Soils and groundwater are considered one of the riskiest elements of most construction projects. The International Building Code (IBC) and International Residential Code (IRC) allow designers to skip the detailed site investigation by making conservative assumptions for engineering parameters like bearing capacity (2000 psf, for example), but anyone who’s worked with a geotechnical engineer knows in situ conditions can often be drastically better than what the code provisions dictate in prescriptive design. Don’t get too excited yet though - they can also be horrendously worse.
Large public works projects typically mitigate this risk with geotechnical investigation programs. In layman’s terms, they hire a geotechnical engineer to help them collect samples of the ground at regular intervals and key project locations to see what’s going on. These programs easily run six figures, and often seven for very large projects. The upshot is the designers gain a clearer picture of the subsurface risk of the project and can help the owner make informed decisions on how best to mitigate that risk (through design, contract provisions, contractor means and methods, or some other means).
Now consider your homeowner looking to build a modest home on an empty lot. The entire budget for the project will likely be six figures – so how about that IRC prescriptive design? Let's see what it says...
IRC 2021addresses "Soil Tests" in section 401.4 and geotechnical evaluation in subsection 401.4.1. To paraphrase, section 401.4 states that the "building official" (typically the local building inspector) may require a soil boring be performed at a site if they deem there is any potential to encounter soils that are expansive, compressive, shifting, or have otherwise questionable characteristics. Otherwise, the code continues into section 401.4.1 "Geotechnical Evaluation" where presumptive bearing capacities are presented with the qualifier that these values may be used in lieu of a complete geotechnical evaluation. It should be noted that the code does not provide any guidance on how a "complete geotechnical evaluation" should be carried out, whom should be directing it, or what it should present save for footnote a in table 401.4.1. The code is implicitly leaving this between a qualified engineer and the building official.
So, your options are to engage a qualified engineer to undertake a "complete geotechnical evaluation" or use the presumptive values in section 401.4.1 of the code and hope the building official doesn't come out to your site, decide your soils look questionable, and require borings. Many times, building officials first visit a site after design is completed and a permit is issued - far too late to identify problem soils and allow for a design change. Additionally, building officials are not all trained soils experts - they come with a wide variety of trainings and backgrounds. The code empowers them to ask for site investigation when it appears warranted, but this still leaves lots of risk on the Homeowner and the Contractor.
How, then, can the homeowner identify and plan for those “horrendous” conditions during design when change is cheap and the code official is not yet involved?
Many times, homeowners will get wind of poor soils early in the project either through word of mouth from a Contractor that walked the site, a city official who knows the area, or just a “gut feeling” from stepping in some marshy stuff near the back of the property.
A knowledgeable Contractor is often keenly aware when a site has potentially bad soils because they are the ones who will encounter the problem in the field and have to deal with it. Through their experience, they have learned the signs of trouble; poor drainage, softness underfoot, observable clay, and whatever else they’ve learned from digging up properties up the street or a few blocks over. Completing a site walk with a knowledgeable local Contractor during design can yield tremendous dividends in identifying bad conditions early.
Getting a soil boring done is also something to consider during design. There are many situations that crop up where a boring is specifically required by code or engineering practice to complete a design; on-site septic treatment is one example; poor soils (see IRC section 401.4 again) are another. If there are surprises underground (there commonly are), the boring log provides the best chance of foiling the surprise before digging begins. In this case, going above and beyond the code minimum requirement offers value to the homeowner in terms of early identification of a potentially design-changing condition.
It should be noted, though, that soil borings are only economic in small numbers and that soil conditions can vary considerably over relatively short distances. For example, a clay lens can show up in a boring on one property and easily disappear two properties away. This reality underscores that geotechnical investigation is as much an art as it is a science and that borings do not eliminate risk. However, as owners of large public works projects have learned, there’s tremendous value in that small amount of data returned from a soil boring.