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Lecture 5 soil compaction

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Soil Mechanics

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Lecture 5 soil compaction

  1. 1. INTERNATIONAL UNIVERSITY FOR SCIENCE & TECHNOLOGY ‫وا‬ ‫م‬ ‫ا‬ ‫و‬ ‫ا‬ ‫ا‬ CIVIL ENGINEERING AND ENVIRONMENTAL DEPARTMENT 303322 - Soil Mechanics Soil Compaction Dr. Abdulmannan Orabi Lecture 2 Lecture 5
  2. 2. Dr. Abdulmannan Orabi IUST 2 Das, B., M. (2014), “ Principles of geotechnical Engineering ” Eighth Edition, CENGAGE Learning, ISBN-13: 978-0-495-41130-7. Knappett, J. A. and Craig R. F. (2012), “ Craig’s Soil Mechanics” Eighth Edition, Spon Press, ISBN: 978- 0-415-56125-9. References
  3. 3. In the construction of highway embankments, earth dams, and many other engineering structures, loose soils must be compacted to increase their unit weights. To compact a soil, that is, to place it in a dense state. The dense state is achieved through the reduction of the air voids in the soil, with little or no reduction in the water content. This process must not be confused with consolidation, in which water is squeezed out under the action of a continuous static load. Introduction Dr. Abdulmannan Orabi IUST 3
  4. 4. Compaction increases the strength characteristics of soils, which increase the bearing capacity of foundations constructed over them. Compaction also decreases the amount of undesirable settlement of structures and increases the stability of slopes of embankments. Compaction of Soil Dr. Abdulmannan Orabi IUST 4
  5. 5. Compaction of Soil Higher resistance to deformation Higher resistance to frost damage Increased stability Decreased permeability Increased bearing capacity Increased durability Poor compaction Good compaction Poor compaction Good compaction Poor compaction Good compaction Dr. Abdulmannan Orabi IUST 5
  6. 6. 1) Increased Shear Strength This means that larger loads can be applied to compacted soils since they are typically stronger. Increased Shear Strength => increased bearing capacity, slope stability, and pavement system strength 2) Reduced Permeability This inhibits soils’ ability to absorb water, and therefore reduces the tendency to expand/shrink and potentially liquefy Purposes of compacting soil Dr. Abdulmannan Orabi IUST 6
  7. 7. 3) Reduced Compressibility This also means that larger loads can be applied to compacted soils since they will produce smaller settlements. 5) Reduce Liquefaction Potential 4) Control Swelling & Shrinking Purposes of compacting soil Dr. Abdulmannan Orabi IUST 7
  8. 8. Compaction, in general, is the densification of soil by removal of air, which requires mechanical energy. Simplistically, compaction may be defined as the process in which soil particles are forced closer together with the resultant reduction in air voids. Compaction of Soil Definition: Dr. Abdulmannan Orabi IUST 8
  9. 9. compacted Soil before compacted compacted Compaction of soils is achieved by reducing the volume of voids. It is assumed that the compaction process does not decrease the volume of the solids or soil grains· Soil before compacted Principles of Compaction Dr. Abdulmannan Orabi IUST 9
  10. 10. Solids Air Water Solids Air Water Loose soil Compacted soil Compaction Effect Principles of Compaction Dr. Abdulmannan Orabi IUST 10
  11. 11. Principles of Compaction The degree of compaction of a soil is measured by the dry unit weight of the skeleton. The dry unit weight correlates with the degree of packing of the soil grains. The more compacted a soil is: • the smaller its void ratio (e) will be. • the higher its dry unit weight ( ) will be = 1 + Dr. Abdulmannan Orabi IUST 11
  12. 12. Compaction Curve The compaction curve is relationship between a soil water content and dry unit weight. Soil sample was computed at different water contents in a cylinder of volume 1000 cc and dry unit weight were obtained. = 1 + Compaction curve is plotted between the water content as abscissa and the dry density as ordinate. Dr. Abdulmannan Orabi IUST 12
