In this article we will discuss about:- 1. Introduction to Soil Compaction 2. Meaning of Soil Compaction 3. Compaction of Cohesionless Soils 4. Compaction of Cohesive Soils 5. Compaction of Moderately Cohesive Soils 6. Soil Compaction Specification 7. Standard Proctor Method 8. Method 9. Effect.
- Introduction to Soil Compaction
- Meaning of Soil Compaction
- Compaction of Cohesionless Soils D
- Compaction of Cohesive Soils
- Compaction of Moderately Cohesive Soils
- Soil Compaction Specification
- Standard Proctor Method of Compaction
- Method of Compaction Used in Field
- Effect of Compaction on Properties of Soils
1. Introduction to Soil Compaction:
Rearrangement of soil particles and packing close together by mechanical methods is known as compaction. For the improvement of soil properties, e.g., density, water content or gradation, compaction is necessary in the construction of embankment, road, sub-grades, earth dams, etc.
Compaction helps to reduce the settlement, permeability, seepage, etc. During compaction, air is expelled from the void spaces in the soil mass which increases the density. Compaction generally increases the shear strength, bearing capacity and makes the soil more stable against the structure failure.
2. Meaning of Soil Compaction:
Compaction means pressing the soil particles close to each other by mechanical methods. During compaction air is expelled from the void spaces in the soil mass and, therefore, the mass density is increased.
To improve the engineering properties –
(i) Shear strength –
(b) Bearing capacity
(ii) Reducing compressibility and permeability
(iii) Decrease in volume changes
(iv) Increase the density
It is a rapid process of reduction of volume by mechanical means.
Compaction is required for –
(a) Construction of earth dams
(b) Canal embankments
(c) Highways, runways
Compaction is a function of three variables, namely:
(i) Moisture content
(ii) Compaction effort
(iii) Type of soil
The air voids of a soil go on decreasing as compaction proceeds with increasing moisture content (MC) upto the optimum value. With further increase in MOIST, the air voids do not decrease appreciably because the remaining air takes the form of small bubbles, entirely surrounded by water and held in position by surface tension.
It is not possible to expel all the air and make the sample fully saturated by compaction alone.
If all the air could be expelled, the soil would become fully saturated and the density would be the maximum possible value for that MC at saturation. This value of dry density is called the ‘saturation dry density’ or ‘zero air voids dry density’.
Since full saturation is unattainable in practice by compaction, the relationship between dry density and MC at saturation (Sr = 1) is known as theoretical ‘saturation line’ or ‘zero air voids’ line which can be plotted from the expression.
3. Compaction of Cohesionless Soils:
The soil which is devoid of cohesion does not display a marked optimum moisture content. The most effective method of compacting them is by imparting vibrations. Vibrations can be imparted in a simple manner by ramming or pneumatic tamping.
The next method in order of effectiveness is watering. The seepage force of water percolating through the sand makes the grains occupy more stable positions. For this method water has to be used liberally.
Rolling is the least effective method of compacting sands. The sand should be almost fully saturated for best advantage. Six to eight passes on 30 cm thick layers are usually needed.
Existing deposits of loose sand will also compact and settle if subjected to vibrations. Before putting any important structure on them, it is desirable to compact them in advance. A convenient method for doing so is by driving piles through them.
4. Compaction of Cohesive Soils:
Being a cohesive material, clay is taken out from the borrow pits in chunks. Except for small scale manual work it is not economically feasible to break these clods into finer pieces. Further, it is difficult to alter the moisture content of the clay after it has been excavated, the only convenient method of doing so is to sprinkle water on the borrow area sufficiently in advance to excavate when the clay is close to the required moisture content.
For clay deposited in chunks, there is no effective method of reducing the void ratio within a chunk—neither vibration nor pressure of short duration applied by rolling can accomplish this. All that can be done to reduce the space between separate chunks is satisfactorily accomplished by sheepsfoot rollers.
Highly pre-compressed clay, or clays with a high value of swelling index are unsuitable for use in embankments as they swell on subsequent contact with water. As the expansion can seldom be uniform, this results in cracking of the embankment.
5. Compaction of Moderately Cohesive Soils:
These soils are best compacted by rolling in layers. Two types of rollers are commonly employed, the choice between the two being governed by the plasticity of the soil.
For silty soils of low plasticity, pneumatic tyred rollers are preferable. These consist of a number of pneumatic tyres mounted on two axles, one in front of the other. The assembly is loaded to exert a pressure of 20 to 25 tonnes per metre along the axle. The layers should not be more than 25 cm thick and 4 to 6 passes of the roller are required for good compaction.
