rippability of rocks

8/10/2019 Rippability of Rocks

1/4

eological factors significant

n th

assessment of rippability

J.

M.

WEAVER

Member)

Synopsis

The geological factors

that

are significant in the evalua-

tion

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 e xcavation operations.

Case

histories

are presented which illustrate the

point

that. although seis

mic wave

velocities

may provide an indication of the

rip-

pability 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

ust 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

dis

ciplines

to

the same problem is often widel y

different

. The geolo

gist

analyses condit ions

as

he finds them; the engineer considers

how

he can change ex.sting 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 res

ponsibility

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 geo logis t can be

of

great prac

tical 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. trans

por1


2/4

Table 4 illustrating these values is presented below.

Rock hardness

For a visual a55essment

of

rock hardness al lied

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 structure

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 engin

eering application

of

the materials on the site under investigation.

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

Table 3

Joint

spacing

classification

Joint spacing

Spacing

Rock m ss

Excavation

description

of oints

grading

characteristics

mm

Very close

>5

Crushed /

Easy

ripping

shattered

Close 50

-

300 Fractured Hard ripping

Moderately

close

300

- 1

000

Blocky/seamy

Very hard

ripping

Wide

1

000

- 3

000 Massive Extremely

hard

ripping

and

blasting

Very wide

>3

Solid/sound Blasting

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.

o

I 2 3 4

Velocity in Meters Per Second l

1000

I I I I I

Velocity

in Feet Per Second

l 10000

2 3 4 5 6 7 8 9 10

II 12

13

14

15

Table 2

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

RIPPABLE _

MARGINAL c:::J

Rock hardness and excavation characteristics

Rock hardness

Identification criteria

deS ription

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.

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.

Hard rock

Cannot be scraped with a knife; hand

specimen can be broken with

pick

with a

single firm blow; rock rings under hammer.

Very

hard

rock

Hand

specimen

breaks with

pick after more

than one

blow;

rock rings

under hammer.

Extremely hard rock

Specimen requires

many

blows with

geological pick

to break through

intact

material; rock rings under hammer.

I

NON RIPPABLE

Unconfined

Seismic

compression strength

waVI

velocity

MPa m/s

1 7 -

3 0

450

- 1

200

3 0

-

10 0

1

200

- 1

500

10 0

-

20 0

1

500

- 1 850

20 0

-

70 0

1 850 -

215

>70 0

>215

Excavation

characteristics

Easy ripping

Hard

ripping

Very hard

ripping

Extremely hard

ripping or blasting

Blasting

314

DIE SIVIELE INGENIEUR in Suid-Afrika - Desember 1975


3/4

The spacing

of

discontinuities

is

of

great

importance

in asses

sing rippability. The very presence

of joints

reduces

the

shear

strength of a rock mass and

their

spacing governs the degree of

such a reduction. A classification

for

joint spacing by Deere

8

is

presented in Table 3, and the effect of such discontinuities on

rippability

is included.

Strike and dip

orientation

The strike and

dip

orientation

of the

dis

continuities

and bedding may be either favourable

or

unfavourable

in

terms of

rippability. Ripping

may

prove easier and more pro

d u c t i ~ e if carried out parallel to such planes of weakness in certain

rock types. Ripping

at right

angles

to

strike

could

assist in re

moving resistant bands

that may

occur

within

an easily ripped

material.

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

mas

sive 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, land

slides

or

talus usually contain

little or

no

matrix

and,

depending

on

the

degree

of compaction

and consolidation, are usually doze

able, .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

v e l o ~ i t y 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 condi

tions

between

the two

types

ofiormation.

It is

to lower the

veloci

ties for

ripping

and blasting

below the

values ordinarily used for

normal weathering processes. These relative figures are shown

in Table 4.

3m

2 13

HAMMER

IMPACT

GEOPHONE

366

GRANITE BOULDERS REQUIRING BLASTING ARE SEEN IN AN

88

MATRIX

OF

RIPPABLE DECOMPOSED GRANITE. THE P ~ R E N T ROCK IS A MODERATELY

CLOSE JOINTED FORMATION.

Fig 1: Typical

boulder

formation

DIE SIVIELE INGENIEUR in Suid-Afrika - Desember 1975

Table 4

Velocity ranges for ripping wiih a heavy tractor

Excavation

Velocity for

Velocity for

characteristics

normally weathered

boulder situations

profile

m/s-

m/s

Easy

ripping

450

- 1

200

450 -

900

Hard ripping

1 200 - 1 500

900 - 1 200

Very hard ripping

1

500

- 1 850

1 200 - 1

500

Extremely hard ripping

1 850 - 2150

1 500 - 1 850

or blasting

Blasting

>2150

> 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

generaliza

tions 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 colour

ed, heavy, iron and magnesium rich minerals. An acidic igneous

rock, such as granite, is

composed

of feldspar

with

light co

loured, 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.

ippability

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

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 pro

posed,

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 m s

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 1 OO-mm

wide

grooves into

the

sur

face. No brecciation

or

fracturing occurred

at

all.

, . From the rippability rating chart the

following

values are ob

tained 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.

Coal seams Witbank :

Seams

comprise

soft

rock, unweathered,

fractured, horizontally bedded coal. Seismic wave velocity

for the

material is 1

520 m s which

classifies

the

material as hard rip

rock, rippable by a D8 tractor. Mater ial 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


4/4

Rippability

rating chart

Rock class

I

II

III IV V

Description

Very

good

rock Good rock

Fair rock

Poor rock Very

poor

rock

Seismi c velocity m/s) > 2150

2

150

- 1

850

1

850

- 1

500

1

500

- 1

200

1

200

-

450

Rating

26 24

20

12

5

Rock hardness

Extremely hard rock Very hard rock Hard rock

Soft

rock Very

soft

rock

Rating

10

5 2 1 0

Rock weathering Unweathered

Slightly

weathered Weathered Highly weathered

Completely

weathered

Rating

9 7

5 3

1

Joint spacing

mm)

> 3000

3000

- 1

000

1

000

-

300 300

-

50

rippability of rocks

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