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IS 2720-31 (1990): Methods of test for soils, Part 31:

Field determination of California Bearing Ratio [CED 43:
Soil and Foundation Engineering]

“!ान $ एक न' भारत का +नम-ण”

Satyanarayan Gangaram Pitroda
“Invent a New India Using Knowledge”

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है”
ह
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“Knowledge is such a treasure which cannot be stolen”
 IS 2720 ( Part31 ):1990
( Reaffirmed 2006 )

WV 31 4+ivfxTFsr 8fW wgm

(cr$mpMJT)

Indian Standard
METHODSOFTESTFOR SOILS
PART 31 FIELD DETERMINATION OF CALIFORNIA BEARING ~RATIO

( First Revision )

UDC 624*131*383

@ BIS 1991

BUREAU OF INDIAN STANDARDS

MANAK BHAVAN, 9 BAHADUR SHAH ZAFAR MARG
NEW DELHI 110002

March 1991 Price Group 3

Soil and Soil Engineering Sectional Committee, CED 23

FOREWORD

This Indian Standard ( Part 31 ) ( First Revision ) was adopted by the Bureau of Indian Standards,
after the draft finalized by the Soil and Soil Engineering Sectional Committee had been approved by
the Civil Engineering Division Council.

The bearing ratio test ( generally known as the California bearing ratio test ) is an ad hoc penetration
test used for the evaluation of the strengths of sub-grade and bases for roads and runaway pavements.
The results obtained from these tests are used in conjunction with the empirical curves, based on
experience for the design of flexible pavements. The test gives empirical strength values which may
not be directly related to fundamental properties governing the strength of soil. The test is either
performed in the laboratory [ see IS : 2720 ( Part 11 ) : 1971 ] or directly in the field. The test may
also be performed in the laboratory on undisturbed sample or sample re-compacted to the field density.
The laboratory procedure has been covered in IS 27.0 ( Part 16 ) : 1987.

This standard covers the method of test to be conducted in the field. This standard was first published
in 1969. The principal modifications made in the revision are:
a) Revising the method of procedure based on the experience gained in the use of this test in the
past 20 years;
b) incorporating the references of various Indian Standards some of which have been revised
and some of them have been brought out as new standard; and
C) Incorporating SI units in place of metric units.
 IS272O(Part31):1990

Indian Standard
METHODSOFTESTFORSOILS
PART 31 FIELD DETERMINATION OF CALIFORNIA BEARING RATIO

( First Revision )
1 SCOPE 4.3 Jacks
1.1 This Indian Standard ( Part 31 ) ( First Two track-type jacks of 50 to 120 kN capacity,
Revision ) covers the method for the determina- having double acting combination trip and
tion of the bearing ratio ( generally known as automatic lowering in cases where loaded truck
the California bearing ratio ) of soils in place or tractor is used for providing the necessary
for the evaluation of strengths of sub-grade and reaction.
bases for roads and runaway pavements.
4.4 Proving Ring
2 REFERENCES One calibrated proving ring of suitable capacity
2.1 The Indian Standards as given in Annex A having an accuracy of not more than one per-
are necessary adjuncts to this standard. cent of the anticipated load shall be used. The
calibration of the proving ring shall be checked
periodically at least once a year.
3 TERMINOLOGY
3.0 For the purpose of this standard, the defini- 4.5 Metal Penetration Piston
tions given in IS 2809 : 1972 and the following 50 f 0-I mm in diameter and not less than
definitions shall apply. 100 mm long.
3.1 Bearing Ratio (Generally known as California 4.6 Extensions
Bearing Ratio or CBR )
Internally threaded pipe or rod extensions not
The ratio of the force per unit area required to less than 200 cm long furnished in the following
penetrate a soil mass with a standard circular
quantities and lengths:
piston at the rate of I.25 mm/min to that requi-
red for corresponding penetration of a standard Length of Extension Number of
material. ( see Note ) Extensions
cm
3.2 Standard Load
5 2
Load which has been obtained from the test on 10 2
crushed stone which is defined as having a
bearing ratio of 100 percent ( see 6.2 ). 30 1
50 1
4 APPARATUS 100 1
4.1 Loading Device NOTE - Other convenient lengths may also be used.
A mechanical screw loading jack with swivel 4.7 Connectors
head for applying load to the penetration piston.
The device should have an arrangement for For coupling the penetration piston and proving
attachment to truck, tractor, truss or any ring assembly either directly or through exten-
other equipment used to provide load reaction. sion pieces.
The jack should be such that a uniform pene- 4.8 Dial Gauge
tration rate of 12.5 mmlmin can be achieved.
The capacity of the jack should not be less than Reading to 0.01 mm having a travel of 25 mm,
50 kN. for measuring the penetration of the piston.
4.2 Equipment for Providing Reaction for 4.9 Dial Gauge Support
Loading
Rigid and of steel, angle welded construction
Truck, tractor, truss or any other suitable or light alloy pipe construction about 2 m long,
equipment. If truck or tractor, is used they of overall height 30 cm and 45 cm wide at the
should be loaded suitably to give the necessary feet with universal or ordinary dial gauge
reaction. If truss is used it should be suitably holder adjustable anywhere along the length of
anchored. the support. r
IS 2720 ( Part 313 z%J90

