Methods of test for soils, Part 31: Field determination of California Bearing Ratio
1990 Edition

According to IS 2720 Part 31 (1990), the essential equipment for performing the field California Bearing Ratio test includes: a penetration piston fitted with a dial gauge or proving ring to measure penetration resistance; a mould and collar for preparing soil samples on site; a loading frame to apply load; and various tools such as a spirit level for surface leveling, pick, spade, scoop, brush for soil preparation, along with apparatus for moisture and density determination as specified in related IS codes (Parts 2, 28, and 29). These tools collectively ensure accurate sample preparation, loading, and measurement during testing.

The procedure for plotting and correcting the load-penetration curve as per IS 2720 Part 31 involves recording load readings at multiple penetration depths (0.5 mm up to 12.5 mm). The load is plotted against penetration depth to form a curve which initially may show a concave shape at the start due to surface irregularities. To correct this, the point of maximum slope on the curve is located, and a tangent line is drawn at this point. The initial concave portion is replaced by this tangent, and the origin is shifted to where this tangent intersects the penetration axis. This corrected curve eliminates irregularity effects and allows for precise calculation of the bearing ratio using the formula: Bearing Ratio = (Corrected Test Load / Standard Load) × 100%.

IS 2720 Part 31 specifies that bearing ratios should be calculated primarily at penetration depths of 2.5 mm and 5.0 mm. The value at 2.5 mm penetration is generally preferred for design purposes. However, if the bearing ratio at 5.0 mm penetration exceeds that at 2.5 mm, the test must be repeated. If the repeated results confirm the higher 5.0 mm value, then it is adopted for design. This ensures consistent and reliable evaluation of soil strength for engineering applications.

As per Clause 8.1 of IS 2720 Part 31, a minimum of three in-place bearing ratio tests are recommended at each location. If the initial three test results do not fall within acceptable tolerance limits (which vary based on bearing ratio value), three additional tests should be conducted. The average of all six tests is then used as the representative bearing ratio for that site. This approach ensures the reliability and consistency of the results.

The standard procedure for soil surface preparation before testing involves clearing the test area of any loose or dried soil particles and debris, followed by careful leveling without disturbing the soil structure. Test points should be spaced at least 50 cm apart to prevent mutual interference. If specified, the surface may be soaked to achieve desired moisture content with surcharge weights applied. After soaking, free water should be drained off, the surface leveled again, and allowed to rest for at least 15 minutes prior to testing. Post-test, soil samples should be collected near the penetration point for moisture content and in-place density determinations as per relevant IS codes.