This contains methods of exploration in rock. How the rock samplers are taken. Quality of rock samples and its reporting. Along with the laboratory tests conducting on these rock samples.

1. ROCK DRILLING,
SAMPLING & TESTING
Prepare by:-
Pathariya Saraswati (MG007)
DEPARTMENT OF CIVIL ENGINEERING
FACULTY OF TECHNOLOGY, DDU (NADIAD)

2. TOPICS TO BE COVERED
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 INTRODUCTION
 METHODS OF EXPLOXATION IN ROCK
 ROCK SAMPLERS
 QUALITY OF ROCK SAMPLE
 REPORTING OF ROCK SAMPLE
 LABORATORY TESTING
 REFERENCES

3. INTRODUCTION
 In rock, except for very soft or partially decomposed sandstone or
limestone, blow counts are at refusal level (N > 100).
 If samples are required then replace the soil drilling with rock drilling
equipment.
 If rock is close to ground surface, it will be necessary to conform that it is
rock strata or a suspended boulder(s).
 When rock is involved it is necessary to know its geological background.
 A knowledge of area geology is also useful to determine both type of rock
& probable quality. This may save the expensive.
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4. SUBSURFACE EXPLORATION IN ROCKS
 Test pits
 Percussion drilling
 Rotary drilling
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5. TEST PITS
 It is most satisfactory for determining foundation conditions in rock,
since the exposed bed rock surface can be closely inspected.
 The strength of the rock & ease of excavation can be determine by
trial with a pick or compressed-air tools.
 Blocks or cylinders of the rock can be cut for laboratory tests.
 It is economical when bed rock lies within 3 m of ground surface.
 It should be used instead of boreholes when rock level is shallower
than 2m below ground level, but for depths between 2 and 3m a few
pit can be dug.
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6. Fig:- Surface Sampling (a) by advance trimming (b) by block sampling
Source:- “FOUNDATION ENGINEERING” by Ralph B. Peck, Hanson & Thornburn
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7. PERCUSSION DRILLING
 In this method breaking up of formation (rocky strata) is done by
repeated blows from a bit or chisel.
 Water should be added to the hole at the time of drilling and debris
is scooped out at an interval.
 By driving the sampler into the rocky strata, cores can be obtained
at interval using suitable tools.
 This method is suitable only for drilling bore holes in boulderous or
gravelly strata.
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8. ROTARY DRILLING
 It is most rapid method for penetrating highly resistant material
unless the deposit is very loose or badly fissured.
 A rapidly rotating drilling bit grinds the material at the bottom of the
hole into small particles.
 The particles are removed by circulating water or drilling fluid same
as wash boring.
 Usually casing is not required, but if hole tends to collapse a drilling
fluid consisting of thixotropic volcanic clay in water is used.
 Rotary boring in rock vary in diameter from about 1 inch to more
than 36 inch.
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9. Fig:- Rotary drilling rig
Source:- “FOUNDATION ENGINEERING” by Ralph B. Peck, Hanson & Thornburn
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10. ROCK SAMPLERS
 The samples obtained from rock drilling
are known as “Rock Cores”.
 Rock coring is the process in which a
sampler consisting of a tube (core
barrel) with the cutting bit at its lower
end cuts an annular hole in a rock mass.
 Thereby creating a cylindrical or core of
rock which is recovered in the core
barrel.
 Core barrels does function of both
drilling & retaining the sample attached
to drilling rod.
 Core barrels may consist of a single tube
or a double tube as shown in fig.
Fig:- Core barrel:- (a) single tube ,(b) double
tube
Source:- “FOUNDATION ENGINEERING” by
Ralph B. Peck, Hanson & Thornburn10

11.  Sample taken in single tube barrel are likely disturbed due to torsion, swelling &
contamination by drilling fluid.
 This can be avoided in hard, intact, rocky strata.
 In double tube barrel, core is protected from circulating fluid because the inner
tube of the double tube prevents washing away of core due to prolonged flow of
water.
 Cutting edge called coring bit at end of the barrel are generally of diamond bit &
tungsten carbide bit.
Diamond
core
bit(from soft
to extreme
hard rock)
Tungsten
carbide core bit
(soft to
medium hard
rock)
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12.  Core barrel can take core up to 2m length.
 Standard core range from about 1.25 inch to nearly 6 inches in dia.
Rock cores
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13. Fig:- Standard sizes of core barrels, drill rods and compatible casing.
Source:- “GEOTECHNICAL ENGINEERING” by V.N.S Murthy
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EX- Casing will fit into a hole drilled by AWX or AWM barrel & EWX or EWM barrels will fit inside EX
casing.

