Geotechnical Logging Techniques - Mine Design Wiki

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Contents

1 Core logging procedure

1.1 Core photographs and preparation

1.2 Intervals

1.3 Core description

1.3.1 Colour and rock description

1.3.2 Total core recovery length

1.3.3 Rock quality designation (RQD) length

1.3.4 Number of discontinuities1.3.5 Number of sets

1.3.6 Strength grade

1.3.7 Weathering/alteration grade

1.4 Discontinuity description

2 Discontinuity orientation

2.1 Methods for determining orientation

3 See also

ore logging procedure

e following steps are suggested during the core logging process:

Clean the core of drilling fluids or mud.1.

Mark major structures, proposed point load testing locations, and depths (every 1-2 metres) on undisturbed core in

splits.

2.

Photograph the core in the splits (if using triple tube method) with a scale placed in the picture and a whiteboardindicating what depth the core has been obtained from.

3.

Complete the Discontinuity and core description logs.4.

Transfer the core from the splits to a labelled core box.5.

Once a core box is full, take a single photograph of the core box with a scale.6.

e steps are detailed in the following sections.

ore photographs and preparation

e of the most important things to do at the drill rig is photograph the undisturbed core in the splits. These photos may be

d later to confirm televiewer images and will be an invaluable resource on the rock mass and for review of the design

rk.



oper core photos require that the core be cleaned prior to photographing. When core is covered in drilling mud, structural

ormation can be obscured making it difficult to determine lithologies. Take the time to properly clean the core. The core

ould be wet if possible as some structural features do not show up on dry core so make sure to wet it down with a spray

tle or paint brush. The following are also required in all core photographs:

A scale - make sure the measuring tape you use is in focus and readable in the photos

A white board with the project name, project number, hole ID, date, interval number(s), and depth from

and to written out

The depths with 1 meter increments marked on the core using a paint pen or grease pencil

Labels of major structures, including type and depth. Examples of colours and symbols are outlined in

Table 1.

Table 1: Examples of core symbols

em Colour Symbol

hole meter depth White

ajor structures Red

LT samples Yellow

echanical breaks Blue

tes:

Major structures should be identified using their corresponding logging code. In the example above the symbol is for

a Fault; if the structure was a Shear the “F” would be substituted for an “S”.

1.

The numbers in the Whole meter depth and Major structures symbols indicate the depth in meters.2.

tervals

e properties used in the core description are recorded by intervals for each run of core. An interval represents a change

ithology, alteration, and/or rock mass quality. The benefit of logging on an interval basis is that it allows for distinctly

ferent units within one run of core to be assigned their own properties. This prevents the need for averaging different

ts over the length of a run, which can lead to overestimating / underestimating material properties. Photograph 1 below

vides an example of a change in rock mass quality. Intervals should be at least 30 cm in length and will be at most the

gth of the core run. Intervals are also numbered sequentially. For example if you have 100 runs you should have

ween 100 and 500 intervals, assuming a maximum run length of 1.5 m (5 feet). The start depth (i.e. depth from) and the

d depth (i.e. depth to) for each interval should be recorded as measured from the top of the hole. This is strictly a

nction of where the drill bit started and ended during the run, and may be less than the length of the maximum run i

cking of the core barrel occurs, the bit requires replacement, etc.



lor

odifier Code Pattern Code Primary/secondary color CodeLight L Banded BA Pinkish PK

Dark D Streaked SK Reddish RD

Blotched BL Yellowish YW

Mottled MT Brownish BR

Speckled SP Olive OL

Table 2: Examples of rock core desciption codes

gure 1: Example of rock core broken into intervals

pths of zones of substantial core loss may have to be estimated. For example, if a 1.5 m run of core is completed and

y 1.0 m of core is returned with 0.4 m of crushed rock at the start (first interval) and 0.6 m of competent rock following

cond interval), it is likely that the core loss occurred in the crushed zone. Consequently, the end depth of the first

erval and the start depth of the second interval should be adjusted to account for the 0.5 m loss of core in the crushed

ne. If a downhole survey (e.g. with a televiewer) has been completed, the zone of core loss may be more accurately

fined by the images from the survey, requiring modification of the logs during the matching process with the televiewer

d core logging data.

