Geotechnical logging techniques

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Core logging procedure


The following steps are suggested during the core logging process:


  1. Clean the core of drilling fluids or mud.
  2. Mark major structures, proposed point load testing locations, and depths (every 1-2 metres) on undisturbed core in splits.
  3. Photograph the core in the splits (if using triple tube method) with a scale placed in the picture and a whiteboard indicating what depth the core has been obtained from.
  4. Complete the Discontinuity and core description logs.
  5. Transfer the core from the splits to a labelled core box.
  6. Once a core box is full, take a single photograph of the core box with a scale.


The steps are detailed in the following sections.

 

Core photographs and preparation

 

One of the most important things to do at the drill rig is photograph the undisturbed core in the splits. These photos may be used later to confirm televiewer images and will be an invaluable resource on the rock mass and for review of the design work.

Proper core photos require that the core be cleaned prior to photographing. When core is covered in drilling mud, structural information can be obscured making it difficult to determine lithologies. Take the time to properly clean the core.  The core should 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 bottle 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
Item Colour Symbol
Whole meter depth White Whole meter depth.jpg
Major structures Red Major structures.jpg
PLT samples Yellow Plt.jpg
Mechanical breaks Blue Mechanicalbreaks.JPG


Notes:

  1. 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”.
  2. The numbers in the Whole meter depth and Major structures symbols indicate the depth in meters.


Intervals

The properties used in the core description are recorded by intervals for each run of core. An interval represents a change in lithology, alteration, and/or rock mass quality. The benefit of logging on an interval basis is that it allows for distinctly different units within one run of core to be assigned their own properties. This prevents the need for averaging different units over the length of a run, which can lead to overestimating / underestimating material properties. Photograph 1 below provides an example of a change in rock mass quality. Intervals should be at least 30 cm in length and will be at most the length of the core run. Intervals are also numbered sequentially. For example if you have 100 runs you should have between 100 and 500 intervals, assuming a maximum run length of 1.5 m (5 feet). The start depth (i.e. depth from) and the end depth (i.e. depth to) for each interval should be recorded as measured from the top of the hole. This is strictly a function of where the drill bit started and ended during the run, and may be less than the length of the maximum run if blocking of the core barrel occurs, the bit requires replacement, etc.

Rock core broken into intervals.jpg 

Figure 1: Example of rock core broken into intervals

 

Depths of zones of substantial core loss may have to be estimated. For example, if a 1.5 m run of core is completed and only 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 (second interval), it is likely that the core loss occurred in the crushed zone. Consequently, the end depth of the first interval and the start depth of the second interval should be adjusted to account for the 0.5 m loss of core in the crushed zone. If a downhole survey (e.g. with a televiewer) has been completed, the zone of core loss may be more accurately defined by the images from the survey, requiring modification of the logs during the matching process with the televiewer and core logging data.

Core description

The Core description portion of the log covers the lithology, interval determination data, and the rock mass classification. Below is a description of each logging parameter. Both the top and the bottom of the interval are to be recorded.

Colour and rock description


Colour and rock descriptions should be logged as part of the core logging procedure to indentify the lithologies and alteration sequences encountered. Logging should be based on easy to identify attributes that will in most cases allow rock type to be determined quickly and easily. Such attributes include:

  • Pattern
  • Colour
  • Grain size
  • Texture
  • Fabric
  • Lithology 
  • Alteration

Logging these parameters separately and on an interval basis will allow for recognition of subtle variations that would normally be smoothed over in the summary log, and will ensure that the descriptions produced for final reporting are clear, concise, and repeatable. Codes describing the above should be decided upon in advance, and kept as simple as possible for ease of data entry and for consistency. An example of possible codes for a geotechnical core logging scheme is included in Table 2 below.


