Reproduced by Sabinet gateway under licence granted by the Publisher (dated 2011).

Geological factors significant in the

assessment of rippability
J. M. WEAVER (Member)

Synopsis
The geological factors that are significant in the evaluation of excavation characteristics of earth and rock materials
are described and a guide to the assessment of rippability
by tractor mounted rippers is provided. A rippability rating
chart is proposed. utilizing the geological parameters which
influence ripping and excavation operations. Case histories
are presented which illustrate the point that. although seismic wave velocities may provide an indication of the rippability of a rock mass. the geological conditions must also
be considered. The term assessment is used deliberately
since it must be appreciated that very often a conclusive
answer as to whether a rock can be ripped or not just cannot
be obtained. In such a situation. only a field test will decide
the issue.
Introduction
Leggat 10 points out that the union between the civil engineer
and the geologist. the practical builder and the man of , Clence is
often a partnership of great value. The approach of the two disciplines to the same problem is often widely different. The geologist analyses conditions as he finds them; the engineer considers
how he can change ex.isting conditions so that they will suit his
plans. From his analysis. the geologist cites problems that exist
and suggests troubles that may arise; the engineer's main task is
to solve the problems and overcome the troubles. The final responsibility for decisions involved must rest always with the
engineer. but in coming to his conclusions he will be gu ided by
and will probably rely upon the factual data given to him by the
geologist.
A field in which the engineering geologist can be of great practical assistance to the engineer through his working knowledge of
the historical development of landforms and bedrock formations
and the geological processes involved in the formation. transpor1<;1tion and depOSition of rocks. is in the evaluation of excavation characteristics 2
To the engineer. civil or mining. fac~d with the responsibility
of moving vast quantities of soil and rock. the geologist can
furnish helpful data for estimating excavation costs and methods.
For igneous rock. 5uch as granite and basalt. he can indicate the
spacing of the joints. the <legi"ee of weathering and the hardness
of the rock. which control the cost of dozing. ripping. drilling and
blasting. Sedimentary rock types vary greatly in cost of excavation. but the engineering geologist. familiar with such formations.
can fairly safely predict the expectable degree of difficulty.
Significant geological factors
The geological factors which are likely to influence the assessment of rippability are as follows :
1.
2.
3.
4.

Rock type
Seismic wave velocity
Rock hardness
Rock weathering
5. Rock structure
6. Rock fabric
Rock type
When classified in terms of origin there are three rock types 5 :

Igneous rocks - are formed by cooling of molten magma or lava

originating within the earth. Igneous rocks may be identified by
the silica or mineral content and almost never have the stratified.
banded or foliated characteristics of other rocks. Granite. syenite.
norite. dolerite and basalt are igneous rocks commonly encoun-

DI.E SIVIELE INGENJEUR in Suid-Afrika - Desember 1975

John M . Weaver is Head of the Engineering Geology Division of

Steffen. Robertson & Kirsten. Johannesburg. After matriculating
at Pretoria Boys High School he completed a BSc (Hons) degree in
engineering geology at the University of the Witwatersrand in
1963. He started his career in industry as an earthmoving engineer
with Barlows Tractor and Machinery Co, Isando. In 1965 he joined
Van Niekerk. Kleyn and Edwards in Pretoria.
During 1967 and 1968 he attended the University of California,
Berkeley, where he received an MS(Eng) degree in Soil Mechanics.
The following year he worked in Los Angeles for Moore & Taber,
Fullerton. On his return to South Africa he rejoined Van Niekerk,
Kleyn & Edwards as Senior Engineering Geologist and in 1970 was
appointed Director of Geodata (Pty) Ltd, a subsidiary firm specializing in foundation drilling and testing. He joined his present firm in
June 1974.
He is active on the Committee of the Association of Engineering
Geologists and has represented the Geological Society of South
Africa on SANCOT. His main fields of interest are subsurface exploration for underground construction, dam sites and foundations.

