Rock Mass Strength Parameters Research Articles

The stabilization of the rock mass of the wieliczka salt mine through the backfilling of the witos chamber with the use of injection methods / Stabilizacji górotworu kopalni soli „wieliczka” poprzez likwidację komór „witos” z zastosowaniem metod iniekcji

The Wieliczka Salt Mine is the most famous and the most visited mining industry monument in the world and it requires modern methods to ensure rock mass stability and tourists’ security. Both for conservation and tourism organization reasons, the group of Warszawa-Wisla-Budryk-Lebzeltern-Upper Witos Chambers (Photo. 1, 2. 3) located the Kazanów mid-level at a depth of 117 m underground is extremely important. Discontinuous deformation occurring in this Chamber complex was eliminated by comprehensive securing work with anchor housing, but their final securing and stability is conditioned by further backfilling and sealing the Witos Chambers situated directly beneath. In the 1940s and 1950s, the Witos Chamber was backfilled with slag from the mine boilerhouse. However, slags with 80% compressibility are not backfilling material which would ensure the stability of the rock mass. The chambers were exploited in the early nineteenth century in the Spizit salts of the central part of the layered deposit. The condition of the Upper Witos, Wisla, Warszawa, Budryk, and Lebzeltern Chambers is generally good. The western part if the Lebzeltern Chamber (Fig. 1), which was threatened with collapse, was backfilled with sand. In all the chambers of the Witos complex, local deformation of ceiling rock of varying intensity is observed as well as significant destruction of the side walls of pillars between chambers. No hydrogeological phenomena are observed in the chambers. It has been attempted to solve the problem of stability of the rock mass in this region of the mine by extracting the slag and backfilling with sand, erecting concrete supporting pillars, backfilling the voids with sand, anchoring the ceiling and the side walls, the use of the pillar housing. The methods have either not been applied or have been proved insufficient to properly protect the excavation situated above. In order to select the optimal securing method, a geomechanical analysis was conducted in order to determine the condition of the chambers with particular emphasis on the pillars between the chambers. The analysis demonstrated the need to backfilling the Witos Chambers in order to improve the strength parameters of the pillars and the cross-level ledge. The next step consisted of selecting the sealing mix and testing how the additional burden and improving the slag strength parameters shall affect the stability of the excavations of the Kazanów mid-level. In order to determine the optimal composition of the backfilling mixtures, formulas of sealing brine slurries have been developed. Laboratory tests were also conducted concerning the strain parameters specifications of slags extracted from the Witos Chamber. Taking into account the slurry tests, and in particular, the density, strength and strain parameters, the optimal composition of the sealing mix was selected. The analysis of the results of numerical recalculations demonstrate that even the use of highest-density mixtures, backfilling(sealing) of the Witos Chambers should not cause significant disturbance of the current tension in the surrounding rock mass. The longterm impact of sealing should lead to improvement of the strain levels on the ledges between Level III and Kazanów mid-level chambers. The positive results of applying in the Mine of injection slurries for sealing and stabilizing the rock mass and the construction of the injection node on the surface of the Kosciuszko shaft area have allowed resuming work in the Witos Chambers. The main injection over 1,000 m long pipeline was constructed from the injection node through the Kosciuszko Shaft and along Level III of the mine. The sealing of the Witos Chambers complex was divided into three areas (Fig. 2) separated by backfilling dams. Each region was connected to an injection and venting pipeline, and areas of possible injection material off-flow from backfilling locations were secured. Once that the Chambers are sealed with the use of the pipeline seven bore holes will be drilled from excavations situated above through which the sealing slurry will be administered. The operation will serve to eliminate any voids and re-seal the slag, and it will be conducted until pressures of approximately 0.5 MPa on the bore hole collar is achieved. As past experience indicates, injection slurry formula can be regularly adjusted adequately to the changing geomechanical parameters and the type of sealing work at the Wieliczka Mine. Once that the backfilling and sealing process in the Witos Chambers complex is completed, it shall be necessary to conduct monitoring activities in order to determine the processes occurring in the rock mass after the backfilling. The properties of sealing mixtures qualify those for use in the environment both of salt mines and other mineral ore mines to stabilize the rock mass in the mining-geomechanical context precluding the possibility of weakening the rock mass strength parameters and at the same time sealing the rock mass and the loose material deposited in the excavation.