  13. 13. It is observed that the dry density increases with an increase in water content till the max. density is attained. With Further increase in water content, the dry density decreases. Compaction Curve 16 18 14 12 20 8 14 16 18 20 221210 Dryunitweight()γ Water Content (Wc) Dr. Abdulmannan Orabi IUST 13
  14. 14. Compaction Curve 16 18 14 12 8 14 16 18 20 221210 OMC Dryunitweight()γ Water Content (Wc) Optimum moisture content (OMC) : The water content corresponding to maximum dry unit weight is called optimum moisture content. Note that the maximum dry unit weight is only a maximum for a specific compactive effort and method of compaction. Dr. Abdulmannan Orabi IUST 14
  15. 15. Compaction Curve Optimum moisture content (OMC) : Each compactive effort for a given soil has its own OMC. As the compactive effort is increased, the maximum density generally increases and the OMC decreases. Water Content (Wc) 16 18 14 12 8 14 16 18 20 221210 OMC1 Dryunitweight()γ OMC2 15
  16. 16. Compaction Curve 16 18 14 12 8 14 16 18 20 221210 OMC Dryunitweight()γ Water Content (Wc) Zero air voids curve or saturation line Theoretical unit weight is given as = 1 + ∗ "Zero Air Voids" S = 100% The curve represent the fully saturated condition ( S= 100%). ( It can not be reached by compaction ) Dr. Abdulmannan Orabi IUST 16
  17. 17. Compaction Curve 16 18 14 12 8 14 16 18 20 221210 Dryunitweight()γ Water Content (Wc) Line of Optimums "Zero Air Voids" S = 100% A line drawn through the peak points of several compaction curves at different compactive efforts for the same soil will be almost parallel to a zero air voids curve , it is called the line of optimums Line of Optimums Dr. Abdulmannan Orabi IUST 17
  18. 18. Factors affecting Compaction • Water content of the soil • Amount of compaction • Type of soil being compacted • The amount of compactive energy used • Method of compaction • Thickness of layer • Saturation line • Admixtures • Stone content Dr. Abdulmannan Orabi IUST 18
  19. 19. Factors affecting Compaction Water content of the soil As water is added to a soil ( at low moisture content) it acts as a softening agent on the soil particles and becomes easier for the particles to move past one another during the application of the compacting forces. As the soil compacts the voids are reduced and this causes the dry unit weight ( or dry density) to increase. Dr. Abdulmannan Orabi IUST 19
  20. 20. As the water content increases, the particles develop larger and larger water films around them, which tend to “lubricate” the particles and make them easier to be moved about and reoriented into a denser configuration. 16 18 14 12 20 8 14 16 18 20 221210 Dryunitweight()γ Water Content (Wc) Water content below OMC OMC Factors affecting Compaction Dr. Abdulmannan Orabi IUST 20
  21. 21. Water content at OMC The density is at the maximum, and it does not increase any further. 16 18 14 12 20 8 14 16 18 20 221210 Dryunitweight()γ OMC Water Content (Wc) Factors affecting Compaction Dr. Abdulmannan Orabi IUST 21
  22. 22. Water starts to replace soil particles in the mold and the dry unit weight starts to decrease. Water content above OMC Factors affecting Compaction 16 18 14 12 20 8 14 16 18 20 221210 Dryunitweight()γ OMC Water Content (Wc) Dr. Abdulmannan Orabi IUST 22
  23. 23. Factors affecting Compaction Soil type Soil type, grain size, shape of the soil grains, amount and type of clay minerals present and the specific gravity of the soil solids, have a great influence on the dry unit weight and optimum moisture content Uniformly graded sand or poorly graded in nature is difficult to compact them. Dr. Abdulmannan Orabi IUST 23
  24. 24. Factors affecting Compaction Soil type In poorly graded sands the dry unit weight initially decreases as the moisture content increases and then increases to a maximum value with further increase in moisture content. At lower moisture content, the capillary tension inhibits the tendency of the soil particles to move around and be compacted. Dr. Abdulmannan Orabi IUST 24
