6 SOIL, ROCK AND GROUNDWATER
6.2 Rock identification, description and classification
Rock description and classification shall distinguish between:
(a) Rock material The intact rock that is bounded by defects.
(b) Defect A discontinuity, fracture, break or void in the material or materials across which there is little or no tensile strength.
(c) Structure The nature and configuration of the different defects within the rock mass and their relationship to each other.
(d) Rock mass The entirety of the system formed by all of the rock material and all of the defects that are present.
Rock mass behaviour is generally controlled by the nature and configuration of defects and describing the type, character and distribution of defects is an essential part of the description of many rock masses. Rock mass behaviour may also be related to the nature and scale of the project elements.
6.2.2 Overview
The following characteristics shall be described:
NOTE: Characteristics should be reported in the order given below.
(a) Description of rock material:
(i) Rock NAME (BLOCK letters).
(ii) Grain size and type.
(iii) Colour.
(iv) Fabric and texture.
(v) Inclusions or minor components.
(vi) Moisture content.
(vii) Durability.
(b) Classification of the rock material condition:
(i) Strength.
(ii) Weathering and/or alteration.
(c) Description of defects.
(d) Interpreted stratigraphic unit.
(e) Geological structure.
The following characteristic may also be described:
(i) Parameters related to core drilling.
(ii) Classification of the rock mass:
(A) Rock mass weathering.
6.2.3 Description of rock material 6.2.3.1 Rock name
Simple rock names should be used to provide a reasonable engineering description rather than a precise geological classification. The rock name should be chosen by considering the nature and shape of the grains or crystals, the texture and fabric of the rock material, the geological structure and setting, information from the geological map of the area and the following guidelines:
(a) Sedimentary rocks are deposited in beds, have grains that are cemented together and which are often rounded and there may be interbedded combinations of different sediment types, different beds and bedding partings and sedimentary structures such as cross bedding. They often have significant porosity. A guide to the naming of sedimentary rocks for engineering purposes is provided in Table 15.
(b) Igneous rocks are formed from molten rock and have a crystalline texture, i.e. interlocking crystals. Most igneous rocks are massive, however a few exhibit flow banding. They typically have low porosity, unless they contain bubbles. A guide to the naming of igneous rocks for engineering purposes is provided in Table 16.
(c) Metamorphic rocks are formed when rocks are subject to heat and/or pressure and commonly have a directional fabric (a foliation which may be specifically a cleavage and/or a schistosity) although some are massive. They typically have low porosity and may have a crystalline texture. A guide to the naming of metamorphic rocks for engineering purposes is provided in Table 17.
(d) Duricrust rocks are formed as part of a weathering profile and show evidence of having been cemented in situ. The cementation is often irregular and exhibits replacement textures. A guide to the naming of duricrust rocks for engineering purposes is provided in Table 18.
If a rock type cannot be identified, the material should be given a distinctive interim name until an observation by a more experienced observer or a petrographic assessment is available. Engineering properties should not be inferred directly from the rock names in Tables 15 to 18 but the use of a particular name does indicate a likely range of characteristics. The rock names given in this Standard are sufficient to describe most of the rocks that are likely to be encountered. However, they are provided as a guide only, and other names may be used where available information or local knowledge can be used to justify a more appropriate name.
If alternative rock names to those provided in Tables 15 to 18 are used, then the geological characteristics of the rocks shall be briefly summarized in the report.
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TABLE 15
GUIDE TO THE NAMING OF SEDIMENTARY ROCKS
Grain size
mm Deposited rock type
At least 90% of rock is
>2 CONGLOMERATE (larger rounded grains in a finer matrix)
BRECCIA (angular or irregular rock fragments in a finer matrix)
LIMESTONE or
0.06–2 SANDSTONE(Notes 1,2) CALCARENITE TUFF
0.002–0.06 MUDSTONE(Note 5) silt and clay ARKOSE (consisting mainly of feldspar) and QUARTZOSE SANDSTONE (quartz grains and siliceous cement). In some cases the proportions of the different grains in the sandstone may be estimated.
2 Sandstones may be described as fine, medium or coarse when their grains are identified as comprising fine, medium or coarse sand, respectively.
3 Where carbonate content is 50–90% the names provided should be used preceded by the word IMPURE.
4 LIMESTONE (predominantly calcium carbonate – CaCO3) should be distinguished from DOLOMITE (predominantly calcium magnesium carbonate – CaMgCO3) where possible.
5 SHALE is a fissile mudstone with preferential weakness parallel to bedding.
6 Rocks displaying alternating fine inter-laminations of different grainsize (e.g. SILTSTONE/CLAYSTONE or SILTSTONE/FINE SANDSTONE) may be referred to as LAMINITE.