ore description

e Core description portion of the log covers the lithology, interval determination data, and the rock mass classification.

low is a description of each logging parameter. Both the top and the bottom of the interval are to be recorded.

lour and rock description

lour and rock descriptions should be logged as part of the core logging procedure to indentify the lithologies and

eration sequences encountered. Logging should be based on easy to identify attributes that will in most cases allow rock e to be determined quickly and easily. Such attributes include:

Pattern

Colour

Grain size

Texture

Fabric

Lithology

Alteration

gging these parameters separately and on an interval basis will allow for recognition of subtle variations that wouldrmally be smoothed over in the summary log, and will ensure that the descriptions produced for final reporting are clear,

ncise, and repeatable. Codes describing the above should be decided upon in advance, and kept as simple as possible for

e of data entry and for consistency. An example of possible codes for a geotechnical core logging scheme is included in

ble 2 below.



Stained ST Greenish GR

None NO Blueish BL

Greyish GY

ain size

ode TermParticle

SizeExamples

VCVery

course> 60 mm Porphyries-w measureable grains

C Coarse 2 -60 mmCongromerate,Breccia,Gneiss-w/measureable

grains

M Medium 0.06- 2 mmSandstone, Gabbro, Granite, Schist - having

clearly visible grains

F Fine0.002 - 0.06

mm Tuff, Siltstone, Claystone, Mudstone, Basalt

VF Very fine <0.002 mm

xture

ode Texture Description

AP Aphanitic Grains cannot be seen with naked eye

Q Equigranular All grains are the same size

M Bimodal Two sizes of crystal exist in rock

TR TrachyticAlignment of grains in a volcanic rock parallel to flow

direction

AC Acicular Crystals are needle shaped

M Diamitic Gap graded, matrix supported clasts (sedimentary)

bric

ode Fabric Description

GN Gneissic Alternating layers of different colour or texture

X Brecciated Angular fragments that have been healed

D Bedded Deposited in layers, can be in sedimentary or volcaniclastic rocks

B Interbedded Beds alternating with others of a different character

MA Massive No crystal or grain fabric (homogeneous)

R PorphyriticIntrusive texture where large phenocrysts are present in

a much finer grained groundmass

TU Tuff Lithified pyroclasic sediments

LT Lapilli tuff Lithified pyroclastic sediments with large clast

inclusions

VC Volcaniclastic Clastic rock containing volcanic materialO Foliated Mineral are aligned due to shearing or metamorphism

hology

ode Lithology Description



MZ Monzonite

Intrusive rock with a low quartz content and equal

amount of plagioclase and alkali feldspar (k-spar),

mafic minerals may or may not be present

GR Granitic intrusion

Intrusive rock, approx. equivalent content of

quartz, plagioclase and alkali feldspar, mafic

minerals may or may not be present

MD Mafic dykeDyke containing mostly mafic minerals. Typically

have bleached contacts at KSM

DT Diorite Intrusive rock, mainly plagioclase feldspar

FPFeldspar

porphyry

Lath shaped feldspar crystals make up a significant

percentage of the rock mass (>=20%)

