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: Identifying the Core Concept:
The question asks for the classification of a clay based on its stress history, specifically comparing its historical maximum pressure (\(\sigma'_{p}\)) with its current pressure (\(\sigma'_{o}\)).
Step 2: Defining Soil Categories based on Stress History:
- Normally Consolidated Clay (NCC): Soil that is currently experiencing the maximum effective stress it has ever encountered in its history. In this case, \(\sigma'_{p} = \sigma'_{o}\), and the Over Consolidation Ratio (OCR) is 1.
- Over Consolidated Clay (OCC): Soil that has been subjected to a higher effective stress at some point in the past compared to its current state. This happens if overlying material was eroded or if water tables shifted. Here, \(\sigma'_{p}>\sigma'_{o}\), and the OCR is greater than 1.
- Lightly Consolidated Clay (LCC): A specific type of OCC where the OCR is only slightly above 1. While this describes the magnitude, the general classification remains Over Consolidated.
- Quick Clay: A sensitive clay characterized by a sudden loss of strength when disturbed; this classification is related to sensitivity rather than the \(\sigma'_{p}\) vs \(\sigma'_{o}\) ratio.
Step 3: Conclusion:
The condition provided is \(\sigma'_{p}>\sigma'_{o}\). This indicates the soil has "memory" of a higher past stress, which is the definition of an Over Consolidated Clay (OCC).