For plastic soils of moderate cohesion, sheepsfoot rollers are more suitable. The surface of sheepsfoot rollers is covered with projections or “feet”, there is approximately one “foot” for each hundred square cm of the surface area of the roller. Representative characteristics of heavy rollers used for earth dam construction are → 5 ft in diameter, 8 ft length and loaded weight 17 tonne.
The feet extend to a distance of 6 to 9 inches and have a surface area of 7 to 12 sq. inches. The contact pressure varies from 300 to 600 psi. For smaller embankments, lighter and smaller rollers are used. Approximately, 8 to 12 passes of the roller on 10-inch to 12-inch thick layers are required for satisfactory compaction.
6. Soil Compaction Specification:
An exact reproduction of field compaction is not possible in any laboratory compaction test, although in many cases there may be reasonable agreement between the results obtained by a laboratory compaction apparatus and those obtained by the field compaction equipment.
Therefore, the values of the optimum water obtained in the laboratory should be considered only a rough guide to the water content that will give a maximum state of compaction in the field.
For important works, a full-scale test is conducted in the field to determine the placement water content, the thickness of layer mass and speed of roller and the number of passes.
Sometimes, in case of small, unimportant work, the placement water content is taken equal to the optimum water content of the Standard Proctor Test for light compaction and equal to that of the modified proctor test for heavy compaction.
However, the field water content is sometimes kept intentionally different from the optimum water content in order to achieve or to improve a specific engineering property of the soil.
1. To avoid large expansions and swelling pressure under pavement and the floors, cohesive soils in such cases are generally compacted at a water content more than the optimum water content with the resulting dry density less than the maximum dry density.
2. The clayey soil in the impervious core in an earth dam is also compacted on the wet side of the optimum to reduce swelling pressure.
3. On the other hand, the highway embankments of cohesive soils are generally compacted at a water content somewhat lower than the optimum WC in order to achieve high shear strength and low compressibility.
4. Likewise, the soil in the outer shells of earth dams is compacted dry of the optimum to obtain high shear strength, high permeability and low pore pressure.
The cohesionless soils do not exhibit a well-defined OMC.
For such soils, the MDD is achieved either in completely dry condition or in completely saturated condition. In the field, completely saturated condition is preferred for practical reasons to achieve the maximum compaction.
Shear strength increases with an increase in the compactive effort till a critical degree of saturation is reached.
With further increase in the compactive effort, the shear strength decreases. Shear strength depends on soil type, moisture content, drainage conditions, method of compaction, etc.
7. Standard Proctor Method of Compaction:
Air dried sample passing 4.75 mm sieve about 5% of water — thoroughly mix.
25 blows each in 3 layers
The top layer should not project more than 6 mm into the collar.
Heavy rammer 4.9 kg.
Drop of 450 mm.
5 layers, 25 blows each
For soil upto 37.5 mm size larger mould-2250 cm3 is used.
5 layers-55 blows each
Standard – 60.45 x 103 kg m/m3
Modified – 275.6 x 103 kg m/m3
It is 4.56 times of standard method
8. Method of Compaction Used in Field:
If the water content of the soil in the borrow area is less than the required placement water content, water is sprinkled over the area.
On the other hand, if it is more than the desired values, the soil is excavated from the borrow pit, spread and allowed to dry.
However, in wet weather, if becomes rather difficult to decrease the water content and the work has to be stopped.
For cohesive soils the dry density of the order of 95% of the max. Dry density of the standard Proctor test (i.e., 95% relative compaction of the Standard Proctor test) can be achieved using a sheepsfoot roller or a pneumatic-tyred roller.
However, if the soil is very heavy clay, only sheepsfoot rollers are effective. For moderately cohesive soils, the dry density of the order of 95% of that in the modified Proctor test can be achieved using pneumatic tyred roller with an inflation pressure of 600 kN/m2 or more.
For cohesionless soils, the dry density of the order of 100% or even more of that in the modified Proctor test can be obtained using pneumatic-tyred rollers, vibratory rollers and other vibratory equipment.
This needle consists of a rod, which works in a piston against the compression of a spring. Different sized needle points — (circular needles or tips of surface area 0.25, 0.5, 1.0, 2.5 and 5.0 sq. cm are generally available) can be attached to the needle shank which, in turn, is attached to a spring loaded plunger. The penetration force can be read on a loaded gauge fixed over the handle.
After the soil has been compacted at given water content during the compaction test in the lab., the rod with a suitable needle point is forced into the soil mass by 7.5 cm, @ approximately 1.25 cm/sec.
