MODULE-3 Compaction

Compaction: Soil compaction is defined as the method of mechanically increasing the density of soil by reducing volume of air. gsoil (2) > gsoil (1) Load Air Air Water Water Soil Matrix Compressed soil Solids Solids WT1 gsoil (1) = WT1 gsoil (2) = VT1 VT2

Water is added to lubricate the contact Why Soil Compaction: 1- Increase Soil Strength 2- Reduce Soil Settlement 3- Reduce Soil Permeability 4- Reduce Frost Damage 5- Reduce Erosion Damage Factor Affecting Soil Compaction: 1- Soil Type 2- Water Content (wc) 3- Compaction Effort Required (Energy) Water is added to lubricate the contact surfaces of soil particles and improve the compressibility of the soil matrix Types of Compaction : (Static or Dynamic) 1- Vibration 2- Impact 3- Kneading 4- Pressure

Applications of Compaction The following are the situations in which compaction will improve the existing field condition. 1. Compaction of foundation soil for house construction. 2. Compaction of soil/gravel/crushed rock/asphalt in road & pavement construction. 3. Compaction of soil in earth embankments. 4. Compaction of soil behind retaining walls. 5. Compaction of soil backfill in trenches. 6. Dam construction 7. Construction of clay liners for waste storage areas 8. Ground improvement

Compaction curve In 1933, R.R. proctor showed that there existed a definite relationship between the soil water content and dry density. Soil sample was compacted at different water contents in a cylinder of volume 100 cc and dry densities were obtained.

EFFECTS OF COMPACTION ON SOIL PROPERTIES Now we will discuss about effects of compaction on the properties of soil. The following properties are effected… 1) Soil structure 2) Permeability 3) Swelling 4) Pore Water Pressure 5) Shrinkage 6) Compressibility 7) Stress-Strain Relationship 8) Shear Strength a) Shear strength at moulded water content b) Shear strength after saturation

EFFECT ON SOIL STRUCTURE The water content at which the soil is compacted plays an important role in soil structure. Soils compacted at water content less than optimum water content have flocculated structure. Soils compacted at water content more than optimum water content have dispersed structure.

EFFECT ON SOIL STRUCTURE

EFFECT ON SOIL STRUCTURE At Point A, the water content is low and attractive forces are predominant, so results in flocculated structure. As the water content is increased beyond optimum, the repulsive forces increase and particles get oriented into a dispersed structure.

EFFECT ON PERMEABILITY Permeability of soil depends on void size. As water content increases, there is an improved orientation of particles resulting in reduction of void size and permeability. Above optimum water content ,the permeability slightly increases. If compactive effort is increased, the permeability decreases due to increased dry density.

EFFECT ON SWELLING The effect of compaction is to reduce void space. Hence swelling is slightly reduced. Further soil compacted dry of optimum exhibits greater swell than compacted on wet side because of random orientation and deficiency of water.

EFFECT ON PORE WATER PRESSURE It is defined as pressure of ground water held within a rock or soil, in gaps between particles (pores). The pore water pressure for soil compacted dry of optimum is therefore less than that for the same soil compacted wet of optimum.

EFFECT ON SHRINKAGE Soils compacted dry of optimum shrink less when compared to compacted wet of optimum. The soils compacted wet of optimum shrink more because the soil particles in dispersed structure can pack more efficiently.

EFFECT ON COMPRESSIBILITY The flocculated structure on the dry side of optimum offers greater resistance to compression than the dispersed structure on wet side. So, the soils compacted dry of optimum are less compressible.

EFFECT ON STRAIN-STRESS RELATIONSHIP The soil compacted dry of optimum have steeper stress-strain curve than those on wet side. The strength and modulus of elasticity of soil on dry side of optimum will be high. Soil compacted dry of optimum shows brittle failure. And soils compacted on wet side experience increased strain

EFFECT ON SHEAR STRENGTH In general, the soils compacted dry of optimum have a higher shear strength than wet of optimum at lower strains. However at large strains the flocculated structure of soil is broken and ultimate strength will be equal for both dry and wet sides.

EFFECT ON SHEAR STRENGTH

SUMMARY DRY SIDE WET SIDE STRUCTURE MORE RANDOM MORE ORIENTED PERMEABILITY MORE PERMEABLE LESS PERMEABLE COMPRESSIBILITY MORE COMPRESSIBLE IN HIGH PRESSURE RANGE MORE COMPRESSIBLE IN LOW PRESSURE RANGE SWELLING SWELL MORE SHRINK MORE STRENGTH HIGHER LESSER

Objectives of Laboratory Compaction Tests 1. To simulate field condition 2. To provide data for placement conditions in field 3. To determine proper amount of mixing water 4. To determine the density in field The fundamentals of compaction of fine-grained soils are relatively new. R.R. Proctor in the early 1930’s was building dams for the old Bureau of Waterworks and Supply in Los Angeles, and he developed the principles of compaction in a series of articles in Engineering News-Record. In his honor, the standard laboratory compaction test which he developed is commonly called the Standard Proctor Test.

STANDARD PROCTOR’S COMPACTION TEST Refer IS 2720 – Part VII – 1987 Apparatus Cylindrical metal mould with detachable base plate (having internal diameter 101.6 mm, internal height 116.8 mm and internal volume 945000 mm3) 2. Collar of 50 mm effective height 3. Rammer of weight 2.5 kg f (25 N) with a height of fall of 304.8 mm

Procedure 1. About 3 kg of dry soil, with all lumps pulverized and passing through 4.75 mm sieve is taken. 2. The quantity of water to be Corse grained soil and more for Fine grained soil). 3. Mould without base plate & collar is weighed (W1). 4. The inner surfaces of mould, base plate and collar are greased. 5. Water and soil are thoroughly mixed. 6. Soil is placed in mould and compacted in three uniform layers, with 25 blows in each layer. Blows are maintained uniform and vertical and height of drop is controlled. 7. After each layer, top surface is scratched to maintain integrity between layers. 8. The height of top layer is so controlled that after compaction, soil slightly protrudes in to collar.

