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Chapter Seven soil mechanics-I.pdf

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The document discusses soil compaction, including definitions, laboratory testing methods, and benefits. It defines compaction as the densification of soil through expulsion of air using mechanical means. Laboratory tests like the standard and modified Proctor tests are used to determine maximum dry unit weight and optimum moisture content for compaction. Benefits of proper compaction include increased soil strength, reduced settlement, and decreased water flow through the soil.

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Chapter Seven Soil Compaction BY . DUMESSA GUDISSA (M.Sc.) June, 2016 G.C MADDA WALABU UNIVERSITY Soil Mechanics I Lecture Note
1. INTRODUCTION Soil compaction is the densification- reduction in void ratio-of a soil through the expulsion of air. This is normally achieved by using mechanical compactors, rollers, and rammers with the addition of water. Soil compaction is the least expensive method of improving soils. Thus, compaction results in an increase in the density of the soil, improves the engineering properties of soils, increases the shear strength of the soil & consequently the bearing capacity, reduces the compressibility & permeability of soil. Definitions of Key Terms Compaction - is the densification of soils by the expulsion of air. Maximum dry unit weight(乞) is the maximum unit weight that a soil can attain using a specified means of compaction. Optimum water content (情) is the water content required to allow a soil to attain its maximum dry unit weight following a specified means of compaction.
The difference b/n compaction & consolidation is as follows even though both cause reduction in volume. 1. Compaction is a rapid process by which a reduction in volume takes place by mechanical means where as consolidation is a gradual process of volume reduction by static and uniform load. 2. Compaction is the reduction in volume of a partially saturated soil mass which takes place as a result of expulsion of air from the voids at the same water content where as consolidation is the reduction in volume of a saturated soil mass as a result of expulsion of water from the soil. 3. Compaction is done artificially to increase the engineering properties of the soils where as consolidation takes naturally when soils are subjected to static loads. 1. INTRODUCTION
Basic concepts 乞 = + 乞 = + = + 乞 The extreme-right-hand term was obtained by replacing by = How can we increase the dry unit weight? by reduce the void ratio, that is must be reduced since is constant. The theoretical maximum dry unit weight is obtained when = = % , that is, =
2. Compaction Test In The Lab The laboratory test generally used to obtain the maximum dry unit weight of compaction and the optimum moisture content is called the Proctor compaction test(Proctor, 1933). Objective of compaction test: determining the optimum moisture content and maximum dry density achievable with a given compactive effort. 1. Standard Proctor Test method In the Standard proctor test, the soil is compacted in a mold that has a volume of 944 cm3. The diameter of the mold is 101.6 mm (4 in.) During the laboratory test, the mold is attached to a baseplate at the bottom and to an extension at the top (Figure a). The soil is mixed with varying amounts of water and then compacted in three equal layers by a hammer(Figure b) that delivers 25 blows to each layer.
The hammer has a mass of 2.5 kg(6.5 lb) and has a drop of 305 mm (12 in.). Figure c is a photograph of the laboratory equipment required for conducting a standard Proctor test. 2. Compaction Test In The Lab
2. Compaction Test In The Lab
2. Modified Proctor Test With the development of heavy rollers and their use in field compaction, the standard Proctor test was modified to modified Proctor test which better represent field conditions. For conducting the modified Proctor test, the same mold is used with a volume of 944 cm3 (1/30 ft3), as in the case of the standard Proctor test. However, the soil is compacted in five layers by a hammer that has a mass of 4.54 kg(10 lb). The drop of the hammer is 457 mm (18 in.). The number of hammer blows for each layer is kept at 25 as in the case of the standard Proctor test. The compaction energy for this type of compaction test can be calculated as 2700 kN-m/m3 (56,000 ft-lb/lb3). Because it increases the compactive effort, the modified Proctor test results in an increase in the maximum dry unit weight of the soil. The increase in the maximum dry unit weight is accompanied by a decrease in the optimum moisture content. 2. Compaction Test In The Lab
3. Interpretation of Proctor Test Results the optimum water content and the maximum dry unit weight of soils is very important for construction specifications of soil improvement by compaction. Specifications for earth structures (embankments, footings, etc.) usually call for a minimum of 95% of Proctor maximum dry unit weight. This level of compaction can be attained at two water contentsone before the attainment of the maximum dry unit weight, or dry of optimum, the other after attainment of the maximum dry unit weight, or wet of optimum. Normal practice is to compact the soil dry of optimum. Compact the soil wet of optimum for swelling (expansive) soils, soil liners for solid waste landfills, and projects where soil volume changes from changes in moisture conditions are intolerable.
Chapter Seven soil mechanics-I.pdf
Example 1. The wet mass of one of the standard Proctor test samples is 1806 grams at a water content of 8%. The volume of the standard Proctor test sample is 9.44 1043. Determine the bulk and dry unit weights. 2. The results of a standard compaction test are shown in the table below. a. Determine the maximum dry unit weight and optimum water content. b. What is the dry unit weight and water content at 95% standard compaction, dry of optimum? c. Determine the degree of saturation at the maximum dry density d. Plot the zero air voids line. Water content (%) 6.2 8.1 9.8 11.5 12.3 13.2 Bulk unit weight (kN/m3) 16.9 18.7 19.5 20.5 20.4 20.1
4. Benefits of Soil Compaction The benefits of compaction are: 1. Increased soil strength 2. Increased load-bearing capacity. 3. Reduction in settlement (lower compressibility). 4. Reduction in the flow of water (water seepage). 5. Reduction in soil swelling (expansion) and collapse (soil contraction). 6. Increased soil stability. 7. Reduction in frost damage. Improper compaction can lead to: 1. Structural distress from excessive total and differential settlements. 2. Cracking of pavements, floors, and basements. 3. Structural damage to buried structures, water and sewer pipes, and utility conduits. 4. Soil erosion.