14. METHODS FOR TYPE OF SAMPLES
Type of sample Method of sampling
ROCK
DISTURBED
Wash samples from
percussion or rotary
drilling.
UNDISTURBED
- Core samples
- Block samples
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15. QUALITY OF ROCK SAMPLE
 The quality of rock core encountered in boring from the barrel are influenced by
drilling technique & type and size of barrel used.
 The length of core recovered inside the core barrel is measure of the soundness
of the rock.
 Core recovery in % =
Length of rock piece collected
Length drilled
∗ 100
 Better estimate of in-suit rock quality is obtained by a modified core recovery
ratio known as Rock Quality Designation (RQD).
 RQD in % =
Length of core in piece of 101.6 mm or 4 in. and above
Total length of drilling
∗ 100
15Source:- IS : 11315 (Part 11)- 1985

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Source:- IS : 11315 (Part 11)- 1985

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REPORTING OF ROCK SAMPLES

18. LABORATORY TESTS
Rock cores are subjected to various laboratory testing such as:-
1. Uniaxial compressive strength test
2. Point load test
3. Tensile strength test
4. Slake durability test
5. Sound velocity test
6. Direct shear test
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19. UNIAXIAL COMPRESSIVE STRENGTH TEST
 Compressive strength is the maximum stress that a rock specimen can sustain.
 The purpose is to determine the compressive strength of cylindrical specimen
with slenderness ratio (height/diameter) of 2to 3.
 When no adequate core length is available, a slenderness ratio of 1 or more may
be adopted and suitable correction to compressive strength is made.
σc = σc1 0.778+
0.222
(𝐿/𝑑)
Where, σc1 = compressive strength for L/d = 1
σc = compressive strength for required L/d > 1 and ≤ 2.
 The compressive strength of intact rock at failure is given by:-
σc i =
𝑃𝑓
𝐴0
Where, Pf = failure or peak load
A0 = initial c/s area of the specimen perpendicular to the direction of
loading.
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20.  The diameter of specimen shall be more than 10 times the largest grain size in
rock, preferable > 45mm but not < 35 mm.
 The load is applied to specimen through a spherical seating platen at constant
rate of 0.5 to 0.1 MPa/s until peak load and failure are obtained and test is
completed in 5 to 10 min.
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Table:- Strength of rock material base on uniaxial compressive testing
Source:- “FOUNDATION DESIGN MANUAL” by Narayan Nayak
TERM COMPRESSIVE STRENGTH (MN/m2)
Very weak Less than 1.25
Weak 1.25 to 5
Moderately weak 5 to 12.5
Moderately strong 12.5 to 50
Strong 50 to 100
Very strong 100 to 200
Extremely strong Greater than 200

22. POINT LOAD TEST
 Perform on irregular pieces of rock with diameter ranging from 30mm to 50mm.
 Test is conducted in a point load tester.
 Specimen is tested between two hardened conical tips having 5mm curvature &
60 ̊ conical angle in a rigid frame.
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23.  Point load strength index is given as: Is50 =
P
d2
where, P= failure load
d= size of specimen measured after placing between conical tips.
 The compressive strength is given by
σci = K*Is50
 Value of K varies from 15 to 35 (ISRM 1985) for most rocks; often taken in range
of 20 – 25.
 When 50 mm size particle are not available, correction is to be made by
Is50 = F
P
d2
Where, F=
𝑑
50
0.45
 Used for classification of rocks and not for design purpose.
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24. TENSILE STRENGTH TEST
 Difficulties associated with performing direct uniaxial tension tests have led
indirect method such as Brazilian split tension test.
 Diameter of disc and ring specimen for Brazilian split tension test shall not be
less than 45mm and thickness shall be approximately equal to half.
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Fig:- Setup of Brazilian split tension test
Source:- IS : 10082-1981

