Review of Current Empirical Approaches for Determination of the Weak Rock Mass Properties

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ScienceDirect Procedia Engineering 191 (2017) 908 – 917

Symposium of the International Society for Rock Mechanics

Review of Current Empirical Approaches for Determination of the Weak Rock Mass Properties Hao Zhai, Ismet Canbulat*, Bruce Hebblewhite, Chengguo Zhang School of Mining Engineering, The University of New South Wales, Sydney, NSW 2052, Australia

Abstract Weak rock mass strength estimation is a long-lasting challenge associated with geotechnical engineering due to its complex nature and limited definition. Weak rock masses normally refer to low strength, highly fractured decomposed and tectonically disturbed rocks which have properties intermediate from brittle rocks to ductile soils. Since the behavior of weak rock mass has not been fully understood, it is a common practice to apply existing empirical approaches, which are developed for competent rock masses influenced by joints, to determine their mechanical properties. This paper reviewed the current empirical approaches, and detailed weak rock mass strength calculations based on rock matrix, joint layout, joint condition and external factors. The limitations associated with these methods are discussed, and suggestions are provided for the selection of suitable methods. ©2017 2017The TheAuthors. Authors. Published by Elsevier © Published by Elsevier Ltd. Ltd. This is an open access article under the CC BY-NC-ND license Peer-review under responsibility of the organizing committee of EUROCK 2017. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of EUROCK 2017 Keywords: Weak rock; Rock mass; Empirical methods

1. Introduction Determination of weak rock mass properties is a significant challenge in geotechnical engineering. In general, weak rocks are considered to be the transitional material between competent rocks and soil, therefore, their behavior converges to competent rock at its upper bound and soil at the lower bound. Despite significant amount of research, methods to estimate in-situ behavior and strength of weak rock masses remain to be relatively fragmented and incomplete. The difficulty of determining their behaviour is mostly caused by the complex nature and inadequate definitions. Different origin and alteration process of weak rocks result in variant properties that inevitably influence their overall behaviour. Hence, it is important to understand the differences in their property that inherited from both

* Corresponding author. Tel.: +61-293-850-721; fax: +61-293-137-269. E-mail address: [email protected]

1877-7058 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of EUROCK 2017

doi:10.1016/j.proeng.2017.05.261

Hao Zhai et al. / Procedia Engineering 191 (2017) 908 – 917

previous phase and alteration process and to adopt suitable approaches to estimate their strength according to these features. In practice, the term weak rock commonly refers to both young sedimentary rocks with low compressive strength and heavily altered hard rock with intense structures [1–3]. Based on the origin and geological alterations, weak rock can be classified as young sedimentary rock, weathered competent rock and tectonically disturbed competent rock as shown in Fig. 1. Young sedimentary rocks such as mudstone and claystone contain poor lithification and weak particle cementation. The strength of them can be described by the ISRM definition of weak rocks with uniaxial compressive strength (UCS) being 0.5 MPa to 25 MPa [2, 3]. Weathered competent rocks such as sandstone can also be considered as weak rock. During prolonged exposure, some rock mass components start to break down and crack along pre-existing micro fractures. As a result of weathering, the well-developed, interconnected defect fabric deteriorates the integrity of the rock mass, thus lead to a reduction of the overall mechanical strength. This type of rock is well represented in Rock Mass Rating (RMR) and Geological Strength Index (GSI) classification systems as poor quality rocks with ratings lower than 25 and 20 respectively or less than 0.1 in Q system. In practice, there is a tendency to consider tectonically disturbed competent rocks, which preserves limited original structures formed in lithification, as weak rock mass [3]. Due to destruction of original structure during folding and shearing, it’s common to observe widely existing intensive fractures. Thus, this type of rock has very low mechanical properties similar to other types of weak rock masses. Marinos and Heok’s study of flysch in 1998 provides a good example of such weak rock [4–6].

Fig. 1. Illustration of formation and evolution of weak rock mass.

This paper attempts to address some critical questions in the determination of weak rock mass properties. A review of difficulties associated with characterisation of rock mass is firstly presented. Then, the current empirical weak rock mass downgrading (also referred as upscaling) systems is reviewed and their suitability to low strength rocks, highly fractured decomposed rocks and tectonically disturbed weak rock masses are investigated.