Distinct element simulation of ultimate bearing capacity in jointed rock foundations

In this paper, distinct element method numerical modeling is applied to evaluate bearing capacity of strip footing rested on anisotropic discontinuous rock mass. As yet, a little work has been carried out to investigate the effect of joint set orientation on the bearing capacity of rock mass. Generally, the overall behavior of rock mass under loading is very complicated and such analysis should include deformation determination, sliding along discontinuities and failure of rock material. Failure mechanism of rock mass depended on both geometrical parameters of joint sets and strength parameters of rock mass. In this research, it is assumed that rock mass contains one joint set, and therefore the anisotropy in bearing capacity and rock behavior is only due to the existence and orientation of the joint set. In this study, by assuming constant strength parameters and using Mohr–Coulomb failure criterion for the single joint set and nonlinear Hoek–Brown failure criterion for rock material, variation of the bearing capacity values and the type of failure mechanism of rock mass with different joint set dips is investigated. The obtained results indicate that the ultimate bearing capacity of rock mass containing one joint set varies between 27 and 86 % of intact rock.

Understanding Etna flank instability through numerical models

As many active volcanoes, Mount Etna shows clear evidence of flank instability, and different mechanisms were suggested to explain this flank dynamics, based on the recorded deformation pattern and character. Shallow and deep deformations, mainly associated with both eruptive and seismic events, are concentrated along recognised fracture and fault systems, mobilising the eastern and south-eastern flank of the volcano. Several interacting causes were postulated to control the phenomenon, including gravity force, magma ascent along the feeding system, and a very complex local and/or regional tectonic activity. Nevertheless, the complexity of such dynamics is still an open subject of research and being the volcano flanks heavily urbanised, the comprehension of the gravitative dynamics is a major issue for public safety and civil protection. The present research explores the effects of the main geological features (in particular the role of the subetnean clays, interposed between the Apennine–Maghrebian flysch and the volcanic products) and the role of weakness zones, identified by fracture and fault systems, on the slope instability process. The effects of magma intrusions are also investigated. The problem is addressed by integrating field data, laboratory tests and numerical modelling. A bi- and tri-dimensional stress–strain analysis was performed by a finite difference numerical code (FLAC and FLAC3D), mainly aimed at evaluating the relationship among geological features, volcano-tectonic structures and magmatic activity in controlling the deformation processes. The analyses are well supported by dedicated structural–mechanical field surveys, which allowed to estimate the rock mass strength and deformability parameters. To take into account the uncertainties which inevitably occur in a so complicated model, many efforts were done in performing a sensitivity analysis along a WNW–ESE section crossing the volcano summit and the Valle del Bove depression. This was mainly devoted to evaluate the effect of topography, geometry and rheological behaviour of the structural units. The 3D numerical model, extended 40×60km, was implemented to simulate the volcano deformation pattern. First, the role of the Pleistocene subetnean clays was investigated, then, two “structural weakness zones” – the Pernicana Fault system and the NE rift – were introduced and their effects on the flank instability evaluated. Two extreme hydrogeological conditions, drained and undrained, were analysed. The results are expressed in terms of stress–strain field, displacement pattern, plasticity states and shear strain increments. Two main instability mechanisms were identified: one at shallow depth, with the sliding surface located inside the subetnean Quaternary clay, and another deep-seated mechanism with a not continuous and less evident sliding surface, developed inside the Apennine–Maghrebian Chain flysch, bordered by active structures. Both mechanisms contribute to explain the present deformation pattern and some of the main structures of the Etna flank. The effect of magma pressure exerted on the active dyke walls during eruptions was then simulated and relations between magmatic activity and flank instability were preliminarily investigated.

