Understanding the geological model in a chalk environment: an example from the High Speed Two (HS2) railway

Standard geological modelling based on boreholes and geological maps can be strengthened using geophysical data. Constrains such as gravity, magnetic and seismic data have already been used. We propose a novel method combining boreholes and the resistivity model resulting from inversion of airborne time domain electromagnetic data. First, the “geophysical” top of the chalk has been identified in the resistivity model after a detailed cross-analysis of resistivities versus boreholes. Then, we jointly interpolated slopes extracted from this geophysical surface together with the top of the chalk in boreholes. Comparison of uncertainties between this model together with pure geological and geophysical models shows that the joint modelling yields the most accurate top of the chalk. A cross-section, intersecting five boreholes (two used as control boreholes) and displaying each of the three surfaces, highlights the usefulness to take into account “geophysical slopes” when modelling. The proposed joint modelling improves what is commonly obtained with geological or geophysical data. This makes the method very attractive for detailed 3D geological modelling.

Combining the requirements for refined modeling and dynamic update of fault slope geological models in open-pit coal mines, we systematically elaborated on the elements and rules of slope 3D geological modeling and proposed a fine modeling and dynamic updating method based on digital elevation model (DEM) and half-edge boundary representation (B-Rep) data structures. Initially, the stratigraphic division of the study area is conducted based on borehole and section data, combined with the geological evolution history and the level set theory. Applying inverse distance weighting (IDW) for estimation, the geological interface triangular irregular network (TIN) is constructed using the TIN algorithm. Secondly, fine reconstruction of fault surfaces through Morphing transformation improves the accuracy and smoothness of fault surface shape reconstruction under sparse data conditions. Then, the intersection-cutting processing method of fault-fault and fault-stratum for slope geological ground state model modeling under complex fault segmentation is investigated. Finally, constructing 3D geological models of fault slopes and the integrated correction of surface and subsurface are realized through dynamic updating methods for 3D surfaces and strata. Taking the 3D geological modeling of a fault slope of an open-pit coal mine in China as an example, the feasibility of the technical method is verified.

In tunnelling, a reliable geological model often allows providing an effective design and facing the construction phase without unpleasant surprises. A geological model can be considered reliable when it is a valid support to correctly foresee the rock mass behaviour, therefore preventing unexpected events during the excavation. The higher the model reliability, the lower the probability of unforeseen rock mass behaviour. Unfortunately, owing to different reasons, geological models are affected by uncertainties and a fully reliable knowledge of the rock mass is, in most cases, impossible. Therefore, estimating to which degree a geological model is reliable, becomes a primary requirement in order to save time and money and to adopt the appropriate construction strategy. The definition of the geological model reliability is often achieved by engineering geologists through an unstructured analytical process and variable criteria. This paper focusses on geological models for projects of linear underground structures and represents an effort to analyse and include in a conceptual framework the factors influencing such models. An empirical parametric procedure is then developed with the aim of obtaining an index called “geological model rating (GMR)”, which can be used to provide a more standardised definition of a geological model reliability.

Introduction. Land subsidence is a visible consequence of deformation processes in the rock mass as a result mining. Deformation processes originate at producing horizons and propagate in the rock mass, usually upwards. The presence of natural physical and mechanically weakened zones in the rock mass complicates the situation creating risks of rock fracture propagation. The main problem is that subsidence of the earth's surface can lead to negative consequences for facilities within the undermining zone. Research objective is to carry out geomechanical modeling to adequately display the current situation on the earth's surface in accordance with real actual observational data. The created geological and geomechanical model can later be used to analyze and predict potential fracture zones in the rock within the rated section. Methods of research. The geological and geomechanical model is based on strata geological description, salt and oil wells geophysical exploration data, current position of workings, as well as physical and mechanical properties of rocks building up the rock mass. Results. The paper presents the results of geomechanical modeling of land subsidence at a site of the Upper Kama potassium-magnesium salt deposit in the vicinity of shaft pillars. Calculations were based on a geomechanical model of salt rock deformation and the developed geological and geomechanical model of an orefield site, including shaft barrier pillars regulated by the relevant reference documents. Timely measures for preventing negative consequences are recommended.

Canada, along with other countries that are considering the permanent disposal of high-level radioactive wastes from nuclear power generation, is undertaking a program of research into deep geologi...

Traditionally, geological interpretation is undertaken by manually annotating cross-sections, and updating them when new information becomes available. This is a time-consuming process and goes against the principles of good digital engineering. A workflow was developed for High Speed Two (HS2) so that raw ground investigation information forms a database that is then used to generate an up-to-date 3D geological BIM model. This model contains the geological strata covering a ∼200m wide corridor along the HS2 alignment. Intelligent geometry represents the borehole sticks used to derive the surfaces, so the user can view the factual data behind the interpretation. This approach automates the process of geological model updates with low manual effort, based on a series of algorithmic rules, and available on a Common Data Environment (CDE), allowing baselining of versions and easy transfer of information between disciplines. This offers important time and cost savings to the project as most of the process is automatic, and the result only needs to be quality checked once the model is set up. Having a centralised, intelligent, up-to-date 3D ground model as a single source of truth allows easy access for all design disciplines using ground data, with improved consistency, quality and cost. This paper discusses each step of the workflow, including how the geological modelling was undertaken. The paper then goes on to describe how the model has been checked, and some of its uses and benefits. Finally, possible future developments are discussed.