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2. Weak rock mass characterization Table 1. Factors influencing the rock mass behaviour (modified from Palmstrom [7]). FACTORS 1 2 ROCK MATRIX - Origin - Anisotropy - Density - Porosity - Hardness - Strength - Deformability - Swelling - Weathering sensitivity JOINT DISTRIBUTION - Persistence - Length - Spacing/Frequency - Number of joint sets - Relative orientation - Pattern - Structure JOINT CONDITION - Weathering - Aperture - Roughness - Infilling EXTERNAL FEATURES - Water - Stress - Blasting damage - Excavation span LEGEND

CLASSIFICATION SYSTEM #

3

4

5

6

7

D

CLASSIFICATION SYSTEM 8 9 10 11 12 D

D

Count 13

14

D

15

16

17

18

A D

I

I

R

I I I

D

D

D

D R

A A

D

A A

D

D

D

A

D

D

D I R I

I

I

I

I R R

R D

R R D

D D

D

D D D

D

D I

D

I I

D R

I

I

P

A D

D

D A A D

R

7 5 15 4 3 5 3

R R R

7 5 7 6

D D

D

A

D

R

D

D D D

D

I R

D R A

R D D

D

D D D D

I D D – well-defined inputs I – included, but not defined A – as additional information (e.g. RMR adjusted value) 1 – Terzaghi 2 – Lauffer 3 – NATM 4 – Rock mechanical system 5 – Unified system 6 – RQD 7 – Size strength system 8 – Rock structure rating 9 – RMR

D R I

D

D D D

D D D D

D D D

A

D

A D R – roughly defined or included P – partly included

D D

4 1 2 1 2 13 3 2 2

D I

6 7 3 4

10 – Q 11 – Typological system 12 – Unified system (rock) 13 – Basic geotechnical system 14 – RMi[7] 15 – GSI[8] 16 – Field weak rock characterization [9] 17 – Dinc et al [10] 18 – CMRR[11]

It is well accepted that the behavior of rock mass is not controlled by a single factor and an exhaustive inclusion of all parameters would be neither practical nor useful in describing the rock mass behavior [7, 12–14]. Therefore, selection of suitable parameters is critical in performing weak rock mass analysis. Table 1 summarizes twenty-three important factors from a number of classification systems. These factors are considered to be relevant in determining the rock mass behavior. Based on the total number of appearances in previous rock mass classification systems, nine factors (underlined in Table 1) are considered to be fundamental or baseline factors that determine the behaviour of weak rock mass. As shown in Table 1, not all factors are considered in existing rock mass rating

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systems and the selection of them is based on rock mass type and mining method (i.e., underground or open cut). For instance, weathering has been removed from Coal Mine Roof Rating (CMRR) which is designed for underground coal mining applications [11] while anisotropy has been investigated during the study of flysch [15]. Challenges for characterization of general weak rock mass properties have been discussed in the following paragraphs, and further elaborations on issues related to each weak rock types are provided in the next section. Strength is the only intrinsic rock matrix parameter widely acknowledged by nearly all rock mass classification systems. In practice, the strength of intact rock is determined by UCS tests of laboratory-scale samples that have a height to diameter ratio of 2.5 to 3.0 and a diameter greater than 54 mm [12]. Even though the strength parameter is a very basic rock property, it is not always easily obtainable for some weak rocks. For example, volcanic rocks have been reported to completely disintegrate when immersed in water while some mudstones types are likely to fracture when moisture content is dropped [3, 4]. This problem not only causes difficulties in sample preparation but also leads to concerns of bias toward survived samples. Aside from improved drilling technics, alternative methods such as point load test, Schmidt hammer, geophysical prospection, block punch index have been considered as additional assessments of intact strength [3, 13]. Moreover, many weak rocks possess prominent bedding planes. Most of the empirical relationships may not hold for anisotropic weak rocks as failures tend to occur along the bedding planes [14]. The importance of discontinuities has been addressed by many researchers [7, 15-19]. It is however debatable which parameter or which sets of parameters best describe the behavior of rock mass. Joint spacing and persistence (measured as joint length according to ISRM) are the most commonly adopted parameters in describing the joint distribution due to the convenience in measurement [12]. More complex measurements of joint distribution such as volumetric joint count and fracture intensity has been introduced but not widely adopted [20, 21]. With the recent advancement in photogrammetry and laser mapping, detailed joint distribution information can be readily obtained and processed [22, 23]. Hence, it is reasonable to use measurements with relatively higher level of accuracy in describing the joint distribution rather than converting high-resolution information into one-dimensional joint spacing measurements. However, despite the rapid advancement of surveying accuracy on the rock face, the mapping capability beyond the excavation face is still very limited [24]. It is unlikely to fully understand defect fabrics and joint interaction until technology for large-scale, high precession prospection beyond excavation face is developed. Four joint condition parameters, namely weathering, aperture, roughness and infilling, are commonly assessed to the shear strength of rock joints. They are rarely assessed individually as each parameter describes a different aspect of joint property. Over the years, some joint condition assessment methods have been developed and adopted in practice. These methods can be grouped as a qualitative assessment similar to GSI and RMR and graphic, qualitative methods such as ISRM suggested JRC–JCS methods [12, 25, 26]. A common assumption in joint condition assessment is that all rock joints within a geotechnical domain should have similar properties that can be denoted as a single number or a range. However, this assumption may not be applicable to some weak rocks such as folded flysch in which bedding, joints and shear zones are introduced during diagenesis, metamorphism and tectonic disturbance [27]. For these weak rock masses, it is necessary to assign different joint properties to different discontinuities. The variation of joint condition further implies violation of the isotropic material assumption. 3. Empirical methods for weak rock mass property determination Empiricism is one of the major approaches in the assessment of rock mass properties. In 1993, Franklin defined empirical methods as a quantified judgment based on experiences [28]. Essentially, most empirical evaluations of rock mass are achieved by generation of a rock mass quality rating from a set of fundamental factors and coupling the rating with a failure criterion. Rock masses with similar ratings are considered to have similar mechanical behaviors. Based on the rating, a set of rock mass parameters for the empirical failure criterion can be derived from intact rock properties based on empirical downgrading relationships. The outputs for rock mass parameters are commonly used as input for subsequent analytical and numerical models. For weak rock, empirical methods are particularly valuable due to the variability and the complexity of the material. However, it is emphasized that an empirical system can only generate reliable results within the range

