tugas geomekanik

7/27/2019 tugas geomekanik

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Engineering geology

Logging geologi teknik dari core pemboran

Drill core provide valuable information about the geological conditions. The cores should

therefore be described and logged in a proper manner. The description can be carried out in

various ways, and different companies may have their own procedures. However, there are

some parameters a core log should always contain. There are RQD and joint frequency in

addition to a general geological description. A rock mass classification system such as the Q-

system or RMR should be used. This report describes a method for systematic logging of

cores where most needs should be covered. Tables for estimation of different Q- and RMR-

parameters are enclosed.

Introduction

Core drilling is a method often used during preinvestigations for tunnels, cavern and dams.

Description core is important that this description is done systematically to get an uniform

core log. The parameters that are needed will differ according to the project type. During

boring in a dam foundation for example, thepermeability of the rock masses will be of great

interest, whereas for a tunnel project the joint spacing will be very important.

The core diameter varies, but is usually between 30 and 100 mm. The cores are placed in

boxes which may contain from 3 to 10m of cores. Often it will be necessary to take out

samples from the cores for laboratory testing, however, no samples should be taken out

before the logging has been finished.

2 EQUIPMENT

2.1 Special forms should beused

For core logging any advanced equipment is usually not necessary. Most of the work

can be done by a ruler, paper and pencil. For a systematic logging special forms are

necessary. Fig. 1 shows a form which can be used for the presentation of the logging data.

Fig. 2 shows an example of the same form infilled In addition it is practical to have a form

to be used directly during logging, see Fig. 3. The last form is only a sketch of the core

box where the different parametervalues can be filled in, the joints can be markedwhere they are found in the core, and the length of the different core pieces can be

noted.

2.2 The cores should be photographed

Before the logging starts, the core boxes should be photographed in order to have a

documentation of the intact cores. A ruler should be used as a scale in the photographs,

2.3 Deluted hydrochloric acid can be used to identify calcite


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Calcite is often found as a joint infill. When diluted hydrochloric acid is added to this

mineral, it starts to foam, and this can be used to identify calcite. In addition a little

knife is practical to test the mineral hardness. More exactmineralogical analyses will

sometimes be necessary, and for this purpose samples must be picked out.

2.4 Profilometer and protractor can be useful to measurejointgeometry

The joint roughness can be measured by a profilometer, and a geological compass or a

protractor can be used for measuring the angle between the joint and the core.

3 LOGGING UNITS

Before the core logging starts, all the cores from a borehole should be inspected and divided

into practical sections which then will be logged as a unit. A logging unit should be some

metres long and should have an uniform rock mass quality. A section in a core box often

contains one metre of core. The core length i. e. the depth in the borehole is often marked

for each metre. Sometimes the marking is related to each time the core has been taken up

from the borehole i. e. usually for lengths between two and three metres.

Several of the engineering geological parameters can be determined for each metre, but for

some parameters one metre will be too short. A convenient length of a logging unit may be

5-10 m i. e. a core box. However, in a logging unit the rock mass quality should be rather

uniform, and in some cases it may be necessary to use units shorter than 5 m. Marked

weakness zones must be described as own units regardless of thickness. If such zones are

Jogged together with rocks of good quality, they may more or less "disappear" in the log.

If different rocks occur in a borehole, it will often be convenient to divide the core into units

along rock type boundaries. In the log it must clearly be pointed out which unit the different

parameter values belong to.

4 GEOLOGICAL DESCRIPTION

4.1 Microscopic study of rocks may be useful

The rock should be described in terms ofmineralogy, grain size, texture and weathering.In

order to get an exact description, it will be useful to prepare thin section and carry out a

microscopic study.

4.2 The rock distribution in the borehole must be described

the rock distribution is marked by different symbols in its own column. It is appropriate to

use the special symbols recommended by the national geological survey. In the column

"Geological description" in the form additional information about the rocks can be given. A

short review of the geological history of the region should also be given in the report.

5 ENGINEERING GEOLOGICAL LOGGING

5.1 Generally


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The form, Fig. 1 may serve as a memory list during logging. On the left hand side of the form

there are columns for bore length and box number. In the heading of the form information

about the project, location, elevation and borehole orientation is noted. This information is

partly found on the core boxes and partly in the report from the boring company.

During the logging the form shown in Fig. 3 can be used. The core should be systematically

studied metre by metre. Each fracture is marked and the length of the different core pieces

are measured to the nearest centimetre. This length should be measured along an

estimated line through the centre of the core, see Fig. 4. To go faster on with the logging, it

may only be necessary to measure the pieces longer than 10 cm. It must be pointed that it is

only pieces between natural joints that are considered.

