The type of soil sample collected using wash boring technique is __________.
zero-representative
Understanding Soil Sample Types in Geotechnical Investigations
Geotechnical investigations involve collecting soil samples to understand the subsurface conditions and properties. Different drilling and sampling techniques yield different types of samples with varying degrees of disturbance and representativeness.
What is Wash Boring?
Wash boring is a drilling method commonly used for advancing boreholes, particularly in softer cohesive soils and granular deposits. It involves driving a casing into the ground and then using a drill rod with a chopping bit to loosen the soil at the bottom of the hole. Simultaneously, water (or drilling mud) is pumped down the drill rod and circulates upwards in the annular space between the rod and the casing, carrying the loosened soil cuttings to the surface. This process effectively 'washes' the soil out of the borehole.
Soil Samples from Wash Boring Technique
In wash boring, obtaining undisturbed or even truly representative samples of the soil layers encountered is difficult or impossible for several reasons:
- The process of washing uses high-velocity fluid circulation, which mixes the soil particles from different depths and washes away finer particles.
- Samples collected are primarily cuttings brought up by the wash water. These cuttings are highly disturbed, lose their original structure, and their moisture content is altered.
- Identifying precise layer boundaries from these mixed cuttings is challenging.
While the cuttings can give an indication of the presence of certain soil types at different depths, they do not preserve the in-situ properties like strength, compressibility, permeability, or accurate stratification.
Representative vs. Undisturbed Samples
- Undisturbed Sample: A sample taken in a manner that preserves the in-situ structure, moisture content, and properties of the soil as much as possible. These are essential for tests like consolidation and shear strength tests. Techniques like using Shelby tubes are designed to obtain undisturbed samples.
- Representative Sample: A sample that reflects the true nature and composition of the soil stratum from which it was taken, even if its structure is disturbed. Split-spoon samples, though disturbed, are often considered representative of the soil composition at a specific depth and are used for classification and index property tests.
The soil cuttings obtained from wash boring are significantly altered from their original state. They are mixed, potentially segregated (fines washed away), and completely lose their in-situ structure and moisture content. Therefore, they are not representative of the soil's engineering properties or precise composition and stratigraphy.
Conclusion on Wash Boring Sample Type
Because the wash boring technique severely disturbs and alters the soil, the sample collected as cuttings is considered zero-representative. It provides minimal reliable information about the actual in-situ soil conditions or properties needed for detailed geotechnical analysis.
Hence, the type of soil sample collected using wash boring technique is zero-representative.
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 _______.
