Consider the following statements : 1. Undisturbed samples may be obtained with the help of augers. 2. Auger drilling is most effective in clayey soils. 3. Hollow stem augers are sometimes used to drill holes in silty sand. Which of these statements are correct ?
2 and 3 only
Statement Analysis Auger Drilling Techniques
This question asks to evaluate three statements regarding the use and effectiveness of augers in soil sampling and drilling. Let's break down each statement:
Statement 1: Undisturbed samples may be obtained with the help of augers.
Augers are primarily used for boring holes and collecting soil samples. However, the process of drilling with an auger, especially continuous flight augers, tends to disturb the soil structure significantly. While augers can help advance a borehole, obtaining truly undisturbed samples usually requires specific sampling tools like thin-walled samplers (e.g., Shelby tubes) or piston samplers, which are designed to minimize disturbance. Therefore, this statement is generally considered incorrect as augers are mainly associated with disturbed sampling.
Statement 2: Auger drilling is most effective in clayey soils.
Augers, particularly continuous flight augers, work by rotating and screwing into the ground. They are very effective in cohesive soils like clays, where they can efficiently cut and lift the soil. They are also useful in softer, less abrasive granular soils. While other methods might be needed for very hard soils or rock, auger drilling performs well in clayey formations. Thus, this statement is considered correct.
Statement 3: Hollow stem augers are sometimes used to drill holes in silty sand.
Hollow stem augers (HSAs) are widely used in geotechnical investigations. They consist of hollow tubes with internal spiral flights. As the auger rotates and penetrates the soil, it acts like a casing, helping to keep the borehole stable, especially in formations that might otherwise collapse, such as silty sands or loose sands. This stability is crucial for subsequent operations like sampling or monitoring well installation. Therefore, this statement is also considered correct.
Conclusion on Auger Sampling Statements
Based on the analysis:
- Statement 1 is incorrect because augers typically yield disturbed samples.
- Statement 2 is correct as auger drilling is effective in clayey soils.
- Statement 3 is correct because hollow stem augers are suitable for drilling in silty sand, providing borehole stability.
Therefore, the correct statements are 2 and 3.
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
Shelby tube is one of the most widely used devices for:
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 _______.
