Soil Compaction
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Compaction of soil involves mechanically rearranging soil particles to reduce voids and increase dry density, which improves engineering properties like strength and reduces settlement. Standard compaction tests determine the optimum water content and maximum dry density for a given soil and compactive effort. Factors like water content, compactive effort, soil type, and method of compaction influence the engineering behavior of compacted soils.
Soil Compaction
- 1. CE6405 SOIL MECHANICS V.Nageshwaran, M.E., Assistant Professor, Department of Civil Engineering, UCET
- 3. COMPACTION OF SOIL Compaction is the application of mechanical energy to a soil so as to rearrange its particles and reduce the void ratio, thus increase its Dry Density. It is applied to improve the properties of an existing soil or in the process of placing fill such as in the construction of embankments, road bases, runways, earth dams, and reinforced earth walls. Compaction is also used to prepare a level surface during construction of buildings. There is usually no change in the water content and in the size of the individual soil particles. 1. To increase soil shear strength and therefore its bearing capacity. 2. To reduce subsequent settlement under working loads. 3. To reduce soil permeability making it more difficult for water to flow through.
- 4. CONT… Proctor – Definite relationship between the Soil Water Content and Degree of Dry Density to which a Soil might be Compacted and Specific Amount of Compaction Energy applied on the Soil – Optimum Water Content – Maximum Dry Density. Methods of Compaction – Dynamic or Impact, Kneading, Static and Vibration. Laboratory Water-Density Relationships – Compaction Tests – Standard & Modified Proctor Compaction, Harvard Miniature Compaction, Abbot Compaction and Jodhpur-Mini Compaction test.
- 5. CONT… Laboratory Compaction – The variation in compaction with water content and compactive effort is first determined in the laboratory. There are several tests with standard procedures such as: 1. Indian Standard Light Compaction Test (Similar to Standard Proctor Test) 2. Indian Standard Heavy Compaction Test (Similar to Modified Proctor Test) Indian Standard Light Compaction Test – Soil is compacted into a 1000 cm3 mould whose internal diameter is 100 mm & internal effective height of 127.5 mm. (IS : 2720 Part VII 1980/87) in 3 equal layers, each layer receiving 25 blows of a 2.6 kg rammer dropped from a height of 310 mm or 12 inches above the soil. Compactive Energy used for the Test is 6065 kg.cm/1000 ml. The compaction is repeated at various moisture contents.
- 6. STANDARD PROCTOR COMPACTION TEST
- 7. STANDARD PROCTOR COMPACTION TEST
- 9. CONT… 3 kg of Air-dried & Pulverized Soil – Passing 4.75 mm Sieve – Mixed thoroughly with a Small Quantity of Water. Leave the Mixture by covering it with Wet Cloth for about 5 to 30 mins (more for clay soil). Water to be added – Probable. Initial Water Content – 4% for Coarse Grained Soils & 10% for Fine Grained Soils. Weigh Empty Mould with Base Plate without the Collar. Attach the Collar to the Mould – Place the Matured Mixture into the Mould upto 1/3rd of its height and provide 25 No. of Blows. Similarly, fill the 2nd and 3rd part into the mould with 25 blows each. Final layer should project 6 mm higher than the collar. Volume of Mould – 945 ml.
- 10. CONT… Remove Excess Soil by trimming off. Weigh the Mould with Base Plate & Soil. Take Representative Sample from the Centre of the Compacted Specimen and Determine its Water Content. Calculate the Bulk Density & Dry Density. Remove Soil from the Mould – Break it with Hand and Remix with raised Water Content (2 or 4 %). Allow to Mature – Repeat the Process – Determine Bulk, Dry Density and Water Content. Plot Compaction Curve – Water Content in the Abscissa and Dry Density as Ordinate. V M ρ w1 ρ ρd
- 13. CONT… Air Voids Line – Line showing Water Content-Density Relation for the Compacted Soil Containing a Constant % Air Voids. Zero Air Voids Line/Saturation Line – Line showing Water Content Density Relation for the Compacted Soil Containing 0 % Air Voids or No Air Voids. Theoretical Maximum Compaction for any given Water Content. s wsa d wG1 ρGn1 ρ s ws d wG1 ρG ρ 0na S wG e s 1S
- 14. CONT… Indian Modified Heavy Compaction Test – It was found that the Light Compaction Test (Standard Test) could not reproduce the densities measured in the field under heavier loading conditions, and this led to the development of the Heavy Compaction Test (Modified Test). Heavier Transport & Military Aircraft – Modified Proctor Compaction Test. Standardized by AASHO. Volume of Mould – 945 ml. No. of Layers – 5 Nos. No. of Blows/Layer – 25 Nos. Weight of Rammer – 4.5 kg. Drop Height – 18 in. Compactive Energy – 27260 kg.cm/1000 cm3. 4.5 times > Std. Proctor Test.
