Tilt, Soak, Slide: When Does a Model Hillside Fail?

Investigation question: How does the amount of water in sand change the slope angle at which a model hillside fails, and can keeping rain out of a slope or letting it drain away delay the failure?

Your role today

• Lifter: Raises the upper end of the tray slowly and evenly, about 1 cm every 5 seconds, and holds it still when the Watcher calls.
• Reader: Holds the ruler upright at the raised end and reads the height of the tray's bottom edge at each call, to the nearest 0.5 cm.
• Watcher and timer: Counts the 5-second steps aloud, calls 'crack' at the first crack or rolling grains and 'fail' when sand passes the tape line, and describes the kind of movement.
• Recorder: Writes every reading, converts heights to angles with the table, computes the means and posts them on the class chart.

Safety

• Wet sand is heavy: a tray with a liter of saturated sand weighs about 2 kg, and the bucket of used sand much more. Keep trays low on the tabletop and carry buckets with two people.
• Wear goggles throughout. Sand can fly when a slope fails, and grit in the eyes is the most likely injury. Never blow on sand to clear it.
• Use washed play sand, not crushed silica, construction sand or sandblasting sand. Keep the sand damp where possible and do not pour dry level A sand from a height; fine dry dust can irritate lungs, so a student with asthma should handle only damp bags.
• Spilled sand and water make floors slippery. Work over paper towels and sweep or wipe spills at once.
• Do not pour sand or sandy water down a sink; it clogs drains. Let it settle in the bucket and pour the clear water off outside.
• Pushpins for the drains are sharp; only the student assigned the drain kit handles them, and they go back in a closed box. Wash hands after the lab; no eating or drinking at the trays.

Procedure

1. Session 1. Each student writes a prediction for q1. Then put on goggles, lay the liner in the tray, and set the tray flat with its lower end butted against a heavy book so it pivots without sliding.
2. Open your first bag, knead it, and spread the sand in the upper 20 cm of the tray. Level it with a straightedge to about 2.5 cm thick and press it flat with five light pats of an open hand. Build every bed the same way.
3. Tilt test: the Reader holds the ruler upright at the upper end. The Lifter raises that end about 1 cm every 5 seconds as the Watcher counts. The Watcher calls 'crack' at the first crack or rolling grains and 'fail' when sand crosses the tape line by 2 cm. The Reader reads the height at each call.
4. The Recorder converts both heights to angles with the height table and records how the sand moved: grains rolling down the surface, a block sliding, a slump that bulges at the toe, or a flow like wet cement.
5. Lower the tray, scoop the sand back into the zone, rebuild the bed and repeat for three trials. Then do three trials with your second bag. Reseal each bag between trials.
6. Compute the mean failure angle for each of your levels, post both on the class chart, and copy the class means into the class table. Plot the class means on the graph and answer q2.
7. Session 2. Time how long 50 mL takes to drain from your rain cup and record it. Build a bed from a fresh bag of level B sand and prop the upper end on books at 15.0 cm, which is 30 degrees.
8. Rain test, bare slope: hold the rain cup about 10 cm above the upper half of the bed and pour in 50 mL. When it has drained, that is one dose. Keep dosing, up to 10 doses. Record the dose number at the first crack, the first water seeping out at the toe, and failure (sand crossing the tape line by 2 cm).
9. Rebuild with your second bag of level B sand and add your assigned mitigation. Cover and gutter: lay plastic wrap over the upper two-thirds of the bed and fold the index card into a gutter along its uphill edge that carries water off to the side. Drains: poke holes every centimeter along three straws and push them into the base of the bed from the toe, sloping slightly downhill, with their ends sticking out. Toe wall: tape eight craft sticks side by side and push them upright into the sand along the toe.
10. Repeat the rain test on the mitigated slope. Post both results on the class chart, copy the class results, clean up as directed, and answer q3 through q10 individually.

Record your observations

Lab 1 recording sheet: Tilt, Soak, Slide

Your bed is a model hillside. The failure angle is the slope at which sand crosses the tape line by 2 cm. Find the angle from the height of the raised end with the table below. Water levels are mL of water mixed into 1,000 mL of dry sand: A 0, B 50, C 125, D 250, E saturated (amount written on the bag).

Analysis questions