Table 2: Examples of rock core desciption codes
Color
Modifier Code Pattern Code Primary/secondary color Code
Light L Banded BA Pinkish PK
Dark D Streaked SK Reddish RD
Blotched BL Yellowish YW
Mottled MT Brownish BR
Speckled SP Olive OL
Stained ST Greenish GR
None NO Blueish BL
Greyish GY


Grain size
Code Term Particle Size Examples
VC Very course > 60 mm Porphyries-w measureable grains
C Coarse 2 -60 mm Congromerate,Breccia,Gneiss-w/measureable grains
M Medium 0.06- 2 mm Sandstone, Gabbro, Granite, Schist - having clearly visible grains
F Fine 0.002 - 0.06 mm Tuff, Siltstone, Claystone, Mudstone, Basalt
VF Very fine <0.002 mm





Texture
Code Texture Description
AP Aphanitic Grains cannot be seen with naked eye
EQ Equigranular All grains are the same size
BM Bimodal Two sizes of crystal exist in rock
TR Trachytic Alignment of grains in a volcanic rock parallel to flow direction
AC Acicular Crystals are needle shaped
DM Diamitic Gap graded, matrix supported clasts (sedimentary)


 

Fabric
Code Fabric Description
GN Gneissic Alternating layers of different colour or texture
BX Brecciated Angular fragments that have been healed
BD Bedded Deposited in layers, can be in sedimentary or volcaniclastic rocks
IB Interbedded Beds alternating with others of a different character
MA Massive No crystal or grain fabric (homogeneous)
PR Porphyritic Intrusive 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 material
FO Foliated Mineral are aligned due to shearing or metamorphism



 

Lithology
Code 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 dyke Dyke containing mostly mafic minerals. Typically have bleached contacts at KSM
DT Diorite Intrusive rock, mainly plagioclase feldspar
FP Feldspar porphyry Lath shaped feldspar crystals make up a significant percentage of the rock mass (>=20%)
SC 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
SH Shale Fine grained sedimentary rock, laminated, fissile
SS Sandstone Clastic sedimentary rock, grains are sand sized and may be cemented with clay / silt sized particles
AR Argillite Very fine grained sedimentary rock, indurated, lacks the fissility of shale
UD Undistinguishable This term should be used as little as possible




 

Alteration codes
Code Alteration type Site specific description from exploration logs Literature description Mineral assemblage Diagnostic features
ARG Argillic -introduces a wide variety of clay minerals, includes kaolinite, smectite, and illite
-can also have kaolinite + quartz + hematite + limonite assemblage
clay minerals -feldspar grains have been replaced with clay
-slippery feel on discons
CHL Chloritic -darkened groundmass when pervasive chlorite, muscovite, quartz, albite
CAR Carbonate -greater than 3% k veins -addition of any carbonate minerals, typically calcite, ankerite, dolomite calcite, dolomite, malachite -veins / matrix react with acid
HFS Hornfels -indurated and strengthened -thermal alteration, seems "baked" resulting in stronger and more indurated rock mass than parent hornblende, plagioclase,  chlorite, biotite
HEM Hematite/Iron Oxide oxide minerals -red/brown/orange
PHY 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
POT 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
PRO 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
SIL 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



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

Total core recovery length

The total length of core recovered by the drillers from an interval is measured. In addition to providing a first indication of zones of poor rock mass quality or drilling problems, total recovery can be used to check the run block depths provided by the drillers. Also, it is not uncommon for the recovery length to be greater than the drilled length; this often happens when the core breaks above the bottom of the hole on the previous run, or as a result of errors in the measurement of the length drilled. Where multiple intervals and core loss (or gain) occur between two run blocks, a judgement must be made for which 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 difference
  • select the interval that contains the most fractured rock as having the difference

A mix of strategies may be appropriate; however, a single strategy is generally recommended for consistency and ease of comparison of data. For programs where corehole televiewer programs are conducted, the downhole images may assist in identifying the major zones of core loss.

Rock quality designation (RQD) length

The rock quality designation (RQD) is a modified core recovery measurement (Deere and Deere, 1989). For each interval, the total length of all core pieces longer than 10 cm (4 in) as measured along the core centerline, should be determined and recorded as shown in Figure 2.

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

When 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 length
  • The 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

Number of discontinuities

The number of geological discontinuities (fractures, joints, shears, bedding, etc.) within each interval is counted and recorded. Breaks in the core from the process of drilling or boxing the drill core (mechanical breaks) are not included in this count.

Mechanical breaks are identified by sharp core edges at the break and will often have clean breakage surfaces with no infilling and no discolouration. If the cause of the break in the core is in doubt, treat the break as a natural feature and include it in the discontinuity count. The core shown in Figure 3 has this kind of clear breakage.

