Question

25. Choose the correct answer:

• A. Height of capillary rise is more in clay soil than in sandy soil.
• B. Height of capillary rise is less in clay soil than in sandy soil.
• C. Height of capillary rise in clay soil is same as that in sandy soil.
• D. Height of capillary rise is zero in both clay soil and sandy soil.

Hint:

Capillary rise height is inversely proportional to pore size (\(h_c \propto 1/d\)).
Because clay has much smaller pore channels than sand, it supports a much higher capillary water column.
However, the rate of water rise is faster in sand due to its high permeability.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Capillary rise is the upward movement of water above the free water table through interconnected pore spaces, which act as a network of fine capillary tubes.
This movement is driven by surface tension forces acting at the contact boundary between the water, air, and soil solid surfaces.
Key Formula or Approach:
The theoretical height of capillary rise (\(h_c\)) in a cylindrical tube of equivalent diameter (\(d\)) is calculated as:
\[ h_c = \frac{4 T \cos\theta}{\gamma_w d} \]
where \(T\) is the surface tension of water, \(\theta\) is the contact angle, \(\gamma_w\) is the unit weight of water, and \(d\) is the pore diameter.
This shows that the capillary rise is inversely proportional to the pore diameter:
\[ h_c \propto \frac{1}{d} \]

Step 2: Detailed Explanation:
The average diameter of pore spaces in any soil is directly related to the size of its individual soil grains.
Sandy soils consist of larger particles, creating relatively large pore channels with a large equivalent diameter (\(d\)).
Because the pore spaces are large, the capillary pulling force remains weak, and water rises to only a small height, typically ranging from a few centimeters to a maximum of about \(1\text{ meter}\).
In contrast, clay soils consist of extremely fine particles with highly complex, microscopic pore networks (very small \(d\)).
Since the pore diameter is extremely small in clay soils, the capillary suction force is remarkably high.
Consequently, the potential height of capillary rise in clay soils is significantly greater, theoretically reaching values between \(10\text{ meters}\) to over \(30\text{ meters}\), although the actual rate of water movement is slow due to the low permeability of clay.

Step 3: Final Answer:
The height of capillary rise is more in clay soil than in sandy soil.