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of its root database [10]. Therefore, it is important to assess the original design purpose and database of empirical system and make decisions accordingly. 3.1. Weathered competent rock Competent rock can chemically decompose and physically disintegrate under the exposure of air and water [29]. In the macro scale, weathering can open existing joints and induce the formation of micro-fractures. Erosion of weak rock substance, such as feldspar, and preservation of strong minerals, such as quartz, are the main features in the micro scale. In1969, Dixon studied weathered Australian granite in laboratory scale and suggested that there is a linear relationship between intact granite UCS and average number of fractures per unit length [30]. The impact of joint opening and development of micro-fractures could be considered as deterioration of defect fabric network in the macro scale. Therefore, this type of weak rock is well contained in general rock mass classification systems and it is expected to have lower strength but higher isotropy. Hoek-Brown failure criterion was introduced in 1980 for the description of failure envelope for both intact rock and rock mass based on tests on Panguna andesite [31]. Initially, Hoek-Brown parameters m, s and a for different rock masses were estimated primarily based on ratings of RMR and Q systems. RMR generates reasonable ratings for blocky rock masses and it is sometimes preferred in back analysis [13, 32]. GSI was introduced in 1994 to address the inadequacy of RMR for poor quality rocks with ratings less than 25 [1]. The layout of GSI went through some major adjustments and the level of downgrading has been fine-tuned to fit the field observation [1, 8, 33]. For the first three versions of GSI, the downgrading level (designated as ୠ Ȁ୧ ratio) has been increased for lowquality rock mass while decreased for high-quality rock masses as shown in Fig. 2. Subsequent updates focused on development of GSI for specific rock types. Considering the original design purpose and the history of evolution, GSI system is considered to be the most suitable tool for the determination of weathered competent rock types. The qualitative description with the aid of graphic examples captures the geological features of weathered competent rock that is impractical to adequately define by quantitative measures. In terms of the reliability of GSI, mixed comments have been expressed based on back analysis and numerical simulation [24, 32, 34, 35]. These contrary results are believed to be partially caused by the fact that GSI has been used regularly outside its designed prediction capacity. GSI system is not a universal tool in determination of weak rock mass strength, and should be applied with caution. 0.8 1992

0.7

1994

0.6

Rock mass quality 1 2 3 4 5 6 7 8

1997

mi/mb

0.5 0.4 0.3 0.2

GSI and equivalent 10 to 20 20 to 30 30 to 40 40 to 50 50 to 60 60 to 70 70 to 80 80 to 100

0.1 0 1

2

3

4

5

6

7

8

Rock Mass Quality Group

Fig. 2. Comparison downgrading level among 1992, 1994 and 1997 versions of GSI.

