L.R. Alejano

FDM predictive methodology for subsidence due to flat and inclined coal seam mining

Abstract A subsidence estimation methodology to predict subsidence troughs due to flat and inclined coal seam exploitation is described. The foundations of the procedure are the correct definition of the rock mass behaviour model, its adequate characterization, and its implementation in the code “FLAC” (Fast Lagrangian Analysis of Continua was written by P.A. Cundall and is marketed by ITASCA Cons. Group Inc. of Minneapolis, MN, USA), which is based on the “Finite Difference Method” or FDM numerical modelling technique. The method is first validated for flat coal seam longwall mining with caving in British basins –because of the large number of data from this area– which includes subsidence measurements and characterization parameters. The method is then applied to gently inclined seams, and the results are seen to fit empirical observations. Finally, for steeply inclined and sub-vertical seams the proposed methodology allows observation of two different movement trends in the overlying strata; the former is perpendicular to the strata direction, in the same way as the one observed in flat seam subsidence; and the latter, which becomes the most important one in seams dipping by more than 75°, is parallel to the strata. These two different trends produce subsidence troughs presenting two relative subsidence maxima and extending along large surface areas. Thus, it would be unlikely to predict and assess these troughs, this may be a possible reason why so little coherent data about this kind of subsidence exists. These numerical observations of steeply inclined coal seams need to be validated empirically.

Application of geostatistical techniques to exploitation planning in slate quarries

When planning workings of slate extraction by mechanical means, it is important to know the quality of the rock mass to optimize the results of the cuttings in the extraction bank, because techniques to be used depend on relative percent of first quality or second quality slate. In this paper, a methodology for quality evaluation applied to slate extraction is developed in three phases, as a tool of support to the planning. The first phase consists of defining the geotechnical parameters that affect the bank and to appraise them in the visible faces. The second phase consists of applying multivariate statistical techniques related to discriminant analysis data to evaluate a quality function. This function can be seen as a recovery index of the slate mass, being its variance intervals established in the analyzed faces. The third phase consists of a forecast of the values of the index from visible faces to the inside of the extraction bank, using geostatistics to evaluate the quality of the mass and to plan the mechanical cutting works, in order to get the best results.

Drucker–Prager Criterion

List of Symbols k Drucker–Prager material constant j Drucker–Prager material constant J2 Second invariant of the stress deviator tensor I1 First invariant of the effective stress tensor r1 Major principal effective stress r2 Intermediate principal effective stress r3 Minor principal effective stress soct Octahedral shear stress roct Octahedral effective normal stress C0 Uniaxial compressive strength T0 Uniaxial tensile strength h Lode angle b MSDPu parameter that defines the shape of the criterion in the p-plane (usually, b % 0.75) a1 MSDPu parameter a2 MSDPu parameter / Angle of internal friction c Cohesion

Study of a rockfall in a limy conglomerate canyon (Covarrubias, Burgos, N. Spain)

In this paper, a rockfall of some tenths of cubic metres of rock, occurred in a conglomerate canyon in the northern Castilian plateau (N-Central Spain), is described in detail. This includes the basic data (geology, mapping, meteorological data and rock involved), the mechanical mechanism of the instability, the propagation and reach of the fallen rock boulders and the final location of the pieces. The study is based on detailed geological, aerial photography and geotechnical studies and advance topographical measurements. Geological engineering techniques are used to analyze the instability initiation and rockfall propagation. This study contributes to a better understanding of the geological processes leading to the occurrence of calcareous canyons and cliffs in the drainage network of the upper Castilian plateau. It is also helpful to improve our knowledge on natural rockfall phenomena for risk engineering purposes aiming to quantify, control and mitigate associated hazards.

