Principle:
This method covers the laboratory determination of the California Bearing Ration (CBR) of a compacted or undisturbed sample of soil. The principle is to determine the relationship between force and penetration when a cylindrical plunger of a standard cross-sectional area is made to penetrate the soil at a given rate. At certain values of penetration the ratio of the applied force to a standard force, expressed as a percentage, is defined as the California Bearing Ratio (CBR). The procedure for soaking the specimen, if required, is included
Test Limitations
Because of the size of the sample and of the plunger the test is appropriate only for materials having a maximum particle size not exceeding 20 mm.
Test conditions
The following test conditions shall be specified before a test is started.
a) The method of preparation of each test specimen.
b) The density and moisture content to be achieved, if applicable.
c) Whether the test is to be carried out on one end or on both ends of the specimen.
d) Whether a test is required on the specimen in the soaked condition.
e) The amount of surcharge to be applied to the specimen for the test, and during soaking if applicable.
Preparation of Test Sample
The sample can be prepared by static compaction or by dynamic compaction by hand or mechanical rammer, or by vibrating hammer, to achieve a specified density to provide a specified compactive effort.
Material
The CBR test shall be carried out on material passing the 20 mm test sieve. If the soil contains particles larger than this the fraction retained on the 20 mm test sieve shall be removed and weighed before preparing the test sample. If this fraction is greater than 25 % the test is not applicable. The moisture content of the soil shall be chosen to represent the design conditions for which the test results are required. Alternatively, where a range of moisture contents is to be investigated, water shall be added to or removed from the natural soil after disaggregation.
Initial Preparation
After bringing it to the required moisture content the soil shall be thoroughly mixed and shall normally be sealed and stored for at least 24 h before compaction into the test mould.
Note: With cohesionless soils it may be possible to reduce or omit the curing period if trial tests indicate that this has negligible influence on the results.
Mass of Soil for Test
The mass of soil required for the test can be obtained by dry density specification, air voids specification or compactive effort specification
(a) Dry Density Specification
In this method, the mass of soil, m1 (in g), required to just fill the CBR mould of volume Vm (in cm3) is given by the equation
Where,
w is the moisture content of the soil (in %); and
ρd is the specified dry density (in Mg/m3).
(b) Air Voids Specification
In this method, the dry density, ρd (in Mg/m3), corresponding to an air voids content of Va (in %) is given by the equation.
Where,
Va is the air voids expressed as a percentage of the total volume of soil;
ρs is the particle density (in Mg/m3);
w is the soil moisture content (in %);
ρw is the density of water (in Mg/m3), assumed equal to 1
The corresponding mass of soil to just fill the CBR mould is calculated from the equation in the dry density specifications.
(c) Compactive Effort Specification
In this method,about 6 kg of soil shall be prepared for each sample to be tested. The initial mass shall be measured to the nearest 5 g so that the mass used for the test sample can be determined after compaction by difference, as a check.
Note: Preliminary trials may be necessary to ascertain the required mass more closely.
Apparatus (for compactive effort specification only)
1.Test sieves, of aperture sizes 20 mm and 5 mm.
2. A cylindrical, corrosion-resistant, metal mould, i.e. the CBR mould, having a nominal internal diameter of 152 ± 0.5 mm. The mould shall be fitted with a detachable baseplate and a removable extension. The internal faces shall be smooth, clean and dry before each use.
3. A metal rammer, either the 2.5 kg rammer or the 4.5 kg rammer, depending on the degree of compaction required. A mechanical compacting apparatus may also be used.
4. An electric, vibrating hammer and tamper.
5. A steel rod, about 16 mm in diameter and 600 mm long.
6. A steel straightedge, e.g. a steel strip about 300 mm long, 25 mm wide and 3 mm thick, with one bevelled edge.
7. A spatula.
8. A balance, capable of weighing up to 25 kg readable to 5 g.
9. Apparatus for moisture content determination.
10. Filter papers, 150 mm in diameter, e.g. Whatman No. 1 or equivalent.
Sample Preparation by Rammer Compaction with Specified Effort
Step 1: The specified effort of compaction shall correspond to the 2.5 kg rammer or to the 4.5 kg rammer method or to an intermediate value. When the 2.5 kg rammer method is used the soil is compacted in three layers. When the 4.5 kg rammer method or the intermediate compaction effort is used the procedure is similar except that the soil is placed and compacted in five layers instead of three.
Note: In certain circumstances, it is required to obtain an intermediate density between that given by the 2.5 kg rammer method and the 4.5 kg rammer method at a given moisture content. This can be obtained by using an intermediate compactive effort between these two levels of compaction. In order to reduce the variations in compactive effort to a minimum, it is suggested that this intermediate effort should be obtained by compacting the specimen in five equal layers, giving each layer 30 blows of a 4.5 kg rammer falling through 450 mm.
Step 2: Divide the prepared quantity of soil into three (five) portions equal to within 50 g and seal each portion in an airtight container until required for use, to prevent loss of moisture.
