Expansive clay around Castro Valley
Clear Lake 土，黏土 50%，線性延展 lep=7.5（≥6 屬高脹縮）; 土壤排水等級「Poorly drained」.
Clear Lake 土，黏土 50%，線性延展 lep=7.5（≥6 屬高脹縮）
土壤排水等級「Poorly drained」
Soil sampled near Castro Valley runs 50 percent clay with a linear extensibility of 7.5, which is the standard measure of how much that ground changes volume with moisture.
Why this matters for a footing
A footing is designed for a soil that stays roughly the same size. When the ground beneath one part of the house changes volume and the ground beneath another part does not, the house is asked to bridge the difference, and concrete is not good at that.
The median home in Castro Valley was built in 1966, which sets what the footing is likely to be and how deep it sits.
What you can do without spending much
- Get roof water away from the wall - extensions on downspouts are the cheapest intervention there is
- Keep the soil moisture around the perimeter even rather than swinging wet to bone dry
- Watch tree roots close to the footing, which draw moisture out of one side only
- Mark and date crack ends, then photograph them each season
Castro Valley averages 663 mm of rain a year, and it is the swing between the wet and dry parts of that total, not the total itself, that moves a footing.