tered on earthmoving jobs. They are the most difficult to rip

because they lack the stratification and cleavage planes essential
to ripping hard rock.
Sedimentary rocks - consist of material derived from destruction
of previously existing rocks. Water action is responsible for the
largest percentage of sedimentary rocks although some are
formed by wind. glacial ice or chemical action. Their most prominent characteristic is bedding or stratification.
They are built up by successive layers of material differing in
type, texture, colour, thickness or all of these properties. Individual
layers which are uniform in texture. colour and composition may
be found within a stratum. These are called beds and may vary in
thickness from paper thin to several hundred metres.
Sandstone. dolomite. tillite, shale, calcrete and ferricrete are
among the most common sedimentary rocks. These generally
are the most easily ripped.
Metamorphic rocks - are formed from pre-existing rocks which
have been changed in mineral composition, texture. or both. The
agents which cause metamorphism in rocks are shearing stresses, pressure. chemical action, or liquids and gases, and temperature. Common metamorphic rocks are gneiss, quartzite, schist
and slate. They vary in rippability with their degree of lamination
or cleavage. All are found on or near the earth's surface and occur
as homogeneous or as disturbed masses.
Seismic wave velocity
One of the basic principles for assessing rippability of earth
and rock. materials is that seismic shock waves travel through
different materials at different velocities. The velocity of a shock
wave depends on the density and degree of compaction of the
materials. Hard rock, sound bedrock conducts a shock wave at a
high velocity; loose sands. of lower density. conduct shock waves
at much lower velocities.
Using a refraction seismograph, the seismic wave velocity
through vari,)us layers of material is measured from which the
degree of consolidation, including such factors as rock hardness.
stratification, degree of fracturing and degree of weathering. can
be determined. From this information an indication of the equipment necessary and method of excavation is obtained.
Average values for the velocities of shock waves in different
materials are available from a variety of sources and tables.

313

Reproduced by Sabinet gateway under licence granted by the Publisher (dated 2011).

Table 14 illustrating these values is presented below.

Table 3
Joint spacing classification

Rock hardness

Joint spacing
description

For a visual a55essment of rock hardness, allied to simple field

tests, the guide as described by Jennings and Robertson 9 should
apply. The seismicwave velocities and excavation characteristics,
utilizing a heavy tractor, relative to the different rock hardness
categories, are presented in Table 2.

Rock mass
grading

Spacing
of joints

Excavation
characteristics

mm
Very close

Close

Rock structure

Moderately close

The following factors are often the most difficult to assess

owing to lack of exposures. Field examination of all available
exposures in the vicinity of the site, such as rock outcrops, hillside
faces, dongas, river banks, borrow pits, road and railway cuttings,
coupled with studies of available geological maps and aerial
photographs, all contribute relevant information. This data can
then be applied to permit a geological interpretation and engineering application of the materials on the site under investigation.

>50

Crushed /
shattered

Easy ripping

50 - 300

Fractured

Hard ripping

Blocky/seamy

Very hard
ripping

Massive

Extremely
hard ripping
and blasting

Solid/sound

Blasting

300 - 1 000

1 000 - 3 000

Wide

Very wide

>3000

discontinuity. The term discontinuity refers to faults, shear zones,

joints, bedding planes, cleavage or foliation surfaces or other
similar surfaces caused by movement or displacement.

Discontinuities - Any structural or geological feature that changes

or alters the homogeneity of a rock mass can be considered as a
Table 1
Ripper performance relative to seismic wave velocity through soils and rocks

Velocity in Meters Per Second l 1000 I

Velocity in Feet Per Second l 10000

10

II

12

13

14

15

TOPSOIL
CLAY'
GLACIAL TILL
.. IGNEOUS ROCKS
GRANITE
BASALT
. TRAP ROCK

SEDIMENTARY ROCKS
SHALE
SANDSTONE
SILTSTONE
CLAYSTONE
CONGLOMERATE
BRECCIA
CALICHE
LIMESTONE

METAMORPHIC ROCKS
SCHIST
SLATE

MINERALS

a ORES

COAL
IRON ORE

I
RIPPABLE

MARGINAL

c:::J

NON-RIPPABLE ~

Table 2
Rock hardness and excavation characteristics

Rock hardness
deSCription

Identification criteria

Unconfined
compression strength

Seismic
waVI1 velocity

MPa

Excavation
characteristics

m/s

Very soft rock

Material crumbles under firm blows with

sharp end of geological pick; can be peeled
with a knife; too hard to cut a triaxial
sample by hand. SPT will refuse. Pieces up
to 3 cm thick can be broken by finger
pressure.

