2FINE ANALYSIS: CONSISTENCY LIMITS With fine cohesive soils the size of the particles is not as important, relatively speaking, as the shape of the particles. Fine cohesive soils behave in a plastic manner (in general an increase in stress bringing about an irrecoverable deformation with a constant or reducing volume and no cracking) whereas course non cohesive soils do not exhibit plasticity to any marked degree. The plasticity of a soil has a marked effect on the engineering properties of a soil - shear strength, compressibility etc.In the coarse analysis (sieve tests) anything finer than 63m was recorded as clay and silt. To obtain an indication of the engineering properties of this clay/silt material the water content at which certain changes in the physical properties of the soil take place are measured. As the particles which make up the clays and silts tend to be “flaky” in nature, this together with changes in the water content gives rise to an inherently variable material.
3FINE ANALYSIS CONSISTENCY LIMITS As moisture is removed from these fine grained soils they pass through four states, solid, semi-plastic solid, plastic, liquid, all of these states are water content dependent.Total VolumeWater contentLiquidPlasticBrittle solidSemi plastic solidThis change from one state to another is a gradual process, however for convenience we chose to define three water content “limits” at which the changes occur. These limits are commonly referred to as the Consistency Limits of the soil.
4FINE ANALYSIS CONSISTENCY LIMITS The three limits in question are :-1) Liquid Limit (WL) - this is the water content at which the soil changes from a liquid to a plastic state. It is the minimum water content at which the soil will flow under it’s own weight.Total VolumeWSWPWLWater content
5FINE ANALYSIS CONSISTENCY LIMITS The three limits in question are :-2) Plastic Limit (WP) - this is the water content at which the soil ceases to be entirely plastic and becomes a semi-plastic solid.Total VolumeWSWPWLWater content
6FINE ANALYSIS CONSISTENCY LIMITS The three limits in question are :-3) Shrinkage Limit (WS) this is the water content below which further loss of moisture does not result in a decrease in the soil volume.Total VolumeWSWPWLWater content
7FINE ANALYSIS CONSISTENCY LIMITS Of these three limits the most important as far as we are concerned are the liquid limit and the plastic limit.The range of water contents over which the soil is in a plastic condition is referred to as the Plasticity Index (IP)Total VolumeWSWPWLWater content
8FINE ANALYSIS CONSISTENCY LIMITS The Liquidity Index (IL) expresses the natural water content of the soil in terms of the consistency limits.IL < 0 soil is in a semi-plastic or solid state0 < IL < 1 soil is plasticIL > 1 soil is in a liquid state and will thus flow (i.e. a quick clay)Natural Water Content, wTotal VolumeWSWPWLWater content
9DETERMINATION OF CONSISTENCY LIMITS (LIQUID LIMIT - penetrometer method)Firstly the soil is dried and then broken up using a pestle and mortar. The sample is then sieved and the material passing the 425 m sieve mixed with distilled water to a paste of stiff consistency. This is then left for 24 hours in an air tight container to allow for the water to fully penetrate the soil. After this time a portion of the soil is placed in the penetrometer cup and the soil struck off level with the top of the cup (care must be taken not to entrap any air in the cup when placing the soil). The cup is then placed on the penetrometer stand and the point of the cone lowered such that it just touches and marks the top surface of the soil sample in the cup.
10DETERMINATION OF CONSISTENCY LIMITS (LIQUID LIMIT - penetrometer method)The dial gauge reading is then taken and noted and then the clamp released. The cone is allowed to penetrate the soil sample for 5 sec. (timed with a stop watch) after which the clamp is re-tightened and a second dial gauge reading taken and again noted. The difference between the second and first dial gauge readings gives the penetration. The same procedure is repeated several times on the same soil sample and an average penetration computed. A small sample of the soil sample is then taken for water content determination.
11DETERMINATION OF CONSISTENCY LIMITS (LIQUID LIMIT - penetrometer method)The whole procedure is then repeated five or six times with the successive addition of amounts of distilled water (i.e the soil sample will have an increasing water content)
12DETERMINATION OF CONSISTENCY LIMITS (LIQUID LIMIT - penetrometer method)Line of best fit25x20xCone penetration (mm)15xx10Data points5Liquid Limit (WL)Water content (%)A graph of cone penetration against water content is then plotted with a “best fit” straight line drawn between the points. The liquid limit is then the water content which corresponds to a cone penetration of 20 mm.
14DETERMINATION OF CONSISTENCY LIMITS (PLASTIC LIMIT)Take approximately 20 g of soil paste (prepared in the same way as for the liquid limit test) and roll it into a ball in the hands until slight cracks appear in it’s surface. Divide the ball into two halves and then one of these halves into four equal portions. Take one of these portions, roll it into a ball and then into a thread on a glass plate. When the diameter of the thread becomes 3 mm knead it again into a ball, this process of handling the soil sample effectively drying out the soil sample (i.e. decreasing the water content). Again roll the soil ball out into a thread. Repeat the process of rolling into a ball and then into a thread until the thread just starts to crumble at the 3 mm dia. Once this has occurred place the thread pieces into an air tight container. The whole process should be carried out on the remaining three portions of the first half of the 20g sample with all thread pieces put into the same container.The test is then repeated on the other 10g of soil sample.The water content of the two 10g’s is then determined and the average of the two reported as the plastic limit of the sample ( % ).
15SOIL CLASSIFICATION SYSTEMS Using the relationship between the liquid limit and the plastic limit it is possible to establish sub - groups for the fine soils. The most commonly used classification in the UK is the British Soil Classification System and this is based on the standard Plasticity Chart.
16SOIL CLASSIFICATION SYSTEMS The liquid limit is plotted against the plasticity index of the soil and depending where this point lies a sub - group for the soil can be determined. The ‘A’ line on the plasticity chart gives an arbitrary division between silts and clays with the vertical lines defining five (5) levels of plasticity:- low(L), intermediate (I), high (H), very high (V) and extremely high (E).
17SUB-GROUP SYMBOLS FOR THE BRITISH SOIL CLASSIFICATION SYSTEM Primary letterSecondary letterCoarse grained soilsG = GRAVELW = well gradedS = SANDP = poorly gradedPu = uniformly gradedPg = gap gradedFine grained soilsF = FINESL = low plasticityM = SILTI = intermediate plasticityC = CLAYH = high plasticityV = very high plasticityE = extremely high plasticityOrganic soilsPt = PEATO = organic
18SUB-GROUP SYMBOLS FOR THE BRITISH SOIL CLASSIFICATION SYSTEM GPu – uniformly graded GRAVELCV – very high plasticity CLAYML – low plasticity SILT