California Bearing Ratio Test - Soil Properties: Testing, Measurement, and Evaluation - page 398

(2.4) Place the surcharge weights on the perforated plate and
adjustable stem assembly and carefully lower onto the compacted
soil specimen in the mold. Apply a surcharge equal to the weight of
the base material and pavement within 2.27 kg (5 lb), but in no case
shall the total weight used be less than 4.54 kg (10 lb). If no pave-
ment weight is specified, use 4.54 kg. Immerse the mold and
weights in water allowing free access of water to the top and bot-
tom of the specimen. Take initial measurements for swell and allow
the specimen to soak for 96 h. Maintain a constant water level
during this period. A shorter immersion period is permissible for
fine-grained soils or granular soils that take up moisture readily, if
tests show that the shorter period does not affect the results. At the
end of 96 h, take final swell measurements and calculate the swell
as a percentage of the initial height of the specimen.

(2.5) Remove the free water and allow the specimen to drain down-
ward for 15 min. Take care not to disturb the surface of the specimen
during the removal of the water. It may be necessary to tilt the spec-
imen in order to remove the surface water. Remove the weights, per-
forated plate, and filter paper, and determine and record the mass.

The CBR test is designed to simulate conditions that will exist at the
surface of the subgrade. A surcharge (weight) is placed on the surface of
the compacted specimen to represent the weight of pavement above the
subgrade. Furthermore, the specimen is soaked to approximate the
poorest field conditions. After soaking, the force required to push a stan-
dard piston into the soil a specified amount is determined and is used to
evaluate the CBR.
The actual step-by-step procedure is as follows (ASTM D 1883-05 [1]):

PROCEDURE

(1) Place a surcharge of weights on the specimen sufficient to pro-
duce an intensity of loading equal to the weight of the base mate-
rial. If no pavement weight is specified, use 4.54 kg mass. If the
specimen has been soaked previously, the surcharge shall be equal
to that used during the soaking period. To prevent upheaval of soil
into the hole of the surcharge weights, place the 2.27-kg annular
weight on the soil surface prior to seating the penetration piston,
after which place the remainder of the surcharge weights.
(2) Seat the penetration piston with the smallest possible load, but in
no case in excess of 10 lb (44 N). Set both the stress and penetration
gages to zero. This initial load is required to ensure satisfactory seat-
ing of the piston and shall be considered as the zero load when deter-
mining the load penetration relation. Anchor the strain gage to the
load measuring device, if possible; in no case attach it to the testing
machine's support bars (legs).

Note 4 —At high loads the supports may torque and affect
the reading of the penetration gage. Checking the depth of
piston penetration is one means of checking for erroneous
strain indications.