Competent Rock Research Articles

Rock Mass Classification of Chalk, a UK Perspective

AbstractInvestigating the geomechanical engineering behaviour of Chalk is challenging since it is a generally soft rock formation and presents in cases a different behaviour from other competent rocks. Additionally, Chalk is widespread throughout the United Kingdom and other parts of the world. Thus, an in-depth understanding of Chalk is paramount when considered in engineering projects. In this research work, the geomechanical behaviour of Northern Province Chalk is examined. This study mainly involves characterising two significant units of Northern Province Chalk, namely Flamborough and Burnham Chalk, through intensive fieldwork and laboratory testing. Multiple scanline surveys are conducted along the cliffs of Flamborough to characterise the jointed nature of the Chalk. The samples collected are assessed in the lab to determine the physical properties of the intact Chalk. Moreover, the Chalk mass properties are compared to those estimated from empirical classification systems. Finally, the limitations of applying rock mass classification systems to Chalk have also been highlighted.

Design of Rock Support in Low Overburden and Hard Rock Conditions with the use of Rock Mass Classification Systems and Numerical Analyses: a Study Based on the Construction of the Hestnes Railway Tunnel, Norway

AbstractEngineering rock mass classifications are used to describe rock masses and to assist in the design of rock support in hard rock tunneling. In most of the encountered geological- and rock mechanical conditions, from hard and competent rock to poor and fractured rock masses, the application of classification systems has normally resulted in successful and economic designs. However, complex ground conditions as these derived from tunneling in low rock overburden pose several challenges to rock mass classification and support design. In the present study, an analysis of the ground behavior and tunnel stability in a portion with very low rock overburden during the construction of the Hestnes railway tunnel (Norway) was conducted to investigate the performance of the current classification practice for tunnel rock support design. The results have revealed several challenges, which are related to conservative rock mass and rock support assessments due to the lack of consideration of the important stabilizing effect that rock arching has on tunnel stability. It was also found that with the application of an integrated methodology able to combine classification methods with numerical simulations and detailed information of the ground properties and behavior, more optimal designs in the form of rock reinforcement with bolts and shotcrete can be achieved when rock mass quality Q < 1. On this basis, a set of design recommendations was developed for the integration of classification systems with more elaborated engineering design analyses to provide guidance in design optimization for hard rock tunnels subject to low overburden.

Thrust v. stylolite

Mesostructures in subhorizontal thinly bedded calcareous mudstones of the Permian Bell Canyon Formation in the western Delaware Basin (Guadalupe Mountains foothills, west Texas) include vertical tectonic stylolites kinematically linked with low-angle thrust faults. Schmidt rebound is high (38–70) for all but a few beds, indicating competent rock. Competent beds exhibit contrasting behavior ranging from (i) no visible tectonic stylolites and numerous small-displacement (<1 cm) thrust faults, to (ii) tectonic stylolites with rare thrust faults. Thrust fault intensity correlates positively with total carbonate, and negatively with clay, clay + quartz, quartz + feldspar, and total organic carbon (TOC). Tectonic stylolites are abundant in beds with 3.5–6.5% clay. Stylolite intensity correlates positively with TOC. Lowest-rebound beds exhibit ductile flowage or internal folding rather than stylolite formation, and tend to terminate small-displacement thrust faults in adjacent beds. Tectonic stylolites and thrust faults are consistent with formation in a thrust faulting stress regime during NE-directed Laramide shortening. Fluid inclusion analyses of vein calcite from thrust faults indicate maximum burial depths of 3.3–4.2 km at the time of deformation. Deformation behaviors are highly sensitive to composition, with coeval thrust faulting in clay-poor beds versus stylolite formation in slightly more clay-rich beds.