Geotechnical Logging Figure 3.jpg

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

Where discontinuities with thick infillings, faults, or zones of soil-like material are encountered count 1 discontinuity per 1 cm of infilling, fault zone, or soil zone thickness along the core centre line.

Figure 4 shows a sample where discontinuities must be estimated.

Geotechnical Logging Figure 4.jpg

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

For intervals that have closely spaced discontinuities too numerous to count (possible in bedded, laminated, or foliated rocks), resulting in “discs” of core, the number of discontinuities can be estimated. This is done by measuring the average size of the core pieces and dividing the interval length by the average piece size. For example, a 30 cm zone of broken rock where the average particle size is 3 cm in diameter would count as 10 discontinuities. “Default” numbers for highly fractured rock should not be used

Number of sets

The number of discontinuity sets present in the rock mass is used to determine the joint set number (Jn), a parameter used in Barton’s Q rock mass classification system. The most accurate way to determine the number of joint sets is to process core orientation data and determine the number of sets from stereographic projection of the discontinuity data. In the absence of core orientation data it may take several drill core runs to see all of the sets present, particularly if there are widely spaced sets present. This parameter can be extrapolated forwards and backwards in the drill core from zones where the set numbers are obvious; however, make sure that if there are changes occurring in the structural fabric they aren’t missed by averaging the sets over long sections of hole. Use whole numbers only, so if you have 2 sets and no other distinct features, use 2, if there are other features use 3. This is a slightly conservative approach that is acceptable because it is difficult to determine at the core scale whether widely spaced discontinuities form sets. It is important not to include mechanical breaks in this number.

Strength grade

The strength grade (Table 4), sometimes referred to as hardness grade, is a field estimate of the strength of the intact material. It is important to use your hands, knife and rock hammer when estimating the strength of a sample. When using the hammer remember that only a firm blow need be applied. Also, make sure that the induced break does not occur along a discontinuity, otherwise the strength test is invalid.


A single value should be used for the strength grade. If the grade within the interval ranges from one hardness grade to another (e.g. is between 3 and 4), use half values (e.g. 3.5). If the hardness is extremely variable, consider splitting the run into two or more intervals to accurately capture the variability.

 

Table 4: Field strength grades (ISRM 1978)

Grade Field identification Description UCS (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
R3 Cannot 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
R1 Crumbles under firm blow with point of geological hammer; can be peeled by a pocket knife Very weak 1-5
R0 Indented by thumbnail Extremely 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 Soft 0.012 - 0.025
S1 Easily penetrated several centimeters by fist Very soft < 0.012


Weathering/alteration grade

The weathering/alteration grade is a measure of how the core properties (i.e. strength, mineralogy, etc.) have been changed from their original form.  Although these two characteristics are often paired together, it is important to make a distinction between weathering and alteration.  Weathering is the result of exposure to and infiltration by surface agents (i.e. surface water, ice, air, freeze-thaw cycles, organic activity, etc.) and is limited by proximity to the ground surface. Weathering is a relatively recent geologic process affecting the rock mass.  Alteration is a result of the geological formation of the rock mass itself, resulting in physical or chemical changes. The effects of alteration generally pre-date weathering effects; however, it may be very difficult to distinguish the two. In addition, alteration, in the context of geotechnical logging, is generally used to downgrade the strength of the rock mass (e.g. sericitization, chloritization, argillization, etc.).  However, there are alteration types that can increase the strength of the rock mass (e.g. silicification, phyllic, etc.).  More sophisticated systems to define alteration type and intensity are often employed by geologists when characterizing the ore deposit, and should be evaluated to determine their relationship to the geomechanical properties of the rock mass.


Table 5 provides suggested weathering/ alteration grades and their associated descriptions.  As for hardness values, a single weathering/alteration value should be used.  If the weathering/alteration is extremely variable, consider splitting the run into two or more intervals to accurately capture the variability.

Table 5: Weathering/alteration grades

Grade Description Field identification
A1/W1 Fresh and Unweathered Parent rock showing no discoloration, loss of strength or any other weathering effects.

