Step 1: Understanding the Concept:
Mole drainage is a method of subsurface drainage where an unlined, circular channel (the "mole drain") is formed in a suitable subsoil without excavating a trench. This is done by pulling a bullet-shaped plow (a mole plow) through the soil at a specific depth. The question asks for the common installation depth for these drains.
Step 2: Detailed Explanation:
1. Suitability: Mole drainage is only suitable for heavy, cohesive soils (clays or clay loams) that have the structural stability to maintain the open channel after the mole plow has passed. It is not suitable for sandy or unstable soils.
2. Depth of Installation: The depth is a critical design parameter.
The drain must be deep enough to be in a stable, plastic clay layer that will not collapse.
It must be deep enough to provide adequate drainage for the crop root zone and to be safe from damage by surface tillage operations.
It should not be excessively deep, as this would increase the draft requirement of the mole plow and could place the drain in a less permeable soil layer.
3. Common Practice: Based on these factors, the common and recommended depth for installing mole drains for agricultural purposes is in the range of 40 cm to 70 cm (0.4 to 0.7 m).
4. Let's review the options:
0.2 and 0.3 m: This is too shallow. The drains would be prone to collapse and could be damaged by ploughing.
0.4 and 0.7 m: This is the standard, accepted range for mole drain depth.
0.8 and 1.0 m, and 1.1 and 1.3 m: These depths are generally too deep for mole drainage. The draft force would be excessively high, and it might place the drain below the layer that needs draining. These depths are more typical for conventional pipe drainage systems.
Step 3: Final Answer:
Mole drains are commonly installed at depths between 0.4 and 0.7 meters. Therefore, option (B) is the correct answer.