Influence of soil - foundation - tank interaction on buckling strength of liquid storage tanks

Large earthquakes in the last 25 years have caused significant damage to buildings and infrastructure, including the partial or total collapse of storage tanks in various industries. Elephant foot buckling, or local buckling at the base, is one of the main failure modes observed in these structures, and this failure mode can lead to their collapse and/or complete loss of contents. Although hydrostatic and hydrodynamic loads typically affect the seismic response of tanks, the effect of soil type on tank buckling behavior has not been widely studied or recognized. This research aims to evaluate the effect of soil type on seismic fragility of tanks by analyzing typical storage tanks used in the wine industry. The work focuses on elephant foot buckling for tanks with both unanchored and anchored bases and compares the influence of three different types of soil and two different tank geometries. The approach uses the capacity spectrum method, as opposed to the more commonly used incremental dynamic analysis, to determine a critical peak ground acceleration to cause buckling at the tank. The tanks were subjected to 21 Chilean seismic records with three different soil types and a no-soil condition. From the results a lognormal fragility curve, and its median and standard deviation, are calculated. The results indicate that unanchored tanks built softer soils exhibit poorer performance, while tanks in competent soils and rock exhibit good performance. Anchored tanks show less sensitivity to soil types than unanchored tanks. The study demonstrates the importance of considering soil-foundation-structure interaction for wine storage tanks, but the results indicate that many comparable storage structures will be similarly affected.

Microseismic event locations and source mechanisms using dominant guided waves recorded in an underground potash mine

Microseismic event locations and moment tensors (MT) in underground mines can provide insights into the subsurface deformation and current state of stress. However, reliable estimation of these source parameters is rather challenging due to high-frequency waveforms and a low signal-to-noise ratio for negative magnitude events. We study microseismicity in an underground potash mine in Saskatchewan, Canada, recorded between 1 March and 30 June 2021, by a network of broadband seismometers. The active mining is carried out in low-velocity evaporites at depths of approximately 1 km below the ground level. The theoretical dispersion curves show that guided waves in the form of leaky P and P-SV/SH normal modes can exist in a 1D velocity model representing mine geology. These guided waves are detected as high-energy dispersive arrivals on the seismograms recorded at the underground receivers. We locate the events using the arrival times of the guided waves and their mean group velocities. Most (approximately 80%) of the detected events cluster around the mine layout between depths of 0.95 to 1.05 km. Next, we compute MT for 92 events using waveforms of guided phases. The MT show nondouble-couple components with only 28 events having double-couple percentages greater than 50%. These events occur near mined-out cavities with source mechanisms corresponding to the layer delamination in the roof and floor or pillar yield related to the closure of cavities. No abnormal microseismicity is detected away from the mine levels in the more competent carbonate rocks above or below the evaporite formations. Thus, guided waves enable the detection of microseismic events up to large distances and can provide high-resolution event locations and MT inversion. These can be interpreted in the context of local geology and mining activities to identify the dominant factors affecting microseismicity.

Numerical and Analytical Determination of Rockburst Characteristics: Case Study from Polish Deep Copper Mine

A simplified analytical method useful for ductile ground support design in underground mine workings is presented. This approach allows for maintaining the stability of sidewalls in rectangular openings extracted in competent and homogeneous rocks, especially in high-pressure conditions, favoring rockburst event occurrence. The proposed design procedure involves the typical assumptions governing the limit equilibrium method (LEM) with respect to a triangular rock block expelled from a sidewall of a long mine excavation subjected to normal stresses of the values determined based on the Maugis’s analytical solution concerned with stress distribution around the elliptical opening extracted within the homogeneous infinite elastic space. This stage of the local assessment of rock susceptibility to ejection from the walls of the excavation allowed for determining the geometry of the block whose ejection is most likely in a given geological and mining situation. Having extensive information about the geometry of the excavations and the properties of the surrounding rocks, it was possible to make an exemplary map of the risk from rockburst hazard, developed as the 2D contours of safety indexes’ values, for special-purpose excavations such as heavy machinery chambers, main excavations, etc. in conditions of selected mining panel of the deep copper mine at Legnica-Głogów Copper Basin, Poland. Another important element of the obtained results is the calculated values of the horizontal forces potentially pushing out the predetermined rock blocks. These forces are the surplus over the potential of frictional resistance and cohesion on the surfaces of previously identified discontinuities or on new cracks appearing as a result of overloading of the sidewalls. Finally, the presented algorithm allows us to perform quantitative tracking of rockburst phenomena as a function of time by determination of acceleration, velocity, and displacement of expelled rocks. Such information may be useful at the stage of designing the support for underground workings.

