Improving Clay Soil and Building a Stable Berm for Drainage Control

A homeowner removed an old pool, leaving a sand layer over clay subsoil. They know mixing sand into clay creates concrete-like soil. They also plan a berm at the property rear to redirect runoff from a neighbor's yard and ask whether pine chips, sand, rock, or clay can be used in the berm.

Lawn Area: Don't Mix Sand into Clay

Leave the sand layer undisturbed if it's well-draining. To improve clay porosity:

• Incorporate organic matter (compost, aged manure, leaf mold) – 2–3 inches tilled 6–8 inches deep.
• Add coarse sand or fine gravel (not fine sand) if needed, but organic matter is primary.
• Core aerate annually and top-dress with compost.

Berm Construction: What NOT to Use

A berm must be stable, low-permeability, and resist erosion. Do not use:

• Organic matter (pine chips, mulch, topsoil) – decomposes, creates voids, increases seepage.
• Sand, rock, or gravel – lacks cohesion, piping failure risk.
• Cracking clay (high shrink-swell), sodic soil, calcitic clay, fine silt – unstable.

Proper Berm Material & Design

• Fill: Homogeneous, stable clay with moderate plasticity (CL or CH per USCS). Target 50%+ clay content.
• Keyway: Trench 2–3 ft deep at base, filled with impervious clay, to cut off seepage.
• Upstream face protection: Riprap (large stone) to prevent wave/current erosion.
• Vegetation: Creeping bentgrass or meadow grass – shallow roots, dense cover. No shrubs or trees (roots create pipes).

Legal Warning

You cannot legally block natural drainage flow onto your property. If the neighbor's runoff is natural, you must accept it. If they altered grading or added impervious surface, you may seek a court order compelling them to fix it. Blocking flow yourself can flood their property and create liability.

Expert: Mark Harshman | www.prestoexperts.com/expert/markland