Question

A clay deposit is found to have a natural water content and void ratio that corresponds to a pre-consolidation pressure (\(\sigma'_{p}\)) that is greater than the present effective overburden pressure (\(\sigma'_{o}\)). This clay is best classified as:

• A. OCC (Over Consolidated Clay)
• B. LCC (Lightly Consolidated Clay)
• C. NCC (Normally Consolidated Clay)
• D. Quick Clay

Hint:

The key to consolidation questions is the Over Consolidation Ratio (OCR). If OCR>1 (\(\sigma'_{p}>\sigma'_{o}\)), it's Over Consolidated. If OCR = 1 (\(\sigma'_{p} = \sigma'_{o}\)), it's Normally Consolidated. The clay "remembers" the maximum stress it has ever experienced.

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
The question describes a clay deposit where the maximum past effective pressure (pre-consolidation pressure, \(\sigma'_{p}\)) is greater than the current effective pressure it is subjected to (present effective overburden pressure, \(\sigma'_{o}\)). We need to classify this type of clay.
Step 2: Detailed Explanation:
The classification of clay based on its stress history is as follows:
- Normally Consolidated Clay (NCC): A soil deposit that has never been subjected to an effective pressure greater than the current overburden pressure. For NCC, \(\sigma'_{p} = \sigma'_{o}\). The Over Consolidation Ratio (OCR), defined as \(OCR = \frac{\sigma'_{p}}{\sigma'_{o}}\), is equal to 1.
- Over Consolidated Clay (OCC): A soil deposit that has been subjected to a higher effective pressure in the past than its current overburden pressure. This could be due to the removal of overlying soil or glaciers, or desiccation. For OCC, \(\sigma'_{p}>\sigma'_{o}\). The Over Consolidation Ratio (OCR) is greater than 1.
- Lightly Consolidated Clay (LCC): This is a sub-category of OCC where the OCR is slightly greater than 1, typically in the range of 1 to 4. However, it is still fundamentally an Over Consolidated Clay.
- Quick Clay: This is a type of highly sensitive clay that experiences a drastic loss of shear strength upon disturbance. Its classification is based on sensitivity, not directly on the relationship between past and present pressures.
Step 3: Final Answer:
The problem states that \(\sigma'_{p}>\sigma'_{o}\). This is the defining characteristic of an Over Consolidated Clay (OCC). Therefore, the clay is best classified as OCC.