Inversion of extensional basins parallel and oblique to their boundaries: inferences from analogue models and field observations from the Dolomites Indenter, European eastern Southern Alps

Abstract. Polyphase deformation of continental crust is analysed through physical analogue models for settings wherein platform–basin geometries at passive continental margins are subject to subsequent shortening and orogenesis. In a first stage, segmentation of the brittle and brittle–ductile models into basins and platforms is achieved by extension. Basins are partly filled with brittle material to allow for a strength difference between basin and platform realms, simulating relatively weaker, incompetent deposits of grabens surrounded by competent pre-rift basement or carbonate platform rock, respectively. In a second stage of deformation, contraction parallel and oblique (10 to 20∘) to the basin axes has been applied, leading to the inversion of basins formed earlier. The experiments show that strength contrasts across platform–basin transitions control the localisation and overall style of compressional deformation, irrespective of the nature of the basal décollement (frictional versus viscous), the rheology of the basin fill, or changing platform–basin thickness ratios. Orientations of thrust faults change laterally across inherited platform–basin transitions throughout all experiments; higher obliquity of basin inversion leads to stronger alignment of thrust curvature with the orientation of pre-existing rift axes. At individual thrust faults, variations in the strike of thrust fronts are accompanied by changes in the shortening direction during incremental phases of deformation. Reactivation of normal faults occurs in oblique basin inversion settings only, favourably at platform–basin transitions where the normal faults face the shortening direction. The amount and style of fault reactivation depend on the material used. Our experiments are relevant for natural cases such as the Dolomites Indenter of the eastern Southern Alps, underlining the importance of inherited geologic features for the subsequent shortening geometries. Field structural data from the western segment of the Belluno thrust of the Valsugana fault system support predicted variations of thrust fault orientation and a lateral change in shortening direction (from SSW to SSE along-strike) along one single fault. Based on our modelling results, we suggest that this variability of thrust fault orientation and shortening directions, controlled by inherited structures, is consistent with strain partitioning during a single phase of deformation and does not necessarily reflect different deformation phases.

Toward a Complete Kinematic Description of Hydraulic Plucking of Fractured Rock

Scour by hydraulic plucking is a fundamental process in landscape evolution in which large, competent rock blocks are eroded from a fractured rock mass by flowing water. This process also affects engineered structures interacting with water, such as dams and bridges, and often leads to operational and safety concerns because erosion of large volumes of material can compromise structure foundations and serviceability. To assess potential scour at a site, present methods either are empirically derived, assume a specific failure mode, or significantly simplify the geometry of potentially eroding rock particles. This limits the broader applicability of these methods and their ability to offer actionable insight into scour risk. Therefore, the discrete-element method coupled with the lattice Boltzmann method was applied to assess hydraulic plucking of fractured rock. In this approach, the three-dimensional shape of rock particles was considered explicitly, including how each particle interacts dynamically with fluid. Additionally, the highly turbulent flow conditions at which plucking often occurs were modeled using large-eddy simulation. The results show that this modeling methodology was able to capture the correct kinematic failure mode in block removal in two example scenarios without restricting the potential failure mechanism, and naturally captures the governing response. This capability makes this scour assessment technique broadly applicable since site-specific characteristics can be input directly into scour risk assessments to understand the influence of local features on the plucking process.

