Stepping Stones and Single-Log Crossings, Sized Honestly: Spacing, Tread, Stability and the Days the Crossing Does Not Exist

Stepping stones are usually placed by eye and judged after someone falls. This paper sizes them in advance for a stated case: a 100 kg total load (an 80 kg person carrying 20 kg) crossing a 12 m stream at a design depth of 0.35 m and a design current of 1.2 m/s. A loaded, wet step shortens to 0.40 m, so the crossing needs 29 stones at 0.40 m centre spacing — far closer than the spacing most people would guess — each with a tread at least 0.45 m long and 0.29 m wide. A 0.60 by 0.50 by 0.30 m stone weighing 234 kg is very stable: its factor of safety against sliding is 7.71 at the design current, and it does not fall to the prudent threshold of 1.5 until the current reaches 2.66 m/s, or to 1.0 until 3.21 m/s. Anchoring or cabling is therefore required only above 2.66 m/s. Slipperiness, not stability, is the binding constraint at ordinary currents: push-off requires a friction coefficient of 0.47, which algae-covered wet rock at 0.25 and even clean wet rock at 0.45 do not reliably provide, while a grooved tread at 0.60 does. Under stated flow statistics the crossing is unavailable on 169.3 days per year and available on only 195.7 days, because the stone tops sit only 0.40 m above the bed. For a single log spanning 8.0 m, bending is not the problem at any practical diameter; balance and surface are. A 0.35 m diameter log is recommended, flattened by a 0.03 m cut to a 0.20 m wide tread, debarked and grooved, with bending stress of only 1.95 MPa and midspan deflection of 0.008 m under the loaded case. A bare round wet log of any diameter examined is stated plainly to be an unreasonable risk for a loaded crossing.