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An introduction to drilling and sampling in geotechnical practice -- 2nd Edition
Ross W. Boulanger
Overview
This video introduces fundamental drilling and sampling techniques used in geotechnical engineering. It explains how the project's requirements, site conditions, and soil types influence the choice of exploration methods and equipment. The video details various drill rig types, four primary drilling methods (solid stem auger, hollow stem auger, mud rotary, and coring), and common sampling tools like the split spoon sampler and Shelby tubes. It emphasizes the impact of drilling and sampling procedures on test results and the importance of understanding these effects for accurate subsurface characterization.
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Chapters
- Subsurface investigations are crucial for planning and executing engineering projects.
- The depth and type of exploration depend on project needs (e.g., road vs. high-rise foundation).
- Site accessibility, soil type, and variability dictate the investigation approach.
- Understanding geological complexity is key to effective exploration.
Properly defining the scope and method of subsurface investigation ensures that the necessary information about soil conditions is obtained efficiently and cost-effectively for safe and stable structure design.
Designing a county road might only require investigation to 20 ft, while a high-rise foundation needs information potentially hundreds of feet deep.
- Drill rigs are categorized into highway, off-road, overwater, and portable types based on mobility and site access.
- Highway rigs offer high mobility on accessible sites.
- Off-road rigs are designed for difficult terrain, often track-mounted.
- Overwater rigs are used for drilling in bodies of water.
- Portable rigs are for extremely limited access and shallower depths.
Selecting the correct type of drill rig ensures that drilling operations can be performed safely and effectively, regardless of the site's location or terrain.
A highway rig mounted on a truck is common for easily accessible, level sites.
- Solid stem continuous flight augers are simple, lifting soil cuttings as they drill.
- This method is suitable for cohesive soils that can support an open borehole.
- Hollow stem continuous flight augers allow sampling through the hollow center without removing the auger.
- Hollow stem augers provide borehole support, preventing collapse, especially in sands below the water table.
Auger drilling methods are efficient for advancing boreholes and obtaining soil samples, with hollow stem augers offering the advantage of in-situ sampling without borehole collapse.
A hollow stem auger allows a sampler to be lowered through the center to obtain a sample while the auger remains in the ground.
- Mud rotary drilling uses a circulating fluid (mud) to remove cuttings and support borehole walls.
- Side discharge bits are preferred to minimize disturbance at the borehole bottom.
- Coring is used for rock or very hard soils, employing a core barrel to recover intact cylindrical samples.
- The type of drilling fluid and bit design significantly impact sample quality and drilling efficiency.
Mud rotary and coring methods are essential for drilling in challenging conditions, such as below the water table or in rock, and for obtaining high-quality samples.
A diamond coring bit is used to cut a cylindrical sample of hard rock, which is then recovered inside a core barrel.
- The goal of sampling is to obtain representative soil specimens for laboratory testing.
- Disturbed samples (e.g., from split spoon) are suitable for identification tests but not for engineering property tests.
- Undisturbed samples (e.g., from Shelby tubes) aim to preserve the soil's in-situ structure for strength and compressibility tests.
- Sample disturbance is inevitable and affects test results, especially for cohesive soils.
Understanding the difference between disturbed and undisturbed samples is critical because the quality of the sample directly impacts the reliability of laboratory test results and subsequent engineering design.
A split spoon sampler provides a disturbed sample useful for visual classification and grain size analysis.
- The SPT uses a split spoon sampler driven by a standardized hammer blow count.
- The 'N-value' (blows for two 6-inch penetrations) estimates soil compactness or stiffness.
- Factors like borehole diameter, drilling method, hammer energy, and blow rate significantly influence N-values.
- SPT results are an indirect measure and require careful interpretation by an experienced engineer.
The SPT is a widely used, cost-effective field test that provides valuable, albeit indirect, information about soil density and strength, guiding preliminary design decisions.
Counting the number of blows required to drive a split spoon sampler 18 inches into the ground yields the N-value.
- Thin-walled Shelby tubes are used for obtaining relatively undisturbed samples of cohesive soils.
- These tubes are pushed into the soil at a constant speed, avoiding hammer impact.
- Piston samplers utilize a piston to control sample entry and minimize disturbance, especially in soft clays.
- The quality of undisturbed samples is often inversely related to cost and time.
Advanced sampling techniques like Shelby tubes and piston samplers are necessary when high-quality, undisturbed samples are required for critical projects involving soft or sensitive soils.
A piston sampler is used to obtain a high-quality sample of soft clay by preventing the soil from rising too quickly into the sampling tube.
- All sampling methods introduce some degree of disturbance to the soil structure.
- Disturbance significantly affects laboratory test results, particularly for clay compressibility and strength.
- Disturbance can occur during sampling, transportation, storage, extrusion, and testing.
- The cost of obtaining higher quality samples generally increases with improved sample integrity.
Recognizing and quantifying sample disturbance is crucial for accurately interpreting laboratory data and making sound engineering judgments, especially for critical structures or problematic soil conditions.
Consolidation test results on a block sample, borehole sample, and remolded sample of clay show progressively less stiffness due to increasing levels of disturbance.
Key takeaways
- The selection of drilling and sampling methods is a critical decision driven by project requirements, site conditions, and soil characteristics.
- Different drill rig types are designed for varying site accessibility, from highways to remote or offshore locations.
- Auger drilling methods (solid and hollow stem) are effective for soil advancement, with hollow stem offering advantages for in-situ sampling.
- Mud rotary and coring techniques are vital for drilling in challenging formations like rock or below the water table.
- The distinction between disturbed and undisturbed samples is fundamental, as sample quality directly impacts the reliability of laboratory test results.
- The Standard Penetration Test (SPT) provides an indirect but valuable measure of soil compactness, though its results are influenced by numerous factors.
- Advanced sampling tools like Shelby tubes and piston samplers are employed to obtain high-quality samples when needed, but at a higher cost.
- Understanding and accounting for sample disturbance throughout the process—from drilling to testing—is essential for accurate subsurface characterization.
Key terms
Subsurface InvestigationDrill RigSolid Stem AugerHollow Stem AugerMud Rotary DrillingCoringDisturbed SampleUndisturbed SampleSplit Spoon SamplerStandard Penetration Test (SPT)N-valueShelby TubePiston SamplerSample Disturbance
Test your understanding
- How do project requirements and site conditions influence the choice of drilling and sampling methods?
- What are the primary differences between solid stem and hollow stem auger drilling, and when would each be preferred?
- Explain the role of drilling fluid in mud rotary drilling and its impact on borehole stability and sample quality.
- What is the fundamental difference between a disturbed and an undisturbed soil sample, and why is this distinction important in geotechnical practice?
- What factors can significantly influence the N-value obtained from a Standard Penetration Test, and how should an engineer account for them?