Response by the Authors to S. R. Hencher’s Discussion of the Paper “Comparison of Different Techniques of Tilt Testing and Basic Friction Angle Variability Assessment

In a recent publication (Alejano et al. 2012), some issues concerning the evaluation of the basic friction angle /b of saw-cut rock surfaces were addressed. In his discussion, S.R. Hencher provides a series of useful references with some low values for the basic friction angle, refers to the variability of this parameter and puts forward a series of comments regarding the paper. The authors would like to acknowledge S.R. Hencher’s interest in the paper and thank him for extending the scope of the research and for providing the opportunity to present these additional comments, clarify some aspects of the work and highlight some of the main conclusions. The main aim of the author’s work was to facilitate the laboratory estimate of /b to input a reliable value in the formulation of Barton’s peak shear strength criterion. As stated in the first paragraph of the introduction to the paper, it is important to note that this criterion applies to natural unfilled rough rock joints. A priori it therefore does not apply to filled discontinuities or to those that have suffered previous shear (typically mismatched), where polished or slicken-sided surfaces appear and where flour rock or fill can be encountered. Concerning the variability of /b, it is important to note that the tilt tests presented in the paper were performed using samples from the same rock types. Moreover, tilt tests were performed using the lateral surfaces of prismatic samples cut from the same rock block. So, the standard deviation values presented in the paper only take into account the intrinsic variability of this kind of test. Tilt tests for the evaluation of /b and the remaining index tests for the characterization of rock joint shear strength proposed by Barton (1999) are very simple and straightforward. As a consequence, inexperienced designers may overlook the variability in the resulting basic friction angles and this, in turn, may lead to hazardous safety assessments. It should also be stressed that the dispersion of the empirical equations that support Barton’s peak shear strength criterion has always been mentioned, from the earliest works to present days (Barton and Choubey 1977; Barton 2011). The paper presents results of tilt tests that rendered low basic friction values. Such low values of /b are generally provided by unweathered rocks with high compressive strength (e.g., in excess of 150 MPa). These low friction values should be anticipated, as saw-cut surfaces and cores of these rocks often show very smooth and sometimes polished surfaces. In the particular case of porphyritic granite, the values displayed in Fig. 1 of the paper refer to all tilt tests repetitions performed with all possible combinations of the lateral faces (120 mm long and 45 mm wide) of two prismatic blocks, resulting in 384 values corresponding to 3 repetitions 9 (8 9 8 combinations) 9 2 tilting directions. To address the issue of the low /b values, it is preferable to study just the result of each tilt test, considering this as the median of the three repetitions. These values, presented in Fig. 1, show clearly that around 50 % of the values are below 20 . In fact, the median is exactly 19 , the mean (arithmetic) of the friction angles is 19.2 and the standard deviation is 4.46 . L. R. Alejano (&) J. González Department of Natural Resources and Environmental Engineering, University of Vigo, Vigo, Spain e-mail: alejano@uvigo.es

Crack Damage Parameters and Dilatancy of Artificially Jointed Granite Samples Under Triaxial Compression

A database of post-peak triaxial test results was created for artificially jointed planes introduced in cylindrical compression samples of a Blanco Mera granite. Aside from examining the artificial jointing effect on major rock and rock mass parameters such as stiffness, peak strength and residual strength, other strength parameters related to brittle cracking and post-yield dilatancy were analyzed. Crack initiation and crack damage values for both the intact and artificially jointed samples were determined, and these damage envelopes were found to be notably impacted by the presence of jointing. The data suggest that with increased density of jointing, the samples transition from a combined matrix damage and joint slip yielding mechanism to yield dominated by joint slip. Additionally, post-yield dilation data were analyzed in the context of a mobilized dilation angle model, and the peak dilation angle was found to decrease significantly when there were joints in the samples. These dilatancy results are consistent with hypotheses in the literature on rock mass dilatancy.

