Geotechnical Engineering II GEN22P1 DR R MBUGUA COMPACTION Definition of compaction • Compaction is a process by which the soil particles are artificially rearranged and packed together into a closer state of contact by mechanical means in order to decrease the porosity (voids ratio) and thus increase in soil dry density or unit weight, accompanied by a decrease in air volume. • Compaction can be Achieved by : Rolling Tamping Vibration CONSOLIDATION • Consolidation - is a gradual process of volume reduction under sustained loading • Compaction is more or less rapid reduction mainly in air voids under loading of short duration There is usually no change in water content. The size of the individual soil particles does not change, neither is water removed The degree of compaction is measured by dry unit weight and depends on the water content and compactive effort (weight of hammer, number of impacts, weight of roller, number of passes). Compactive effort is standardized by specifying number of blows to be applied by a given mass of hammer falling through given height Maximum compaction is neither achieved when the soil is very dry nor it is very wet Consolidation cont’ Maximum dry density is found to correspond to some intermediate moisture content known as optimum moisture content For a given compactive effort, the maximum dry unit weight occurs at an optimum water content. Protector showed there is relationship between the soil water content and degree of dry density • Purposeful compaction is intended to improve the strength and stiffness of soil. Consequential (or accidental) compaction, and thus settlement, can occur due to vibration (piling, traffic, etc.) or self-weight of loose fill. Determination of maximum dry density and optimum moisture content take air dried sample sieved through 19mm sieve approx. 35kg the material passing is divided into five portions weigh first portion is weighed and mixed with predetermined amount of water and covered for 30 minutes so that moisture is evenly distributed (maturing) • (initial water content may be taken 4% for coarse grained 10% for fine grained soils) soils and other four potions are treated in the same way using different amounts of water to obtain sufficient points on either side of the optimum moisture content compaction test clean dry mould is weighed and assembled to base plate The moist material from first test is remixed and compacted with rammer in 5 layers each receiving 55 blows After second layer is compacted in the mould, a representative sample is taken from the material and moisture content determined After the final layer the surface of material should project between 5mm to 15mm above the mould with the collar removed. Cut off excess with straight edge Fine material may be used to fill in gaps Mass of material in the mould is determined (m) Volume of mould is determined by determining the mass of water required to fill it (v) Procedure is repeated for the other 4 tests Bulk density 𝛾 = • Dry density 𝛾𝑑 = 𝑀 𝑉 𝛾 1+𝑚 • Plot compaction curve dry density versus moisture content. compaction test cont.’ • Plot compaction curve dry density versus moisture content. • From this the maximum dry density and optimum moisture content can be determined Objectives of compaction • Compaction can be applied to improve the properties of an existing soil or in the process of placing fill. The main objectives are to: increase shear strength and therefore bearing capacity increase stiffness and therefore reduce future settlement decrease voids ratio and so permeability, thus reducing potential frost heave Factors Affecting Compaction of Soil – Effect on Different Soil Types DRY DENSITY • Normal soil consists of solids, water (moisture) and air. Usually we calculate bulk density of soil by finding out the weight of soil per unit volume. Dry density is bulk density -moisture content • Following the different factors affecting compaction of soil: • Water content Amount of compaction Types of soil Methods of soil compaction Effect of Water Content on Compaction of Soil Factors Affecting Compaction of Soil cont.’ • Water content • At low water content, the soil is stiff and offers more resistance to compaction. As the water content is increased, the soil particles get lubricated. The soil mass becomes more workable and the particles have closer packing. • The dry density of the soil increases with an increase in the water content till the optimum water content in reached. At that stage, the air voids attain approximately a constant volume. With further increase in water content, the air voids do not decrease, but the total voids (air plus water) increase and the dry density decreases. • Thus the higher dry density is achieved upto the optimum water content due to forcing air voids out from the soil voids. After the optimum water content is reached, it becomes more difficult to force air out and to further reduce the air voids. • The effect of water content on the compaction of soil can also be explained with the help of electrical double layer theory. At low water content, the forces of attraction in the adsorbed water layer are large, and there is more resistance to movement of the particles. • As the water content is increased, the electrical double layer expands and the inter-particle repulsive forces increase. The particles easily slide over one another and are closely packed. This results in higher dry density. Factors Affecting Compaction of Soil cont.’ • Amount of Compaction • The compaction of soil increases with the increase in amount of compactive effort. With increase in compactive effort, the optimum water content required for compaction also decreases. At a water content less than the optimum, the effect of increased compaction is more predominant. • At a water content more than the optimum, the volume of air voids become almost constant and the effect of increased compaction on soil is not significant. • It may be mentioned that the maximum dry density does not go on increasing with an increase in the compactive effort. For a certain increase in the compactive effort, the increase in the dry density becomes smaller and smaller. Finally a stage is reached beyond which there is no further increase in the dry density with an increase in the compactive effort. • The line of optimums which join the peaks of the compaction curves of different compactive efforts follows the general trend of the zero-air void. This line corresponds to air voids of about 5%. Factors Affecting Compaction of Soil cont.’ • Type of Soil: • The compaction of soil depends upon the type of soil. The maximum dry density and the optimum water content for different soils are shown in figure. In general, coarse grained soils can be compacted to higher dry density than finegrained soils. • With the addition of even a small quantity of fines to a coarse-grained soil, the soils attain a much higher dry density for the same compactive effort. • However, if the quantity of the fines in increased to a value more than that required to fill the voids of the coarse-grained soils, the maximum dry density decreases. A well graded sand attains a much higher dry density than a poorly graded soil. • Cohesive soils have high air voids. These soils attain a relatively lower maximum dry density as compared with the cohesionless soils. Such soils require more water than cohesionless soils and therefore the optimum water content is high. Heavy clays of very high plasticity have very low dry density and a very high optimum water content. Factors Affecting Compaction of Soil cont.’ • Method of Soil Compaction: • The dry density achieved depends not only upon the amount of compactive effort but also on the method of compaction. For the same amount of compactive effort, the dry density will depend upon whether the method of compaction utilizes kneading action, dynamic action or static action. • • For example, in Harvard Miniature compaction test, the soil is compacted by the kneading action, and therefore, the compaction curve obtained is different from that obtained from the other conventional tests in which an equal compactive effort is applied. • • Different methods of compaction curve give their own compaction curves. Consequently, the lines of optimums are also different. Calculation of dry density • • • • • • • • • • • • Expressions for calculating density A compacted sample is weighed to determine its mass: M (grams) The volume of the mould is: V (ml) Sub-samples are taken to determine the water content: w The calculations are: Worked example A compacted soil sample has been weighed with the following results: Mass = 1821 g Volume = 950 ml Water content = 9.2% Determine the bulk and dry densities. Bulk density r = 1821 / 950 = 1.917 g/ml or Mg/mł Dry density rd = 1.917 / (1+0.092) = 1.754 Mg/mł Worked example • Determine the dry densities of a compacted soil sample at a water content of 12%, with air-voids contents of zero, 5% and 10%. (Gs = 2.68).
