CHP 9.1 (RQD)

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CHAP 9: Classification of rock mass: RQD, RMR & Q-system:Soils are classified according to types & properties e.g. granular soil (φ-soil) & clay (c-soil). Rocks are also classified based on properties. This is to help in understanding their characteristics as construction materials & components of engineering structures thus, helping in design & construction work. Classification of rocks based on geological aspects are subjective: igneous, sediment & metamorphic. For design & construction, objective classification (numerical values) is more appropriate – classification of rock based on prevailing weakness planes, number of joint set, & engineering properties like strength, weathering grade & permeability.

Rock Quality Designation, RQD:The most basic engineering classification introduced by Deere (1964), is an index of assessing rock quality quantitatively.

It is more sensitive index of the core quality than the core recovery ((length of core/length of core barrel) × 100 %)

The RQD is a modified per cent core recovery which incorporates only sound pieces of rock core that are 100 mm or greater in length along core axis.

RQD = {(Σ Xi) / (total length of core, L)} x 100%.Xi = core length ≥ 100 mmL = length of core recovered (1.5m if barrel is full)

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Core samples obtained from rock drillingCore samples obtained from rock drilling

Wash boring machine YBM 2Wash boring machine YBM 2

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Double tube core barrel is used to Double tube core barrel is used to obtain rock core samples during obtain rock core samples during wash boring. Length of barrel is wash boring. Length of barrel is 1500 mm.1500 mm.

If core barrel is full with rock If core barrel is full with rock sample (100 % recovery, R) then, sample (100 % recovery, R) then, the total length of core is 1500 the total length of core is 1500 mm.mm.

Double tubeDouble tube core barrel core barrel to obtain rock core to obtain rock core samples during wash samples during wash boring.boring.

Triple tubeTriple tube core barrel core barrel ensures minimal ensures minimal disturbance to the core disturbance to the core samplesample

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Method of obtaining RQD:

(1) Direct method:

CORE SAMPLES OF IN SITU ROCK MASS: ISRM recommends a core size of at least NX size (54.7 mm dia.) drilled with double-tube core barrel using diamond coring bit.

Artificial (not natural) fractures or joints (that occurs during drilling) can be identified by close fitting (matched joint surface) of cores and fresh (unstained) surfaces.

All the artificial joints are ignored while counting the core length for RQD.

A slower drilling rate will also give a better RQD

Correlation between RQD and Rock Mass QualityCorrelation between RQD and Rock Mass Quality

S. No. Rock mass quality RQD (%) 1 Very poor 0 - 25 2 Poor 25 - 50 3 Fair 50 - 75 4 Good 75 - 90 5 Excellent 90 - 100

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1500 mm (core length)

KEY: --------- Fractures induced by drilling. ______ Joints.

130 mm

90 mm

90 mm

75 mm

100 mm

120 mm

135 mm

220 mm

75 mm

95 mm

70 mm

70 mm

230 mm

1500 mm (core length)

KEY: --------- Fractures induced by drilling. ______ Joints.

130 mm

90 mm

90 mm

75 mm

100 mm

120 mm

135 mm

220 mm

75 mm

95 mm

70 mm

70 mm

230 mm

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Example of RQD calculationExample of RQD calculation

Fractures induceby drilling operation

Safe bearing pressure Safe bearing pressure –– based on rock based on rock strength & fracturingstrength & fracturing

SAFE BEARING PRESSURE – guidance values

100

4

8

12

UCS (MPa)

25

1

3

5

SBP (MPa)

10

0.2

1

2

RQD (%) 25 70 90

Fracture spacing (mm) 60 200 600

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RQD is perhaps the most commonly used method to characterisethe degree of jointing in borehole cores, although this parameter also may implicitly include other rock mass features like weathering and ‘core loss’.

(2) Indirect method:

SEISMIC PROPERTIES OF ROCK: The seismic survey method makes use of the variations of elastic properties of the rock strata that affect the velocity of the seismic waves travelling through them, thus providing useful information about the subsurface materials (e.g. cavities, dense rock, jointed rock).



