Shrinkage Limit Test
- 1. Determination of Physical Properties of Soil Shrinkage Limit Test IS: 2720 Part 6 – 1972 Reaffirmed 2016 CENGRS GEOTECHNICA PRIVATE LIMITED ISO 9001:2015 accredited by JAS-ANZ ISO 17025:2017 certified geotechnical laboratory (NABL)
- 2. What is Shrinkage limit • A saturated soil is slowly dried, capillary menisci* form between the individual soil particles. • As a result, the interparticle (effective) stresses increase, and the soil decreases in volume. • A point is eventually reached where this volume change stops, even though the degree of saturation is still essentially 100%. • The moisture content at which this occurs is defined as the shrinkage limit (wSL) * Menisci are a manifestation of capillary action, by which surface adhesion pulls a liquid up to form a concave meniscus or internal cohesion pulls the liquid down to form a convex meniscus.
- 3. Need & Scope of Shrinkage Limit • As the soil loses moisture, either in its natural environment, or by artificial means in laboratory it changes from liquid state to plastic state to semi-solid state and then to solid state. • The volume is also reduced by the decrease in water content. But, at a particular limit the moisture reduction causes no further volume change. • A shrinkage limit test gives a quantitative indication of how much moisture can change before any significant volume change and to also indication of change in volume.
- 4. Continued…. • The shrinkage limit is useful in areas where soils undergo large volume changes when going through wet and dry cycles (e.g. expansive soils) • The shrinkage factor helps in the design problems of structure made up of this soil or resting on such soil. It helps in assessing the suitability of soil as a construction material in foundations, roads, embankments, and dams. • This limit is needed for studying the swelling and shrinkage properties of cohesive soil.
- 5. Principle for Determining the Shrinkage Limit of Soil: • Figure shows the schematic diagram in which a fully saturated soil in stage I having volume • V1 undergoes shrinkage and on complete drying reaches stage III, where the entire water is evaporated. • Between stages I and III lies stage II, where the soil is at shrinkage limit water content. • In stage II, the soil is fully saturated, but a further decrease in the water content does not cause any decrease in the volume of the soil and air occupies the space of the evaporated water.
- 6. Continued…. • The volume of the soil at shrinkage limit is equal to the total volume of oven-dried soil. It is to be noted that the volume of soil solids is constant throughout the shrinkage process, and the decrease in volume occurs only due to decrease in volume of voids. So we get – Weight of water in stage I = – W1 – Wd Loss of water from stage I to II = (V1 – V2)γw Weight of water in stage II = (W1 – Wd) – (V1 – V2)γw Shrinkage limit = Water content of soil in stage II • Where W1 is the initial total weight of soil in stage-I, Wd is the dry weight of soil, V1 is the initial volume of the soil in stage-I, Vd is the volume of soil in dry state (stage III), ω1 is the initial water content of soil in stage-I, and γw is the density of water.
- 7. Lab Test of Shrinkage Limit Lab procedure is in accordance with IS:2720 (Part 6)-1972
- 8. Apparatus 1. Cylindrical stainless steel shrinkage dish with 45-mm internal diameter and 15-mm internal height. 2. Cylindrical glass cup with 50-mm internal diameter and 25-mm internal height. 3. Porcelain evaporating dish. 4. Square acrylic plastic plate of size 75 mm x 75 mm with three metal prongs. 5. Square plain plastic plate of size 75 mm x 75 mm. 6. Thermostatically controlled oven. 7. Mercury 8. calibrated Sieve. 9. Spatula & Calibrated balances.
- 9. Procedure 1. Coat the inside of the shrinkage dish with a thin layer of silicon grease to prevent adhesion of soil to the dish. Determine its empty weight. 2. Determine the capacity of the shrinkage dish by filling to overflowing with mercury, removing the excess by pressing the plain glass plate firmly flush over the dish, ensuring no air is entrapped. Weigh the mercury held in shrinkage dish and divide this by unit weight of mercury (13.6 g/cc) to obtain the volume, which is also the volume of wet soil pet (V). 3. For test on remoulded sample fill the dish in three layers by placing soil paste about one third the capacity of the dish at a time and tapping the dish gently on a firm surface with proper cushioning by a rubber sheet so that the soil flows to the edges. 4. The last layer should stand a little above the run and care should be taken not to trap air within the soil . Strike off the excess soil in level with the top of the dish and clean the outside. For undisturbed soil pat, trim it from undisturbed soil sample approximately 44 mm in diameter and 15 mm in height. Round off their edges to prevent the entrapment of air during mercury displacement. Measure the dimension of the pat to calculate volume (V).