Thrust force requirements in fault zones with squeezing ground

Shield jamming constitutes a critical hazard in mechanized tunnelling under squeezing conditions. These are often encountered within narrow fault zones interspersed between competent rock layers, where the competent rock exhibits much smaller deformations, thus mobilising shear stresses at the interfaces. This so-called “wall-effect” results in substantially reduced deformations and rock pressures in the fault zone and is thus favourable with respect to shield jamming. Building on existing design nomograms for evaluating the required thrust force in homogeneous ground, this paper proposes two novel, simple and practical thrust force estimation methods that additionally consider the favourable influence of the wall-effect in narrow fault zones. The first method performs an a priori modification of the nomogram input via an equivalent improved ground stiffness that depends on the fault zone length. The second method performs an a posteriori correction of the nomogram output via an analytically determined coefficient, established via multiparametric regression that considers the results of 3450 numerical analyses. Both methods allow for a fast evaluation of the required thrust force in fault zones with reasonable accuracy, thereby providing a valuable tool for design.

Fault-controlled barite veins of the eastern Anti-Atlas (Ougnat, Morocco), a far-field effect of the Central Atlantic opening? Structural analysis and metallogenic implications

Tectonic and micro-tectonic studies at different scales are of primary importance in a modern metallogenic investigation. In the Ougnat massif of the eastern Anti-Atlas belt (Morocco), the barite ore deposit is hosted within mafic to felsic rocks of the Ediacaran Saghro and Ouarzazate groups together with their folded Paleozoic cover. Here, we provide for the first time new field-based data on the tectonic control of barite mineralization, and their relation with regional tectonic events. The mineralization occurs in brittle-ductile structures as a vein-type system, mainly hosted within NE-SW to E-W and NW-SE strike-slip-normal faults. Economic orebodies spread frequently along the Precambrian-Cambrian contact zone thus acting as open conduits for mineralized fluid flow. The geometry of hosted barite sigmoidal lenses corresponds to “pull-apart” or “tension gashes” openings, commonly arranged en echelon arrays along the bearing transcrustal faults. The occurrence of predominant massive and breccia internal texture implies a tectonic-mineralizing collapse process in an extensional tectonic context. Kinematic pattern, directional distribution and host-rock age relationship point to a syntectonic barite control probably occurred during the NW-SE Atlantic rifting, as already attributed to similar deposits in the neighboring Ougarta and Atlas-Meseta domains.The barite ore is quantitatively and qualitatively more preserved in competent host rocks than those with ductile behavior, thus offering a useful hint for further barite exploration in this part of the Anti-Atlas fold and thrust belt.

A new high-pressure high-temperature deformation apparatus to study the brittle to ductile transition in rocks

Understanding the micro-mechanisms underlying the localized–ductile transition (LDT) as well as the brittle–plastic transition (BPT) has become crucial for our wider understanding of crustal processes and seismicity. Given how difficult in situ observations of these transitions are to perform, laboratory experiments might be our only way to investigate the processes active under these conditions (high T and high P). Here, we present Triaxial AppaRatus for GEoThermal energy, a new gas-based triaxial apparatus located at EPFL in Switzerland that was specifically designed to operate under conditions where both the LDT and BPT can occur in geomaterials. We show that the machine is capable of deforming rock samples at confining pressures of up to 400 MPa, temperatures of up to 800 °C, and pore pressures (liquid or gas) of up to 300 MPa while keeping the temperature gradient along samples of 40 mm in length and 20 mm in diameter minimal (less than 30 at 700 °C). Most importantly, the maximum load is 1000 kN (stresses as high as 2.2 GPa on 24 mm samples and 3 GPa on 20 mm samples), allowing for the deformation of very competent rock samples. Moreover, during deformation, the pair of syringe pore pressure pumps allow for continuous permeability or dilatancy recording. We benchmarked our machine against existing data in the literature and show that it accurately and precisely records stress, strain, permeability, pressure, and temperature.

