1.0
INRTODUCTION
In the early 1990s, a Swedish scientist named Atterberg dev eloped a method to describe the consistency of fine-grained soils with varying moisture contents. Atterberg limits are defined as the water corresponding to different behavior conditions of finegrained soil (silts and clays). The four states of consistency in Atterberg limits are liquid, plastic, semisolid and solid. The dividing line between liquid and plastic states is the liquid limit; the dividing line between plastic and semisolid states is the shrinkage limit. If a soil in the liquid state is gradually dried ou t, it wills past through the liquid limit, plastic state, plastic limit, semisolid state and shrinkage limit and reach the solid stage. The liquid, plastic and shrinkage limits are therefore quantified in terms of the water content at which a soil changes from the liquid to the plastic state. The difference index. Because the liquid limit between the liquid limit and plastic limit is the plasticity the plasticity index. and plastic limit are the two most commonly used Atterberg limits, the following discussion is limited to the test procedures and calculation for these two laboratory tests. The liquid limit is that moisture content at which a soil changes from the liquid state to the plastic state. It along with the plastic limit provides a means of soil classification as well as being useful in determining other soil p roperties. As explained, plastic limit is the dividing line between the plastic and semisolid states. From a physical standpoint, it is the water content at which the soil will begin to crumble when rolled in small threads.
1.1
OBJECTIVE
•
To determine the liquid limit of a soil.
•
To determine the liquid limit of a soil.
1.2
LIQUID LIMIT TEST
1.2.1
APPARATUS
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Cone penetrometer
•
Flat glass plate
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Metal cups
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Spatula
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Containers
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Distilled water
Cone penetrometer and metal cup
1.2.2
PROCEDURE
1. The apparatus is checked so that:
Mass of falling cone assembly to 0.1g
Stem falls freely when released.
Tip of cone can be felt through gauge when brushed with finger.
2. The soil is mixed (paste with the spatulas for at least 10 minutes, distilled water must be added in successive stages to give a cone penetration of about 15mm and mix well in. Note: Through mixing and kneading is the most important feature of the test and must never be overlooked. 3. The soil paste is pressed against the side of the cup to avoid trapping air. Press more paste well into the bottom of the cup, without an air pocket. The small spatula is convenient for these operations. The top surface is finally smoothed off level with the rim using the straight edge. 4. The tip of the cone is adjusted with a few millimeters of the surface of the soil in the cup. Hold the cone, the release button is pressed and the height of the cone is adjusted so that the tip just touches the soil surface. 5. The stem of the dial gauge is lowered to make contact with the top of the cone shaft. Record the reading of the dial gauge to the nearest 0.1mm (R1). Alternatively if the pointer is mounted on a friction sleeve, the pointer is adjusted to read zero. 6. The timer is set to 5 second and then the button is pressed and it is released immediately. Automatic re-locking of the stem is indicated by a click. The apparatus must remain steady and must not be jerked. 7. The dial reading is recorded to the nearest 0.1mm (R2). Record the difference between R1 and R2 as the cone penetration. If the pointer was initially set to read zero, the reading R2 gives the cone penetration directly. 8. The cone is lifted out and it is cleaned carefully. Avoid touching the sliding stem. Add a little more wet soil to the cup, without entrapping air, smooth off and repeat procedure (5), (6) and (7). 9. Two consecutive penetrations should be within 0.5mm or three within 1mm.
10. Take a moisture content sample of about 10g from the area penetrated by the cone using the tip of a small spatula. Placed in a numbered moisture content container, which is weighed, oven dried and weighed as in the standard moisture content procedure in experiment 1. 11. The soil remaining in the cup is remixed with the rest of the sample on the glass plate together with a little more distilled water, until a uniform softer consistency is obtained. 12. Penetration range of the soil sample should be within 15 – 25mm. The procedure (3) – (10) is repeated with different volume of water to get at least 4 points evenly spaced. 13. The moisture content of the soil from each penetration reading is calculated from wet and dry weighing as in the moisture content test. Each cone penetration (mm) is plotted as ordinate, against the corresponding moisture content (%) as abscissa, both to the linear scales. The best straight line fitting these points is drawn. 14. From the graph, the moisture content corresponding to a cone penetration of 20mm is read off to the nearest 0.1%. The result is reported to the nearest whole number as the liquid limit (cone test).