1,7 - 3,0

450 - 1 200

Easy ripping

Soft rock

Can just be scraped with a knife; indentations

1 mm to 3 mm show in the specimen with
firm blows of the pick point; has du11 sound
under hammer.

3,0 - 10,0

1 200 - 1 500

Hard ripping

Hard rock

Cannot be scraped with a knife; hand

specimen can be broken with pick with a
single firm blow; rock rings under hammer.

10,0 - 20,0

1 500 - 1 850

Very hard rock

Hand specimen breaks with pick after more

than one blow; rock rings under hammer.

20,0 - 70,0

1 850 - 2150

Extremely hard
ripping or blasting

Extremely hard rock

Specimen requires many blows with

geological pick to break through intact
material; rock rings under hammer.

>70,0

>2150

Blasting

314

, Very hard ripping

DIE SIVIELE INGENIEUR in Suid-Afrika - Desember 1975

Reproduced by Sabinet gateway under licence granted by the Publisher (dated 2011).

The spacing of discontinuities is of great importance in assessing rippability. The very presence of joints reduces the shear
strength of a rock mass and their spacing governs the degree of
8
such a reduction. A classification for joint spacing by Deere , is
presented in Table 3, and the effect of such discontinuities on
rippability is included.

Table 4
Velocity ranges for ripping wiih a heavy tractor"

Strike and dip orientation: The strike and dip orientation of the discontinuities and bedding may be either favourable or unfavourable
in terms of rippability. Ripping may prove easier and more producti~e if carried out parallel to such planes of weakness in certain
rock types. Ripping at right angles to strike could assist in removing resistant bands that may occur within an easily ripped
material.

Easy ripping

450 - 1 200

Hard ripping

1 200 - 1 500

900 - 1 200

Very hard ripping

1 500 - 1 850

1 200 - 1 500

Extremely hard ripping

or blasting

1 850 - 2150

1 500 - 1 850

Continuity: The continuity of a joint or set of joints, or bedding

planes, within a rock mass has a marked effect on the strength of
the mass and influences excavation characteristics. Penetration of
a ripper shank into a cOlltinuous major joint could weaken a massive or sound rock formation so as to break out large boulders or
blocks of rock.
Gouge: The effect of gouge on the strength properties of a joint is
of outstanding importance. If the gouge is sufficiently thick for
example, the joint walls will not touch and the strength properties
of the joints will be those of the gouge. In assessing rippability,
the greater the amount of gouge or of 50rt material between joints
or boulders, the easier it becomes to penetrate the formation and
the easier it becomes to rip.
Boulder formations: Imbedded boulders, massive or columnar
formations, consisting of large blocks or spheroids in a matrix of
soil or very soft rock, occ Jrfrequently in sedimentary, igneous and,
metamorphic rocks, This condition creates marked exceptions to
the standard seismic survey profile where dozeable material alters
through easy rip to hard rip to blast conditions.
Rock types which are particularly inclined to weather into a
boulder formation are the basic igneous rocks such as basalt,
dolerite, diabase, gabbro and norite, also andesite and granite.
The sedimentary rocks which weather to this condition are most
commonly dolomites, limestones, tillite and sandstone. Boulder
beds such as occur in stormbeach gravels, stream deposits, landslides or talus usually contain little or no matrix and, depending
on the degree of compaction and consolidation, are usually dozeable, .although with considerable difficulty.
The presence of a layer of boulders in a soil matrix affects the
seismic wave velocity between the hard rock bedrock below
(velo~ity 3 660 m/s) and the soil matrix above (velocity 1 220 m/s),
to yield an average seismic wave velocity that is marginal in terms
of rippability (eg 1 830 m/s). Note that boulders are detected in
the intermediate zone from th'Ei scattered time - distance points
on the seismic graph. The con'dition described above is illustrated
in Fig. 1.
;
Church 6 has advocated a method to compensate for the conditions between the two types ofiormation. It is to lower the velocities for ripping and blasting below the values ordinarily used for
normal weathering processes. These relative figures are shown
in Table 4.
HAMMER
IMPACT

GEOPHONE

3m
2,13

Excavation
characteristics

Velocity for
normally weathered
profile

m/s-'

Blasting

>2150

Velocity for
boulder situations

m/s
450 -

900

> 1850

Tractor-ripper with a working mass of45 to 49,5 t and a 280 to 360 kW

engine.

