datos q

8/12/2019 Datos Q

1/13

Geological controls on rock mass classification of coal from

Huntly East Mine, New Zealand

Vicki Moon*, Trisha Roy

Department of Earth Sciences, University of Waikato, Private Bag 3105, Hamilton, New Zealand

Received 18 December 2003; accepted 24 May 2004

Available online 10 July 2004

Abstract

Rock Mass Rating (RMR) measurements from 65 sites within Huntly East underground coal mine are presented. All

measurements are in coal, for which the dominant discontinuities are vertical cleat. Basic RMR values using two

discontinuity spacings are presented: overall RMR based on the average spacing of all individual discontinuities; and cleat

zone RMR based on the average spacing between zones of cleat. Cleat orientations are highly variable, but on average

approximately parallel horizontal stress axes (face cleat follows maximum horizontal stress axis, butt cleat follows minimum

horizontal stress axis).

Contours of RMR variations throughout the mine are used to compare rock mass conditions with geological structure. It

is apparent that: (1) RMR is least within downthrown fault blocks, and particularly immediately on the downthrown sidesof faults, and greatest in upthrown fault blocks; and (2) RMR contours parallel horizontal stress axes within fault-bounded

blocks, and bend to parallel faults at block boundaries. From similar contours for parameters contributing to RMR, the

Rock Quality Designation (RQD), groundwater rating, and discontinuity condition rating create most of the observed

variations in RMR. RQD is determined from the measured discontinuity frequency and hence is a measure of the degree of

fracturing of the rock mass. This is interpreted as influencing the groundwater and condition parameters directly by

allowing greater water ingress. Discontinuity frequency is greatest (least spacing) in the immediate vicinity of faults, and in

downthrown fault blocks, generating low RMR values. Within fault blocks RQD varies little, so RMR contours align with

cleat orientations.

As RMR contours, faults, stress field and cleat orientation are clearly interrelated, there is unequivocally a connection

between RMR and structural geology; this allows some predictive capacity in terms of ground conditions. If geological

features can be accurately defined through either drilling programs or seismic surveys, then ground conditions may be

predicted before panels are driven.

D 2004 Elsevier B.V. All rights reserved.

Keywords:Rock mass classification; Coal; Cleat; In situ stress

1. Introduction

Rock mass classification is a commonly used

technique for assessing rock mass conditions in many

mining and tunnelling applications. The Rock Mass

0013-7952/$ - see front matterD 2004 Elsevier B.V. All rights reserved.doi:10.1016/j.enggeo.2004.05.007

* Corresponding author. Tel.: +64-7-856-2889; fax: +64-7-856-

0115.

E-mail address:[email protected] (V. Moon).

www.elsevier.com/locate/enggeo

Engineering Geology 75 (2004) 201213

8/12/2019 Datos Q

2/13

Rating (RMR) is one such classification system that

has gained wide acceptance in mining, and has

previously been applied to coal mining by many

authors (for example, Ghose and Raju, 1981; Abadet al., 1983; Unal, 1983; Kendorski et al., 1983;

Newman, 1985; Venkateswarlu, 1986; Newman and

Bieniawski, 1987; Faria-Santos and Bieniawski, 1989;

Ghosh and Ghose, 1992; Trueman et al., 1992;

Vutukuri and Hossaini, 1992). In these cases, RMR is

mostly used as an intermediate step to assist engineer-

ing design or to aid in design of support requirements.

Recent work in weak rock masses has concentrated on

the relevance of RMR to clay-rich materials, and

modifications to the RMR to better apply to weak

rocks (Gokceoglu and Aksoy, 2000; Yasar, 2001;

Moon et al., 2001; Sen and Sadagah, 2003).

This paper presents RMR data measured from 65

sites within the Huntly East underground coal mine,

New Zealand, in order to establish relationships

between structural geology and the rock mass con-

ditions of the coal. The aim is to identify controls on

the measured RMR to allow prediction of rock mass

conditions from knowledge of the general structure

and stress regime of the area.

2. Regional setting

2.1. Waikato coal region

The Waikato Coal Region includes 13 coalfields

and is New Zealands major coal producing region

(Edbrooke et al., 1994).The Waikato Coal Measures,

of Late Eocene age (Schofield, 1978), overlie Meso-

zoic basement rocks unconformably and are typically

30 to 100 m thick. With gradual marine encroachment,

the coal measures were overlain by a succession of

marginal marine and shelf sediments comprising theTe Kuiti Group (Kear and Schofield, 1959; King,

1978; White and Waterhouse, 1993).

The Waikato Region is characterised by block

faulting with steeply dipping normal faults. Two main

fault sets can be recognised: a set of north to north-

west (strikef 338j) trending faults(Edbrooke et al.,

1994); and a much more common set of northeast to

north northeast (strikef 034j) trending faults that

may offset the north trending set. This block-faulting

and uplift occurred during the late Tertiary Kaikoura

Orogeny and has resulted in a regional seam dip of

10j to the NW, though this varies (up to 25j)(Barry

et al., 1994; Yardley, 1994), as well as considerable

faulting of the coalfields(Kirk et al., 1988).Huntly East Mine is within the Huntly Coalfield

which occupies an area about 20 km long and up to

9 km wide. A number of coal seams exist within the

Huntly Coalfield; two, the Kupakupa and Renown

seams, are relevant to the Huntly East Mine. The

Renown is the upper seam and ranges in thickness

from 0.5 to 5 m, whereas the Kupapkupa seam, the

main extraction seam, is approximately 6 to 10 m

thick. The coal is sub-bituminous B to high sub-

bituminous C in ASTM rank, with low to medium

ash and low-sulfur content(Barry et al., 1994).Floor

and roof rocks are weak (25 MN m 2), with slightly

higher coal strength (525 MN m 2)(Yardley, 1994).