7 BRECCIA is any sedimentary rock composed of angular fragments in a finer matrix.
8 COAL is a mostly organic rock that consists of indurated accumulations of plant debris.
9 The term carbonaceous may be added to the names in the table where a rock is assessed to contain a significant carbon content.
10 EVAPORITES are rocks that consist mainly of salts such as halite, anhydrite or gypsum.
11 FLINT and CHERT are amorphous or cryptocrystalline quartz, from any origin.
12 Cements may be, for example, siliceous, calcareous, limonitic, carbonaceous, argillaceous (clay), or zeolite and where identified this should be noted.
13 The depositional origin of the sediment may be indicated by prefixes such as aeolian, glacial, or marine.
TABLE 16
GUIDE TO THE NAMING OF IGNEOUS ROCKS
Grain size
Coarse (>2) GRANITE DIORITE GABBRO
Medium (0.06–2) MICROGRANITE MICRODIORITE DOLERITE
Fine (<0.06) RHYOLITE ANDESITE BASALT
NOTES:
1 PEGMATITE is an igneous rock consisting of large crystals often forming a dyke or vein.
2 OBSIDIAN and VOLCANIC GLASS are rocks that have cooled too quickly for crystals to develop and consequently have an amorphous (glassy) texture.
3 APLITE may occur as light coloured veins of quartz and feldspar in other igneous rocks.
4 PORPHYRY is an igneous rock consisting of large crystals in a much finer matrix.
TABLE 17
GUIDE TO THE NAMING OF METAMORPHIC ROCKS
Grain size
mm Foliated Non-foliated
Coarse (>2) GNEISS—well developed but often widely spaced foliation sometimes with schistose bands
MARBLE—crystalline calcium carbonate
QUARTZITE—fused quartz grains SERPENTINITE—usually a grey and green rock formed by the alteration of mafic igneous rocks HORNFELS—usually a fine grained rock formed by thermal metamorphism
Medium (0.06–2) SCHIST—well developed foliation with much mica, some micas larger than 2 mm
Fine (<0.06) PHYLLITE—slightly undulose foliation sometimes spotted.
SLATE—well developed planar cleavage
NOTE: Foliated metamorphic rocks normally form by regional metamorphism and non-foliated metamorphic rocks form by contact or thermal metamorphism.
TABLE 18
GUIDE TO THE NAMING OF DURICRUST ROCKS
Dominant cementing mineralogy Iron oxides and
hydroxides Silica Calcium carbonate Gypsum
FERRICRETE SILCRETE CALCRETE GYPCRETE
NOTES:
1 Refer to rock mass grades in Table 25, Clause 6.2.10.2 for a classification of duricrusts.
2 Field differentiation of LIMESTONE and CALCRETE should be based on observation of textures, fabric and defects with LIMESTONE being dominated by sedimentary features and CALCRETE being dominated by replacement features.
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6.2.3.2 Grain size and type
In sedimentary rocks with predominantly sand sized grains, the following terms should be used to describe the dominant or average grain size:
(a) Coarse grained—mainly 0.6 mm to 2 mm.
(b) Medium grained—mainly 0.2 mm to 0.6 mm.
(c) Fine grained—mainly 0.06 mm (just visible) to 0.2 mm.
In igneous and metamorphic rock types, where significant, the following terms should be used to describe the dominant or average grain size and/or the grain size may be recorded in millimetres:
(i) Coarse grained—mainly greater than 2 mm.
(ii) Medium grained—mainly 0.06 mm to 2 mm.
(iii) Fine grained—mainly less than 0.06 mm (just visible).
If readily identifiable, the minerals should be described.
6.2.3.3 Colour
The colour of a rock shall be described in the moist condition, using simple terms such as black, white, grey, red, brown, orange, yellow, purple, green, blue, etc.
These may be modified as necessary, e.g. by ‘pale’, ‘dark’ or ‘mottled’. Borderline colours may be described as a combination of these colours.
6.2.3.4 Texture and fabric
The texture of a rock describes the arrangement of, or the relationship between, the grains and/or crystals that make up the rock. Terms such as porphyritic (larger crystals—
phenocrysts—set in a finer groundmass), crystalline (consisting of interlocking crystals having a distinctive colour and habit), amorphous (having no definite crystalline structure), glassy (looking like manufactured glass) should be used to describe the texture where it is significant.
