RockMassDescription-DescriçãoMaciçosRochosos

Quarterly Journal of Engineering Geology and Hydrogeology

doi: 10.1144/GSL.QJEG.1977.010.04.01 1977; v. 10; p. 355-388Quarterly Journal of Engineering Geology and Hydrogeology

by the Geological Society Engineering Group Working PartyThe description of rock masses for engineering purposes : Report

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Q. JIEngng Geol. 1977. Vol. 10. pp. 355-388, 4 figs, 10 tables. Printed in Great Britain

THE DESCRIPTION OF ROCK MASSES FOR ENGINEERING PURPOSES

Report by the Geological Society Group Working Party

Engineering

Contents I. Terms of reference 356

2. Introduction 356

3. Previous work 357

4. Requirements of a scheme of description 358 4.1 Identification of indices 358 4.2 Division of study 358

5. Rock material description 359 5.1 Choice of approach 359 5.2 Rock material indices 359

5.2.1 Scope 359 5.2.2 Rock type 360 5.2.3 Colour 361 5.2.4 Grain size 361 5.2.5 Texture and fabric 361 5.2.6 Weathered and altered state 361 5.2.7 Strength 362

6. Rock mass description 365 6.1 Rock mass indices 365 6.2 Discontinuities 366

6.2.1 Definition 366 6.2.2 Types 366 6.2.3 Numbers of discontinuity sets 366 6.2.4 Location and orientation 366 6.2.5 Spacing between adjacent

discontinuities 367 6.2.6 Aperture of discontinuity surfaces 367 6.2.7 Infilling 367 6.2.8 Persistence or extent 368 6.2.9 Nature of surfaces 368 6.2.10 Additional information 369

6.3 Weathered and altered state 369 6.4 Discontinuity spacing in

three-dimensions 370

6.5 Permeability (secondary) 371 6.6 Seismic velocity 371

7. Techniques for carrying out classification tests 372 7.1 Choice of method 372 7.2 Schmidt rebound hammer test 372 7.3 Point load test 373 7.4 Portable shear box 374 7.5 Slake durability test 375 7.6 Permeability tests 375

7.6.1 Packer or Lugeon test 375 7.6.2 Other permeability tests 376

7.7 Seismic survey 376

8. Techniques for obtaining data 377 8.1 Observations 377 8.2 Discontinuity surveys 378

8.2.1 Techniques available 378 8.2.2 Direct measurements 378 8.2.3 Measurements from drillholes 380

8.2.3.1 Core orientation 380 8.2.3.2 Down-hole methods 380

8.2.4 Surface photographic methods 381 8.2.5 Comparison of discontinuity

survey methods 381

9. Procedures for recording and presenting descriptive data on rock masses 9.1 Recording of data 9.2 Presentation of data

10. Conclusion

11. Membership of the working party

12. Acknowledgements

13. References

382 382 383

386

386

386

386

356 R E P O R T BY E N G I N E E R I N G G R O U P W O R K I N G P A R T Y

I. Terms of reference

The Geological Society Engineering Group Working Party on the Description of Rock Masses for Engineering Purposes was established to produce a report which would provide a format for the collection of descriptive data from the rock mass to permit assessment and hence rational design and construction of structures or excavations, on, or within, that mass. It was further intended that the report should facilitate communication in the description of rock masses betweent the various branches of geology concerned with mining and civil engineering.

2. Introduction

It is very necessary that standardisation should be achieved in the description and assessment of rock masses to facilitate the communication of facts between interested parties. It is thus essential that any system recommended should find wide acceptance.

Only the description of rock masses is considered in this report; assessment of the engineering properties and behaviour of rock masses is beyond the terms of reference. These aspects are specifically excluded both because of their wide range and because the type and scale of the engineering operation has considerable bearing on the way the mass should be assessed. As description forms the first step in the process of assessment it is essential that uniformity and consistency of description should be introduced into the subsequent process of rock mass assessment. It is recognised, however, that it may be possible to reduce this process of description by considering fewer parameters in an endeavour to produce a rock mass 'rating' as a quick guide to particular engineering performance.

For many years the discipline of soil mechanics has utilised the concept of index properties for discriminating between different types of soil in any given category, and classification tests for determining those index properties (Terzaghi & Peck 1967); this is now considered to be a fundamental procedure in soil mechanics. It is considered that it is equally necessary that a similar system should be introduced into the description of rock masses. This report proposes a system of rock mass description in which the rock mass 'indices' or parameters, analogous to the soil indices, are identified and described. Where possible a system of classification tests by which the indices may be determined is recommended. Some indices may only be indirectly quantified by testing, and others (e.g. colour) are amenable to description alone.

A rock mass may be considered as in situ rock material which has been made discontinuous by planes of weakness generally of natural origin such as joints, faults and bedding planes, which may be broadly referred to as discontinuities. The mass can therefore only be fully described by the use of various indices which define aspects of the rock mass together with indices which define the rock material. Many rock mass and material indices may be quantified either directly or indirectly by carrying out classification tests.

It is recommended that the description of a rock mass should aim to assemble data in a form amenable to subsequent processing and analysis. From this point onwards the data

T H E D E S C R I P T I O N O F R O C K M A S S E S F O R E N G I N E E R I N G P U R P O S E S 357

may be selected, collated, weighted or treated in any other way in order to establish new classifications or methods of assessment for the particular end-use required. If only selected indices are defined initially, as in the case of some rock mass rating systems, this greatly restricts the subsequent use of the data. The precise use to which the rock mass data might be put cannot always be established early on in the life of a project.

3. Previous work

There is a broad division between those authors who have considered rock mass description from the largely geological point of view and those authors who have been stimulated by practical need to rationalise the description of rock masses. Indeed, in recent years, the greatest stimulus has come from those rock mechanics engineers involved in underground problems. A long and continuing need to determine the ability of rocks to stand unsupported and to determine the type and degree of support needed, has produced many different schemes for rock mass classification.

Terzaghi (1946) was the first to attempt any type of rock classification for engineering purposes. He recognised the significance of discontinuities, their spacing and their filling materials, and used this information to define certain types of ground which would influence the load imposed on steel arches in tunnels. Stini (1950) and Lauffer (1958) carried on this work and related the 'stand-up' time in tunnels to rock quality. John (1962), Coates (1965) and Coates & Parsons (1966) further developed rock classifications and although these were useful it is considered that they placed insufficient importance on the geological aspects.

In 1964 Deere endeavoured to produce the first rock mass rating based on his 'Rock Quality Designation' index. This index determined from rock cores found wide practical application especially when considered along with other rock properties, although limitations on its use are recognised.

Detailed studies of rock material and rock mass parameters were made in the U.S.A. by Deere & Miller (1966) and Deere, Merritt & Coon (1969) with the particular purpose of rock assessment for military installations. Although more geological factors were taken into account than in previous studies they were less concerned with recording the total description of the rock than in recording the parameters most relevant to their engineering problems. These authors were the first to make extensive use of correlations between parameters.

Work has been carried out by several authors to obtain easily identifiable rock parameters from simple tests (Duncan 1966, Franklin 1970, Cottiss et al. 1971, Franklin et al. 1971, 1972). Correlations have been obtained between the test results and rock properties determined by more complex methods.

Piteau (1970) has described the significant factors in rock mass description for the stability of slopes cut in rock.

The search for rock classifications useful for underground work has continued with Bieniawski (1973, 1975) and Barton et al. (1974). These authors have tried to obtain classifications of jointed rock masses which depend on various weighted aspects of both the rock material and the rock mass. As their objective has been to obtain rock mass ratings which could be used for design, their classifications should be truly regarded as assessments.


Dearman (1974a, b) has considered rock mass description and has recommended a system largely based on that produced by the Geological Society of London Engineering Group (Anon. 1972). He has introduced a useful rock material classification as an aid to identification and has endeavoured to distinguish between different types of weathering. His paper represents the most extensive attempt to date to provide a multi-functional rock classification for engineering purposes and this report is in agreement with his approach in many respects.

The International Society for Rock Mechnaics has set up a series of working parties to report on the classification and characterisation of rocks and engineering design tests as part of their standardisation of laboratory and field tests. Draft copies of relevant sections (Category 1, 9 and 10, rock mass field observations) are currently available. Similar studies are also being made by the Institution of Civil Engineers in their revision of the British Standard Code of Practice CP 2001.

