Soil, Grading, and Drainage
Soil fundamentals
Know texture (sand, silt, clay) and how it drives pore space, infiltration, and water holding capacity. Structure, bulk density, and compaction determine root growth; construction traffic can raise bulk density enough to stop roots and pond water. Test pH, organic matter, and cation exchange capacity, and amend by volume based on the test rather than by habit. A typical bed amendment is two to four inches of compost tilled into the top six to eight inches, blended across the whole bed rather than dumped into planting holes.
Grading
Finished grades must move water away from structures. Two percent slope equals about one quarter inch of fall per foot. Use swales, berms, and area drains rather than relying on soil infiltration where subsoil is clay. Never bury existing topsoil under fill; strip and stockpile it.
Drainage and erosion
French drains need a filter fabric, washed drain rock, and perforated pipe with the holes down, daylighting to a legal discharge point. Catch basins and channel drains need sumps and cleanouts. On disturbed sites over an acre, Oregon DEQ stormwater permitting and an erosion control plan apply; silt fence, straw wattles, and stabilized construction entrances protect the downstream system.
Rule of thumb
Two percent minimum slope for turf and planting beds, one to two percent for paving; verify with a level or laser, not by eye.
Traps: assuming sandy loam percolates like sand, grading to one percent and calling it positive drainage, and installing a French drain without fabric.