The load should be increased until there is an

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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 45-cm or even a diameter plate with a J-tonne 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. Xl-cm NOTE 2-The 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 75-cm diameter from Fig. 3 and thus equivalent value of Kr, for ai 75-cm diameter plate can be evaluated. 5-t-2 Correction of Load-Dejection 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 load-deflection curve is plotted. In these cases the load-deflection curve shall not be a straight line and hence a correction should be made. Generally, the load-deflection curve shall approximate a straight line between unit loads of 31 kN ( 3 100 kgf) and 93 kN ( 9 300 kgf) [ O-07 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 straight-line portion of the load-deflection 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 PLATE-cm 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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