Chapter 15

Question 1

Explain why moist sand is typically stronger than either dry sand or saturated sand.

Answer 1

In moist sand the grains are each surrounded by an envelope of water, and the water envelopes overlap. The attractive surface tension of the water holds the grains together.

Question 2

In the context of mass wasting, how does a flow differ from a slide?

Answer 2

In a flow, the material moves like a fluid (individual particles move independently). In a slide the mass moves as an intact unit, with little or no relative motion between grains or clasts.

Question 3

If a large rock slide starts moving at a rate of several metres per second, what is likely to happen to the rock, and what would the resulting failure be called?

Answer 3

If a large rock slide starts moving at a rate of several metres per second, the rock is very likely to break into smaller pieces. If the pieces are small and numerous enough that the material can flow, then it becomes a rock avalanche.

Question 4

In what ways does a debris flow differ from a typical mudflow?

Answer 4

A debris flow is composed mostly of sand-sized and larger clasts, while a mudflow is composed mostly of sand-sized and smaller clasts.

Question 5

In the situation described in the chapter regarding lahar warnings at Mount Rainier, the residents of the affected regions have to assume some responsibility and take precautions for their own safety. What sort of preparation should the residents make to ensure that they can respond appropriately when they hear lahar warnings?

Answer 5

Residents at risk from Mt. Rainier lahars need to know what the warnings mean and roughly how much time they have between receiving a warning and being in actual danger. They need to create a plan to exit their residence quickly, and they need to know which way to go to get to safety as efficiently as possible.

Question 6

What factors are likely to be important when considering the construction of a house near the crest of a slope that is underlain by glacial sediments?

Answer 6

Some of the important factors include:
The steepness of the slope
Any existing erosion processes happening at the base of the slope (e.g., wave or stream erosion)
The nature of surface or shallow sub-surface drainage in the upper part of the slope, and any effects that the construction might have on the drainage
The weight of the building (unless it is to be constructed in an excavation that represents more mass than the building itself)