**Introduction:**
Cut-off is an important component in the design and construction of earthen dams and rockfill dams. It is a barrier constructed within the foundation of the dam to prevent the loss of stored water through seepage and to reduce the uplift pressure in the base of the dam. This helps in ensuring the stability and safety of the dam structure.

**Explanation:**

1. **Reducing seepage loss:**
- Seepage is the movement of water through the foundation of the dam, which can result in the loss of stored water.
- A cut-off is constructed to reduce the seepage by providing a barrier that restricts the flow of water through the foundation.
- It is usually constructed using impermeable materials like concrete, clay, or plastic sheets, which prevent or minimize the seepage of water.
- By reducing the seepage loss, the cut-off helps in maintaining the required reservoir levels and ensuring the availability of water for various purposes.

2. **Preventing sub-surface erosion:**
- Sub-surface erosion, also known as piping action, is a phenomenon that can occur when seepage water erodes the soil particles in the foundation, creating channels or pipes.
- These channels can weaken the foundation and lead to the failure of the dam structure.
- The cut-off acts as a barrier to prevent the seepage water from eroding the foundation soil and forming pipes.
- By preventing sub-surface erosion, the cut-off enhances the stability and integrity of the dam.

3. **Increasing grouting efficiency:**
- Grouting is a process used to improve the foundation's properties by injecting grout material into the soil.
- The presence of a cut-off in the foundation can help in increasing the efficiency of the grouting operation.
- The cut-off acts as a guide for the grout injections, ensuring that the grout material is distributed uniformly within the target zones.
- This helps in improving the overall effectiveness of the grouting process and enhancing the foundation's strength and stability.

4. **Reducing uplift pressure:**
- Uplift pressure is the hydrostatic pressure exerted by the seepage water on the base of the dam.
- The cut-off reduces the seepage flow, thereby reducing the amount of water that exerts pressure on the base of the dam.
- By reducing the uplift pressure, the cut-off helps in minimizing the risk of base instability and the potential for dam failure.

In conclusion, the cut-off in earthen dams and rockfill dams serves multiple purposes, including reducing seepage loss, preventing sub-surface erosion, increasing grouting efficiency, and reducing uplift pressure. These functions collectively contribute to the stability, safety, and effectiveness of the dam structure.