4. Requirements of a scheme of description

4.1 Identification of indices

The complete specification of a rock mass requires descriptive information on the nature and distribution in space of both the materials that constitute that mass (rock, soil, water and air-filled voids), and the discontinuities that divide up the mass. As the behaviour of the rock mass is determined in large measure by the type, spacing, orientation and characteristics of the discontinuities, accurate collection of data concerning them is of importance. In order to describe the mass it is necessary to decide which indices need to be measured. As stated earlier in this report it is recommended that all, or most, of the selected indices should be recorded. For many engineering purposes some parameters may be more relevant than others and some selection may be made. Combinations of parameters may be collected by means of some classification tests.

4.2 Division of study

The study of rock masses should be divided into two parts; a study of the rock material (often referred to in earlier literature as intact rock) and a study of the discontinuous rock mass. Some authors give scant attention to consideration of the rock material in rock description but it is emphasised that this important aspect must be treated adequately.

The rock material may be considered as a continuum or polycrystalline solid, consisting of a natural aggregate of minerals. The properties of the rock material depend on the physical properties of the constituent minerals and their type of bonding one to another (Deere & Miller 1966). Parameters which could be included in a description of the rock material are colour, grain size, texture, density, porosity, hardness, weathering, alteration, durability, minor lithological characteristics, strength, primary (or intrinsic) permeability, seismic velocity, modulus of elasticity, swelling, slake-durability, and petrological name. It is not necessary, however, to describe all these parameters.

T H E D E S C R I P T I O N OF R O C K MASSES FOR E N G I N E E R I N G P U R P O S E S 359

The rock mass may be considered as a discontinuum consisting of rock material rendered discontinuous by planes of weakness (or discontinuities). Parameters which could be included in a description of the rock mass relate to the nature and distribution in space of its structure (e.g. bedding, lamination) and discontinuities, as well as its strength, deformation modulus, secondary permeability (or hydraulic conductivity) and seismic velocity.

Whilst these studies result in a rock description, it is recommended that arbitrary distinctions between rock and soil should be disregarded and the system of description could be used for any discontinuous material, whether it be a jointed granite or cleaved slate, a fissured London Clay or a jointed glacial till. The generally accepted engineering distinctions between soil and rock-- 'soil is a natural aggregate of mineral grains that can be separated by such gentle mechanical means as agitation in water. Rock is a natural aggregate of minerals connected by strong and permanent cohesive forces'--(Terzaghi & Peck 1967) have no particular relevance in this context. The grades of compressive strength have therefore been continued below the arbitrarily accepted limit on rocks of 1.25MN/m ~ (MPa).

5. Rock material description

5.1 Choice of approach

Rock material may be described in two ways: (i) from a petrographic, or geological point of view, in which the prime consideration

is the mineral content of the rock, the interaction (physical/chemical) between the grains, and the processes which have affected the rock after its formation; or

(ii) in a manner which places the main emphasis on those aspects of the rock material which concern the engineering properties.

The first approach is rejected for the purposes of this report as it involves a detailed consideration of the petrography of the rock, much of which is considered to be incidental to rock mass description. Moreover, it provides little information on those rock properties important to engineering behaviour. However, it must be emphasised that a basic petrographic description is essential and it is strongly recommended that any petrographic data should be provided by a geologist although it is recognised that rock masses are often described for engineering purposes by engineers. Dearman (1974b) has endeavoured to satisfy this requirement by producing a classification of rock material to provide a basic minimum of information without utilising detailed petrographic descriptions.

5.2 Rock material indices

5.2.1 Scope. In addition to the petrographic name the rock material may be described in terms of other characters, some acquired, some inherent, and some inherent but modified by processes subsequent to lithification. Deere & Miller (1966) have examined fully the parameters of rock material description and have recommended a selection of useful


engineering index properties which can be easily determined. However, this report recommends that full material descriptions should be obtained rather than the few chosen by Deere & Miller.

The following indices provide a full description of the rock material.

Group 1 Rock type Colour Grain size Texture and fabric Weathering Alteration Strength

Group 2 Hardness Durability Porosity Density Strength Sonic velocity

Descriptive indices

Indices which can be determined by classification tests requiring little or no sample preparation

Group 3 Young's modulus of elasticity Poisson's ratio Primary permeability

Indices for design normally only determined by complex testing or requiring extensive sample preparation, or both

Properties included in Group 1 are purely descriptive. Group 2 properties may be determined by simple classification tests but direct results may not be obtained; thus some of these properties may be regarded as being only semi-quantifiable. Group 3 properties may be determined directly by testing and are quantifiable properties although the tests may only be carried out for design purposes.

Sections 5 and 6 of the report deal with those properties of rock material and mass respectively which can be described in the field. Section 7 deals with the classification tests. Design tests covered by Group 3 are considered to be beyond the scope of this report.

Although it is recommended that the data collected should be recorded on data sheets (Section 9) it will often be necessary to produce a general description of the rock mass. In this event it is proposed that the same scheme of description should be used as in the Geo- logical Society Engineering Group, Working Party Report on The Preparation of Maps and Plans in Terms of Engineering Geology (Anon. 1972), section, 5.2, i.e. suffixes to the rock name should relate to the main engineering properties.

5.2.2 Rock type. Because of the difficulties described in section 5.1 it is recommended that Dearman's scheme of petrographic description should be adopted. His classification has been amended slightly and is shown in Table 1. It should be used as a guide to providing a


broad petrographic name for the rock. Each major rock type has been given an identification number for use in recording on the data sheets.

5.2.3. Colour. The overall rock colour is difficult to quantify and unaided assessments can be very misleading. It is, however, a property which can be significant and therefore should be given due importance. A rock colour chart has been produced by the Geological Society of America (1963); the Munsell Soil Color Chart is obtainable from Tintometer Ltd., Waterloo Road, Salisbury, England. Colours are based on the choice of three diagnostic parameters: hue, chroma, and value. A simplified scheme (Anon. 1972) is shown in Table 2 and its use is recommended. A colour from Column 3 may be supplemented if necessary by a term from Column 2 or Column 1, or both.

5.2.4 Grain size. The grain sizes used in the description of rocks should be the same as those in widespread use for the description of soils as shown in Table 3.

5.2.5 Texture and fabric. The texture of a rock refers to the geometrical aspects of, and the mutual relationship among the component particles or crystals. Examples of these aspects are the size, shape and arrangement of the constituent elements of a sedimentary rock; the crystallinity and granularity of the constituent elements of an igneous rock.

The texture of a rock, therefore, refers to individual grains. The arrangement of grains is referred to as the rock fabric, which may show a preferred orientation. In sedimentary rocks, fabric is the orientation (or lack of it) in space of the elements (discrete particles, crystals, cement) comprising the rock. The term is used in igneous and other crystalline rocks for the pattern produced by the shapes and orientation of the crystalline and non- crystalline parts of the rock. It is dependent on the relative sizes and shapes of the parts and their positions with respect to one another and to the groundmass (which may be glassy) when present (Gary et al. 1972).

Terms frequently used include porphyritic, crystalline, crypto-crystalline, granular, amorphous and glassy.

Examination of the rock texture may require the use of a hand lens, or the microscopic examination of a thin slice of the rock.

5.2.6 Weathered and altered state. Weathering refers to those destructive processes, caused by atmospheric agents at or near the surface, that produce an in situ mantle of waste that has undergone little or no transport (saprolite). The effects may be separated into mechanical disintegration and chemical decomposition including solution (Dearman 1974a). Mechanical weathering results in the opening of discontinuities, the formation of new discontinuities by rock fracture, the opening of grain boundaries, and the fracture or cleavage of individual mineral grains. Chemical weathering leads eventually to chemical changes in the original minerals, often to form clay minerals; some minerals resist this action and survive unchanged. Early stages of chemical weathering result in discoloration of the rock material.

Alteration comprises those changes in the chemical or mineralogical composition of a rock produced by the action of hydrothermal fluids. Typical forms of alteration would be kaolinisation or mineralisation. It may be difficult to distinguish between the effects of weathering and alteration in some rocks, although the effects of weathering invariably die out in depth whereas alteration may have originated at very considerable depths within the rock m a s s .