Introduction. Slope stability analysis reliability depends on the level of schematizing the results of engineering-geological survey, hydrogeological monitoring, electrical sounding, and mine surveying data generalization. In order to improve the stability analysis accuracy, it is relevant to use the geomechanical models, which take into account the spatial variability of the adjacent rock mass physical and mechanical properties, with further search for the most hazardous section in the plan. Research aim is to substantiate the relevance of slopes physical and mechanical monitoring by the methods of downhole control and further stability analysis with the use of the geological and geophysical models. Methodology includes the search for the most hazardous section of the rock mass by the ratio of the shearing and restraining forces within the limits of the established zones characterized by the variability of physical and mechanical properties. Results. As a result of generalizing the databases of the engineering and geological study, hydrogeological monitoring, electrical sounding and mine surveying, the volumetric geological and geophysical models were built of the technogenic massif formed at the dump, as well as the slopes of the hydraulic overburden face, represented by the three-level fill of dry overburden. Test areas stability has been analyzed for the actual position, and the comparative analysis of the results was made. Conclusions. When building volumetric geological and geophysical models of man-made rock masses formed at the areas of filled hydraulic waste diposals, in addition to the results of the traditional survey measurements and direct engineering and geological study, it is important to create informative databases on spatial and temporal variations of physical and mechanical properties of alluvial rock conditioned by their fluid loss and consolidation, which with in details at cross hole intervals may be obtained based on the statistical dependences on electrical conductive properties variation by the methods of electrical sounding or tomography; and when making enclosing embankments (bridges) of dry rocks – information about excess pore pressure under these facilities. When using the hydromechanized method for the alluvial rocks, relatively homogeneous in strength properties, the hydraulic overburden face slope stability is determined to the greatest extent by the absolute values of the accumulated strength indicators in the mined area and the unfavorable combination of the edge shape (the presence of protruding sections and face unevenness in the plan) with the entry height. The established range of the stability coefficient was n = 1.03–3.76. The stability of the enclosing structures made of dry rock to the greatest extent depends on weak alluvial layer depth and thickness and the absolute value of excess pore pressure in this layer. The established range of stability coefficient variation for this rock mass section is much narrower and amounted to n = 1.29–1.59.

Engineering geology model of the Crater Lake outlet, Mt. Ruapehu, New Zealand, to inform rim breakout hazard

Geotechnical stability analysis in open-pit mines requires access to a representative geotechnical model. The confidence level in the collected geotechnical data influences slope design. This paper investigates the influence of the number of boreholes, drilled to collect geological information, on the quality of one component of the geotechnical model, the geological model. The number of boreholes influences the number of rock core samples collected for the identification of rock type, and the definition of geotechnical domains and their boundaries within the rock mass. A challenge in the definition of the geotechnical domains is the determination of the drill hole density that minimizes the variation in the interpreted geological model from the actual rock mass. To quantify the influence of the drill hole density, boreholes are simulated in the most recently updated geological model for three mine sites. The simulated drill hole density is increased progressively until the variation of the interpreted section, compared with the original section, is minimized. A classification strategy was developed to determine the complexity level for each geotechnical domain. Furthermore, a series of empirical quantitative guidelines are presented prescribing the minimum drill hole density per domain complexity, while limiting variations from the actual rock mass.

3-D geological models are built with data collected in the field such as boreholes, geophysical measurements, pilot shafts or geological mapping. Unfortunately, these data are always limited in number. It implies that geological information is sparse and subsurface models are thus always built of both subjective interpretation and mathematical interpolation/extrapolation techniques. These models are therefore uncertain and this uncertainty is rarely pointed out in a geological prognosis. Our study proposes to bring a new methodology for the evaluation of geological uncertainties related to 3-D subsurface models and to test its potential use. The methodology we propose is based on the 3-D subsurface model, which is here considered as the most probable prediction (notion of best guess). The various geological interfaces that compose the subsurface model are handled individually as Gaussian random fields. At each location of an interface, the random function Z(u) describing the position of this interface is composed of a deterministic part m(u) which represents the expected position, and a random part σ(u)e(u) which describes fluctuations around the predicted position. Then, a model of spatial variability (a variogram function γ(h)) is proposed in order to condition the random field according to available observations. Several structural constraints, such as the shape of folds and the thickness of layers can also be accounted for in this model. At this point, we are able to estimate the local variance all over the study area by the application of the kriging technique. Finally, the variability is converted into three-dimensional information by calculating probabilities, this describes the occurrence of the various rock masses that are present in the study area. The probabilities are calculated according to intersection rules that govern the stratigraphic sequence of the subsurface model, and they allow us to build a probabilistic model of subsurface structures in the form of a three-dimensional probability field. All of this has been incorporated in a computer program.