The depth of the coal below the surface is typically

150300 m in the Huntly East Mine(Yardley, 1994).

2.2. In situ stress field

The in situstress field inthe Huntly East Mine was

measured by Mills (1986). A vertical stress of ap-

proximately 4 6 MN m 2 was equivalent to the

expected lithostatic stress (Mills, 1986). Regionally,

the major and minor horizontal stresses were found tobe approximately 4.5 and 1.5 MN m 2 and lie in

northeast/southwest and northwest/southeast direc-

tions, respectively. This relation is valid for depths

ranging from 120 to 250 m, implying the stress has a

tectonic origin, and is in agreement with the regional

faulting and geomorphic features observed (Mills,

1986). Locally however, the stress field can vary in

both magnitude and direction, the major influence

being faults and basement relief(Mills, 1986).

Tan (1988) reported data from Mills for in situ

stresses within a portion of Huntly East Mine near thestudy area. These values are:

4.9 MPa, bearing 028j, plunge 81j 2.5 MPa, bearing 188j, plunge 9j 1.6 MPa, bearing 278j, plunge 3j

The magnitude of these stresses may not be exactly

appropriate to this study area, but it is assumed that

the orientation of the stress field ought to be similar to

these values.

V. Moon, T. Roy / Engineering Geology 75 (2004) 201213202

8/12/2019 Datos Q

3/13

3. Methods

3.1. Site selection

Initial structural mapping identified major discon-

tinuities (persistence >2 m) within the mine, allowing

identification of a number of discrete fault blocks

(Fig. 1). Sites for RMR assessment were then selected

with the intention of ensuring that all identified fault

blocks were represented, and that the full range of face

conditions (both near and distant from faults) was

sampled. Site selection for RMR assessment com-

monly relies on division into lithological domains: in

this case there is no obvious lithological variation so

structural features were used to define sampling

philosophy. A total of 65 RMR sites was measured

(Fig. 1). The methods for determining each of the

RMR input parameters are described below.

3.2. Intact rock strength

The NCB cone indenter was used to determine

intact strength of the coal as uniaxial compressive

strength and point load testing were impractical due to

the highly fractured nature of the material. Tests were

performed in accordance with the procedure outlined

by MRDE (1977), for which the weak rock cone

indenter number (Iw) is calculated and converted to

an equivalent compressive strength for 25 mm diam-

eter cores (rc(25)) by means of a calibration equation(rc(25) = 16.5Iw). For each RMR site 15 specimens

were tested and the mean strength determined. RMR

ratings were assigned using the classification chart of

Bieniawski (1989).

3.3. Discontinuities

Scanline surveys (Brady and Brown, 1985) were

used to acquire statistically valid discontinuity data.

Priest and Hudson (1976) found that to estimate the

number of discontinuities per metre to reasonable

precision requires a scanline length of at least 50

times the mean discontinuity spacing. To achieve this,

scanline lengths of 3 to 10 m were required, giving an

average number of discontinuities measured in a

single scan line of approximately 50.

For each discontinuity the following data were

recorded: distance along scanline (m); orientation

(dip/dip direction); persistence (length of discontinu-

ity above scanline (m)); roughness; separation follow-

ing the classification of Romana (1993) that sees

divisions at 0.1, 1.0 and 5.0 mm; and infill in terms

Fig. 1. Map of study portion of Huntly East Mine. Two portions of the mine are studied: M&M includes the main drives in the northern

portion of the mine, and the South-4 extraction area lies to the west of the Okowhao Fault.

V. Moon, T. Roy / Engineering Geology 75 (2004) 201213 203

8/12/2019 Datos Q

4/13

Table 1

Field observations of rock conditions used to derive rock mass classification numbers

Site Compressive Discontinuities Groundwater Overall Cleat

strength

(MN m 2) Mean spacing(m)

Mean cleatspacing (m)

Est.RQD

Length(m)

Width(mm)