Rock material tends to deteriorate in quality due to the effects of weathering and/or

362 R E P O R T B Y E N G I N E E R I N G G R O U P W O R K I N G P A R T Y

alteration. The effects of these changes may be detected by systematic measurements of parameters such as the strength of rock material or fracture spacing, but a qualitative assessment may be made on the basis of a visual estimate of the degree to which the rock material has been affected. Although weathering and alteration occur in the rock material, the processes are governed to a large degree by the discontinuities in the rock mass and can often only be appreciated on the scale of rock exposures or excavations (6.3).

5.2.7 Strength. It may not be essential to know the strength of the rock material with

TABLE 1" Rock type classification

GENETIC GROUP

Usual Structure

Composition

DETRITAL SEDIMENTARY

BEDDED

Grains of rock, quartz, feldspar

and minerals

At least 5000 of grains are of

carbonate

Very Grains are of rock fragments coarse

Rounded grains: grained 60 �9 t~ CONGLOMERATE (10) CALCI-

Coarse ~ Angular grains: RUD1TE grained 2 ~ BRECCIA (11) (21)

er

I SANDSTONE Grains are mainly "~ mineral fragments '~

~ QUARTZ SANDSTONE (12): 95% ~ quartz, voids empty or cemented

ARKOSE (13): 75%, quartz, upto250, ~ CALC- !~ ~ feldspar: voids empty or cemented ~ ARENITE Z :_ ~ ARGILLACEOUS SANDSTONE Z (22)

~ (14): 75~ quartz, 15% _L fine 0 0 detrital material.

0.06

0.002

Medium grained

Fine grained

Very fine grained

GLASSY

I~ MUDSTONE (15) ;u~ SHALE (16): fissile mudstone

SILTSTONE (17): 50% fine-grained CALCi- ~ particles SILTITE O CLAYSTONE (18): 50% very fine- (23) ~ grained particles ~ CALCAREOUS MUDSTONE (19) 2~ CALCI-

�9 < ,d LUTITE (24)

PYROCLASTIC

BEDDED . . . .

At least 50% of grains are of fine-grained

volcanic rock

Rounded grains A G G L O M E R A T E (31) Angular grains VOLCANIC BRECCIA

(32)

CHEMICAL/ ORGANIC

T U F F (33)

Fine-grained T U F F (34)

Very fine-grained T U F F (35)

SALINE ROCKS Halite (41) Anhydrite

(42) Gypsum

(43)

FLINT (45)

COAL (46)

OTHERS (47)

CHERT (44)


great accuracy as the strength of the mass is largely governed by the discontinuities within the mass. However, it is useful to know the rock material strength in assessing the shear strength of discontinuities (Barton 1974), and is essential when describing massive rocks without discontinuities.

Estimation of the mechanical strength of the rock material in a rock mass will generally require some form of laboratory or in situ test usually involving determination of the unconfined compression strength or point load strength, possibly supplemented by the Schmidt

METAMORPHIC IGNEOUS

FOLIATED MASSIVE MASSIVE ..

Quartz, feld- spars, micas, acicular dark

minerals

MIGMATITE (51)

GNEISS (52) Alternate layers of granular and flakey minerals

SCHIST (53)

PHYLLITE (54)

SLATE (55)

MYLONITE (56

HORNFELS (61)

MARBLE (62)

GRANULITE (63)

QUARTZITE (64)

AMPHIBOLITE (65)

Light coloured minerals are quartz, feldspar, mica and feldspar-like minerals

Acid rocks Intermediate rocks

PEGMATITE (81)

GRANITE (71)

MICRO-GRANITE (72)

RHYOLITE (71)

DIOR1TE (82)

MICRO-DIORITE (83)

ANDESITE (84)

OBSIDIAN and PITCHSTONE (74) (85)

Basic rocks

..

GABBRO (92)

DOLERITE (93)

BASALT (94)

TACHYLYTE (95)

Dark minerals

., Ultrabasic rocks

PYROXENITE (o~) and

PERIDOTITE (02)

SERPENTINE (03)

364 REPORT BY ENGINEERING GROUP WORKING PARTY

1

light dark

TABLE 2: Rock colour

2 3

pinkish pink reddish red yellowish yellow brownish brown olive olive greenish green bluish blue

white greyish grey

black

TABLE 3 : Grain size

Term

Retained on B.S. Particle Sieve No. (approx.

Size equivalent Equivalent Soil Grade

Very coarse-grained > 60 mm 2 in Coarse-grained 2-60 mm 8 Medium-grained 60 microns-2 mm 200 Fine-grained 2-60 microns Very fine-grained < 2 microns

Boulders and Cobbles Gravel Sand Silt Clay

Note: grains > 60 microns diameter are visible to the naked eye

TABLE 4: Rock material strength

Term

Unconfined compressive

strength MN/m 2 (MPa) Field estimation of hardness

Very strong > 100

Strong 50-100

Moderately 12.5-50 strong

Moderately weak 5.0-12.5

Weak 1.25-5.0

Very weak rock or hard soil 0.60-1.25

Very stiff 0.30-0.60* Stiff 0.15-0.30 Firm 0.08-0.15 Soft 0.04-0.08 Very soft <0.04

Very hard rock--more than one blow of geological hammer required to break specimen. Hard rock--hand held specimen can be broken with single blow of geological hammer. Soft rock--5 mm indentations with sharp end of pick.

Too hard to cut by hand into a triaxial specimen. Very soft rock--material crumbles under firm blows with the sharp end of a geological pick.

Brittle or tough, may be broken in the hand with difficulty. Soil can be indented by the finger nail. Soil cannot be moulded in fingers. Soil can be moulded only by strong pressure of fingers. Soil easily moulded with fingers. Soil exudes between fingers when squeezed in the hand.

* The compressive strengths for soils given above are double the unconfined shear strengths.

T H E D E S C R I P T I O N OF R O C K MASSES F O R E N G I N E E R I N G P U R P O S E S 365

rebound value. The point load test is very useful in this respect and has been correlated with compressive strength (7.3). The strength of rock material determined in unconfined compression is dependent on moisture content of the specimen, anisotropy of the material, and the test procedure used. Where rock strengths are not measured, they can be estimated by following the modified scheme of Piteau (1970) given in Table 4. Such field estimates can only be very approximate and the actual criteria used must be established on the basis of individual experience.

6. Rock mass description In addition to description of the rock material, description of rock in the mass involves the determination of structure and the distribution of the different rock types present. It also is concerned with the description of the weathering profile and the distribution of any effects due to alteration.

In contrast to texture and fabric which are features of the rock material, s t ruc tu re refers to the larger features of the rock mass that can only be examined at outcrop or in artificial excavations. It includes such features as bedding, foliation, flow banding, jointing, cleavage, and brecciation, and their interrelationships.

6.1 Rock mass indices

When the rock material has been fully described it is necessary to recognise and describe those features dividing the mass. The following indices of the mass may be measured directly in the field or can be obtained by the use of relatively simple classification tests. They are grouped in a similar way to the rock material indices:

Group 1 Discontinuities

type number of discontinuity orientations location and orientation frequency of spacing between discontinuities aperture or separation of discontinuity surfaces persistence and extent in filling nature of surfaces additional information

Weathered and altered state

Descriptive indices


Group 2 Permeability (secondary)* Seismic velocity* Shear strength*

Indices which can be determined by relatively simple classification tests

Group 3 Modulus of deformability Permeability (secondary)* Seismic velocity* Shear strength*

Indices for design normally only determinable by complex testing.

* Where indices have been included in both Groups 2 and 3 it is because at the design stage tests may be repeated by an alternative method to gain more reliable data on the same index.

6.2 Discontinuities

6.2.1 Definition. A discontinuity is considered to be a plane of weakness within the rock across which the rock material is structurally discontinuous and has zero or low tensile strength, or a tensile strength lower than the rock material under the stress levels generally applicable in engineering. Thus a discontinuity is not necessarily a plane of separation.

6.2.2 Types. Discontinuities may have a wide variety of origins and forms but they tend to fall into two basic types; those that occur in sets or systems, e.g. joints, cleavages, bedding planes, and which are amenable to statistical analysis; and those that are unique, e.g. faults, and have to be considered on an individual basis. Where possible it is desirable to differentiate between the origins of the various types of discontinuity because the engineering properties may be strongly related to the genesis. Goodman & Duncan (1971) stress the importance of this especially in respect of those planes formed by extension and those by shear. The major types of discontinuity are faults, joints, fissures, cleavages, schistosity, shear planes, tension cracks, foliations, bedding planes, lamination and veins.