- 16. FIELD COMPACTION CONTROL Consists of (i) OMC Determination (ii) Dry Density & Degree of Saturation Determination achieved. Rapid Method – used. Rapid Method of Water Content Determination – Calcium Carbide & Proctor Needle Method (Field). Placement Water Content can be Found.
- 18. FACTORS AFFECTING COMPACTION Effect of Increasing Water Content As water is added to a soil at low moisture contents, it becomes easier for the particles to move past one another during the application of compacting force. The particles come closer, the voids are reduced and this causes the dry density to increase. As the water content increases, the soil particles develop larger water films around them. This increase in dry density continues till a stage is reached where water starts occupying the space that could have been occupied by the soil grains. Thus the water at this stage hinders the closer packing of grains and reduces the dry unit weight. The maximum dry density (MDD) occurs at an optimum water content (OMC), and their values can be obtained from the plot.
- 20. CONT… Effect of Increasing Compactive Effort The effect of increasing compactive effort is shown. Different curves are obtained for different compactive efforts. A greater compactive effort reduces the optimum moisture content and increases the maximum dry density. An increase in compactive effort produces a very large increase in dry density for soil when it is compacted at water contents drier than the optimum moisture content.It should be noted that for moisture contents greater than the optimum, the use of heavier compaction effort will have only a small effect on increasing dry unit weights. It can be seen that the compaction curve is not a unique soil characteristic. It depends on the compaction effort. For this reason, it is important to specify the compaction procedure (light or heavy) when giving values of MDD and OMC.
- 23. CONT… Method of Compaction – Weight of Compacting Equipment, Mode of Operation (Dynamic or Impact, Static, Kneading or Rolling), Time and Area of Contact. Type of Soil – Well Graded Coarse Grained Soil – High Dry Density & Low OMC – Fine Grained Soil – More Water – Lubrication – Greater Specific Surface. The water content of a compacted soil is expressed with reference to the OMC. Thus, soils are said to be compacted dry of optimum or wet of optimum (i.e. on the dry side or wet side of OMC). The structure of a compacted soil is not similar on both sides even when the dry density is the same, and this difference has a strong influence on the engineering characteristics.
- 25. EFFECT OF COMPACTION ON SOIL PROPERTIES Soil Structure – For a given compactive effort, soils have a flocculated structure on the dry side (i.e. soil particles are oriented randomly), whereas they have a dispersed structure on the wet side (i.e. particles are more oriented in a parallel arrangement perpendicular to the direction of applied stress). This is due to the well-developed adsorbed water layer (water film) surrounding each particle on the wet side. Swelling – Due to a higher water deficiency and partially developed water films in the dry side, when given access water, the soil will soak in much more water and then swell more.
- 26. EFFECT OF COMPACTION OF SOIL PROPERTIES Shrinkage – During drying, soils compacted in the wet side tend to show more shrinkage than those compacted in the dry side. In the wet side, the more orderly orientation of particles allows them to pack more efficiently. Construction Pore Water Pressure – The compaction of man-made deposits proceeds layer by layer, and pore water pressures are induced in the previous layers. Soils compacted wet of optimum will have higher pore water pressures compared to soils compacted dry of optimum, which have initially negative pore water pressure.
- 27. CONT… Permeability – The randomly oriented soil in the dry side exhibits the same permeability in all directions, whereas the dispersed soil in the wet side is more permeable along particle orientation than across particle orientation. Compressibility – At low applied stresses, the dry compacted soil is less compressible on account of its truss-like arrangement of particles whereas the wet compacted soil is more compressible. The stress-strain curve of the dry compacted soil rises to a peak and drops down when the flocculated structure collapses. At high applied stresses, the initially flocculated and the initially dispersed soil samples will have similar structures, and they exhibit similar compressibility and strength.
- 28. CONT… Stress-Strain Characteristics – Soil Compacted Dry Side of Optimum – Steeper Stress- Strain – Higher Modulus of Elasticity. Soil Compacted Wet Side of Optimum – Gradual Stress-Strain Brittle Failure. Shear Strength – Depend on Dry Density, Moulding Water Content, Soil Structure, Method of Compaction, Strain used to define Strength, Drainage Condition and Type of Soil.