Occasionally I am asked “How accurate is the CPT?” In short – generally it’s more accurate than most other in-situ and laboratory tests for a wide range of soils. What do I mean by that? The other primary in-situ test for soils is the SPT, which (as we know) is not very accurate. There are many variables that influence the SPT (e.g. borehole disturbance, variable hammer/anvil energy, non-standard sampler, loose rod connections). For example, you may measure an SPT N value of 12, but if the energy ratio of the hammer/anvil system was 80%, the corrected N60 is 16.
The main advantage of the modern electronic CPT is that the test is operator independent. In essence, different testing companies can test the same ground and should measure the same CPT tip resistance (qc) profile to within the tolerances set by the electronic transducers, about ± 0.1% of the full-scale output (FSO) using available commercial equipment (e.g. ASTM Standard D5778). Most commercial cones have a maximum capacity or full-scale output (FSO) of about 1,000 tsf for the tip resistance (qc). These cones are designed to survive pushing through very hard ground (including soft rock). Hence, they have an accuracy of ± 1 tsf for qc. In most soils, this is an excellent level of accuracy that is often better than ± 1% of the measured values, especially in sands where qc > 100 tsf. Research has shown that even university research laboratories cannot achieve this level of repeatability in simple triaxial compression testing of reconstituted samples.
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