Sedimentary Rock Masses Research Articles

A new approach for defining Slope Mass Rating in heterogeneous sedimentary rocks using a combined remote sensing GIS approach

Engineering rock mass classification is usually the first stage in the analysis and characterization of rock slopes. However, when dealing with sedimentary/heterogeneous rock masses, the use of existing classification methods can be difficult and often misleading, especially when used to define rockfall risk areas and appropriate slope mitigation works. In this research, we describe a novel approach for geomechanical rock slope analysis based on the combined use of remote sensing, geographic information systems (GIS), and the Slope Mass Rating (SMR) classification system. The Montagna dei Fiori area (Italian central Apennines), which is characterized by the sedimentary rocks of the Umbria Marche heterogeneous succession, is used as a case study to demonstrate the application of the proposed approach. Conventional geomechanical scanlines are integrated with photogrammetric techniques to increase the amount of data collected, especially in inaccessible areas. In particular, a new fast and low-cost method of georeferencing 3D photogrammetric models is presented. GIS are used to manage all the data acquired using remote sensing techniques and geomechanical analyses, and a semi-automatic tool developed to allow calculation of the SMR along a major highway, the SP52, which crosses the study area. Finally, a modification of the SMR procedure is proposed to enable definition of the most appropriate mitigation works in folded heterogeneous sedimentary rock masses comprising alternating marls and limestones.

The 1985 earthquake-triggered North Nahanni rockslide, Northwest Territories, Canada: The co-seismic movement of a sedimentary rock mass conditioned by residual strength

The North Nahanni rockslide (volume ~7.6 Mm3), Northwest Territories, Canada was triggered by the Mw 6.6 Nahanni earthquake of October 5, 1985; the earthquake was followed by a Mw 6.8 earthquake at approximately the same location in December 1985. The landslide occurred in a Palaeozoic carbonate sequence sliding along a thrust fault, which partly follows bedding and partly cuts across bedding in the source rock mass. The sliding surface thus consisted of two planes; the lower plane dipped at 20° while the upper plane dipped at 35°. Tilting table test of block samples from the sliding surface and field-measurements of large-scale sliding surface roughness indicated that the friction angle of the sliding surface was 24°. Two-dimensional (2D) slope stability analysis was performed for the slope before and during the earthquake using a discontinuum numerical technique (the distinct element UDEC code). 2D static slope stability analyses indicate that the simple-structured rock mass was marginally stable for sliding surface friction angles of 24°, and was constrained by the residual shear strength of the pre-sheared fault surface. 2D dynamic analyses of the co-seismic movements are conducted by comparing seismically-induced slope displacements computed in both UDEC and SLAMMER (a sliding-block analysis using real earthquake ground motions records of the December 1985 Nahanni earthquake). Both results are in good agreement. The dynamic analysis indicates that the slope becomes unstable for given seismic inputs that correspond closely to those estimated for the October earthquake at a friction angle of 24° for the sliding surface indicating that co-seismic rock slope movement was only possible on low-friction pre-sheared surfaces at residual strength. It also showed that the deformation behavior of the North Nahanni rock masses was dependent on the frequency of the seismic signals. Because the static slope stability analysis showed that the slope was close to instability prior to seismic shaking, the October 1985 Nahanni earthquake operated as a trigger event that accelerated the occurrence of the slide.

Rippability Assessment of Weathered Sedimentary Rock Mass using Seismic Refraction Methods

Rippability or ease of excavation in sedimentary rocks is a significant aspect of the preliminary work of any civil engineering project. Rippability assessment was performed in this study to select an available ripping machine to rip off earth materials using the seismic velocity chart provided by Caterpillar. The research area is located at the proposed construction site for the development of a water reservoir and related infrastructure in Kampus Pauh Putra, Universiti Malaysia Perlis. The research was aimed at obtaining seismic velocity, P-wave (Vp) using a seismic refraction method to produce a 2D tomography model. A 2D seismic model was used to delineate the layers into the velocity profile. The conventional geotechnical method of using a borehole was integrated with the seismic velocity method to provide appropriate correlation. The correlated data can be used to categorize machineries for excavation activities based on the available systematic analysis procedure to predict rock rippability. The seismic velocity profile obtained was used to interpret rock layers within the ranges labelled as rippable, marginal, and non-rippable. Based on the seismic velocity method the site can be classified into loose sand stone to moderately weathered rock. Laboratory test results shows that the site’s rock material falls between low strength and high strength. Results suggest that Caterpillar’s smallest ripper, namely, D8R, can successfully excavate materials based on the test results integration from seismic velocity method and laboratory test.

