Sub grade
The layer constructed above an embankment is called the sub grade. It is an essential component of the embankment structure and plays a crucial role in providing stability and support to the overlying layers.

Function of Sub grade
The sub grade acts as the foundation for the subsequent layers of the embankment. It distributes the load from the embankment and any traffic passing over it to the underlying soil or subsoil. The sub grade also helps in preventing excessive settlement and deformation of the embankment.

Construction of Sub grade
The construction of the sub grade involves several steps:

1. Excavation: The area where the embankment is to be constructed is excavated to the required depth. The excavation is carried out to remove any unsuitable or weak soil that may affect the stability of the sub grade.

2. Compaction: After excavation, the sub grade is compacted to increase its density and strength. Compaction is usually done using compaction equipment such as rollers or plate compactors. This process helps in reducing the potential for settlement and improving the load-bearing capacity of the sub grade.

3. Moisture control: Controlling the moisture content of the sub grade is crucial to ensure its stability. Excessive moisture can lead to softening of the soil, while inadequate moisture can result in poor compaction. Proper moisture control is achieved by adding or removing water as required during the compaction process.

4. Grading: The sub grade is graded to achieve a smooth and even surface. This helps in providing uniform support to the overlying layers of the embankment.

Importance of Sub grade
The sub grade is a critical component of the embankment structure because:

1. Load distribution: It distributes the load from the embankment and any traffic passing over it to the underlying soil or subsoil. This helps in preventing localized failures and excessive settlement.

2. Stability: The sub grade provides a stable foundation for the overlying layers of the embankment. It helps in maintaining the integrity and long-term stability of the embankment structure.

3. Drainage: The sub grade also plays a role in facilitating proper drainage of water from the embankment. It should be properly graded to ensure that water does not accumulate and cause detrimental effects to the embankment.

In conclusion, the layer constructed above an embankment is called the sub grade. It is an essential component of the embankment structure and provides stability, load distribution, and drainage capabilities.