The following information of the rock masses can be inferred from seismic data:

(a) Location & configuration of bed rock and geological structures in the subsurface.

(b) The effect of discontinuities in rock masses may be estimated by comparing the in situ compressional wave velocity with sonic velocity of intact drill core obtained from the same rock mass.

[Since in situ rock are fractured and jointed hence, compressional wave velocity is lower compared to intact core]

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Based on seismic data of in situ rock mass and intact rock sample, RQD can be estimated:

RQD (%) ≈ Velocity ratio≈ (VF / VL)2 × 100

Where VF is in situ compressional wave velocity (obtained from seismic refraction method in the field), and VL is compressionalwave velocity in intact rock core (obtained from ultrasonic velocity test in laboratory).

Sonic velocity test on core sample (nonSonic velocity test on core sample (non--destructive destructive test) to give test) to give VpVp of rock sample in laboratoryof rock sample in laboratory

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Typical Seismic Velocity PlotsTypical Seismic Velocity Plots

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3660-6100Unweathered limestones, granites, gneiss, other dense rocks.

2440-3660Hard shales and sandstones, interbedded shales and sandstones, slightly fractured hardrocks.

1680-2440

Hardpan; cemented gravels; hard clay; boulder till; compact, cobbly and bouldery materials; medium to moderately hard shales and sandstones, partially decomposed granites, jointed and fractured hard rocks.

1460-1830Compacted, moist clays; saturated sands and gravels; soils below water table; dry medium shales, moderately soft sandstones, weathered, moist shales and schists.

1460-1520Water, saturated silts or clays, wet gravels.

910-1460Dry, heavy, gravely clay; moist, heavy clays; cobblymaterials with considerable sands and fines; soft shales; soft or weak sandstones

460-910Dry gravels, moist sandy and gravely soils; dry heavy silts and clays; moist silty and clayey soils.

300-490Dry sands, loams; slightly sandy or gravely soft clays.

180-370Dry, loose topsoils and silts.

P-wave velocity m/s

Type Of Rocks



VOLUMETRIC JOINT COUNT OF IN SITU ROCK MASS: Where cores are not available, RQD may be estimated from number of joints (discontinuities) per unit volume Jv.

A simple relationship which may used to convert Jv into RQD for clay-free rock masses is:

RQD = 115 − 3.3 Jv

Where Jv represents the total number of joints per cubic meter or the volumetric joint count.

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Jv has been described by Palmstrom (1986) as a measure for the number of joints within a unit volume of rock mass defined by:

Where Si is the average joint spacing in metres for the ith joint set and J is total number of joint sets except the random joint set.

∑=

=J

1i)1(

iv S

J

Take 1 mTake 1 m3 of a rock of a rock mass, with 2 joint sets, mass, with 2 joint sets, J1 & J2J1 & J2Avg. spacing, S1 = 0.2 Avg. spacing, S1 = 0.2 Avg. spacing, S2 = 0.3 Avg. spacing, S2 = 0.3

JJvv = = ∑∑JJii=1=1 (1 / (1 / SiSi))

JJvv = (1/0.2) + (1/0.3)= (1/0.2) + (1/0.3)JJvv = 5 + 3.3= 5 + 3.3JJvv = 8.3= 8.3

RQD = 115 RQD = 115 −− 3.3 3.3 JJvvRQD = 115 RQD = 115 −− 3.3 3.3 ×× 8.38.3RQD = 88 %RQD = 88 %

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Joint sets in rockJoint sets in rock

Joint sets in granite Joint sets in granite –– usually 3 sets, almost usually 3 sets, almost perpendicular to each other. perpendicular to each other.

