Question

The top 500 mm of soil sub grade should be compacted at its

• A. optimum moisture content
• B. maximum dry density
• C. dry density
• D. saturated density

Hint:

The key to effective soil compaction is the moisture-density relationship. Compaction effort drives out air from voids. At OMC, lubrication by water allows maximum particle packing, leading to MDD. Both OMC and MDD are crucial parameters determined from compaction tests, but OMC is the condition for field compaction.

Answer 1

The Correct Option is A

Step 1: Understand soil compaction and its objectives in road construction.
Soil subgrade is the foundation layer upon which pavement layers are built. Compaction is a process that increases the density of soil by reducing air voids, thereby improving its engineering properties. The main objectives of compacting the subgrade are to increase its strength (CBR value), reduce settlement, minimize permeability, and enhance stability.
Step 2: Relate compaction to moisture content and density.
The relationship between moisture content and dry density of a soil during compaction is typically determined by a Proctor compaction test. This test shows that for a given compactive effort, there is an optimum moisture content (OMC) at which the soil achieves its maximum dry density (MDD).

• If the moisture content is too low, the soil particles resist compaction due to high friction.
• If the moisture content is too high, water fills the voids, preventing close packing of soil grains and reducing the density.
• At OMC, water acts as a lubricant, allowing soil particles to slide past each other and achieve the densest possible packing for that compactive effort.

Step 3: Analyze the options in the context of subgrade compaction.

• (1) Optimum moisture content: Compacting the subgrade at or near its optimum moisture content ensures that the soil achieves its maximum dry density for the given compactive effort. Achieving MDD provides the desired engineering properties (strength, stability, reduced settlement). This is the standard practice.
• (2) Maximum dry density: While the goal of compaction is to achieve maximum dry density, this is the result of compaction, not the condition at which it should be compacted. The condition refers to the moisture content. One compacts at OMC to achieve MDD.
• (3) Dry density: This is a general term for the density of soil solids. Compacting at any dry density is not sufficient; the aim is to achieve the maximum possible dry density.
• (4) Saturated density: Saturated density refers to the density of soil when all voids are filled with water. Compacting at saturated conditions would result in a lower dry density than MDD, as water would prevent efficient packing of soil particles. This is not desirable for subgrade.

Step 4: Conclude the correct condition for compaction.
For proper compaction of soil subgrade to achieve desired engineering properties (like strength and stability), the soil should be compacted at its optimum moisture content to achieve its maximum dry density. The question asks for the condition at which it should be compacted. $$\boxed{\text{optimum moisture content}}$$