Prof. B V S Viswanadham, Department of Civil Engineering, IIT Bombay

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2 Soil Compaction -1

3 Activity (After Bell, 1993) Swell-Shrinkage response of clay = f (Period, magnitude of precipitation and evapotranspiration) Kaolinite Smallest swelling capacity Illite May swell up to 15% and intermixed illite and montmorillonite may swell up to % Swelling of Ca Montmorillonite swell up to % and Na Montmorillonite swell up to 2000% Range of the degree of expansiveness of a clay based on the activity

4 Example 1 For a soil specimen, given: Passing 2 mm sieve = 100 %; Passing mm sieve = 85 %; Passing No. 200 sieve = 38 % LL = 20 % and PI = 12 % Classify the soil by the Unified soil classification system

5 Solution for Example 1 - Soil is a coarse grained soil (Percent passing No. 200 sieve < 50). - Sands (percent of coarse fraction passing No. 4 sieve > 50) - Since more than 12 % passes No. 200 sieve, it must be SM or SC - PI = = 8 > 7 [above A-line] Hence the soil classification is SC

6 Example 2 For a soil specimen, given: Passing No. 4 sieve = 92 %; Passing No. 40 sieve = 78 %; Passing No. 10 sieve = 81 %; Passing No. 200 sieve = 65 % LL = 48 % and PI = 32 % Classify the soil by the unified Soil classification system No. 4 sieve = 4.75 mm No. 10 sieve = 2 mm; No. 40 sieve = mm sieve

7 Solution for Example 2 Since more than 50 % is passing through a No. 200 sieve, it is a fine-grained soil, i.e., it could be ML, CL, OL, MH, OH, CH or OH. Now, if we plot LL =48 and PI = 32 on the plasticity chart, it falls in the zone CL. So the soil is classified as CL

8 Example 3 Limit tests performed on a clay indicate a liquid limit of 67 and a plastic limit of 32. From a hydrometer analysis to determine particle sizes, it is found that 80 % of the sample consists of particles smaller than mm. From this information, indicate the activity classification and the probable type of clay mineral.

9 Solution for Example 3 PI = LL PL = = 35 C A c = 35/80 = 0.44 A c = PI The clay mineral is Kaolinite as A c :

10 Example 4: Use the grain-size distribution curve shown below to classify soils A and B using the USCS. Soil B s Atterberg limits are LL = 49% and PL = 45%.

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12 For soil A, G = 2%, S = 98%; M = 0% & C = 0%. C U = 1.4/0.5 = 2.8; C c = /(1.4)*(0.5) = 1.29 Soil A is a poorly graded sand (SP) For soil B, G = 0%, S = 61%; M = 35% & C = 4%. C U = 0.45/0.005 = 90 Soil A is a very well graded silty sand (SM)

13 Compaction In many situations soil itself used as a construction material. - Such as: Highway embankments Railway embankments Earth dams Highway/Airfield pavements Backfilled trenches Landfills When soil is used as a foundation material, it is desirable that the in-place material possess certain properties. The purpose of compaction is to produce a soil having physical properties appropriate for a particular project.

14 Compaction Compaction is defined as the process of increasing the unit weight of soil by forcing the soil solids into a dense state and reducing the air voids (No significant change in volume of water in the soil) This is achieved by applying static or dynamic load to the soil. Compaction is measured quantitatively in terms of the dry unit weight γ d of the soil.

15 Compaction generally leads to the following desirable effects on soils:

16 Purpose of Compaction 1. Maximum shear strength occurs approximately at minimum void ratio. 2. Large air voids may lead to compaction under working loads, causing settlement of the structure during service. 3. Larger voids if left may get filled with water which reduces the shear strength. 4. Increase in water content is also accompanied by swelling and loss of shear strength in some clays.

17 Advantages of Compaction 1. Settlements can be reduced or prevented. 2. Soil strength increases and stability can be improved. 3. Load carrying capacity of pavement sub-grades can be improved. 4. Undesirable volume changes (by frost action, swelling, shrinkage) may be controlled.

