Comparison of Failure Risk in Rock Slopes Using Kinematic and Empirical Methods (Case Study)

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Comparison of Failure Risk in Rock Slopes Using Kinematic and Empirical Methods (Case Study)

https://doi.org/10.21203/rs.3.rs-4811133/v1

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In engineering projects related to rock mechanics such as open pit mines, evaluating and ensuring the stability of mine walls is a crucial aspect of these structures. Understanding the types of failures that may occur is a key focus during the study phase of open pit mines. The occurrence of failure phenomena in both natural and artificial rock slopes, like those found in open-pit mines and road trenches, poses significant challenges for such projects. The Chadormalu mine is known as one of the largest open-pit iron mines in Iran, with part of the mine's pit affected by fractures. In this study, field studies were conducted on the fracture systems of mine walls and spatial assessment for their location in three blocks, B-2, B-12 and B-15, for conducting kinematic analyses using RockPack III program and for empirical methods, Slope Mass Rating (SMR) method has been utilized. A case study of the Chadormalu iron ore mine, where the west wall ruptures have been analyzed using kinematic and empirical methods, reveals that the presence of various fault surfaces and fractures in the rock mass can lead to different types of failures such as planar, wedging, and toppling failures in the mine wall. The results obtained from this analysis, utilizing both kinematic and empirical methods, demonstrate a strong correlation between the data obtained from these two approaches.

Open-pit mines

Rock Slopes

Kinematical and Empirical Methods

Chadormalu Mine

RockPack III software

Slope failures are major natural hazards that occur in many areas throughout the world. Slopes expose two or more free surfaces because of geometry. Plane, wedge, toppling, rockfall and rotational (circular/non-circular) types of failure are common in slopes (Fig. 1). The first four are more predominant in rock slopes and are primarily controlled by the orientation and the spacing of is continuities planes with respect to the slope face. The pattern of the discontinuities may be comprised of a single discontinuity, or a pair of discontinuities that intersect each other, or a combination of multiple discontinuities that are linked together to form a failure mode.

Among the stability analysis methods for geotechnical engineering, the development of slope stability analysis method is the most remarkable. Especially, the stability of high-steep rock slopes has always been a problem that many researchers have been trying to solve but not satisfactorily solved yet (Tang, C., 2015). Considering analytical procedures, analysis of rock slope stability is usually undertaken by a combination of kinematic analysis and limit equilibrium analysis. The conventional continuum approaches like limit equilibrium methods, finite element methods and finite difference methods have certain disadvantages to model the behavior of jointed rock masses, owing to the complex internal deformational mechanism (Kveldsvik, et al., 2009, Rasouli Maleki., 2009). Kinematic analysis involves interpreting the geometry of the geological discontinuities and the slope to determine whether failure by a given mechanism is feasible (Babiker, et al., 2014). Kinematic analysis was performed to illustrate the potential for various modes of rock slope failures (plane, wedge, and toppling failures) that occur due to the presence of unfavorably oriented discontinuities (Siddique, et al., 2015).

Structural analyses are related to rock environments, which always take effects of various joints and fractures. The purpose of this study is to compare results of failure s analysis according to current empirical methods with those of kinematic ones. In this study, analysis procedure was selected so that; first, joint studies are done on the west wall of Chadormalu mine; then, Slope Mass Rating (SMR) system was performed and its results were analyzed. On the other hand, failures analysis was performed by stereonet methods based on orientation of joints and discontinuities in the area to the slope direction; and finally, results of these two methods were analyzed and compared.