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Joints and joint sets in rock Joints and joint sets in rock –– Joint spacing is the Joint spacing is the horizontal distances between horizontal distances between each joint in a seteach joint in a set and and

measured along a horizontal line measured along a horizontal line

Take 1 mTake 1 m3 3 of rock mass with three joint sets, J1, J2 and of rock mass with three joint sets, J1, J2 and J3 (major joint set only). Measure the spacing between J3 (major joint set only). Measure the spacing between

each joint (in a given set) along a horizontal line each joint (in a given set) along a horizontal line

Joint set 1, J1

Joint set 2, J2

Joint set 3, J3

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Joint spacing for set Joint spacing for set J1: 135, 225, 300 & J1: 135, 225, 300 & 260 mm. 260 mm. Average spacing, SAverage spacing, S11 = = 230 mm 230 mm

Joint spacing for set Joint spacing for set J2 : 350 & 450 mm. J2 : 350 & 450 mm. Average spacing, SAverage spacing, S22 = = 400 mm.400 mm.

Joint spacing for set Joint spacing for set J3: 250, 270, 280 mm. J3: 250, 270, 280 mm. Average spacing, SAverage spacing, S33 = = 267 mm267 mm

260300 225 135

280 270 250

350 450

Joint Joint spacingsspacings J1: 135, 225, 300 & 260 mm. J1: 135, 225, 300 & 260 mm. Average spacing, SAverage spacing, S11 = 230 mm = 0.23 m = 230 mm = 0.23 m

Joint Joint spacingsspacings J2 : 350 & 450 mm. J2 : 350 & 450 mm. Average spacing, SAverage spacing, S22 = 400 mm = 0.4 m.= 400 mm = 0.4 m.

Joint spacing J3: 250, 270, 280 mm. Joint spacing J3: 250, 270, 280 mm. Average spacing, SAverage spacing, S33 = 267 mm = 0.267 m= 267 mm = 0.267 m

(Note: unit for average joint spacing is in (Note: unit for average joint spacing is in metremetre) )

JvJv = 1/0.23 + 1/0.4 + 1/0.267 = 1/0.23 + 1/0.4 + 1/0.267 = 10.60 m= 10.60 m

RQD (%) = 115 RQD (%) = 115 –– 3.3 3.3 ×× JvJv= 115 = 115 –– 34.9834.98= 80 % = 80 %

∑=

=J

1i

)1(i

v SJ

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Compared to direct method (RQD using core sample), VOLUMETRIC JOINT COUNT gives an indication on discontinuities orientation (dip & strike).

During drilling and transportation of cores, orientation of discontinuities is lost (rotation and movement of core samples),unless directional drilling is used (very expensive & usually used in petroleum exploration).

Classification of Volumetric Joint Count, Classification of Volumetric Joint Count, JvJv((PalmstromPalmstrom 1996)1996)

S. No. Term for Jointing Term for Jv Jv

1 Massive Extremely Low < 0.3 2 Very weak jointed Very Low 0.3 – 1 3 Weakly jointed Low 1 – 3 4 Moderately jointed Moderately high 3 – 10 5 Strongly jointed High 10 – 30 6 Very strongly jointed Very high 30 – 100 7 Crushed Extremely high > 100

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Rock quality designation, RQDRock quality designation, RQD:

Though the RQD is a simple and inexpensive index, Though the RQD is a simple and inexpensive index, when considered alone it is not sufficient to provide an when considered alone it is not sufficient to provide an adequate description of a rock mass because it adequate description of a rock mass because it disregards joint orientation, joint condition, type of disregards joint orientation, joint condition, type of joint filling and stress condition.joint filling and stress condition.

Which rock mass is easier to excavate, a rock with a Which rock mass is easier to excavate, a rock with a higher or lower RQD? higher or lower RQD?

Which rock mass is more suitable as a foundation for Which rock mass is more suitable as a foundation for a structure, rock with RQD = 90% or RQD = 30% ?a structure, rock with RQD = 90% or RQD = 30% ?

Which rock mass will display rock fall, is the one with Which rock mass will display rock fall, is the one with a higher or lower RQD?a higher or lower RQD?

How can we differentiate between natural fractures How can we differentiate between natural fractures and those induced by drilling?and those induced by drilling?

Some frequently asked questionSome frequently asked question:

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CHP 9.1 (RQD)