SOIL LAB REPORT: The liquid limit test.

SOIL LAB REPORT

The liquid limit test

 

Abstract: This blog post is a report of laboratory soil analysis to find the liquid limit which is a coefficient needed for soil classification.

Apparatus:  We use a brass cup and a hard rubber base, spatula, wet soil, balance, oven.

This device consists of a brass cup and a hard rubber base.

The brass cup can be dropped onto the base by a cam operated by a crank.

Objective: Determine the liquid limit.

 

Procedures:

1.  To perform the liquid limit test, one must place a soil paste in the cup.

 

2. A groove is then cut at the center of the soil pat with the standard grooving tool.

 

3. By the use of the crank-operated cam, the cup is lifted and dropped from a height of 10 mm (0.394 in.).

 

4. The moisture content, in percent, required to close a distance of 12.7 mm (0.5 in.) along the bottom of the groove.

 

5.  After 25 blows is defined as the liquid limit. But It is difficult to adjust the moisture content in the soil to meet the required 12.7 mm (0.5 in.) closure of the groove in the soil pat at 25 blows. Hence, at least three tests for the same soil are conducted at varying moisture contents, with the number of blows, N, required to achieve closure varying between 15 and 35.

 

6. The moisture content of the soil, in percent, and the corresponding number of blows are plotted on semi-logarithmic graph paper

 

7. Place a portion of the previously mixed soil into the cup of the liquid limit apparatus at the point where the cup rests on the base. Squeeze the soil down to eliminate air pockets and spread it into the cup to a depth of about 10 mm at its deepest point. The soil pat should form an approximately horizontal surface

 

8. Place a portion of the previously mixed soil into the cup of the liquid limit apparatus at the point where the cup rests on the base.

 

9. Squeeze the soil down to eliminate air pockets and spread it into the cup to a depth of about 10 mm at its deepest point. The soil pat should form an approximately horizontal surface.

10. Use the grooving tool carefully cut a clean straight groove down the center of the cup. The tool should remain perpendicular to the surface of the cup as a groove is being made.

 

11. Turn the crank of the apparatus at a rate of approximately two drops per second and count the number of drops, N, it takes to make the two halves of the soil pat come into contact at the bottom of the groove along with a distance of 13 mm (1/2 in.)

12. Place the soil into a moisture can. Immediately weigh the moisture can containing the soil

 

13. Place the can into the oven for at least 16 hours.

14. Determine the mass of dry soil.

15. Repeat steps for at least two additional trials producing successively lower numbers of drops to close the groove. One of the trials shall be for a closure requiring 25 to 35 drops. Determine the water content from each trial by using the same method.

Discussion:

The relationship between moisture content and log N is approximated as a straight line. This line is referred to as the flow curve.

The moisture content corresponding to N = 25, determined from the flow curve, gives the liquid limit of the soil.

The slope of the flow line is defined as the flow index and may be written as:  

Number of cans	Mass of can without soil	Mass of can + wet soil	Number of blows (N)	Mass of can + dry soil	Mass of wet soil	Mass of dry soil
1	4.21 g	27.08 g	14	20.52 g	22.87 g	16.31 g
2	4.23 g	35.5 g	29	26.19 g	31.27 g	21.96 g
3	4.16 g	22.28 g	38	16.95 g	18.12 g	12.79 g

 

Calculation of water content: w = (Mw) / (Ms)

Mass of water = mass of wet soil – mass of dry soil

W1 = (22.87 – 16.31) *100 / 16.31 = 40 %

W2 = (31.27 – 21.96) * 100 / 21.96 = 42.39 %

W3 = (18.12 – 12.79) *100 / 12.79 = 41.673 %

From the graph, for N = 25, w = LL = 41.75%

 

References:

1)   Soil lab (BAU).

2)   Book: principles of geotechnical engineering (25th edition).