Chadormalu is located at center of Persia (Iran) Desert, at north of gray chahmohammad mountains. At the south of Saghand salt marsh, at the 180Km of north eastern of Yazd and 300 Km of south of Tabas city (Fig. 2), where no human life effects can be seen except Chadormalu members. The mine is connected to the Iranian Rail Road network and Via Asphalt Road to the Main Yazd – Mashhad, Road. In summary, the specifications of the mine can be introduced as follows:

Mine discovered: 1940
Exploration works from geological investigation, air born magnetic up to detail exploration: 1963 – 1978
Total Core drilling: 36,500 M
Drillings for water recourse investigation: 4,700M
Geological Ore reserve: 399 Mio ton
Economical reserve: 320 Mio ton
Average Fe – grade: 55.3%
Average P – content of Ore: 0.94%
Average S – content of Ore: 0.19%
Average stripping ratio: waste/ore: 1.5
Recent annual ore excavation: 12 Mio ton

The mountain ranges south and southwest of Chadormalu consist of fine-grained granites and metamorphic rocks cut by basic dykes such as amphibolites. In general, the lithology of the area mainly consists of gabbro diorite. Diabase-granite is prophyry and metamorphic schists. Metamorphic schists appear in dark gray and brownish gray colors. The type of these schists makes an angle of 40 degrees with the southern part. The intrusive igneous rocks mostly belong to the gabbro, rhyolite and diorite family, which have been replaced under the schist layers. The geological Map of Chadormalu Iron ore mine has shown in Fig. 3.

According to the geology studies performed in this region, it was cleared that Chadormalu fault between the plain and high lands is the major factor of create ore in the region formed in two forms of northern and southern anomaly. Petrography studies on the mine rocks shows that major rocks in Chadormalu mine area are Albitite, Metasomatite, Granite, and Diorite. It should be mentioned that, performance of different faults in this area makes the mine rocks to be severely tectonics and provide suitable conditions for different failures of the mine walls. Fig. (4) shows the geological profile perpendicular in the western wall of the together with its lithological combination and in the Fig. (5) shows the geological map in per selected blocks.

4.1 Study of Discontinuities and Joints

In this study, for engineering classification of the west wall rock masses, three blocks with fixed 15m length were selected. Doing field studies, the engineering characteristics of discontinuities in each block were noted according to ISRM Standard (ISRM, 1981). Of all these parameters, orientation, spacing, persistence, roughness, aperture and filling and underground water conditions are the most important parameters studied in each of the joints that have been applied after selection for doing various ratings.

4.2 Engineering Properties of Rock Mass

As mentioned, in engineering works, especially in the analysis of rock slopes, the purpose of classifying rocks is to estimate and correctly measure engineering and geomechanical properties of rock mass.

In this study, in the first step, physical and mechanical properties of the various rock units in each block such as unit weight, uniaxial compressive strength, young's modulus, tensile strength, shear strength parameters (c and φ) and … were determined by laboratory tests. So, uses results of rocks classification and Roclab software (Rocscience Inc., 2005) for each of the rock pieces enclosed in blocks to detect those parameters that have been introduced by empirical methods established by researches throughout the world. The results shown in Table 1.

Table 1. Physical and mechanical properties of the rock units in the each rock groups

Today's, classification systems of rock masses are very important, as these systems not only discuss on characteristics such as rock mass quality, but also give data on the rocks quantity. In this study, in order for examining the rocks quality of the west wall of Chadormalu mine; first, characteristics of the discontinuities were studied, then two systems of classification following were used.

5.1 Rock Mass Rating (RMR)

The Rock Mass Rating (RMR) system is a geomechanical classification system for rocks, developed by Z. T. Bieniawski between 1972 and 1973 (Bieniawski, 1989) Rock Mass Rating has found wide application in various types of engineering projects such as tunnels, slopes, foundations, and mines. The following six parameters used to classify a rock mass using the RMR system. This classification system combines the most significant geologic parameters of influence and represents them with one overall comprehensive index of rock mass quality, which is used for the design and construction of excavations in rock, such as tunnels, mines, slopes, and foundations.

Uniaxial compressive strength of rock material
Rock quality designation (RQD)
Spacing of discontinuities
Condition of discontinuities
Groundwater conditions
Orientation of discontinuities

RMR is determined as an algebraic sum of ratings for all of the parameters. RMR values for a given engineering structure, the rock mass is sorted into five classes: very good (RMR 100–81), good (80–61), fair (60–41), poor (40–21), and very poor (<20).