6.2.3 Number of discontinuity sets. The presence of discontinuities within a rock mass is likely to change its properties,e.g, reduce its strength or increase its deformability or permeability. The degree to which this occurs depends not only on the direction of the discontinuities but also on the number of sets present and their spacing. The number and orientations of intersecting sets or unique discontinuities govern the ability of the rock to deform or fail without involving fracture of rock material.

Discrete orientations of discontinuities, or sets of discontinuities, may be recognised visually in the field, or alternatively may only be apparent when later plotted out onto a stereogram.

6.2.4 Location and orientation. Sufficient information is required to locate each discontinuity in space. This may consist of ground co-ordinates plus elevation or the relative position along a fixed datum line. Information should preferably be recorded visually on a map or plan.

The orientation of each discontinuity surface should be expressed in terms of its dip and dip direction. The use of strike rather than dip direction should be discouraged.

Dip and dip direction is normally measured with a clinometer and compass and should

THE D E S C R I P T I O N OF R O C K MASSES FOR E N G I N E E R I N G P U R P O S E S 367

be expressed to the nearest degree. Since the majority of natural discontinuity surfaces are rough, a significant amount of scatter may be expected to occur in the measurements if these are made over a small area of the discontinuity surface. Some of this scatter can be avoided if desired by using a 200 mm diameter aluminium measuring plate laid on the discontinuity before any measurements are made. Particular care is required in measuring the orientation of discontinuities which dip at a low angle.

6.2.5 Spacing between adjacent discontinuities. The spacing between adjacent discontinuities should be measured by counting the number of discontinuities which cut a traverse line of known length and expressed as a mean fracture spacing in metres or milli- metres (as in core logging). The mean and range of spacings between discontinuities in each set should also be measured and recorded. In order to allow for sampling bias resulting from measurements along a single line, measurements should ideally be made along three mutually perpendicular axes.

The descriptive terms for discontinuity spacing in Table 5 are recommended. Although a separate set of terms for bedding spacing has generally been used, it is felt that there should be one common set of terms for both bedding and all other forms of discontinuity spacing.

TABLE 5: Discontinuity spacing

Term Spacing

Extremely wide > 2 m Very wide 600 m m - 2 m Wide 200-600 mm Moderate ly wide 60-200 mm Moderate ly narrow 20-60 mm Nar row 6-20 mm Very narrow < 6 mm

6.2.6 Aperture of discontinuity surfaces. The degree to which a discontinuity is open, or to which the faces of the discontinuity have been separated and the space subsequently in- flled (such as in a vein or fault) is of major importance (Piteau 1970, Bieniawski 1973). The mean separation should be measured and described using the terms in Table 6.

TABLE 6 : Aperture of discontinuity surfaces

Aperture (discontinuities) Thickness (veins, faults)

Wide > 200 mm Moderate ly wide 60-200 mm Moderate ly nar row 20-60 mm N a r r o w 6-20 mm Very nar row 2-6 mm Extremely narrow > 0-2 mm Tight Zero

6.2.7 InfiHing. The infilling between discontinuity surfaces may be gouge or breccia in the case of faults, or materials introduced into the opening, for example clay and other soils,


calcite and other mineral matter. The resistance to sliding along a discontinuity can be either increased or decreased depending on such factors as the degree of separation, the thickness of the infilling, the nature (type and hardness/consistency) and the character of the discontinuity walls. If, for example, the infilling is sufficiently thick, the discontinuity walls will not touch and the strength of the discontinuity will be that of the infiUing material.

The thickness of the infilling is taken as the width of the material separating the host rock surfaces. Measurements are made in the same way as for discontinuity aperture (Table 6). H o e k & Bray (1974) suggest that the influence of the infilling is a function of the relationship between its thickness and the amplitude of the asperities on the discontinuity walls. The influence of the wall rock on the strength of the discontinuity may not become insignificant until the infilling is over 100 mm thick.

The nature of the infilling material should be identified and a complete engineering geological description given according to the recommended system for the description of rock and soil.

The unconfined compressive strength of the infilling material should be either assessed visually according to the system described in Anon. 1972, or should be measured using a pocket penetrometer or pocket vane tester in the case of soils and a point load tester or Schmidt Hammer in the case of rocks.

6.2.8 Persistence or extent. Persistence is an expression of the extent to which any particular discontinuity can be traced. It may thus give some measure of the percentage of the rock material which would have to be sheared during failure along a surface. Alterna- tively it may demonstrate the degree to which failure would have to take place along en echelon joints, or stepped surfaces.

Persistence is one of the most important factors in discontinuity description and is also one of the most difficult to quantify. Often, particularly in the case of major joints, the planes are continuous beyond the confines of the rock exposure and it may be impossible to estimate their actual persistence. It is recommended that the maximum trace length should be measured. Some comment may be made on the data sheet to indicate whether the total trace length can be seen, and whether the discontinuity terminates in solid rock or against another discontinuity.

6.2.9 Nature o f surfaces. The nature of a discontinuity surface may be considered to have three aspects, viz. waviness, roughness and condition of the walls. Waviness and roughness differ from each other in terms of scale and their effect on the shear strength of the discontinuity or the characteristics of the discontinuity during shearing (Piteau 1970) and will depend to a large degree upon the normal stress across the plane. Waviness refers to those first order wall asperities which appear as undulations of the plane and would be unlikely to shear off during movement. Roughness refers to second order asperities of the plane which because they are sufficiently small would be sheared off during movement.

The effects of waviness do not change with displacements along the discontinuity surface since no shearing takes place through the asperities. Waviness therefore modifies the apparent angle of dip but not the frictional properties of the discontinuity. On the other hand, increased roughness of the discontinuity walls results in an increased effective friction angle along the discontinuity; however these effects are found to diminish or disappear in the presence of infilling.

Waviness may be measured by means of a standard tape or rule placed on the exposed


discontinuity surface in a direction normal to the trend of the waves and this direction should be recorded. In this way the mean wave length and maximum amplitude are determined. Fecker & Rengers (1971) have described an alternative method in which a large number of dip angle and dip direction measurements are made. The mean scatter of these measurements gives a measure of the waviness angle.

A variety of techniques are used to measure roughness. A seven point visual classification modified from the table given by Piteau (1970) is recommended (Table 7) for use when quantitative measurements are not made.

It should be stressed that such a table is meaningful only when the direction of the irregularities in the surface is in the least favourable direction to resist sliding. It can be envisaged that the angle of frictional resistance along a surface being sheared normal to the axis of slickensiding would be higher than along a surface with defined ridges where the movement was parallel to the ridges. It is therefore necessary to specify the trend of the lineation on the surface of the discontinuity in relation to the direction of shearing.

TABLE 7: Roughness categories

Category Degree of roughness

1 Polished 2 Slickensided 3 Smooth 4 Rough 5 Defined ridges 6 Small steps 7 Very rough

In order to ensure such a classification is as objective as possible it is recommended that at each site where it is used typical examples of each category should be identified and pho to graphed to maintain uniformity of assessment. Quantitative measurements of roughness using a profilometer or photogrammetric methods have been described by Ross-Brown & Walton (1975).

The condition or strength of the rock material forming the walls of the discontinuity can have a significant effect on the shear strength of the discontinuity especially where the infilling is thin or absent and roughness becomes of importance. The rock material should be described or measured as recommended in Section 5. Where the effects of weathering have penetrated deeply inwards from a discontinuity, a wide weak zone may be present.

6.2.10 Additional information. Any information considered relevant, but not covered in any of the other sections is recorded in this section. This may include such information as the presence of groundwater seepages, water levels in wells or other excavations, the presence of swelling materials, indications of recent or past movement or instability.

6.3 Weathered and altered state

The terms weathering and alteration have been defined in section 5.2.6 in terms of their effects on rock material. The weathering of the rock mass may be described relative to the distribution of the weathered materials within it and the effect of weathering on discon-


tinuities. It may only be possible to appreciate weathering profiles f rom recently formed natural exposures, e.g. along shorelines, or from artificial excavations.

