Soil Reports and Deep Foundations

Inadequate Soil Reports and Deep Foundations

This article will examine why most soil reports are inadequate for deep foundations.

The usual soil report generated for a domestic site or for a small industrial / commercial
building is conducted with a small drilling rig attached to a 4wd Ute or similar. The soil
types are catalogued and density readings taken with a DCP rod or Pocket Penetrometer.
The report is limited, often by budget, and will usually just classify the top couple of metres
of soil.

If the soil is classed as suitable for high level footings then the report is adequate and
useable for a footings design.

If the soil is classed as “P” or incapable of supporting high level footings, then the report
will probably recommend deep foundations (such as bored piers, hammered timber or steel
piles or steel screw piles).

These reports may identify a very dense layer for the piers to found on if it is close to the surface; but being constrained by budget, will not investigate any deeper. Herein is the problem with the soil reports that caused grief to many builders using deep foundations when they ended up much deeper (and costlier) than expected.

The soil reports do not usually clearly identify that a further and deeper investigation is needed.

The sub-soil types seen by screw pile installers in S.E. Queensland on the Gold and
Sunshine Coasts are many different layers of sands and clays or mixtures; interspersed with
very thin hard layers of cemented sands or shells that resist an excavators ripper or digging
bucket. Erroneously, footings or bored piers or screw piles are founded on these hard layers
without any knowledge of what is underneath.

In our experience what is underneath is most often soft clay and sometimes compressible
estuarine or marine clay that is unsuitable for a foundation.

We can torque up a screw pile in a dense layer until it meets its load requirement and
appears to satisfy the criteria of the Torque Vs SWL formula. This is meaningless if it is
sitting 200mm above a soft layer as in time it can just punch through and building
settlement occurs. Only an expensive static load test will identify that potential problem.

If your soil report recommends piers and does not identify the soil classification for 2+
metres beneath their base; then you need to commission a deeper examination. Your
Geotechnical Surveyor will advise the appropriate methods, such as a continuous CPT
probe with Pore Pressure and Sleeve Friction readings or deep Borelogs with SPT density
sampling every metre, taken with a heavy drill truck.

Finally, don’t waste your money on foundations that could potentially fail. Find out what is beneath your deep foundations and be sure that they will support your building.

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