Study of fractured rock masses deformation in Boukhadra (Tebessa) underground mine empirical and numerical approach (N-E Algeria)

The purpose of this paper is to assess the engineering geological characteristics of Boukhadra’s iron mine rock mass. Several problems of deformation and stability were frequently encountered during the progress of the exploitation. This phenomenon affects seriously the safety of the miners and the progress of the work. In this research project, we try to estimate the behavior of the host rock mass, its deformability and stability. Field studies consisted of geological mapping, discontinuity surveying, core drilling and sampling for laboratory studies. Physical properties, uniaxial compressive strength and shear strength tests were conducted in the laboratory. Rock mass classes and rock mass strength and deformability parameters were determined based on the AFTES guidelines, rock quality designation (RQD), rock mass rating (RMR) and Q rock mass classification systems. The deformations were then numerically determined using Plaxis Software. The modeling results were in good agreements with the rock mass qualities as assessed by RMR, Q-system, AFTES guidelines. Key words: Engineering geology, finite element method, rock mass classification, iron mine, Boukhadra, Algeria.

음향방출기법을 이용한 KURT 화강암의 균열 발생 특성에 관한 실험적 연구

방사성폐기물처분장 주변 암반의 수많은 불확실성을 이해하기 위해서는 무결암에서 발생하는 균열의 성장과 거동 분석은 필수이다. 이에 본 연구에서는 처분장과 유사한 지질적 구조적 특성을 지닌 한국원자력연구원 내에 위치한 지하처분연구시설에서 채취한 화강암 시료를 이용하여 균열의 성장과 이에 따른 손상도를 AE parameter와 모멘트텐서해석법을 이용하여 분석하였다. 시료의 균열개시 균열결합 균열손상응력은 최대강도의 0.45배, 0.73배, 0.84배인 것으로 나타났다. 모벤트텐서해석법을 이용한 결과 응력 초기에는 인장균열의 발달이 우세하였으나 응력 수준이 증가함에 따라 전단균열이 발달하였다. 또한 시료에 균열손상응력 이상의 응력이 가해지면 파괴면을 중심으로 불안정한 전단균열이 발생하였으며 이는 파괴에 직접적인 역할을 하는 것으로 해석되었다. The first step in improving our understanding of uncertainties suclt as rock mass strength parameters and deformation modulus in rock masses around high-level radioactive waste disposal repositories, for improved safety, is to study the process of crack development in intact rock. Therefore, in this study, the fracture process and crack development were examined in samples of KURT granite taken from the KAERI Underground Research Tunnel (KURT), based on acoustic emission (AE) and moment tensor analysis. The results show that crack initiation, coalescence, and unstable crack occurred at rock uniaxial compressive strengths of 0.45, 0.73, and 0.84, respectively. In addition, moment tensor analysis indicated that during the early stage of loading, tensile cracks were predominant. With increasing applied stress, the number of shear cracks gradually increased. When the applied stress exceeded the stress level required for crack damage, unstable shear cracks which directly result in failure of the rock were generated along the failure plane.

Macro-mechanical properties of columnar jointed basaltic rock masses

From the geological structure of the columnar jointed rock mass, a visual model was established in software AUTOCAD by programming based on the algorithm of the Voronoi diagram. Furthermore, a program to convert the AUTOCAD model into 3DEC (3-dimensional distinct element code) model was developed, and a numerical model was established in 3DEC. Moreover, the results of triaxial compression tests of columnar jointed rock masses were simulated numerically. The REV (representative element volume) scale was studied, and the result shows that the REV size is 3 m × 3 m. The proposed approach, the established model and the numerical simulation were applied to study the macro-mechanical properties and the equivalent strength parameters of the columnar jointed rock mass. The numerical simulation results are in good accordance with the in-situ test results.

