Compiled by: Prof.B.S.Chawhan M.Tech(Geo-Tech Engg), Asst.Professor,CED,Government.Engineering College,Haveri-581110(12/4/2011-Till date) UNIT-06 COMPACTION OF SOILS Definition of Compaction: The densification of soil by the application of mechanical energy is known as compaction. Compaction is the most common and important method of soil improvement. It is a process by which the soil grains get arranged more closely, the value of air voids gets reduced and the density of soil increased. Δh A A V2 W V1 W S S (a) (b) Fig.1. a).Before compaction. b).After compaction. Compaction generally leads to an increase in shear strength and helps improve the stability and the bearing capacity of a soil. It also reduces the compressibility and permeability of the soil. Detrimental settlements can be prevented and undesirable volume changes through swelling and shrinkage can be controlled. Laboratory Compaction Test: The compaction characteristics and the degree of compaction can be obtained from the laboratory. In these tests, a specified amount of compactive effort is applied to a constant volume of soil mass. The compactive energy is reported in J/m3. In the laboratory, impact compaction is most commonly used; a hammer (rammer) is dropped several times on a soil sample in a mould. In the field, compactive effort is the number of passes or coverages of a roller of a certain kind and eight on a given volume of soil. In the laboratory two types of tests are conducted for compaction test. a. Standard proctor Test b.The Modified proctor test. a. Standard proctor Test. In 1933 proctor first introduced a laboratory compaction test which is still the most widely used test. The Proctor test, adopted by the BIS the light compaction test(As per IS: 2720 (Part-7&8), 1974), consists in compacting soil at various water contents into a cylindrical metal mould, having an internal diameter of 10cm, and internal effective height of 12.73cm and a capacity of 1000cm3. the soil is compacted in three equal layers, each layer being given 25 blows of a 2.6kg rammer dropped from a height of 31cm above the soil. The following figure shows a mould and a rammer for the Proctor test. By knowing the weight of compacted soil and its water content, the dry density for each test is determined. The following figure shows about standard proctor test; 5cm 5cm 2.5kg 10cm or 2.6kg 31 cm capacity= 1000cm3 12.73cm Fig.2. Mould and rammer . Compiled by: Prof.B.S.Chawhan M.Tech(Geo-Tech Engg), Asst.Professor,CED,Government.Engineering College,Haveri-581110(12/4/2011-Till date) MDD Apex point Dry density (gm/cc) Dry Side Wet Side OMC w(%) water content Fig.3. Water content-dry density curve. Density= weight of compacted soil Volume of the soil mass γ= W/V 3 3 Dry density, γd= γ/1+w in KN/m or gm/cc or t/m . The result of the compaction test are presented in the form of a compaction curve plotted between water contents as abscissa(X- axis) & corresponding dry density as ordinates(Y-axis). The dry density corresponding to the maximum point on the water content-dry density curve obtained for a specified amount of compaction is called the maximum dry density (MDD). The water content at which a physical amount of compaction produces a maximum dry density is known as the optimum water content (OMC). Air Voids line (ηa=0): Y Sr=100% Dry density (gm/cc) Water content w(%) Fig.4. water content Vs dry density. Compiled by: Prof.B.S.Chawhan M.Tech(Geo-Tech Engg), Asst.Professor,CED,Government.Engineering College,Haveri-581110(12/4/2011-Till date) A line which shows the water contents, dry density relation for the compaction acting soil containing a constant percentage air voids is known as an air-void line and can be obtained for the following relation ()1−η a Gγ w γ = (1) d 1+ wG V n = v (2) V V and η = v (3) a V where, ηa=percentage air voids. w= water content for compaction soil in %. 3 3 γw= density of water in g/cm or KN/m . G=Specific gravity of soil solids. *The theoretical max compaction for any given water content corresponds to zero air voids condition ηa=0* Zero Air voids line (ηa=0): The line showing the dry density is a function water content for soil containing no air voids is called zero air voids line or saturation line which is given by the relation i.e., ()1−η a Gγ w γ = (4) Put ηa=0 d 1+ wG Gγ w *γ = * (5) d 1+ wG Alternatively, a line showing the relation between water content and dry density for a constant degree of saturation (Sr) is given by the equation, Gγ w γ = d 1+ wG Gγ w γ = (6) d wG 1+ S r Modified proctor test: Fig.5. Typical standard and modified proctor test shown above and below. Y 80%Degree of saturation 100%Degree of saturation Zero air voids line i.e ηa=0 Sr=100% Line of optimum Modified proctor test B Standard proctor test Dry density (gm/cc) A