  25. 25. Soil type Factors affecting Compaction At a given moisture content, a clay with low plasticity will be weaker than a heavy or high plastic clay so it will be easier to compact. Dr. Abdulmannan Orabi IUST 25
  26. 26. DryUnitWeight Water Content High Compactive Effort Low Compactive Effort Flocculated Structure, or Honeycomb Structure, or Random Intermediate structure Dispersed Structure or parallel Factors affecting Compaction Structure of Compacted Clay Dr. Abdulmannan Orabi IUST 26
  27. 27. Effect of Compaction effort Factors affecting Compaction The compaction energy per unit volume used for the standard Proctor test can be given as Dr. Abdulmannan Orabi IUST 27 = . × . × !"ℎ$ ℎ %% × ℎ !"ℎ$ & ' (% % &
  28. 28. Increased compactive effort enables greater dry unit weight. It can be seen from this figure that the compaction curve is not a unique soil characteristic. It depends on the compaction energy. Factors affecting Compaction Effects of increasing compactive effort Water Content (Wc) 16 18 14 12 8 14 16 18 20 221210 OMC1 Dryunitweight()γ OMC2 High compactive effort curve Low compactive effort curve Dr. Abdulmannan Orabi IUST 28
  29. 29. Factors affecting Compaction Effects of increasing compactive effort For this reason it is important when giving values of (γdry)max and OMC to also specify the compaction procedure (for example, standard or modified). From the preceding observation we can see that 1. As the compaction effort is increased, the maximum dry unit weight of compaction is also increased. 2. As the compaction effort is increased, the optimum moisture content is decreased to some extent. Dr. Abdulmannan Orabi IUST 29
  30. 30. Coarse-grained soils Fine-grained soils Rubber-tired rollers LaboratoryField Vibration Vibrating hammer Kneading Static loading and press General Compaction Methods Hand-operated vibration plates Motorized vibratory rollers Rubber-tired equipment Free – falling weight dynamic compaction Hand-operated tampers Sheep-foot rollers Falling weight and hammers Kneading compactors Dr. Abdulmannan Orabi IUST 30
  31. 31. Laboratory Compaction Tests Laboratory compaction tests provide the basis for determining the percent compaction and molding water content needed to achieve the required engineering properties, and for controlling construction to assure that the required compaction and water contents are achieved. Dr. Abdulmannan Orabi IUST 31
  32. 32. Laboratory Compaction Tests The aim of the test is to establish the maximum dry unit weight that may be attained for a given soil with a standard amount of compactive effort. When a series of samples of a soil are compacted at different water content the plot usually shows a distinct peak. Dr. Abdulmannan Orabi IUST 32
  33. 33. Laboratory Compaction Tests The fundamentals of compaction of fine- grained soils are relatively new. R.R. Proctor in the early 1930’s developed the principles of compaction. The proctor test is an impact compaction. A hammer is dropped several times on a soil sample in a mold. The mass of the hammer, height of drop, number of drops, number of layers of soil, and the volume of the mold are specified. Dr. Abdulmannan Orabi IUST 33
  34. 34. There are several types of test which can be used to study the compactive properties of soils. Laboratory Compaction Tests 1. Standard Procter Test is not sufficient for airway and highways, 2. Modified Procter Test was later adopted by AASHTO and ASTM Dr. Abdulmannan Orabi IUST 34
  35. 35. Soil is compacted into a mould in 3-5 equal layers, each layer receiving 25 blows of a hammer of standard weight. The energy (compactive effort) supplied in this test is 595 kJ/m3. The important dimensions are Volume of mould Hammer mass Drop of hammer 1000 cm^3 2.5 kg 300 mm Standard Procter Test Dr. Abdulmannan Orabi IUST 35