The carbonate-hosted Gortdrum Cu-Ag(±Sb-Hg) deposit, SW Ireland: C-O-Sr-Nd isotopes and whole-rock geochemical signatures

The Gortdrum Cu-Ag(±Sb-Hg) deposit, consisted of a fault-controlled orebody (3.8 Mt @ 1.19 % Cu and 25.1 g/t Ag) formed at the base of the Irish Midlands basin, in Lower Carboniferous rocks laterally time equivalent to Navan Group rocks hosting the giant Navan Zn-Pb deposit, and form the base of the Irish Midlands basin. The ore body is hosted on the hanging-wall of the Gortdrum Fault either along strata or within a wedge of brecciated carbonate rocks. Vertical zonation based on predominant host rock, ore textures, sulfide assemblages, and whole-rock geochemistry allowed the detailing of three ore types: a) the lower ore, representing Cu sulfides hosted within basal carbonated siliciclastic rocks; b) the upper ore, representing Cu, Cu-Sb, and Hg sulfides hosted within upper calcareous rocks and; c) the vein-associated ore, dominantly hosted in the more competent upper carbonate rocks. The origin of the deposit is unambiguously related to the development of the Gortdrum Fault and its associated permeability, which allowed basement/basin-derived fluids to react with carbonates and induce copper-silver mineralization. The mineralogy, ore shoot geometry, and geochemical association of Gortdrum are shared with classic Zn-Pb Irish-type deposits such as Navan, Lisheen, Silvermines, and Tynagh (sub-seafloor replacement). In these Irish-type deposits, copper-silver mineralization is associated with the late stage of Zn-Pb mineralization and shares a common geochemical footprint with Gortdrum of anomalous Ag, As, Sb and Hg. Additionally, the C-O and Sr-Nd isotope range of Gortdrum and Navan samples overlap, indicating that mineralization processes in both deposits were ineffective in modifying the original host-rock signature. These similarities suggest that Gortdrum could represent a variation of Irish-type mineralization where late-stage Cu-Ag-Sb-bearing fluids succeeded in forming a deposit. Hypothetical early-stage Zn-Pb fluids a) never existed; b) deposited disseminated sulfides in country-rocks, c) formed an undiscovered resource or, d) deposited ore concentrations that were eroded off.

Multiphysics Simulations of Microwave Induced Damage Applied to Rock Samples of Varying Strength and Absorptivity

AbstractIn this work, we show the development of a numerical model to investigate the 3D interactions between microwave radiation and basalt, granite, and sandstone rock samples. In particular, we assign sample heterogeneity based on the Weibull statistical distribution, and invoke a damage model for elemental tensile and compressive stresses based on the maximum tensile stress and the Mohr–Coulomb theories, respectively. Model implementation is facilitated by the use of comsol for use in coupling the electromagnetic, thermal, and solid displacement relations. Various parametric studies are conducted related to variable input power and waveguide port alignment, with model validation conducted with respect to damage resulting from a uniaxial compression test. The results indicate that relatively high induced temperatures will promote damage potential, but its impact must be placed within the context of the sample strength to quantify the true potential damage evolution of a given rock mass. As observed herein, a mechanically weaker rock may be prone to mechanical damage; however, it may also possess a relatively large relative permittivity, enabling it to absorb the least amount of microwave radiation thus yielding comparatively low overall damage profiles compared to a more mechanically competent rock mass.

Evaluation of Engineering Site and Subsurface Structures Using Seismic Refraction Tomography: A Case Study of Abydos Site, Sohag Governorate, Egypt

