Geotechnical Units Research Articles

Classification of geotechnical units and their associated slope movements for application to civil engineering in volcanic territories

Slope movements are one of the natural hazards that most affect volcanic islands and often cause alarm to society, as they produce considerable material losses, affect communications and basic community services and, on occasion, involve the loss of human lives. Based on the classification of geotechnical units of the Canary Islands and the characteristic slope movements observed in each of them, the main goal of this article has been to elaborate a classification of movements versus units, as a tool for the management landslides hazards in volcanic environments. The applied methodology was based on the types of slope movements defined by the USGS, and on the research team three decades of experience of the study of geotechnical characterisation and slope instability of volcanic formations. A graphical classification of the possible movements affecting the volcanic geotechnical units has been prepared to make a preliminary estimate of the types of movements that will affect a slope or hillside.

Identification of geotechnical units in soil exploration through principal component analysis and clustering

AbstractObtaining accurate information on soil characteristics is essential for large‐scale construction projects. Geotechnical exploration methods are commonly used to obtain information on soil characteristics. However, integrating the characteristics obtained through different exploration methods can be challenging. To address this problem, this study proposes the use of Principal Component Analysis (PCA) to combine geotechnical variables obtained from cone penetration tests (CPT), standard penetration tests (SPT) with soil sampling, and Suspension PS Logging tests. PCA and clustering analysis were applied to the data collected from a study area, and the results were highly effective in identifying the stratigraphy and soil type, leading to the definition of geotechnical units. This approach provides valuable information for geotechnical engineering projects, enhancing the accuracy of stratigraphic investigations and improving our understanding of soil characteristics and their influence on construction projects. Moreover, the proposed technique can be applied in other areas and different types of soils, making it a valuable tool for evaluating soil characteristics.

Automatic coal mine roof rating calculation using machine learning

The Coal Mine Roof Rating (CMRR) is an essential input parameter for roof support design. The current manual calculation process brings evident subjectivity in selecting rock mechanical properties, roof unit classification, and fracture spacing calculation, which is needed for CMRR calculation. In this paper, state-of-the-art machine learning and computer vision techniques were applied to provide a repeatable and consistent data-driven solution to reduce the subjectivity of CMRR calculation through integration of geophysical logging data, drill core images, and laboratory data. Firstly, the group-based machine learning approach, which has demonstrated better performance than the conventional machine learning methods, was introduced to predict the uniaxial compressive strength (UCS) of roof strata. Then, an advanced computer vision model trained by transfer learning technique was adopted to extract the dimensions of the core pieces for the automatic rock quality designation (RQD) and fracture spacing calculation based on the drill core images. The geotechnical units within the roof bolted interval were classified in terms of the adjacent sedimentary layers, which share similar geotechnical properties. The predicted quantities (UCS, RQD, and fracture spacing) by the machine learning methods were converted into the corresponding ratings, which were further used for the unit ratings and machine learning based automatic CMRR calculation. The automatic CMRR values from machine learning models show a promising linear correlation (R2 = 0.78) with the manually calculated CMRR values, demonstrating that this new approach has the potential to be applied as an alternative method, which delivers a benchmark of CMRR values to enhance its calculation confidence and objectivity.

Remote sensing applied to geological, structural, and mass movements characterization in the connection between Curral Homocline and Moeda Syncline, Quadrilátero Ferrífero Region, Brazil

