If a weir is built on ground that lets water seep through too easily, it can cause some problems:

Leaks: Water can sneak through the foundation, which might weaken the weir and make it unstable over time.

Erosion: The flowing water can wash away the soil under or around the weir, which can cause it to shift or get damaged.

Uneven Settling: Water moving through the ground can make the weir settle unevenly. This means parts of it might sink or tilt, leading to issues.

More Pressure: If there’s extra water behind the weir because of seepage, it can put too much pressure on the weir and make it more likely to fail.

So, a foundation that lets water through too much can really mess with a weir’s stability, making it important to keep an eye on the foundation and design it right.