Because of the strategic importance of the Abydos archaeological site in Egypt as a source of wealth for Egyptian tourism, this study was concerned with carrying out geophysical measurements to detect subsurface succession and measure variations in the geotechnical engineering features of the soils/rocks in order to protect this significant area. The findings will assist geologists and seismologists in collaborating with archaeologists for future site development, revitalization, and investment. The primary objectives of this work were to determine the subsurface lithology, evaluate the engineering geotechnical properties of soils/rocks, identify the layer thicknesses, and identify the site class by calculating Vs30. To achieve these goals, seventeen (17) seismic refraction tomography (SRT) P- and S-wave measurements were executed in front of the Osirion location. SeisImager Software was used for the processing and interpretation of the outcomes. The results were the travel time–distance curves, which were used for building the 2D seismic models that exhibited the velocity and the depth of the layered models. These models were validated by our previous works using electric resistivity tomography and borehole data. The results indicated that this site consisted of three geoseismic subsurface layers. The first layer was the surface that was made up of wadi deposits, which were a mixture of gravel, sand, and silt and were characterized by incompetent to slightly competent materials. The second layer corresponded to the sand and muddy sand deposits of competent rock that was of fair to moderate quality. The third layer (clay deposits) had a higher velocity and was more compact and may be employed as a bedrock layer. The elastic moduli, Vs30, petrophysical, and geotechnical properties of the three geoseismic layers were appraised as essential parameters. Integration of petrophysical and geotechnical parameters and elastic moduli revealed that the third layer was composed of competent clays, which were characterized by low values of porosity, void ratio, Poisson ratio, and stress ratio. It also had a high rigidity, Young’s and bulk moduli, concentration and material indexes, N-value, ultimate bearing capacities, and high density values, and vice versa for the first layer. The standard NEHRP site class was B (rocks). These parameters are ordinarily used as key indications and serve as significant inputs for any future work.

TECTONIC DEFORMATIONS AND PALEOSTRESS FIELDS IN THE ROCK COMPLEXES OF THE PIENINY KLIPPEN BELT IN THE CITY OF SZAWNYCTA (POLAND)

Purpose of the paper is study of faults and folds and reconstruction of paleostress fields in rock complexes of the Pieniny Klippen Belt in Pieniny Mts. area (Southern Poland). Methods of field geological and structural research were involved, including the kinematic method with subsequent reconstruction of paleostress fields using the program Win-Tensor. The results. On the right bank of the Danube River in the south-western outskirts of the city of Szczawnytsia (Southern Poland), numerous fold and fault deformations, their kinematic types and structural patterns were studied in the large rootless block of the PKB. Paleostress fields of the thrust, normal and strike-slip kinematic types have been restored. In the inverse stress field, thrusts were formed on the border of competent and incompetent rocks. Drug-folds, mélange and broken formations zones were formed in the more ductile rocks of the lower block. In the normal-type stress field, the former thrusts were activated as normal faults and cuted by the ortogonal and oblique sinistral strike-slip faults. The kinematic sence of the thrusts changed to an oblique normal faults with a component of the sinistral strike-slip, in the bodering zones of the activated faults, drag folds ans s-type folds were formed. In the youngest stress field, the former thrust surfaces were activated as strike-slip ones. Shear deformations were also recorded along thrust-limiting orthogonal and oblique surfaces. Kinematic types of tectonic deformation and folded structures in the different parts of the studed rootless block vitnesses the rotation of the fault wings in the horizontal and vertical planes. The wavy shape of the surface of the movement of the rootless block may be the reason for the simultaneous existence of a field of compression and tension in its various segments. Scientific novelty. As a result of the research, the kinematic types of fault, their relationships and features of development in various paleostress fields were determined. The features of the structure of the thrust and the faults that cut them, the relationships of folded and faulted deformation are established. Changes of the kinematic types of faults in time from the inverse to the strike-slip type have been established. Practical impact. The obtained results complement the history of the formation of the Pieniny Klippen Belt and can be used to refine tectonic maps and select a geodynamic model of the region. Key words: Pieniny Klippen Belt, paleostress, thrust, strike-slip, mélange.

