Bligh’s creep theory for seepage under hydraulic structures: Along which path is the percolating water assumed to travel under a weir or barrage floor when computing creep length?

Introduction / Context:
Bligh’s creep theory is a classical method for estimating seepage and uplift under hydraulic structures such as weirs and barrages. The method introduces the concept of creep length to estimate pressure dissipation, assuming a simplified seepage path.

Given Data / Assumptions:

• Permeable foundation beneath a horizontal floor with upstream and downstream heads.
• Cutoffs or sheet piles at upstream/downstream edges.
• Homogeneous pervious foundation in a simplified sense.

Concept / Approach:

Bligh assumes that the percolating water follows the contact surface between the structure (floor and cutoffs) and the foundation. The total length of this contact surface is the creep length, and head loss is taken proportional to this length, enabling practical safety checks against piping and uplift.

Step-by-Step Solution:

Verification / Alternative check:

Although more refined theories (e.g., Khosla’s method, potential flow solutions) exist, Bligh’s approach remains instructive for preliminary checks and historical designs.

Why Other Options Are Wrong:

Straight or circular paths ignore the role of structural contact surfaces; random tortuous paths cannot be quantified in this theory; vertical-only path is not representative of overall seepage.

Common Pitfalls:

Applying Bligh indiscriminately to stratified or anisotropic foundations; forgetting the effect of floor thickness and intermediate cutoffs on creep length.

Final Answer:

Along the outline (contact) of the base of the floor and cutoffs (the creep path)