Conventional CPT compression cones, though great tools for many things, have a hidden dirty secret – they cannot reliably measure teeny tiny fs values that one might associate with very soft natural soils, sediments and tailings; say stuff with shear strength below 10kPa or so. This is an industry “elephant in the room” – preferably not talked about much, if at all.
The cause is friction/stiction in seals inside the cone’s friction sleeve – these seals induce some small internal friction in this measuring system, and this has to be overcome before the sleeve can load its load cell and register the tiny forces involved in such measurements.
This is most noticeable in tests where qc is less than (say) 150-200kPa.
It matters not what capacity of cone is used, 100MPa, 25MPa or even 10MPa – the sleeve still has the seals and the small friction/stiction still applies.
IGS conceived a solution, and our (wonderful) supplier Geomil built this for us; an innovative cone design using a different sleeve set-up and non-standard materials in the load cells.
We are now able to repeatably measure sleeve friction fs values of < 1-2kPa.
(a) The red plots are from a very sensitive 10MPa compression cone; our previous best option.
(b) The blue and green plots are from IGS’s new cones.
(c) The three tests are within about 1.5m of each other, in very soft “ooze” like tailings.
(d) All three cones are measuring similar fs below about 4m depth; i.e. in materials with qc of (say) 200kPa or greater.
(e) The effect of the seal friction/stiction on fs can be clearly seen in the red plot above 4m depth, in materials with qc less than (say) 200kPa.
These new cones have now been used on several projects, including both tailings and very (extremely) soft natural clays. The results are consistent and repeatable.
Note that calibration of these cones, for both qc and fs is done using dead weights.
For more information please contact any of us at IGS:
Why not use a tbar probe for soft materials like tailings…
We do use full flow penetrometers on these materials and they work very well, however, they do not provide measurements of Sleeve Friction.
Our clients are finding measuring the sleeve friction has advantages in these very low strength tailings particularly in allowing assessment of friction ratio, material classification etc.
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