- 10. Continued…. 5. Weigh the dish full of wet soil. Calculate the weight of wet soil pat (W). Allow it to dry in air until the colour of soil pat turns light and the soil starts to leave the sides of the dish. Then, dry in an oven at 105-110o C, cool the dish with dry soil pat in a desiccators and weigh immediately after removal from the desiccators. By weighing the shrinkage dish and dry soil calculate the weight of dry soil pat (Wo). 6. Keep the glass cup in the large porcelain on stainless steel dish, fill it to over flowing with mercury and remove the excess by pressing the plain glass plate firmly over the top of the cup, taking care not to entrap any air. 7. Wipe off any mercury adhering on the side and then transfer the cup full of mercury to another large dish taking care not to entrap any air. 8. Place the dry soil pat on the surface of mercury and submerged it under the mercury by pressing with the “glass plate with prongs”, taking care not to entrap air. 9. Transfer the mercury displaced by the dry pat to the mercury-weighing dish and weigh and determine the volume of dry soil pat by dividing this weight by the unit weight of mercury (Vo).
- 11. Calculations • Calculate the shrinkage limit (remoulded soil) using the following formula: Where: Ws = Shrinkage limit in percentage w = moisture content of wet soil pat in percentage, V = volume of wet soil pat , Vo = volume of dry soil pat , W = weight of wet soil pat Wo = weight of oven-dry soil pat in g.
- 12. • Calculate the shrinkage limit (Undisturbed soil) using the following formula: Where : Wsu = shrinkage limit (undisturbed soil) in percentage, Vos = volume of oven-dry specimen in ml, Wos = weight of oven-dry specimen in g, G = specific gravity of soil
- 13. TABULATION AND RESULTS S.No Determination No. 1 2 3 1 Wt. of container in gm,W1 2 Wt. of container + wet soil pat in gm,W2 3 Wt. of container + dry soil pat in gm,W3 4 Wt. of oven dry soil pat, W0 in gm (W3-W1) 5 Wt. of water Ww in gm (W2-W3) 6 Moisture content (%), w = (Ww/Wo)*100 7 Volume of wet soil pat (V), in ml 8 Volume of dry soil pat (V0) in ml By mercury displacement method a. Weight of displaced mercury (Wm) b. Specific gravity of the mercury (Gm) V0 = Wm/Gmγw 9 Wt. of Shrinkage dish + Oven dry specimen (g) 10 Wt. of oven dry specimen, Wos, (g) 11 Volume of oven dry specimen, Vos, ml 12 Specific gravity of soil, G 13 Remoulded Soil: Shrinkage limit (Ws%) 14 Undisturbed Soil: Shrinkage limit (Ws%) 15 Shrinkage Index (Is) = Ip - Ws 16 Shrinkage ratio (R)
- 14. Engineering Description of soil
- 15. Foundation Damage • The most obvious way in which expansive soils can damage foundations is by uplift as they swell with moisture increases. Swelling soils lift up and crack lightly-loaded, continuous strip footings, and frequently cause distress in floor slabs.
- 16. Damage to foundation from expansive soil A rectangular slab, uniformly loaded, will tend to lift up in the corners because there is less confinement
- 17. Damage to foundation from expansive soil Figure : Damage to home supported on shallow piers. (1) At the beginning of the rainy season, the piers are still supported by friction with the soil. When it begins to rain, water enters deep into the soil through the cracks. (2) After 5 to 10 large storms, the soil swells, lifting the house and piers. (3) In the dry season, the groundwater table falls and the soil dries and contracts. As tension cracks grow around the pier, the skin friction is reduced and the effective stress of the soil increases (due to drying). When the building load exceeds the remaining skin friction, or the effective stress of the soil increases to an all-time high, adhesion is broken by this straining, and the pier sinks.
- 18. Mitigation Measures • The best way to avoid damage from expansive soils is to extend building foundations beneath the zone of water content fluctuation. • The reason is twofold: 1. First, to provide for sufficient skin friction adhesion below the zone of drying 2. Second, to resist upward movement when the surface soils become wet and begin to swell.