Strength may be increased by some alteration types.

A2/W2 Slightly 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.
A3/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.
A4/W4 Highly 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.
A5/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.
A5/W6 Residual Soil Original rock fabric is completely destroyed.


Discontinuity description

Engineering in rock provides different challenges than those faced when using soil or concrete as engineering materials because rock is a discontinuous material: the rock mass is made of blocks defined by joints, bedding, faults, etc. (discontinuities). The geological and engineering properties of the discontinuities are important for excavation design. While the detail of observations commonly required for a single discontinuity will be familiar to exploration geologist, the volume of data to be collected over a drilling program can be overwhelming. The level of effort required in discontinuity data collection must be determined with consideration of the detail required for the level of design of the study, and the practical limitations of the site conditions and the field program schedule.

For detailed engineering studies is it not uncommon for every individual discontinuity to be logged and described to a level of detail that includes all of the observations outlined in the sections below.  However, for a geotechnical data collection program running concurrently with exploration and relying on the site geology staff at, say a preliminary assessment level, provision of this amount of detail may not be practical.  During the early rock engineering investigation phases of projects or 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.

At least one example of the “typical” discontinuities for the interval should be logged.  Where oriented core is conducted, it will be useful to log one or two representative discontinuities of each type in each interval.  The following sections describe the observations that should be made for logged discontinuities.

Discontinuity depth

The discontinuity location should recorded as the total downhole distance along the core from the collar or other zero reference point used for the program (drill deck, top of stick-up, ground surface) to the intersection of the structure with the core axis to the nearest centimetre. The locations of discontinuities should not be recorded with reference to the geotechnical interval from or to. Instead, the depth to the discontinuity along the core centre line is recommended. Where multiple discontinuities occur at the same depth, it is useful to add another digit to the depth measurement to differentiate between the features. For example, two discontinuities at 352.21 m could be recorded as 352.211 and 352.212. Since many databases and 3D geological modelling software tools interpret over lapping depths as errors, this can be avoided with a small modification to the data collection approach recommended.

Discontinuity orientation

Methods for determining orientation


The orientation of discontinuities encountered in a drillhole can be determined a variety of ways. The general concept is to mark the core with reference to a known direction or location (generally the top or bottom of the core or magnetic north), depending on the method used to survey the core hole, then measure the relative orientation of the discontinuities to the reference line. The most common core orientation methods currently in use include the following methods:

  1. Spear
  2. Cralieus
  3. Ezy-Mark
  4. Clay-Imprint
  5. Ball-Mark
  6. ACT
  7. Scribe
  8. Acoustic or optical televiewer

Downhole surveys of the drillhole, where optical or acoustic images are taken of the discontinuities in the walls of the core hole (method 8) have more recently become popular as technology has become more advanced and the costs for these methods have decreased. The main advantage of this type of discontinuity orientation method is that because it does not require recovery of the core to get a measurement, it is less labour intensive. The main disadvantage of the televiewer system is that it still requires characterization of the discontinuity so that the orientations calculated can be assigned to the appropriate structure type, thus requiring some post-processing of the images and development of relational databases to match the data from the core logging to the downhole survey data.

Alpha and beta angles

For drilling programs where more traditional methods of core orientation are undertaken, the orientation of individual discontinuities and geological structures can be calculated by measuring the alpha (α ) and beta (β) angles of the discontinuity and the orientation of the drillhole at the location of the discontinuity, as determined by a downhole survey data for the core hole.

The alpha angle (α ) is the angle of intersection of between the discontinuity surface and the core axis (Figure 5). This can be measured with a goniometer, carpenter’s protractor, or even a Douglas-style protractor. The alpha angle is always a positive angle between 0o to 90o.

The beta angle (β) is measured around the circumference of the core, clock-wise from the reference line provided from the core orientation method (Ball-Mark, Ezy-Mark, ACT, clay-imprint, various scribe systems, etc.) to the tip of the discontinuity farthest down-hole (Figure 1). The beta (β) angle is measured using a linear protractor which has been sized for the diameter of the core.

Oriented core measurements.jpg
Figure 5: Oriented core measurements

See also

Discontinuity characterization
Geotechnical drilling
Geotechnical site investigation

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