3.2. Disturbed competent rock Some rock masses contain strong competent rock but have undergone severe tectonic alteration. These disturbed rock masses normally have low mechanical properties and are considered as “weak” in geotechnical engineering

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practice [3, 36]. Due to the presences of folding, faulting and shearing planes, the original structural integrity of the rock mass reduces and intact pieces are separated by widely existing joints, shear zones and voids. In 1998, Hoek, Marinos and Benissi studied the disturbed schist formation in Athens and suggested that this type of rock mass can be characterized by highly complex structural patterns and high heterogeneity with frequently changes of lithological faces and materials [25]. A qualitative approach was found to be most effective for describing the quality of disturbed competent rock mass. Many attempts have been made by Hoek, Marinos and their colleagues using qualitative methods to provide downgrading ratings based on tunneling experiences in Greece and Italy. A brief summary of their advancements is provided below: x In 1998, in addition to blocky/disturbed category, which was considered inadequate, a foliated /laminated/sheared category was added at the bottom of the original GSI table [25]. x In 2000, the heterogeneous GSI system was introduced to further elaborate ratings for flysch like rock mass [37]. x In 2005, GSI system for molasses was introduced for both confined and unconfined conditions [38]. x In 2006, a guideline for rating ophiolite was provided [39]. x In 2007, an updated heterogeneous GSI system was published with adjustments of ratings, inclusion of new rock types and rewording of descriptions [40]. x In 2010, GSI system for brecciated sandstone and gneiss was introduced [41]. These improvements have identified common types of disturbed rock and they have significantly enhanced the knowledge in estimating the disturbed rock mass strength. However, exhaustive attempts also caused lack of validation and difficulty in selection of the appropriate system. In the engineering practice, the 1998 version of GSI is still the preferred method unless prominent heterogeneous feature is observed. While the heterogeneous GSI system (2007 version) can be used with caution for disturbed weak rocks, the isotropy of rock masses always needs to be assessed as the Hoek-Brown failure criterion is inapplicable to anisotropic rock masses. As suggested by Marinos, first five types of rock masses in heterogeneous GSI system (2007 version) are anisotropic [42]. The application of GSI systems that were developed for a specific type of rock masses are limited to the underlying databases, thus recommendations can only be made for the assessment of relevant rock types. 3.3. Young sedimentary rock Young sedimentary rocks are poorly lithified earth surface materials that can be considered as the transition between stiff soil and rock [10, 43, 44]. In general, their behavior converges to hard rock to stiff soil at upper and lower boundaries, respectively. Considering that GSI-system is a discontinuity-based system and it is essentially hard rock oriented, GSI is likely to underestimate the mass strength of young sedimentary rocks. Hence it is unsuitable for the evaluation of young sedimentary weak rocks. This type of rock fits well in the conventional perception of weak rock which has joint features like competent rock but lower strength, and its behavior as a rock mass is poorly understood. The range of young sedimentary weak rocks has been studied extensively; they are defined as rocks with UCS of 0.5 MPa to 25 MPa by ISRM [3, 12, 13, 45, 46]. However, there is a tendency to reduce the upper bound of weak rock to 20 MPa or even lower to 15 MPa due to noticeable behavior differences [44, 45, 47]. In addition to the baseline rock mass properties, degree of lithification and weathering sensitivity are considered important in determination of young sedimentary rock mass properties. Intact strength, void ratio, smectite percentage and slake durability index are key features for determining the degree of lithification and weathering sensitivity [13, 45–47]. A key argument related to this type of weak rock mass is the role of rock matrix. For young sedimentary rocks with poor lithification, failures are postulated to be more likely to occur on the rock matrix rather than fracture fabrics due to less strength differences [10, 44, 48, 49]. Hence, less discontinuity based strength penalty and downgrading should be applied to the rock mass, and young sedimentary rock mass is expected to behave close to intact rock if the intact rock strength is low enough. Following this argument, a transition function has been developed by Carvalho, Carter and Diederich [43, 44] for GSI system as a correction for low strength rock as in the following form:

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்݂ ሺߪ௖௜ ሻ ൌ  ൝

ͳǡ ିሺ௎஼ௌ೔ ିହ௣ೌ ሻమ ݁ ଶହ଴௣ೌ ǡ

ߪ௖௜ ൑ ‫݌‬௔ ‫ ݔ‬൐ ͷ‫݌‬௔

‫ כ ݏ‬ൌ ‫ ݏ‬൅ ሺͳ െ ‫ݏ‬ሻ்݂ ሺߪ௖௜ ሻ

(2)