Step 3: Stand the mould assembly on a solid base, e.g. a concrete floor or plinth.
Step 4: Place the first portion of soil into the mould and compact it, so that after 62 blows of the appropriate rammer the layer occupies about or a little more than one-third (one-fifth*) of the height of the mould. Ensure that the blows are evenly distributed over the surface. Alternatively, the mechanical compacting apparatus may be used.
Step 5: Repeat step 4 using the other two (four) portions of soil in turn, so that the final level of the soil surface is not more than 6 mm above the top of the mould body.
Note: Preliminary trials may be necessary to judge the amount of soil required for each layer.
Step 6: Remove the collar and trim the soil flush with the top of the mould with the scraper, checking with the steel straightedge.
Step 7: Weigh the mould, soil and baseplate to the nearest 5 g (m3).
Step 8: Seal and store the sample.
Preparation of Undisturbed Sample
1. Take an undisturbed sample from natural soil or from compacted fill using a weighed CBR mould fitted with a cutting shoe.
2. After removing the cutting shoe from the mould, cut and trim the ends of the sample so that they are flush with the ends of the mould body. Fill any cavities with fine soil, well pressed in.
3. Attach the baseplate and weigh the sample in the mould to the nearest 5 g (m3). Unless the sample is to be tested immediately, seal the exposed face with a plate or an impervious sheet to prevent loss of moisture.
Soaking
Additional soaking Apparatus
1. A perforated baseplate, fitted to the CBR mould in place of the normal baseplate
2. A perforated swell plate, with an adjustable stem to provide a seating for the stem of a dial gauge.
3. Tripod, mounting to support the dial gauge.
4. A dial gauge, having a travel of 25 mm and reading to 0.01 mm.
5. A soaking tank, large enough to allow the CBR mould with baseplate to be submerged, preferably supported on an open mesh platform.
6. Annular surcharge discs, each having a mass known to ±50 g, an internal diameter of 52 mm to 54 mm and an external diameter of 145 mm to 150 mm. Alternatively half-circular segments may be used.
7. Petroleum jelly.
Soaking Procedure
Step 1: Remove the baseplate from the mould and replace it with the perforated baseplate.
Step 2: Fit the collar to the other end of the mould, packing the screw threads with petroleum jelly to obtain a watertight joint.
Step 3: Place the mould assembly in the empty soaking tank. Place a filter paper on top of the sample, followed by the perforated swell plate. Fit the required number of annular surcharge discs around the stem on the perforated plate.
Note: One surcharge disc of 2 kg simulates the effect of approximately 70 mm of superimposed construction on the formation being tested. However, the exact amount of surcharge is not critical. Surcharge discs of any convenient multiples may be used.
Step 4; Mount the dial gauge support on top of the extension collar, secure the dial gauge in place and adjust the stem on the perforated plate to give a convenient zero reading.
Step 5: Fill the immersion tank with water to just below the top of the mould extension collar. Start the timer when the water has just covered the baseplate.
Step 6: Record readings of the dial gauge at suitable intervals of time, depending on the rate of movement.
Step 7: Record the time taken for water to appear at the top of the sample. (This may not necessarily indicate the end of the swelling stage.) If this has not occurred within 3 days, flood the top of the sample and leave to soak for a further day, giving the normal soaking period of 4 days. A longer period may be necessary to allow swelling to reach completion.
Step 8: Plot a graph of swelling (as indicated by the dial gauge movement) against elapsed time or square-root time. Flattening of the curve indicates when swelling is substantially complete.
Step 9: Take off the dial gauge and its support, remove the mould assembly from the immersion tank and allow the sample to drain for 15 min. If the tank is fitted with a mesh platform leave the mould there to drain after emptying the tank.
Step 10: Remove the surcharge discs, perforated plate and extension collar. Remove the perforated baseplate and refit the original baseplate.
Step 11: Weigh the sample with mould and baseplate to the nearest 5 g if the density after soaking is required.
Step 12: If the sample has swollen, trim it level with the end of the mould and reweigh.
Step 13: The sample is then ready for test in the soaked condition.
Penetration Test Procedure
Apparatus
1. A cylindrical metal plunger, the lower end of which shall be of hardened steel and have a nominal cross-sectional area of 1935 mm2, corresponding to a specified diameter of 49.65 ± 0.10 mm. A convenient size would be approximately 250 mm long.
2. A machine for applying the test force through the plunger, having a means for applying the force at a controlled rate. The machine shall be capable of applying at least 45 kN. The unloaded machine approach speed shall be 1.2 ± 0.2 mm/min.
3. A calibrated force-measuring device complying with 4.2.1.6 of BS 1377-1:1990. The device shall be supported by the crosshead of the compression machine so as to prevent its own weight being transferred to the test specimen.
Note: At least three force-measuring devices should be available, having the following ranges.