Wellbore Stability Analysis of Coal Seam Based on Hoek-Brown Criterion

Stable evaluation of coal strength is needed in coal well-bore stability analysis. The regular analysis method of wellbore stability adopts Mohr-Coulomb strength criteria to judge the collapse pressure. Coal is dual porosity structure and contains joint fissures richly. Hoek-Brown criterion is much more reasonable to estimate the strength of jointed and fractured rock. So Hoek-Brown criterion is used to analyze the stability of multi-lateral horizontal coal bed methane well. Considering the GSI (geological strength index), structural and surface conditions of coal, the coal and rock mass strength parameters are converted into the underground coal mechanical parameters based on the triaxial test of intact coal. According to the stress state of multi-lateral horizontal well, the borehole collapse formulas are established based on Hoek-Brown Criterion. The effects on the wellbore stability, due to the joint fissures of coal, borehole size, drilling disturbance, are also discussed in this paper.

Rock Mass Characterization and Rock Property Variability Considerations for Tunnel and Cavern Design

Geotechnical design input parameters, such as in situ stress field, rock mass strength parameters and deformation modulus, are never known precisely. There are always uncertainties involved in these parameters, some are intrinsic and others are due to lack of knowledge or understanding of these parameters. To quantify the effects of these uncertainties on tunnel and cavern design, it is necessary to utilize probabilistic analysis methods. In the present study, a quantitative, probabilistic approach to use the Geological Strength Index (GSI) system for rock mass characterization is presented. It employs the block volume and a joint condition factor as quantitative characterization factors to determine the GSI values. The approach is built on the linkage between descriptive geological terms and measurable field parameters such as joint spacing and joint roughness, which are random variables. Using GSI values obtained from field mapping data, and in combination with the intact rock strength properties, the probability density distributions of rock mass strength parameters and elastic moduli of the jointed rock mass can be calculated using Monte Carlo method. Furthermore, probabilistic analysis of tunnel and cavern stability based on the variable input parameters is conducted employing the point estimate method. One example is given to illustrate how to consider the variability of in situ stress and the rock mass properties in tunnel and cavern design. The method presents an approach for systematic assessment of uncertainty in rock mass characterization in rock engineering, and it can assist us to better understand how uncertainty arises and how the rock support system design decision may be affected by it.

Stability of an undercut sea-cliff along a Cilento coastal stretch (Campania, Southern Italy)

This paper deals with a finite element stability analysis performed on a sea-cliff affected by a deep notch. The cliff is about 200 m long with height ranging between 8 and 19 m. The notch at the cliff base is about 1 m above sea level, nearly stretching along the entire cliff toe. Detailed geo-structural and geo-mechanical surveys were performed in situ, and rock samples were tested in laboratory. The relevant physical and mechanical properties were used for determining rock mass strength parameters. A finite element approach was implemented using a two-dimensional elasto-plastic finite-element stress analysis program. The stability analysis shows that the notch causes increasing shear strains spreading from the notch end towards the top cliff. Decreasing strength reduction factors affect the cliff as the notch reaches increased depth into the rock mass, and when its depth is about 2 m the analysis results do not converge. At this stage, an uninterrupted shear plane develops along which a roughly prismatic rock block can slide and then topple.

An analytical expression of reliability solution for Druker-Prager criterion

Strength parameters of rock mass are taken as random variables. Based on the reliability theory, an analytical expression of reliability solution for Drucker-Prager criterion is given. The Monte-Carlo method and FORM (first order reliability method) are used to test the correctness of the solution. Some influences of variation of strength parameters on the judging results of Drucker-Prager criterion are discussed by the use of the solution. Conclusions of discussion show that variations of strength parameters have different influences on the reliability probability of Drucker-Prager criterion. When the coefficients of variation of strength parameters are great, their influences on the reliability probability of Drucker-Prager criterion are obvious and can not be neglected. The conclusions of this paper provide a new way for the reliability judgment of yield on rock mass.