‫כ‬

ܽ ൌ ܽ ൅ ሺͳ െ ܽሻ்݂ ሺߪ௖௜ ሻ ݉௕‫כ‬

ൌ ሾ݉௕ ൅ ሺ݉௜ െ ݉௕ ሻ்݂ ሺߪ௖௜

(1)

(3) ሻሿȀሺͶܽ ‫כ‬

െ ͳሻ

(4)

where ்݂ ሺߪ௖௜ ሻ is the correction factor for transition material, ‫݌‬௔ is atmospheric pressure, ݉௕ ,݉௜ , s, and a are original Hoek-Brown parameters and ݉௕‫ כ‬, ‫ כ ݏ‬and ܽ‫ כ‬are corrected Hoek-Brown parameters. For very weak rocks, this modification of GSI system enables the predicted rock mass failure envelope to approach the intact rock failure envelope. This modification also generates a quasi-linear failure envelope similar to Mohr-Coulomb criterion. However, this transition function, which is an empirical model by itself, is relatively unrefined and should be considered as a conceptual model for following reasons. Firstly, this model requires more field calibration as the original model is developed based on simplified numerical modelling simulations. Secondly, the role of intact rock strength is poorly defined and it may be double counted, as ܷ‫ܵܥ‬௜ is also an input parameter in Hoek-Brown failure criterion. Thirdly, it fails to consider the influence of weathering sensitivity. Last but not least, its prediction conflicts with observations on fissured clay at lower boundary of its prediction capacity. Defect fabrics in soil are considered to have similar implications to strength and permeability as in rock mass [46]. Based on back-analysis of English clay, Skempton (1977) suggested that fully softened strength could be considered as the approximation of fissured clay mass strength [50]. Fully soften strength can be obtained by testing crushed and remolded samples and it is essentially cohesionless with similar or lower frictional angle comparing to intact samples. This implies that some level of downgrading is required for hard clay when considering its mass strength. Skempton’s approximation have been widely verified and proved valid for soil masses with prominent structures [51–53]. Due to the observation difficulty, no conclusive guidelines are available for less intensively fractured soil masses, and their strengths are commonly considered the same as laboratory strengths. Dinc et al. (2011) have established an empirical model for estimation of rock mass Hoek-Brown parameters that considers both roles of rock matrix and defect networks [10]. This system includes impacts of joint spacing, joint condition, blasting damage, deformability modulus, intact rock strength and degree of interlocking. In terms of the database, this system adopted nine back analysed and large scale testing cases, including three cases for weak rock masses. This system can be considered as an alternative to GSI but its prediction capability for young sedimentary rock mass requires further validation.

Fig. 3. Comparison among intact Hoek-Brown, GSI, transition function and Dinc's approach for weak rock mass with UCS of 5 MPa, blocky rock mass and fair joint condition.

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Fig. 3. compares the intact rock and rock mass failure envelopes generated using the original GSI transition function and Dinc’s approach for a blocky marlstone with a fair joint condition. While the transition function predicts a better mechanical behavior, the rock mass failure envelope generated using Dinc’s approach is similar to the result of GSI approach. Currently, existing empirical approaches including GSI system, transition function and Dinc’s approach are considered incomplete for low strength rocks due to the contrary between various theories, models and field observations. Hence, it appears that back-analysis of failed cases and numerical approaches are more suitable for evaluation of rock mass parameters for such rocks. 4. Conclusion Determination of weak rock mass properties is a challenging subject in the areas of soil and rock mechanics. Complex nature and vague definitions are the major concerns associated with this type of rock masses. Based on differences in the origin and the alteration process, weak rock masses can be subdivided into three categories, namely low strength rock mass, highly fractured decomposed rock mass and tectonically disturbed rock mass. Although the mechanical behavior of weak rock mass has not been fully understood, empirical methods are still used extensively. This paper summarised some challenges in the weak rock mass characterization and the current empirical approaches for the determination of weak rock mass properties. While the GSI system is appropriate for highly weathered rock mass, current empirical approaches for low strength young sedimentary rocks and tectonic disturbed rock mass are somewhat inadequate. Empirical evaluation of last two types of rock masses is either rock type specific or inadequately verified. All these challenges call for further studies towards the understanding of fundamental properties of weak rock mass and more effective rock mass classification systems.

Acknowledgement Authors would like to acknowledge the Australian Coal Industry’s Research Program (ACARP) for providing the support and funding for this research.

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