PLASTIC LIMIT TEST APPARATUS •
Glass plate
•
A separate glass plate for rolling of threads
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Spatulas
•
Moisture content apparatus
PROCEDURE
1. Take about 20g of the prepaid soil paste and spread it on glass mixing plate so that it can partially dry. Mixed it occasionally to avoid local drying out. 2. When the soil is plastic enough, it is well kneaded and then shaped into a ball. Mould the ball between the fingers and roll between the palms of the hands so that the warmth of he hands slowly dries it. 3. When slight cracks begin to appear on the surface, divide the ball into two portions each of about 10g. Further divide each into four equal parts, but keep each set of four parts together. 4. One of the parts is kneaded by the fingers to equalize the distribution of moisture and then formed into a thread about 6mm diameter, using the first finger and thumb of each hand. 5. The thread is rolled between the fingers of the one hand and the surface of the glass plate by uniform pressure. The pressure should reduce the diameter of the thread from 6mm to about 3mm after between five and ten back-and-forth movements of the hand. Note: It is important to maintain a uniform rolling pressure throughout; do not reduce pressure as the thread approaches 3mm diameters. 6. The soil is dried further by molding between the fingers again, not by continued rolling which gives a dried crust. From it into a thread and roll out again as before, repeat this procedure until the thread crumbles when it has been rolled to 3mm diameter.
7. Crumbling of the thread include falling apart in small pieces; breaking into a number of short pieces tapered towards the ends; longitudinal splitting from the ends towards the middle and then falling apart. Note: Crumbling must be the result of the d ecreasing moisture content only, and not due to mechanical breakdown caused by excessive pressure or oblique rolling or detachment of an excessive length beyond the width of the hand. 8. As soon as the crumbling stage is reached, gather the crumbled threads and place them into a weighed moisture content container. The lid is replaced immediately. 9. Procedure (4) – (8) is repeated for other pieces of soil and place in the same container. The container is weighed and soil as soon as possible, dried in the oven overnight, cool and weigh dry, as in the standard moisture content procedure. 10. Procedure (4) – (9) is repeated on the other set of four portions of the soil, using a second moisture content container. 11. The moisture content of the soil in each of the two containers is calculated. Take the average of the two results. If they differ by more than 0.5% moisture content, the test should be repeated. 12. The average moisture content referred to above is expressed to the nearest whole number and reported as plastic limit (w1) of the soil. The method of the preparation of the soil is reported and so is the percentage of material passing the 425µm sieve if it was sieved. The result is usually reported on the same sheet as the liquid limit test. 13. The difference between the liquid limit and the plastic limit is calculated to give the plasticity index ( I p ) of the soil : I p = W L – W P
The value is also reported to the nearest whole number. If it is possible to perform the plastic limit test, the soil is reported as non-plastic (NP). This also applies if the plastic limit is equal to or greater than the liquid limit: the latter can occur in same soils with high mica content (Tubey and Webster, 1978).
RESULTS Liquid limit test and result
Test number Cone penetration (mm)
17. 4
Average penetration (mm) Container no. Wet soil & container (g) Dry soil & container (g) Container (g) Dry soil (g) Moisture loss (g) Moisture content (%)
1 17.6
17.5
20.4
2 20.8
20.7
27.3
3 27.3
17.5
20.6
27.2
1 15.40
2 15.20
3 15.90
14.00 9.2 4.8 1.4 29.2
14.00 9.5 4.5 1.2 26.7
14.00 9.4 3.6 1.9 52.8
Plastic limit test and results
Test number Container no. Wet soil & container (g) Dry soil & container (g) Container (g) Dry soil (g) Moisture loss (g) Moisture content (g) Average (g)
1 PL - 1 11.50 10.00 9.20 0.80 1.50 18.75
2 PL - 2 11.10 10.00 9.30 0.70 1.10 15.71 14.70
3 PL – 3 10.60 10.00 9.30 0.70 0.60 9.57
27.2
CALCULATION
Plastic limit, PL = 18.75 + 15.71 + 9.57 3 = 14.70 From the graph, liquid limit, LL = 31.3% (when the penetration is equal to 20mm) Plasticity index, PI = LL – PL = 31.30 – 14.70 = 16.6% Liquidity index, LI = w – PL x 100 PI = 40 – 14 x 100 16.6 = 156.6%
DISCUSSION •
The plastic limit is defined as the moisture content in percent, at which the soil crumbles, when rolled into threads of 3.2 mm (1/8 in.) in diameter.
