To prevent undercutting in channels, what should be installed under rock materials?

Installing filter fabric under rock materials is essential for preventing undercutting in channels. Filter fabric acts as a geotextile barrier that allows water to flow through while inhibiting the movement of soil particles. This function is crucial because it prevents soil erosion and the potential for undercutting, which can destabilize the channel and lead to structural failures.

By utilizing filter fabric, the integrity of the channel is maintained over time, as it helps to retain the underlying soil while providing a stable base for rock materials. This stabilizing effect is enhanced by the filtration properties of the fabric, which prevents the clogging of water drainage pathways that could lead to increased erosion.

The other options may not provide the same level of protection against soil movement and erosion. For example, concrete barriers may obstruct water flow and create additional challenges in managing stormwater, while steel mesh may not effectively prevent soil erosion on its own. Gravel beds, although they may provide some drainage, do not adequately address the underlying soil stability needed to prevent undercutting.