C Schist

Foliated metamorphic rock, mica typical on

foliation separation planes, foliation usually

undulating, sometimes poorly defined

VB Volcanic breccia

Cemented angular fragments. Cause of brecciation

is volcanic, either by injection of melt or the

breccia is composed of pyroclastic debris

TU Tuff

General term for all consolidated

pyroclastic/volcaniclastic material, flow lines / bedding may be visible

LT Lapilli tuff Large clasts (2-64 mm) are visible in the tuff beds

VC Volcanic Fine grained, flow lines may be visible, mafic

H Shale Fine grained sedimentary rock, laminated, fissile

SS SandstoneClastic sedimentary rock, grains are sand sized and

may be cemented with clay / silt sized particles

AR ArgilliteVery fine grained sedimentary rock, indurated,

lacks the fissility of shale

UD Undistinguishable This term should be used as little as possible

teration codes

odeAlterationtype

Site specificdescription fromexploration logs

Literature descriptionMineralassemblage

Diagnosticfeatures

RG Argillic

-introduces a wide variety of clayminerals, includes kaolinite,

smectite, and illite

-can also have kaolinite + quartz +

hematite + limonite assemblage

clay minerals -feldspar grainshave been replaced

with clay

-slippery feel on

discons

HL Chloritic

-darkened groundmass

when pervasive

chlorite,

muscovite, quartz,

albite

AR Carbonate

-greater than 3% k

veins

-addition of any carbonate

minerals, typically calcite, ankerite,

dolomite

calcite, dolomite,

malachite

-veins / matrix

react with acid

FS Hornfels

-indurated and

strengthened

-thermal alteration, seems "baked"

resulting in stronger and more

indurated rock mass than parent

hornblende,

plagioclase,

chlorite, biotite

EMHematite/Iron

Oxide

oxide minerals -red/brown/orange



HY Phyllic

-pyrite concentrations

2% and greater

-sericite and quartz

altered feldspars

-sericite typically pale

green

-greenish pale grey

groundmass

-typically formed from decomp of

feldspars, sericite and quartz

replace large feldspar grains, and

feldspar in the groundmass

-can be associated with high pyrite

concentrations

-softens rock, easily scratchable

-greasy feel

-occurs in acidic conditions

sericite,

quartz, pyrite

-feldspars

decomposed to

sericite and quartz

OT Potassic

-matrix has been

replaced by fine

grained hydrothermal

k-feldspar

-typically dark grey -

purplish grey

-high temperature alteration, results

from potassium enrichment

-can occur during crystallization of

magma

biotite, k-feldspar,

magnetite, +/-

epidote

specularite

-potassium feldspar

present in

groundmass or as

veining

-dark grey /

purplish grey

RO Propylitic

-dark green to green

-magnetic

-turns rocks green, usually

alteration minerals replace Fe-Mg

bearing minerals (biotite,

amphibolite, pyroxene) but can also

replace feldspar

-low temperature, distal to other alteration types

chlorite, epidote,

pyrite, actinolite

+/- carbonate

-green rock matrix

IL Siliceous

-silica flooded/lots of

veins

-hardened

-addition of secondary silica

(quartz)-most common silica

flooding: replacement of the rock

with microcrystalline quartz

-another style is stockwork:

formation of closely spaced

fractures in a network filled in with

quartz

quartz -strong to v strong

-sometimes

stockwork of

quartz veins

tes:

More than one alteration type may be present in a zone. This should be indicated in the core log.

tal core recovery length

e total length of core recovered by the drillers from an interval is measured. In addition to providing a first indication o

nes of poor rock mass quality or drilling problems, total recovery can be used to check the run block depths provided by

drillers. Also, it is not uncommon for the recovery length to be greater than the drilled length; this often happens when

core breaks above the bottom of the hole on the previous run, or as a result of errors in the measurement of the length

lled. Where multiple intervals and core loss (or gain) occur between two run blocks, a judgement must be made for

ich interval(s) will be recorded as having the difference. Appropriate strategies include:

distribute core loss (or gain) over all intervals between the core blocks

record the interval nearest the first core block as having the differenceselect the interval that contains the most fractured rock as having the difference

mix of strategies may be appropriate; however, a single strategy is generally recommended for consistency and ease o

mparison of data. For programs where corehole televiewer programs are conducted, the downhole images may assist in

ntifying the major zones of core loss.



ck quality designation (RQD) length

e rock quality designation (RQD) is a modified core recovery measurement (Deere and Deere, 1989). For each interval,

total length of all core pieces longer than 10 cm (4 in) as measured along the core centerline, should be determined and

orded as shown in Figure 2.

