Lane’s weighted creep theory for weir foundations: What relative weightages are assigned to vertical and horizontal creeps when computing the effective percolation path length?

Introduction / Context:
Lane’s weighted creep theory improves upon Bligh’s creep theory by recognizing that vertical percolation through cutoff walls is more effective in reducing uplift than horizontal percolation along the base.

Given Data / Assumptions:

• Weir/barrage foundation on pervious alluvium.
• Cutoffs and floor lengths provide percolation path (creep).
• Design seeks sufficient weighted creep length to ensure safety against piping and uplift.

Concept / Approach:
Lane assigned different effectiveness factors (weights): vertical creep is given three times the weight of horizontal creep because downward percolation through cutoffs dissipates head more efficiently.

Step-by-Step Solution:
Define weighted creep length Lw = Lh * 1 + Lv * 3.Compare Lw with required length derived from head and soil properties.Therefore, vertical : horizontal weightage = 3 : 1.

Verification / Alternative check:
Design examples compute Lw by multiplying vertical cutoff depths by 3 and adding horizontal floor lengths to obtain effective creep length.

Why Other Options Are Wrong:

• Equal or greater weight to horizontal creep contradicts Lane’s central postulate.
• Doubling horizontal over vertical underestimates the value of cutoffs.

Common Pitfalls:
Forgetting to multiply vertical components by 3; confusing Lane’s weighting with Khosla’s flow net method (a different approach).

Final Answer:
triple weightage to vertical, single to horizontal