Roughness

basic

RMR

basic

RMR

M5 58F 10 0.03F 0.01 0.4F 0.2 63 1 3 < 0.1 sl. rough dry 61.6 66.1

M8 64F 6 0.04F 0.01 0.5F 0.3 70 < 1 < 0.1 sl. rough dry-sl.damp 63.8 68.5

M14 63F 15 0.04F 0.01 0.4F 0.2 64 < 1 < 0.1 sl. rough dry-sl.damp 62.8 66.4

M17 68F 7 0.04F 0.01 0.9F 0.3 72 1 3 < 0.1 rough dry-sl.damp 64.5 72

M20 72F 14 0.07F 0.01 0.8F 0.2 70 < 1 < 0.1 sl. rough damp 59.4 65.7

M23 56F 10 0.03F 0.01 0.4F 0.2 58 < 1 < 0.1 sl. rough dripping 51.2 54.4

M29 38F 8 0.05F 0.01 0.6F 0.2 68 < 1 < 0.1 rough damp-sl.drip. 57.0 62.3

M32 38F 13 0.05F 0.01 1.0F 0.5 74 < 1 < 0.1 sl.rgh-rgh dry-sl.damp 63.5 72.3

M34 20F 29 0.09F 0.01 1.1F 0.8 85 1 3 0.1 1.0 rough dry 62.9 71.5

M38 41F 22 0.03F 0.01 0.3F 0.2 68 1 3 < 0.1 sl.rgh-rgh dry 62.1 65.4

M39 49F 10 0.10F 0.01 1.0 69 < 1 none smooth dry 64.7 72.1

M42 35F 6 0.05F 0.01 1.0F 0.5 69 1 3 none smooth dry 60.9 68.8

M45 47F 3 0.08F 0.03 4.6 85 1 3 none smooth dry-sl.damp 63.2 76.7

M48 27F 7 0.08F 0.02 0.7F 0.3 75 < 1 < 0.1 sl. rough sl. damp 61 67

R1 65F 18 0.09F 0.01 1.5F 0.5 76 < 1 < 0.1 rough dry 69.1 79.6

B3 36F 12 0.08F 0.03 0.8F 0.3 67 1 3 none smooth dry-sl.damp 58.2 63.6

BA1 44F 15 0.05F 0.01 0.6F 0.6 63 1 3 < 0.1 sl.rgh-rgh dry-sl.damp 60 66.7

XC13 11F12 0.08F 0.01 1.2F 0.5 87 1 3 none sl.rgh-rgh dry-sl.damp 59 68.3

XC17 58F 9 0.05F 0.01 1.3F 0.6 72 1 3 none sl.rgh-rgh dry 65.6 75.9

XC24 23F 4 0.03F 0.01 0.7F 0.3 74 1 3 none sl.rgh-rgh dry 66.1 72.7

XC28 66F 9 0.04F 0.01 0.4F 0.1 68 1 3 none smooth dry 63.3 67.6

XC32 24F 6 0.07F 0.01 1.5F 0.4 79 < 1 none smooth dry 64.6 75.8

XC37 69F 8 0.02F 0.01 0.06F 0.05 74 1 3 none smooth dry 64 64.8

SMa 43F 7 0.05F 0.01 0.30F 0.05 81 1 3 none smooth damp 59.1 62.2

SMb 51F 7 0.08F 0.02 2.8F 1.3 86 1 0.1 1.0 rough sl. damp 64.1 77.6

SMc 70F 7 0.14F 0.02 1.5F 0.8 86 < 1 0.1 sl.rgh-rgh sl. damp 67.4 77.8SMd 43F 12 0.07F 0.01 0.7F 0.3 66 1 3 0.1 sl.rgh-rgh damp 56.4 62.9

SMe 55F 10 0.05F 0.01 1.5F 0.6 77 1 0.1 1.0 sl. rough sl. damp 60 71.5

SBa 47F 9 0.10F 0.02 0.8F 0.2 81 < 1 none smooth dry-sl.damp 64.7 71.3

SBb 56F 9 0.08F 0.01 0.8F 0.3 80 1 0.1 smooth dry 64.4 70.7

SBc 50F 5 0.04F 0.01 0.7F 0.2 78 1 3 < 0.1 sl. rough dry 64 70.7

SBd 50F 9 0.07F 0.02 0.7F 0.5 87 < 1 0.1 sl. rough dry 67 72.7

SBe 46F 9 0.18F 0.05 2.4F 0.7 91 1 3 0.1 sl. rough dry 66.9 79.1

SBf 50F 12 0.10F 0.02 5F 1 85 1 3 0.1 sl.rgh-rgh dry 66 79.1

SRa 51F10 0.13F 0.02 0.7F 0.2 87 < 1 < 0.1 sl. rough dry-sl.damp 66.8 72

SRb 60F 7 0.17F 0.05 2F 1 91 1 3 none smooth dry 69 81.3

SRc 29F 5 0.07F 0.01 0.9F 0.5 84 < 1 none sl.rgh-rgh dry 68.6 76.1

SRd 41F 4 0.15F 0.02 0.8F 0.4 84 1 none rough dry-sl.damp 66.7 72.2

SRe 50F 7 0.16F 0.03 2F 1 91 < 1 0.1 sl.rgh-rgh dry-sl.damp 66.8 77.8

S41 56F 6 0.14F 0.02 1.6F 0.6 87 < 1 < 0.1 sl.rgh-rgh dry 71.1 81.8S42 43F 6 0.22F 0.05 2.2F 0.9 93 < 1 0.1 1.0 rough dry-sl.damp 67.8 79.5

S43 34F 8 0.09F 0.02 1.0F 0.5 92 1 3 0.1 sl. rough dripping 54.3 61.6

S44 47F 11 0.14F 0.02 0.9F 0.4 87 < 1 none smooth dry-sl.damp 67.9 74.1

S45 58F 6 0.16F 0.02 2.1F 0.7 89 < 1 none smooth dry-sl.damp 68.6 80.9

S46 41F 6 0.11F 0.01 1.2F 0.4 83 < 1 < 0.1 sl.rgh-rgh dry-sl.damp 67.3 76.1

S47 43F 4 0.15F 0.05 1.4F 0.7 88 < 1 < 0.1 rough dry 69.8 79.2

S31 38F 7 0.16F 0.05 3F 1 94 1 < 0.1 sl.rgh-rgh dry-sl.damp 66.1 78.4

S32 54F 9 0.12F 0.02 0.7F 0.3 86 < 1 < 0.1 sl.rgh-rgh damp 66.4 71.7

S33 32F 7 0.13F 0.02 0.5F 0.1 87 < 1 none smooth dry 68.2 72

S34 37F 7 0.03F 0.01 0.4F 0.2 87 1 3 < 0.1 sl.rgh-rgh dry 66.5 71

S35 44F 9 0.07F 0.02 1.5F 0.5 90 < 1 < 0.1 sl.rgh-rgh dry 68.9 80.1


8/12/2019 Datos Q

5/13

of the type (none, high friction material, soft gouge)

and thickness of infilling material. Mining-induced

fractures encountered were included within the dis-

continuity measurements.