Mass movements naturally occurring along slopes, especially after heavy rains, cause heavy losses of materials and life and affect highways, pipelines, and bridges, among other structures. Among the most common mass movements, landslides and subsequent mass flows especially stand out. Remote sensing and geoprocessing techniques are useful tools for geological and geotechnical analysis, here applied to characterization of mass movements from the analysis of the influence of slopes, lineaments or fractures, and geological-geotechnical at the junction of the Curral homocline and the Moeda syncline, Quadrilátero Ferrífero, Southeast Brazil. The area consists of Archean granitic-gneissic basement and the greenstone belt sequence of the Rio das Velhas Supergroup with Proterozoic metasedimentary rocks of the Minas Supergroup covering these two older successions. Based on satellite images, a preliminary map of lineaments was created, and fractures were grouped according to their direction before being rasterized. Each lineament group was associated with a previously known geological and/or structural feature. Geological units mapped in the area were numerically classified according to their mass movement susceptibility and related geotechnical units. Using a Shuttle Radar Topography Mission (SRTM) image, a terrain slope raster was generated. The objective was to obtain a raster indicating low and high probabilities of mass movements. Four sets of lineaments were identified: the first set associated with the northeast strike stratification of the rocks of the Moeda Formation; the second set related to eastward-trending warp axis lineations; the third set related to the southeast trend of stretch lineaments or mafic dykes directions; and the fourth set evidently associated with the watershed pattern of the Curral Range crest, perpendicular to the strike of the rocks in the area. Simulations with different combinations of parameters produced eight scenarios were generated were constructed to classify the area as to its susceptibility to rupture through changes in slope, lithotype, and lineament density; the first four (1 to 4) were classified by using the slope percentage raster without interval reclassification, so its values in the cartographic algebra were not grouped. The last for (5 to 8), consider the slope percentage classified. This methodology successfully combines mathematical models for predicting mass movements and determining areas in the urban landscape most susceptible to these phenomena. As such, it comprises a useful tool for government planning of preventative actions for areas of high risk.

Geotechnical characterization of Viña del Mar for areas where buildings were affected by mega-earthquakes in Central Chile

Over the course of little more than a century, several large-magnitude earthquakes have struck Central Chile. In the area within the Marga-Marga basin in the city of Viña del Mar, notable damage to buildings suggests the occurrence of local seismic site effects. While seismic microzonation studies have attempted to explain such effects, the geotechnical characteristics adopted are generally oversimplified. This article addresses this with a thorough geotechnical characterization of the areas in Viña del Mar that sustained outsized damage to structures. The article provides a robust and updated geotechnical and geophysical database of the area, with additional analysis from records of the city’s urban development, geomorphology, and geology features. The study builds on geotechnical data from previous seismic zonation studies following the 3 March 1985 earthquake; building design projects undertaken between 1977 and 2018, including 89 standard penetration test (SPT) boreholes, 58 dynamic cone penetration tests (DCPT), 18 refraction microtremor surveys (ReMI), and 16 Down-Hole tests; and rock depth estimates from 63 horizontal to vertical spectral ratio (HVSR) tests. These data were used in data spatialization techniques to model geotechnical units along one (1-D) and two-dimensional (2-D) profiles, for the foundations of buildings damaged after the 1906, 1985, and 2010 earthquakes. The results show the existence of deep the existence of deep soft soils, high impedance contrast between geotechnical units shape of rock, and the dynamic soil properties can explain the damage observed in the studied area damage in the studied area. The study provides significant background and experimental information for the area, where there is enough seismic potential available to produce a large earthquake in the coming decades of similar characteristics as the 1730 Valparaiso earthquake (9.1–9.3 Mw).

Internal and External Collapse Analysis of Twin-Tubes Tunnel in the Initial Support Stage

The characterization of rock mass along the tunnel alignment based on physical, geological and geotechnical data of the project area was used in this work. The support systems were recommended for all geotechnical units using RMR and tunneling quality index (Q-system) support chart. The paper also consists in making a two-dimensional numerical study of the stability of the left lateral slope of the Portal of the tunnel with the Slide 6.0 software and inside provisional support with the Phase.2 2D program. The study shows good results of the internal rock mass stability of the tunnel and satisfactory safety factor (FoS) in terms of collapse analyzes of the lateral slope of the tunnel portals.

Hydrothermally induced edifice destabilisation: The mechanical behaviour of rock mass surrounding a shallow intrusion in andesitic lavas, Pinnacle Ridge, Ruapehu, New Zealand