A rock possesses a fabric where the arrangement of grains shows an alignment, a preferred orientation or a layering that is visible at the scale of outcrop or core. Where a fabric is visible it shall be described. The following are common terms for describing the type of fabric in the rock material, but other terms may be used:
(a) Sedimentary rocks:
(i) Bedding Layering produced by changes in sedimentation, which may be defined by grain size, colour, or other features.
(ii) Lamination Similar to bedding but developed in layer thicknesses of less than 20 mm.
(b) Metamorphic rocks:
(i) Foliation The parallel arrangement of minerals due to metamorphic processes.
(ii) Cleavage A type of foliation developed in fine grained metamorphic rocks such as slates.
The most important observation to make is the effect that the fabric has on rock strength.
That is, does the fabric have no significant effect on the strength, which is therefore isotropic, or does the fabric cause the strength to be anisotropic, which means that the rock will have different strengths in different directions. The visual appearance of the rock is not necessarily a good indicator of the influence of any fabric on strength and the rock should be broken to establish the influence of the fabric. The degree of development of the fabric shall be described using the following terms:
(i) Indistinct fabric There is little effect on strength properties.
(ii) Distinct fabric The rock may break more easily parallel to the fabric.
NOTE: Where ‘fabric’ is any appropriate geological term for the relevant rock type such as those described above.
The orientation and thickness of the layers defining the fabric shall be described directly, e.g. distinct bedding dipping at 30°, 30 mm to 100 mm thick.
6.2.3.5 Features, inclusions and minor components
Features, inclusions and minor components within the rock material shall be described where those features could be significant, i.e. the features could influence engineering behaviour. Examples of features which could be significant under certain circumstances include—
(a) gas bubbles (vesicles if empty; amygdules or amygdales if mineralized) in igneous rocks;
(b) veins of quartz, calcite or other minerals;
(c) pyrite crystals and nodules or bands of ironstone or carbonate;
(d) cross-stratification in sandstone; and
(e) clast or matrix support in conglomerates and breccia.
The general proportions and dimensions of features and inclusions should be described directly, e.g. ‘about 30% vesicles from 2 mm to 5 mm in size’.
6.2.3.6 Moisture content
Where significant, this shall be described by the feel and appearance of the rock using one of the following terms:
(a) Dry Looks and feels dry.
(b) Moist Feels cool, darkened in colour, but no water is visible on the surface.
(c) Wet Feels cool, darkened in colour, water film or droplets visible on the surface.
The moisture content of rock cored with water may not be representative of its in situ material may not have adequate durability, this shall be noted and described.
6.2.4 Classification of rock material condition 6.2.4.1 Rock material strength
The strength of the rock material shall be classified using Table 19. It should be based on the uniaxial compressive strength (UCS). The UCS for classification purposes should be based on specimens tested at close to their in situ moisture content. Where strength is measured at another moisture condition, this shall be clearly stated.
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Where it is not practical to conduct UCS tests, or adequate UCS test data are not available, classification of strength may be made on the basis of the point load strength index (Is(50)) measured on specimens close to their in situ moisture condition.
Table 19 implies a correlation between Is(50) and UCS that should be used for classification, unless a correlation is or has been developed for specific rock types at the location of the investigation. Established and documented correlations between UCS and Is(50) may also be used for classification purposes.
If point load strength tests are used to assess the strength of rock with a distinct fabric, the strength perpendicular to the planar anisotropy shall be used for classification purposes and the strength anisotropy index (Ia(50)) shall be reported where possible.
For preliminary field classification, or where testing is not practical, the field assessment of strength in Table 19 provides guidance on methods and interpretation of results, which may be adopted for strength classification.
Any correlation implied in Table 19 shall not be relied upon for design purposes without supporting evidence.
TABLE 19 with sharp end of pick; can be peeled with knife; too hard to cut a triaxial sample by hand. Pieces up to 30 mm thick can be broken by finger pressure.
Low Strength L 2 to 6 0.1 to 0.3 Easily scored with a knife; indentations 1 mm to 3 mm show in the specimen with firm blows of the pick point; has dull sound under hammer. A piece of core 150 mm long by 50 mm diameter may be broken by hand. Sharp edges of core may be friable and break during handling.
Medium Strength M 6 to 20 0.3 to 1 Readily scored with a knife; a piece of core 150 mm long by 50 mm diameter can be broken by hand with difficulty.
High Strength H 20 to 60 1 to 3 A piece of core 150 mm long by 50 mm diameter cannot be broken by hand but can be broken by a pick with a single firm blow; rock rings under hammer.
Very High Strength
VH 60 to 200 3 to 10 Hand specimen breaks with pick after more than one blow; rock rings under hammer.
Extremely High Strength
EH more than
200
more than 10 Specimen requires many blows with geological pick to break through intact material; rock rings under hammer.
