The Hillside Behind the Gym

Ranking, strongest to weakest: new tension cracks, muddy seep and new basement water, tilted pole, burn scar and clay layer, forecast intensity, then the parent's 30-year statement.

The cracks and seep are direct signs that the slope is already deforming and that water is moving through it. The tilt of the pole is consistent with shallow downslope movement. The burn scar matters because fire destroys root cohesion and can create a water-repellent layer, increasing runoff and concentrated infiltration. The clay-rich soil expands when wet, loses strength, and can perch water above the less permeable sandstone. A 3 m thick clay layer over fractured rock is a plausible shallow slide geometry.

The forecast is relevant through pore pressure. At 10 mm/h for several hours, water can infiltrate cracks and raise pore pressure, reducing effective stress and frictional resistance. Sixty millimeters in 24 hours is not extreme for all soils, but on a burned, cracked clay slope it is a credible trigger.

The parent's statement is not strong evidence because the slope's conditions changed after the fire. 'Thirty years without movement' describes a different, vegetated slope. The correct conclusion is that the hazard is elevated and the risk is high because 600 people would occupy the gym.

Decision: cancel or relocate the tournament. With active cracks, a seep, a tilted pole, a burn scar, and intense rain forecast, the probability of movement is high enough that a 600-person occupancy is not justified. The gym should be closed until a geotechnical engineer inspects the slope.

Monitoring plan:

• Install at least two piezometers and two inclinometers or extensometers across the cracks.
• Map and mark crack widths daily; photograph the same reference points.
• Record rainfall at the site, not only 2 km away.
• Set conservative triggers: any measurable crack widening, a rise in pore pressure, new seepage, or rainfall intensity above an engineer-set threshold should trigger evacuation and closure of the downslope area.
• Establish an evacuation route that does not cross the toe of the slope.

Why visual monitoring alone is insufficient: a slope can accelerate between inspections, at night, or during heavy rain when observers cannot safely remain. Cracks may widen below snow or debris, and the most dangerous period is often during or shortly after the rain, when visibility is poor. Instrumented thresholds and a clear chain of command are needed. The rain gauge 2 km away is useful context but cannot replace on-site measurement.

Hazard is the probability and potential size of the slope failure itself. On this hillside, the hazard is elevated because of the clay layer, the burn scar, the cracks, the seep, and the forecast rain. The hazard would still exist even if the gym were empty.

Risk combines hazard with exposure and vulnerability. Exposure is who or what is in the path-here, 600 people in the gym. Vulnerability is how easily they could be harmed-an indoor event with limited warning and a downslope exit route. The same slope creates low risk when the gym is empty and high risk during a tournament. Closing or relocating the event reduces risk without changing the hazard.

This distinction helps the principal explain why 'the slope has not moved in 30 years' does not settle the decision. The hazard has changed since the fire, and the exposure is unusually high tomorrow. The appropriate response is to reduce exposure now, then investigate and monitor the hazard before reopening. The principal should also state the uncertainty honestly: the exact failure time cannot be predicted, but the combination of evidence and forecast is enough to act conservatively.