The maximum force used to penetrate the needle is read out on the scale. The force when divided by the needle area will give the penetration resistance in kg/cm2 or N/cm2 or kN/m2.
The Proctor needle, thus in fact measures the penetration resistance offered by the given soil mass. The resistance offered by the compacted soil to the needle, when penetrated into it, in turn, measures the moisture content of that soil mass.
A number of such measurements are made in the lab during the compaction test; a calibration curve is obtained between the penetration (R) and the moisture content.
The penetrative resistance depends only on the MC. It is possible to prepare in lab, a chart between the Penetration Resistance and MC. Once the penetration resistance is worked out on the Proctor needle, the corresponding values of MC can be easily read out from this calibration curve. It is found that R decreases with an increase in moisture content.
This test is used for the evaluation of the supporting power of subgrade soils. This is a penetration test developed by the California division of highways by O.J. Porter and a design methodology was evolved from survey of pavement conditions carried out in California in 1929. The basis of design of flexible pavements in India is the CBR method. This test has been standardized by the Bureau of Indian Standard.
The CBR apparatus consists of a mould of 150 mm diameter with a base plate and a collar, a loading frame with a cylindrical plunger of 50 mm diameter, and dial gauges to measure axial deformation or penetration of the soil specimen as shown in Fig. 10.10.
The rate of penetration is maintained at 1.25 mm per minute. The load values to cause 2.5 mm and 5.0 mm penetration are recorded or obtained from a plot of penetrations versus load as shown in Fig. 10.11.
Load values are recorded at penetration values of 0.5 mm intervals by means of a calibrated proving ring. Two typical curves are shown: the first in which the curve is convex up throughout and the second in which the initial portion is concave up; the corrected origin is the second case which is obtained by drawing a tangent from the steepest portion of the curve as shown.
The load values with respect to the origin (or the corrected origin, as the case may be) corresponding to 2.5 mm and 5.0 mm are noted from the curve. The initial concavity may be obtained due to irregular contact between the plunger and the soil or due to a soft patch at the top, requiring a little pressure for proper seating.
These load values are compared with those for standard crushed stone material at the corresponding penetration values from a calibration test or a readymade chart. The ratios expressed as percentage are called the CBR values.
Normally, the value at 2.5 mm penetration is higher than that at 5.0 mm penetration and is reported as the CBR value; if, however, that at 5.0 mm penetration is higher, the values are checked by repeating the test, and the higher value is specified as the CBR value.
The average of at least three tests on the material, corrected to the first decimal place, is reported as the CBR value. The material passing through a 20 mm sieve alone is only used for achieving reproducibility. Invariably, the specimen is soaked for at least 24 hours to test in the worst condition.
The appeal of the CBR test lies in the following advantages:
(i) Its widespread recognition all over the world is because of its simplicity.
(ii) The procedures for design of pavements based on CBR are very easy.
(iii) It can be performed both in the laboratory and in the field.
(iv) It can be used for designing a new pavement as well as an overlay.
(v) It can be used for analysis of existing pavements, layer by layer, in respect of their strength and load-carrying capacity.
(vi) It helps in identifying the causes of failure of road pavements and for devising measures to rectify these defects.
9. Effect of Compaction on Properties of Soils:
The dry of optimum means when the water content is less than the optimum and the wet of optimum means when the water content is more than the optimum.
Soil compacted at a water content less than the optimum water content generally has a flocculated structure, regardless of the method of compaction.
Soil compaction → more than OMC, usually has a dispersed structure.
If the compaction efforts are increased, there is a corresponding increase in orientation and higher densities.
There is corresponding reduction in the size of voids which cause a decrease in permeability.
OMC — Minimum permeability
After — Permeability slightly increases
If the compactive effort is increased the permeability decreases due to increased dry density and better orientation of particles.
The minimum permeability occurs at or slightly above the optimum water content. After that stage, the permeability slightly increases.
Soil samples compacted dry of optimum and therefore having essentially flocculated structures shrink appreciably less than those of equal densities compacted wet of optimum.
Dry side compacted samples exert greater swelling characteristics and swell to higher water content than samples of the same density obtained from wet side compaction, if both are initially saturated and have the same void ratio.
Because soils at wet side are dispersely oriented.
Whereas on dry side, extra pressure is required to cause particle orientation from a flocculated structure.
A sample compacted dry of the optimum has low water content. The pore water pressure developed for the soil compacted dry of the optimum is therefore less than that for the same soil compacted wet of the optimum.