The CPT probes will not penetrate boulders, cobbles, rubble, well compacted sediment, or sound bedrock. The dummy probe has no sensing capability, and consists of a cone and push rods which can be pushed to a desired depth to clear a path for the LIF or resistivity probe. The 1.44 inch dummy rods are smaller in diameter than the sensor rods while the 1.75 inch dummy rods are the same diameter as the sensor rods. A dummy probe is used by pushing the dummy to the desired depth, retracting it, and reinserting a probe with sensors into the same hole and pushing to a greater depth than the maximum depth reached by the dummy probe. All stratigraphic information in the interval penetrated by the dummy probe is lost because the soil material in that interval has been permanently altered.
Data taken below the terminal depth of the dummy push are valid.
When the large diameter dummy rod is used the LIF and resistivity data collected in the interval penetrated by the dummy rods are questionable because it is uncertain whether the instrumented probe sensor had good contact with the soil. However, when the small diameter dummy is used, the resistivity data and LIF data collected in the interval penetrated by the dummy rods may still be valid because the smaller dummy rod diameter should allow for good contact between the soil and sensor. Because of the cohesive nature of clays, the hole may remain open after the rods are removed, but sandy material may collapse into the hole as the push rods are removed. Accordingly, even a sensor probe push following a small diameter dummy pre-push should be viewed with some caution. First, the CPT is less intrusive than conventional drilling because the CPT hole is relatively small and there are no drill cuttings to dispose of.