gure 2: Measuring RDQ length from rock core (after Deere and Deere, 1989)

hen measuring the RQD, the following should be taken into consideration:

The total length of core must include all lost core sections

When summing up the lengths, the breaks created by the driller during removal from the core barrel

(often referred to as “mechanical” breaks) must be ignored

Before measuring the RQD, apply slight pressure with your hands along the length of the core to check

that all the discontinuities have opened. This will help ensure that “tight” joints are properly accounted

for

A “soundness check” should be carried out for weathering / alteration and hardness (R) grades; if W/A

>4 or R≤1, then that length of core does not get counted in the RQD lengthThe RQD length is measured along the axis of the core

If RQD can be measured in the split tubes (if triple tube drilling has been carried out) before the core is

put into the box this will result in a more accurate estimate of RQD

mber of discontinuities

e number of geological discontinuities (fractures, joints, shears, bedding, etc.) within each interval is counted and

orded. Breaks in the core from the process of drilling or boxing the drill core (mechanical breaks) are not included in

s count.

chanical breaks are identified by sharp core edges at the break and will often have clean breakage surfaces with no

illing and no discolouration. If the cause of the break in the core is in doubt, treat the break as a natural feature and

lude it in the discontinuity count. The core shown in Figure 3 has this kind of clear breakage.



gure 3: Example of when the number of discontinuities can be counted directly from the core

here discontinuities with thick infillings, faults, or zones of soil-like material are encountered count 1 discontinuity per 1

of infilling, fault zone, or soil zone thickness along the core centre line.

ure 4 shows a sample where discontinuities must be estimated.

gure 4: Example of when the discontinuity number must be estimated

r intervals that have closely spaced discontinuities too numerous to count (possible in bedded, laminated, or foliated

ks), resulting in “discs” of core, the number of discontinuities can be estimated. This is done by measuring the average

e of the core pieces and dividing the interval length by the average piece size. For example, a 30 cm zone of broken rock

ere the average particle size is 3 cm in diameter would count as 10 discontinuities. “Default” numbers for highly

ctured rock should not be used

mber of sets

e number of discontinuity sets present in the rock mass is used to determine the joint set number (Jn), a parameter used

Barton’s Q rock mass classification system. The most accurate way to determine the number of joint sets is to process

e orientation data and determine the number of sets from stereographic projection of the discontinuity data. In the

ence of core orientation data it may take several drill core runs to see all of the sets present, particularly if there are

dely spaced sets present. This parameter can be extrapolated forwards and backwards in the drill core from zones where

set numbers are obvious; however, make sure that if there are changes occurring in the structural fabric they aren’t

ssed by averaging the sets over long sections of hole. Use whole numbers only, so if you have 2 sets and no other distinct

tures, use 2, if there are other features use 3. This is a slightly conservative approach that is acceptable because it is

ficult to determine at the core scale whether widely spaced discontinuities form sets. It is important not to include

chanical breaks in this number.

ength grade

e strength grade (Table 4), sometimes referred to as hardness grade, is a field estimate of the strength of the intact

terial. It is important to use your hands, knife and rock hammer when estimating the strength of a sample. When using

hammer remember that only a firm blow need be applied. Also, make sure that the induced break does not occur along

iscontinuity, otherwise the strength test is invalid.

single value should be used for the strength grade. If the grade within the interval ranges from one hardness grade to

other (e.g. is between 3 and 4), use half values (e.g. 3.5). If the hardness is extremely variable, consider splitting the run

o two or more intervals to accurately capture the variability.

ble 4: Field strength grades (ISRM 1978)



rade Field identification DescriptionUCS

(MPa)

R6 Specimen can only be chipped with flat end of geological hammer Extremely

strong> 250

R5 Specimen requires many blows of flat end of geological hammer to fracture Very strong 100-250

R4 Specimen requires more than one blow of flat end of geological hammer to fracture Strong 50-100