Experimental study of genesis of a geologically activated and self-sealing cementing material for deep wellbore plugging and abandonment under acidic conditions

Harsh subsurface conditions pose a widespread challenge for permanently plugging and abandoning (P&A) oil and gas wells. A new, cement-free plugging material is proposed and demonstrated using nature-inspired geochemical processes to generate stable and resilient, rock-like material that is ideally suited to acidic geofluid, high-pressure, and high-temperature (AG-HP-HT) conditions. These conditions accelerate hydration and carbonation reactions that turn granular ultramafic raw materials into competent rock, dubbed here as “Geologically Activated Cement” (GAC). The experiments firstly demonstrate the generation of GAC in an acidic environment, using a pressure of 13.7 MPa and temperature of 180 °C as typical representative values relevant to P&A operations in high temperature reservoirs. The results show that within hours to days, and hence relevant to P&A operational timeframes, a solid, rock-like material can be produced from Mg2SiO4–CO2 mixtures. The reaction products are analyzed by X-ray powder diffraction (XRD), scanning electron microscope (SEM) and Energy-dispersive X-ray spectroscopy (EDS). These analysis shows the rock to be comprised of unreacted olivine particles that are bound together by magnesium carbonate. Because of the presence of unreactive olivine, the reaction is set in motion again by exposing the olivine to CO2-enriched fluid by thermally damaging (cracking) the sample. The damaged material is then subjected to flow-through of carbonic acid for 8–14 hours. Under these conditions, GAC experiences a 75% reduction in permeability over the hours immediately following damage. Control experiments using class H cement experience steady to slightly increasing permeability under the same conditions and over a similar time frame. We therefore show that GAC can self-seal cracks within a few hours when subjected to damage and subsequent flow of acidic fluid under HPHT conditions. This self-sealing behavior provides the potential for a resilient cementing system over geological timescales.

Lithospheric strength and stress revisited: Pruning the Christmas tree

Whether lithospheric stress can reach the maximum level predicted by the Christmas-tree strength envelope is a fundamental question but with controversial answers. There is little controversy that a deforming lithosphere in high heat flow regions is likely critically stressed, i.e., at full yield at all depths, as described by the envelope. But different conceptual frameworks offer opposite views for very cold lithosphere, either at full yield or far below yield. Here, we use simple numerical models to investigate stresses in end-member cold cratonic lithosphere (e.g., Canadian Craton) in comparison with end-member warm plate-boundary lithosphere (e.g., Canadian Cordillera). The two key elements of our modelling are (1) that lithospheric stress builds up elastically with horizontal tectonic loading not only in the elastic–frictional brittle regime but also in the viscoelastic ductile regime, and (2) that the stress level is limited by the available tectonic force. In a cratonic lithosphere, the limiting tectonic force is sustained by competent rock material over a large depth range, represented by the competent thickness Tc that exceeds 90 km. The lithosphere undergoes mostly elastic deformation at a stress level of a few tens of MPa. While weakly stressed strong lithosphere can still produce limited earthquakes at shallow depths due to structural and stress heterogeneity, the lithospheric stress under horizontal tectonic loading is theoretically predicted to be orders of magnitude lower than predicted by the Christmas-tree envelope. Stresses in a real lithosphere may substantially deviate from this theoretical level because of spatiotemporal variations in rheology and structure. For example, the stress memory of past loading history in cold lithosphere may or may not be erased by more recent tectonic stresses. Because much of previous scientific debates on lithospheric stress levels and comparison with seismicity were focused on topographically induced flexural stress, we also investigate the effect of vertical loading. We show that the effective elastic thickness Te derived from the flexural response is a reasonable proxy for Tc derived from horizontal tectonic loading; a very large Te such as > 80 km is generally associated with very low tectonic stress far below yield. However, the flexure-induced bending stress is not directly comparable with seismicity because it may either enhance or suppress seismogenic stress in the crust.

Kinematics of fault-propagation folding: Analysis of velocity fields in numerical modeling simulations

