4.3 Sub-Grade Soil, Its Importance and Requirements for Practical Use

  

Road pavement consists of four major parts and are named as subgrade, sub base course, base course and wearing course. The sub grade consists of natural soil or gravel material, sub course consist of graded natural gravel, base course consists of graded crushed materials and wearing course consists of bituminous of concrete materials. Subgrade is the lowest part of road pavement, and is the foundation of pavement. Due to the function of foundation, subgrade shall be strong enough. If subgrade soil is of poor quality and is not capable to bear the load transferred by sub base course, failure of road will occur. Pot holes, longitudinal ruts, patches are the failures due to the poor subgrade soil. Various in situ methods are conducted for testing various properties of subgrade soil. Study of sub grade soil has following importance:

• Nature and type of sub grade soil determines the strength of subgrade and which determines the strength and durability of the road pavement.
• Type and nature of subgrade soil determines the cost of the road construction.
• The quality of subgrade soil determines the thickness of road pavement, which determines the cost of construction.
• The quality of subgrade soil determines the durability of road pavement, so it to determines the probable future maintenance of Highway and its cost.
• Quality of subgrade soil determines the drainage quality of subgrade, so it determines the sustainability of road.
• The quality of soil also determines its properties like expansion, shrinkage etc. which determines the strength and durability of Road pavement.
• The quality of subgrade soil determines the load carrying capacity of subgrade. In overall, quality of subgrade soil is responsible for settlement of subgrade during movement of vehicles over it.

Requirement of Subgrade Soil

The subgrade soil is the natural or improved soil layer on which the road pavement is constructed. It plays a important role in supporting the road pavement and transferring the loads from the pavement to the underlying layers. Therefore, it is essential to meet certain requirements for subgrade soil in road construction, including:

A. Bearing capacity: The subgrade soil must have adequate bearing capacity to support the weight of the road pavement and any traffic loads. The bearing capacity of soil can be measured using standard laboratory tests.

B. Density of soil: Density must be adequate so that it can withstand the resistance from pavement. Compaction and moisture variation of soil somehow depends on the density of subgrade soil.

C. Moisture content: The subgrade soil must be compacted to a specified density and moisture content to provide adequate support and prevent excessive settlement.

D.  Uniformity in composition: The subgrade soil should be uniform in composition and free from any soft or weak spots that could lead to differential settlement and uneven pavement.

E. Effective Drainage: The subgrade soil must be adequately drained to prevent the accumulation of water, which can weaken the soil and reduce its bearing capacity. The drainage can be achieved by constructing suitable subgrade drains.

F. Compatibility: This compatibility can be achieved by selecting appropriate materials and thicknesses for the pavement layers and ensuring that the subgrade soil is properly prepared and compacted.