Use Due-diligence, before you design
The first step in the process of building a custom home is choosing an appropriate building lot. Before you get locked into a building plan, research your site, because site conditions, lot size, set-back and side-yard requirements can affect the size of 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 where it’s being built and what’s under it.
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 geotechnical, or soils engineer. Ask neighbors, they’ll probably know if there’s a ledge, a high water table or problem soils. Get information on set-back and side-yard requirements from the local building department. Take a good look at the site yourself and notice if the lot sits high or low compared to other homes and properties in the immediate area. If it appears to be lower, it will probably require fill-dirt, which can be a costly added expense. 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 fifteen feet, or greater below the depth of your planned foundation.
Foundation codes are written for sandy soils, which are the best natural soils for construction. Heavier silts and softer clays are not ideal and may not meet the minimum code requirements. Most local building departments will require information on the foundation design, which should be based on the soils engineer report and site conditions.
A soils report is based on a site survey, “soil borings” and possibly test pit samples. The report should also contain a construction recommendation, as to the type of foundation that is recommended or required.
Quite often, the excavator will discover 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 (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 slabs should sit above the water table. On a site with a high water table, you may have to bring in fill to raise the grade.
Drainage Is Essential
Soil drainage varies depending on the type of soil. Sandy soils drain better than silts and clays. If the native soil is sand, you can use soil from your excavation to backfill the site. Some silts or clays, which don’t drain well, may be used as fill if sand is mixed with them. However, if the excavated soil is not suitable, the appropriate fill will have to be trucked-in. In all cases, the lot should be properly graded, so the water will drain away from the home.
If your house needs a septic system, the water table and soil drainage are issues once again. Septic disposal or “leach” fields are issues that should be investigated by a soils analyst. In some circumstances you may need to build up the lot with fill to meet the local requirement, which can be complicated and expensive, since trucking clean fill is very costly.
The money and time spent on site investigation is an excellent investment and good insurance against trouble later on, when the foundation really counts.