4.10 Surcharge Weight .:pipes or rods. It should be ensured that the

entire assembly is plumb and the loading jack
One annular metal weight of mass 5 kg and of should be clamped in position.
250 mm diameter with a central hole 53 mm in
diameter. Two circular slotted weights of mass 5.4 The surcharge annular weight of mass 5 kg
5 kg and of diameter 215 to 250 mm with a should be kept in position on the surface to be
central hole and slot width of 53 mm. Two tested so that when the piston is lowered, it
circular slotted weights of mass 10 kg and of will pass through the hole in the annular weight.
diameter 215 to 250 mm with a central hole The penetration piston should be seated with
and slot width of 53 mm. the smallest possible load not exceeding a total
load of 40 N ( or unit load of 0.02 MPa) so that
4.11 Miscellaneous Apparatus full contact is established between the piston
Other general apparatus, such as spirit level, and the surface to be tested. For materials with
pick, spade, scoop and brush, apparatus for irregular surface the piston may be seated
moisture determination [ see IS 2720 ( Part 2 j : on a thinnest practical layer of fine limestone
1973 ] and density determination [ see IS 2720 screening or plaster of Paris spread over the
ip;; 28 ) : 1974 and IS 2720 ( Part 29 ) : surface.
5.5 While the seating load is on the piston, a
5 PROCEDURE 3 to 6 mm layer of clean sand should be spread
over the surface to be covered by the surcharge
5.1 The general surface area to be tested should annular weight. This helps in distributing the
be exposed, cleaned of all loose and dried surcharge load over the surface uniformly.
material and levelled. Extreme care shall be
taken not to disturb the test surface. The 5.6 Surcharge weights, sufficient to produce an
spacing of the tests should be such that opera- intensity of loading, equal to the weight of the
tions in one area do not disturb the soil in the base material and pavement, except that the
other area. For testing operations this spacing minimum weight applied should be 150 N
may be 50 cm for the penetration piston used including that of the annular weight [ this
in the test. weight gives an intensity of loading approxi-
mately equal to that in the laboratory bearing
5.2 If actual service conditions in the field ratio test [ see IS 2720 ( Part 16 ) : 1987 ] should
warrant, the surface to be tested may be soaked be applied. The penetration indicating dial
to the desired degree. During the process of should be suitably fixed for reading the pene-
soaking the required surcharge weights should tration and the dial set to zero. A diagrammatic
be kept in place. The test surface should be set up of the test is shown in Fig. 1.
drained of all free water, levelled and allowed
to stand for at least lfimminutes before starting 5.7 Load shall be applied on the penetration
further operations. piston so that the penetration is approximately
1.25 mm/min. The load readings shall be recor-
5.3 The equipment used to provide load ded at penetration of 0.5, 1.0, 1.5, 2.0, 2.5, 3.0,
reaction ( truck, tractor, truss etc ), should be 4.0, 5.0, 7.5, 10.0 and 12.5 mm. The maximum
so located that the centre of the beam against load and penetration shall be recorded if it
which the loading jack will work is over the occurs for a penetration less than 12.5 mm.
centre of the surface to be tested. If loaded The set up may then be dismantled.
truck or tractor is used for providing the 5.8 After the completion of the test, a sample
necessary reaction, the rear wheels of the truck shall be collected from the point of penetration,
or tractor should be completely raised by for moisture content determination. The
means of the track type jacks placed below the moisture content shall be determined in accord-
frame of the body near the wheels in order to ance with IS 2720 ( Part 2 ) : 4973. Besides the
avoid the loss of loading effort which would moisture content, the in-place density shall be
otherwise be spent on the flexing of the axial determined in accordance with IS 2720 ( Part
springs of the vehicle at the time of testing. In ~28) : 1974 or IS 2720 ( Part 29 ) : 1975 about
order to avoid accidents due to the failure of 15 cm away from the point of penetration.
jacks near the wheels and the lifting of the
vehicle at higher loads, the rear side of the 6 CALCULATIONS
body of the vehicle should be placed over two 6.1 Load Penetration Curve
rigid supports. The screw jack with swivel
should be installed to the underside of the The load penetration curve shall be plotted
equipment -providing reaction, at. the correct (see Fig. 2). This curve may be convex
position for the test. The proving ring should upwards although the initial portion of the
be connected to the bottom end of the jack and curve may be concave upwards due to surface
the piston connector to the bottom of the irregularities. A correction shall then be applied
proving ring. The piston should then, be connec- by drawing a tangent to the curve at the point
ted using, if necessary, lengths of extension of maximum slope. The corrected curve shall