0 to 2 kN readable to 2 N for values of CBR up to 8 %
0 to 10 kN readable to 10 N for values of CBR from 8 % to 40 %
0 to 50 kN readable to 50 N for values of CBR above 40 %
4. A means of measuring the penetration of the plunger into the specimen, to within 0.01 mm. A dial gauge with 25 mm travel, reading to 0.01 mm and fitted to a bracket attached to the plunger is suitable.
Note: A dial gauge indicating 1 mm/r is convenient since a nominal rate of penetration of 1 mm/min can be controlled conveniently by keeping the hand of the dial gauge in step with the second hand of a clock or watch.
5. A stopclock or stopwatch readable to 1 s.
6. The CBR mould
7. Surcharge discs
Testing Procedure
Step 1: Place the mould with baseplate containing the sample, with the top face of the sample exposed, centrally on the lower platen of the testing machine.
Step 2: Place the appropriate annular surcharge discs on top of the sample.
Step 3: Fit into place the cylindrical plunger and force-measuring device assembly with the face of the plunger resting on the surface of the sample.
Step 4: Apply a seating force to the plunger, depending on the expected CBR value, as follows.
For CBR value up to 5 % apply 10 N
For CBR value from 5 % to 30 %, apply 50 N
For CBR value above 30 % apply 250 N
Record the reading of the force-measuring device as the initial zero reading (because the seating force is not taken into account during the test) or reset the force-measuring device to read zero.
Step 5: Secure the penetration dial gauge in position. Record its initial zero reading, or reset it to read zero.
Step 6: Start the test so that the plunger penetrates the sample at a nominal rate of 1 mm/min, and at the same instant start the timer.
Step 7: Record readings of the force gauge at intervals of penetration of 0.25 mm, to a total penetration not exceeding 7.5 mm (see form 4.F of Appendix A).
Note: If the operator plots the force penetration curve as the test is being carried out, the test can be terminated when the indicated CBR value falls below its maximum value. Thus, if the CBR at 2.5 mm were seen to be 6 % but by 3.5 mm penetration it could be seen to have fallen below 6 %, the test could be stopped and the result reported as:
CBR at 2.5 mm penetration = 6 %
CBR at 5.0 mm penetration = < 6 %
Step 8: If no further test is to be made on the sample, proceed to step 13.
Step 9: If a test is to be carried out on both ends of the sample, raise the plunger and level the surface of the sample by filling in the depression left by the plunger and cutting away any projecting material. Check for flatness with the straightedge.
Step 10: Remove the baseplate from the lower end of the mould, fit it securely on the top end and invert the mould. Trim the exposed surface if necessary.
Step 11: If the sample is to be soaked before carrying out a test on the base follow the procedure on soaking.
Step 12: Carry out the test on the base by repeating steps 1 to 8.
Step 13: After completing the penetration test or tests, determine the moisture content of the test sample as follows.
a) For a cohesive soil containing no gravel-sized particles, take a sample of about 350 g from immediately below each penetrated surface, but do not include filling material used to make up the first end tested. Determine the moisture content of each sample.
Note: If the sample has been soaked the moisture content after soaking will generally exceed the initial moisture content.
Because of the possibility of moisture gradients, the determination of dry density from the moisture content after soaking may have little significance. If required, the dry density after soaking can be calculated from the initial sample mass and moisture content and the measured increase in height due to swelling.
b) For a cohesionless soil or a cohesive soil containing gravel-sized particles, extrude the sample, break in half and determine the moisture contents of the upper and lower halves separately.
Calculation and Plotting (see Chart below)
Step 1: Calculate the force applied to the plunger from each reading of the force-measuring device observed during the penetration test.
Note: Alternatively, readings of the force-measuring device may be plotted directly against penetration readings. Forces are then calculated only at the appropriate penetration values after correction if necessary.
Step 2: Plot each value of force as ordinate against the corresponding penetration as abscissa and draw a smooth curve through the points.
The normal type of curve is convex upwards, and needs no correction. If the initial part of the curve is concave upwards, the following correction is necessary.
i. Draw a tangent at the point of greatest slope, i.e. the point of inflexion, S, and produce it to intersect the penetration axis at say, Q.
ii. The corrected curve is represented by QST, with its origin at Q from which a new penetration scale can be marked.
If the graph continues to curve upwards as for test 3, and it is considered that the penetration of the plunger is increasing the soil density and therefore its strength, the above correction is not applicable.
Step 3: The CBR value obtained from a test is the force read from the test curve (after correction and calculation if necessary) at a given penetration expressed as a percentage of the force corresponding to the same penetration on the standard curve. Penetrations of 2.5 mm and 5 mm are used for calculating the CBR value. From the test curve, with corrected penetration scale if appropriate, read off the forces corresponding to 2.5 mm and 5 mm penetration. Express these as a percentage of the standard forces at these penetrations, i.e. 13.2 kN and 20 kN respectively (see Table below). Take the higher percentage as the CBR value.
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