5.2 Geological Strength Index (GSI)

The GSI system was developed in the 1980s by Evert Hoek et. Al (Hoek, et al., 1998). The GSI value expresses an approximate of the compressive strength of rock masses. This system is now widely used for the estimation of the rock mass strength and the rock mass deformation parameters. The GSI system concentrates on the description of two factors, rock structure and block surface conditions. The guidelines given by the GSI system are for the estimation of the peak strength parameters of jointed rock masses. There are no guidelines given by the GSI, or by any other system, for the estimation of the rock mass’ residual strength that yield consistent results (Hoek, et al., 1998). Summerly, Table 2 shows the value of RMR and GSI values for each of these blocks (Rasouli Maleki, et al., 2011a, 2011b).

Table 2. The value of RMR and GSI obtained for each of the blocks of the mine west wall

Results obtained show that the quality of the rock masses in each of the blocks in the west wall is poor and they are rated among poor quality rocks.

6.1 Introduction

The slope stability analysis is one of the complex subjects in geomechanics. Today, despite the advancement of the frontiers of engineering science among capable researchers around the world, the slope stability analysis remains unsolvable. In general, slope stability can be defined as the resistance of sloping surfaces against failure (sliding or falling). Slope stability analysis methods are tools used to evaluate the balance and safety of natural and man-made slopes (such as embankments, open-pit mines). , excavations, etc.) are used against sliding. The main purpose of these analyzes is to find the critical areas, investigate the possible failure mechanism, determine the sensitivity of the slope to different mechanisms, design the optimal slopes with regard to safety, achieving high reliability is to consider economic aspects and design possible options for slope reinforcement.

Today, there are several methods for slope stability analysis, each has its own advantages and disadvantages. Empirical and Kinematical analyses methods are the most common ones that used for rock slope analyses. One of the software’s that can analyze rock slope stability in a kinematical way is powerful software called RockPack III (Watts, C.F., ROCKPACK III, 2012). This software used in the present study for analyses of stability in the west wall of Chadormalu mine.

6.2 Empirical Method

One of the methods, today used commonly for analysis of failures in rock environments is to use empirical methods of rock mass classification. So far, various classification systems have been introduced by great researches throughout the world for rock slopes. Of all these systems, one can refer to a rock system named Slope Mass Rating (SMR) which was introduced by Ramana for the first time (Romana, 1995).

Ramana, in 1985, established a new rock engineering rating for the first time which paid especial attention to rock slopes stability on the basis of geomechanical rating (RMR) that is called Slope Mass Rating (SMR). The basis of this classification system was rock mass rating which was the result of analysis on 87 rock slopes in Valencia. In this rating system, there is a new factor which is called vicinity factor related to the relationship between joints directions to the slope direction. In brief, quantity of SRM values is calculated as follows:

SMR = RMRB + (F1×F2×F3) + F4 (1)

Where:

RMRb is the RMR index resulting from Bieniawski's Rock Mass Classification without any correction.
F1 depends on the parallelism between discontinuity, αj (or the intersection line, αi, in the case of wedge failure) and slope dip direction.
F2 depends on the discontinuity dip (βj) in the case of planar failure and the plunge, βi of the intersection line in wedge failure. As regards toppling failure, this parameter takes the value 1.0. This parameter is related to the probability of discontinuity shear strength.
F3 depends on the relationship between slope (βs) and discontinuity (βj) dips (toppling or planar failure cases) or the immersion line dip (βi) (wedge failure case). This parameter retains the Bieniawski adjustment factors that vary from 0 to −60 points and express the probability of discontinuity outcropping on the slope face for planar and wedge failure.
F4 is a correction factor that depends on the excavation method used. (Table 3).

Although SMR is worldwide used, sometimes some misinterpretations and imprecisions are made when applied. Most of the observed inaccuracies are related to the calculation of the ancillary angular relationships between dips and dip directions of the discontinuities and the slope required to determine F1, F2 and F3 factors. Summerly, the empirical description of SMR classification system of rock mass has been shown in the Table 4.