Fault-propagation folding occurs when a shallow fold is created by an underlying propagating thrust fault. These structures are common features of fold and thrust belts and hold key economic relevance as groundwater or hydrocarbon reservoirs. Reconstructing a fault-propagation fold is commonly done by means of the trishear model of the forelimb, a theoretical approach that assumes simplistic rheological rock properties. Here we present a series of numerical models that elucidate the kinematics of fault-propagation folding within an anisotropic sedimentary cover using complex visco-elasto-plastic rheologies. We explore the influence of different parameters like cohesion, angle of internal friction, and viscosity during folding and compare the velocity field with results from the purely kinematic trishear model. In the trishear paradigm, fault-propagation folding features a triangular shear zone ahead of the fault tip whose width is defined by the apical angle that in practice serves as a freely tunable fitting parameter. In agreement with this framework, a triangular zone of concentrated strain forms in all numerical models. We use our models to relate the apical angle to the rheological properties of the modeled sedimentary layers. In purely visco-plastic models, the geometry of the forelimb obtained can be approximated using a trishear kinematic model with high apical angles ranging between 60° and 70°. However, additionally accounting for elastic deformation produces a significant change in the geometry of the beds that require lower apical angles (25°) for trishear kinematics. We conclude that all analyzed numerical models can be represented by applying the theoretical trishear model, whereby folds involving salt layers require high apical angle values while more competent sedimentary rocks need lower values.

Ground surface uplift during hardcoal longwall mining

Ground surface uplift was detected during longwall mining in the Lazy Mine, a hard coal mine in the Czech part of the Upper Silesian Coal Basin, Czech Republic. The greatest uplift was 218 mm, based on surface point levelling by comparing surface points with heights of non-mined areas. This uplift occurred during subcritical mining at an average depth of 740 m below the surface. The mined coal seam is situated below the previously mined part of the multi-seam deposit. This study aims to provide a geomechanical explanation for ground surface uplift observed during longwall mining. The source of surface uplifts were deep inside the rock mass, therefore, enabling only the examination of geological, geomechanical, seismic, and technical mining data for uplift occurrence analysis. Therefore, the spatiotemporal evaluation of the observed vertical displacement and induced seismicity data was processed, and the observed ground surface was compared with empirical subsidence . Two time periods with surface uplifts during longwall mining were evaluated. Thick, competent rock layers, together with specific geomechanical conditions, were identified as the main characteristics responsible for the uplift.

Application of Multi-Channel Analysis Surface Waves and Electrical Resistivity Tomography Methods to Identify Weak Zones at University of Mosul, Northern Iraq

Cracks have been developed in a large lecture hall on the University of Mosul Campus. These cracks exist along two sides of the building. Multi Analysis Surface Wave and Electrical Resistivity Tomography surveys were conducted at the surrounding building to investigate the nature and distribution of shallow subsurface soil/rock. The MASW’s data show three layers: the first layer is characterized by low shear wave velocity ranges between 210 and 420 m/s which is attributed to infill materials, the next layer is attributed to a river terrace (fairly competent rock) which has Vs ranging from 430m/s to 840 m/s. It is recognized by a gradual increase in mechanical characteristics as depth increases. In-depth between about 2.5m and 6.0 m the shear wave velocity drops in the top contact of this layer. The causes of low velocity may be due to weathering of the medium because of a sinkhole. The Electrical Resistivity Tomography profile shows four electrical zones, the first zone has 50-70 Ω.m. with a variable thickness from 0.5-1.5 m which indicates infill materials. The second zone has very low resistivity value, and a depth ranges 0.5m - 3.5 m, which might be interpreted to be increased clay, silt and water content (high water-saturated zone). The third zone is characterized by a high-resistivity value (&gt;100 Ω.m) that could be related to a dry conglomerate rock and gravels belong to river terraces. The fourth zone has a low-resistivity value (&lt;20 Ω.m) associated with a water-saturated marl bed. We could be definitively correlated the resistivity and velocity anomalies to sinkhole activity in were identified and characterized using combined geophysical methods. The Multi Analysis Surface Waves and Electrical Resistivity Tomography were shown to complete each other in the evaluation. The variations in Vs (low velocity) and resistivity (conductive zone) within the river terrace were detected and proposed to be indicative of dissolution and the subsidence responsible for structural damage implied by the change in the velocity and resistivity. The roughness of the top terrace surface strongly influences the nature of the velocity and resistivity values. This roughness is suggestive of dissolution or erosion.