Constant head method is preferred for coarse grained soil because coarse grained have higher permeability than fine grained soil and constant head method can give discharge in a small time.

Constant head method
The constant head method is the best-suited method for determining the permeability of coarse grained soils. This method involves the measurement of the flow rate of water through a soil sample under a constant head condition. Here is an explanation of why this method is preferred:

Principle:
- In the constant head method, a steady flow of water is maintained through the soil sample.
- The flow is considered to be laminar, and Darcy's law is applicable in this case.
- According to Darcy's law, the flow rate (Q) is directly proportional to the hydraulic gradient (i) and the cross-sectional area (A) of the sample, and inversely proportional to the length (L) of the sample and the coefficient of permeability (k).
- By measuring the flow rate and the other parameters, the coefficient of permeability can be determined.

Advantages:
1. Suitable for coarse grained soils: The constant head method is particularly suitable for coarse grained soils, such as sands and gravels. These soils typically have high permeability, and the flow of water through them can be easily maintained under a constant head condition.
2. Simple setup: The setup for the constant head method is relatively simple and straightforward. It usually involves a constant head tank, a soil sample, and a measuring device to determine the flow rate.
3. Quick results: The constant head method provides relatively quick results compared to other methods, such as the falling head method. This can be advantageous when time is a constraint.

Limitations:
1. Applicable to high permeability soils: The constant head method is not suitable for low permeability soils, such as clays, which require a different testing method.
2. Limited accuracy: The accuracy of the constant head method may be limited due to factors such as leakage around the sample or the presence of preferential flow paths.

In conclusion, the constant head method is the best-suited method for determining the permeability of coarse grained soils due to its suitability for high permeability soils, simplicity, and relatively quick results. However, it is important to note that for low permeability soils or when higher accuracy is required, alternative methods may be more appropriate.