TITLE: ATTERBERG
LIMIT TESTS
OBJECTIVES: To
determine the liquid limit, the plastic limit and the linear shrinkage and plasticity index of the given soil sample.
INTRODUCTION
The Atterberg limits are basic measure of the critical water contents of a fine-grained soil. fine-grained soil. When clay minerals are presents in fine grained soils, that soil can be remolded in the presence of some moisture without crumbling. This This cohesive nature is caused by the adsorbed water surrounding the clay particles. At very low moisture content, soil behaves more like solid and when the moisture content is very high, the soil and water may flow like a liquid. The Atterberg limits can be used to distinguish between silt between silt and clay, and clay, and and it can distinguish between different types of silts and clays. These distinctions in soil are used in assessing the soils that are to have structures built on. Soils when wet retain water and some expand in volume. The amount of expansion is related to the ability of the soil to take in water and its structural make-up (the type of atoms present). These tests are mainly used on clayey or silty soils since these are the soils that expand and shrink due to moisture content. Clays and silts react with the water and thus change sizes and have varying shear strengths. Thus these tests are used widely in the preliminary stages of designing desi gning any structure to ensure that the soil will have the correct amount of shear shear strength and not too much change in volume as it expands and shrinks with different moisture contents.
THEORY
In the early 1900s, a Swedish agricultural scientist named A. Atterberg developed a method to describe the consistence of fine grained soils with varying moisture content. Depending on the moisture content, the behavior of the soil was divided into four basic states: solid, semisolid, plastic and liquid and defined the boundaries of four states in terms of limits as follows. Shrinkage Limit (SL) – (SL) – The The moisture content below which no further soil volume reduction take place. Boundary between the semi-solid and solid state. Plastic Limit (PL) – (PL) – The The moisture content below which the soil shows non plastic behavior. Boundary between the plastic and semi-solid state. Liquid Limit (LL) – (LL) – The The moisture content below which the soil behaves as a plastic material. At this moisture content the soil is on the verge of becoming a viscous fluid. Boundary between the liquid and plastic state.
Water content =( (weight of water)/(weight of solid)) * 100% The liquid limit (LL) is the water content, at which the soil has such small shear strength that it flows to close 13 mm distance of a groove of standard width, when jarred in a specified manner. The liquid limit is read from the flow curve as the water content at 25 blows. (Figure 2a). The plastic limit (PL) is the water content of the soil, at which soil begins to crumble when it is rolled into a thread of 3 mm in diameter. Usually PL is cal culated as the mean value of three water content determinations (figure 2b).
The shrinkage limit (SL) is the water content that is just sufficient to fill the pores when the soil is at the minimum volume, which will attain by drying. Linear shrinkage (LS), which aids to estimate the shrinkage limit, characterizes the change in length induce by drying a cylindrical sample of soil initially at about its liquid limit.
LS = (L – L0 ) 100% L
Where, L is the original length of the sample at about the limit and L 0 is the length of dried sample. The amount of water, which must be added to change a soil from its plastic limit to its liquid limit, is an indication of the plasticity of the soil. The plasticity is measured by the plasticity index (PL), which is equal to the liquid limit minus the plastic limit. Plasticity index (PI) = Liquid limit (LL) – Plastic limit (PL)
APPARATUS
Liquid limit device and grooving tool
Large glass plate
Half cylindrical linear shrinkage mold
Distilled water
Balance (sensitive to 0.01g)
Drying oven with temperature range from 105 0C to 110 0C
Desiccators
Drying containers with lids
Mixing dish or bowl
Spatula
Sieve of No. 40
Figure 3: Liquid limit apparatus
PROCEDURE
Liquid limit determination a) The liquid limit apparatus was calibrated to 10 mm falling height using the space gauge on the grooving tool handle. b) About 100 g of moist soil passing through sieve no 40 was taken and mixed it thoroughly with distilled water and a uniform paste was formed. c) A portion of the paste was placed in the cup of the li quid limit device and smoothed the surface off to a maximum depth of ½ inch. d) The grooving tool was drawn through the sample along the symmetrical axis of the cup, holding the tool perpendicular to the cup at the point of contact. e) The crank handle was turned at a rate of about two revolutions per second, and counted the blows necessary to close the groove in the soil for a distance of ½ inch. f) Approximately 10g of soil from near the closed groove was taken for a water content determination. g) The water of the soil was altered and repeated steps (b) to (f) h) Five water content determination in the range of t he ten to fifty blows was obtained. Plastic limit determination a) About 15g of the moist soil (soil which was used for LL test) was mixed thoroughly. b) The soil was rolled on a glass plate with the hand until it was approximately 3mm in diameter. c) The step (b) was repeated with changing the water content until a 3 mm diameter thread showed sign of crumbling. d) Some of the crumbled material was taken for the water content determination. e) The steps (b) to (d) was repeated and obtained three determination, average of the three was taken as the plastic limit.
CONCLUSION
The Atterberg limits represent the water content where the consistence of a fine grained soil is transformed from a plastic state to a liquid state and from a semisolid state to plastic state. The quantity of water at the Atterberg limits and for the other physical properties depends on the same, mostly compositional factors, such as the type of minerals, the amount of each mineral, the shapes and size distribution of the particles and the pore-water composition, which leads to the conclusion that the exactly defined relationships between the values of the Atterberg limits and the soil’s other properties must exist. Liquid limit, plastic limit, and shrinkage limit tests of fine-grained soil are indicators of the nature of its plasticity. The difference between the liquid limit and plastic limit is called the plasticity index. Liquid limit and plasticity index are required parameters for classification of fine-grained soils. Therefore the objectives were determined successful in this lab.
REFERENCES
Factors affecting permeability of soils - Wikipedia, the free encyclopedia. 2016. Factors affecting permeability of soils - Wikipedia, the free encyclopedia. [ONLINE] Available at:https://en.wikipedia.org/wiki/Factors_affecting_permeability_of_soils. [Accessed 30 August 2016].
Why is it necessary to use distilled water in Atterberg limit tests? | Yahoo Answers. 2016 [ONLINE] Available at:https://answers.yahoo.com/question/index?qid=20140620115142AAR6rf6. [Accessed 1 September 2016]. Liquid limit devices (Casagrande), Soil testing equipment, Controls. 2016. Liquid limit devices (Casagrande), Soil testing equipment, Controls. [ONLINE] Available at: http://www.controls-group.com/eng/soil-testing-equipment/liquid-limitdevices-casagrande.php . [Accessed 29 August 2016].
