Undisturbed and Remoulded Shear Strength in Cohesive Soil

Undisturbed shear strength is the shear strength of a soil sample that has not been disturbed from its in situ state. Remoulded shear strength, on the other hand, is the shear strength of a soil sample that has been disturbed and remoulded in a laboratory.

Ratio of Undisturbed Shear Strength to Remoulded Shear Strength

The ratio of undisturbed shear strength to remoulded shear strength is an important parameter in geotechnical engineering. It is denoted by the symbol Su/Sr. The value of Su/Sr depends on the type of soil and the degree of disturbance. In general, the value of Su/Sr is greater than 1 for cohesive soils under undrained conditions.

Explanation

When a soil sample is disturbed and remoulded in a laboratory, its structure and fabric are altered. This results in a decrease in the shear strength of the soil. The remoulded soil has fewer particle contacts and a higher water content, which leads to a lower shear strength. Therefore, the remoulded shear strength is always lower than the undisturbed shear strength.

In cohesive soils under undrained conditions, the pore water pressure cannot dissipate quickly enough due to the low permeability of the soil. This results in an increase in the effective stress, which in turn increases the shear strength of the soil. Since the undisturbed soil has a well-developed structure and fabric, it has a higher shear strength than the remoulded soil. This results in a Su/Sr ratio greater than 1.

Conclusion

In summary, the ratio of undisturbed shear strength to remoulded shear strength in cohesive soils under undrained conditions is greater than 1. This is because the undisturbed soil has a well-developed structure and fabric, which leads to a higher shear strength compared to the remoulded soil.