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Chapter Seven soil mechanics-I.pdf

  • 1. Chapter Seven Soil Compaction BY . DUMESSA GUDISSA (M.Sc.) June, 2016 G.C MADDA WALABU UNIVERSITY Soil Mechanics I Lecture Note
  • 2. 1. INTRODUCTION Soil compaction is the densification- reduction in void ratio-of a soil through the expulsion of air. This is normally achieved by using mechanical compactors, rollers, and rammers with the addition of water. Soil compaction is the least expensive method of improving soils. Thus, compaction results in an increase in the density of the soil, improves the engineering properties of soils, increases the shear strength of the soil & consequently the bearing capacity, reduces the compressibility & permeability of soil. Definitions of Key Terms Compaction - is the densification of soils by the expulsion of air. Maximum dry unit weight(乞) is the maximum unit weight that a soil can attain using a specified means of compaction. Optimum water content (情) is the water content required to allow a soil to attain its maximum dry unit weight following a specified means of compaction.
  • 3. The difference b/n compaction & consolidation is as follows even though both cause reduction in volume. 1. Compaction is a rapid process by which a reduction in volume takes place by mechanical means where as consolidation is a gradual process of volume reduction by static and uniform load. 2. Compaction is the reduction in volume of a partially saturated soil mass which takes place as a result of expulsion of air from the voids at the same water content where as consolidation is the reduction in volume of a saturated soil mass as a result of expulsion of water from the soil. 3. Compaction is done artificially to increase the engineering properties of the soils where as consolidation takes naturally when soils are subjected to static loads. 1. INTRODUCTION
  • 4. Basic concepts 乞 = + 乞 = + = + 乞 The extreme-right-hand term was obtained by replacing by = How can we increase the dry unit weight? by reduce the void ratio, that is must be reduced since is constant. The theoretical maximum dry unit weight is obtained when = = % , that is, =
  • 5. 2. Compaction Test In The Lab The laboratory test generally used to obtain the maximum dry unit weight of compaction and the optimum moisture content is called the Proctor compaction test(Proctor, 1933). Objective of compaction test: determining the optimum moisture content and maximum dry density achievable with a given compactive effort. 1. Standard Proctor Test method In the Standard proctor test, the soil is compacted in a mold that has a volume of 944 cm3. The diameter of the mold is 101.6 mm (4 in.) During the laboratory test, the mold is attached to a baseplate at the bottom and to an extension at the top (Figure a). The soil is mixed with varying amounts of water and then compacted in three equal layers by a hammer(Figure b) that delivers 25 blows to each layer.
  • 6. The hammer has a mass of 2.5 kg(6.5 lb) and has a drop of 305 mm (12 in.). Figure c is a photograph of the laboratory equipment required for conducting a standard Proctor test. 2. Compaction Test In The Lab
  • 7. 2. Compaction Test In The Lab
  • 8. 2. Modified Proctor Test With the development of heavy rollers and their use in field compaction, the standard Proctor test was modified to modified Proctor test which better represent field conditions. For conducting the modified Proctor test, the same mold is used with a volume of 944 cm3 (1/30 ft3), as in the case of the standard Proctor test. However, the soil is compacted in five layers by a hammer that has a mass of 4.54 kg(10 lb). The drop of the hammer is 457 mm (18 in.). The number of hammer blows for each layer is kept at 25 as in the case of the standard Proctor test. The compaction energy for this type of compaction test can be calculated as 2700 kN-m/m3 (56,000 ft-lb/lb3). Because it increases the compactive effort, the modified Proctor test results in an increase in the maximum dry unit weight of the soil. The increase in the maximum dry unit weight is accompanied by a decrease in the optimum moisture content. 2. Compaction Test In The Lab
  • 9. 3. Interpretation of Proctor Test Results the optimum water content and the maximum dry unit weight of soils is very important for construction specifications of soil improvement by compaction. Specifications for earth structures (embankments, footings, etc.) usually call for a minimum of 95% of Proctor maximum dry unit weight. This level of compaction can be attained at two water contentsone before the attainment of the maximum dry unit weight, or dry of optimum, the other after attainment of the maximum dry unit weight, or wet of optimum. Normal practice is to compact the soil dry of optimum. Compact the soil wet of optimum for swelling (expansive) soils, soil liners for solid waste landfills, and projects where soil volume changes from changes in moisture conditions are intolerable.
  • 11. Example 1. The wet mass of one of the standard Proctor test samples is 1806 grams at a water content of 8%. The volume of the standard Proctor test sample is 9.44 1043. Determine the bulk and dry unit weights. 2. The results of a standard compaction test are shown in the table below. a. Determine the maximum dry unit weight and optimum water content. b. What is the dry unit weight and water content at 95% standard compaction, dry of optimum? c. Determine the degree of saturation at the maximum dry density d. Plot the zero air voids line. Water content (%) 6.2 8.1 9.8 11.5 12.3 13.2 Bulk unit weight (kN/m3) 16.9 18.7 19.5 20.5 20.4 20.1
  • 12. 4. Benefits of Soil Compaction The benefits of compaction are: 1. Increased soil strength 2. Increased load-bearing capacity. 3. Reduction in settlement (lower compressibility). 4. Reduction in the flow of water (water seepage). 5. Reduction in soil swelling (expansion) and collapse (soil contraction). 6. Increased soil stability. 7. Reduction in frost damage. Improper compaction can lead to: 1. Structural distress from excessive total and differential settlements. 2. Cracking of pavements, floors, and basements. 3. Structural damage to buried structures, water and sewer pipes, and utility conduits. 4. Soil erosion.