Determination of rock mass modulus : Chappell, B A Proc 4th Australia-New Zealand Conference on Geomechanics, Perth, Western Australia, 14–18 May 1984V2, P514–518. Publ Barton: Inst of Engineers, 1984

Extensive high, steep and perilous rocky slopes are present on both sides of the Three Gorges Reservoir area, with numerous such slopes interspersed. The rock mass structure of this type of dangerous rock slope is complex, strongly affecting the stability of the rock masses in this region. Hence, it is necessary to establish a detailed 3D geological model in combination with the internal structural characteristics of the rock mass. Taking the Jianchuandong 3# dangerous rock mass (JDRM 3#) in the Three Gorges Reservoir area as an example, this study integrated UAV photogrammetry technology and 3D laser scanning technology to establish models of the surface of the dangerous rock mass, internal dissolution cavities and random fissures. A 3D geological model of JDRM 3# was constructed by integrating these models. Based on the constructed model, the evolution process of the deformation of JDRM 3# under the influence of long‐term water‐level fluctuation was simulated. The simulation results showed that long‐term water‐level fluctuation can lead to a deterioration in the strength of the base of the rock mass, with deformation and failure of the dangerous rock mass initially occurring around the internal dissolution cavities. A change in the internal structure of the rock mass played a vital role in the failure process of JDRM 3#, which aggravated the eccentric compression state of the base of the rock mass. It will eventually lead to a collapse failure of JDRM 3#. When the compressive strength of the base of the rock mass decreased by approximately 70.3%, the factor of safety (FOS) for JDRM 3# was <1. By combining different geological investigation methods to establish a 3D geological model and using it for numerical calculations, this study provides a novel method to assess the stability of high‐steep bank slopes in reservoir areas.

Downhole temperature distribution in horizontal wells can be an important source of information that helps us characterize the reservoir and understand the bottom-hole flow conditions. The temperature measurements are obtained from permanent monitoring systems such as downhole temperature gauges and fiber optic sensors. Also, production history and bottomhole pressures are usually readily available and are routinely used for history matching to improve the initial geological models. Combining the downhole temperature distribution and the production history, we can extract more reliable information about the reservoir permeability distribution and bottomhole flow conditions that help us optimize the wellbore performance, particularly in horizontal wells. In this paper, we use a thermal model and a transient, 3D, multiphase flow reservoir model to calculate the wellbore temperature distribution in horizontal wells. By comparing the simulated temperature and the observed data, we first derive large-scale permeability trends in the reservoir. These permeability trends are then incorporated as ‘secondary’ information in the geologic model building and history matching. Finally, the updated permeability models from history matching are used to infer the downhole flow conditions along horizontal wells. The final outcome is a geologic model that is consistent with reservoir static and dynamic information, and also the wellbore temperature measurements. We present several synthetic cases to illustrate the procedure. The results show that with only production history matching without distributed data along the wellbore, the water entry location in horizontal wells can not be detected satisfactorily. Combining production history matching with the temperature distribution in the wellbore, we can get an improved geological model that can match the production history and also, locate the water entry correctly. Based on the downhole flow conditions and the updated geological model, we can now optimize the well performance by controlling the inflow rate distribution, such as shutting the high water inflow sections.

Using downhole temperature measurement to assist reservoir characterization and optimization

The intact rock properties and discontinuity properties for both DRC and DP rock formations that exist in the selected open pit mine were determined from tests conducted on rock samples collected from the mine site. Special survey equipment which has a total station, laser scanner and a camera was used to perform remote fracture mapping in the research area selected at the mine site. From remote fracture mapping data, the fracture orientation, spacing and density were calculated in a much refined way in this study compared to what exist in the literature. Discontinuity orientation distributions obtained through remote fracture mapping agreed very well with the results of manual fracture mapping conducted by the mining company. GSI rock quality system and Hoek–Brown failure criteria were used to estimate the rock mass properties combining the fracture mapping results with laboratory test results of intact rock samples. Fault properties and the DRC–DP contact properties were estimated based on the laboratory discontinuity test results. A geological model was built in a 3DEC model including all the major faults, DRC–DP contact, and two stages of rock excavation. The built major discontinuity system of 44 faults in 3DEC with their real orientations, locations and three dimensional extensions were validated successfully using the fault geometry data provided by the mining company using seven cross sections. Numerical modeling was conducted to study the effect of boundary conditions and lateral stress ratio on the stability of the considered rock slope. For the considered section of the rock slope, the displacements obtained through stress boundary conditions were seemed more realistic than that obtained through zero velocity boundary conditions (on all four lateral faces). Stable deformation distributions were obtained for k 0 in the range of 0.4–0.7. Because the studied rock mass is quite stable, it seems that an appropriate range for k 0 for this rock mass is between 0.4 and 0.7. The displacements occurred between July 2011 and July 2012 due to the nearby rock mass excavation that took place during the same period were compared between the field monitoring results available from the mining company and the predicted numerical modeling results; the best agreement was obtained for k 0 = 0.4. Therefore, k 0 = 0.4 can be decided as the most appropriate value for the studied mine site. In overall, the successful simulation of the rock excavation during a certain time period indicated the possibility of using the procedure developed in this study to investigate rock slope stability with respect to expected future rock excavations in mine planning.