18 Compaction When loose are soils are applied to a construction site, compressive mechanical energy is applied to the soil using special equipment to densify the soil (or reduce the void ratio). Densification Reduction in Volume of Air Voids It is almost an instantaneous phenomena and soil is always partially saturated. Typically applies to soils that are applied or reapplied to a construction site. Compaction is a old technique adopted in Ancient China/India (e.g. Great Wall of China/Tajmahal)

19 Compaction of cohesion-less soils When speaking of cohesion-less soils, gravelly soils, there are many possibilities: Loose, Angular soil Dense, Angular soil Honeycombed soil, Very Loose Loose Dense

20 Compaction of cohesion-less soils Soils in loose or honeycombed state are avoided, or compacted before being built upon, since they are prone to densification when subjected to vibratory or shock loading (as from earthquakes or vibrating machinery)

21 Compaction of cohesion-less soils The relative looseness of a soil in its natural, in-situ state is determined by measuring/computing its relative density, D r The smaller D r is for a given soil deposit, the more prone that soil deposit will be to densification and settlement. For uniformly (poorly graded) spherical grained soils, the theoretical range of void ratios is 0.35 < e <0.90.

22 Compaction of cohesion-less soils For non-uniform, well-graded soils, the possible range of void ratios is much smaller. Well-graded, sub-angular sand: 0.35 < e < 0.75 Well-graded, silty sand: 0.25 < e < 0.65 The range of void ratios for well-graded soils is less than that for uniformly graded soils. That is why it is generally preferred to use wellgraded soils in geotechnical applications as opposed to uniform soils.

23 Compaction of cohesion-less soils Cohesion-less soils are compacted by vibration. Static load produces very little compaction of loose sand. Medium and fine sands do not get compacted easily when moist because of the shear strength developed by capillary forces. Dry or submerged sands can be compacted by Vibration.

24 Compaction of Clayey soils Clays cannot be compacted by vibration. Shaking or vibration does not change the volume. A very small amount of static pressure produces a large volume decrease of the platelet particles (like mica flakes). In compacting the clay, position of the particles must be changed by forcing the contact points along adjacent surfaces to positions nearly more parallel with reduced voids.

25 Compaction of Clayey soils Thickness of adsorbed water + Free water = f(water content) Platelet particles Loose structure of clay before compaction Dense structure of clay after compaction

26 Compaction of Clayey soils When the clay has a higher water content less than saturation, a thick layer of free water surrounds the particles (low viscosity). Under this condition only a small amount of pressure is required to force the particles to new positions. But a high degree of compaction cannot be produced with this high water content because the thick layer of free water prevents the particles from being forced close together.

27 Proctor s Theory - After R.R. Proctor (1930) Proctor showed that: 1. There exists a definite relationship between the soil moisture content and the degree of dry density to which a soil may be compacted. 2. That for a specific amount of compaction energy applied on the soil there is one moisture content termed Optimum Moisture Content (OMC) at which a particular soil attains maximum dry density.

28 Proctor s Theory Proctor proposed tests to determine relationship between w, γ d or e of a compacted soil in a standard manner and to determine the OMC (optimum moisture content) for the soil. Compaction = f [ γ d, compactive effort, and soil type (gradation, presence of clay minerals, etc.) ] Compactive effort is a measure of mechanical energy applied to a soil mass.

29 Measuring compaction of soils in the laboratory 1. Standard Proctor compaction test 2. Modified Proctor compaction test

30 Standard Proctor Test Scope This method covers the determination of the relationship between the moisture content and density of soils compacted in a mould of a given size with a 2.5 kg rammer dropped from a height of 305 mm.

31 Compactive Energy E applied to soil per unit volume NnWh E = V N = No. of blows per layer n = No. of layers W = Hammer weight h = Height of drop V = Volume of mould Compactive Effort kg m m = = = kj m 3

32 Standard Proctor Test Procedure Dry unit weight Calculation Volume of Proctor mould Bulk unit weight of soil,γ b Dry unit weight of soil, γ d = V = W/V = γ b / (1+w)

33 Typical moisture content-dry unit weight relationship γ d, max OMC

34 Principle of compaction and moisturedensity relations Compaction of soils is achieved by reducing the volume of voids. It is assumed that the compaction process does not decrease the volume of solids or soil grains.

