abbs the use of the point load index in weak carbonate rocks

8/17/2019 ABBS the Use of the Point Load Index in WEAK CARBONATE ROCKS

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The Use of

the Point

Load Index

in

Weak

arbonate Rocks

REFERENCE: Abbs, A. F •

"'T'

ht Ust o t t Point Load lndtx in \Vtak Carbonate

Rocks, .. Strt11gtl1Ttsting o J.fari' t Sedime s: laboratory and In-Situ tltas1u-emenJs.

ASTM STP 883 R. C. Ch

ane

y and K. R. Demars. Eds. , American Society for Testing and

Ma.1erials, Philadelphia. 1985. pp. 413- 421.

ABSTRACT:

There

has been inet'easing use of the poinl 1000 index as a

l1 1C1SUrc

of rock

streng

th

since i

ts

in1roduc1ion in 1972 .The tes1 rcsut1s ha\

C be

en correla1ed 10 in 1he un

iaxia

l

C Omprcssive strcnglh

t

of the rock . The correlations be1v.un the

ra

tes of point load

ind

ex

f at1d

the

u

niaxi.al

strcn,gth


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3/6

• limtstnes

These

are often encountered as thin layel'S that rarely exceed

1.5 1n in thickness and may be encount-ered :11 any depth.

• Co

ra

ls -

Vario

us

t

ypes

of

corn

l are

cncoun1ered:

they

coukl be simply

described

as

tree or branching coral. 11Wesicular coral, and mass.i e solid coral.

Porous corals are nonnally infilled with sand or sill si1.ed m:11erial in ..,arying

pro

port

ions.

•

Calcareflites

The

majority

of

tbe

caJc

arc

nitcs enco

un

tec-ed

are

mainly

oo

n1posed ofsand

sized frag1nen

ts ofshe.Us and coral.

The

1nateriaJ s ...ary consid

·

erably in both strength and constirution: in some cases the n1atcrials arc almos1

cn1rely con1posed of she

ll

fragmen1s.

•

Calcisillites

The materiaJs arc predominantly sih sized though they often

contai n minor qu

an

1i1icsof sand size particles. Al the ti ne

end

of the particle size

spectrum calcisihitc may a

ppe

ar as a unifo

nn

ma1e rial.

•

Calcilutites and Hard

a b o a t ~

1fuds

-

Very

fine

grai

ned

(clay

size)

paniclcs com

pri se

these materials. hey are typically very unifonn

wi

th no larger

si:rod

minor components.

In add it ion it should be noted that gypsum is often encountered at various

depzh

s.

The gyps

um

can occur as

vcin. S,

layers

o

m

assi\'C

gypsum.

and as

a

secondary

corriponen1

filling the

pore

space of calcarcni1 e s .

The rock types described above exhibit vary ing strength and porosity. The

streng

Lh of

1he

tQck

gener

alJ)'

li

es in the range 0.5 to

S.O mPa

and

is often

very

variable in situ .

The majority or the materials that h

aYe

been encountered

in

offshore in·

vestigations in the Arabian

Gulf

fa

ll

into the categories ''eak calcisiltite and

calcareni

1e.

h should be noted that 1hesc

dcsc rip

ti\'e terms a

lo

ne give no direct

iocHea

tion of he strengthorengineering

pr

o

pe

rtiesof the n1a1crials. Th is isin fac1

a disadvan1age

or

lhis geological classification

sys.icm

.

Corrt-lation Bel 'etn Point Load lndtx and UniaxJal Con1prtss:h•e Stre-ngth

In order to study this re lationship resul ts were assembled from

togs

of

O\'Cr

128

borings selected from widely distributed areas of the Arabian Gulf.

The

bo rings were

only sel

ec

ted if they had

uniax

ial compressi

on test

s

and p


4/6

•

'

. '"'

t 3 t S 1 ~

n•

~

0 0 0 0

•

I

•

~

•

t;

•

•

•

l •

•

c

•

•

8•

•

•

•

•

0

< •

•

•

•••

•

,

••

0

••

•

•

2

••

•

0

0 I

•

•

HT

"""

I

HOEX

1

1

eaMN/•:2.

on

Fig. 2.

Ahhough it is possible that a bcner

fit

co

ul

d

be

obta.iocd

using

a cutvc

fining process. it was

dec

ided to use linear regression in order to compare the

results with published relalionsbips bet\\•cen point lood index

and

unjaxial

streng

th

.

A summary of the results

of

these analiyses are presented in Fig. 3 in the fom1

of

the u

i

nte,rccpt at

/

1

;;;;

0

and

the gradient

of

the regression line. The

cor·

relation coefficient and

the

nu

n1be

r of

tests

are also

sh

own in Fig. 3.

ln general it

was

found that the

best

f i f

lines

did

not in fact JXlSS through the

originu

= 0 , /

1

= 0. Thisdiscortion is panly due 1o thc seance

of

results

but

also

the fact

lha1

there

are no test

results fof'

n

ear

zero strength

Jnatcrial. (The

cor·

rclat:ions are also significantly influenced by small numbers

of

tesLs on hi.ghcr

s1rength ma1erials for which a simple linear relationship

is

probably inap-

propriate.) h is hov.'evcr inconsistent jn a physical sense to

ad

opt a "be.st fi ' ' that

does nOt pass

1h ro

ug_h the origin of

the

graph. f Or 1his reason a line passing


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.,

FI

O.

3 Corrtlo1ion bt:1wttn u

a d

l

ol HJ.

was calculated, and the gradientof th is line is also presented in Fig. 3. The

results

show

this ratio varying

from

2.69 to 8.81.

Follo"'ing the analysis of results from each ind

ivi

dual investigation the analy

sis was repc,ated for each material type

1asing

all the test results combined. For

lhese larger sets of data the distribu tion of resul ts was

quire

uneven with a larger

propo

rti

on

of

the resuhs from very weak samples. In order

to

reduce

this

dis·

proportionate influence

the

rc ultS were g.roupe


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12

I I

10

9

8

7

' 6

bO

4

3

•

2

•

•

•

•

•

•

•

•

•

o

o 2 3 •

t, :5.0

/m2

J 1 i

C < H ' r ~ l a t i o s

darafrom all

in

l

estigat/011.t

alci.siltire

When the data

are

reduced to a more convenient fonn. it is possible to define

a relationship of

abbs the use of the point load index in weak carbonate rocks

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