1. The challenge of tailings and waste rock piles
A recurring challenge in recent campaigns is drilling on tailings and waste rock piles. The material is heterogeneous: many loose blocks alternating with soft layers. In this scenario the CPTu test is demanding, because the piezocone must get past impenetrable layers to advance in depth.
2. From two rigs to one
Until then, the client handled this scenario with two rigs: a penetrometer unit for the CPTu and a rotary rig to drill out the impenetrable layers. The arrangement put more people on the work pad, more exposed, and meant higher cost.
With the multipurpose rig, the drill-out is done by the rig itself, which also collects samples in those layers. The test, the drill-out and the sampling are concentrated in a single machine and a single borehole.
3. Complete borehole stratigraphy
There is an additional gain: the soil tested by CPTu is also cored, meaning the advance through the layers is accompanied by core barrel sampling. Besides the continuous piezocone profile, samples are recovered along the entire borehole, resulting in the complete stratigraphy of the vertical.
4. Care with the cone tip
The operation demands attention. Alternating between drill-out and piezocone pushing in the same borehole requires an attentive, experienced operator, so the cone tip is not damaged in the transition between layers. It is this care that preserves test quality along the entire vertical.
5. Result: CPTu with drill out
The DRILL OUT bands on the profile mark the stretches where the cone met impenetrable layers and the rig itself drilled them out by rotary, resuming the push just below. The continuous record of tip resistance, sleeve friction and pore pressure is preserved along the entire vertical.
6. Conclusion
One rig pushed the CPTu, drilled out the impenetrable layers and collected samples along the entire borehole: complete stratigraphy of the vertical, with fewer people exposed on the work pad and lower cost, without giving up test quality.