The generalised descriptive terms for a scale of weathering grades of the rock mass given in Table 8 are recommended. The terms given are general and may be modified to suit part icular situations; subdivision may be necessary. It should be noted that this scheme is not applicable to all types of rock masses and ad hoe schemes such as those devised for Keuper Marl (Chandler 1969) and chalk (Ward et al. 1968) may be necessary.

It should be noted that all grades of weathering may not be seen in a given rock mass, and that in some cases a part icular grade may be present to a very small extent. Distr ibution of the various weathering grades of rock material in the rock mass may be related to the porosity of the rock material and the presence of open discontinuities of all types in the rock mass. Examples of 'fossil ' weathering overlain by fresh younger material, as in some volcanic sequences, may be encountered.

In logging cores the distribution of weathering grades of rock material may be recorded; distr ibution of weathering grades of the rock mass from which the cores were obtained has to be inferred from this type of evidence.

TABLE 8" Weathering~alteration grades

Term Description Grade

Fresh Faintly weathered Slightly weathered

Moderately weathered

Highly weathered

Completely weathered

Residual soil

No visible sign of rock material weathering. IA Discoloration on major discontinuity surfaces. IB Discoloration indicates weathering of rock material and discontinuity I1 surfaces. All the rock material may be discoloured by weathering and may be somewhat weaker than in its fresh condition. Less than half of the rock material is decomposed and/or disintegrated III to a soil. Fresh or discoloured rock is present either as a continuous framework or as corestones. More than half of the rock material is decomposed and/or disintegrated IV to a soil. Fresh or discoloured rock is resent either as a discontinuous framework or as corestones All rock material is decomposed and/or disintegrated to soil. The V original mass structure is still largely intact. All rock material is converted to soil. The mass structure and material VI fabric are destroyed. There is a large change in volume, but the soil has not been significantly transported.

6.4 Discontinuity spacing in three dimensions

Although the shape and size of the blocks of rock material formed by the discontinuities can be defined by the spacings and orientations of the discontinuities, a series of general descriptive terms are also recommended.

The following terms define shape:

blocky - - approximately equidimensional tabular - - one dimension considerably shorter than the other two columnar - - one dimension considerably larger than the other two

THE DESCRIPTION OF ROCK MASSES FOR ENGINEERING PURPOSES 371

The orientation of the long or short dimensions should be specified in tabular and columnar blocks respectively. Additionally it may be helpful to state the ratio of the orthogonal dimensions, e.g. 1 vertical: 2 north: 6 east. The block size may be described using the terms in Table. 9.

TABLE 9: Block size

Block Term size

Equivalent discontinuity spacings

in blocky rock

Very large > 8 m a Large 0.2-8 m a Medium 0.008--0.2 m 3 S m a l l 0.0002q3.008 m 3 Very small < 0.0002 m 3

Extremely wide Very wide Wide Moderately wide Less than moderately wide

6.5 Permeability (secondary)

The permeability of a rock mass (i.e. secondary permeability) results from flow through discontinuities or solution voids under the influence of hydraulic pressure. A great variety of techniques have been described for measurement but it is beyond the scope of this report to consider methods in any detail. However. the most common quick method for determining rock mass permeability is the Lugeon or packer test which is considered in Section 7.6.1. The permeability of the rock material, the primary permeability, is generally often small in comparison with the secondary permeability and may not need to be considered for rock mass description. It is usually necessary to measure primary permeability in the laboratory.

Although the secondary permeability is much affected by the size of the apertures and degree of infilling of the discontinuities, some estimate of the permeability can be made based on the frequency of discontinuities. However, any such estimates should be treated with extreme caution and cannot be applied to rocks susceptible to solution. It should be emphasised that there is no adequate substitute for in situ permeability testing. (See Section 7.6).

6.6 Seismic velocity

Seismic velocity normally refers to the velocity of propagation of pressure waves through a rock mass.

Unlike many other indices seismic velocity is not an independent variable; it is a function of many rock properties including density, porosity, mineral composition, cementation and degree of fracturing.

In the case of a fractured rock mass it is often possible to correlate the seismic velocity with variations in the degree of fracturing throughout the rock mass. Examples of this are described by Grainger et al. (1973) in a study of the Chalk at Mundford and by Knill (1970) in a study of grout-take in rock. It is also possible to obtain an index of fracturing based on the ratio of the laboratory and field seismic velocity values (see Section 7.7).


7. Techniques for carrying out classification tests

7.1 Choice of method

This report is concerned with description of rock masses and an arbitrary line has had to be drawn between classification tests which may commonly be used to describe a rock mass (including the rock material) and more specific design tests; some classification tests may, however, also be used as specific design tests. The suitability of any test for the purposes of general description is governed to a large degree by the interrelated factors of cost, speed, convenience and the nature of the engineering project.

The following recommended classification tests have been found to provide quantitative or semi-quantitative data rapidly and at low cost. Some of the field tests, however, require considerable organisation of equipment and staff and thus must be the subject of an operation separate from the general collection of geological data. Their singular advantage lies in the large number of tests which can be carried out to give a better indication of the variability of the rock than might be achieved by more subjective methods; they generally yield indirect rather than direct results. Care must be taken that these results are not used in isolation from other rock mass indices.

7.2 Schmidt rebound hammer test

The Schmidt Rebound Hammer determines the hardness of a rock material by measuring the degree to which a steel hammer will rebound from a prepared rock surface. A 'rebound number' is indicated which has been shown to correlate with uniaxial compressive strength after taking dry density of the rock into account. (Hucka 1965, Deere & Miller 1966, Hendron 1968, Dearman 1974). Typically, however, there is only a 75 per cent probability that the laboratory determined uniaxial compressive strength will lie within 50 per cent of the strength determined from the correlation charts prepared by Deere & Miller and re- produced with minor modification as Fig. 1.

The equipment is light and easy to use in the field but care must be exercised in selecting the places at which the measurements are made. It is necessary for normal use that surface coatings and unevenness should be removed before testing. However, where it is desired to obtain the strength of the rock forming the wall of the discontinuity, no preparation should be carried out. At least ten readings should be taken at each place after disregarding off- shots which deviate from the mean by more than five rebound units. The method is only applicable to rocks in the strong to very strong strength range.

It should be emphasised that the test relates to a rock material index and that it is essential to ascertain that the results are not being influenced, for example, by local discontinuities, weathering effects or large crystals.

T H E D E S C R I P T I O N OF R O C K M A S S E S F O R E N G I N E E R I N G P U R P O S E S 373

"E

e-

E o

Dispers;on MN/m=

o +1 +1 +1 +1 +1

t

/ 200 i !

150 ,,- - - - - -

100 9C

8g

7O

60

50 L :

20 5

/

9",,.5,

10 15 20 25 30 35 40

Schmidt Hardness, R (L -Hammer)

/;r / , /

Z,

"/// / X . ~-

J/HI'l,."

I I 1 I I I

45 50 55 60

FIG. 1. Rock strength chart based on Schmidt hardness using the L. Hammer (after Deere & Miller 1966). Note, hammer vertical downwards, and dispersion limits defined for 75% confidence.

7.3 Point load test

A second method for determining rock material strength is the point load test which enables a large number of determinations to be made in the field (Franklin, Broch & Walton 1971, Broch& Franklin 1972, Bieniawski 1973 & 1975), particularly on rock cores.

The core is usually loaded to failure between two points across a diameter, but other directions may also be tested. For the diametral test the length of the core should be at least 0.7 times the diameter. If a length of core is tested along the core axis the length of the core should be approximately 1.1 times the diameter. If core is not available an irregular lump test may be performed with loading along the longest axis which should be about 50 mm long compared with the shortest axis of about 40 mm. Results from the latter type of test are likely to display considerably more scatter than those where core is used. Sufficient tests should be performed to give a statistically meaningful result.

The uncorrected point load index Is is obtained by dividing the force at failure by the square of the length of the loaded axis. Various corrections may be made to this figure for shape and size effects and if such corrections are made their nature should be stated. Follow- ing Bieniawski (1975) this report recommends that results should be expressed as equivalent


compressive strengths ~c~ using the empirical conversion factor which varies with the length of the axis loaded during the test (Fig. 2) for the range shown. The method of testing and the source of the correlation factor should, however, be quoted.

A significant strength reduction is sometimes observed with increasing moisture content. Except in special circumstances, such as when dealing with rocks from an arid environment, or with argillaceous specimens which have previously been dried, tests should be carried out as far as practicable with water saturated specimens. In practice, however, there is usually relatively little strength difference between fully and partially saturated specimens.