. This recasting of velocity ranges results in. relatively more

volume in the hard ripping and blasting classifications.
Rock fabric
From experience and observations, the following generalizations can be made:

1. Coarse grained rocks with a large grain size (> 5 mm) such as
pegmatites, coal, conglomerates, gritstones, calcretes and
sandstones can be more easily ripped than fine grained rocks
1 mm) such as quartzites, tillites, basalts, chert, dolomite
and limestone.
2. Basic igneous rocks will tend to yield a higher seismic wave
velocity than acid igneous rocks. A basic igneous rock, such
as norite, is composed essentially of feldspar with dark coloured, heavy, iron and magnesium rich minerals. An acidic igneous
rock, such as granite, is composed of feldspar with light coloured, light, silica and aluminium rich minerals. Basic igneous
rocks therefore have a higher specific gravity and density than
acidic igneous rocks and seismic wave velocity in basic rocks
will be higher than in acidic rocks.

Rippability classification
Bieniawski 3 in his classification of rock parameters has assigned
ratings to each parameter by a weighted numerical value. The final
rock class rating is the sum of the weighted parameters. The rating
system was originally proposed by Wickham, Tiedemann and
Skinner 11 to assess support requirements in tunnels. Utilizing the
geomechanics classification system, it is possible to produce a
rating for the assessment of rippability once one recognizes that
the rock class which may be rated as very poor rock for tunneling
is, in terms of rippability, a very good rock.
The rippability rating chart shown overleaf is therefore proposed, utilizing the rock parameters already described.
Case studies
Silica sand, Hartebeestpoort: The deposit comprises soft rock,
highly weathered, massive, horizontally bedded quartzite. Seismic
wave velocity for the material is 1 300 mls which classifies the
rock as a hard rip rock, rippable by a D8 tractor. Material could be
cut from a vertical face by a Cat 966 front end loader. Using a
D9G the rock could not be ripped and the ripper succeeded only
in cutting 300-mm deep by 1OO-mm wide grooves into the surface. No brecciation or fracturing occurred at all.
, . From the rippability rating chart the following values are obtained forthis material: SWV = 12; hardness = 1 ;weathering = 3;
joint spacing = 30; continuity = 5; gouge = 5; strike and dip = 15.
Total rating = 73. Analysis = Extremely hard ripping.

3660

GRANITE BOULDERS REQUIRING BLASTING ARE SEEN IN AN 88% MATRIX OF

RIPPABLE DECOMPOSED GRANITE. THE P~.RENT ROCK IS A MODERATELY
CLOSE JOINTED FORMATION.

Fig 1:

Typical boulder formation

DIE SIVIELE INGENIEUR in Suid-Afrika - Desember 1975

Coal seams, Witbank: Seams comprise soft rock, unweathered,

fractured, horizontally bedded coal. Seismic wave velocity for the
material is 1 520 mls which classifies the material as hard rip
rock, rippable by a D8 tractor. Material can be easily cut and loaded
from vertical face by a Cat 966 front end loader. Using a D8H the
coal could not be ripped and the grousers slipped, producing

315

Reproduced by Sabinet gateway under licence granted by the Publisher (dated 2011).