Joint sets were identified by plotting the orientation

data stereographically using STEREO (McEachran,

1986). This was undertaken for all scanline data andpole concentrations were contoured. The maximum

density points on the contour diagram were selected as

the best representation of the orientation of each

discontinuity set. Mean discontinuity spacing was

calculated for each recognised discontinuity set as

the average distance between adjacent discontinuities

making up the set, corrected for directional bias

following the method described byPriest (1985).This

correction calculates the perpendicular distance be-

tween adjacent parallel discontinuities, hence remov-

ing the influence of the orientation of the scanline.It was noted during preliminary work that cleat were

not present along entire scanlines, but often occurred in

zones. To account for cleat within an RMR assessment,

the start, finish, and number of cleat within each zone

were recorded, and a cleat zone defined as a series of at

least four recognizable cleat with regular orientation

and spacing. Two discontinuity spacing variations

were calculated: one overall mean spacing considering

every discontinuity that crossed the scanline (over-

all spacing); and a mean cleat zone spacing (cleat

spacing). For eachof these, the classification chart of

Bieniawski (1973)was used to assign ratings.

Discontinuity condition is a complex parameter

that includes several sub-parameters: roughness, fill-

ing, persistence, separation, and weathering. The

RMR classification combines these parameters into

one grouped component of the classification. In orderto assign values each sub-parameter was rated sepa-

rately and the values summed to a total rating (after

Bieniawski, 1989).

3.4. Rock Quality Designation (RQD)

Core data for calculation of RQD were not avail-

able. Therefore, RQD was estimated from mean

discontinuity frequency (total number of discontinu-

ities/scanline length) using the method of Priest and

Hudson (1976). RQD values were converted to ratingsusing the classification chart ofBieniawski (1989).

3.5. Groundwater and weathering

Groundwater flow was expressed using a descrip-

tive classification proposed by Brown (1981) for the

estimation of seepage from individual unfilled and

filled discontinuities. Ratings based on these descrip-

tive terms were assigned using the chart ofBieniawski

(1989). All sites classified as unweathered.

Table 1 (continued)

Site Compressive Discontinuities Groundwater Overall Cleat

strength

(MN m 2)Mean spacing

(m)

Mean cleat

spacing (m)

Est.

RQD

Length

(m)

Width

(mm)

Roughness basic

RMR

basic

RMR

S36 42F 6 0.09F 0.01 0.7F 0.2 82 < 1 none smooth dry 68.7 74.2

S37 58F 8 0.11F 0.01 1.1F 0.3 85 1 3 0.1 1.0 sl. rough dry 64.3 72.7

EST6690 36F 10 0.06F 0.01 2.0F 0.9 83 < 1 < 0.1 sl. rough dry 71.1 79.8

EST7475 46F 9 0.12F 0.05 2F 1 90 < 1 0.1 1.0 sl. rough damp 64 69.4

EST7440 64F 9 0.08F 0.01 1.5F 0.7 88 1 0.1 smooth dry 71.1 79.1

EST7476 54F 8 0.05F 0.01 1.1F 0.3 75 1 < 0.1 smooth dry 71.8 71.1

EST7444 55F 12 0.08F 0.02 0.6F 0.3 84 < 1 0.1 1.0 sl. rough sl. damp 61.3 69.7

EST7442 53F 7 0.08F 0.01 0.5F 0.1 84 1 3 0.1 1.0 sl.rgh-rgh sl. damp 66.2 75.3

EST7474 47F 10 0.13F 0.01 1.2F 0.8 87 < 1 0.1 slickensided damp 61.6 72.7

EST7462 78F 5 0.13F 0.02 1.8F 0.5 89 < 1 0.1 1.0 sl. rough dry 72 79.4

EST6693 64F 7 0.09F 0.02 1.9F 0.4 90 < 1 0.1 sl.rgh-rgh damp-sl.drip. 67.2 75.9

EST6677 50F 9 0.10F 0.02 3F 1 92 < 0.1 sl. rough damp 63.5 76.1

EST6672 61F

9 0.10F

0.03 0.5F

0.3 86 < 1 0.1 sl. rough dry-sl.damp 66.3 70.2EST6692 57F 6 0.07F 0.01 1.5F 1 89 < 1 none rgh-v. rgh dry 71.9 76.6

All sites classify as unweathered, and all sites recorded no infill except for site EST6692 where there was < 5 mm of hard infill. Calculated

rock mass classification numbers using both the average discontinuity spacing for all discontinuities measured (overall basic RMR), and the

average spacing between cleat zones (cleat basic RMR) are included.


8/12/2019 Datos Q

6/13

4. Results

4.1. RMR values

Information was recorded regarding the rating

parameters for each site(Table 1), from which ratings

numbers were assigned and total ratings calculated. As

there are two spacing variations, two basic RMR

values have been generated, designated overall

(overall mean spacing) and cleat (cleat zone spac-

ing). Adjusted values are not presented as they repre-

sent both the intrinsic rock conditions and the face (or

roadway in this case) orientation. The aim is to

examine intrinsic properties of the rock mass, so

adjusted values are not considered appropriate.

Histograms of the distributions of basic RMR

values are presented in Fig. 2. These indicate that

the basic overall RMR gives a strongly peaked distri-

bution with the values clustered tightly around a meanof 64.8 and a small standard deviation of 4. The

values range across two of the five possible classes,

with ratings in the good rock and fair rock categories.

In contrast, the basic cleat zone ratings show a greater

variation, with a higher mean of 72.4 and a greater

standard deviation of 6. In this case, the values vary

across three of the five categories, very good rock,

good rock and fair rock. The greater mean for the cleat

zone spacing is expected, as the spacing of the cleat

zones is, by definition, greater than the average

discontinuity spacing, and this is carried through the

RMR calculation.