2
 IS ,272t-B
( .Part:3$ ) : 1990

-be taken to be this tangent, together with the follows:

convex portion of the original curve, with the
.origin of strains shifted to the point where the Bearing ratio = - Pt x 100 percent
PS
tangent cuts the horizontal axis for penetration, where
as illustrated in Fig. 2. P, = corrected unit ( or total ) test load
6.2 Bearing Ratio corresponding to’ the chosen penetra-
tion value read from the load penetra-
Corresponding to the penetration value at which tion curve, in MPa ( or N );
the bearing ratio is desired, corrected load P, = unit ( or total ) standard load for the
values shall be taken from the load penetration same depth of penetration as per Pt,
Gurve and the bearing ratio calculated as taken from Table 1, in MPa ( or N ).

TOP PLATE’ ,#i# \-TRUCK ATTACH-

MENT CLAMP

-GEAR BOX

PROVING RING

r DIAL G,AUGE

EXTENSION ROD

rADJUSTABLE POST PENETRATION

DIAL GAUGE

MAGNETIC CLAMP
SLAT TED SUR-
CHARGE WEIGHT /-FOLDING DATUM

LANNULAR k JOINT \-UNIVERSAL DIAL

SURCHARGE NIPPLE OF GAUGE CLAMP
WEIGHT DATUM BAR

FIG. 1 FIELD CBR APPARATUS

IS 2720 ( Part 31) ~: 1990

6.2.1 The bearing ratios are usually calculated’

for peretration of 2.5 mm and 5 mm. Generally
the bearing ratio at 2.5 mm penetration will
be greater than that at 5 mm penetration and m
such a case the former shall be taken as the
bearing ratio for design purposes. If the bearing
ratio corresponding to a penetration of 5 mm
exceeds that for 2.5 -mm, the test shall be
repeated. If identical results follow, the bearing.
ratio corresponding to 5 mm penetration shall
be taken for design.

7 REPORT

7.1 The bearing ratio shall be reported correct

to the first decimal place. The details in the-
recommended proforma for the record of test
results given in Annex B shall be given.

