Be Careful How You Access Soft Sites

IGS Technical Note

IGS-Low Contact Pressure-CPT-Rig-“Lizzy”

“Be Careful How You Access Soft Sites

(Especially Tailings Dams- Don’t build Roads to Do CPTs)

There are two ways to access very soft sites such as tailings dams to undertake in situ testing:


  1. Build road-like fill embankments over the surface and use conventional test machines that drive on these embankments, or
  2. Use specialist testing equipment and techniques that were specifically designed and built to safely and efficiently access very soft sites (without having to build such embankments).

See our Rigs and Services pages and our Tailings Brochure. IGS has evolved its business and equipment to work the second way, for two reasons:

  1. The consolidation effect under embankments can radically change results of in situ tests such as CPTs. The critical soft profile to be tested will be preloaded and stiffened by the embankment, making test data through the zone “optimistically misleading”.
  2. Access embankments cost money that is simply wasted when there are other options which are also technically better. Why not spend the access budget on better testing?

The Approach To Avoid Is Depicted Below:

The red dashed line on the qc Vs depth graph, below, is the potential impact on qc under this scenario.

Based on a current result from a current IGS project.

It assumes that the access embankment has been in place for a few days/weeks before CPT testing (see note marked * below the figures).

(apologies to Boussinesq for the simplifications made)

* Even if the embankment has not been in place long enough to cause the significant qc changes depicted, the embankment’s mass will significantly elevate pore pressures and in this way similarly “spoil” the test data over its zone of influence.

For more information please contact any of us at IGS:

Mark Chapman
Mick O’Rourke
Sergey Skrobotov
Wyn Binmore
Allan McConnell