The relatively restricted range of RMR values for

the overall spacing calculation indicates that there is a

relatively small range of rock mass conditions encoun-

tered within the mine. This is not surprising as all of

the measurements are made within a single lithology

and within a restricted area. However, the changes

observed are large enough to impact on mining oper-

ations, so identifying or predicting these relatively

small variations is important.

4.2. RMR contours

To facilitate comparison between structural and

rock mass classification information, the RMR val-

ues were contoured. Production of these contours

involved firstly defining the polygon describing the

boundary of the contour plot, then triangulating the

data and generating the contour plot. In order to

ensure that contouring did not extend beyond the

area of reliable data, the polygon was set close to the

actual limit of the roadways so as to limit the degree

of extrapolation that would inevitably occur. Thecontours are presented in five-point increments in

Figs. 3 and 4.

The two sets of contours produce similar patterns,

with the cleat zone spacing contours showing some-

what more complexity due simply to there being a

greater range of values and hence more contours. There

is an apparent distinction between the M&M (northern

portion of mine) and the South-4 extraction area

(southwestern portion) of the mine in that the contours

show a much simpler pattern in the M&M area.Fig. 2. Histograms of calculated basic RMR values for (A) overall

discontinuity spacing, and (B) cleat zone spacing.


8/12/2019 Datos Q

7/13

Simple east west contours defining elongated

zones of high rock mass characteristics exist in the

M&M area. These contours, bearing 273j, approxi-

mately parallel the direction of the minor horizontal

stress (278j). Near the faults this simple contour

pattern is clearly disrupted and the contours are bent

to follow the faults. This is particularly true of the Te

Ohaki and Okowhao Faults.

The South-4 extraction area is structurally more

complex than the M&M area, and this structural

Fig. 4. Contours of cleat zone basic RMR values.

Fig. 3. Contours for overall basic RMR values.


8/12/2019 Datos Q

8/13

complexity is reflected in the RMR contour patterns.

The main feature of this region is a marked area of

poor rock mass quality (low RMR) within the block

between the Ralph, Kear, Okowhao, South-6, and

South-4 Faults. A similar, but less distinct, low exists

along the South-6 Fault. Higher RMR values areencountered to the south of each of these zones.

Simplification of the RMR contours (Fig. 5) sug-

gests that in the South-4 extraction area two sets of

contour patterns are apparent:

in the southern portion contours running approx-

imately parallel to the minimum horizontal stress

(at 291j); in the northern portion contours running at 019j

approximately parallel the maximum horizontal

stress (188j

or 008j

).

These patterns are not as well developed as in the

M&M, and the overall trends are broken into a series of

highs and lows by the tendency for the contours

to curve parallel to faults in their immediate vicinity.

4.3. Fault blocks

All faults in Huntly East Mine area are normal or

extensional faults. Fault blocks(Fig. 1)are classified

according to the direction of throw of the boundary

faults; if all boundary faults are downthrown around

the block it is classified as downthrown, similarly for

upthrown blocks. However, the throw of a bounding

fault is not always consistent around the block. When a

block is bounded by faults with different throw ori-entations it is classed as a combination block. Table 2

lists the fault blocks, their classification, and ranks

them from 1 = fully upthrown to 10 = fully down-

thrown. The maximum or minimum value of the

Fig. 5. Simplified RMR contour patterns showing general zones of good (high RMR) and poor (low RMR) rock mass quality.

Table 2

Fault blocks, their orientation, ranking, and RMR values based on

contour patterns

Fault

block

Type Ranking Overall

RMR

Cleat

RMR

1 upthrown 2 70 75

2 downthrown 9 55 55

3 upthrown 1 70 80

4 combination 2 up, 1 down 4 70 75


6 combination 1 up, 2 down 7 75

7 downthrown 10 50 65



10 combination 3 up, 1 down 3 75

A means that no contour loops were formed within that fault

block.


8/12/2019 Datos Q

9/13

overall discontinuity RMR contours for each block is

also listed, according to the contour in the centre of

any contour loop within the block. Plotting these

values(Fig. 6)indicates that upthrown blocks have ahigher overall RMR value than downthrown fault

blocks, and combination blocks fall within the range

between these two extremes. The cleat zone RMR

values also show a general tendency for downthrown

blocks to have lower RMR values, but this is not a

strong effect.

It thus appears that high RMR values occur on the

upthrown side of faults and hence within upthrown

blocks. Correspondingly, lower values of RMR, and

therefore weaker rock masses, exist on the down-

thrown side of faults and hence in downthrown

blocks.

4.4. In situ stress and cleat orientation

It is apparent from Figs. 3 and 4 that the RMR

contours approximately parallel the minor horizontal

stress direction, suggesting that the RMR is influ-

enced by the stress regime in the mine. As cleat is the

major source of discontinuities contributing to the

RMR values, the interrelationship between cleat, in

situ stress, and RMR contours is investigated.

The presence and orientation of cleat was recordedduring fieldwork. Average cleat orientations were

determined by using stereonet analysis combining

the cleat measurements from all sites measured. This

generated:

dominant (face) cleat = 87/117 (dip/dip direction;

8% concentration) secondary (butt) cleat = 89/022 (dip/dip direction;

3.5% concentration)

These are not high concentrations due to a very

wide variability in cleat orientation across this por-

tion of the mine. This variability is also reported

elsewhere within Huntly Coalfield (Mills, 1986); a

F 20j variability around the average orientations

accounts for most of the variation observed in the

dataset.

