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In your mind you’ve got a dream house, but in reality you have a building lot. Before you get locked into a building plan, research your site, because site conditions affect your design and the cost to build it. No designer should draw house plans for you without a detailed site plan, and no builder should estimate the construction costs without knowing what’s under foot.
It’s best to have complete site information before you build, but you can gather a lot of good data on your own before you hire a civil or geotechnical engineer. Ask neighbors; they’ll probably know if there’s a ledge, a high water table, or problem soils. Get a local soils map from the building department or local library. Take a good look at the site, noticing exposed rock, water plants, or new plant growth that may indicate fill.
Start with Soil
Since you may have layers of different soil types on site, your builder and designer need to know what’s there. The critical layers go from the surface down to about eight feet below the depth of your planned foundation.
Foundation codes are written for sand or gravel soils, which are the best natural soils for construction. Heavier silts and softer clays are not ideal and may require more than the minimum code requirements. Most building departments will want information on soils before they sign off on a permit; they may even require an engineer’s site report or stamp on your foundation design.
An engineering report is based on a site survey and test pit samples. If real problem soils are suspected, the engineer may do “soil borings,” but they are usually reserved for commercial projects.
Watch for Water
Quite often, the excavator discovers water when digging the foundation hole or test pit. This is not necessarily a problem, since water levels fluctuate from season to season in response to rainfall, drought, and melts. Engineers and site planners do, however, need to identify the water table (the depth where water sits year-round) and its high point. They do this by analyzing the color or “mottling” of the soil in the pit.
Foundation footings and basement slabs should sit above the water table so that groundwater will not put pressure on the foundation or cause a dampness problem. On a site with a high water table, you may prefer to build a shallow foundation, or bring in fill to raise the grade.
Drainage Is Essential
Soil drainage varies depending on the type of soil. Sands and gravels drain better than silts and clays, and this affects the project. If the native soil is sand or gravel, you can use soil from your excavation to backfill the foundation, placing it back against the foundation walls. But silts or clays, which don’t drain well, should not be used as fill because they tend to hold water against the foundation. This added pressure creates a structural load in addition to the obvious moisture concern. So if the original soil was a poorly draining silt or clay, it’s best to bring in gravel or sand for backfilling, and dispose of the original soil elsewhere.
If your house needs a septic system, the water table and soil drainage are issues once again. Septic disposal or “leach” fields are usually four feet above the water table. You may need to build up with fill to meet that requirement, which is complicated and expensive since trucking clean fill is very costly.
Your septic permit will also depend on a “perc test,” which is done by filling a test pit with water and measuring the time it takes to drain. For a septic field to work, the wastewater has to seep through the “treatment zone” fast enough to dissipate easily, but slowly enough to give soil bacteria time to break down the wastes. Make sure your site will pass a perc test; otherwise, you can’t build there.
Building On Bedrock
Rock outcroppings on or near your property are a sign that there’s rock near the surface, commonly known as “ledge.” Blasting rock requires an expert, and costs far more than standard excavation—on the order of $20,000 a day. You may opt to forgo the full basement if your site has ledge, building instead on what you find.
The good news is that rock is strong. As long as the whole house rests on rock, settling is unlikely to be a concern. If you fill, put the whole house on engineered fill (preferably gravel); if you don’t fill, put the whole house right on the rock. At all costs, avoid uneven settling.