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Unformatted text preview: eflection again noted when there is no
perceptible increase in settlement.
This procedure should be repeated
until a total settlement of not less than 1.75 mm has been produced.
For
recording observations pro forma is given at Appendix B.
NOTE 1  Rapid and less accurate tests may be made with a 45cm or even a
diameter plate with a Jtonne loaded lorry to provide the load reactions, a
pressure gauge to measure the load and at least 3 dial gauges to measure the vertical deflection of the hearing plate.
In such cases, the KP value applied should be
corrected for the standard 75 cm plate as explained in 5.
Xlcm NOTE 2The
test may be conducted with a plate of diameter smaller than
75 cm when adequate reaction is not available.
When a smaller plate is used, the
Kp value should be corrected to get the K value corresponding to a standard 75 cm
diameter plate as explained in 5. EVALUATION
OF SUBGRADE
TEST RESULTS
5.1 The corrections mentioned in 5.1.1 to 5.1.4 should be applied before
a final value of subgrade reaction K is evaluated.
5.1.1 Correction when Using Plates Smaller than 75 cm Diameter  Theorktical relationship may be established between the modulus of subgrade
reaction and the plate diameter.
This value for a plate of particular
diameter can be expressed as a percentage of the equivalent modulus of
subgrade value of a 75cm diameter from Fig. 3 and thus equivalent value
of Kr, for ai 75cm diameter plate can be evaluated.
5t2 Correction of LoadDejection
Curve  The correction
should be
necessary if the value of K,, is 0.555 MPa/cm ( 5.55 kgf/cme/cm ) or more.
In such a case unit loads up to 93 kN ( 9 300 kgf) in 15.5 kN ( 1 550 kgf )
increments should be applied and a loaddeflection curve is plotted.
In
these cases the loaddeflection curve shall not be a straight line and hence
a correction should be made.
Generally, the loaddeflection curve shall
approximate a straight line between unit loads of 31 kN ( 3 100 kgf) and
93 kN ( 9 300 kgf) [ O07 to 0.21 MPa ( 0.7 to 2.1 kgf/cms ) 1. The correction
should then consist of drawing a straight line, parallel to the straightline
portion of the loaddeflection curve through the origin. The deflection for
computing the Kd value shall then be determined at a unit load of 31 kN
( 3 100 kgf) ( 0.07 MPa/O*7 kgf /cm* ) and the Kd computed by the formula given in 4.3.1.
In case no straight portion on the curve is observed,
then at least three points in the region having the least curvature should
be selected ( see Fig. 4 ).
5. 10 IS : 9214 1979 600 co 0 200 0
DIAMETER FIG. 3 OF BEARI NO PLATEcm RELATIONSHIP
BETWEEN MODULUS SUBGRADE EACTION
OF
R
AND DIAMETER OF BEARINGPLATE 5.1.3 Correction, for Bending of the Plates  The bending of the bearing
plate is greater at the centre than at the rims, even when nests ofplates are
used,
A method for the correction
ding is shown in Fig. 5. of modulus of subgrade for plate ben The value of uncorrected
subgrade reaction
should be used as the
ordinate to enter the curve in Fig. 5. The value obtained on the abscissa in
the curve of Fig. 5 should be the K;, value corrected for bending of plate.
NOTE plate  bending When llncorrecttd value is less than
is negligible
and may be ignored. 0.275 MPa/cm, the correction for 5.1.4 Correction for Saturation  The moisture content of the subgrade
at the time of the plate bearing test may increase after the pavement
has
been constructed and the worst condition may be covered by converting
the value of modulus of subgrade obtained from bearin
test to a value for
It is impracticable
to do this directly by artifithe subgrade when soaked.
cially wetting the test area.
Hence correction should be applied on the
basis of consolidation
tests on the subgrade material.
11 lS:9214...
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 Spring '13
 PROF.DEEPANKARCHOUDHURY

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