Volcanic edifices are composed of an assortment of materials with varying mechanical behaviours. The range in mechanical behaviours of these rock materials presents a critical unknown element to volcanic stability assessments. In this study, we explore the deformation behaviour (e.g. uniaxial, triaxial, and tensile deformation behaviour) of volcanic materials from seven geotechnical units associated with a small, shallow intrusion at Pinnacle Ridge, Ruapehu, New Zealand. First, we provide a complete characterisation of the rock masses using field (e.g. rock mass characterisation) and lab (compressive and tensile strength analyses) data for each geotechnical unit. The new data show a range of tensile and triaxial compressive strengths dependent largely on porosity, which is further modified by hydrothermal alteration. More altered rocks are generally weaker (with the exception of the brecciated lava margins) under triaxial conditions. Altered rocks generally transition from brittle to ductile behaviour at lower confining pressure, although this depends on the porosity.Using the stratigraphy and geomorphology of Pinnacle Ridge in conjunction with the rock property data we collect, we then use finite element modelling to depict the deformation behaviour of these units under different pore pressure and seismic conditions. The modelling shows that a relatively minor increase in pore pressure (e.g. < 3 MPa above hydrostatic conditions) is sufficient to change the slope failure type from deep-seated rotational sliding to extensional rupture. The presence of a low-permeability, low-rock mass strength material, even if very localised like a hydrothermal vein, can reduce slope stability by locally increasing pore pressure and providing a weakness plane that facilitates failure. The modelling also shows that a relatively minor increase in pore pressure (e.g. < 6 MPa above hydrostatic conditions) in a shallow hydrothermal system is sufficient to cause localised failure in the material, especially in the hydrothermal vein material. Additionally, the low-permeability hydrothermal vein material can cause pore pressure compartmentalisation, decreasing the strength factor in all materials in the compartment. While Pinnacle Ridge and its associated hydrothermal veins are >1 km from the currently active hydrothermal system, similar veins may be present near active parts of the hydrothermal system at Ruapehu and are an important consideration for future stability models at Ruapehu and other volcanoes.

The possible use of equivalent homogeneous subsoil models for 1D seismic response analyses in seismic microzonation studies

The possibility is here explored to use an ‘equivalent’ homogeneous configuration to simulate 1D seismic response of heterogeneous engineering-geological bodies when relatively weak seismic impedance contrasts internal to the bodies (it was assumed a shear wave velocity variation between the alternating layers equal to 150 m/s) only exist above the seismic bedrock. This equivalent configuration is obtained by considering an equivalent Vs value the harmonic average of the actual Vs values and a linear combination of G/G0 and D curves relative to the lithotechnical components present in the actual configuration. To evaluate feasibility of this approach, a wide set of numerical simulations was carried out by randomly generating subsoil layering including sequences of alternating thin layers of geotechnical units (e.g. sands and clays) each characterized by a characteristic nonlinear curve. Outcomes of these simulations are compared with those provided by considering a single homogeneous layer characterized by equivalent nonlinear curves obtained as a weighted average of the original curves. By comparing the heterogeneous and the homogeneous columns seismic response in terms of amplification factors and fundamental period, the results confirm the possibility to model a 1D column characterized by a generic lithostratigraphic succession with an equivalent one without introducing significative errors that, at least for the studied cases, do not exceed the 6%. This conclusion is substantially confirmed by extending the comparison to a real case, i.e., the 113 m-thick heterogeneous soil profile at Mirandola site (Northern Italy), presented in the last part.

The Relevance of Geotechnical-Unit Characterization for Landslide-Susceptibility Mapping with SHALSTAB

Given the increasing occurrence of landslides worldwide, the improvement of predictive models for landslide mapping is needed. Despite the influence of geotechnical parameters on SHALSTAB model outputs, there is a lack of research on models’ performance when considering different variables. In particular, the role of geotechnical units (i.e., areas with common soil and lithology) is understudied. Indeed, the original SHALSTAB model considers that the whole basin has homogeneous soil. This can lead to the under-or-overestimation of landslide hazards. Therefore, in this study, we aimed to investigate the advantages of incorporating geotechnical units as a variable in contrast to the original model. By using locally sampled geotechnical data, 13 slope-instability scenarios were simulated for the Jaguar creek basin, Brazil. This allowed us to verify the sensitivity of the model to different input variables and assumptions. To evaluate the model performance, we used the Success Index, Error Index, ROC curve, and a new performance index: the Detective Performance Index of Unstable Areas. The best model performance was obtained in the scenario with discretized geotechnical units’ values and the largest sample size. Results indicate the importance of properly characterizing the geotechnical units when using SHALSTAB. Hence, future applications should consider this to improve models’ predictivity.

