Subgrade and Earth Work of Road Construction >> Civil-Engineerings.com

Subgrade preparation is a very important step in road construction; it involves preparing the natural soil (subgrade) to provide a stable foundation for the road. The subgrade must be properly compacted and graded to support the weight of the road and prevent settlement or deformation, subgrade preparation process is as follows:

Clearing and Grubbing: Before preparing the subgrade, the area must be cleared of vegetation, debris, and other obstructions.
Excavation: If the existing soil is not suitable for the subgrade, excavation may be required to remove unsuitable material and reach a suitable, stable soil layer.
Compaction of Natural earth: The subgrade soil is then compacted using heavy machinery such as rollers or compactors. Compaction increases the soil's density and strength, reducing its susceptibility to settling under the weight of the road. Upto a depth of twenty (20) cm below the natural ground, all solids and vegetation shall be removed and clear surface shall be broken up by ploughing and scarifying. The natural ground will be compacted upto a depth of 20 cm to a degree as defined below :-

Compaction of Natural Ground for Road
S.No.	When Height of the embankment is above Natural GL (cm)	Percent of MDD (As determined by AASHTO T-180)
1	0 to 30	95
2	30 to 75	93
3	Over 75	90
4	Below the foundation of structures for Roads	95

Compaction of Embankment: Material for embankment shall be placed in horizontal layers of uniform thickness and in conformity with the lines, grades, sections and dimensions as per defined in drawings. Loose material, other than rock shall not be more than 20 cm thick, placed in layers and scarified to the designated depth for formation of embankment. Embankment shall be compacted to the density specified Below:-

Compaction of Embankment in Layers for Road
S.No.	When Depth of the Embankment Layer below Sub grade (cm)	Percent of MDD (As determined by AASHTO T-180)
1	0 to 30	95
2	30 to 75	93
3	Over 75	90

Grading: The subgrade is graded to achieve the desired slope and cross-section of the road. Proper grading ensures effective drainage and a smooth road surface.
Moisture Control: Proper moisture content is crucial for achieving optimal compaction. Excessive moisture can weaken the soil, while insufficient moisture can make compaction difficult. Moisture levels are carefully controlled during the compaction process.
Stabilization: In some cases, the subgrade may require stabilization to improve its strength and durability. This can be achieved through the addition of stabilizing agents such as lime, cement, or fly ash.
Testing: Throughout the subgrade preparation process, testing is conducted to ensure that the soil meets the required specifications for density, moisture content, and strength. Proper subgrade preparation is essential for the long-term performance of the road. A well-prepared subgrade provides a stable foundation that minimizes settlement and deformation, ensuring a smooth and durable road surface.