Correlation of Resistivity Value with Geotechnical N-Value of Sedimentary Area in Nusajaya, Johor, Malaysia

Electrical resistivity survey and the geotechnical SPT blow counts (N-value) were carried out simultaneously on the tropically weathered sedimentary rock mass for an excavation project at Nusajaya, Johor, Malaysia. This study aims to determine subsurface profile by using 2D-resistivity methods and correlate with N-value derived from boring works. Four boreholes were investigated in five survey lines that revealed the site is underlain by moderately to completely weathered sandstone, clay, silt and shale. Data analysis from 2D-resistivity image shows that zones with high resistivity value generally have high N-value, and vice versa. Five zones have inversed the proportional relation between N-value and resistivity Ωm value due to different types of soil lithology. It indicates that 2D-resistivity is significance to detect bodies of anomalous materials or estimating the depth of bedrock. As a conclusion, the integration of geophysical and geotechnical analysis provides a promise approach to understand some relationship concerning the subsurface subsurface ground through the combination of 2D-resistivity and N-value.

Influence of counter-tilted failure surface angle on the stability of rock slopes subjected to block toppling failure mechanisms

In rock slopes where sedimentary rock masses dip into the face of the slope, failure may occur by block toppling. In traditional analytical models, the failure surface is assumed to be a single plane running from the upper columns to the toe of the slopes, which may be inconsistent with the physical situation, where the weak plane has undergone counter-tilting within the rock slope due to variations of lithology and weak plane characteristics. To better reflect the physical situations, the failure surfaces ought to be determined instead of basing it on assumptions and incorporated in the existing analytical methods for stability analyses. Therefore, a searching technique for determining the counter-tilted failure surface angle has been proposed and traditional analytical models for evaluating the stability of rock slopes subjected to block toppling failure mechanisms have been modified by incorporating the counter-tilted weak plane angle. The physical slope with counter-tilted failure surface was comprehensively analyzed using the modified analytical model and the results were validated using numerical simulation models. The simulated failure mode zones are consistent with the failure mode zones obtained by the modified analytical method. The influence of relative angles of the counter-tilted failure surface on the slope stability has been studied and the results show that progressive increase of the counter-tilted failure surface angles lead to a gradual increase in slope instability. The proposed analytical method could provide precise applications to evaluate the slope instability in rock slopes with counter-tilted failure surface.

Simulating mining-induced strata permeability changes

Mining processes fracture the surrounding strata and may modify the flow of groundwater by inducing new fractures or changing the permeability of existing defects. The result of mining-induced permeability changes can be disturbance to aquifers or other surface or sub-surface water bodies. Traditional methods for predicting mining-induced fracture connectivity and enhanced permeability based on empirical strain-based criteria may not satisfy modern regulatory demands, nor adequately reflect local geological, geotechnical and hydrogeological conditions. Standard continuum numerical methods may indirectly estimate permeability enhancement from plastic strains however they are not able to track aperture on flow paths or predict fracture connectivity. This paper presents a numerical approach that is demonstrated to be capable of representing longwall mining induced fracturing in sedimentary rock masses. By initiating and propagating fractures, determining connectivity and calculating aperture in a piecewise manner on flow paths, we have estimated permeability enhancement from first principles. Fracture intensity and porosity metrics are calculated and identify the height of the enhanced permeability fractured zone above a longwall goaf. Permeability within the overburden is estimated from the Kozeny-Carman permeability–porosity equation. At a mine site studied in detail in this paper a permeability increase from the in situ state is predicted to range from approximately eight orders-of-magnitude in the caved zone to one to two orders-of-magnitude in the strata above the fractured zone. Realistically simulating cracking, fracturing and crushing of rock strata remains numerically intensive and challenging at the scale of a longwall panel. It is demonstrated in this paper and provides valuable insights into the rockmass response to mining.

