What soils are hard to build on here?
Shallow, stony soils over bedrock on steep Blue Ridge slopes are the main problem: Virginia Cooperative Extension names steep slopes, stoniness and shallow depth over bedrock as the Blue Ridge's overriding soil constraints. In Wythe County's Ridge and Valley ground, sinkholes can trouble development where karst is near the surface. A soil map only screens a parcel; its actual soils are confirmed on site.
Measure the distance to rock before anything else
Start the diagnosis with one number: how many inches of soil sit on top of bedrock. Virginia Cooperative Extension sorts soils into depth classes, and the class says a good deal about what a homesite on that ground will be up against.
| Class | Depth to bedrock |
|---|---|
| Very shallow | Less than 10 inches |
| Shallow | Less than 20 inches |
| Moderately deep | 20 to 40 inches |
| Deep | 40 to 60 inches |
Depth matters twice on a homesite. A foundation has to reach or sit on something, and a septic system needs soil to treat what it receives. Extension says that for a conventional septic tank and drainfield, the soil must have no restrictions to drainage in the upper 36 inches. A shallow soil, by definition, runs out of depth well before 36 inches.
The overriding soil use constraints in the Blue Ridge are steep slopes, stoniness, and shallow depth over bedrock.
Virginia Cooperative Extension, Soils of Virginia
Floyd County's comprehensive plan describes the layer underneath: in the Blue Ridge the topsoil is generally underlain by weathered bedrock called saprolite, fractured enough to store and move water but without the filtering capacity of the soil above it. How the rock of each province becomes its soils, and how to read a county soil survey, is the subject of Blue Ridge soils and what they mean.
Very stony, channery soils on Carroll County's dissected uplands
Carroll County's comprehensive plan maps the county's soils into 11 associations, and two of them read like a list of the problems above.
One is Ramsey-Wiekert-Hazel, which the plan describes as shallow to moderately deep, very stony and channery, and sloping to very steep. The other is Manor-Talladega-Watauga: shallow to moderately deep, micaceous, and also sloping to very steep. The plan places both on strongly dissected uplands.
An association is mapped across a broad area, so a parcel inside one can still differ from the description at a particular spot. What the descriptions do tell you is which questions to ask first: how deep, how stony and how steep.
Extension counts either one as a severe limitation for a homesite. Steep slopes or shallow-to-bedrock conditions are severe limitations in its words, excessive relief can restrict basic construction and force compromises on access and utilities, and septic drainfields are difficult to place on steep slopes. Slope is its own building question, answered under whether you can build on a steep lot; the point here is that both associations pair shallow soil with slope and grade that run from sloping to very steep. What that does to the area a house site needs is under how much land you need to build a house here.
Across the county line, sinkhole risk where karst lies close to the surface
In Wythe County the problem can change from thin soil over hard rock to ground that opens from below.
Wythe County's comprehensive plan says the county lies primarily within the Ridge and Valley Province and borders the Blue Ridge. It notes that most of Virginia's karst is found in the Ridge and Valley, and it names the risk plainly.
Sinkholes can also be a problem for development when the karst formations are close to the surface.
Wythe County Comprehensive Plan
The same plan describes the other side of Wythe's ground. Frederick-Hagerstown soils, which it puts at approximately 31 percent of the county's soils, are very deep, well-drained and permeable soils found on uplands, and slope and erosion are their main limitations. So a deep soil in Wythe still leaves the slope, erosion and karst questions open. Which kinds of land turn up across that county is covered under what kinds of land are common in Wythe County.
Expansive clay, and why a soil map screens a parcel without deciding it
The last hard soil on the list is a statewide one, and the last step in the diagnosis is the one that settles all of them.
Shrink-swell clay. Extension explains that soils with a high content of shrink-swell, or expansive, clays change volume as they wet and dry, and that they can cause severe damage to foundations, footings, sidewalks and roads. It recommends an on-site evaluation by a qualified soil scientist when such soils are encountered or expected. That is a statewide educational statement, not a finding that expansive clays occur on any particular ground in these counties.
The map screen. A county soil survey, served online through the Natural Resources Conservation Service's Web Soil Survey, maps depth to bedrock, slope and wetness and rates shrink-swell potential for soils high in clay, and NRCS itself says onsite investigation is still needed in some cases.
Screen the parcel on Web Soil Survey
Note the mapped soil names, the depth to bedrock, the slope class and any shrink-swell rating.
Walk the ground
Rock outcrops, stones at the surface and wet spots either agree with the map or tell you it is off here.
Put the septic question to a soil evaluation
The local health department takes the request, and the evaluation may come from a licensed onsite soil evaluator. Virginia's regulation weighs physiographic province, topography, available area, degree of slope and the soil profile; a percolation test may be required and is one of many criteria.
Put the foundation question to the professionals who own it
Where clay or karst is expected, the building official and a design professional decide what the footing has to do.
Well-drained soils are generally the most suitable for building sites, Extension says, and it also says alternative systems can be designed and built for less than ideal soil and site conditions. On thin ground a perc test is one input among many; the soil evaluation procedure is in how perc tests work, and the options on a site that did not pass for a conventional drainfield, including an alternative septic system, are under whether you can build on land that failed a perc test.
Weighing a homesite and unsure what the soils under it will allow?
Send the parcel and what you have seen on the ground, and Lowell will walk through what the soil survey shows and which evaluation settles the rest. No cost, no obligation.
Got it. Thank you.
We'll be in touch shortly with an honest read.
Questions this raises
Is saprolite soil or rock?
Weathered rock. Floyd County's comprehensive plan describes saprolite as weathered bedrock under the Blue Ridge topsoil: the same bulk rock as the bedrock below, weathered into a highly fractured material capable of storing and transmitting water. It says that layer of broken-up rock does not have the filtration properties of the soil above it.
Does a county soil survey rate shrink-swell clay?
Yes. The Virginia Department of Conservation and Recreation says county soil surveys rate the shrinking and swelling potential of soils high in clay content, along with erodibility, slope, permeability, wetness and depth to bedrock. A mapped rating describes the soil type, not a particular footing, and Virginia Cooperative Extension recommends an on-site evaluation by a qualified soil scientist where expansive soils are encountered or expected.
Are the deep Frederick-Hagerstown soils in Wythe County free of building limits?
No. Wythe County's comprehensive plan describes them as very deep, well-drained and permeable soils found on uplands and names slope and erosion as their main limitations. The same plan places the county mainly in the Ridge and Valley, where most of Virginia's karst is found, and says sinkholes can be a problem for development where karst is close to the surface.
Where this page's facts come from
Every rule, office and figure above traces to one of these. Rules change; check the office before you act on one.
- Soils of Virginia (SPES-299): Blue Ridge constraints, depth classes, drainfields and expansive clays Virginia Cooperative Extension
- 2024 Comprehensive Plan: soil associations Carroll County, Virginia
- Comprehensive Plan: saprolite under Blue Ridge topsoil Floyd County, Virginia
- Comprehensive Plan: karst, sinkholes and Frederick-Hagerstown soils Wythe County, Virginia
- Virginia soil surveys Virginia Department of Conservation and Recreation
- Web Soil Survey Natural Resources Conservation Service
- 12VAC5-610-450: soil evaluations, and the percolation test as one of many criteria Virginia Department of Health