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The following is a general overview of the soils report process. It may vary from state to state, engineer to engineer, and is subject to change based on best practices and regulation if applicable.
Soil testing is typically done for all types of construction. Consider a residential subdivision as an example. There would be a series of tests done over the entire building site to get a better understanding of the expected soil on the site. This exploratory information is used determine what, if any, additional testing is needed for infrastructure (roads and sidewalks) and what tests are needed for a home foundation. In both cases the goal is to determine what loads the soil can support. If the soil does not support the expected loads (trucks, cars, homes, etc.), modifications can be made to the soil. An alternative would be to build suitable foundations to support the proposed loads. Expected loads are calculated and evaluated for each site and use.
The soils report for a single family dwelling typically consists of two soil borings, 20’ and 35’. These help to determine what the soils could look like on that lot/job site. The limitation to soils test is the test only tells you what the soil looks like in that bore hole. That hole may not represent the entire site. To overcome that limitation, once the basement or crawl space is excavated an open-hole inspection is performed by the soils engineer. They verify that all the soil supporting the proposed foundation is consistent with the two borings that were done. The soils engineer visits the job site and looks at the soil for variations of soil stratification from the test borings. If variations in soil types occur, the engineer can recommend solutions to accommodate the soil type variation. In many cases the poor soil can be over-excavated and replaced with a structural soil designed to support the proposed loads on that site.
The borings are done using standard industry procedures and are outlined in the test report. The field samples are logged and labeled in the field and taken to a lab for further testing. Based on field observations additional test samples can be done immediately if the engineer determines they may be beneficial.
The soils information is then used to suggest the type(s) of foundation that is/are used as well as recommended sizes of foundation components. This includes the bearing capacity of the soil typically stated as some amount of load per square foot. These recommendations are then forwarded to a foundation engineer who designs the building foundation.
The other issue the soils test reviews is the presence of groundwater on the site. This is done by an initial observation looking for water in the bore hole at the time of the boring and a second observation 24 hours later. In some cases, the presence of groundwater could negate the design of a basement for the home. There can also be regulations for the minimum distance between the foundation and the groundwater.
Answered by Scott Glick, Colorado State University