Stability Analyses of Jointed Rock Slopes with Counter-tilted Failure Surface Subjected to Block Toppling Failure Mechanisms

In rock slopes where sedimentary rock masses dip into the face of the slope, failure may occur by block toppling. In traditional analytical models, the failure surface is assumed to be single plane running from the upper columns to the toe of the slope which may be inconsistent with the physical situation where the weak plane has undergone counter-tilting within the rock slope due to variation of lithology and weak plane characteristics. To better reflect the physical situations, the failure surfaces ought to be determined instead of basing it on assumptions and incorporated in the existing analytical methods for stability analyses. Therefore, an analytical technique for determining counter-tilted failure surface angle has been proposed and traditional analytical model for evaluating the stability of rock slopes subjected to block toppling failure mechanisms has been modified by incorporating the counter-tilted weak plane angle. The physical slope with counter-tilted failure surface was comprehensively analysed using the modified analytical model, and results were validated using the numerical simulations models. The simulated failure mode zones are consistent with the failure mode zones obtained by the modified analytical method. The influence of relative angles of counter-tilted failure surface on the slopes’ stability has been studied, and the results show that progressive increase in the counter-tilted failure surface angles leads to gradual increase in slope instability. The proposed analytical method could provide precise applications to evaluate the slope instability in rock slopes with counter-tilted failure surface.

Subcritical crack growth in sandstone in aqueous environment with different calcium ion concentration

Investigating the time-dependent fracturing in sandstone is crucial to consider the long-term integrity of a sedimentary rock mass surrounding structures. Subcritical crack growth is one of the main causes of the time-dependent fracturing in rock materials. For the long-term integrity, it is essential to investigate subcritical crack growth in rock, which is influenced by the surrounding environmental conditions. Numerous cementitious materials are used to construct various structures such as underground repositories for radioactive waste, underground power plants, etc. In these cases, the calcium ion concentration in the ground water in a rock mass surrounding these structures is high. Therefore, investigation on the subcritical crack growth in rock in water with a high calcium ion concentration is essential. In this study, subcritical crack growth in sandstone is measured in distilled water and calcium hydroxide solution (Ca(OH)2) to investigate the influence of calcium ions on the time-dependent fracturing in rock. Berea sandstone was used as a rock sample. The load-relaxation method of the double-torsion test was used to measure the crack velocity and the stress intensity factor. All measurements have been carried out under controlled temperature and relative humidity. It was shown that the crack velocity in a calcium hydroxide solution is lower than that in distilled water even though the pH was high. In previous researches, it has been considered that the crack velocity in rock increased when the pH was high from the measurements in water and sodium hydroxide solution (NaOH). Therefore, it is considered that calcium ion affects the decrease of the crack velocity. It is concluded that a water environment with a high calcium ion concentration is suitable for the long-term integrity of a sedimentary rock mass.

Geomechanical principles for designing crosscuts in steep sedimentary rock mass

Geomechanical principles for designing crosscuts in steep sedimentary rock mass

Clay, Water, and Salt: Controls on the Permeability of Fine-Grained Sedimentary Rocks.