Plotting these averages together with regional fault

orientations (Section2.1)and the seam dip gives the

stereonet shown in Fig. 7, where the cleat and faults

are represented by strike directions. From this it is

apparent that:

1. the faults appear to form a conjugate set (56jacute

angle) almost symmetrically around the maximum

horizontal stress direction;

2. the dip of the seam is approximately normal (79j

difference) to one of the fault sets;3. the face and butt cleat are at approximately right

angles (95j difference);

4. the strike of the face cleat set is roughly (within the

shaded zones representing the variability) parallel

to the trend of the maximum horizontal stress (19j

difference), while the strike of the butt cleat set

approximately parallels the trend of the minimum

horizontal stress (14j difference).

The overall stress field in the region is extensional

(as evidenced by the predominance of normal faults),as a result of the vertical stress being greater than the

horizontal stresses. However, the horizontal stresses

themselves are compressive (Mills, 1986). The con-

jugate fault set is thus an expected response to the

stress orientations given, and lends credence to the

assumption that the stresses measured by Mills

(1986)adequately estimate the regional stress pattern

(at least in orientation if not magnitude) and hence

can be used as indicators of the likely stress field at

Huntly East.Fig. 6. RMR value against fault block number where fault blocks

are ranked from upthrown (1) to downthrown (10).


8/12/2019 Datos Q

10/13

The dip of the seam at right angles to the strike

of one of the faults indicates overall downthrow

across this fault orientation. Individual faults are not

necessarily downthrown in this direction, but this is

the trend across the region.

Cleat are believed to form during coalification as a

response to hardening and shrinkage of the coal(Mills,

1986). They thus represent tensional failure of the

developing coal, and should reflect the regional stress

patterns at the time of coal formation in the same waythat other tensional joints do. Two approximately

normal cleat sets are commonly reported (Bachu and

Bell, 2001), although it is noted that there is high

variability in local cleat orientations(Mills, 1986).The

cleat orientations determined here are thus in keeping

with expected results, and the variability is similar to

that reported from other areas of the Huntly Coalfield

where cleat is seen to vary with position with respect to

faults and other structures in the mine (Mills, 1986;

Cameron, 1995).

In terms of stress orientations, the main fractures

(face cleat) are expected to form perpendicular to

the minimum tensile stress (parallel to the maxi-

mum compressive stress); the butt cleat will form

normal to this (Mills, 1986; Bachu and Bell, 2001).

In this case, the cleat orientations are approximate-

ly 17j clockwise of the stresses indicated, probably

representing local variations in the stress field;

local variations of at least 20j were recorded by

Mills (1986), generally reflecting proximity tofaults that were seen to alter the local stress

orientations.

As the RMR contours have been observed to lie

approximately parallel to the minimum horizontal

stress in many parts of the mine, it follows that they

also approximately parallel the strike of the butt

cleat. Likewise, in the northern part of the South-4

extraction area, block 7, the RMR contours parallel

the maximum stress, and hence the face cleat

strike.

Fig. 7. Stereonet showing regional fault orientations, horizontal stress axes, regional seam dip, and average cleat orientations (including F 20j

variation in dip direction).


8/12/2019 Datos Q

11/13

5. Discussion

From the preceding sections, the main patterns

discernable in the RMR results are:

1. RMR values are lowest in downthrown blocks, and

highest in upthrown blocks, especially with a very

deep low value in block 7;

2. RMR contours tend to follow the minimum

horizontal stress axis (or butt cleat orientation) in

the M&M and the southern part of the South-4

extraction area, and the maximum horizontal stress

axis (or face cleat orientation) in the northern part

of South-4;

3. RMR contours bend to parallel the faults, partic-

ularly the Okowhao and Te Ohaki faults in the

M&M.

By drawing similar contours for each of the com-

ponents contributing to RMR, both in terms of the

original measurements and the ratings values, some of

the characteristics of the rock mass causing these

variations can be deduced. In particular:

1. RQD, which is based on discontinuity frequency,

shows contour patterns running parallel to the

stress axes in the same ways as described for theRMR contours, with low RQD values generally in

downthrown blocks (7 and 2) or immediately to the

downthrown side of faults;

2. groundwater and condition ratings show low (wet,

poor condition) areas in block 7 at the position of

the low point of the RMR contours, and ground-

water rating is also low parallel to the Okowhao

and Te Ohaki Faults in the M&M;

3. strength and cleat zone spacing, and overall

discontinuity spacing (corrected for directional

bias) show no obvious patterns in the mine area.

5.1. Fault blocks

In the downthrown blocks, particularly block 7, the

discontinuity frequency is high, and hence the RQD is

low compared with surrounding blocks. This indicates

a greater degree of shattering of the coal mass in

blocks that have been downthrown. This is generally,

however, restricted to the areas immediately surround-

ing the faultsin the centres of the blocks the differ-

ences in comparison with surrounding blocks are

small. Greater discontinuity frequency on the down-

thrown side of faults, and hence in downthrownblocks, indicates that more of the extensional strain

is being accommodated by fracturing of downthrown

blocks. This is in keeping with observations of greater

structural complexity in downthrown blocks.

Several very low groundwater rating values

(damp or dripping descriptions) in blocks 2

and 7 dominate the groundwater contour patterns

observed. These coincide with the low RQD areas,

indicating that closer fracturing in downthrown blocks

results in greater groundwater movement through

these areas. For block 7, this is accompanied by a

poor condition rating for the discontinuities, indi-

cating greater joint aperture. Similar effects are noted

along the Te Ohaki and Okowhao Fault zones, but

they are not as strongly developed as in the down-

thrown blocks. RQD, groundwater, and condition

components together make up some 65% of the total

RMR classification, so the patterns created by these

impact strongly on the RMR ratings.