Rippability rating chart

Rock class

II

III

IV

Description

Very good rock

Good rock

Fair rock

Poor rock

Very poor rock

Seismic velocity (m/s)

> 2150

2 150 - 1 850

1 850 - 1 500

1 500 - 1 200

1 200 - 450

Rating

26

24

20

12

Rock hardness

Extremely hard rock

Very hard rock

Hard rock

Soft rock

Very soft rock

Rating

10

Rock weathering

Unweathered

Slightly weathered

Weathered

Highly weathered

Completely
weathered

Rating

Joint spacing (mm)

> 3000

3000 - 1 000

1 000 - 300

300 - 50

<50

Rating

30

25

20

10

Joint continuity

Non continuous

Slightly continuous

Continuous no gouge

Continuous some gouge

Continuous with gouge

Rating

Joint gouge

No separation

Slight separation

Gouge - <5 mm

Gouge -

Rating

Strike and dip orientation

Very unfavourable

Unfavourable

Slightly unfavourable

Favourable

Very favourable

Rating

15

13

10

Total rating

100 - 90

90 - 70

70 - 50

50 - 25

<25

Rippability assessment

Blasting

Extremely hard
ripping and blasting

Very hard ripping

Hard ripping

Easy ripping

Tractor selection

DD9G/D9G

D9/D8

D8/D7

D7

Horsepower

770/385

385/270

270/180

180

Kilowatts

575/290

290/200

200/135

135

Separation < 1 mm

>5

mm

Original strike and dip orientation now revised for rippability assessment.
Ratings in excess of 75 should be regarded as unrippable without pre-blasting.

undesirable coal and fines.

From the rippability rating chart the following values are obtained: SWV = 20; hardness = 1; weathering = 9; joint spacing
= 10; continuity = 5; gouge = 5; strike and dip = 15. Total rating
= 65. Analysis = Very hard ripping.
Summary
The geological features which influence ripping may be summarized as follows:
1. Rock type: Sedimentary and metamorphic rocks are more
easily ripped than igneous rocks.
2. Rock hardness: The softer the rock the more easily ripped.
3. Rock weathering: the greater the degree of weathering the
easier the ripping.
4. Rock structure: Discontinuities in the form of faults, fractures,
joints, cleavages, schistocity, bedding, laminations all act as
planes of weakness.
5. Rock fabric: Coarse grained rocks rip more easily than fine
grained rocks. Basic igneous rocks will tend to yield a higher
seismic wave velocity than acidic igneous rocks.
6. Seismic wave velocity alone does not provide the answer and
results obtained must be tempered by cognisance of geological
factors for correct analysis of rippability.

References
1. Ass of Engng Geol (SA Section). Committee on core logging. Private
communication.
2. Bean, E.F. Engineering geology of highway location, Construction and
Materials - Berkey Volume. Geol Soc of Amer, 1958, pp181 - 194.
3. Bieniawski, Z.T. Engineering classification of jointed rock masses.
Trans SA Instn of Civ Engrs, Vol 1 5,No 12, Dec 1973, pp335 - 344.
4. Caterpillar Tractor Co. Caterpillar performance handbook, 3rd Ed, Ja n
1973.
5. Caterpillar Tractor Co. Handbook of ripping - a guide to greater profits. 4th Ed Apr 1972.
6. Church, H.K. Two exceptions to seismic principles. World Construction, Vol 27, No 5, May 1974 pp26 - 32.
7. Deere, D.U. Technical descriptions of rock cores for engineering purposes. Felsmechanik and Ingeniergeologie, Vol 1, No 1,1963, pp17 22.
8. Deere, D.U., et al. Design of tunnel liners and support systems. Report
for Office of high speed ground transportation. US Dept of Trans, Wash
DC. Clearing-house for Federal scientific and technical information,
No PB 1 83799, Springfield, Va pp 11 -1 2.
9. Jennings, J.E., and Robertson, A. Macg. The stability of slopes cut
into Qatural rock. Proc Int Symp on Soil Mechanics. Mexico City, 1970,
pp585 - 590.
10. Leggat, R.F. Geology and Engineering, 2nd Ed, McGraw-Hili Book Co,
New York, 1962.
11. Wickham, G.E., Tiedemann, H.R., and Skinner, E.H. Support determinations based on geologic predictions. Proc 1st Amer rapid excavation andtunnellingconf. AIM E, New York, 1972, pp43 - 64.

Acknowledgements
The contributions of A. Roberts and A. Blenkinsop are gratefully acknowledged. Special appreciation is due to P. Stone who
suggested that the above observations should be presented formally. Thanks to P. van der Poel who prepared the drawings.

316

DIE SIVIELE INGENIEUR in Suid-Afrika - Desember 1975