Expanded Clay Aggregate (ECA ®) is an ideal material for building tunnels. Most commonly, it’s used to fill in the gaps between the rock mass that is excavated, and the liner of the tunnel. Expanded Clay Aggregate is extremely strong, but some types of ECA ® can collapse at load levels that are lower than the mechanical resistance of the liner. This allows for even pressures to be exerted on the tunnel liner, even if the surrounding rock or liner begins to shift – preserving the structure and ensuring proper mechanical loads throughout the tunnel. In addition, ECA ® has excellent drainage properties, which also helps preserve tunnel integrity, even in extremely wet climates or biomes.