x w(%) Compiled by: Prof.B.S.Chawhan M.Tech(Geo-Tech Engg), Asst.Professor,CED,Government.Engineering College,Haveri-581110(12/4/2011-Till date) Modified proctor test as to better simulate the compaction required for airfields to support heavier aircraft. The test employed a heavier hammer,4.5kg with a height of fall of 457.2mm and 5 layers tamped 56 blows into a modified proctor mould of internal diameter-150mm and effective height-127.3mm. The Indian Standard equivalent of the Modified proctor test is called the heavier compaction test(IS:2720-partVIII-1983). Application of Compaction: Compaction of soils increases their density, shear strength, bearing capacity but reduces their void ratio, porosity, permeability and settlements. The results of the compaction test are useful in the stability of field problems like earthen dams, embankments, roads and airfields. In such constructions, the soils are compacted. The moisture content at which the soils are compacted in the field is controlled by the value of optimum moisture content determined by the laboratory proctor compaction test. The compaction energy to be given by the field compaction unit is also controlled by the maximum dry density determined in the laboratory. In other words, the laboratory compaction tests results are used to write the compaction specification for field compaction of soils. Factors affecting compaction: Following are the factors affecting compaction. a. Water content. b. Compactive effort. c. Type of soil. d. Method of compaction. e). Admixtures. a. Water content (w): As the water content increases, the particles develop large and larger water films around them, which tend to lubricate the particles and make them easier to be worked around, to move close into a denser configuration, resulting in a higher dry unit weight and lower air voids. The dry unit weight continues to increase till the optimum moisture content is reached, a stage when the lubrication effect is the maximum with further increase in moisture content, however, the water starts to replace the soil particles and since γw<< γsat, the unit weight starts decreasing. The dry unit weight can also be related to the water content and degree of saturation by following equation, Gγ w γ = d 1+ wG Gγ w γ = (7). d wG 1+ S r For a given water content, the theoretical maximum value of dry unit weight for a compacted soil is obtained corresponding to the situation when no air voids are left, i.e., when the degree of saturation becomes equal to 100%. If the zero air void density is calculated for different water content values and plotted along the compaction curve. It is more convenient to draw lines corresponding to different percentage air voids, ηa. From following equation, Gγ w γ = (8). d 1+ wG The zero air void line obtained for Sr=100% in eqn (7) and for ηa=0% in Eqn(8) are identical. However, it can be seen that 100% air void line & 90% saturation line are not identical. b. Compactive effort: For all types of soil and with all methods of compaction, the effect of increasing the compactive energy is to increase the maximum dry density and to decrease the optimum water content. According to figure (5), compaction curve B corresponding to the higher compactive effort in a Modified proctor test. Comparing it with the compaction curve A for a standard proctor test, one can see that the compaction curve shifts to the top and to the left when the compactive effort is increased. The margin of increase becomes smaller and smaller even on the dry side of OMC (Optimum Moisture Content), while on the wet side of OMC, there is hardly any increase at all. If the peaks of compaction curves for different compactive efforts are joined together, a’ line of optimums’ is obtained in figure (5). The line of optimum is nearly parallel to the zero air void curve. Compiled by: Prof.B.S.Chawhan M.Tech(Geo-Tech Engg), Asst.Professor,CED,Government.Engineering College,Haveri-581110(12/4/2011-Till date) c. Type of soil: Following figure shows the different type of soils Y 1 2 3 4 5 6 Dry density 7 (gm/cc) 8 X w(%) water content Fig.6. Description of different types of soil compaction curve. Compaction curve Description of soil Group Symbol Proctor compaction MDD(gm/cc) OMC(%) 1 Well graded to loamy sand(SW-SM) SW 1.907±0.080 13.3±2.5 2 Well graded sandy loam(SM) SM 1.827±0.016 14.5±0.40 3 Med-graded sandy loam(SM) CL-ML 1.747±0.032 16.3±0.70 4 Lean sandy silt clay(CL) CL 5 Lean silty clay(CI) CI 6 Loessial silt(ML) ML 1.651±0.016 19.2±0.7 7 Heavy clay 8 Poorly graded sand(SP) SP 1.763±0.032 12.4±1.0 Y Fullsaturation Air-Dry Dry density (gm/cc) X Water content ( %) Compiled by: Prof.B.S.Chawhan M.Tech(Geo-Tech Engg), Asst.Professor,CED,Government.Engineering College,Haveri-581110(12/4/2011-Till date) Fig.7.