Conduct a Preliminary Site Investigation

The following actions help to identify potential site problems that can be accounted for in planning and design. While an expense to the overall project, the preliminary site investigation will ultimately prove to be a cost saving measure as potential problems will be addressed prior to becoming an issue for the home that is eventually built. If the site is part of a larger development, those costs could be spread over many dwelling units.

As part of a preliminary site investigation, typically bore holes or test pits are used to verify subsurface soil conditions. An illustration of a typical bore hole used to explore subsurface conditions is shown in Figure 3–1. Test pits are another way to identify subsurface conditions on a site but usually require an excavator or back hoe. If either piece of equipment is available, a test pit can be a quick and informative way to understand the local soils and should be dug to 2’ below the footing level.

• Survey the surface conditions and local plant species for signs of seasonal or constant high ground water levels. (See USDA resources at end of chapter.)
• Consider the lay of the land and surface water flow onto and off of the site to ensure that proper surface water drainage can be achieved around the building site.
• Check soil maps from USDA’s Natural Resources Conservation Service. (See additional resources listed at the end of the chapter.)
• Use a hand auger to bore one or more test holes at the proposed building location and determine general soil type/characteristics and ascertain the water table level; be sure to factor in any seasonal or recent climate conditions such as the amount of precipitation over the previous month or so (see Figure 3–1). At least one hole should be at the building location and extend at least a couple of feet below the proposed footing elevation. If deeper subsurface problems are expected (as by local experience), then a geotechnical engineer may need to use special drilling equipment to explore deeper below grade to ensure that adequate support and stability exists.
• If possible, test the soil for bearing capacity at the depth and location of proposed footings. A simple hand-held penetrometer (e.g., a standardized metal rod and drop weight) used in accordance with the manufacturer’s instructions serves this purpose.
• If fill or questionable soil conditions are suspected (as on a steep slope), the services of a geotechnical engineer and knowledgeable foundation contractor may be needed to appropriately prepare the site (e.g., compaction) or design a suitable foundation system.
• Do not use basement foundations on sites with high ground water table. If a basement is a must, build the basement using waterproof construction methods.
• Observe other homes being constructed in the area and talk with those performing that construction to garner a better understanding of the local conditions.
• Review the conditions of the site during or immediately after a large rainfall to observe the runoff patterns.