The angle between the joints and the core must be estimated, eventually it canbe measured

exactly by means of a protractor. The joint angle for the most prominent joint sets is filled

into a column in the form in Fig. 1. The joint roughness and the joint filling must also be

described. The mineral type ofthe infill should be marked by special abbreviations. A

legend for these abbreviations should be enclosed

RQD and joint frequency is based on natural joints. Therefore each fracture must be

studied to find out if it is a natural joint or a artificial fracture causedby the drilling or the

handling of the cores.If artificial fractures are marked in the form, Fig. 3, they should be

given a special symbol Usually it is easy to divide between natural joints and artificial

fractures. Natural joints are often weathered and may have a mineral coating or filling.

Artificial fractures are usually very irregular and rough without any sign ofweathering.

In rocks with schistosity or foliation it may be difficult to distinguish between natural

joints and artificial fractures. These types of rock often have a high number of parallel

weakness planes, and the number of fractures occurring along these planes may be more or

less accidental depending on theboring equipment and handling of the cores. During

logging fractures along schistosity and foliation usually are considered as natural joints,

but this shouldbe evalutated for each rocktype.

Sometimes core loss occurs. shorter or longer sections of the core are missing. Core loss is

usually recorded in the report from the boring company. It is of course impossible to

determine the different parameter values where the core is missing, but the RQD will usually

be zero since core loss will as a rule will be caused by very jointed or altered rocks.

In strongly jointed rocks, the core consists only of small pieces. The length of the crushed

section should be noted in a column in the form in Fig. 1. One must be aware of that such

crushed sections may take up more space in the core box than the real core length it

corresponds to. The length of the crushed section should therefore be calculated by

comparing it to the total length of intact core in the logging unit underconsideration.

5.2 Q-values

By using the Q-method the stability of a rock mass can be described numerically. Thecalculation of the Q-value is based on 6 parameters, During core logging the values of the


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parameters RQD, Jn, Jr, and Jn can be determined rather exactly, and the values are noted in

special columns in the logging form, Fig. 1.

RQD is defined as "the sum of the length of all core pieces (between natural joints) longer

than 10 cm in per cent of the total core length." During logging it is often practical to

calculate the RQD for every metre or for every take up. Joint frequency which is the number

of natural joints per metre should also be calculated for the same sections as RQD.

In rocks with well developed foliation or schistosity the determination of RQD and joint

frequency may be uncertain. The apparent number of joints may here depend on the boring

equipment, the handling of the cores and the core diameter. There will usually be a higher

number of breaks in a thin core than in a thick one. The point of time of logging may also

have effect on the result.

In a shale for example there may be very few breaks just after the core has come up from the

borehole and RQD may be 100%. After some weeks, when the core bas dried out, a diskingeffect may have taken place, and the core may consist of only thin disks, and RQD = 0%. As

a rule all fractures along the foliationare considered when the RQD-value is calculated,

however, these problems should be taken into consideration during the evaluation of the

logging results.

Jnis a parameter describing the number of joint sets,In the cores the Jn - value can be estimated

by considering the angle between the joints and the core which is noted in a column in the form, Fig.

1. The Jn -values should usually be estimated for core sections of several metres, because if a joint

set apparently is missing in some metres of core, it may be caused by large joint spacing or that the

joints are more or less parallel to the core. The estimated Jn value from drill cores will therefore

usually be a minimum value.

Jris describing the joint roughness, see Table 1 and Fig. 5. A problem during core logging

may be that the joint faces often are so short that it will be difficult to see if the joints are

planar or undulating. Field observation will therefore be a useful supplement.

Ja is describing the joint filling and can be determined by observing the thickness and type of

minerals in the infill. The Ja - value can easily be underestimated in the cores because soft

mineral fillings for example of clay minerals may be washed away during drilling.

Jwcan in principle only be determined from observations in a tunnel. Permeability tests are

often carried out associated with core drilling. These tests are carried out by isolating sections,

for example 5m, of theborehole by packers. Water with a fixed pressure is then pressed into

this isolated section, and the leakage of water into the rock is measured.

As a unit for water loss Lugeon is used. Lugeon is defined as leakage in litreper minute per

metre with an overpressure of 1MPa. The Lugeon values are noted in a column in the form,

Fig.l.A conversion of the leakage to 1 MPa overpressure by supposing that the leakage are

increasing proportionally with increasing pressure will not always be valid. By a certain


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pressure the fractures may start to open which will lead to a rapid increase in leakage. In

such cases it will be informative to note the real leakage by the used pressure.

