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The Importance of a Suitable Geotechnical Investigation.

Omitting an investigation can alter plans, not always for the better. This could be as simple as a more conservative design because you can’t confirm the soil parameters, or it could be significantly more costly and time consuming. There have been cases of completing a build only to have to remove the construction, do an investigation, redesign with appropriate parameters and then begin the build again.

Highlighting a worst-case scenario is the recent collapse of a bridge in Saskatchewan, Canada just 6 hours after opening (read the full article here).

The Dyck Memorial Bridge in the Rural Municipality of Clayton was to carry approximately 1000 cars per year. The decision was made to construct this bridge as cheaply as possible and do away with things that would take time and money.

In this case, a geotechnical investigation was done for each landing on shore, with the riverbed being assumed to be the same. Their excuse for omitting a riverbed investigation was that it is “impossible to drill through water and take underground samples”.

Given the bridge piles had to be drilled through water into the riverbed, this statement makes no sense! Even deep sea oil wells do this, at much greater depths than a river bridge pier would need.

This lack of investigation resulted in the bridge collapsing hours after it was opened. The 5 piles supporting the central span of the bridge settled approximately 4 feet (1.2m). The bridge spans could not reach across this new, longer distance and collapsed.

Most construction work is not a road bridge, but even a simple house build carries risk if there is no geotechnical information available.

  • How do you know what’s underneath your site without an investigation?
  • Is there peat, pumice, tomos, or voids?
  • Is the soil a soft mud (like walking out across an estuary when the tide is out)?
  • Is there rock or good strong silt/clay or dense sands there?
  • Has the soil been placed by earthworks?
  • Does the soil contain hazardous material?
  • Has it been compacted properly?
  • Will the weight of your building compress (squash) the soil that is there?
  • Will what you want to build be able to stay where you want it, or will it sink so low you can’t open the front door?
  • Will your build slide into the neighbour’s property?
  • Is the slope on your site stable, or will it move?

Everyone relies on the ground being there to support their build, but how do you know it will be strong enough or that it will stay where it is now?

Buildings on flat ground are at risk of sinking as well as bridges. If only one part of the structure is on suitable ground, and the other is on soft material, it could start to tilt as it sinks. You could try to make a tourist destination from the result (like the Leaning Tower of Pisa), but it might be a bit intrusive to have people taking photos of themselves holding up your home. If your home does start to settle, the local Council could deem it unsafe and force you to evacuate.

Ultimately, these issues can mostly be avoided by getting a geotechnical investigation done. Be honest with your engineer – we want to be as thorough as possible, but if you tell us you’re building a single-storey one-bedroom bach, the amount of investigation we do would be different than if you plan to build a five-storey apartment building with 3 levels of underground parking. Conversely, tell us if you are working to a tight budget – we want to get the most thorough investigation for you that we can, but if you cannot afford some testing we can provide a conservative estimate for some parameters, we just need to know so the investigation can be adjusted.

We will let you know the absolute minimum that must be done to be able to provide you with a report that will help put your next build on solid ground.

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