Don’t take the ground for granted

Expect the unexpected.  This is relevant when designing foundations to support a structure.  Also, when investigating the cause of a failure in the built environment – a forensic engineering investigation.

I sometimes wonder how often the designer does this – takes the ground for granted, and gets away with it because the foundation soils in Nova Scotia are often very strong.  That includes well constructed, man-made filled ground.

I thought of this when an experienced structural engineering friend of mine exclaimed recently, “There’s sand everywhere in Moscow..!!”.  He was describing his trip to the Scandinavian countries and Russia this summer.  He went on to describe the size of the grains, the expanse of the deposit and the level terrain.  It was like he was seeing a natural, undisturbed deposit of sand for the first time.

That might be understandable.  My friend has practised structural engineering in Nova Scotia since the 1970s.  Much of the province is covered by dense mixtures of gravel, sand, silt and clay – natural, undisturbed glacial soils.  This is what my friend probably expects to find beneath a site when he’s designing the foundations, and the steel, concrete or timber supporting the structure above.

(Everything in the built environment can be thought to have three major components:

  • the Structure above the ground surface,
  • the Foundations at or near the ground surface, and,
  • the Foundation Soils below.

The lower you go the less glamour there is, and too often the less attention paid to what’s below.  “There’s no glamour in the ground”. Ref. 1)

Such an expectation by my friend would be dangerous.  The comfort we feel in Nova Scotia about what is below the ground surface can get you in trouble.  It’s better to “expect the unexpected” as I learned practising geotechnical engineering – a civil engineering specialty, in the U.K., Australia and eastern Canada.

We have loose sand and silt in Nova Scotia and also soft clay – poor foundation soils.  Not a lot but it’s out there.  We also have poorly constructed fills – “un-natural, disturbed“ mixtures of different materials.  Fill is material brought from elsewhere and placed on the ground to raise the level.

Poorly constructed fill can cause problems for low-rise structures like one and two story buildings.  In fact, when I investigate the cause of a foundation failure, as an initial hypothesis, I “expect” to find a poorly constructed fill beneath the foundations.  Or one of the other poor foundation soils that occasionally show up on our construction sites.  This expectation is reasonable because I would have evidence suggesting something is wrong down below when most of the time in Nova Scotia all’s good there.

Appendix

Good fill in engineering usually consists of well compacted mixtures of soil – gravel, sand, silt and clay, placed on undisturbed, natural soil.

Poor fill can consist of these materials plus varying amounts of topsoil, peat, roots, boulders and/or debris.

Geotechnical engineering is a civil engineering specialty that identifies the types of soils and rocks below the ground surface -  below proposed foundations, measures and tests their physical properties, and analyses and calculates how they will perform when used as engineering materials by design engineers.

Reference

1. A quite well known comment by Karl Terzaghi, considered the father of soil mechanics, the science underlying geotechnical engineering.

 

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