New Hampshire Soil

The soil in NH at depth underneath loam (i.e. where it matters for supporting structures) is largely the result of ancient glaciers.  The colloquial term for most glacial till is “hard pan.”  This is a hard to very hard mixture of silt, sand, gravel and cobbles/boulders that when undisturbed is good for bearing of foundations, but soft and unsuitable for use under foundations when disturbed.  So careful excavation by hand at the bottom of a trench is needed if you don’t want to pay for over-excavation and replacement of disturbed glacial till (a.k.a., “muck).

Glaciers were also responsible for lacustrine (a.k.a., lake) deposits of silts and clays.  Lakes were formed when glaciers melted.  In the quiet water, small particles of silts and clay gently floated to the bottom.  It’s interesting to see varves (a.k.a thin bands) of alternating clays and silts.  Silts and clays will compress over large periods of time.  Building on them should not be done without the input from a geotechnical engineer to prevent significant settlement and foundation cracking.

There were also streams in glacial times.  Fluvial (a.k.a. river) deposits can be uniform sand (mostly the same size grains like beach sand) or mix of gravel, sand and silt.  Sand and gravel mixtures with trace silt are generally considered the best soil to use in construction.  The amount of silt is important, because more silt make the material more susceptible to frost heaving when there are freezing temperatures and moisture present.  Uniform sand drains well but is difficult to compact.

Then there is organic soil.  Organic soil can have widely ranging consistency, based on size and formation of the organic material within it.  Organic soil will decompose when there is oxygen present.  Building on organic soil always leads to trouble for a structure if there are no engineered deep foundation units.

Residential projects in NH rarely involve a geotechnical engineer during design and construction, which puts the responsibility on the contractor for identifying problems and requesting engineering expertise when needed.  Commercial projects are required by code to have geotechnical explorations.

Playing in dirt is fun for us.  Call us before unsuitable soils have caused havoc with your structure.