What is the standard rate of penetration for the test?

The standard rate of penetration is 1.25 mm per minute (Clause 4.3).

What surcharge weight should be used?

A surcharge weight of 10 kg should be used, which is equivalent to the effect of the pavement overlying the subgrade (Clause 4.2).

What happens if the CBR at 5.0 mm is greater than at 2.5 mm?

If the CBR value at 5.0 mm penetration is higher, the test should be repeated. If the repeat test yields a similar result, the CBR value corresponding to 5.0 mm penetration is used (Clause 5.1).

What is the minimum distance between two test locations?

The code does not specify a minimum distance, but general practice suggests spacing tests sufficiently far apart (e.g., 10-15m) to avoid influencing subsequent tests.

IS 8763

: 1978

Code of practice for in-situ California bearing ratio (CBR) test for soils

CurrentFrequently UsedTesting MethodGeotechnical · Surveying and Geotechnical Investigation

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This standard details the procedure for conducting the California Bearing Ratio (CBR) test on soils in-situ. It is used to determine the load-bearing capacity of subgrade soils for designing and evaluating pavements, roads, and airfields. The code specifies the apparatus, test procedure, data recording, and calculation methods.

Lays down the procedure for conducting the in-situ California Bearing Ratio (CBR) test on soils.

Quick Reference Values

Standard rate of penetration1.25 mm/min

Penetration plunger diameter50 mm

Penetration plunger length100 mm

Standard load for 2.5 mm penetration1370 kg

Standard load for 5.0 mm penetration2055 kg

Minimum reaction load/vehicle weight1000 kg heavier than max test load

Key Formulas

CBR (%) = (P_t / P_s) * 100 — Where P_t is the test load and P_s is the standard load for a given penetration.

Practical Notes

Ensure the reaction vehicle is significantly heavier than the maximum anticipated test load to prevent lifting, which invalidates the test.

The test area must be leveled precisely to ensure the plunger is perpendicular and fully in contact with the soil before applying the initial seating load.

If the load-penetration curve is initially concave, a correction must be applied by shifting the origin, as described in the standard.