It is envisaged that this test would be used for the majority of cases when only the compressive or tensile uniaxial strength of the rock material is required.

7.4 Portable shear box

A portable shear box has been developed for use in the field in order to obtain quickly and simply index shear strengths along discrete discontinuities or prepared surfaces (Hoek & Bray 1974, Ross-Brown & Walton 1975). Advantages of the apparatus are that many tests can be done, and repeated, all within a limited budget, allowing a more representative range of results to be obtained than in the conventional in situ test.

There are several relatively simple portable shear boxes commercially available which permit testing of specimens up to 140 mm cube, or cores up to 140 mm diameter. The apparatus consists of a split box in which the two halves of the specimen are mounted so that the discontinuity is parallel to the split. One hydraulic ram provides a normal force and another the shear force. A third ram may be provided to permit reversal of the shear direction. Displacements are measured using dial gauges and a total cumulative displacement of 100 mm is normally required to obtain the residual shear strength along a surface; this

O~ '

20 0 0 60

Length of Loaded Axis d (ram)

FIG. 2. Size correlation chart for point load strength to unconfined compressive strength conversion (Oc should read acs).


involves several displacement cycles. In dealing with irregular discontinuity surfaces a correspondingly irregular load-displacement relationship will be obtained, sometimes lead- ing to uncertainty about the proper value for the loads, and in such cases a range of friction angles should be calculated.

7.5 Slake durability test

The durability of a rock, or its ability to withstand weathering processes likely to cause short-term disintegration or chemical breakdown, can be estimated by the slake durability test. Although it is often possible to predict durability from a study of the mineralogy and micro-structure of the rock it is considered to be quicker and more reliable to carry out a test.

After giving consideration to the large variety of techniques available to assess that property, Franklin & Chandra (1972) developed an apparatus for measuring a slake durability index. Their method involves the selection of ten rock samples weighing 40-60 grams each which are subjected to oven-drying and wetting cycles. After oven-drying the rock is placed in a standard test drum half immersed in water and rotated; slaked fragments pass through the 2 mm meshed exterior of the drum. After ten minutes rotation at 20 r.p.m, the sample is again oven-dried, and weighed to determine the loss in weight. The ratio of the weights of the dry material retained to that in the drum at the start of the cycle is known as the slake durability index. Results are claimed to be reproducible to within approximately 5 per cent (using consistent samples) and relate closely to in situ observations of weathering performance. The second cycle slake durability index is recommended as a standard for purposes of classification but samples with second cycle indices from 0-10 per cent should be further characterised by their first cycle index and by their plasticity index. The initial and final rock materials should be described.

The test can only be undertaken within reach of transport in the field and is best carried out in a laboratory. It is quick, cheap and reliable. It should be regarded as a classification test only and not a comprehensive test for durability. There is undoubtedly significant abrasion of weaker rocks; also breakdown, which may occur as the result of causes other than wetting, drying and abrasion, is not covered by the test.

7.6 Permeability tests

The U.S. Department of the Interior Bureau of Reclamation (1974) has published detailed information on permeability testing.

7.6.1 Packer or Lugeon test. The classification test generally utilised for rock mass permeability determination is the packer or Lugeon test sometimes called a water absorption test, so-called because its use was first recommended by Lugeon (1932). This test is carried out in a drillhole in which a single or double packer is inflated by mechanical, pneumatic or hydraulic means to seal off a section of the rock. Water is pumped into the test section under pressure and after allowing for the saturation of the ground the loss of water over a specified period is recorded. By definition, the test should be carried out over a one metre test section at a pressure of 10 kg/cm 2 (145 lb/in2). A rate loss of 1 litre/minute is said to equal one


kugeon unit (or approximately a permeability of 10 -7 m/sec). The standard size of hole is NX but it has been shown that, within the limits of accuracy of the test, the size of hole is not critical.

The test has wide application for providing an initial indication of permeability. The main constraints are as follows: only clean water must be used; allowance must be made for the static head applied by the water column above the water table; a correction should be made for head losses in the pipes; the test must be carried out in stages utilising increasing and decreasing pressures because of silting up, or washing out, of the filling in discontinuities; pressures must not be so high as to cause hydraulic fracture.

Because hydraulic fracture may occur when the applied water pressure exceeds the weight of overburden, pressures lower than those implied by the definition of the Lugeon test must be used at shallow depths. Extrapolation of the results permits the Lugeon value to be determined.

7.6.2 Other permeability tests. Rising, falling or constant head permeability tests, although generally used to determine the permeability of soils, may also be used for the determination of rock mass permeabilities. Specific testing for aquifer properties are considered to be beyond the scope of this report.

Rising and falling head tests involve adding water to a hole or bailing out water from the standing water column in a hole, and measuring the change of level with time. In the case of the constant head test (more suitable for measurement in formations of low permeability) an applied static head of water is kept constant by means of the addition of a measured flow of water using a floating ball valve arrangement.

Although generally quick and cheap to carry out, the tests have a number of inherent disadvantages: they are unable to apply the pressures to the rock mass similar to those which may subsequently be applied by the engineering structure; they may test only a small volume of rock; they depend on the casing to seal off the upper sections of the drillhole, often in- effectively; and the formation may silt up with the falling head test, or internal erosion may occur with the rising head test. However, their advantage is that many tests can be carried out within a short time and an approximate order of magnitude of rock mass permeability can be obtained.

The theories developed for the different conditions of flow in the above tests are based upon the various assumed geometries of the borehole, casing and formation (Cedergren 1967). Thus the results may not be representative of the in situ conditions. The calculated results therefore require careful consideration.

7.7 Seismic survey

Seismic surveys for engineering purposes generally use refraction techniques which are particularly applicable for investigation of the rock mass to shallow depths. These may be used in conjunction with laboratory determinations of the sonic velocity in the rock material as noted in 6.6.

A basic seismic refraction survey (Griffiths & King 1965) consists of transmitting an acoustic pulse into the rock mass and recording the energy received at a geophone array positioned on the surface of the mass at various distances from the acoustic source. In a


typical case, a falling weight is used to generate an acoustic pulse, which is detected by a 12- geophone receiving array coupled to a seismograph, producing a direct print of the received signals. These records may be used to pick out the direct and refracted arrivals and from these results travel time graphs can be constructed. In the ideal case, the travel time graph will consist of a number of straight line sections; the slope of each section corresponds to a velocity of sound characteristic of one particular stratum. From the graph it is possible to compute the depth to each stratum using standard intercept theory. More complex analysis may be required; for example a model having a smooth increase of velocity with depth may be used to investigate certain weathering profiles.

Simplified surveys in which only the first compression wave arrival is recorded are often utilised for determining the velocity of the overburden or depth to bedrock. More complex surveys utilising explosives or multi-geophone arrangements are not covered in this report.

When required for use in conjunction with the sonic velocity in the rock mass, the sonic velocity in the rock material is normally determined by measuring the travel time of an acoustic pulse through a machined rock specimen. In a typical laboratory test, the sample has a nominal diameter of 40 mm and a length of approximately 80 mm with the sample ends ground flat and parallel. The acoustic pulse transmitted through the sample is detected by a receiver, the transmission time recorded and the velocity calculated from the time and length of the specimen. The sample should first be tested in a dry state and then water- saturated. The temperature of the sample should also be recorded.

8. Techniques for obtaining data

8.1 Observations

Observations may be made singly (point observations) or in groups (area observations). It is often desirable to divide a rock mass up into zones of well-defined characteristics from a number of point observations. When the zones have been thus identified, area observations may be undertaken fully to describe the mass within the various zones.

Rock mass zoning may be based on a large number of different parameters depending on the nature of the engineering problem. The most common parameters are likely to be lithology, rock strength, weathering and/or alteration, discontinuity frequency and rock quality. Other parameters such as permeability could be included.

Rock mass zones based on lithology are commonly identified directly by geological mapping or from the interpretation of air photographs. The differences in lithology may or may not have engineering significance; in fact lithological zones have proved to be positively misleading in some cases (Dearman 1974). It is likely that lithology would be only one parameter of several considered in the description of a zone and need not be diagnostic of the engineering characteristics.

