Compaction MCQ Quiz - Objective Question with Answer for Compaction - Download Free PDF

Explanation:

• Shear strength is the ability of soil to resist sliding along internal surfaces when subjected to stress.

• It is fundamental for the stability of slopes, foundations, retaining walls, and earthworks.

The shear strength of soil is composed of three main components:

(i) Structural resistance to displacement

• Comes from the soil’s internal structure or fabric.

• This involves the way soil particles are arranged and interlocked.

• Stronger packing or structure offers more resistance against movement or failure.

(ii) Frictional resistance between individual soil particles

• Arises due to the roughness and angularity of particles (especially in coarse-grained soils like sand and gravel).

• Larger, angular particles generate more friction, enhancing shear strength.

• Dominant component in cohesionless soils.

(iii) Cohesion between the surface of soil particles

• Occurs mainly in fine-grained soils (like clays).

• Due to electrochemical attraction, cementation, or water menisci.

• Allows soil to "stick together," even without frictional resistance.

• Plays a major role in undrained conditions or saturated clays.

 Additional Information

Factors Affecting Shear Strength of Soil

• Granular soils (sand and gravel) mainly derive their shear strength from internal friction between particles.

• Cohesive soils (clays) gain strength from cohesion (electrochemical attractions) and internal friction.

• Silty soils behave in between; they may show some cohesion but are highly sensitive to water.

• Water affects pore water pressure within the soil.

• High pore water pressure reduces effective stress, thus reducing shear strength.

• In well-drained conditions (drained shear), water drains out and soil can resist shear better.

• In undrained conditions (rapid loading or poor drainage), pore pressure builds up and shear strength decreases, especially in clays.

• If loading is applied slowly, water has time to drain out (drained behavior), leading to higher shear strength.

• If loading is rapid (such as during earthquakes or quick construction), the soil behaves undrained and pore pressures may increase, causing a drop in shear strength.

Explanation:

California Bearing Ratio (CBR) Test is used to evaluate the strength of subgrade soil for roads and pavements. The test involves compacting soil into a mold and then penetrating it with a plunger to measure resistance.

The quality of compaction affects the test results, and these factors are key:

Number of Layers 

• The soil is placed in the mold in layers to ensure uniform density throughout the specimen.

• Typical CBR procedures use 3 layers for compaction.

• Uniform layering avoids zones of weakness or excessive density variation in the specimen.

Number of Blows 

• Compaction is achieved by giving a certain number of hammer blows per layer.

• The number of blows ensures consistent energy input for each layer and is crucial for achieving repeatable and accurate test results.

• For example: Standard CBR uses 56 blows per layer with light compaction; Modified CBR uses more blows or heavier compaction.

 Additional InformationStandard CBR Test (Used for evaluating subgrade for flexible pavements like roads)

Soil Compaction:

• Soil is compacted in 3 layers inside a standard mold.

• Each layer receives 56 blows using a 2.6 kg hammer.

• The hammer falls from a height of 310 mm.

• The purpose is to achieve light compaction, simulating ordinary subgrade conditions.

Modified CBR Test (Used where higher pavement loads or airfield pavements are expected)

• Soil Compaction:

  • Soil is compacted in 5 layers.

  • Each layer is compacted with 56 blows using a heavier 4.9 kg hammer.

  • Hammer falls from a greater height of 450 mm.

  • This provides higher compaction energy, simulating more heavily loaded conditions (e.g., airports).

Explanation 

Sheep Foot Roller:

• It is best suited for compacting cohesive soils like clay and silty clay.
• The roller has projecting feet (or lugs) that penetrate and knead the soil, breaking up clods and enhancing moisture distribution.
• This kneading action is highly effective in reducing air voids in plastic soils, leading to higher compaction.

 Additional Information

Vibratory Roller:

• Best suited for granular soils like sand and gravel.

• Uses vibration to rearrange soil particles into a denser configuration.

• Ineffective in cohesive soils due to their plastic nature.

Smooth Wheeled Roller:

• Used mainly for finishing surface layers and for granular base or sub-base layers.

• Not effective in compacting cohesive soils as it lacks penetration and kneading action.

Pneumatic Roller:

• Suitable for subgrades and bituminous layers.

• Offers some compaction for cohesive soils, but less effective than sheep foot rollers.

• Works well when combined with water sprinkling on cohesive materials.

Explanation:

In soil mechanics, the IS light compaction test and the IS heavy compaction test are two standardized methods used to determine the compaction characteristics of soil—specifically the maximum dry density (MDD) and the optimum moisture content (OMC).

• The IS light compaction test applies a lower compactive effort, and hence more moisture is needed to lubricate the soil particles so that they can rearrange and compact effectively.

• The IS heavy compaction test applies a much higher compactive effort, which means the particles are compacted more effectively with less moisture. The higher energy input compensates for the reduced lubrication, allowing air to be expelled even at lower moisture levels.

Thus, for the same type of soil, the optimum moisture content decreases when the compaction effort increases.

Concept:

The following table shows the different compactors and their application in the field.