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 Difficulties associated with performing direct uniaxial tension tests have led
indirect method such as Brazilian split tension test.
 Diameter of disc and ring specimen shall not be less than 45mm and
thickness shall be approximately equal to half.
 In Brazilian test a disc shape specimen of rock is loaded by two opposing
normal strip loads at the disc periphery.
 Radius of jaws shall be 1.5 times specimen radius.
 Width of jaws shall be 1.1 times the specimen thickness.
 Apply continues constant rate of load on specimen such that failure in
weakest rocks occur within 15 to 30 sec. loading rate of 200 N/s is
recommended.
 Tensile strength, σt =2P/∏DL

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Fig:- Mode of failure
Source:- IS : 10082-1981

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SLAKE DURABILITY TEST
 Purpose of the test is to determine the resistance of the rock against
disintegration when subjected to cycles of drying & wetting.
 Select 10 oven dried rock lumps each of 40 -60 g, with total mass of 450 -500 g.
 Place in 140 mm diameter cylindrical drum having 2 mm mesh. Supported on
trough containing tap water at 20˚C.
Fig:-Setup of slake durability test

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 Drum is rotated at a speed of 20 rpm for 10 min. After 200 revolution lumps
are oven dry at 105˚C and weighted.
 These oven dried lumps are again subjected to second cycle of revolutions,
oven-dried and weighed.
 Slake durability index (second cycle), Id2= W3/ W1 * 100 (%)
W1= initial dry mass, W3= final dry mass
 If Id2 is between 0 to 10 % , slake durability index based on first cycle of drying
and wetting is estimated as: Id1= W2/ W1 * 100 (%)
W2 = dry mass after first cycle.
SLAKE DURABILITY INDEX Id2 (%) CLASSIFICATION
0 – 30 Very Low
30-60 Low
60-85 Medium
85-95 Medium high
95-98 High
98-100 Very High

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SOUND VELOCITY TEST
 Use to determine the velocity of elastic wave propagation through rock in
laboratory.
 It is non-destructive, fast & inexpensive test compare to other.
 The main electronic component include:-
Fig:-Layout of electronic components P & S waves

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 Rock core of length more than three times the core diameter are used.
 End of the core are to be smooth & flat. Film of grease is applied on the ends
to have proper contact with the transducers.
 Energy transmission between transducers will enable to determine velocities of
compressional (P wave / longitudinal) and shear wave (S wave/ transverse).
 Velocity of P and S waves are given by:
vp = d/tp and vs = d/ts
vp = velocity of P- wave
vs = velocity of S- wave
d= distance of travel
ts and tp = times taken by S- wave and P-waves

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DIRECT SHEAR TEST
 Test is performed to determine the shear strength parameters ‘c’ and ‘Φ’ along
the joint by shear/ sliding action.
 This test is conducted by shearing the specimen in a direct shear machine with
the failure plane oriented parallel to a shear box.
 Normal load on the vertical plane of shear could be varied and vertical shear
load is determined at failure.
 Plotting of normal stress and corresponding shear stress at failure, the shear
strength parameter could be estimated.
Fig:-Setup of direct shear test

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REFERENCES
1) ‘Engineering in Rocks for Slopes, Foundation and Tunnels’ by T
Ramamurthy
2) ‘Foundation Analysis and Design’ by Joseph E. Bowles; fifth edition.
3) ‘Foundation Design Manual’ by Narayan V. Nayak; fourth edition.
4) ‘Foundation engineering’ by Peck, Hanson, Thornburn; first edition.
5) ‘Geotechnical engineering’ by V.N.S Murthy.
6) IS-8764 : 1998, ‘Method for determination of point load strength index of
rocks’.
7) IS- 10082 : 1981,’Method of test for determination of tensile strength by
indirect tests on rock specimens’.
8) IS- 9143 : 1979, ‘Method for the determination of unconfined
compressive strength of rock materials’.
9) IS : 11315 (Part 11)- 1985, ‘Method for the quantitative descriptions of
discontinuities in rock masses.’

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