Shelby tube is one of the most widely used devices for:
collecting undisturbed soil samples
Understanding the Purpose of a Shelby Tube
The question asks about the primary use of a Shelby tube in geotechnical investigation. Understanding different soil sampling methods is crucial in civil engineering and geology.
What is a Shelby Tube?
A Shelby tube, also known as a thin-walled tube sampler, is a device specifically designed for collecting soil samples. It's a thin-walled metal tube, usually made of seamless steel tubing.
How is a Shelby Tube Used?
The tube is pushed into the soil at the bottom of a borehole using hydraulic pressure. The soil enters the tube as it is advanced. The thin wall and sharp edge minimize disturbance to the soil structure as it enters the tube. After pushing to the desired depth, the tube is twisted or a cutting shoe is used to detach the soil core at the bottom, and the sampler is then retrieved.
Why Use a Shelby Tube?
The main objective of using a Shelby tube is to obtain soil samples that represent the in-situ conditions as closely as possible. These are called undisturbed soil samples. Undisturbed samples are essential for conducting various laboratory tests, such as triaxial compression tests, consolidation tests, and permeability tests, which require the soil structure and moisture content to be preserved.
Analyzing the Options
Let's look at the given options:
- Wash borings and drilling cobbles: Wash boring is a drilling technique, often used for advancing boreholes quickly, but it typically disturbs the soil significantly. Drilling cobbles requires specialized drilling methods, not standard tube sampling. So, this option is incorrect regarding the Shelby tube's primary use.
- Measuring effective stress and plotting Mohr envelope: Measuring effective stress and plotting the Mohr envelope are laboratory procedures performed on soil samples or in-situ tests using other devices (like piezometers for pore water pressure, and shear tests or triaxial tests for shear strength leading to the Mohr envelope). A Shelby tube is used to collect the sample that *might* be used for these tests, but it doesn't perform the measurement or plotting itself. So, this is not its primary use.
- Collecting undisturbed soil samples: As discussed above, the Shelby tube is specifically designed to minimize disturbance during sampling to obtain undisturbed or relatively undisturbed soil samples. This aligns perfectly with its purpose.
- Drilling rocks: Shelby tubes are not designed for drilling rock. Rock coring uses different equipment like core barrels with diamond or carbide bits. The thin walls of a Shelby tube would buckle or be destroyed if attempted in rock. So, this option is incorrect.
Based on the analysis, the most accurate description of the primary use of a Shelby tube is for collecting undisturbed soil samples.
Conclusion on Shelby Tube Use
The Shelby tube sampler is a standard tool in geotechnical investigations when undisturbed samples of cohesive soils (like clays and silts) are needed for detailed laboratory testing. Its design focuses on minimizing sample disturbance during collection.
Revision Table: Shelby Tube Purpose
| Feature | Description | Relevance to Shelby Tube |
|---|---|---|
| Primary Function | Collecting soil samples | Yes |
| Sample Type | Undisturbed or relatively undisturbed | Yes, key purpose |
| Suitable Soil Types | Cohesive soils (clays, silts) | Primary application |
| Method | Pushed into soil | Characteristic method |
| Application | Laboratory testing requiring preserved structure | Directly linked to undisturbed sample need |
Additional Information: Soil Sampling Techniques
Soil sampling is a critical part of geotechnical site investigation. The choice of sampler depends on the soil type and the required quality of the sample (disturbed vs. undisturbed).
- Disturbed Samples: These samples do not preserve the in-situ structure, density, or moisture content. They are suitable for classification tests (like particle size analysis, Atterberg limits) and compaction tests. Examples include samples from auger cuttings or split-spoon samplers (though split-spoon samples are sometimes considered "representative" rather than truly undisturbed).
- Undisturbed Samples: These samples aim to preserve the in-situ structure, void ratio, and water content. They are required for tests like consolidation, shear strength (triaxial, direct shear), and permeability tests. Shelby tubes are a common method for obtaining undisturbed samples in soft to medium cohesive soils. Piston samplers are another type, often used in very soft soils or marine environments. Block samples can also be carefully cut from test pits for undisturbed testing.
Understanding the difference between disturbed and undisturbed samples and the tools used to collect them is fundamental in geotechnical engineering.
Important Questions from Site Investigation and Sub-Soil Exploration
In order to minimize sampling disturbance the area ratio A r,\(\left( {{{\rm{A}}_{\rm{r}}} = \frac{{{\rm{D}}_{\rm{e}}^2 - {\rm{D}}_{\rm{i}}^2}}{{{\rm{D}}_{\rm{i}}^2}}{\rm{\;}}} \right)\) should be
Which one of the following is NOT a soil exploration technique?
If the sampler tube is pushed at the bottom of the bore hole to a distance of 585 mm with length of the sample recovered being 535 mm. Then the value of recovery ratio is ______.
If the inside diameters of the cutting edge and sample tube are 68 mm & 70 mm respectively and 76 mm & 74 mm are the outside diameters of the cutting edge and sample tube respectively, then outside clearance of the sampler is _______.
The type of soil sample collected using wash boring technique is __________.
