Principle and application
Combines SPT in the soil and rotary core drilling in the rock, in the same borehole, from the surface to the bearing elevation.
Mixed drilling is the answer when the borehole has to cross the soil and continue into the rock. It starts as an SPT boring, by percussion, measuring the soil resistance layer by layer until it reaches refusal. From there it switches to rotary core drilling, which cuts the rock and recovers continuous core. A single borehole delivers the full profile, from natural ground to the foundation bearing elevation.
It is the investigation of choice where rock lies within the project's depth of interest: foundations of bridges and tall buildings, dams, works in rock mass and ground with boulders. Part of the Geotechnical Investigation area, it is carried out with the same in-house crews and fleet as the other borings.
The boring in steps
Percussion in soil
Advance by SPT, with measurement of the penetration resistance index (N-SPT) and sampling every metre, to NBR 6484.
Transition
On reaching refusal to percussion, the casing is set and the rig switches to the rotary system.
Rotary in rock
The diamond bit cuts the rock and the core barrel recovers continuous core, to NBR 6502 and rotary practice.
Profile and core
The result is the full geological-geotechnical profile, the core recovery and the RQD of the rock section.
To NBR 6484 and rotary
| Soil section | SPT to NBR 6484, with N-SPT and samples |
|---|---|
| Rock section | Rotary core drilling, with continuous core |
| Parameters | N-SPT, core recovery and RQD |
| Delivery | Single profile from the top of soil to the bearing elevation |
Quality and field operation
Data quality
- SPT to NBR 6484, with calibrated equipment.
- Rock core stored in an identified box, with recovery and RQD recorded.
- Boring log with profile, elevations and water level.
- Chain of custody from sample to laboratory.
Safety and operation
- Qualified for high-compliance sites: mining, industry and ports.
- Organized, signposted work fronts with OHS procedures.
- In-house crews and fleet, including night shifts.
The deliverable
| Profile | Continuous geological-geotechnical profile, from soil to rock |
|---|---|
| Soil | N-SPT index per metre and description of the layers |
| Rock | Core, recovery index and RQD |
| Log | Elevations, water level and identified bearing elevation |
Sectors and project types
Frequently asked questions about mixed drilling
What is mixed drilling?
It is the boring that combines, in a single hole, percussion (SPT) in the soil and rotary core drilling in the rock. It starts by measuring the soil resistance layer by layer and, on reaching refusal to percussion, switches to cutting the rock with a diamond bit, recovering continuous core down to the bearing elevation.
When is mixed drilling needed instead of SPT?
When the project needs to characterize the rock at the depth of interest, not just the soil. SPT alone stops at refusal to percussion. Mixed drilling continues into the rock, which is required for foundations of bridges and tall buildings, dams, works in rock mass and ground with boulders.
What does mixed drilling deliver about the rock?
Beyond the soil profile with the N-SPT index, it delivers the continuous core of the drilled rock section, the recovery index and the RQD (rock quality designation), which characterize the rock mass and guide the foundation design.
Mixed drilling for your project
Tell us about your project and we will put together the investigation plan and the quote.