcome these well-known issues, non-invasive and cost-effective geophysical measurements have been proposed (and used) to supply spatially extensive data that should be integrated with direct investigations. Therefore, in this paper we present the joint use of multichannel analysis of surface waves (MASW), electrical resistivity tomography (ERT), and ground penetrating radar (GPR) to characterize a reconstructed river embankment (made of concrete and tout-venant) prone to serious leakages. We compare common lengthwise ERT profiles, performed from the levee crest, with cross-embankment and cross-river profiles, showing how relying only on the first type of surveys may lead to misinterpretations. Furthermore, we take advantage of a land-streamer for the MASW surveys and of a trans-illumination approach to improve the GPR application. The comparison between geophysical and geotechnical data helps identify the spatial extent of the volume actually invaded by grouting injections, which appears as a highly electrically conductive mean. The lack of homogeneity within this domain, highlighted by the cross-embankment ERT profiles, can be related to the seepage phenomena affecting this artificial levee. Therefore, this case study demonstrates the effectiveness of combining direct and non-invasive investigations for the characterization of river embankments.

Geophysical and geotechnical studies of geology and sediment properties at a quick-clay landslide site at Esp, Trondheim, Norway

Increased urbanisation and development in previous marginal areas have led to the need to protect the population, the environment and infrastructure from hazards such as quick-clay landslides. In this paper the opportunity which was presented following the gathering of a large amount of data in the area of the quick-clay landslide at Esp, Trondheim, Norway in 2012 is explored. The data is of particular interest given the availability of Frequency domain Helicopter-borne ElectroMagnetic data (FHEM) and extensive LiDAR imaging as well as a full range of in situ geophysical and geotechnical soundings and samples. The ground conditions in the area are shown to be complex and it is clear that a multi-disciplinary type approach combining geophysical, geomorphological and geotechnical investigations is necessary in order to understand the hazards posed by potential quick-clay landslides. The investigation shows that very good overview mapping of the area can be obtained from FHEM and that the data obtained from local electrical resistivity tomography (ERT) and resistivity cone (RCPTU) are consistent with each other and with the FHEM data. Use of ERT immediately after the landslide was particularly useful in delineating further risk areas and as an aid for assessment of evacuation of the local population. LiDAR data is particularly useful in understanding the landslide “trigger” mechanisms, the depth of the landslide, the form of the run-out and the nature of the material in the run-out. Geotechnical data collected in the area is characteristic of that of soft sensitive Norwegian marine clays and the various geotechnical and geophysical parameters correlate well with one another and also follow well-established patterns. The data shows that no single investigative method is available to accurately define quick-clay layers but that a number of complimentary in situ and laboratory techniques are necessary for that purpose.

Evaluation of the change in dominant periods in the lake-bed zone of Mexico City produced by ground subsidence through the use of site amplification factors

We compute site amplification functions for several sites in Mexico City using actual accelerograms recorded from 1985 to 2010 and we present field evidence of the change in the dominant period of a given site (Ts) as a consequence of ground subsidence produced by groundwater withdrawal. The changes in Ts are larger in the lake-bed zone where thicker clay deposits exist, although there are sites in the southwest part of the lake-bed zone where Ts has remained constant. With the information obtained from the site amplification functions and available geotechnical soundings we develop an empirical model to estimate the future value of Ts for several sites in Mexico City. Because the practical application of the model requires extrapolation we also present a method to compute the uncertainty of the model when it is used to forecast a future value of Ts at a given site. Our results suggest that significant changes in the dominant period at several sites in Mexico City can be expected in the future.

Geophysical and Geotechnical Investigations of a Landslide in Kekem Area, Western Cameroon

Geophysical and geotechnical surveys were conducted in the Western Cameroon (Kekem area) following a landslide on argillaceous material in order to understand the triggering processes and mechanisms of this landslide and to assess the stability of the slope. The geophysical soundings consisting of vertical electrical soundings with the Schlumberger electrode array configuration were carried out to monitor the behaviour of electrical resistivity in the landslide. Geoelectrical data showed a zone of low resistivity values identified as a clayey sand-filled aquifer. This aquifer played an important role in the triggering process of the landslide. Geotechnical soundings showed that the aquifer had a thickness of 7.0 m. The depth from the landslide crest level to the failure surface reached 3.0 m and 20.6 m. Laboratory tests were then carried out in order to evaluate the cohesion of the soil and the angle of internal friction, and to calculate the safety factor in view of making a stability analysis. The laboratory results exhibited a soil with low consistency, almost doughy. The mean value of the safety factor (1.4) been lower than the slope stability coefficient (1.5), revealed that the slope is unstable, likely to know at any moment a reactivation of the slide. This study showed that electrical soundings coupled with geotechnical surveys are useful tools for the characterization of landslides.

Statistical correlations of shear wave velocity and penetration resistance for soils

In this paper, the correlation between shear wave velocity and standard penetration test blow counts (SPT-N) is investigated. The study focused primarily on the correlation of SPT-N and shear wave velocity (Vs) for several soil categories: all soils, sand, silt and clay-type soils. New empirical formulae are suggested to correlate SPT-N and Vs, based on a dataset collected in a part of Eskişehir settlement in the western central Anatolia region of Turkey. The formulae are based on geotechnical soundings and active and passive seismic experiments. The new and previously suggested formulae showing correlations between uncorrected SPT-N and Vs have been compared and evaluated by using the same dataset. The results suggest that better correlations in estimation of Vs are acquired when the uncorrected blow counts are used. The blow count is a major parameter and the soil type has no significant influence on the results. In cohesive soils, the plasticity contents and, in non-cohesive soils except for gravels, the graded contents have no significant effect on the estimation of Vs. The results support most of the conclusions of earlier studies. These practical relationships developed between SPT-N and Vs should be used with caution in geotechnical engineering and should be checked against measured Vs.

Apport de l'association des méthodes TDEM ( Time-Domain Electromagnetism) et électrique pour la connaissance de la structure du glissement-coulée de Super Sauze (bassin de Barcelonnette, Alpes-de-Haute-Provence, France)

The aim of this study is to determine an investigation protocol that permits the Super Sauze flowslide to be described along continuous lines, through the association of electrical and TDEM (Time-Domain Electromagnetism) methods. The transition from one resistivity range to another often corresponds to the three layer limits defined by geotechnical soundings. The results presented show that the combined methods are well suited to the question investigated and that they are validated by the geotechnical data.