The Soil Test

Soil profile 236x288 38.76 KB. Units are inches.

Soil profile 236×288 38.76 KB. Units are inches. (Photo credit: Wikipedia)

This post will be (yawn…) a bit boring and a bit long, but necessary I think.  The quality and characteristics of the soil are critical to the building process.   As with the survey, there are different types of soil tests for residential construction.  They can be broken down into 3 different types:

1.  Water percolation test, or the “perc test” :  this tests the soil’s water absorption rate.  This information is used to design a septic system.  Necessary for unimproved land (land without access to utilities, water, etc.).  Unnecessary for an improved lot with easy access to the city’s sewage system (which is my lot.  Yay!)

2.  Environmental Site Assessment:  this test evaluates the soil for pollutants/contaminants/harmful substances.  It is typically performed by soil engineers or civil engineers.  This test in particular has many nuances.

  • it may be required by law, particularly for commercial building.
  • there are 3 phases:  Phase 1 is a paper search to determine what was historically on that site.  This information then identifies what specific contaminants the ground may contain, if any.  Phase 2 is actual field testing of the soil, which may require sub-surface investigation,  for the pre-identified contaminants.  For example, if the paper search determined that the land was previously farmed, then it may be contaminated with pesticides.  The soil test would be a targeted test for pesticides.  Phase 3 is remediation of that contaminant.
  • who performs the remediation is interesting:  if you own the property, then it’s you, regardless of who contaminated the soil.  If your purchase is contingent upon this inspection, then it’s the current owner (provided they agree).    The real question is (as with a standard homeowner’s inspection):  does one pay for this test now (before owning the lot) just in case remediation is necessary?

Cracked. (Photo credit: atxryan)

3.  Geotechnical investigation:  this test evaluates soil characteristics, such as soil composition and density.  Designing the proper foundation relies on understanding the properties of the soil.  If the soil cannot bear the load, or settles, the foundation will crack.  The geotechnical engineers use their soil analysis to provide recommendations for foundation depth, size, location as well as recommendations for backfill and grading.

In the Pittsburgh area, most soil/civil/geotech engineering companies do not perform all 3 types of tests (in other words, it is not one stop shopping).  This led me to presume that these tests do require a specific level of expertise and training, which is why a company specializes in just one.  There are very large companies that perform all 3 types of testing under one roof, but they cater to large developers, not the person looking to build a single small house.

Just as an aside, would note how different people in the industry have different opinions regarding the necessity of above tests.  One architect is emphatic that a geotech investigation would be a waste of money, since the load of a residential house is so light that virtually the same type of foundation is used for all single family residential construction in this area.   Another engineer states that it’s absolutely necessary because it’s known my lot was backfilled, this backfill may contain organic material which will decompose with time, so if a foundation sits on this decomposing material, it will crack and sink.  I think this difference of opinion will be common as I continue along this process.  Perhaps the city’s building and planning department would be helpful..  At the very least, the minimum investigation necessary to obtain a building permit is a good starting point ..  Or, as someone else advised, get everything done upfront so there are no future surprises, particularly relating to unanticipated costs.  To which I said, surprises are inevitable in home construction – you can’t plan for them.  That’s why they’re called surprises 🙂


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