  36. 36. Standard Procter Test Standard Proctor test equipment Dr. Abdulmannan Orabi IUST 36
  37. 37. Standard Proctor test equipment Standard Procter Test Dr. Abdulmannan Orabi IUST 37
  38. 38. Standard Procter Test Proctor established that compaction is a function of four variables: • Dry density (ρd) or dry unit weight γd. • Water content wc • Compactive effort (energy E) • Soil type (gradation, presence of clay minerals, etc.) Dr. Abdulmannan Orabi IUST 38
  39. 39. Standard Procter Test Several samples of the same soil , but at different water contents, are compacted according to the compaction test specification The soil is mixed with varying amounts of water to achieve different water contents Dr. Abdulmannan Orabi IUST 39
  40. 40. • Apply 25 blows from the rammer dropped from a height of 305 mm above the soil. Standard Procter Test Dr. Abdulmannan Orabi IUST 40
  41. 41. • Distribute the blows uniformly over the surface and ensure that the rammer always falls freely and is not obstructed. Standard Procter Test Rammer Pattern for compaction in 101.6 mm Mold 4 3 1 2 5 7 6 8 4 etc. The first four blows The successive blows Dr. Abdulmannan Orabi IUST 41
  42. 42. The soil is in mold will be divided into three lifts Standard Procter Test Soil sample 3 layers 2.5 kg (5.5lb) 25 blows per layer 305mm • Place a second quantity of moist soil in the mould such that when compacted it occupies a little over two-thirds of the height of the mould body. Each Lift is compacted 25 times Dr. Abdulmannan Orabi IUST 42
  43. 43. Standard Procter Test Soil sample 3 layers 2.5 kg (5.5lb) 25 blows per layer 305mm • Repeat procedure once more so that the amount of soil used is sufficient to fill the mould body, with the surface not more than 6mm proud of the upper edge of the mould body. Dr. Abdulmannan Orabi IUST 43
  44. 44. Standard Procter Test Derive the dry unit weight from the known unit weight and water content The unit weight and the actual water content of each compacted sample are measured = 1 + Dr. Abdulmannan Orabi IUST 44
  45. 45. Standard Procter Test 16 18 14 12 8 14 16 18 20 221210 OMC Dryunitweight()γ Water Content (Wc) "Zero Air Voids" S = 100% Plot the dry unit weight versus water content for each compacted sample. Determine the maximum dry weight and OMC Dr. Abdulmannan Orabi IUST 45
  46. 46. Standard Procter Test Diameter of mold Volume of mold Weight of hammer Height of hammer drop Number of hammer blows per layer of soil Number of layers of compaction Energy of compaction Method A Method B Method C 101.6 mm 101.6 mm 152.4 mm 943.3 cm^3 943.3 cm^3 2124 cm^3 24.4 N 24.4 N 24.4 N 304.8 mm 304.8 mm 304.8 mm 3 3 3 591.3 kN.m/m^3 591.3 kN.m/m^3 591.3 kN.m/m^3 562525 Specification of standard Proctor test ( Based on ASTM Test Designation 698) Item Dr. Abdulmannan Orabi IUST 46
  47. 47. Standard Procter Test Method A Method B Method C Specification of standard Proctor test ( Based on ASTM Test Designation 698) ( con.) Soil to be used Portion passing No.4 ( 457mm)sieve . May be used if 20% or less by weight of material is retained on No.4 sieve. Portion passing 9.5 mm sieve . May be used if retained on No.4 sieve is more than 20% and 20% or less by weight of material is retained on 9.5 mm sieve. Portion passing 19- mm sieve . May be used if more than 20% by weight of material is retained on 9.5 mm sieve and less than 30% by weight of material is retained on 19- mm sieve. Item Dr. Abdulmannan Orabi IUST 47
  48. 48. •Was developed during World War II •By the U.S. Army Corps of Engineering • For a better representation of the compaction required for airfield to support heavy aircraft. Modified Procter Test Dr. Abdulmannan Orabi IUST 48
  49. 49. Same as the Standard Proctor Test with the following exceptions: The soil is compacted in five layers Hammer weight is 10 Lbs or 4.54 Kg Drop height h is 18 inches or 45.72cm Then the amount of Energy is calculated Modified Procter Test Dr. Abdulmannan Orabi IUST 49
