Liquid Limit
General
Wherever possible the test shall be carried out on soil in its natural state. With many clay soils it is practicable and shall be permissible to remove by hand any coarse particles present, i.e. particles retained on a 425 μm test sieve. In the definitive method the soil shall not be allowed to become dry before testing. Where air drying is required the method used shall be stated.
Procedure
Step 1: Take a sample of about 300 g from the soil paste and place it on the glass plate.
Step 2: Mix the prepared paste for at least 10 min using the two palette knives. If necessary, add more distilled water so that the first blow count is about 50 blows.
NOTE: Some soils, e.g. clays of high plasticity and residual soils, may require up to 40 min of continuous mixing immediately before testing to obtain reliable results.
Step 3: With the cup of the apparatus resting on the base, place a portion of the mixed soil in the cup without entrapping air. Level off the soil surface parallel to the base.
Step 4: Use the grooving tool to divide the soil into two equal parts by drawing the tool from the hinge towards the front in a continuous circular movement. Hold the grooving tool normal to the surface of the cup, with the chamfered edge facing the direction of movement.
NOTE: With soils having low plasticity indices it is sometimes difficult to cut a smooth groove in the soil with the grooving tool. However, it does not necessarily follow that because a smooth groove cannot be obtained, the soil is non-plastic, and it should be recorded that the liquid limit could not be obtained by this method. The cone penetrometer method is the only practicable procedure for soils of this type.
Step 5: Turn the crank handle at the rate of 2 r/s so that the cup is lifted and dropped, counting the number of bumps (blows). Continue until the two parts of the soil come into contact at the bottom of the groove along a distance of 13 mm, measured with the end of the grooving tool or with a ruler. Record the number of bumps at which this occurs.
NOTE: Sometimes the soil flows so as to leave a gap between two areas of contact. The test should continue until there is a length of continuous contact for 13 mm.
Step 6: Add a little more of the prepared soil from the glass plate and mix it with the soil in the cup and repeat steps 2 to 4 until two consecutive runs give the same number of bumps for closure.
NOTE: Some soils tend to slide on the surface of the cup instead of flow taking place within the soil. If this occurs the result should be discarded and the test repeated until flowing does occur. If, after additional increments of water, sliding still occurs, the test is not applicable and a note should be made that the liquid limit could not be obtained by this method. The cone penetrometer method should be used instead.
NOTE: Care should be taken to see that the sample does not dry out between repeat tests as the number of blows for closure will increase gradually as the sample dries.
Step 7: Take about 10 g of soil with a spatula from the portions of the sample that have just flowed together. Place the soil in a suitable container and determine the moisture content as specified.
Step 8: Repeat steps 2 to 7 at least three more times using the same sample of soil to which further increments of distilled water have been added. Proceed from the drier to the wetter condition of the soil. The amount of water added shall be such that when the four or more moisture contents are plotted, they are evenly distributed over the range between 50 bumps to 10 bumps (see Table below for approximate range of bumps require at each phase). Each time the soil is removed from the cup for the addition of water, wash and dry the cup and grooving tool.
Step 9: If at any time during the above procedure the soil has to be left for a while on the glass plate, cover the soil with the evaporating dish or a damp cloth to prevent the soil drying out.
Calculations and Expression of Results (see attached form)
Step 1: Calculate the moisture content of each test sample.
Step 2: Plot the relationship between the moisture content and the corresponding number of bumps on a semi-logarithmic chart, with the percentage moisture contents as ordinates (vertical axis) on the linear scale and the number of bumps as abscissae (horizontal axis) on the logarithmic scale.
Step 3: Draw the best straight line fitting the plotted points (the “flow curve”). From the flow curve read off the moisture content corresponding to the abscissa of 25 blows to the first decimal place. Express this moisture content to the nearest whole number and report it as the liquid limit.
Test Report
The test report shall affirm that the test was carried out in accordance with BS 1377-2:1990 and shall include the following information:
a) the method of test used;
b) the value of the liquid limit;
c) the percentage of material passing the 425 μm test sieve;
d) the history of the sample, e.g. whether tested in the natural state or after wet sieving or after any other process.
Plastic Limit
Procedure
Step 1: Take a sample of about 20 g from the soil paste when the conducting liquid limit test from the mixture that closed at number of bumps between 22 – 28 and place it on the glass mixing plate.
Step 2: Allow the soil to dry partially on the plate until it becomes plastic enough to be shaped into a ball.
Step 3: Mould the ball of soil between the fingers and roll it between the palms of the hands until the heat of the hands has dried the soil sufficiently for slight cracks to appear on its surface. Divide this sample into two subsamples of about 10 g each and carry out a separate determination on each portion. Divide each subsample into four more or less equal parts and treat each part as specified in steps 4 to 8.
Step 4: Mould the soil in the fingers to equalize the distribution of moisture, then form the soil into a thread about 6 mm diameter between the first finger and thumb of each hand.
Step 5: Roll the thread between the fingers, from finger-tip to the second joint, of one hand and the surface of the glass rolling plate. Use enough pressure to reduce the diameter of the thread to about 3 mm in five to 10 complete, forward and back, movements of the hand. Some heavy clays will require 10 to 15 movements when the soil is near the plastic limit because the soil hardens at this stage. It is important to maintain a uniform rolling pressure; do not reduce the pressure as the thread diameter approaches 3 mm.
Step 6: Pick up the soil, mould it between the fingers to dry it further, form it into a thread and roll it out again as specified in step 5.
NOTE: Gradually drying of the soil is affected by alternately rolling and moulding, not by continual rolling, either as a ball or as threads, which produces a dried crust.
Step 7: Repeat step 6 until the thread shears both longitudinally and transversely when it has been rolled to about 3 mm diameter, as gauged by the rod. Do not gather the pieces of soil together after they have crumbled, in order to reform a thread and to continue rolling; the first crumbling point is the plastic limit.
NOTE: With soils that are marginally plastic it is often difficult to obtain the correct crumbling condition.
Step 8: Gather together the portions of the crumbled soil thread, transfer them to a suitable container and replace the lid immediately.
Step 9: Repeat steps 4 to 8 on the other three portions of soil, placing them all in the same container. Determine the moisture content of the soil in the container.
Step 10: Repeat steps 3 to 9 on the duplicate sample formed in step 3.
Calculations and Expression of Results (attached form above can be used)
Step 1: Calculate the moisture content of both samples. If the two results differ by more than 0.5 % moisture content, repeat the whole test.
Step 2: If the two samples does not differ by more than 0.5%, calculate the average of the two moisture content values and express the value to the nearest whole number. This is the plastic limit.
Test report
The test report shall affirm that the test was carried out in accordance with BS 1377-2:1990 and shall include the following information:
a) the method of test used;
b) the plastic limit of the soil sample;
c) the percentage of material passing the 425 μm test sieve;
d) the history of the sample, e.g. whether tested in the natural state, or after wet sieving or after any other process; If it is not possible to determine the plastic limit this fact shall also be reported.