Integrating uncertainty into geotechnical design of underground openings in tectonically undisturbed but lithologically varied sedimentary environments

A systematic but straightforward probabilistic approach was adopted for the preliminary design using a tunnel case in Egnatia Odos Highway. This tunnel was excavated in a heterogenous molassic formation in Northern Greece, and more specifically in tectonically undisturbed but lithologically varied sedimentary series. This research aimed to identify sources of uncertainty and evaluate its governance during the selection of the geotechnical design parameters. Rigorous analyses of the critical parameters and factors were undertaken following a standardised method for designing underground structures based on the probability theory. Initially, the statistical analysis of the intact rock strength parameters and project-specific geological and geotechnical factors allowed the development of understanding in determining these sources of uncertainty and assisted in quantifying them. The lithological units encountered in the tunnel alignment were classified into first, Geological Units and, secondly, into Rock Mass Types (Geotechnical Units). Finally, the latter were grouped into anticipated Rock Mass Behaviour Types, which were numerically investigated using RS2 for the Finite Element Method analysis and Unwedge to assess these Rock Mass Types' failure modes. For this purpose, the Point Estimate Method was used in the FEM analysis to evaluate the effect of uncertainty in the strength parameters. Ultimately, the numerical analysis results subsequently led to the preliminary design of the respective support sections. Although this investigation was limited by the geotechnical information provided, it was concluded that the approach followed proved to be adequate in identifying the primary sources of uncertainty and its impact on geotechnical design. It was also noted that with more detailed geological information, this probabilistic approach could be used to produce a robust final support tunnel design.

A geotechnical stratigraphy for the shallow subsurface in the Southern Central Graben, North Sea

The Integration of conventional 3D seismic data with high-resolution 2D seismic data and cone penetration test (CPT) data from geotechnical jack-up foundation reports has enabled a new late Quaternary geotechnical stratigraphy for the Southern Danish Central Graben area in the North Sea. The shallow subsurface has been subdivided into geotechnical units based on seismic appearance and soil behaviour type. The lateral thickness and geotechnical property variations have been mapped and associated with the existing stratigraphies from adjacent areas in the German and British sectors. The geotechnical variations have also been related to geomorphological features in the subsurface that are a result of the changing glacial conditions over the last 120.000 years. Particularly, buried valleys produce unpredictable geotechnical conditions that require additional evaluation and possibly borehole investigations as well as CPTs. The study synthesizes data from multiple geotechnical surveys and extrapolates the local knowledge from the different sites, providing a better regional understanding of the geology and the geotechnical behaviour of the shallow subsurface. • A geotechnical stratigraphy based on seismics and CPT data. • Correlation of seismic facies and geotechnical property distributions. • Regional stratigraphic input to improve risk assessment in offshore construction. • Geotechnical data improves geological evolution models.

Multi-proxy analysis of boreholes in remolded Quaternary paraglacial deposits (Avignonet landslide, French Western Alps)

The Avignonet landslide in the Trièves area is made of Quaternary paraglacial sediments among which imbricated till, glacio-lacutrine and fluviatil layers. Numerous boreholes (drillings and corings) were conducted during the mid-1980s and the mid-2000s to assess the geological and geotechnical setting of the inhabited, southern part of this slow-moving earthslide. Heterogeneous paramaters available from these different boreholes make interpretation difficult. Gamma-ray logging along with magnetic susceptibility and pocket vane tests on core samples allowed to distinguish between the different lithological and geotechnical units. Mechanical strength obtained from drilling parameter recording acquired during drilling was correlated to corings and their combination, along with outcrop observations, was used to delineate the lithological units and shear surfaces. The joint analysis of the dataset with outcrop observations and chronological data reveals a high geological complexity with the intrication of various sedimentary units. Two ancient landslides, several tens of metres in width, were furthermore identified within the large Avignonet landslide, almost 2 km in width. All the results point toward a high lithological and hydrogeological complexity that impacts the dynamics of the landslide. This work shows the interest of a multi-proxy approach in studies involving Quaternary sedimentary units. The methodology developed here could be applied to other sites with different palaeoenvironmental contexts initially exhibiting sedimentary compaction, such as lacustrine or coastal environments. • The Avignonet landslide affects Quaternary paraglacial sediments. • Ten boreholes and chronological data were used to produce a geological and chronological model. • Logging and measurements on core samples allowed to distinguish between the different units. • The mechanical strength was deduced from drilling parameters and pocket vane tests and allowed to detect remolded layers. • The interpretation of the dataset evidences ancient minor landslides within the large Avignonet landslide.