R3Cannot be scraped or peeled with pocket knife; can be fractured with single firm

blow of flat end of the geologic hammer Medium strong 25-50

R2 Can be peeled by a pocket knife with difficulty; shallow indentation made by firm blow with point of geological hammer Weak 5-25

R1Crumbles under firm blow with point of geological hammer; can be peeled by a

pocket knifeVery weak 1-5

R0 Indented by thumbnailExtremely

weak < 0.2 – 1

S6 Indented with difficulty by thumbnail Hard >0.2

S5 Readily indented by thumbnail Very stiff 0.1 – 0.2

S4 Readily indented by thumb but penetrated only with great effort Stiff 0.050 - 0.1

S3 With moderate effort, penetrates several centimeters by thumb Firm 0.025 -0.05

S2 Easily penetrated several centimeters by thumb Soft0.012 -

0.025

S1 Easily penetrated several centimeters by fist Very soft < 0.012

rade Description Field identification

1/W1Fresh and

Unweathered

Parent rock showing no discoloration, loss of strength or any other weathering effects.

Strength may be increased by some alteration types.

2/W2Slightly Weathered

or Altered

Rock may be slightly discoloured, particularly adjacent to discontinuities, which may be

open and will have slightly discoloured surfaces; the intact rock is may be weaker than the

fresh rock.

eathering/alteration grade

e weathering/alteration grade is a measure of how the core properties (i.e. strength, mineralogy, etc.) have beenanged from their original form. Although these two characteristics are often paired together, it is important to make a

tinction between weathering and alteration. Weathering is the result of exposure to and infiltration by surface agents

. surface water, ice, air, freeze-thaw cycles, organic activity, etc.) and is limited by proximity to the ground surface.

athering is a relatively recent geologic process affecting the rock mass. Alteration is a result of the geological formation

the rock mass itself, resulting in physical or chemical changes. The effects of alteration generally pre-date weathering

ects; however, it may be very difficult to distinguish the two. In addition, alteration, in the context of geotechnical

ging, is generally used to downgrade the strength of the rock mass (e.g. sericitization, chloritization, argillization,

.). However, there are alteration types that can increase the strength of the rock mass (e.g. silicification, phyllic, etc.).

ore sophisticated systems to define alteration type and intensity are often employed by geologists when characterizing

ore deposit, and should be evaluated to determine their relationship to the geomechanical properties of the rock mass.

ble 5 provides suggested weathering/ alteration grades and their associated descriptions. As for hardness values, a single

athering/alteration value should be used. If the weathering/alteration is extremely variable, consider splitting the run

o two or more intervals to accurately capture the variability.

ble 5: Weathering/alteration grades



3/W3

Moderately

Weathered or

Altered

Rock is discoloured; discontinuities may be open and have discoloured surfaces with

alteration starting to penetrate inwards; intact rock is noticeably weaker than W1/A1 rock of

the same unit.

4/W4Highly Weathered

or Altered

Rock is discoloured; discontinuities may be open and have discoloured surfaces, and the

original fabric of the rock near the discontinuities may be altered; alteration penetrates

deeply inwards. The ratio of original rock to weathered rock should be estimated where

possible.

5/W5

Completely

Weathered or

Altered

Rock is discoloured and decomposed/ friable or changed completely to a soil, but original

fabric is visible. The properties of the soil depend in part on the nature of the parent rock.

5/W6 Residual Soil Original rock fabric is completely destroyed.

scontinuity description

gineering in rock provides different challenges than those faced when using soil or concrete as engineering materials

cause rock is a discontinuous material: the rock mass is made of blocks defined by joints, bedding, faults, etc.

scontinuities). The geological and engineering properties of the discontinuities are important for excavation design.

hile the detail of observations commonly required for a single discontinuity will be familiar to exploration geologist, the

ume of data to be collected over a drilling program can be overwhelming. The level of effort required in discontinuity

a collection must be determined with consideration of the detail required for the level of design of the study, and thectical limitations of the site conditions and the field program schedule.