If several parameters are chosen as the basis for zoning, areas of varying rock quality can be determined. The most widely accepted measure of quality is RQD (Rock Quality Designation) recommended by Deere (1964). RQD is defined as the percentage of the total core in a drill-run which is formed by fully circular core of length greater than 100 mm (originally 4 in). It has been found to correlate with some other indices (Deere, Merritt &


Coon 1969). However some correlations, especially those interrelating RQD, seismic velocity and rock quality, should be treated with caution. Nevertheless the measurement of RQD can represent a rapid method of zoning a rock mass so that subsequent area observations can be made. Although RQD is generally measured on core it has also been adapted to surface outcrops and tunnel exposures by using the correlation between discontinuity spacing and RQD (Deere, Merritt & Coon 1969). Based upon a mean discontinuity frequency per metre (A), Priest & Hudson (1976) derived a relationship giving a theoretical equivalent value of the RQD (RQD*) viz. RQD* ~ 100e -~ (0.1A + 1).

8.2 Discontinuity surveys 8.2.1 Techniques available. Within any particular area it may be necessary to carry out

surveys to obtain statistically representative information on discontinuities. Three main techniques may be used:

(i) direct measurements (ii) measurements from drill holes

(iii) surface photographic methods

8.2.2 Direct measurement. Direct measurement on the ground is perhaps the most widely used method for collecting discontinuity data. There are two basic levels at which a survey may be carried out depending upon the amount of detail that is required : a subjective survey in which only those structures which appear to be important are measured and recorded, and an objective survey in which all structures intersecting a fixed line or area of the rock face are measured and recorded.

Any system of measurement of discontinuities which relies on the judgement of the geologist to recognise the relative importance of the discontinuity sets can greatly reduce the volume of data and allow the effort to be concentrated on the apparently significant sets. However in this way there is always the risk of missing or discounting sets which nevertheless might be of importance. The risk is greatly reduced when techniques are used in which all the discontinuities intersecting a face of limited size or a line of limited length are sampled. Before any discontinuity survey is initiated, conventional geological mapping of the area must be carried out to determine rock type, delineate major geological structures such as faults, dykes, lithological contacts, and any other features that form major discontinuities in the rock mass. It is only after becoming familiar with the geology that the most efficient and accurate way of conducting the discontinuity survey can be defined.

Several different methods have been adopted in the past for carrying out systematic discontinuity surveys. Muller (1959) and Pacher (1959) recorded discontinuities that occurred on two mutually perpendicular surfaces. Weaver & Call (1965) and Halstead et al. (1968) used a technique which they called fracture set mapping, which involved recording all discontinuities occurring in 6 m by 2 m bands spaced at 30 m intervals along a face. Da Silveira et al. 0966) suggested that sampling should consist of choosing portions of the surface areas (squares, rectangles or circles) to form a mesh in such a way that the dimensions and distributions of these areas ensured adequate representation. Similarly Knill (1971) ad- vocated the use of area sampling on a rock face. Piteau (1970), Robertson (1970) and Broad- bent & Rippere (1970) all preferred a line sampling method which in the latter case consisted


of stretching a measuring tape at waist height along the exposed face and recording the location and features of interest of every discontinuity that intersected the tape. Measuring distances of 30 m were used and it was suggested that to ensure complete representation the survey should be continuous, i.e. should not be terminated at intervals.

The line sampling approach appears superior to other types of sampling techniques and gives more detail than other methods on the discontinuity intensity and attitude variability. It is also indiscriminate in that all discontinuities that intersect the tape, whether large or small, are recorded. To ensure that all discontinuity sets are recorded, line sampling on three approximately mutually perpendicular axes would be necessary, but this is rarely possible in practice.

When sampling is undertaken on a single line only, or on a planar rock face, errors associated with non-random sampling may arise due to the fact that the frequency of discontinuities lying in approximately the same plane as the face may be under estimated. In such cases it is possible to apply corrections to compensate for the sampling bias (Terzaghi 1965, Robertson 1970). However, these corrections, because they assume the rock face on which the measurements are carried out is planar, tend to over-correct the results since natural and artificial rock faces are invariably somewhat irregular. It is therefore recommended that where non-random sampling is believed to occur, all data should be presented uncorrected, but that measurements made on differently orientated faces should be compared to determine whether there is any sampling bias in the results.

Any completely objective approach to sampling suffers from the major disadvantage that it is time-consuming, and without some form of automatic data processing system is very difficult to analyse effectively. Usually, therefore, discontinuity surveys tend to be conducted in a somewhat subjective manner. This is particularly the case in the early stages of any investigation where the object is to reveal the major structural trends at a site quickly.

Prokopovich (1972) has compared the two approaches and concluded that a subjective approach is adequate to define regional discontinuity trends providing that a sufficient number of observations are made to ensure statistical reliability. A similar conclusion was reached by Broadbent & Rippere (1970) who concluded that a subjective approach saved time and effort and also revealed all discontinuity systems subsequently found by more objective sampling techniques. They do, however, suggest that a subjective set of data does not constitute an adequate input for mechanical stability analyses. John (1968), in contrast, suggests that a discontinuity survey should be limited to absolutely necessary data with the incorporation of comments on the data collection sheets to help identify those features of particular relevance.

With regard to the number of measurements that are necessary to ensure statistical reliability, although opinion seems to differ, a minimum of 200 readings per locality is recommended.

It is concluded that in most cases a subjective survey carried out by a trained and competent engineering geologist or engineer will furnish the necessary data for an adequate rock mass description provided that an adequate number of measurements are made. In particularly complex areas, or where trained staff are not available, objective methods will tend to be used. An optimum method of survey would perhaps consist of the measurement of the apparent discontinuity sets in all natural and artificial exposures, together with detailed


objective surveys in a limited number of more critical areas, to ensure that all discontinuity systems have been identified.

8.2.3 Measurements from drillholes. A limited amount of information can be obtained on discontinuity characteristics and patterns from drillhole cores, thus providing data from otherwise inaccessible areas within the rock mass. The major limitations with such methods are that they are dependent on the quality of the core recovered, the poor quality rock and gouge being most likely to be lost during drilling. Unless orientated core is obtained only information on the dip and not the dip-direction of discontinuities can be obtained from any single vertical hole and it is difficult or impossible to assess the persistence, separation, thickness of infilling or to determine the waviness of the discontinuity faces.

8.2.3.1 Core orientation. Various commercial methods of core orientation are available including the Craelius core orientator and the Rocha integral sampler (Rocha 1971). In the former method the instrument is lowered to the bottom of the hole at the start of a core run and a series of prongs takes up the shape of the core-stub. As the orientation of the instrument can be determined and the upper end of the core on the next drill-run can be related to the instrument, the core can often be orientated on recovery from the hole but this may be difficult or impossible in some cases, for example with fragmented rock. With the Rocha method a pilot hole is drilled and an orientated rigid dowel is grouted into place. Overcoring then permits recovery of the dowel and core as an intact and orientated cylinder. Other methods involve the use of compasses grouted into the base of the hole and the inscribing of reference marks on the core during drillings. No method is fully satisfactory. They all suffer from being inconvenient during the drilling operation; some leave room for doubt in the results and others are expensive.

8.2.3.2 Down-hole methods. Various techniques for borehole inspection and photography have been utilised for discontinuity observations and commented upon by various authors including Broadbent & Rippere (1970), Knill (1971), Hock & Bray (1974); they may form a valuable adjunct to core logging. Problems may arise in connection with turbidity which exists in some holes below the water table. The main methods are as follows:

The borehole periscope consists of a rigid tube which supports a system of lenses and prisms. It is probably the most successful instrument for borehole examination in that it is rapid to use, involves direct inspection and the equipment is simple. A major advantage of the system is that it is orientated from outside the hole but it has the disadvantage that currently it is only effective to depths of approximately 30 m.

A number of borehole cameras have been developed (Le Roy 1951, Burwell & Nesbitt 1954, Hock & Pentz 1968). The orientation of the camera is recorded and an illuminated conical mirror enables a photograph to be taken of a length of the borehole wall. By pro- jecting back over a conical mirror it is possible to obtain a view of the actual borehole wall.

The television camera has the advantage over conventional photography that a direct view of the borehole is obtained and a videotape recording may be made. Although Broad- bent & Rippere (1970) achieved only limited success with such a camera, further advances in technology are likely to render this a much more important tool in the future.

