What material is used for subgrade?

A subgrade is made up of native soil that has been compacted to withstand the loads above it. It is a layer required in many structures such as pavements and slabs, although it needs to have certain characteristics.

What does subgrade material mean?

Subgrade is that portion of the earth roadbed which after having been constructed to reasonably close conformance with the lines, grades, and cross-sections indicated on the plans, receives the base or surface material. In a fill section, the subgrade is the top of the embankment or the fill.

What is the difference between subgrade and subbase?

A.: In concrete paving terminology, the subgrade is the soil the pavement is built on. Usually it is treated by compacting it sufficiently to prevent the pavement from settling. Subbase is composed of the layer(s) of material between the subgrade and the concrete pavement. A subbase may not always be required.

What are the properties of subgrade soil?

Sub grade soil
  • Stability.
  • Incompressibility.
  • Permanency of strength.
  • Minimum changes in volume and stability under adverse conditions of weather and ground water.
  • Good drainage, and.
  • Ease of compaction.

What are the types of subgrade?

There are three basic subgrade stiffness/strength characterizations commonly used in the U.S.:
  • California bearing ratio (CBR). …
  • Resistance value (R-Value). …
  • Resilient modulus (MR).

What is the CBR value for subgrade?

To understand the impact of CBR (subgrade strength) on the pavement, let’s look at some typical CBR values. A clayey soil generally has a low CBR value (less than 8). Sands are more granular and drain better and will generally have CBR values between 15 and 35. Gravel will have the best CBR values, generally 25 and up.

How do you classify soils for highway use?

AASHTO classification, (table-2) is otherwise known as PRA classification system. It was originally developed in 1920 by the U.S. Bureau of Public Roads for the classification of soil for highway subgrade use. This system is developed based on particle size and plasticity characteristics of soil mass.

What is subgrade highway?

In transport engineering, subgrade is the native material underneath a constructed road, pavement or railway track (US: railroad track). It is also called formation level.

What is the thickness of subgrade?

➢The thickness of sub-base should not be less than 150 mm for design traffic less than 10 msa and 200 mm for design traffic of 10 msa and above. ➢The recommended minimum thickness of granular base is 225 mm for traffic upto 2 msa and 250 mm for traffic exceeding 2 msa.

What do you mean by subgrade?

sub·​grade ˈsəb-ˌgrād. : a surface of earth or rock leveled off to receive a foundation (as of a road)

What is subgrade in foundation?

Subgrade—this is the native soil (or improved soil), usually compacted. Subbase—this is a layer of gravel on top of the subgrade. Base (or base course)—this is the layer of material on top of the subbase and directly under the slab.

What is subgrade soil in road construction?

In transport engineering, subgrade is the native material underneath a constructed road, pavement or railway track (US: railroad track). It is also called formation level.

Why is subgrade important in road construction?

The subgrade must be able to support loads transmitted from the pavement structure. This load bearing capacity is often affected by degree of compaction, moisture content, and soil type. A subgrade that can support a high amount of loading without excessive deformation is considered good.

What is the thickness of subgrade?

➢The thickness of sub-base should not be less than 150 mm for design traffic less than 10 msa and 200 mm for design traffic of 10 msa and above. ➢The recommended minimum thickness of granular base is 225 mm for traffic upto 2 msa and 250 mm for traffic exceeding 2 msa.

How do you prepare a road subgrade?

The foundation of the pavement structure is known as subgrade. Preparation of subgrade consists of all operations before the pavement structure is placed over it and compacted. The subgrade may be situated on an embankment, by excavation, or at the existing ground surface.

What is subbase material?

In this publication, a subbase is defined as the layer of material that lies immediately below the concrete pavement. Some engineers call this a base course since that term is used to designate the first layer beneath an asphalt surface.

What does CBR value indicate?

The California Bearing Ratio (CBR) is a measure of the strength of the subgrade of a road or other paved area, and of the materials used in its construction. The ratio is measured using a standardized penetration test first developed by the California Division of Highways for highway engineering.

What is the CBR value for embankment?

The compaction degree K (K is defined as the ratio of dry density to the maximum dry density, as a percentage) of soils to be used as sub-base embankment fillers should exceed 90%, and the CBR value should be at least 3%.