Principle and application
Drilling with hollow stem flight augers allows borings, tests and instrument installation to be carried out dry, with no use of water or drilling fluids. The borehole remains fully cased by the augers, which preserves the walls and the soil to be sampled. Because the method introduces no moisture, it does not alter the pore pressure of the ground being investigated.
Typical applications: dams and tailings structures, undisturbed sampling in soft soils, environmental borings and projects where introducing water into the ground is undesirable.
The drilling system
The system is made up of hollow flight augers, with inner rods and a pilot bit. Drilling advances with the plugged assembly and, once the target depth is reached, the inner assembly is withdrawn, leaving the borehole cased by the augers. The technique is especially suited to borings and tests in dams and to undisturbed sampling with Shelby tubes or stationary piston samplers, even at greater depths.
Plugged assembly
The hollow augers advance with inner rods and a pilot bit; the assembly drills dry with the plugged tip.
Borehole cased by the augers
The flights themselves act as continuous casing, preserving the borehole walls.
Withdrawal of the inner assembly
Once the target depth is reached, the inner assembly is withdrawn, leaving the borehole cased and open for the operation.
Test, sample or instrument
SPT is performed, an undisturbed sample is collected with a Shelby tube or piston sampler, or piezometers are installed through the inside of the borehole.
Quality controls: plug check before each advance, logging of depths and section couplings, bore cleanliness check before sampling, and traceability of every sample by borehole and depth.
The method in the field
Watch hollow stem auger drilling in operation, recorded by the Damasco Penna field team.
Per ASTM D6151
| Method | Hollow-stem flight augers with inner rods and pilot bit, per ASTM D6151 |
|---|---|
| Drilling | Dry, with no water or fluids; borehole cased by the augers themselves |
| Sampling | Undisturbed, with Shelby tube or stationary piston sampler through the stem (NBR 9604, ASTM D1587) |
| Instrumentation | Installation of piezometers and monitoring wells |
| Deliverable | Field log with depths, sections and recovery condition, traceable by borehole and depth |
Executed under the same QA/QC, HSE and site standard as every Damasco Penna service, described in Quality and field operations.
Frequently asked questions about the Hollow Stem Auger
What is the hollow stem auger for?
For drilling dry, with no water or fluids, keeping the borehole cased by the augers themselves. This allows SPT testing, undisturbed sampling and piezometer installation without changing the moisture or the pore pressure of the ground being investigated.
Why does drilling without water matter?
In dams, tailings structures and soft soils, drilling water changes the moisture and the pore pressure of the ground and can compromise both the test data and the sample quality. The dry method preserves the natural condition of the soil.
How is the sample taken through the auger?
At the target depth, the inner assembly (rods and pilot bit) is withdrawn and a Shelby tube or stationary piston sampler goes down the hollow stem, pushed just below the cutting head into soil the drilling never touched.
Which standard governs hollow stem auger drilling?
Hollow-stem auger drilling for geotechnical site investigation follows ASTM D6151. The associated undisturbed sampling follows NBR 9604 and ASTM D1587.
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