NOTES:
1 Material with strength less than ‘Very Low’ shall be described using soil characteristics. The presence of an original rock structure, fabric or texture should be noted, if relevant.
2 The method for measuring the uniaxial compressive strength shall be in accordance with AS 4133.4.2.1.
3 The method for measuring the point load strength index shall be in accordance with AS 4133.4.1.
6.2.4.2 Degree of weathering
The process of weathering involves physical and chemical changes to the rock in response to the changes in pressure, temperature, moisture and chemical environments that result from being exposed at the earth’s surface.
The degree of weathering of the rock material shall be classified. The terms in Table 20 should be used. If an alternative rock material weathering classification scheme is used, it shall be documented. This approach is typically used when logging rock cores. Where it is useful to describe the degree of weathering of the rock mass, rather than just the weathering of the rock material, e.g. when logging outcrops or excavations, the classification system provided in Table 24 may be used.
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TABLE 20
CLASSIFICATION OF MATERIAL WEATHERING
Term Abbreviation Definition
Residual Soil (Note 1) RS
Material is weathered to such an extent that it has soil properties. Mass structure and material texture and fabric of original rock are no longer visible, but the soil has not been significantly transported.
Extremely Weathered (Note 1) XW
Material is weathered to such an extent that it has soil properties. Mass structure and material texture and fabric of original rock are still visible.
Highly Weathered (Note 2)
The whole of the rock material is discoloured, usually by iron staining or bleaching to the extent that the colour of the original rock is not
recognizable. Rock strength is significantly changed by weathering. Some primary minerals have weathered to clay minerals. Porosity may be increased by leaching, or may be decreased due to deposition of weathering products in pores.
Moderately Weathered
(Note 2) MW
The whole of the rock material is discoloured, usually by iron staining or bleaching to the extent that the colour of the original rock is not
recognizable, but shows little or no change of strength from fresh rock.
Slightly Weathered SW
Rock is partially discoloured with staining or bleaching along joints but shows little or no change of strength from fresh rock.
Fresh FR Rock shows no sign of decomposition of individual
minerals or colour changes.
NOTES:
1 The term ‘Extremely Weathered rock’ is misleading as the material has soil properties. The word ‘rock’
should be replaced with the name of the original rock in lower case or the word ‘material’, e.g. Extremely Weathered granite or Extremely Weathered material. Residual Soil and Extremely Weathered material should be described using soil descriptive terms.
2 Where it is not practicable to distinguish between ‘Highly Weathered’ and ‘Moderately Weathered’ rock the term ‘Distinctly Weathered’ may be used. ‘Distinctly Weathered’ is defined as follows: ‘Rock strength usually changed by weathering. The rock may be highly discoloured, usually by iron staining.
Porosity may be increased by leaching, or may be decreased due to deposition of weathering products in pores’. There is some change in rock strength.
6.2.4.3 Degree of alteration (alteration intensity)
Where physical and chemical changes of the rock material are caused by hot gases or liquids at depth the process is called alteration. The distinction between weathered material and altered material is important because they are likely to have different distribution patterns. For example, unlike weathered material, altered material may occur at any depth and show no relationship to topography.
When altered materials are recognized, the terms presented on Table 21 should be used. If alternative rock material alteration classification schemes are used, they shall be documented.
TABLE 21
CLASSIFICATION OF MATERIAL ALTERATION
Term Abbreviation Definition
Extremely Altered XA
Material is altered to such an extent that it has soil properties. Mass structure and material texture and fabric of original rock are still visible.
Highly Altered
The whole of the rock material is discoloured, usually by staining or bleaching to the extent that the colour of the original rock is not recognizable. Rock strength is changed by alteration. Some primary minerals are altered to clay minerals. Porosity may be increased by leaching, or may be decreased due to precipitation of secondary minerals in pores.
Moderately Altered
(Note 2) MA
The whole of the rock material is discoloured, usually by staining or bleaching to the extent that the colour of the original rock is not recognizable but shows little or no change of strength from fresh rock.
Slightly altered SA Rock is slightly discoloured but shows little or no change of strength from fresh rock.
NOTES:
1 The term ‘Extremely Altered rock’ is misleading as the material has soil properties. The word ‘rock’ should be replaced with the name of the original rock or the word ‘material’, e.g. Extremely Altered basalt or Extremely Altered material. Extremely Altered material should be described using soil descriptive terms.
2 Where it is not practicable to distinguish between ‘Highly Altered’ and ‘Moderately Altered’ rock the term
2 Where it is not practicable to distinguish between ‘Highly Altered’ and ‘Moderately Altered’ rock the term