35 Principles of compaction and moisturedensity relations 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. Recall that: γ d G γ = s 1 + e The more compacted a soil is: -The smaller its void ratio will be and thus -The higher its dry unit weight γ d will be w

36 Principle of compaction and moisturedensity relations Water plays a critical role in the soil compaction process: -It lubricates the soil grains so that they slide more easily over each other and can thus achieve a more densely packed arrangement. -While a little bit of water facilitates compaction. Too much water inhibits compaction.

37 Principle of compaction and moisture-density relations At low values of water content most soils tend to be stiff and are difficult to compact. As the water content is increased the soil becomes more workable, facilitating compaction and resulting in higher dry densities. At high water contents, however, the dry density decreases with increasing water content, an increasing proportion of the soil volume being occupied by water.

38 Compaction AIR AIR In practice this dry unit weight is never achieved but it represents theoretical upper bound. WATER WATER WATER SOLIDS SOLIDS SOLIDS Un-compacted soil Compacted soil Theoretically maximum degree of compaction

39 Principles of compaction and moisture-density relations w s r s d G S w G γ γ + = 1 ( ) c r a w s a s d na S n n wg n G = = + = 1 1 ) (1 γ γ Air-Void lines: Saturation lines: a c = V a /V V n a = V a /V

40 Saturation Line or Zero Voids Line 1. Saturation line is a hypothetical line. 2. Points on the line denote density for completely saturated condition at respective water contents. 3. It is the maximum possible dry density for any soil. 4. Practically it is not possible to achieve this density Dry density for saturation line γ d = γ w 1 is given by + Gs w

41 AASHTO or Modified Proctor Test 1. Standard Proctor test is not sufficient for airways and highways. 2. US Army Corps of Engineers developed Modified Proctor Test which used greater levels of compaction and produced higher dry densities. 3. Modified Proctor Test was later adopted by AASHTO & ASTM.

42 Modified Proctor Test Specifications No. of blows No. of layers Wt. of hammer : 25 per layer : 5 layers : 4.5 kg Falling height : 0.45m Compactive Effort = = kg m m 3

43 Dry Density-Moisture content curve γ d = γ w Gs w

44 Importance of Proctor Test 1. It gives the density that must be achieved in the field. 2. Provides the moisture range that allows for minimum compactive effort to achieve density. 3. Provides data on the behaviour of the material in relation to various moisture contents. 4. It is not possible to determine whether a density test passes or fails without it.

45 Factors influencing Compaction Factors Influencing Compaction Moisture Content Soil Type Effect of Compactive Effort Nature of Effort Amount of Effort Load Duration Area of Contact

46 Soil Type 1. Soil type, grain size, shape of the soil grains, amount and type of clay minerals present and the G s of soil solids, have a great influence on the γ d and OMC. 2. In poorly graded sands γ d initially decreases as the moisture content increases, and then increases to a maximum value with further increase in moisture. 3. At lower moisture contents, the capillary tension inhibits the tendency of the soil particles to move around and be compacted.

47 Compaction curve effect of soil type gravel-sand-clay Compactibility or ease with which soils can be compacted will depend on the soil type

48 Soil Type At a given moisture content a clay with low plasticity will be stronger than a heavy or high plastic clay, as it will be easier to compact. The reason is attributed to: For a given compactive effort the air voids can be removed more easily for a low plasticity clay and because it will have a lower moisture content anyway, a higher dry unit weight can be obtained.

49 Dry density-water content curves for a range of soil types

50 Effect of Compactive Effort Amount of compactive effort 1. Maximum dry unit weight increases with increase in compactive effort. 2. Increase in compactive effort decreases optimum moisture content to some extent.

51 Effect of Compactive Effort Applying more energy to a soil will reduce the air voids content further and increase the dry unit weight. Increasing compaction energy More compaction energy can be beneficial especially for soils dry of OMC

52 Effect of Compactive Effort If a soil is already moist, weaker and above OMC then applying more energy is wasteful since air can quickly be removed. Applying large amounts of energy to a very moist soil may be damaging since no more air can be expelled but high pore water pressures can build up which could cause: -Slope Instability during construction - Consolidation settlements as they dissipate after construction.

