Check piping safety under a weir with Lane's weighted creep ratio: creep length, cutoffs and the required ratio for your soil. Try it free on CalcTree.
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About this Lane's Weighted Creep Ratio Calculator
This calculator checks the safety against piping of a weir or other hydraulic structure on a permeable foundation by Lane's weighted creep method. Seepage that follows a vertical contact loses more head than seepage along a horizontal one, so vertical contacts count in full and horizontal contacts at one third. The weighted creep length divided by the head must be at least the ratio Lane recommends for the foundation soil.
- Geotechnical engineer makes a first check of seepage length under a weir, barrage or sluice before a seepage analysis.
- Water resources engineer sizes the floor length and cutoff depths of a diversion structure at concept stage.
- Dam safety reviewer screens an existing structure on an earth foundation against Lane's recommended ratios.
It is an engineering-grade CalcTree calculation with a drawing of the creep path, and every step is visible and live.
More info on Lane's Weighted Creep Ratio
Inputs
The inputs are the head difference across the structure, the horizontal contact length along the floor or apron, the depth of the upstream cutoff below the floor, the depth of the downstream cutoff, and the foundation soil, chosen from twelve classes from very fine sand or silt to very hard clay or hardpan.
Weighted creep length
Each cutoff is counted down one face and up the other, so the vertical contact length is twice the sum of the cutoff depths. The horizontal contact counts at one third. Their sum is the weighted creep length L_w, and dividing by the head gives the weighted creep ratio C_w.
Required ratio and check
Lane's table gives the required weighted creep ratio for each foundation soil. The structure passes when the ratio provided is at least the ratio required, and the page reports the utilisation with a pass or fail.
Limits of the method
Lane's method is empirical, drawn from the performance of existing dams and weirs on earth foundations. Use it as a first check alongside a seepage analysis, not instead of one. Layered foundations and dispersive or internally unstable soils are not covered.
Common Calculation Errors to Avoid
- Counting a cutoff once halves its contribution. Seepage travels down one face of a cutoff and up the other.
- Counting horizontal contacts in full overstates the creep length. Horizontal contacts count at one third.
- Classing a sloping contact wrongly distorts the result. Count a contact as vertical when it is steeper than 45 degrees and as horizontal otherwise.
- Forgetting the floor's own embedment misses vertical contact. Add the upstream and downstream faces of the floor to the vertical length if it is set into the foundation.
- Relying on Lane alone for a layered or dispersive foundation, where the method does not apply.
Engineering templates
Common calculators
Design guides
FAQs
What is Lane's weighted creep ratio?
It is the weighted creep length of the seepage path under a structure divided by the head across it. Lane weighted vertical contacts in full and horizontal contacts at one third, because seepage along a vertical contact loses more head.
Why do cutoffs help so much?
A cutoff adds vertical contact on both of its faces, and vertical contact counts in full. Extending a floor adds only horizontal contact, which counts at one third, so a deeper cutoff usually raises the ratio faster than a longer floor.
Does the position of the cutoffs matter?
Not in this check. The weighted creep ratio depends only on the cutoff depths and the floor length, not on where the cutoffs sit along the floor.
Is Lane's method enough on its own?
No. It is an empirical first check. Pair it with a seepage analysis, and do not rely on it for layered foundations or for dispersive or internally unstable soils.
Which soil class should I choose?
Choose the class that best describes the foundation soil along the seepage path. Finer soils need a higher ratio than coarse gravels and boulders.
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