The ability to predict the permeability of fine-grained soils, sediments, and sedimentary rocks is a fundamental challenge in the geosciences with potentially transformative implications in subsurface hydrology. In particular, fine-grained sedimentary rocks (shale, mudstone) constitute about two-thirds of the sedimentary rock mass and play important roles in three energy technologies: petroleum geology, geologic carbon sequestration, and radioactive waste management. The problem is a challenging one that requires understanding the properties of complex natural porous media on several length scales. One inherent length scale, referred to hereafter as the mesoscale, is associated with the assemblages of large grains of quartz, feldspar, and carbonates over distances of tens of micrometers. Its importance is highlighted by the existence of a threshold in the core scale mechanical properties and regional scale energy uses of shale formations at a clay content Xclay ≈ 1/3, as predicted by an ideal packing model where a fine-grained clay matrix fills the gaps between the larger grains. A second important length scale, referred to hereafter as the nanoscale, is associated with the aggregation and swelling of clay particles (in particular, smectite clay minerals) over distances of tens of nanometers. Mesoscale phenomena that influence permeability are primarily mechanical and include, for example, the ability of contacts between large grains to prevent the compaction of the clay matrix. Nanoscale phenomena that influence permeability tend to be chemomechanical in nature, because they involve strong impacts of aqueous chemistry on clay swelling. The second length scale remains much less well characterized than the first, because of the inherent challenges associated with the study of strongly coupled nanoscale phenomena. Advanced models of the nanoscale properties of fine-grained media rely predominantly on the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, a mean field theory of colloidal interactions that accurately predicts clay swelling in a narrow range of conditions (low salinity, low compaction, Na+ counterion). An important feature of clay swelling that is not predicted by these models is the coexistence, in most conditions of aqueous chemistry and dry bulk density, of two types of pores between parallel smectite particles: mesopores with a pore width of >3 nm that are controlled by long-range interactions (the osmotic swelling regime) and nanopores with a pore width <1 nm that are controlled by short-range interactions (the crystalline swelling regime). Nanogeochemical characterization and simulation techniques, including coarse-grained and all-atom molecular dynamics simulations, hold significant promise for the development of advanced constitutive relations that predict this coexistence and its dependence on aqueous chemistry.

New Rapid Evaluation for Long-Term Behavior in Deep Geological Repository by Geotechnical Centrifuge—Part 1: Test of Physical Modeling in Near Field Under Isotropic Stress-Constraint Conditions

The objective of this study is to evaluate the long-term geomechanical behavior of a geological repository for high-level radioactive waste disposal, using the centrifugal near-field model test. The model consisted of a sedimentary rock mass, bentonite buffer, and model overpack, and was enclosed within a pressure vessel. Tests were conducted with a centrifugal force field of 30 G under isotropic stress-constraint conditions with confining pressures of 5–10 MPa and injection of pore water up through a time period equivalent to about 165 years in the field. Our results showed that the measured values and the temporal changes in the displacement of the overpack, the soil pressure of the bentonite, and the strain of the rock mass were clearly dependent on the confining pressure. These data were not convergent during the test. Our data experimentally revealed that long-term behavior in the near field was changed by the geomechanical interaction between the deformation stress of the bedrock/disposal hole and the swelling behavior of the bentonite buffer.

Estimation of the equivalent elastic modulus in shale formation: Theoretical model and experiment

Anisotropy is prevalent in sedimentary rock masses, such as shale formations that contain numerous inherent bedding planes (BPs) and natural fractures (NFs). This study proposes a new methodology to determine the equivalent elastic properties of shale formations that uses the weak patches stiffness model and fracture mechanics theory to theoretically consider both BP- and NF-induced anisotropy. First, a series of uniaxial compression tests for Longmaxi shale demonstrates that its apparent elastic modulus ranges from 14.9 to 18.8GPa. Then, by collecting experimental data and performing a Monte–Carlo simulation method, the NF network is built within the target 3D shale formation, and the equivalent elastic modulus values along a given wellbore wall are estimated. The results reveal that the elastic parameters of shale formations exhibit high directional dependency associated with the presence of BPs and NFs; a higher NF density is also found to lower the BP-induced anisotropy. Wider application of Monte–Carlo simulations in future studies is recommended to calculate the deformation issues of large-scale numerical petroleum engineering modeling when considering the influence of NFs.

Assessment of Slope Stability for Deep Pits in Sedimentary Strata

With advances in technology increasing the economic depth limit of open-cut mines, including those in sedimentary strata, the design of stable slopes has become more critical. Increased use of geophysical logging to compliment more traditional geological and geotechnical logging provides an unprecedented detailed description of the rock mass from surface to sub-seam. Making full and best use of this data in a quantifiable, repeatable and transparent manner for the stability assessment of slopes requires a step beyond simplifying assumptions during limiting equilibrium and numerical stress analyses. By describing a repeatable deterministic model with parameterised input data we provide the basis for detailed probabilistic studies for the quantification of uncertainty. This paper considers the implication on slope stability, as measured by a Factor of Safety, of describing the strata at near geophysical log resolution. A comparison is made when representing the rock mass at increasing levels of detail with justifiable property estimates. We find that for deep pits in sedimentary rock masses a detailed description of the strata at near log resolution together with explicit representation of the near surface blast damaged zone becomes more critical as pit slope angle is optimised. This approach is demonstrated to capture the composite failure mode in sedimentary strata so that the predicted failure mode becomes a combination of low angle slip on weak units with high angle shear failure through stronger rock units as observed in the field. Compared to models with a simpler description of the rock mass, our model predicts that the mass of rock at failure becomes increasingly shallow and localised and is displaced near horizontally out of the pit slope, a failure mode observed in the field.

