The Slope Stability Rig
A hillside stands for decades, then fails in seconds after rain that was not the heaviest of the year.
Phase 0 · The phenomenon
The storm that was not the biggest
A cross-section of a hillside with a curved scar near the top and a lobe of slumped soil and fallen trees spread across a road at its foot. An inset bar chart of daily rainfall over 30 days shows one 80 mm bar on day 6, a cluster of 12 to 22 mm bars on days 18 to 23, and a 45 mm bar on day 24 marked as the day of failure.
A road cut through a soil-covered hillside has stood for forty years. In one wet month it took an 80 mm downpour without moving a centimetre.
Eighteen days later, after a week of steady showers, a smaller storm of 45 mm arrived — and the slope let go, sending a tongue of mud and trees across the road in seconds.
Why would a hillside survive the biggest storm and fail in a smaller one? Every slope keeps a balance sheet of forces. In this lab you will build that balance one piece at a time.
Why can a hillside survive the biggest storm of the year and then fail during a smaller one?
What this lab works toward
HS-ESS2-1Model how Earth's internal and surface processes, working at different sizes and speeds, build continents and ocean-floor features.HS-ESS2-2Analyze data to show how one change to Earth's surface can trigger feedbacks that change other Earth systems.
Plain-English summaries. The course maps these performance expectations to this lab; the official wording is published at nextgenscience.org.
The stations
- Angle of ReposeAt what angle does a slope of dry, loose grains start to slide?
- Roots and CohesionHow much do roots and cohesion add to a slope's strength?
- Pore PressureHow does water in the soil change the balance of forces?
- Rain RecordWhy did the 45 mm storm trigger the failure when the 80 mm storm did not?
Each station runs the same loop: predict before you see any data, investigate the model and record runs, reveal what the measurements show, then explain the gap between your prediction and the result. A claim-evidence-reasoning checkpoint and an exit ticket close the lab.
- The slope is an "infinite slope": a uniform soil layer of constant thickness over bedrock, failing on a plane parallel to the surface. Real landslides often fail on curved surfaces.
- Soil unit weight is held at 19 kN/m³ whether the soil is dry or wet; real saturated soil is a little heavier.
- Water in the soil flows parallel to the slope. Pore pressure lowers the effective normal stress between grains — it does not change the friction angle itself.
- Station 4 uses a constructed 30-day rainfall record and a simple "leaky bucket" for soil water, so the exact failure day depends on the drainage you choose.