Final answer:
Draping mesh over a slope helps to prevent rock blocks from moving and catches falling debris. It is not intended to prevent major slump failure or retain water within the slope. External factors like water saturation, overloading, and natural disasters can stress slopes and lead to significant failure.
Step-by-step explanation:
Draping mesh over a rock slope is primarily designed to prevent rock blocks from moving, which addresses option B. This technique is also very effective at catching small blocks of falling rock, as stated in option C. By capturing debris, the mesh redirects them into a designated ditch or collection area, away from roads or structures. However, these meshes are not intended to prevent major circular slump failure (A) nor are they designed to hold back water within the slope (D), as their main purpose is to stabilize the surface from rockfalls and minor slippage.
Situations involving slope failure can be exacerbated by materials becoming saturated with water, leading to debris flows or mud flows. Such events may carry along structures and vegetation, causing extensive damage and potentially blocking watercourses. On the other hand, the use of sandbags, as described in related instances, to surround a leak outside a river levee is a method whereby the pressure from the contained water helps balance against the force exerted by the larger body of water behind the levee, thereby preventing further erosion or failure of the slope embankment.
Furthermore, tree roots and vegetation play a crucial role in stabilizing slopes and preventing landslides by binding the soil and reducing saturation. Overloading by natural or anthropogenic means, as well as natural disasters like earthquakes or volcanic eruptions, can compromise slope integrity, leading to major failure events.