A procedure to detect, construct, quantify, numerically simulate and validate fracture networks in coal blocks

Coal mass is a typical sedimentary rock mass with three approximately orthogonal discontinuity sets: bedding planes, face cleats and butt cleats. The mechanical anisotropy of coal masses is dependent on this fabric to a large extent. In this paper, a framework incorporating X-ray computed tomography (CT) scanning, statistical analysis, the fracture tensor based methodology and a numerical stress analysis technique is proposed to detect, construct, quantify and simulate the fracture networks in coal blocks. The CT scanning was first used to obtain the geometric information of the fractures in cubic coal blocks. Then a statistical numerical procedure was proposed to estimate and construct the locations, orientations and sizes of the fractures. An overall assessment of the constructed fracture networks was next made using the fracture tensor based methodology to incorporate all the important features: number of joint sets, joint density, joint orientation and joint size. The fracture networks in cubic blocks were then numerically simulated through a proposed modified fictitious joint procedure to accommodate a large quantity of non-persistent joints with an acceptable numerical stress analysis effort. The framework was validated at different stages to evaluate the reliability. Finally, the implementation of this framework is shown by a numerical investigation of the influence of the fracture networks and confining stresses on the jointed coal mass strength. The proposed framework provides a systematic and innovative method to tackle the fracture network induced mechanical anisotropy of the fractured rock masses.

Experimental and numerical modeling of solute transport through fractured sedimentary rock mass

This paper presents the experimental breakthrough curves for conservative and non-conservative tracers through a fractured porous sandstone rock mass. An experimental model for the fractured rock mass was developed in the laboratory. This model consisted of 4 m long sandstone slabs, placed face to face parallel to each other. A small space of 4 mm wide was created between sandstone slabs, through which the water was allowed to flow. Sodium chloride and sodium fluoride solutes were used as conservative and non-conservative tracers through experiments. A numerical model was also developed for the solution of advective–dispersive transport equation including the equilibrium sorption and the first order degradation constant for reactive solute transport. The developed numerical model was used to simulate the observed experimental breakthrough curves through the fractured rock mass. The observation of the breakthrough curves indicates that the mass of solute reduces in the fracture due to sorption and diffusion of solute within the rock mass. An attempt has also been made to estimate the transport parameters such as effective diffusion, dispersion coefficients and sorption coefficients.

New Rapid Evaluation for Long-Term Behavior in Deep Geological Repository by Geotechnical Centrifuge. Part 1: Test of Physical Modeling in Near Field Under Isotropic Stress-Constraint Conditions

The objective of this study is to evaluate the long-term geomechanical behavior of a geological repository for high-level radioactive waste disposal, using the centrifugal near-field model test. The model consisted of a sedimentary rock mass, bentonite buffer, and model overpack, and was enclosed within a pressure vessel. Tests were conducted with a centrifugal force field of 30 G under isotropic stress-constraint conditions with confining pressures of 5–10 MPa and injection of pore water up through a time period equivalent to about 165 years in the field. Our results showed that the measured values and the temporal changes in the displacement of the overpack, the soil pressure of the bentonite, and the strain of the rock mass were clearly dependent on the confining pressure. These data were not convergent during the test. Our data experimentally revealed that long-term behavior in the near field was changed by the geomechanical interaction between the deformation stress of the bedrock/disposal hole and the swelling behavior of the bentonite buffer.