9. Excess soil is scrapped. 10. Mould and soil are weighed (W2). 11 9. Excess soil is scrapped. 10. Mould and soil are weighed (W2). 11. A representative sample from the middle is kept for the determination of water content. 12. The procedure is repeated with increasing water content. 13. The number of trials shall be at least 6 with a few after the decreasing trend of bulk density.

Modified Compaction Test In early days, compaction achieved in field was relatively less. With improvement in knowledge and technology, higher compaction became a necessity in field. Hence Modified Compaction Test became relevant. It was developed during World War II by the U.S. Army Corps of Engineering to better represent the compaction required for airfield to support heavy aircraft. Difference between Standard & Modified Compaction

Factors affecting Compaction Effect of Water Content 1. With increase in water content, compacted density increases up to a stage, beyond which compacted density decreases. 2. The maximum density achieved is called MDD and the corresponding water content is called OMC. 3. At lower water contents than OMC, soil particles are held by electrical forces that prevents the development of diffused double layer leading to low inter-particle repulsion. 4. Increase in water results in expansion of double layer and reduction in net attractive force between particles. Water replaces air in void space. 5. Particles slide over each other easily increasing lubrication, helping in dense packing. 6. After OMC is reached, air voids remain constant. Further increase in water, increases the void space, thereby decreasing dry density.

Effect of Amount of Compaction 1. As discussed earlier, effect of increasing compactive effort is to increase MDD And reduce OMC (Evident from Standard & Modified Proctor’s Tests). 2. However, there is no linear relationship between compactive effort and MDD

Effect of Method of Compaction The dry density achieved by the soil depends on the following characteristics of compacting method. 1. Weight of compacting equipment 2. Type of compaction 3. Area of contact 4. Time of exposure 5. Each of these approaches will yield different compactive effort. Further, suitability of a particular method depends on type of soil.

Effect of Type of Soil 1. Maximum density achieved depends on type of soil. 2. Coarse grained soil achieves higher density at lower water content and fine grained soil achieves lesser density, but at higher water content.

Effect of Addition of Admixtures 1. Stabilizing agents are the admixtures added to soil. 2. The effect of adding these admixtures is to stabilize the soil. 3. In many cases they accelerate the process of densification.

Field Compaction Control It is extremely important to understand the factors affecting compaction in the field and to estimate the correlation between laboratory and field compaction. Field compaction control depends on (i) Placement water content, (ii) Type of equipment for compaction (iv) Number of passes based on soil type & degree of compaction desired. Placement water content is the water content at which the ground is compacted in the field. It is desirable to compact at or close to optimum moisture content achieved in laboratory so as to increase the efficiency of compaction. However, in certain jobs the compaction is done at lower than or higher than OMC (by about 1 – 2 %) depending on the desired function as detailed in the table.

Proctor’s Needle

1. Used for rapid determination of water content of soil in field. 2 1. Used for rapid determination of water content of soil in field. 2. Rapid moisture meter is used as an alternative. 3. Proctor’s needle consists of a point, attached to graduated needle shank and spring loaded plunger. 4. Varying cross sections of needle points are available. 5. The penetration force is read on stem at top. 6. To use the needle in field Calibration in done on the specific soil in lab and calibration curve is prepared and the curve is used in the field to determine placement water content.

Compaction control in field There are many variables which control the vibratory compaction or densification of soils. Characteristics of the compactor: (1) Mass, size (2) Operating frequency and frequency range Characteristics of the soil: (1) Initial density (2) Grain size and shape (3) Water content Construction procedures: (1) Number of passes of the roller (2) Lift thickness (3) Frequency of operation vibrator (4) Towing speed Degree of Compaction Relative compaction or degree of compaction Correlation between relative compaction & relative density It is a statistical result based on 47 soil samples. Typical required R.C. >= 95%

Types of field Compaction Equipment 1. Smooth Wheeled Steel Drum Rollers 2. Pneumatic Tyred Rollers 3. Sheeps foot Rollers 4. Impact Rollers 5. Vibrating Rollers 6. Hand Operated Vibrating plate & rammer compactors

Smooth wheeled steel drum rollers 1. Capacity 20 kN to 200 kN 2. Self propelled or towed 3. Suitable for well graded sand, gravel, silt of low plasticity 4. Unsuitable for uniform sand, silty sand and soft clay

Pneumatic Tyred Rollers 1. Usually two axles carrying rubber tyred wheels for full width of track. 2. Dead load (water) is added to give a weight of 100 to 400 kN. 3. Suitable for most coarse & fine soils 4. Unsuitable for very soft clay and highly variable soil.

Sheeps foot Roller 1. Self propelled or towed 2. Drum fitted with projecting club shaped feet to provide kneading action. 3. Weight of 50 to 80 kN 4. Suitable for fine grained soil, sand & gravel with considerable fines.

Impact Roller 1. Compaction by static pressure combined with impact of pentagonal roller. 2. Higher impact energy breaks soil lump and provides kneading action

Vibrating Drum 1. Roller drum fitted with vibratory motion. 2. Levels and smoothens ruts

Plate & Rammer Compactor It is used for backfilling trenches, smaller constructions and less accessible locations.