  50. 50. 3 7 2 8 1 6 4 5 9 Rammer Pattern for compaction in 152,4 mm Mold Modified Procter Test Uniformly distribution of the blows over the surface 457.2mm # 1 # 3 # 2 # 5 # 4 44.5 N(10 lb) Dr. Abdulmannan Orabi IUST 50
  51. 51. Modified Procter Test Diameter of mold Volume of mold Weight of hammer Height of hammer drop Number of hammer blows per layer of soil Number of layers of compaction Energy of compaction Method A Method B Method C 101.6 mm 101.6 mm 152.4 mm 943.3 cm^3 943.3 cm^3 2124 cm^3 44.5 N 44.5 N 44.5 N 457.2 mm 457.2 mm 457.2 mm 5 5 5 2696 kN.m/m^3 2696 kN.m/m^3 2696 kN.m/m^3 562525 Specification of standard Proctor test ( Based on ASTM Test Designation 698) Item Dr. Abdulmannan Orabi IUST 51
  52. 52. Method A Method B Method C Soil to be used Portion passing No.4 (457mm)sieve May be used if 25% or less by weight of material is retained on No.4 sieve. If this gradation requirement cannot be met, then Methods B or C may be used. Portion passing 9.5 mm sieve . May be used if soil retained on No.4 sieve is more than 25% and 25% or less by weight of material is retained on 9.5 mm sieve. Portion passing 19- mm sieve . May be used if more than 20% by weight of material is retained on 9.5 mm sieve and less than 30% by weight of material is retained on 19- mm sieve. Item Specification of standard Proctor test ( Based on ASTM Test Designation 698) ( con.) Modified Procter Test Dr. Abdulmannan Orabi IUST 52
  53. 53. Dryunitweight(γd) Standard Procter Test Modified Procter Test Water Content (wc) ( * .) (,- . .) OMC Comparison-Curves Dr. Abdulmannan Orabi IUST 53
  54. 54. Standard Proctor Test Mold size: 943.3cm^3 304.8 mm height of drop 24.4 N hammer 3 layers 25 blows/layer Energy 591.3 kN.m/m^3 Modified Proctor Test Mold size: 943.3cm^3 457.2 mm height of drop 44.5 N hammer 5 layers 25 blows/layer Energy 2696 kN.m/m^3 Comparison-Summary Dr. Abdulmannan Orabi IUST 54
  55. 55. Filed Compaction Compaction Equipment Most of the compaction in the field is done with rollers. The four most common types of rollers are: 1. Smooth-wheel rollers (or smooth-drum rollers) 2. Pneumatic rubber-tired rollers 3. Sheepsfoot rollers 4. Vibratory rollers Dr. Abdulmannan Orabi IUST 55
  56. 56. Smooth-wheel rollers are suitable for proof rolling subgrades and for finishing operation of fills with sandy and clayey soils. These rollers provide 100% coverage under the wheels, with ground contact pressures as high as 310 to 380 kN/m^2. They are not suitable for producing high unit weights of compaction when used on thicker layers. Compaction Equipment Filed Compaction Dr. Abdulmannan Orabi IUST 56
  57. 57. Compaction Equipment Smooth-wheel rollers oone steel drum and rubber tired drive wheels o two steel drums one of which is the driver o effective for gravel, sand, silt soils Dr. Abdulmannan Orabi IUST 57
  58. 58. Pneumatic rubber-tired rollers are better in many respects than the smooth-wheel rollers. The former are heavily loaded with several rows of tires. These tires are closely spaced—four to six in a row. Pneumatic rubber-tired rollers Compaction Equipment Pneumatic rollers can be used for sandy and clayey soil compaction. Compaction is achieved by a combination of pressure and kneading action. Dr. Abdulmannan Orabi IUST 58
  59. 59. Compaction Equipment Pneumatic rubber-tired rollers Dr. Abdulmannan Orabi IUST 59
  60. 60. Sheepsfoot rollers are drums with a large number of projections. The area of each projection may range from 25 to 85 cm2. These rollers are most effective in compacting clayey soils. The contact pressure under the projections can range from 1400 to 7000 kN/m2. Compaction Equipment Sheepsfoot rollers Dr. Abdulmannan Orabi IUST 60
  61. 61. During compaction in the field, the initial passes compact the lower portion of a lift. Compaction at the top and middle of a lift is done at a later stage. Compaction Equipment Sheepsfoot rollers Dr. Abdulmannan Orabi IUST 61
  62. 62. Compaction Equipment Sheepsfoot rollers Dr. Abdulmannan Orabi IUST 62