r detailed engineering studies is it not uncommon for every individual discontinuity to be logged and described to a level

detail that includes all of the observations outlined in the sections below. However, for a geotechnical data collection

gram running concurrently with exploration and relying on the site geology staff at, say a preliminary assessment level,

vision of this amount of detail may not be practical. During the early rock engineering investigation phases of projects

properties, the data collected should focus on:

Estimating the “average” or “typical” properties of the materials at the site

Determining what / where the materials are that may be at the ends of the spectrum of expected

engineering behaviour, i.e. where are the very weak rocks and very strong rocks or the highly plastic

soils and very stiff soils? Those materials that differ significantly from the “general” or “average” site

conditions need to be quickly identified so that they can be explored, because the weaker materials are

most likely the ones that will have the greatest impact on the stability of an excavation.

Identifying and describing the major discontinuity features: faults, weak seams or beds, and/or contacts

between geological units.

least one example of the “typical” discontinuities for the interval should be logged. Where oriented core is conducted,

will be useful to log one or two representative discontinuities of each type in each interval. The following sections

cribe the observations that should be made for logged discontinuities.

scontinuity depth

e discontinuity location should recorded as the total downhole distance along the core from the collar or other zero

erence point used for the program (drill deck, top of stick-up, ground surface) to the intersection of the structure with

core axis to the nearest centimetre. The locations of discontinuities should not be recorded with reference to the

otechnical interval from or to. Instead, the depth to the discontinuity along the core centre line is recommended. Where

ltiple discontinuities occur at the same depth, it is useful to add another digit to the depth measurement to differentiate

ween the features. For example, two discontinuities at 352.21 m could be recorded as 352.211 and 352.212. Since many

abases and 3D geological modelling software tools interpret over lapping depths as errors, this can be avoided with a

all modification to the data collection approach recommended.

iscontinuity orientation

ethods for determining orientation



e orientation of discontinuities encountered in a drillhole can be determined a variety of ways. The general concept is to

rk the core with reference to a known direction or location (generally the top or bottom of the core or magnetic north),

pending on the method used to survey the core hole, then measure the relative orientation of the discontinuities to the

erence line. The most common core orientation methods currently in use include the following methods:

Spear 1.

Cralieus2.

Ezy-Mark 3.

Clay-Imprint4.

Ball-Mark 5.

ACT6.Scribe7.

Acoustic or optical televiewer 8.

wnhole surveys of the drillhole, where optical or acoustic images are taken of the discontinuities in the walls of the core

e (method 8) have more recently become popular as technology has become more advanced and the costs for these

thods have decreased. The main advantage of this type of discontinuity orientation method is that because it does not

uire recovery of the core to get a measurement, it is less labour intensive. The main disadvantage of the televiewer

tem is that it still requires characterization of the discontinuity so that the orientations calculated can be assigned to the

propriate structure type, thus requiring some post-processing of the images and development of relational databases to

tch the data from the core logging to the downhole survey data.

pha and beta angles

r drilling programs where more traditional methods of core orientation are undertaken, the orientation of individual

continuities and geological structures can be calculated by measuring the alpha (α ) and beta (β) angles of the

continuity and the orientation of the drillhole at the location of the discontinuity, as determined by a downhole survey

a for the core hole.

e alpha angle (α ) is the angle of intersection of between the discontinuity surface and the core axis (Figure 5). This can

measured with a goniometer, carpenter’s protractor, or even a Douglas-style protractor. The alpha angle is always a

sitive angle between 0o

to 90o

.

e beta angle (β) is measured around the circumference of the core, clock-wise from the reference line provided from the

re orientation method (Ball-Mark, Ezy-Mark, ACT, clay-imprint, various scribe systems, etc.) to the tip of the

continuity farthest down-hole (Figure 1). The beta (β) angle is measured using a linear protractor which has been sized

the diameter of the core.



gure 1: Oriented core measurements

ee also

scontinuity characterization

otechnical drilling

otechnical site investigation

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