The internal division of the Pliocene Lillo Formation: correlation between Cone Penetration Tests and lithostratigraphic type sections

The highly fossiliferous Lillo Formation (Pliocene, southern margin North Sea Basin) is formally subdivided in five lithostratigraphical members. While these members are generally relatively easily identifiable in outcrops, they are much more difficultly identifiable in boreholes, hampering geological studies. In the Port of Antwerp area, however, an already existing and dense network of Cone Penetration Tests (CPTs) provides an alternative and additional tool for the recognition of the different lithologies in the Lillo Formation. CPTs reveal a clear geotechnical threefold division of the Formation. The lower and upper intervals, herein named CPT units Li-A and Li-C respectively, are characterized by high qc and low Rf values and correspond to shell-bearing, sandy lithologies. CPT unit Li-A can further be divided in a shelly basal subunit Li-A1 and an overlying sandy subunit Li-A2. CPT units Li-A and Li-C are separated by CPT unit Li-B characterized by low qc and high Rf values which correspond to more clayey lithologies. Four CPTs selected in close vicinity of four well-documented outcrops, some of them being formal stratotypes for the Formation and some of its members, allowed to correlate the geotechnical units with detailed lithological descriptions as well as the formal stratigraphy. CPT unit Li-A1 can be correlated to the Luchtbal Member and the basal part of the Oorderen Member, whereas unit Li-A2 corresponds to the superjacent sandy body of the Oorderen Member. The overlying CPT units Li-B and Li-C seem to correlate differently in different outcrops to the clayey upper part of the Oorderen Member, Kruisschans and Merksem Members. This inconsistency hinders any direct correlation between the geotechnical stratigraphy and lithostratigraphy of the Members of the Lillo Formation. Further research is needed to find the reason(s) for this inconsistency.

Applications of Geophysical Logs to Coal Mining—Some Illustrative Examples

Geophysical logs can be used not only for qualitative interpretation such as strata correlation but also for geotechnical assessment through quantitative data analysis. In an emerging digital mining age, such a use of geophysical logs helps to establish reliable geological and geotechnical models, which reduces safety and financial risks due to geological and geotechnical uncertainty for new and existing coal mining projects. This paper presents some examples of geological and geotechnical characterizations from geophysical logs at various coal mines in Australia and India. The applications include rock strength and coal quality estimations, automated lithological/geotechnical interpretation and geophysical strata rating, all based on geophysical logs. These derived parameters could provide input to modelling, control, even ‘digital twin’ generation in a form of geological and geotechnical models as part of the future digital mining. The outcomes can be visualized in 3D space and used for identifying the key geotechnical strata units that are responsible for caving behaviors during longwall mining. This could assist site geologists and planning and production engineers predict and manage mining conditions on an ongoing basis. Both conventional logs such as density, natural gamma and sonic and less common logging data, such as full waveform sonic, televiewer and SIROLOG spectrometric natural gamma logging data are examined for their potential applications. The geotechnical strata classification and rock strengths predicted from the geophysical logs match the laboratory tests, drill core geotechnical strata classification, core photos and the mining condition/behavior observed. These illustrate the usefulness and effectiveness of using geophysical logs for geological and geotechnical characterizations.