Back-analysis of rock mass strength parameters using AE monitoring data

Most back-analyses in geotechnical engineering are based on methods that utilize field displacement monitoring data. In the present study, a novel method is developed to back-calculate rock mass strength parameters from AE (acoustic emission) monitoring data in combination with FEM stress analysis. The method is based on the important concept of generalized AE initiation threshold of rock masses, established from comprehensive data analysis of laboratory test and underground monitoring programs using AE and microseismic (MS) techniques. An easy-to-use Wizard is developed in Microsoft Excel™ to assist site engineers to perform the back-analysis. The efficient solver in Excel is utilized to reach the optimization solution of an objective function with constraints. The Wizard allows the user to complete the analysis process in an interactive fashion. One example is given to demonstrate the back-analysis process using AE monitoring data recorded from a cavern site. The rock mass strength parameters identified from this approach compare well with field test data, suggesting that the tool can be used effectively to back-calculate rock mass strength parameters from AE monitoring data.

Rock mass strength and slope stability of the Hilina slump, Kīlauea volcano, Hawai'i

Stability of the Hilina slump, on the south flank of Kīlauea volcano, Hawai'i, is investigated using slope stability analyses based on rock mass mechanics. The Hilina slump is an active example of the giant landslides that are observed around submarine flanks of ocean island volcanoes. Based on edifice topology, along with derived rock mass strength and deformability parameters, slope stability analyses predict that the geometries of present-day failure surfaces of the Hilina slump are shallow, located at less than ∼3.5 km depth below the upper surface of the edifice. The failure surfaces at the base of the Hilina slump are predicted to be structurally independent from a previously interpreted subjacent detachment at the contact between the Kīlauea edifice and the oceanic crust. Based on these model results, the Hilina slump is envisioned to ride atop the south flank of Kīlauea as it spreads seaward along this slipping basal detachment. Under present-day slope and sea-level configurations, local horizontal ground accelerations greater than ∼0.4–0.6 g are predicted to cause slip along failure surfaces within Hilina slump. Therefore, dynamic stress changes due to slip along the subjacent detachment may potentially trigger slip along failure surfaces of the Hilina slump. Model-derived stability and failure surface geometries can account for the distinct observed distribution of slip along specific Hilina faults following the M fa 7.9 1868 Ka'u and M s 7.2 1974 Kalapana earthquakes. Further, the spatial distribution of south flank aftershocks > M 1.5 recorded between 1950 and 1976 are consistent with the predicted distribution of potential earthquake-induced slip of the Hilina faults.

Engineering geological assessment of the proposed Uruş Dam, Turkey

This paper describes the results of the engineering geological investigations and rock mechanics studies carried out at the proposed Uruş Dam site. Analyses were carried out in terms of rock mass classifications for diversion tunnel, kinematic analysis of excavation slopes, permeability of the dam foundation and determination of rock mass strength parameters. Uruş Dam is a rock-filled dam with upstream concrete slab. The dam will be built on the Suveri River in the central part of Turkey. The foundation rocks are volcanic rocks, which consist of andesite, basalt and tuff of Neogene Age. Studies were carried out both at the field and the laboratory. Field studies include engineering geological mapping, intensive discontinuity surveying, core drilling, pressurized water tests and sampling for laboratory testing. Uniaxial, triaxial and tensile strength tests were performed and deformation parameters, unit weight and porosity were determined on the intact rock specimens in the laboratory. Rock mass strength and modulus of elasticity of rock mass are determined using the Hoek–Brown empirical strength criterion. Rock mass classifications have been performed according to RMR and Q systems for the diversion tunnel. Engineering geological assessment of the proposed dam and reservoir area indicated that there will be no foundation stability problems. Detailed geotechnical investigations are required for the final design of the dam.

