SOIL DENSITY of SOIL GEOTECHNICAL ENGINEERING

Geotechnical

1. Soil density Chapter 6

2. importance Density is an important engineering property of soil . When soils are used in embankment or dams , the object of rolling operation is to obtain a high density of the soil . If density is high , future settlement is minimized , to bearing capacity of shear strength is increased and the permeability is decreased , does the soil is less susceptible to the detrimental effects of water.   It is therefore important for it is therefore important for the engineer to understand the factors which influence density , to be familiar with the methods of measuring it and the procedures by which greater density may be obtained .

3. density In soil mechanics , density is used synonymously with a term , unit weight . It is defined as the weight of soil per unit of volume and is usually expressed in units of pounds per cubic foot or grams per cubic centimeter . 3   Density of water: = 1.94 slugs /cu. Ft.  

4. Dry and Wet Density DRY DENSITY - The weight of solid particles in a unit volume of the soil mass   WET DENSITY - the total weight of solid particles and water contained in the void spaces in a unit volume of the soil mass =   Therefore , a wet soil has two numerical values of density and the wet density will vary as the amount of water in the void varies . The minimum density of a soil is its dry density and the maximum density is the wet density when the void are completely filled with water .

5. Factors affecting Density 5 a.) Relative volume of solid and void spaces . The greater the volume of solids or the less the volume of voids , the greater is the density.   b.) Specific gravity of soil particles . The greater the specific gravity of solids , equator is a density, all other factors being equal.   c.) Amount of water in void spaces . This factor affects only the wet density . It is obvious that the more the amount of water the greater is the wet density .

6. Effect of Size of Uniform Particles Density is practically the same whether the soil is composed of small or large particles , provided to the shape and arrangement remain constant . Effect of Arrangement of Soil Particles Density is affected appreciably by arrangement of soil particles . In rolling of soils in soil structures , the particles are rearranged and brought closer, thus reducing the void volume and increasing the density Effect of Different Sizes of Soil Particles Density is greater when the soil particles are of various sizes ranging from fine to coarse . The void spaces are filled with a smaller particles , does reducing the relative void volume. The highest density is obtained when the voids are filled progressively with smaller particles until the smallest voids are filled with a microscopic soil particles. It must be understood that the variable sizes of particles effects only the potential density of the soil . To obtain a high density even a well graded soil must be compacted by the application of energy and addition of the proper amount of water.

7. Theoretical Maximum Density 7 The density of a soil is maximum when there are no void spaces between the soil particles or when the soil is solid . As soil solid mass having a specific gravity of 2.65 would have a density of 2.65 g/cc or about 165 pcf . However , it is impossible to attain this maximum density in actual soils because of the uneven shade of soil particles and other factors. The density of natural soils in is often from 50 to 60% of the theoretical maximum . In highly porous soil, like peat, the national density may be as low as 15% , why well graded soil aggregate mixtures may be compacted to a high as 90% , of the theoretical maximum density.

8. Maximum Density and Optimum Water Content 8 There is a maximum density to which a soil can be compacted by a given amount of energy , but in order to obtain this maximum density the soil must contain a certain specific amount of water . It is possible to obtain in the laboratory the water content that will give the maximum density with a specific amount of energy. Standard compaction test employees as island racal metal mold 4 inches in diameter with a volume of 1/30 cubic feet ,and rammer which weighs 5-1/2 lbs. and drops height of 12 inches . The soil is compacted in the mold in three equal layers , each layer is compacted by 25 blows of the standard rammer at a number of different water contents from an air-dry to a very wet condition .The dry density and water content are determined in each case . Then the dry densities are plotted against the corresponding water content in the arithmetic scale and a smooth curve is drawn through the point comma see Fig . VI -1 . Pass the water content is increased ,the curve rises to a maximum density and then declines .The highest point on the curve indicates the maximum dry for the soil , the water content corresponding to the maximum density is called the optimum water content .Recently , heavier compacting equipment have been developed to attain higher densities in embankment construction . A laboratory compaction test comparable to the heavier field compaction was therefore introduced . In this modified test, the soil is placed in the standard mode in five equal layers , each layer is compacted by 25 blows of a rammer which weighs 10 pounds. and drops freely through a height of 18 inches . The compact of energy applied in the modified test is about 5 times greater than in the standard test . The application of this greater compactive energy on a given soil results in a higher maximum density and a lower optimum water content . The difference in maximum density and optimum water content obtained from the two compaction test is greater than fine-grained that for coarse-grained soils , see Fig. VI-2. This indicates that the effort-grained soil can be compacted to a fairly high density with the relatively low compactive effort , same is not true in fine-grained soils.

9. Behavior of Soil Under Compaction 10 Water plays an important part in a compaction of soil especially fine grained soils . If water is added to a dry soil a film of water is absorbed by the soil particles . On the addition of more water , this films get thicker and permit the particles to slide over each other more easily . This process is often called “lubrication" . Since the thickness of water film on a ford soil particle is negligible in comparison with a particle diameter , lubrication effects are limited to fine-grained soils . The application a compactive energy on a dry soil will not increase appreciably its density because the resistance three arrangement of particles is high due to friction between particles . If a small increment of water is added, this will be absorbed into the voids, and the friction between particles is still high . As successive increments of water are added, friction between particles is reduced because of lubrication and the compacting force becomes more effective and moving particles closer .Therefore , the voids are decreased and the density increases . This effect is continued until the additional water ceases to replace air from the voids. At this point , the amount of entrapped air remains essentially constant and maximum density is attained .The total amount of water added corresponding to the maximum density is the optimum amount for a given soil and compaction process.

10. Example: the wet density of a soil is 130 pcf , it’s water content is 14% , and the specific gravity of solids is 2.65 . What is the dry density , void ratio , porosity , degree of saturation , and the percent air voids ?

11. 12 Presentation title Percent compaction = dry density of soil/maximum dry density Max. Dry Density = 120 pcf Dry Density = Ws /V V= 0.0169 cu.ft , W= 914g = 2.02lb Dry density = 2.02/0.0169 = 119.53 pcf Percent Compaction = (119.53/120) * 100% = 99.61%