Investigation of the interaction of the construction of building S1 on underlying Thameslink

ABSTRACT Building S1 is a 12-storey office development site that is situated within London, north of King’s Cross Station. The site directly overlies twin Thameslink rail. The construction activities include: ~3 m of surface excavation; insertion of piles either side of – and between – the tunnels; subsequent loading of the piles; and excavation of a 1-storey deep basement adjacent to the tunnels. The tunnels have live monitoring equipment installed around the lining. The aim of this project is to use the live monitoring data to produce a back-analysed finite element model that accurately predicts the tunnel displacement recorded. The latter is compared with the model of the stages of construction covered by the monitoring. The subsequent stages are modelled in order to have a holistic idea of the behaviour of the site. The model uses the assumptions involved with 2D plane strain, along with assuming homogenous and isotropic geotechnical units. Additionally, the London Clay is modelled with the hardening soil with small strain constitutive model (HSS). Sensitivity analyses show that the small strain parameters of HSS for the London Clay had the most significant effect on tunnel displacement compared with the strength, stiffness and permeability.

Hydro-geotechnical modelling of subsidence in the Como urban area

The urban area of Como (Italy) is particularly susceptible to subsidence. The reason is the particular structure of the subsoil, combined with the anthropic modification of the lakeshore lands linked to the historical evolution of the area. Moreover, this phenomenon exposes the lakefront areas to an increasing risk of flooding. The primary purpose of this study was to develop an effective methodology for the assessment of subsidence in urban areas, to be used as a support in groundwater management. The development of a coupled hydrogeological-geotechnical numerical model for the period 2004–2011 allowed recognizing the shoreline results as most susceptible areas to the phenomenon. Model results were critically evaluated through comparison with time series of PSInSAR and high precision levelling data available in the area. The anthropic perturbations of groundwater flow linked to the construction of lake flooding defenses in 2008–2009, enhanced the subsidence phenomenon in a localized area, pointing out the strong inter-dependence of groundwater circulation, lake level oscillations and geotechnical behavior of soils. Thus, the model revealed the most critical zones and geotechnical units, demonstrating to be a potential powerful tool to predict subsidence scenarios (e.g. the future completion of the floods defense works in the shoreline area).

The 1963 Vajont Landslide: A Numerical Investigation on the Sliding Surface Heterogeneity

The 1963 Vajont landslide is a key case in landslide literature, because it was catastrophic and because a lot of accurate data were collected before and after its occurrence. In this paper, the main focus is on the possible heterogeneity of the sliding surface involved by the landslide motion, which is reflected by a heterogeneous distribution of the dynamic basal friction coefficient μ. Assuming a given zonation of the sliding surface, our strategy was to apply a 2D Lagrangian model to compute the landslide motion and to find the values of μ for each zone, leading to the best agreement between the computed and the observed final deposit. Following some hints from the literature, we have explored heterogeneous configurations composed of up to four different zones, including also the homogeneous case, by means of a 2D numerical model (UBO-BLOCK2) that handles the landslide as a mesh of blocks and runs quickly enough to allow the computation of tens of thousands of simulations in a reasonable computing time. It is found that the four-zone zonation produces the best fit (or the least misfit), which is a strong hint that the gliding surface involved different geotechnical units.

Rock mass properties and edifice strength data from Pinnacle Ridge, Mt. Ruapehu, New Zealand

Volcanic edifices exhibit spatially variable physical and mechanical properties. Magmatic intrusions are common at shallow depths within the volcanic edifice and are a poorly-understood contributor to this spatial variability. Intrusion-related alteration has been found to weaken rock mass strength through the development of joints and fractures; however, there is a paucity of research investigating how intrusions affect rock mass strength specific to the geotechnical units that define the rock masses. In this study, we employ a range of field techniques—field permeametry, rock hardness assessment, rock mass classification, and discontinuity mapping—to characterise an exposed fossil geothermal system produced by a shallow intrusion at Pinnacle Ridge, Mt. Ruapehu (New Zealand). We find that intrusions detrimentally affect the rock mass characteristics of altered brecciated lava margins. The resulting change in rock mass strength may be offset by an increase in intact rock strength as a product of alteration mineral precipitation in microfractures. Consequently, the final strength of the rock mass of the altered brecciated lava margins has the potential to be lowest of any of the geotechnical units in the volcanic edifice. We also conclude that these discontinuities increase permeability of the host rock at distances from the intrusion roughly proportional to 1–2 times the thickness of the intrusion itself under near-surface conditions. The data and conclusions presented in this study help to bridge the gap between the lab- and the field-scale and have immediate relevance to engineering geology and geothermal applications worldwide, and to rock mass classification assessments in volcanic environments.