In situ strength criteria for tunnel design in highly-stressed rock masses

Abstract The selection of rock mass strength parameters and the approach to assessing the mechanical and thermal-mechanical rock mass behaviour around tunnels are important aspects of designing underground openings for a spent nuclear fuel waste repository. This paper demonstrates how strength criteria based on in situ observations can be used to design stable underground openings in rock masses subjected to adverse stress conditions. The findings are based upon work conducted at AECL’s Underground Research Laboratory (URL) regarding rock mass strength around tunnels in sparsely fractured granite.

原位置せん断試験で得られる均質な岩盤の変形・強度特性

To investigate the fracture process and to interprete the strength parameters of the homogeneous soft rock mass obtained by the in-situ block shear test, two dimensional finite element simulation study has been conducted under the assumption that the rock mass behaves as the strain softening material. At the same time, three dimensional physical model experiment has been performed using plaster as the modeling material. Main results are as follows:(1) Results of the in-situ shear test obtained both by the simulation and model study were expressed as the linear curve in the σ-τdiagram, and the value of the shear strength indicated from these data lay between the peak shear strength and the residual shear strength esitimated from the triaxial tests.(2) In the model study, three types of shape on the shear load versus vertical displacement curve, depending on the vertical stress level, were recognized.(3) In the simulation study, progressive failure has started from the early stage of loading and in accordance with this, load-displacement curve bent from the straight line observed in the inital loading stage. Similar tendency for the corresponding curve was recognized both in the field tests and in the model study.(4) The corrugated shaped fracture surface located deeper than the assumed shear plane was observed in the field experiment as well as in the model study. It was also found in the model and in the simulation study that the thickness of the yield zone developing under the fracture surface was not uniform.

Geotechnical appraisal of the foundation rock mass behaviour of Narmada Sagar Dam project, central India: a case study

The Narmada Sagar Dam, a concrete gravity structure under construction across the Narmada River in central India, will have a crest length of 653 m and a maximum height of 92 m. The present paper discusses the results of geotechnical investigations undertaken for evaluating the rock mass strength parameters, applying the modern concepts of rock mechanics, especially the rock mass rating, modulus of deformation, shear parameters, and strength and permeability of the dam foundation rocks. The joint systems observed are used to interpret and explain the occasional zones of poor core recovery and to explain the location and orientation of the drilling programme. It is suggested that the estimate of spacing and opening of water-conducting joints may lead to useful estimates of the groutable volume of voids in the rock mass, and inclined drill holes are preferable for optimum grouting. The values of different parameters have been obtained and correlated. The studies have led to the conclusion that the foundation rocks of Narmada Sagar Dam are competent and suitable for designing the proposed concrete gravity dam. Key words: dam, strength, elastic modulus, deformability, rock discontinuity, rock quality designation, rock mass classification.

Application of rock mass weakening coefficient for stability assessment of slopes in heavily jointed rock mass

ABSTRACT A geotechnical investigation in a large quarry was carried out in order 10 evaluate the rock mass strength parameters for designing slopes in jointed rock formations. The quantitative and qualitative parameters of rock miss discontinuities together with the laboratory strength parameters of intact rock were used for modifying rock mass weakening coefficient The RMR system and rock mass weakening coefficient in conjunction to Hoek and Brown empirical criterion was used to predict rock mass strength. It was found that the new rock mass classification system is consistent and cheaper alternative to the RMR rating system.

Empirical Strength Criterion for Rock Masses

An empirical strength criterion for rocks and rock masses is proposed. The nonlinear criterion includes the uniaxial compressive strength of the intact rock material and introduces two dimensionless parameters, m and s. The parameter m varies with rock type, the angle of interblock or interparticle friction and the degree of block or particle interlock. The parameter s varies from 1.0 for intact rock material to zero for granular aggregates and depends on the interparticle tensile strength and the degree of interlock within the rock mass. For anisotropic rock, both m and s vary with the orientations of the planes of weakness to the principal stress directions. Approximate relationships between rock type, rock mass quality indices, and the rock mass strength parameters m and s, are presented. These relationships have been found useful in preliminary design calculations for slopes and underground excavations in jointed rock.