Table 3. Joint adjustment rating for joints (Ramana, 1985)

Table 4. Empirical description of SMR classification system of rock mass.

In order to determine SMR rating for rock masses in the west wall blocks; also, to evaluate the capacity of the wall in rock failures, analysis of major joint of each block was performed after joint studies. With attention to this issue that the main purpose of this empirical method was to determine failure kind of the wall, this study tried to compute SMR quantity value per each of the joint surfaces separately for more certainty. Summarily, Table 5 shows input parameters and output results of SMR rating for each of these blocks.

Table 5. Input parameters and SMR value obtained for each of the blocks

Results obtained from this method indicate that of among all these blocks, No B-15 has better stability than others due to suitable direction of the slope to failures direction, so that it is observable in descriptive table of SMR classification system for rock mass of each block with ease (Table 6).

Table 6. Empirical descriptive of SMR Class for rock masses of the blocks selected from the west wall.

Obtained Results in Table (6) show that rock masses surrounded in blocks B-02 and B-12 were poor and unstable class, so that according to Ramana model, one can say that the kind of probable failures in these blocks are of surface discontinuities and big wedge.

6.3 Kinematical Method

Rock failures analysis by cinematic methods is another method introduced for the first time by Goodman, a researcher from California University Berkeley. According to presence of various discontinuities in rock mass, can be expressed that this analytic method can present a general and detailed view on the kind of the failures due to intersection of joints sets and rock mass discontinuities.

In this study, after determining of joints dominant direction, this study tries to investigate and determine rocks internal friction angle per each concerned block, and discuss about the probability of occurrence of different kinds of failures individually. In brief, results obtained from kinematic analysis for different failures in each of the concerned blocks are as follows (Fig. 6):

Block B-02:

- Planar failure: Very high (on fault surfaces and major joints)

- wedging failure: High (large dimension, due to fault surfaces and major)

- Toppling failure: Nowhere

Block B-12:

- Planar failure: Somewhere (on fault surfaces)

- wedging failure: High (large dimension, due to fault surfaces and major)

- Toppling failure: Nowhere

Block B-15:

- Planar failure: Somewhere (on joints surface)

- wedging failure: High (due to intersection of major joints)

- Toppling failure: Nowhere (probable)

Briefly, comparing the results obtained from both empirical and cinematic methods, one can admit that both methods confirm occurrence of wedge and plate failures in the mine west wall; so that number, dimension, and risk of occurrence of these slides in blocks B-02 and B-12 are more probable than others. As it is seen in Table 7, frequency of wedge and plate failures obtained from cinematic method agrees with SMR rating obtained for both B-12 and B-02 blocks.

Table 7. Comparison of the results of failure s analysis based on cinematic and empirical methods.

Chadormalu iron mine is one of the largest iron mines in Iran, so it is very important to know about the health of the mine's peat. The presence of altered igneous and metamorphic units in the mine site along with the tectonic function of the region has caused mine trenches to be affected by several structural failures. The general results obtained from this study show that the use of empirical and kinematical methods in evaluating of slope stability of trenches could provide more comprehensive and accurate information on the type of possible failures. The results of the studies conducted for the western trenches of Chadormalu mine show that the mineral walls affect the structure, in such a way that there is a possibility of planar and wedge failures.

Conflicts of Interest

The authors declare no conflict of interest.

Author Contribution

Mahdi Rasouli Maleki wrote the main manuscript text Nafiseh Yousefi Mahmoud prepared all figures . All authors reviewed the manuscript."

Acknowledgments:

With special thanks to the friends who expressed their ideas towards this manuscript and most especially to Eskandar and Abbas Rasouli Maleki for their cooperation in accomplishing this manuscript.