53 Effect of Compactive Effort Nature of Effort - Load Duration and Contact Area 1. Longer time duration leads to reduced shear stiffness response and greater compaction. 2. Greater contact area leads to greater depth of influence

54 Effect of Compactive Effort Degree of compaction generally increases with increasing compactive effort. However, beyond a certain point, increased compactive effort produces only very small increase in dry unit weight. i.e. It takes a great deal of additional compactive effort E to see significant increase in dry unit weight.

55 Effect of Compactive Effort

56 Moisture Density Relationships of Cohesion-less Soils Surface tension induces apparent cohesive strength, resisting compaction initially decreasing in dry density. Thin water film grain Bulking phenomena

57 Effect of compaction on soil structure Flocculated Direction of increasing dispersion E B A Highly Flocculated High strength, more k, less shrinkage, more swelling C Highly Dispersed D Dispersed Low strength, low k, more shrinkage, less swelling

58 Effect of compaction on soil structure 1. At low water contents, attractive forces between clay particles predominate, creating a more or less random orientation of plate like particles. (results in low density) 2. The addition of water increases repulsion between particles leading them to assume more parallel orientation near OMC. 3. If compacted wet of optimum parallel orientation is further increased leading to what is described as dispersed structure.

59 Compaction equipment In the field, fill soils are typically imported to a site and applied to the existing grade level in layers which are called lifts. When a lift of soil is placed, it will be very loose. Special compaction equipment is then used to compact this lift of the soil. Rollers, Rammers and Vibrators

60 Types of Rollers Smooth-wheel rollers Vibratory rollers Pneumatic-tire rollers Sheepsfoot rollers

61 Rammers Dropping weight (including piling equipment) Internal combustion type Pneumatic type

62 Smooth Wheeled Rollers 100 % coverage area under wheel with ground contact pressures upto 380 kpa. 1. Conventional three wheel type - 18 tons Tandem rollers Three axle tandem rollers - 1 to 14 tons - 12 to 18 tons Weight increased by ballasting the rolls with water or by a heavy sliding weight. 2. Performance is affected by the load unit width under the compaction rolls, and the width and diameter of the rolls.

63 Smooth Wheeled Rollers 3. Load per unit width and diameter control the pressure in the surface layer of soil; dimension of the rolls affect rate with which this pressure decreases with depth. 4. Suitable for gravels, sands, hardcore, crushed rock and any material where crushing action is needed.

64 Pneumatic-tyred Roller 80 % coverage area (i.e. 80 % of area is covered by tires) With tire pressures upto 700 kpa Suitable for fine grained soils (closely graded sands). Best performance on cohesive soils obtained when moisture content is 2-4% below PL. Depth of compaction: Light rollers (200kN) 150 mm Med. rollers (500kN) 300 mm Heavy rollers (1800kN) 450 mm

65 Sheepsfoot Rollers Sheepsfoot rollers are most suitable for fine soils, both plastic and non-plastic, especially at water contents dry of optimum. Compaction is by tamping and kneading Area of protrusions range from 30 to 80 cm % coverage, very high contact pressures are possible, ranging from 1400 to 7000 kpa

66 Vibrators Out of balance type Pulsating hydraulic type

67 Out of balance type vibrator

68 Vibrators 1. Vibrators consist of a vibrating unit of either the out-of-balance weight type or a pulsating hydraulic type mounted on a plate or roller. 2. Vibrators give maximum dry density much in excess of the corresponding compaction test value at OMC. 3. Frequencies range cycles/min. (Frequency range within natural frequency of most soils)

69 Compaction equipment Equipment type Smooth wheel rollers Pneumatic rubber tired rollers Sheepsfoot rollers Vibratory rollers Vibratory tampers Soil type Sands and Gravels Silts and clays Silts and clays Sands and Gravels Sands and Gravels To increase the compaction energy applied to the soil in the field: a) Increase the mass/weight of the compaction equipment; b) Decrease the thickness of lift thickness; and c) Increase number of machinery passes.

70 Field compaction and specifications Two categories of earthwork specifications: 1. End product specifications 2. Method specifications With End product specifications, a certain relative compaction or percent compaction, is specified.