What should Liam the contractor have installed to prevent the collapse of the excavation?

To prevent the collapse of an excavation, it's essential to ensure that the sides of the excavation are adequately supported. This is where excavation supports play a crucial role. They are specifically designed to reinforce the walls of an excavation to maintain stability and prevent soil from caving in.

While a retaining wall is typically used to hold back soil and prevent erosion on sloped areas, it may not be the most direct solution for excavation stability. In fact, during excavation, using excavation supports such as shoring, bracing, or trench boxes is the standard practice to protect workers and maintain the integrity of the excavation site.

The drainage system serves a different purpose; it prevents water accumulation that could lead to soil erosion or instability but doesn't directly support the walls of the excavation. A safety net is more relevant for protecting workers from falling into the excavation rather than supporting the structure itself.

In summary, the correct answer highlights the importance of using specialized supports designed for the purpose of ensuring that excavations remain safe and prevent collapse.