An asymmetric appearance improves the aesthetics, with an off-center peak and a variety of slopes along the berm’s length.
A flat top reduces runoff and provides more water to plants growing on the side of the berm. [3] X Research source While building your berm, do your best to avoid compacting the soil at the base with heavy equipment. If it’s already compacted, loosen the soil by digging 8 inches (20 cm) deep to improve drainage.
Well-compacted clay soil is recommended for steep or tall berms. Loam will typically collapse at slopes steeper than 35%. Gravel is cheaper, but the soil tends to wash through it — and it is not compliant with some local codes. If you use it, cover it with at least 1 inch (2. 5 cm) of compacted clay.
Building the turn around a tree, large rock, or other obstacle discourages bikers from short-cutting the turn. Make sure the obstacle doesn’t block line of sight, and that it’s not close enough to hit someone’s head when they lean into the turn. For your first berm, avoid a sideslope (slope between the two sides of the turn) steeper than a 4:1 gradient (25%). Steeper slopes require a turning platform, which takes a lot more work.
Flat, square stones weighing at least 50 lbs (23 kg) make the best retaining walls. Lock the stones together with smaller rocks and “deadmen” logs extending into the bank to prevent wobbling, then backfill each layer with small rocks and mineral soil. [5] X Research source Wooden retaining walls take less labor, but rot over time. Use pressure treated wood rated for ground contact (“heavy duty” or “extreme” ground contact in wet climates). Lay deadmen logs into the bank, attached to the wall with notched log cabin construction. [6] X Research source
The soil must compact and cohere well. If it is too loose and sandy, mix in clay fill. [7] X Research source Powered compaction tools are difficult to use for this project, since they flatten the curve of the riding surface.
The rough surface helps the layers attach to each other, improving strength. A light sprinkle of water also helps. You can incorporate small rocks in the lower layers, but the top surface should be pure mineral soil only.