Devices such as the Televiewer reported by Zemanek (1968) produce an acoustic picture of the borehole wall and present information similar to that provided by a borehole camera. One advantage of the acoustic method is that boreholes do not need to be flushed before surveying.


8.2.4 Surface photographic methods. A large amount of information relating to discontinuity distribution, spacing, persistence, planarity, and zones of influence is given by photographs of a rock mass exposure. Such photographs can show the relationship between several discontinuity systems and the influence of discontinuity sets upon the rock mass structure.

Photographs are generally either taken from the air looking vertically (or occasionally obliquely) down on the rock mass (air photographs), or from the ground generally looking horizontally at the mass (terrestrial photographs). Such photographs may or may not have survey control.

Uncontrolled photographs are usually taken of rock faces using standard hand-held cameras. Stereo-pairs can easily be obtained by taking two photographs of the same face from positions approximately 5 per cent of the distance to the face apart along a line parallel to the face. Such photographs provide a good basis for delineating major discontinuity patterns and for preliminary subdivision of the face into structural zones. The major disadvantages with such photographs is that data cannot be accurately transferred from them onto maps and plans, other than visually, and no detailed, accurate measurements can be made.

Controlled photographs are taken in such a way that information on them can, with the necessary equipment, be accurately located in space and plotted on to maps and plans. Such photographs are normally obtained by either air photography with complementary ground control or ground based phototheodolite surveys. Air and ground based photography is normally carried out using panchromatic film but the use of colour and infra-red 'false colour' techniques is becoming more widespread.

The interpretation of aerial photographs using the techniques of photogeology and photogrammetry is now considered to be a basic tool in structural geology. The technique has proved to have considerable potential in the investigation of natural discontinuity patterns in rocks. Existing air photographs may be used but generally the scales are too small for detailed discontinuity studies. For major schemes, specially taken photographs are usually used, and the scale may be selected. In this way other specialised requirements may also be introduced such as low-sun-angle photographs.

Terrestrial photogrammetry is now a conventional surveying technique and can readily be adapted for use as a photogeological tool particularly during project construction (Ross- Brown et al. 1972, 1973). In this way accurate data can be collected easily and at relatively low cost. Photographs obtained with a phototheodolite can be viewed in a stereo-com- parator, or similar instrument, which produces a stereoscopic model. Measurements of the location in space of points in this 3-dimensional model can be made to an accuracy of about 1 part in 5 000 of the mean object distance. Hence a point on a face photographed from 50 m can be located to an accuracy of 10 mm. Thus the orientation, frequency and persistence of discontinuities can be readily and accurately determined. Such techniques are of particular use when inaccessible or unsafe faces are being investigated. Terrestrial photogrammetry is especially suited to steep surfaces, as for example in a gorge, where air photographs would yield only poor and inaccurate data.

8.2.5 Comparison of discontinuity survey methods. The various methods of carrying out discontinuity surveys vary in the quality and type of information they are capable of producing. The various types of survey are contrasted in Table 10. It is apparent from this table


TABLE 10: Quality of information from different types of discontinuity survey

Orientated Type of Direct Surface drillhole Drillhole Acoustic

information measurement photography core camera methods

Location Good Good Good Good Medium Type of Discontinuity Good Medium Good Good Poor Description of rock

material Good Poor Good Poor None Orientation: Dip Good Medium Medium Medium Poor Orientation: Dip direction Good Good Medium Medium Poor Frequency Good Good Medium Medium Poor Persistence Good Good Poor Poor Poor Aperture Good Poor Medium/Poor Medium Medium Gouge: nature Good Poor Medium Poor None Gouge thickness Good Poor Medium Medium Medium Surface asperities:

waviness Good Medium Poor Poor Poor Surface asperities:

roughness Good Medium Medium Poor None

Ranking: Good feature measured reliably Medium feature measured but not easily and often with poor reproducibility Poor feature difficult to measure None impossible to identify feature

how much data are to be gained from direct measurements in comparison with other methods. It is strongly recommended that every discontinuity survey should include at least some direct measurements, possibly supplemented by measurements carried out by other

techniques.

0 Procedures for recording and presenting descriptive data on rock masses

This section deals with the format for recording and presenting data so that they can subsequently be used for a particular design or construction purpose.

9.1 Recording of data

It is strongly recommended that all data should be recorded on data sheets amenable to subsequent automatic processing. In this way not only is the informat ion recorded in a s tandard fashion, using accepted terminology, but it ensures that important features are not overlooked. A previous working party (Anon. 1970) has considered how data from drill cores should be regarded as complementary. The process of data recording should be a stage between those of data collection and data presentation.


Two types of data sheet are recommended for recording data on rock mass description, the rock mass description sheet and the discontinuity survey data sheet (Figs. 3 and 4). All the data considered in the previous sections of the report, using the nomenclature recommended, can be plotted on to one or other of these forms. The forms are simple and have been designed to facilitate the subsequent production of punched-cards. It is not suggested that the forms have to be fully completed. In many cases it will be necessary to collect only a certain amount of the information shown. However, the fact that the complete range of rock mass indices is specified should ensure that no parameter is overlooked.

9.2 Presentation of data

When the data have been recorded, either in the form of data sheets or drillhole/borehole logs, then they have to be presented in some form amenable to easy assimilation and assessment. Because geological data often have strong spatial interrelationships they are usually presented in a cartographic, graphical or tabulated form. A previous working party (Anon. 1972) considered how geotechnical information from all sources, including rock mass data, might be presented in the form of maps and plans. Whilst maps and plans can be used solely for presenting rock mass data they do not by themselves easily permit the orientation or importance of discontinuities one to another to be compared.

The three methods most commonly employed to present angular relationships are the rose diagram, the histogram and the spherical projection. The rose diagram can indicate the ranges of both dip and dip-direction but is relatively insensitive; the histogram only shows the distribution of any single feature; the spherical projection permits the consideration of both linear and planar features and their angular relationships.

Spherical projections or the representation of three-dimensional data on a two-dimensional figure, fall into two main types:

(i) the equal-angle projection in which the angular relationships between features are accurately represented (as plotted on the Wulff net).

(ii) the equal-area projection in which the spatial distribution of data is accurately represented (as plotted on the Lambert or Schmidt net).

Hoek& Bray (1974) have summarised the methods of presenting data using such projections and have discussed the interpretation of the resulting diagrams within the context of slope stability studies. The reader is referred to their work for detailed consideration of spherical projections in geotechnical engineering. The projection most commonly used is the equal-area projection. This permits the assessment of statistical distributions, whilst still permitting planes and lines to be plotted but with reduced accuracy.

Where a statistical assessment of data is required it is usual to plot orientation measurements of planes as a series of points representing normals or poles to planes on a Schmidt equal-area net. Contouring of the density of the pole concentrations on the resuIting scatter diagram allows the rapid assessment of these data although it is recognised that the contouring process itself can produce inaccurate weightings of the data. It is recommended that where contoured diagrams are presented, the original point diagrams should always accompany them.

Various types of data can be plotted on the same scatter diagram by the use of different

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symbols. The data can also be plotted and contoured automatical ly if required either from the basic data sheets or from punched cards produced from them. However, where only small quantities of data are involved, or the results are needed in the field, it is preferable to plot manually.

I0. Conclusion In conclusion, it is emphasised that the necessity for the introduction of clear, concise and uniform methods in rock mass description is long overdue. Whilst it will never be possible to produce terminology which is acceptable to all branches of geotechnical engineering in various parts of the world, s tandardisat ion of accepted practice in Britain should go a long way to help in this respect. It is thus hoped that the foregoing recommendat ions will be used in practice.

II. Membership of the working party R. Chaplow, Sir Alexander Gibb & Partners, Reading, U.K. (formerly Lecturer in

Engineering Geology, Imperial College, London). J. M. Edmond, Wimpey Laboratories Ltd., Hayes, Middlesex, U.K. D. McCann, Institute of Geological Sciences, London. U.K. G. E. Rawlings, Golder Brawner & Associates Ltd., Vancouver, Canada. (Chairman),

(formerly of Binnie & Partners, London, U.K.) Although not a formal member of the Working Party, Prof. W. R. Dearman (Geology

Dept, University of Newcastle upon Tyne) made a substantial contr ibution to the report.

Acknowledgements: The working party wish to acknowledge the valuable assistance and advice which they have received from individuals and organisations during the preparation of this report.

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