Understanding Undisturbed Soil Samples and Area Ratio

Obtaining undisturbed soil samples is crucial in geotechnical engineering for accurate laboratory testing. These tests help determine the soil's in-situ properties, such as shear strength, compressibility, and permeability, which are essential for designing foundations, retaining walls, and other structures. An undisturbed sample represents the soil's state in the ground as closely as possible, minimizing changes to its structure, density, and moisture content caused by the sampling process.

What is Area Ratio in Soil Sampling?

The area ratio is a key parameter used to assess the potential disturbance to a soil sample during collection using a sampler. It is defined based on the dimensions of the sampling tube. Specifically, it relates the area of the soil displaced by the walls of the sampler to the area of the sample being collected.

The formula for the area ratio ($\text{A}_r$) is:

$$\text{A}_r = \frac{D_w^2 - D_i^2}{D_i^2} \times 100\%$$

Where:

• $D_w$ is the outer diameter of the sampler cutting edge.
• $D_i$ is the inner diameter of the sampler cutting edge.

A higher area ratio means that the walls of the sampler are relatively thick compared to the sample size. This causes significant soil displacement and compaction (or expansion) as the sampler is pushed into the ground, leading to a high degree of sample disturbance.

Minimizing Disturbance: The Role of Low Area Ratio

To obtain good quality undisturbed soil samples, it is essential to minimize the disturbance caused during the sampling process. A primary factor influencing disturbance is the area ratio of the sampler. A lower area ratio indicates that the sampler walls are thin relative to the inner diameter, resulting in less soil displacement and thus less disturbance to the sample structure.

Geotechnical standards and practices recommend specific limits for the area ratio to classify samples as undisturbed or of good quality. For good quality undisturbed soil samples, the area ratio should be kept as low as possible.

Generally, for obtaining high-quality undisturbed samples, the area ratio is required to be less than a certain percentage. A commonly accepted criterion for relatively undisturbed samples is that the area ratio should be below 10%.

Let's look at the given options:

• greater than 10%: This would indicate a high degree of disturbance.
• greater than 25%: This would indicate a very high degree of disturbance.
• less than 25%: While better than greater than 25%, this range still includes values (like 20%) that might lead to significant disturbance, not necessarily "good quality undisturbed" samples.
• less than 10%: This range is typically specified for samplers designed to collect good quality undisturbed samples, as the low area ratio minimizes disturbance.

Therefore, for obtaining good quality undisturbed soil samples, the area ratio should be less than 10%.

Conclusion

The area ratio is a critical factor in assessing soil sample disturbance. A lower area ratio leads to less displacement of soil and results in a more undisturbed sample. For good quality undisturbed soil samples, the area ratio should be less than 10%.