How To Keep Your Home From Subsiding (for Less Than the Cost of a TV)

Italian Torre Pendente di Pisa. Aka – The Leaning Tower of Pisa. A medieval structure in Pisa, Italy, started in 1173. Famous for the settling of its foundations which caused it to lean 4.5 metres from its base.

In the late 20th century the structure was still subsiding. In 1990, the tower was closed due to being unsafe.

Remedial work took 10 years and the structure was reopened to visitors in 2001. Engineers now expect the tower to remain stable for at least 200 years.

Why the story? To understand how critical the role of the underlying soil strength is to the performance of a structure.

Soil has strength?

A geotechnical report is an analysis of sub soil strength by a chartered professional engineer, qualified in the field of geotechnical engineering. Ok, I know that’s a mouthful.

What it means is that someone who knows what they are doing will touch, test, prod and poke to establish the strength of soil. Effectively, a mud doctor.

Mud doctor to the rescue

A series of deep holes are drilled in the ground, typically 100 mm in diameter and soil samples are extracted.

The samples are tested, probed, and prodded in a lab and voila, the doctor determines what structure the soil can support and what type of foundations are necessary. Aha!

That brings me back to the Leaning Tower of Pisa

I’m sure you’d have seen a picture of this 1000-year-old monumental icon. Pretty dramatic isn’t it? Well, put it this way, if a proper Geotech report had been done, then the leaning tower probably wouldn’t be leaning.

When should an assessment be done?

Well, I shouldn’t even need to say so, but before any design work is done! To find out why a tower is leaning, almost a thousand years after it’s built, is somewhat worthless.

Don’t get me wrong, the Italians have done a fabulous job with the means they had at that time, even to keep the tower standing till now.

But you don’t have an excuse

The technical means available now, well exceeds the actual testing required. Just in case you are wondering what it costs, well, for starters far less than the cost of fixing a leaning or subsiding building, typically say 0.5%(that’s a half of a percent) of the cost of building a house.

You can do the Maths. It is not a huge cost by any means.

Will a Geotech report guarantee the stability of my house?

In one small word. No. Tell me something in this world that is guaranteed. (except taxes of course). The idea is that a professional investigation will minimise your risk.

Now you wouldn’t prefer years of legal battles, an unsafe house and related stress, would you?

Let’s summarise the key points

  1. If you are buying a site to build on, discuss getting a Geotech report with your designer
  2. Do this before you buy or start any design work
  3. Use a professionally qualified firm or engineer (ask your designer for recommendations)

Clear as mud? Next step – call your local Council to get information on your prospective site. A question you can ask them is, ‘Does this site have any soil stability issues?’ And you should be leaned towards to the appropriate department…we hope.

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