8 NUMBER OF FIELD TESTS

2.5 5.0 7.5 10.0 12.5
8.1 Three in-place bearing ratio tests shall be
PENETRATION IN mm -c performed at each location to be tested.
FIG. 2 TYPICAL LOAD PBNBTRATIONCURVES However, if the results of the three tests in any
group do not show reasonable agreement, three
additional tests shall be performed at the same
Table 1 Standard Load, P, location and numerical average of the six tests
( Clause 6.2) shall be used as the bearing ratio at that
location. A reasonable agreement between the
Penetration Unit Standard Total Standard minimum and maximum values of the three
Depth Load Load
mm MPa N tests where the bearing ratio is less than 10%
2’~5 6’86 13 430 permits a tolerance of 3%, from 10% to 30% a
5’0 10’30 20 150
tolerance of 5%, from 30% to 60% a tolerance
of lo%, and greater than SO%, a tolerance of
1’0 13’10 25 790
25%. If it is known that single value is erratic
10’0 15’90 31 180 for any reason, that value should be discarded
12’5 17’90 35 300 and another test performed.

ANNEX A
( Clause 2.1 )
LIST OF REFERRED INDIAN STANDARDS

IS No. Title IS No. Title

2720 Methods of test for soils ( Part 28 ) : -1974 Determination of dry
density ;inyils in-place, by
( Part 2 ) : 1973 Determination of water replacement
the
content ( second revision) method ( jirst revision )
( Part 11 ) : 1971 Determination of the shear
strength parameters of a ( Part 29 ) : 1975 Determination of dry
specimen tested in unconso- density of soils in-place, by
lidated undrained triaxial the core cutter method
compression without the (first revision )
measurement of pore water
pressure 2809 : 1972 Glossary of terms and
( Pa& 16 ) : 1987 Laboratory determination _ symbols relating to soil.
of CBR ( secohd re&iolz-7 j/ 1 engineering ( jirst revision )J
 IS 2720 ( Part 31 ) : 1990

ANNEX B
( Clause 7.1 )
PROFORMA FOR IN-PLACE BEARING RATIO TEST

~Location .......................................... Tested by ..................................................................

Material at the test point ............................. Date ............................................................
Depth of tests point .....................................................................................................
soaked
Condition of test
unsoaked
Period of soaking, if any . . . . . . .., ._.._*.......... m. . . . . . . . . - . . . . . . . . . . . . . . . . . . ..,................ - I...................... __.

Surcharge weight used during soaking .. . .. . . --... .. . . ..I........ .-. . . . . . . ._ . . . . . . . . . . . . . . . *. . . . . . . . . .. . . . . . . . . . . . . . . .

Moisture content ._*.... -. I-.__......._.. - . . . . . . . . . ..a....... -.. -.- . . . . . . . . . . . . . . -.-----*.----- . . . . . . . . . . . . . . . . . . . . . . .

Density . . . . . . . . . . . . . . . . . . . . . . . . --- .I.. -. . . . . . . . . . . . . . . . . . . . _...I . . . . . . . . . . -..., . . . . - . . . . . -1.-..- . . . . . -. . . . . . - . . . . . . . ._ ...

Method used for determination of density U.._ . . . . . . . . . . . . - . . . . . . . . . _.. .U._. .I... *-.. . . . . . . . . . . -1.. . . . . .
Penetration test
Surcharge weight used -.. . . . . . . . . . . . . . . . . . . . . . - . . . . . .._.... -_. ._. . . . . -- . . . . .. . . . . . . . . .._._....... - . . . . . . . . . -...... ._.

Penetration Proving Ring Dial Load Corrected Load

mm Gauge Readings N N(see6.1)
O-5
1.0
1.5
2.0

2.5
3.0
40
---
5.0

7.5
10.0
12.5

Bearing ratio at 2.5 mm penetration 1 1,

2 > Average
3j

Bearing ratio at 5 mm penetration 1 7

2 ? Average
3j

Reasons if test is rejected:

Result of repeat test, if conducted . . . . . . . . . . . . _ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .._.......... . .._ . . . . . . . . . . . . . . I.C.
Standard Mark
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marked products are also continuously checked by BIS for conformity to that standard as a
further safeguard. Details of conditions under which a licence for the use of the Standard Mark
may be granted to manufacturers or producers may be obtained from the Bureau of
Indian Standards.
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Revision of Indian Standards

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issued from time to time. Users of Indian Standards should ascertain that they are in possession of the
latest amendments or edition. Comments on this Indian Standard may be sent to BIS giving the
following reference:

. Dot : No. CED 23 ( 4450 )

Amendments Issued Since Publication

Amend No. Date of Issue Text Affected

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