Guide bank protection: The water face (river-side) of guide banks is generally protected using what class of stone pitching?

Introduction / Context:
Guide banks are vital river training works used near barrages and bridges. Their water face is exposed to high velocities, turbulence, and attack from eddies. Appropriate stone pitching provides erosion resistance and stability against wave action and drawdown.

Given Data / Assumptions:

• Alluvial river with seasonal floods.
• Standard practice categories of hand-placed stones (e.g., one-man, two-man stones) based on size and handling.
• Proper underlayers (filter/graded bedding) provided beneath pitching.

Concept / Approach:
“Two-man stone” pitching uses stones large enough that two workers are typically needed to handle one piece. This size provides better resistance to dislodgement under attack than one-man stones, yet is still practical for placement over large areas of guide bank slopes.

Step-by-Step Solution:
1) Identify hydraulic loading on the water face of guide banks.2) Select stone size class adequate to resist design shear and uplift.3) Two-man stones are a commonly adopted standard for water face pitching, with appropriate thickness and toe protection.

Verification / Alternative check:
Design manuals often specify pitching thickness and stone weight based on velocity and depth. For many Indian river projects, two-man stone pitching on the water face has proven effective when laid over filter layers and combined with toe aprons.

Why Other Options Are Wrong:

• One-man stones: may be too small and susceptible to movement.
• Three-, four-, five-man stones: heavier than typically necessary and costlier unless very severe conditions demand them.

Common Pitfalls:

• Lack of filter/graded bedding under pitching leading to piping of fines.
• Insufficient toe protection causing undermining of pitching.

Final Answer:
Two-man stone pitching.