5.2. RMR contour orientations

RQD contours run parallel with horizontal stressaxes, as observed for the RMR contours. As noted in

Section 4.4 the face and butt cleat orientations are

approximately parallel to the maximum and minimum

horizontal stress axes, respectively.

Observed cleat characteristics are that:

1. their spacing is not regular, rather they occur in

zones of closely spaced cleat separated by zones

with few cleat (Section 3.3);

2. although cleat generally align with the stress axes,

they are quite variable in their orientation (Section4.4), and in particular they will bend to align

locally with faults(Mills, 1986);

3. near faults their frequency increases (from above).

As RQD appears to be exerting the over-riding

influence on the RMR patterns observed, it is thus

most likely the frequency or spacing of cleat that

impacts most significantly on the final RMR value for

this mine. Assuming the face and butt cleat have

similar zone spacings, then development of high and


8/12/2019 Datos Q

12/13

low contours for RQD is dependent on the sampling

of cleat zones. If the measurement points are prefer-

entially aligned parallel to either the butt or face

cleats, then the spacing of these cleat will dominate

discontinuity frequency measurements(Fig. 8).

In the M&M the tunnels are aligned sub-parallel to

the minimum horizontal stress, so the butt cleat

frequencies are sampled preferentially. As the butt

cleat align with the minimum horizontal stress, so the

RQD and hence RMR contours also align with the

minimum horizontal stress. Likewise, in the South-4

area there are two main tunnel orientations: in thenorthern part the tunnels are sub-parallel to the max-

imum horizontal stress, as are the RMR contours,

whereas in the southern part the tunnels and RMR

contours both align with the minimum horizontal

stress axis. Sampling patterns are thus an important

control on the resulting RMR contours. If a full square

could be sampled, it would be expected that the RMR

values should be similar throughout the block, and

swing to parallel the faults bounding the block. Cleat

orientations within the block reflect the state of stress

of the block, and cleat at the boundaries reflect thefault characteristics, as described by Mills (1986).

6. Conclusions

It is interpreted that the downthrown/upthrown

structure of the fault blocks within the mine exert the

most important influence on measured RMR as this

determines cleat frequency. A high cleat frequency (or

close spacing) in turn allows water ingress, and hence

impacts on groundwater and condition rating values.

Recognition of major structures thus gives a good

indication of overall rock mass conditions to be

expected.A secondary effect is for RMR contours to parallel

stress axes. This is the direct result of cleat tending to

parallel these axes, meaning that cleat provide a

simple indication of stress orientation (as suggested

by Mills, 1986), though sampling bias must be con-

sidered as likely to be having an impact on any

measurements undertaken.

Therefore, RMR contours, faults, stress field and

cleat orientation are clearly interrelated; there is un-

equivocally a connection between RMR and the

structural geology of the region. The existence of this

relationship has practical relevance for Huntly East

Mine. The connection between RMR and geologic

features allows some predictive capacity in terms of

ground conditions. When geological factors such as

faults have been identified, the variation in rock mass

conditions within mining blocks can be anticipated.

For example, lower RMR, and therefore poorer

ground conditions, may be expected of a downthrown

side of a fault, or within a downthrown block.

Therefore, if geological features can be accurately

defined through either drilling programs or seismic

surveys, then ground conditions, and hence reinforce-ment costs, may be predicted before panels are driven.

Acknowledgements

Solid Energy North is thanked for financial and

logistic contributions to the study on which this paper

is based. The Foundation for Research Science and

Technology funded part of the research through a

GRIF scholarship.

References

Abad, J., Celada, E., Chacon, E., Gutierrez, V., Hildago, E., 1983.

Application of geomechanical classification to predict the con-

vergence of coal mine galleries and to design their supports.

Proceedings 5th Congress of the International Society for Rock

Mechanics, Melbourne, Australia, vol. 2. Balkema, Rotterdam,

pp. E15 E19.

Bachu, S., Bell, S., 2001. Stress regime in the Cretaceous succes-

sion of the Alberta Basin: a predictor for coal bed methane

Fig. 8. Schematic diagram of sampling discontinuity frequency (for

RQD calculation). Assuming the cleat is arranged in zones, derived

contours will be parallel to either the face or butt cleat orientations.

The layout of the sampling sites will determine which set of

possible contours resultin this case the sampling is undertaken

sub-parallel to the butt cleat so this is the orientation of the contours.

H = high discontinuity frequency; L = low discontinuity frequency.


8/12/2019 Datos Q

13/13

production. Rock the Foundation Convention, June 18 22 2001,

Canadian Society of Petroleum Geologists, pp. 003-1 003-5.

Barry, J.M., Duff, S.W., MacFarlan, D.A.B., 1994. Coal resources

of New Zealand. Resource Information Report 16. Energy and

Resources Division, Ministry of Commerce, Wellington, NewZealand. 73 pp.

Bieniawski, Z.T., 1973. Engineering classification of jointed rock

masses. S. Afr. Inst. Civil Eng. 15 (12), 335344.

Bieniawski, Z.T., 1989. Engineering Rock Mass Classifications.

Wiley, New York. 251 pp.

Brady, B.H.G., Brown, E.T., 1985. Rock mechanics for under-

ground mining. Allen and Unwin, London. 527 pp.

Brown, E.T. (Ed.), 1981. Rock Characterization, Testing and Mon-

itoring. ISRM Suggested Methods. International Society for

Rock Mechanics, Pergamon, Oxford. Published for the Com-

mission on Testing Methods.

