The California Bearing Ratio Test (CBR) Test
OBJECTIVE -To introduce the student to a method of evaluating the relative quality of subgrade, subbase, and base soils for pavement.
THEORY
The California Bearing Test was developed in 193, to evaluate Californian highway subgrade strengths and became the basis for the design of road and airfield pavements throughout the world. !t is used in both situ and situ and on prepared samples in the laboratory, but is limited to materials of particles si"es si"es up to a ma#imum of $%mm. The test determine the relationship between force and penetration when a cylindrical plunger 193&
2
mm
in cross'sectio cross'section n is pressed pressed into into soil at at a given given rate of 1 mm(min. mm(min. )or
any given penetration, the ratio is e#pressed as a percentage of a standard force derived for crushed stone. )or CB* tests in the laboratory, the soil specimen is prepared at a predetermined moisture content and is compacted into a cylindrical mould 1&$ mm in diameter and 1$+ mm high either by continuous tamping, compression in three equal layers or dynamic compaction in layers, using either the rammers or the vibrating hammer used in the compaction test. !n all cases, the mass of soil poured into the mould is calculated as that required to provide the chosen dry density or air voids percentage on completion of compaction. These usually correspond to the optimum, determined from compaction tests, or are the values measured on the soil situ.
APPARATUS
1. $. 3. -. &.
old Collar Base plate pacer disc )!lter paper
PROCEURE !" /repare appro#imately -.&0g of fine grained '2 o. - sieve material or &.& 0g of 19.. ma#imum'si"e material to the optimum moisture content of the soil as determined by the appropriated compaction effort compaction test2. !f it is desired to cure the soil for a more uniform moisture distribution, mi# the soil with the necessary percentage of moisture and store in sealed container for about 1$ to $- h prior to the test. #" 4ust before ma0ing the compacted mold of soil ta0e a representative water'content sample at least 1%% g for grained soil2 $" 5eight the mold less base plate and the collar. %" Clamp the mold to the base plate, insert a spacer dis0 the 1&.1 # &.$cm solid round dis02 in the mold, and cover with a piece of filter paper. &" Compact the soil using 6T 789 or 71&&+ ethod B or 7 for the soil used or as specified by the instructor. Ta0e a final represantative of water'content sample from the remaining soil. '" *emove the collar, and trim the specimen smooth and flush with the mold. /atch any holes formed from aggregate displacement when scraping the specimen flush with the mold. se any of the soil particles for patching that are smaller than the hole to be filled. " *emove the base plate and the spacer dis0, weight the mold : compacted soil, and determine the wet unit weight. " /lace a piece of filter paper on the base plate, invert the specimen so the spacer on the top2, and attach the bas plate so the soil is in contact with the filter paper on the base.
)or unsoa0ed specimens, do step 9 to 11 ne#t; for soa0ed samples proceed to step 1$. 9. /lace the sufficient slotted weights but not less than -.& 0g2 on the sample to stimulate the required overburden pressure. 1%. /lace the pecimen in the compression machine and seat the piston using a seating load no greater than -.& 0g. et load and penetration dials to "ero. 11. Ta0e penetration readings as outlined in the <=eneral 7iscussion,> and ta0e the corresponding load'dial readings. ?#trude the sample, and ta0e two additional representative water'content samples. )or soa0ed samples@ 1$. /lace the perforated plate with adAustable stem on the compacted soil and apply sufficient additional slotted weights to obtain the desired surcharge within $.$0g but with a total surcharge weight of not less than -.& 0g. *ecord the total
surcharge weight be sure to include the perforated plate as part of the surcharge weight2. 6lso be sure to use a piece of filter paper between the perforated stem base and the soil to prevent the soil from stic0ing to the stem base. 13. !mmerse the mould and weights in a container of water so the water has access to both the top and bottom of the sample, and attach the dial gage reading to %.%1 mm2 in its holding brac0et. ar0 the mould where the brac0et is placed so that it can be replaced properly between readings. 1-. et the swell gage to "ero and record the time of the start of the test. Ta0e readings at %,1, $, -, , 1$, $-, 38, -, +$ and 98 hours of elapsed time. The swell test may be terminated after - hours if the swell'dial reading has been constant for thee last $- hours. 1&. 6t the end of 98 hours of soa0ing, remove the sample and let it drain for 1& minutes. Blot the free surface water from the top surface of the sample with paper towels. 18. 5eigh the soa0ed sample including the mould. 1+. 7o steps 9 to 11 e#ample, ta0e penetration and readings2 1. Ta0e water content samples of the soa0ed specimen as follows@ Two within the top 3 cm of soil Two within the bottom 3 cm of soil Two at the center of the sample