To some degree of accuracy Jw can be estimated from the permeability tests. in dry holes or

holes with only small leakage (< 1 Lugeon) Jw can be determined to 1. With higher leakage

(1-10 Lugeon) a Jw- value of 0.66 can be used. For even higher Lugeon values and rock

masses with open fractures a lower Jw - value will be relevant. If tunnels are taken into

consideration, the ground water pressure at the tunnel depth must be considered.

SRF is the stress reduction factor and must be related to a fixed tunnel at a fixed depth.In

many cases SRF can be set to 1, and this value can be used in rock masses of good quality in

moderate depth. For a tunnel near the surface the stress will usually be low, and a SRF value of

2.5 will be realistic. If there are marked fault zones in the cores, i. e. sections with crushed core, a

SRFvalue of 5 will be relevant. In cases with rather soft rock or rocks with swelling minerals

even higher SRF must be used. For tunnels with overburden of several hundred metres, high

SRF-value (50-400) may be used in cases of rock burst.

Calculation of Q-values

The Q-values are calculated from the following formula:

When a Q-value based on core logging data shall be calculated, it is necessary to make some

assumption, especially concerning Jwand SRF.To get a review of the rock quality in a borehole,

the spreadsheet shown in Fig. 6 may be used. Here the Q-values and the different parameter

values are shown. Fig. 7 is an example of a completed spreadsheet. Another spreadsheet based on

histograms is shown in Fig. 8. This will be useful to show the variation of the different Q-

parameter values in a borehole. This spreadsheet also calculates the max.-, min.- and mean Q-

values.

The Q-values can be used to describe the support in rock masses nominally. This is based on

the empirical connection between the existing support and Q- values in more than 1000

reference projects, see Fig. 9

5.3 RMR-valuesRMR together with the Q-system are the two most used classification system forthe stability

in rock masses.. The different parameter values for RMR can be filled into the spreadsheet, Fig.

10. The values are determined from Table 2 and Fig. 11, 12 and 13. The RMR-values are

calculated by addition of the different parameter values. It is practical to use the same logging

units as for the Q logging.

Compression strengthThe compression strength for the rock forms the base for the first of theparameters in the

RMR-system. An approximate value for the compression strength can be measured by a

Schmidt hammer, see Fig. 11 and 12.During the measurement, the cores have to be clamped to


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a heavy base. If Schmidt hammer measurements and other tests are missing, the compression

strength has to be stipulated from the petrographic description of the rock.

RQD

Based on the RQD-value the parameter values is calculated from the curve in Fig. 12.

Joint spacing

Based on the average joint spacing in the logging unit the parameter value is determined from the

curve in Fig. 13.

Joint aperture

Based on observations the parameter values is determined from Table 2. Determination of

joint aperture may be difficult from drill cores, and field observation will therefore be a good

supplement.

Joint roughness

The parameter value is determined from Table 2.

Joint infill

The parameter values will depend on the thickness of the infill and whether the infill is soft or

hard, see Table 2.

Grade of weathering

The parameter values are determined from Table 2.

Water condition

The parameter value is based on leakage, see Table 2. As for 1w in the Q-system this

parameter may be stipulated from the Lugeon values in the borehole.

Joint orientation

This parameter is evaluated from the joint orientation related to a tunnel, a foundation or a

slope, see Table 2. If the drill cores cannot be related to a certain tunnel or slope, the

parameter value "fair" can be used as a preliminary value.


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SPECIALTESTS

For some projects special tests may be required on the cores.

In order to give a exact petrographic description concerning mineralogy, grade or alteration,

grain size etc. use of microscope will be necessary. Thin sections must then be prepared.

To study joint infill X-ray diffractometer will be convenient. A problem may be that clay

minerals are easily washed away during drilling, and the remaining material in the cores may

not be representative.

To test the rock strength a Schmidt hammer can be used. A heavy metallic base will then be

necessary to clamp the core. Point load test is another method especially designed for cores.

For point load test the grade of saturation of the samples is important.

The cores may also be used for uniaxial compression strength tests or triaxial tests.

To measure the friction angle along joints the tilt test is used. Special equipment is needed for

tilt tests, and samples consisting of both faces of a joint are required.

To measure the roughness (JRC) a profilometer can be used. With this equipment profiles

along any direction of a joint can be drawn, and the roughness amplitude can be measured.

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