  63. 63. Vibratory rollers are extremely efficient in compacting granular soils. Vibrators can be attached to smooth-wheel, pneumatic rubber- tired, or sheepsfoot rollers to provide vibratory effects to the soil. The vibration is produced by rotating off-center weights. Compaction Equipment Vibratory rollers Dr. Abdulmannan Orabi IUST 63
  64. 64. For field compaction, soil is spread in layers and a predetermined amount of water is sprayed on each layer (lift) of soil, after which compaction is initiated by a desired roller. In addition to soil type and moisture content, other factors must be considered to achieve the desired unit weight of compaction in the field. Factors Affecting Field Compaction Dr. Abdulmannan Orabi IUST 64
  65. 65. These factors include the thickness of lift, the intensity of pressure applied by the compacting equipment, and the area over which the pressure is applied. These factors are important because the pressure applied at the surface decreases with depth, which results in a decrease in the degree of soil compaction. Factors Affecting Field Compaction Dr. Abdulmannan Orabi IUST 65
  66. 66. During compaction, the dry unit weight of soil also is affected by the number of roller passes. Factors Affecting Field Compaction The dry unit weight of a soil at a given moisture content increases to a certain point with the number of roller passes. Dr. Abdulmannan Orabi IUST 66
  67. 67. Specifications for Field Compaction In most specifications for earthwork, the contractor is instructed to achieve a compacted field dry unit weight of 90 to 95% of the maximum dry unit weight determined in the laboratory by either the standard or modified Proctor test. Dr. Abdulmannan Orabi IUST 67
  68. 68. Specifications for Field Compaction This is a specification for relative compaction, which can be expressed as where R = relative compaction / = & ! & & 012 × 100 % For the compaction of granular soils, specifications sometimes are written in terms of the required relative density Dr or the required relative compaction. Dr. Abdulmannan Orabi IUST 68
  69. 69. Relative density should not be confused with relative compaction. Specifications for Field Compaction / = / 1 − 6 1 − /where : / = &(%!7) & 012 Correlation between relative compaction (R) and the relative density Dr Dr. Abdulmannan Orabi IUST 69
  70. 70. Determination of Field Unit Weight of Compaction When the compaction work is progressing in the field, knowing whether the specified unit weight has been achieved is useful. The standard procedures for determining the field unit weight of compaction include 1. Sand cone method 2. Rubber balloon method 3. Nuclear method Dr. Abdulmannan Orabi IUST 70
  71. 71. Sand Cone Method Sand Cone Method (ASTM Designation D-1556) The sand cone device consists of a glass or plastic jar with a metal cone attached at its top Dr. Abdulmannan Orabi IUST 71
  72. 72. Determination of Field Unit Weight of Compaction Nuclear Method Rubber Balloon method Dr. Abdulmannan Orabi IUST 72
  73. 73. The results of a standard Proctor test are given in the following table. Determine the maximum dry unit weight of compaction and the optimum moisture content Also, determine the moisture content required to achieve 95% of (γdry)max . Worked Examples Volume of Proctor Mold (cm^3 ) 944 944 944 944 944 944 944 944 Mass of wet soil in the mold ( kg) 1.68 1.71 1.77 1.83 1.86 1.88 1.87 1.85 Water content ( % ) 9.9 10.6 12.1 13.8 15.1 17.4 19.4 21.2 Example 1 Dr. Abdulmannan Orabi IUST 73
  74. 74. Worked Examples Example 2 Given 1) The in situ void ratio of a borrow pit’s soil is 0.72. 2) The borrow pit soil is to be excavated and transported to fill a construction site where it will be compacted to a void ratio of 0.42. 3) The construction project required 10000 m^3 of compacted soil fill Required Volume of soil that must be excavated from the borrow pit to provide the required volume of fill. Dr. Abdulmannan Orabi IUST 74

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