Final answer:
The statement that accumulating water hazards improve trench stability is not true. Accumulated water increases soil weight, reduces soil cohesion, and increases lateral earth pressure, all of which contribute to decreased stability and can lead to landslides and other problems.
Step-by-step explanation:
The statement related to accumulating water hazards that is not true is "They improve trench stability." When water accumulates, it indeed increases soil weight and reduces soil cohesion. It also increases lateral earth pressure against walls or slopes. These factors contribute to a decrease in trench stability, not an improvement. Water accumulation can lead to a variety of problems, including slope failures, landslides, and soil erosion, as the added weight and reduced cohesion make the soil less stable.
Heavy rainfall can cause hillsides to become saturated, and the additional weight and reduced friction can cause major structural damage. This is why it's crucial to understand the dangers associated with water hazards and their impact on soil stability, especially in areas prone to heavy rainfall or flooding.