•
The plastic limit test is simple and is performed by repeated rolling of an ellipsoidalsize soil mass by hand on a ground glass plate.
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The plasticity index (PI) is the difference between the liquid limit and the plastic limit of a soil, or
•
PI = LL - PL
Burmister (1949) classified the plasticity index in a qualitative manner as follow:
PI 0 1-5 5-10 10-20 20-40 >40
Description Nonplastic Slightly plastic Low plasticity Medium plasticity High plasticity Very high plasticity
→
Since PI = 16.6, the description of the soil is Medium Plasticity.
Advantage of carry out liquid and plastic tests •
Identify the consistency of fine grain soil with varying moisture content. Not suitable for coarse grain soil like sand
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Depending on moisture content of soil, we can determine a soil phase will divided into four basic states. It’s solid, Semi-solid, plastic and liquid.
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Can be able to describe the consistency of fine grained soils with varying moisture content.
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This later is to determine the behavior of soil which is divided into four basic states – solid, semisolid, plastic and liquid.
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Less expensive equipment required and simpler experiment in which provides information about the nature of cohesive soils.
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Can be used extensively for the correlation of several physical soil parameters as well as for soil identification.
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All these are important to make analysis for planning an d design before construction happen or taking place in an foundation
The limitation of liquid and plastic limit tests •
The limitations of this test is the shrinkage limit where with continuing loss of moisture, a stage of equilibrium is reached at which more loss of moisture will result in no further volume change. The moisture content, in percent, at which the volume of the soil mass ceases to change, is defined as the shrinkage limit.
•
The methods described herein are performed only on that portion of a soil that passes the 425-um (No. 40) sieve. Therefore, the relative contribution of this portion of the soil to the properties of the sample as a whole must be considered when using these tests to evaluate properties of a soil.
•
The liquid and plastic limits of many soils that have been allowed to dry before testing may be considerably different from values obtained on no n-dried samples. If the liquid and plastic limits of soils are used to correlate or estimate the engineering behavior of soils in their natural moist state, samples should not be
permitted to dry before testing unless data on dried samples are specifically desired. •
The plastic limit test is performed on material prepared for the liquid limit test
•
The mechanisms controlling the test procedures do not simulate the mechanisms controlling the water-holding capacity of soils which the liquid limit is supposed to represent. Further, the test procedures have their limitation in that they are relatively arbitrary
Five (5) common laboratory errors for liquid and plastic limit tests. •
Didn’t mix the soil properly, causing the soil to be nonuniform soil.
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Using hand when mixing causes some of the water absorbed by our hand.
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The dish didn’t cleaned properly when measurement taken.
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The rolling done in smaller scale in diameter.
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Doesn’t maintain a uniform rolling pressure throughout.
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Penetration needle didn’t fall freely when released.
CONCLUSION
Plasticity Chart
Liquid limit (LL)
= 31.3
Plastic Limit (PL)
= 14.7
Plasticity Index (PI)
= 16.6
Liquidity index (LI)
= 156.6
•
After this experiment, and after few calculations, we obtain the result as shown above.
•
By using moisture content 40%, we have obtained plasticity index (16.6%) and liquid index (156.6%).
•
With all this value, we have obtained the group which this soil is classified. From the chart we found that the type of soil was falling under CL.
REFERENCE •
Fundamental geotechnical engineering (Braja M. Das- Brooks/ Cole Publishing).
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Basic soil mechanics (Roy Whitlow)
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Principle of geotechnical engineering 5th edition (Braja M. Das - Brooks/ Cole Publishing).