A Benchmark Experiment to Assess Factors Affecting Tilt Test Results for Sawcut Rock Surfaces

From the earliest studies on the topic, plane sliding techniques, usually known as tilt tests, have shown contradictory features. On the one hand, they reflect on a small-scale basic principles regarding definition of the friction angle and can reproduce conditions very similar to those of the sliding of blocks on rock slopes; furthermore, tilt angle results agree with friction angles derived from shear and pull tests (Hencher 1977; Muralha 1996). On the other hand, the literature includes several reports of erratic results and it is recognized that even apparently smooth surfaces are actually rough at the microscopic level (Hencher and Richards 2015). Thus, adhesion and textural interlocking may contribute to the variability and the nonreproducibility of tilt test results. A number of authors have carried out simple tilt tests in the past, and several examples illustrate the above-mentioned arguments. Thus, Hencher (2012) observed extreme variability in the results of tilt tests; Nicholson (1994) found that friction angles for sawcut Berea sandstone in direct shear tests varied by 12.5 , despite great attention paid to sample preparation and reproducibility; Coulson (1972) demonstrated that the friction angle of planar surfaces of rock varied with surface finish; Krahn and Morgenstern (1979) reported similar variation for surfaces prepared in different ways and with different surface finishes; and Kveldsvik et al. (2008) found that the basic friction angle for a rock slope, derived from tilt testing of core, varied between 21 and 36.4 . These great differences in measured tilt angles are mainly attributed to different surface finishing and to wear of the rock contacts (Pérez-Rey et al. 2015, 2016), although other reasons, such as testing and ambient conditions, cannot be ruled out. Mehrishall et al. (2016) revealed that the residual friction coefficients of grinded joint surfaces and of rough rock surfaces were almost identical. This manuscript contains interesting findings in rock mechanics practice, derived from large efforts to test the same rocks under different conditions and different laboratories. At the same time, this manuscript presents part of the work which will be used to prepare a Suggested Method on Tilt Testing. It is published in the form of a Technical Note, although its length exceeds what is generally accepted for this type of publication.

Geotechnical characterization of the site and stability of a tailings dam in Ecuador

In this paper we present the design and stability analysis of a tailings dam developed in order to stock up tailings from mineral processing plants located in the left bank of Calera river in the ZarumaPortovelo Mining District, in Ecuador; including a geotechnical study of materials involved. This study first reviews some instability problems that took place in tailings dam in other parts of the world. Then, the geological and geotechnical characterization of various geological materials (soil, rocks and aggregates) that will serve as a foundation or that will be utilized in the dam construction site is presented. Since some of these the materials are ripped and then used to build the dam, they are also characterized according to their granular behavior after set up in the dam construction. Finally, a design fulfilling operational needs is proposed and its stability analyzed in terms of the most probable instability mechanisms (circular failure), and accounting for standard and worst possible hydrogeological and seismic conditions expectable in the area. The ultimate aim of a tailings impoundment is to contain fine-grained tailings. This has to be carried out in a cost-effective manner that provides for longterm stability of the embankment structure and the impounded tailings and the long-term protection of the environment (USEPA,1994). Failure of tailings dams may result in disastrous damages to lives, properties and the surrounding environment. In this way, design, construction and operation of the tailings dams require a high level of care in engineering practice (Ozcan et al., 2013). This article focuses on presenting the background of geotechnical characterization of soils, aggregates and rocks affecting a tailing dams, showing the most relevant variables controlling their stability and presenting an example of a safe particular design in the above named mining zone of Ecuador. 1.2 Problem statement Portovelo is the principal mining region in the south of Ecuador, where, for decades, mining waste management has been a concern. This waste has so far moderately polluted the Puyango river basin. This river flows into Peruvian national territory and finally into the Pacific Ocean, where it flows out in the town of Tumbes. So to mitigate this problem, it was agreed to build a tailings dam, which could help to avoid further polluting and to clean the river basin. Several recent researches provided representative examples to improve material characterization facing toward generate reliable studies of tailing dams design. For example, a study published by Ozcan et al. (2013) analyzes the characterization and stability in a structure like this. It is noted the necessity of performing a complete geotechnical study of the affected materials, the determination of the geotechnical characteristics of the aggregate materials and the assessment of the slope stability. The static and pseudo-dynamic analysis results, based on limit equilibrium methods and numerical approaches, showed that circular failures are the most common critical failure mechanism associated to dam behavior. In this way, the factors of safety tend to diminish as far as the dam is growing in height (Fig. 2). Nevertheless, safety ranges derived by both methods are similar and suitable for dam growing in safety conditions, provided input data are computed in a reasonably accurate way.