Physical and mechanical property relationships of a shallow intrusion and volcanic host rock, Pinnacle Ridge, Mt. Ruapehu, New Zealand

Shallow magmatic intrusions are prevalent in volcanic settings worldwide. Understanding how these intrusions interact and influence their volcanic host rocks is therefore relevant to many engineering geology, geothermal, and volcanological applications. In this study, we present the most comprehensive dataset for a shallow intrusion and its host rock in a volcanic setting to date, detailing the mechanical and physical properties of volcanic rocks from Pinnacle Ridge, Mt. Ruapehu, New Zealand. Based on the geomechanical properties of 194 measured samples, we identify seven geotechnical units: (1) unaltered dense coherent lava, (2) altered dense coherent lava, (3) unaltered brecciated lava margin, (4) altered brecciated lava margin, (5) unaltered intrusion, (6) altered intrusion, and (7) hydrothermal veining. We detail the mineralogy (andesite compositions ranging from primary to an advanced argillic alteration assemblage), porosity (0.7–31%), permeability (10−21–10−12 m2), elastic wave velocities (1994–5615 m/s), uniaxial compressive strength (1–332 MPa) of these geotechnical units. Our laboratory analyses indicate that primary lithology is the predominant control on the physical and mechanical properties of the geotechnical units. Additionally, the data suggest that there is a correlation between distance to the largest intrusion; this is particularly evident for the measurements on the brecciated lava margin samples. Towards the largest intrusion, this breccia shows decreasing porosity (30.92 to 5.49%) and permeability (10−12 to 10−17 m2) and increasing elastic wave velocities (1994 to 4157 m/s) and uniaxial compressive strength (3 to 61 MPa). Thin-section analysis suggests that these correlations are due to mineral precipitation within fractures and pores in the brecciated lava margins. These correlations with distance to the largest intrusion are not shared by the altered intrusions or dense coherent lavas. We suggest that the high primary permeability of the unaltered breccia facilitated efficient hydrothermal fluid circulation and mineral precipitation adjacent to the intrusion. The other geotechnical units are less affected because hydrothermal fluid flow, alteration, and mineral precipitation were limited due to low initial permeability (10−21–10−16 m2). Our study shows that the initial properties of the host rock (i.e. porosity and permeability) control the extent of hydrothermal alteration and the susceptibility to modifications of rock geomechanical properties. Modifications to porosity and permeability can influence edifice-scale behaviour; for example, a reduction in permeability can result in pore pressure augmentation, which exerts a primary control on volcanic slope stability, seismicity, and eruptive behaviour. This study provides the most comprehensive and complete geomechanical properties data suite on a shallow intrusion in volcanic host rock to date and will support monitoring and modelling of volcanic hazards associated with shallow igneous intrusions.

Avaliação da Condutividade Hidráulica das Unidades Geológico-Geotécnicas de Santa Maria (RS) com a Utilização de Piezômetros

Determinou-se a condutividade hidraulica saturada das Unidades Geotecnicas sedimentares de Santa Maria/RS, utilizando a tecnica de piezometro de Casagrande. Essa tecnica foi empregada para resolver problemas de alteracao da estrutura, amolgamento, fuga lateral da agua e variabilidade de resultados obtidos atraves de outros metodos descritos na literatura. As Unidades Geotecnicas mostraram diferentes valores de condutividade hidraulica que ocorrem, principalmente, pela variacao granulometrica, cimentacao e estruturas sedimentares desses materiais. Valores baixos de condutividade hidraulica foram obtidos para a Formacao Santa Maria (exceto Arenito Basal) e mais elevados, para Depositos Coluvionares e Fluviais. Os procedimentos adotados para os ensaios foram satisfatorios em relacao ao tempo de ensaio e valores obtidos, confirmando e quantificando as informacoes contidas na Carta Geotecnica de Santa Maria (RS). Os resultados mostram que, em termos ambientais e hidrogeologicos, a infiltracao de aguas nos aquiferos apresenta diferentes condicoes de recarga, o que deve ser considerado na avaliacao e na gestao das reservas de aguas subterrâneas da regiao.