Explanation:

The range of optimum water content for different types of soil is as follows:

Mostly four types of rollers used are:

1. Pneumatic tired roller
2. Tamping roller/ sheep foot roller
3. Smooth wheel rollers
4. Vibratory Roller

Pneumatic tired roller: Pneumatic tired roller has a number of rubber tires at the front and at the rear end. A pneumatic tired roller can be used for highways, construction of dams, and for both fine-grained and non-cohesive soils.

Tamping roller/sheep foot roller: Sheep foot roller also named tamping roller. The front steel drum of the sheep foot roller consists of many rectangular-shaped boots of equal sizes fixed in a hexagonal pattern. In sheep foot, roller compaction is by static weight and kneading of the respective layer. This makes the tamping roller better suited for clay soils.

Smooth wheel rollers: Smooth wheel roller and vibratory rollers are the same. Both have the same characteristics. Only the difference in both is vibratory equipment. The smooth wheel roller has no vibrator attached to the drum. This makes smooth wheel roller best suited for rolling of weaker aggregates, proof rolling of subgrades, and compacting asphalt pavements.

Vibratory Roller: Vibratory-type rollers have two smooth wheels/drums plus the vibrators. One is fixed at the front and the other one is on the rear side of the vibratory roller. Vibration is to reduce the air voids and to cause densification of granular soils. During the vibration of the soil layer, rearrangement of particles occurs due to the deformation of the granular soil because of the oscillation of the roller in a cycle.

Mistake Points

Sheep foot roller is used for compacting clay soil but here proof rolling subgrades and finishing operation of fills with clayey is asked (only finishing operation as compaction is already done) and for finishing operations and proof rolling subgrades, smooth wheel roller is most suitable not sheep foot roller.

Concept:

Compaction Equipment:

• It is often got by using a vibrating or non-vibrating steel drum roller, a pneumatic-tire roller, a grid or cleated roller, or a sheep-foot roller.

Pneumatic tyred rollers:

• Suitable for soil type: Non-plastic silts and fine sands, not suitable for uniformly graded soil
• Nature of Project: Base, sub-base, and embankment compaction for highways, airfields, etc.

Different compaction equipment with suitable soil is as follows:

Explanation:

(i)The Indian Standard Equivalent of the Standard Proctor Test is called the light compaction test (IS: 2720 Part VII - 1974).

(ii) The Indian Standard Equivalent of the Modified Proctor Test is called the heavy compaction test (IS: 2720 Part VIII - 1983).

In compaction test:

Concept-

Compaction of soil is the pressing of soil particles close to each other by mechanical methods. Air during compaction of soil is expelled from the void space in the soil mass and therefore the mass density is increased.

Compaction of soil is done to improve the engineering properties of the soil. Compaction of soil is required for the construction of earth dams, canal embankments, highways, runways and many other structures.

Effect of Compaction on Soil Structure-

Soils compacted at a water content less than the optimum generally have a flocculated structure. Soils compacted at water content more than the optimum usually have a dispersed structure.

Effect of Compaction of Soil on Permeability

The permeability of soil depends upon the size of voids. The permeability of soil decreases with an increase in water content on the dry side of optimum water content.

Given Data and Analysis-

The optimum moisture content of clay soil is 24%, whom compaction test is conducted at 30% moisture content.

The moisture content is more than the optimum moisture content, so the soil will have a dispersed structure of the soil.

Explanation

Vibratory Roller: Vibratory type rollers have two smooth wheels/drums plus the vibrators. One is fixed at the front and the other one is on the rear side of the vibratory roller. Vibration is to reduce the air voids and to cause densification of granular soils. During the vibration of the soil layer, rearrangement of particles occurs due to the deformation of the granular soil because of the oscillation of the roller in a cycle.

Suitability of Compaction Equipment

Using the relation,

\({\gamma _d}_{max} = \frac{{G{\gamma _w}}}{{1 + e}}\)

\(1.65 = \frac{{2.65 \times 1}}{{1 + e}}\)

e = 0.606

Using the relation

se = wG

s = \(\frac{{0.205 \times 2.65}}{{0.606}}\)

s = 0.896

a_c + s = 1

where, a_c = air content

a_c = 0.104 = 10.4 %

Concepts:

S1: True- As the compaction effort is increased; the maximum dry unit weight of the compaction is also increased.

Explanation:

Maximum Dry Density (MDD) with minimum moisture content (OMC) depend on impact energy (compactive effort) given to soil specimens. Higher the impact energy, higher will be MDD and lower OMC.

S2- False The co-efficient of consolidation generally decreases as the liquid limit of soil increases

Explanation:

The coefficient of consolidation of clay is given as:

\(C_v = \frac{K ( 1 + e_0)}{a_v γ_w }\)

On increasing the liquid limit, the compressibility increases due to which coefficient of compressibility (a_v) increases which in turn reduces the C_v as C_v ∝ 1/a_v. 

Concept:

Effect of compaction on properties of soil:

Hence,

The soil at 'A' will have more swelling potential and less shrinking upon moisture variation, compared to 'B'.

The Indian Standard Equivalent of the Standard Proctor Test is called the light compaction test (IS: 2720 Part VII - 1974).

The Indian Standard Equivalent of the Modified Proctor Test is called the heavy compaction test (IS: 2720 Part VII - 1983).

In compaction test:

∴ Option 2 is more appropriate.