Empirical model for estimating groundwater flow into tunnel in discontinuous rock masses

The high volume of water inflow into tunnel plays a significant role in the design of drainage systems and exerts bio-environmental effects. In engineering practice, analytical and empirical methods that are commonly used to estimate water inflow in sedimentary rock masses, lack sufficient accuracy. The geostructural anisotropy in a fractured rock has a great impact on water inflow. In discontinuous media, anisotropy and heterogeneity of the fractured rock masses are highlited. Hence, these methods are not efficient to calculate water inflow to tunnel in such media, due to the assumed isotropic hydraulic coefficient. In this regard, an empirical formula is developed in this study for hydraulic conductivity in the fractured rock masses for analytical methods, alternately used to predict water inflow. To achieve this, a discrete network flow model was performed. The simulation resulted in a dataset that is helpful in developing hydraulic conductivity empirical formula for well-known Goodman equation. The geostructural parameters, such as the joint orientation, aperture, spacing and joint interconnectivity were included to determine this formula. The acquired empirical equation was utilized in the evaluation of groundwater inflow to middle-depth Amirkabir tunnel in north of Iran. In comparison to the observerd flow, analytical methods resulted in higher overestimation, especially in the sites with high anisotropy. However, empirical model led to a better estimation of water inflow to tunnel.

Experimental Investigation of Non-Darcy Flow in Sandstone

A study was conducted to investigate non-Darcy flow for sandstone using an experimental system designed and constructed by the research group. Non-Darcy flow was found to exist for both the pre-peak and post-peak strains; post-peak flow behavior showed a higher level of non-Darcy flow. In addition, the post peak permeability turned out to be about 2–3 times that of the pre-peak permeability. This sudden change in permeability that occurs when sedimentary rock masses around underground excavations develop new fractures may lead to water inrush in coal mines. All three non-Darcy permeability parameters showed significant changes between the pre-peak and post peak strains. The main reason for that is the formation of new cracks around and after the peak stress. Relations are developed among the three non-Darcy permeability parameters.

Modeling deformation modulus of a stratified sedimentary rock mass using neural network, fuzzy inference and genetic programming

This paper investigates a series of experimental results and numerical simulations employed to estimate the deformation modulus of a stratified rock mass. The deformation modulus of rock mass has a significant importance for some applications in engineering geology and geotechnical projects including foundation, slope, and tunnel designs. Deformation modulus of a rock mass can be determined using large scale in-situ tests. This large scale sophisticated in-situ testing equipments are sometimes difficult to install, plus time consuming to be employed in the field. Therefore, this study aims to estimate indirectly the deformation modulus values via empirical methods such as the neural network, neuro fuzzy and genetic programming approaches. A series of analyses have been developed for correlating various relationships between the deformation modulus of rock mass, rock mass rating, rock quality designation, uniaxial compressive strength, and elasticity modulus of intact rock parameters. The performance capacities of proposed models are assessed and found as quite satisfactory. At the completion of a comparative study on the accuracy of models, in the results, it is seen that overall genetic programming models yielded more precise results than neural network and neuro fuzzy models.

Performance evaluation of existing surface excavation assessment methods on weathered sedimentary rock

This paper is intended as a contribution to the discussion of the overview of existing excavatability assessments and their reliability in assessing the state of tropically weathered rocks, specifically in Nusajaya, Johor, Malaysia. For this purpose, case studies were conducted on sedimentary rock masses of the Jurong formation, located in Nusajaya, Johor. The existing available rock mass excavatability assessments were first critically reviewed. Three case studies were then conducted in order to assess the performance of surface excavatability in these sedimentary rock masses by practical excavation tests on site. Comparisons were then made between the assessments of those assessors and that obtained through the practical excavation test. The conclusions made in the paper about the results of the study show many divergent opinions among the assessors. It emphasizes the limitations of existing available assessments in predicting the excavatability of tropically weathered rock masses. Further complication may arise in sedimentary rock masses where different layers of rock are interbedding, producing different weathering profiles. In view of these, a simpler graphical method, which considers only two parameters, namely point load strength and spacing of surface discontinuities, was introduced to overcome the problems, thus might be misjudged during early excavation assessment. This subjectivity becomes more important as the heterogeneity of a rock mass increases with increasing interbedding and inherent anisotropy of the rock material, giving rise to quite different evaluations, even for a rock with the same state of weathering. Most of the existing excavatability assessment methods are found to be less reliable because nature, lithology and weathering state at various rock mass layers is not being taken into account. A more practical and appropriate excavatability assessment method is thus required to economically assess the tropically weathered sedimentary rock during the preliminary stage.