Babiker, A.F.A., Smith, C.A., Gilbert, M., Ashby, J.P., (2014), “Non-associative limit analysis of the toppling-sliding failure of rock slopes”, International Journal of Rock Mechanics & Mining Sciences 71 1–11.
Bieniawski, Z. T., (1989), “Engineering Rock Mass Classifications,” Wiley, New York, pp. 251.
Hoek, E., Brown, ET., (1998), “Practical estimates of rock mass strength,” Int J Rock Mech Min Sci, 34(8):1165–1186.
Hoek, E., Marinos, P., Benissi, M., (1998), “Applicability of the geological strength index (GSI) classification for very weak and sheared rock masses,” Bull. Eng. Geol. Env. 57, pp. 151-160.
ISRM, (1981),“Suggested methods for the quantitative description of discontinuities in rock masses. Rock characterization, testing and monitoring” London. Pergamon, Oxford, pp 221.
Kveldsvik, V., Kaynia, A.M., Nadim, F., Bhasin, R., Nilsen, B., Einstein, H.H., (2009). “Dynamic distinct element analysis of the 800 m high Aknes rock slope”. International Journal of Rock Mechanics and Mining Sciences 46 (4), 686e698.
Rasouli Maleki, M., Mahyar, M., (2011a), “Assessment and comparison of occurring probability of rock failures based on empirical and kinematical methods”, 4th National Conference on Rock Mechanics, Tehran.
Rasouli Maleki, M., Mahyar, M., Meshkabadi, K. (2011b), “Design of Overall Slope Angle and Analysis of Rock Slope Stability of Chadormalu Mine Using Empirical and Numerical Methods” Journal of Scientific Research, Engineering, Vol.3, pp. 965-971.
Rasouli Maleki, M., (2009), “Engineering geological studies of the diversion tunnel, focusing on stabilization analysis and support design, Iran,” International, Journal of Engineering Geology 108 208–224.
Rocscience Inc. (2005), “Roclab – Rock Mass Strength Analysis Using the Hoek-Brown failure criterion”.
Romana, M., (1995), “The SMR geomechanical classification for slopes: A critical ten years review” Proc 8th Int Conf Landslides, Granada (Spain), (Eds. Chacon, Irigaray & Fernandez), AA Balkema, Rotterdam, pp. 255–267.
Siddique, T., Masroor Alam, M., Mondal, M.E.A., Vishal, V., (2015), “Slope mass rating and kinematic analysis of slopes along the national highway-58 near Jonk, Rishikesh, India”, Journal of Rock Mechanics and Geotechnical Engineering 7 600-606.
Tang, C., Li, L,, Xu, N., Ma, K., (2015), “Microseismic monitoring and numerical simulation on the stability of high-steep rock slopes in hydropower engineering”, Journal of Rock Mechanics and Geotechnical Engineering (7) 493-508.
Watts, C.F., ROCKPACK III, (2012). Grandview Drive, Radford, Virginia 24142 & 4589 Mallard Point Way #22, Dublin, Virginia 24084.

Tables 1,2,3,5,6 and 7 are available in the Supplementary Files section.

Table 4 is not available with this version.

Table 4. Empirical description of SMR classification system of rock mass.

No competing interests reported.

Table1.png
Table 1. Physical and mechanical properties of the rock units in the each rock groups
Table2.png
Table 2. The value of RMR and GSI obtained for each of the blocks of the mine west wall
Table3.png
Table 3. Joint adjustment rating for joints (Ramana, 1985)
Table5.png
Table 5. Input parameters and SMR value obtained for each of the blocks
Table6.png
Table 6. Empirical descriptive of SMR Class for rock masses of the blocks selected from the west wall.
Table7.png
Table 7. Comparison of the results of failure s analysis based on cinematic and empirical methods.

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Comparison of Failure Risk in Rock Slopes Using Kinematic and Empirical Methods (Case Study)

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Abstract

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1. Introduction

2. Chadormalu Iron Ore Mine

3. Geological Condition of Site

4. Engineering Geology Studies

5. Engineering Classification of Rock Masses

6. Slope Stability Analysis

7. Comparison of the Results of both Empirical and Kinematic Methods

8. Conclusions

Declarations

Conflicts of Interest

Author Contribution

Acknowledgments:

References

Tables

Additional Declarations

Supplementary Files

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