OCR: Understanding Over Consolidation Ratio in Geotechnical Design

Geotechnical design is an important aspect of civil engineering that deals with the analysis and design of structures that interact with the ground. One of the critical parameters that play a crucial role in geotechnical design is the Over Consolidation Ratio (OCR). It is the ratio of the maximum effective vertical stress that a soil has experienced in the past to its current effective vertical stress. OCR values are used to understand the past loading history and current state of stress of the soil, which is essential in predicting soil behaviour and stability analyses in geotechnical design.

Oedometer

There are various methods to determine OCR values, the most common being the consolidation test. The test involves applying incremental loads to a soil sample and measuring its settlement over time, which can be conducted by performing oedometer tests and the Rowe cell tests. The oedometer test is widely used for fine-grained soils, while the Rowe cell test is suitable for both fine-grained and coarse-grained soils. Below is an example of our Automated Oedometer system, as well as a Rowe Cell Setup.

CRS (Constant Rate of Strain)

Another commonly used test to study OCR is the Constant Rate of Strain (CRS) test. The CRS test involves subjecting a soil sample to a constant rate of deformation or strain, while measuring the corresponding change in stress. The test is performed using a Load frame, Pressure Controller, and enclosed Pressure Chamber, where the sample is enclosed in a cylindrical cell and loaded with an axial pressure. The CRS test is particularly useful for studying the stress-strain behaviour of soils that are subjected to cyclic or repeated loading, such as those found in earthquake-prone regions. A picture of a typical VJ Tech CRS Setup can be seen below (note: the Multi-Purpose Cell can also be used for SWCC testing).

During the CRS test, data is collected on the stress-strain behaviour of the soil sample, including the initial modulus of elasticity, shear strength, and deformation characteristics such as strain-hardening and strain-softening behaviour. These data can be used to calculate the OCR value using the same formula as the consolidation test method.

OCR values can range from less than 1 (normally consolidated soil) to greater than 1 (over-consolidated soil). Factors such as the history of past loads, duration of load application, and soil composition can affect OCR values. Interpreting OCR values is important in understanding soil behaviour, as over-consolidated soils tend to exhibit lower compressibility and higher shear strength compared to normally consolidated soils. Hence, OCR values play a significant role in predicting soil behaviour and stability analyses in geotechnical design.

Ignoring the OCR value can lead to inaccurate predictions of soil behaviour and potentially result in design failures. Therefore, considering OCR in geotechnical design and construction is essential for accurate soil behaviour predictions and ensuring the safety and reliability of structures. Over-consolidated soils with high OCR values are less susceptible to settlement and can exhibit higher shear strength, which can impact foundation design, slope stability analysis, and overall geotechnical design.

In conclusion, understanding OCR and over-consolidation ratio is crucial in geotechnical engineering for predicting soil behaviour, settlement, and stability analyses. OCR values are determined using various tests such as consolidation and CRS tests, and interpreting OCR values is essential in geotechnical design. Considering OCR values in geotechnical design and construction is necessary for accurate soil behaviour predictions and ensuring the safety and reliability of structures.