Cameron, M.J., 1995. Cleat Structure, Waikato Coal Region. MSc

Thesis, University of Auckland. 101 pp.

Edbrooke, S.W., Sykes, R., Pocknall, D.T., 1994. Geology of the

Waikato Coal Measures, Waikato Coal Region, New Zealand

Institute of Geological and Nuclear Sciences, Lower Hutt, New

Zealand Monograph 6.

Faria-Santos, C., Bieniawski, Z.T., 1989. Floor design in under-

ground coal mines. Rock Mech. Rock Eng. 22, 249271.

Ghose, A.K., Raju, N.M., 1981. Characterisation of rock mass vis-

a-vis application of rock bolting in Indian coal measures. Proc.

22nd Int. Symp. Rock Mech., 422427.

Ghosh, C.N., Ghose, A.K., 1992. Estimation of critical convergence

and rock load in coal mine roadwaysan approach based on

rock mass rating. Geotech. Geol. Eng. 10, 185202.

Gokceoglu, C., Aksoy, H., 2000. New approaches to the character-

ization of clay-bearing, densely jointed and weak rock masses.Eng. Geol. 59, 123.

Kear, D., Schofield, J.C., 1959. Te Kuiti Group. N. Z. J. Geol.

Geophys. 2, 685717.

Kendorski, F., Cummings, R., Bieniawski, Z.T., Skinner, E., 1983.

Rock mass classification for block caving mine drift support.

Proceedings 5th Congress of the International Society for

Rock Mechanics, Melbourne, Australia. Balkema, Rotterdam,

pp. B51 B63.

King, P.R., 1978. Sedimentology of the Waikato Coal Measures,

South Auckland, New Zealand. MSc Thesis, University of

Waikato. 280 pp.

Kirk, P.A., Sherwood, A.M., Edbrooke, S.W., 1988. Waikato Coal

Region: a summary of geology and coal resources. New Zealand

Geological Survey Record, vol. 34. DSIR, Lower Hutt. 28 pp.McEachran, D.B., 1986. StereoRThe stereographic program for

the Macintosh. Rockware, USA.

Mills, K.W., 1986. In-Situ Mechanical Behaviour of Huntly Coal.

School of Engineering Report No. 406. University of Auckland.

480 pp.

Mining Research and Development Establishment (MRDE), 1977.

NCB Cone Indentor. MRDE Handbook no. 5.

Moon, V., Russell, G., Stewart, M., 2001. The value of rock mass

classification systems for weak rock masses: a case example

from Huntly, New Zealand. Eng. Geol. 61, 5367.Newma n, D.A. , 1985. The design of coal mine roof support

for longwall mines in the Appalacian coalfield. PhD thesis,

Pennsylvania State University. 401 pp.

Newman, D.A., Bieniawski, Z.T., 1987. Modified version of the

Geomechanics Classification for entry design in underground

coal mines. Transactions of the Society of Mining Engineering,

vol. 280. AIME, New York, pp. 21342138.

Priest, S.D., 1985. Hemispherical Projection Methods in Rock

Mechanics. Allen and Unwin, London. 124 pp.

Priest, S.D., Hudson, J.A., 1976. Discontinuity spacings in rock.

Int. J. Rock Mech., Min. Sci. Geomech. Abstr. 13, 135148.

Romana, M.R., 1993. A geomechanical classification for slopes:

slope mass rating. In: Hudson, J.A. (Ed.), Comprehensive Rock

Engineering, vol. 3. Pergamon, Oxford, pp. 575599.

Schofield, J.C., 1978. Tertiary Stratigraphy, Auckland, South

Auckland, and Coromandel Range. In: Suggate, R.P., Stevens,

G.R., Te Punga, M.T. (Eds.), The Geology of New Zealand,

vol. 2. Government Printer, Wellington, pp. 449456.

Sen, Z., Sadagah, B.H., 2003. Modified rock mass classification

system by continuous rating. Eng. Geol. 67, 269280.

Tan, J.K., 1988. Geomechanics of Wongawilli extraction in Huntly

East Mine, Huntly, New Zealand. ME Thesis, University of

Auckland. 179 pp.

Trueman, R., Thin, I.G.T., Tyler, D.B., 1992. Rock mass classifi-

cation as an aid to estimating the strength of coal pillars. Pro-

ceedings of the 11th International Conference on Ground

Control in Mining, Wollongong, Australia, Australasian Instituteof Mining and Metallurgy, pp. 2229.

Unal, E., 1983. Design guidelines and roof control standards for coal

mine roofs. PhD Thesis, Pennsylvania State University. 355 pp.

Venkateswarlu, V., 1986. Geomechanics classification of coal mea-

sure rocks vis-a-vis roof supports. PhD thesis. Indian School of

Mines, Dhanbad. 251 pp.

Vutukuri, V.S., Hossaini, S.M.F., 1992. Assessment of the applica-

bility of strength criteria for rock and rock mass to coal pillars.

Proceedings of the 11th International Conference on Ground

Control in Mining, Wollongong, Australia, Australasian Institute

of Mining and Metallurgy, pp. 17.

White, P.J., Waterhouse, B.C., 1993. Lithostratigraphy of the Te

Kuiti Group: a revision. N. Z. J. Geol. Geophys. 36, 255266.

Yardley, W., 1994. Thick seam mining study. Unpublished Consul-tant Report Prepared for the Coal Corporation of New Zealand,

1994. 32 pp.

Yasar, E., 2001. A new rock mass classification for Coal Measures

Rocks. Eng. Geol. 62, 293300.


datos q

Documents