CA*CU*ATIO+ A+ ATA PRESE+TATIO+
!" /lot a curve of penetration resistance in 0ilopascals 0/a2 vs. penetration in millimeters for both the freshly compacted and the soa0ed samples. !f the curve is not essentially through the origin, e#tend a line from the straight line portion to intersect the abscissa. The difference between this value and "ero penetration is a correction to apply to compute the CB* value. Curves for both samples soa0ed and unsoa0ed2 should be plotted on the same graph and cle arly identified, together with the curve correction values, so that one may readily observe the effect of soa0ing the sample. se a sheet of graph paper from the data sheet section. #" btain the penetration resistance from $.& and &.% mm from the curve and compute the CB* number. $" Compute the water contents and dry densities of both samples before soa0ing and of the soa0ed sample and its final state. %" Compute the percentage of swell based on the nominal initial height of the sample. /lot a curve of percent swell vs. elapsed time on a separated sheet of graph paper. &" The report should compare the CB* values, present a summary of the water contents properly identified, and include the required curves. 7iscuss the significance of a large or small CB* value, any changes in CB* with soa0ing, and any swell that occurred. Dow might the swell be reduced or eliminated for this soilE Be sure your report includes classification data and the 66DT classification of the soil.
ISCUSSIO+
The CB* tests was developed by the California 7ivision of Dighways in 19$9 as a means of classifying the suitability of a soil for use as a subgrade or base course material in highway construction. 7uring 5orld 5ar !!, The ,, Corps of ?ngineers the test for use airfield construction. The CB* test 6T calls the test simply a bearing'ratio test2 measures the shearing resistance of a soil under controlled moisture and density conditions. The test yields a bearing'ratio number, but from the previous statement it is evident this number is not a constant for a given soil but applies only for the tested state of soil. The test can be performed in field on an in'place soil, but this is beyond the scope of this laboratory manual. The CB* number is obtained as the ratio of the unit load in pounds per square inch2 required to effect a certain depth of penetration of the penetration piston with an area of 19.cm
2
2 into a compacted specimen of soil at some water content and density to the standard
unit load required to obtain the same depth of penetration on a standard sample of crushed stone. !n equation form, this is
test unit load
CB* F
standard unit load
× 100
)rom this equation it can be seen that the CB* number is a percentage of the standard unit load. !n practice, the percentage symbol is dropped and the ratio is simply noted as a number, such as 3, -&, 9. Galues of standard unit load to use in the equation above. The CB* number is usually based on the load ratio for a penetration of $.& mm. !f, however, the CB* value at a penetration of &.% mm penetration, the CB* for &.% mm is larger, test should be redoneordinarily2. !f a second test yield also a larger CB* number at &.%mm penetration, the CB* for &.% mm should be used. CB* tests are usually made on specimens at the optimum moisture value for the soil as determine using the standard compaction test. e#t, using ethod B or 7 or 6T 789 or 6T 71&&+ the specimen are made up using the compaction energy shown. Two one are often compacted one for immediate penetration testing and one for testing after soa0ing for a period of 98 h. The second specimen is soa0ed for a period of 98 h with a surcharge appro#imately equal to the pavement weight used in the field but ! in no case the surcharge mass less than -.& 0g. well readings are ta0en during this period at arbitrary level selected times, and at the end of the soa0ing period the CB* penetration test is made to obtain a CB* value for the soil in a saturated condition.
!n both penetration test for the CB* values, a surcharges of the same magnitude as for the swell test is placed on the soil sample. The test on the soa0ed sample accomplishes two things@ 1. !t givens information concerning e#pected soil e#pansion beneath the pavement when the soil become saturated. $. !t gives indication of strength loss from field saturation.
/enetration testing is accomplished in a in a compression machine using strain rate of 1.$+mm(min. *eadings of load vs. penetration are ta0en at each %.&mm of penetration to include the value &.% mm, and then at each $.& mm increment thereafter until the total penetration is 1$.+ mm. The CB* number is used to rate the performance of soils primarily for use as bases and subgrades beneath pavements of roads and airfields. any paving design procedures are published in which one enters a chart with the CB* number and reads directly the thic0ness of subgrade, base course, and(or fle#ible pavement subgrade modulus k before entering the paving'design charts.
