Homeowner’s training: alternatives when you can’t do a soil report
This is an article in a series, where we intend to train homeowners to do basic inspections. Because of the coronavirus pandemic, safety and social distancing is a must. However, that does not mean your construction requirements must be overlooked or paused.
Together, we can remotely manage your construction and engineering design work while adhering to social distancing and government safety guidelines without delaying or halting your remodeling requirements.
The foundation of a building is probably the most important part, as the entire structure rests on it. Designing the foundation is a highly specialized task which usually involves inspecting the ground to understand the properties of the soil and the types of foundation suitable for those.
However, the current coronavirus circumstances may bar you from bringing an inspector to write a soil report. So, what are the alternatives? In this post, we will share some techniques that not only will help you to proceed with your projects but are also more cost-efficient.
The techniques we are about to describe are not universal. They are only to be used when you are remodeling a building or designing the foundation for an ADU. Also, they should be used with the guidance of a structural engineer.
In some jurisdictions, soil reports are mandatory by law so you may need to get a soil report anyway. However, you may still use the techniques described here to get the work started, then conduct the soil inspection at a later phase when the circumstances are more appropriate. If you follow these techniques, the adjustments you would need to do at that time would be minimal or none at all.
With that out of the way, let’s have a look at traditional soil reports.
What is a soil report and why do we need one?
A soil report is a process where a licensed geotechnical engineer or civil engineer with experience in soils engineering, surveys the soil to determine what loads it can support. This process may require soil borings to determine what the soil looks like, as well as test the presence of groundwater on the site. Single-family dwellings typically involve two soil borings: 20’ and 35’.
Based on the report, the soil can be “strengthened”, or the type of the foundation may be altered to make it suitable for the expected load.
Soil reports are necessary for designing a structurally sound foundation. However, if you are in remodeling your home or developing an ADU, there are a few alternatives for you.
1. Check the existing building
ADUs and remodeling are typically built on an existing perimeter. As such, you may already have written documents for the main house, for instance, calculation reports or foundation drawings. Those can serve as the perfect reference point, as the bulk of the work is already handled there. By designing a foundation similar to the existing building which has persisted for several years, you can be sure you are on the right track.
Even without written documentation, it is still fairly easy to inspect the existing foundation. Just remove the first-floor sheet or dig a trial pit next to the existing building to learn what the existing foundation looks like.
2. Check geotechnical reports of nearby structures
Similar to checking existing buildings, you may also check nearby structures or ongoing projects for geotechnical reports. Again, those can serve as excellent reference points and will require minimum alterations when the actual soil report is written.
3. Consider the topology
The topology of the land greatly limits the choices. For instance, if you are building on a slope, the foundation options will be severely limited. You only need to send photographs to an experienced engineer, who can then tell you what option to proceed with.
4. Use building codes
You may consult the building codes of your jurisdiction to see the requirements for designing a foundation. Generally, building codes are very conservative and tend to increase construction costs as they may require you to build a foundation that is stronger than necessary. However, on-site geotechnical borings for soil tests tend to be costlier, which makes the building code path a more affordable option.
The International Building Code (IBC) has a 1804.2 table which defines the allowable pressure for foundations built on various types of soils. For clay, sandy clay, silty clay, clayey silt, silt and sandy silt (CL, ML, MH and CH) the allowable foundation pressure is 1500 psf. So, in case we find that kind of soil on the site, the foundation pressure we must design for is already determined.
Another factor we must consider is the Plasticity Index or PI. The PI is the size of the range of water contents where the soil exhibits plastic properties. If we notice any clay on the site, then we can safely assume a PI of about 40%.
5. Use select fill
In some cases (and with the coordination of a structural engineer) you may remove 2’-3’ of the existing soil and replace it with select fill, as that would resolve most settlement issues. Again, the cost of this work is less than two borings on the site, and you would still have a foundation that is capable of enduring the load of the structure.
As you’ve noticed, certain situations dictate the kind of foundation that you should use, regardless of the soil report. A soil report might suggest that a weak foundation is sufficient for your intended load, but building codes and regulations still force you to design for a strong foundation. If you follow the tips above, an experienced engineer can easily start your project’s design and calculations, and make sure you will reach your construction schedule.
Stay safe and healthy.