Effects of Increasing the Normal Effective Stress
So far, we have only used a single normal effective stress in our presentation of the responses of Type I and Type II soils. What is the effect of increasing the normal effective stress? For Type I soils, the amount of compression and the magnitude of the critical state shear stress will increase (Figure ). For Type II soils, the peak shear stress tends to disappear, the critical shear stress increases, and the change in volume expansion decreases (Figure ). If we were to plot the peak shear stress and the critical state shear stress for each constant normal effective stress for Type I and II soils, we would get:
1. An approximate straight line (OA, Figure ) that links all the critical state shear stress values of Type I and Type II soils. We will call the angle between OA and the s9n axis the critical state friction angle, f9cs. The line OA will be called the failure envelope because any shear stress that lies on it is a critical state shear stress.
2. A curve (OBCA, Figure) that links all peak shear stress values for Type II soils. We will call OBC (the curved part of OBCA) the peak shear stress envelope because any shear stress that lies on it is a peak shear stress.
At large normal effective stresses, the peak shear stress for Type II soils is suppressed, and only a critical state shear stress is observed and appears as a point (point 9) located on OA (Figure). For Type II-A soils, the residual shear stresses would lie on a line OD below OA. We will call the angle between OD and the s9n axis the residual friction angle, f9r. As the normal effective stress increases, the critical void ratio decreases (Figure 10.5d). Thus, the critical void ratio is dependent on the magnitude of the normal effective stress.
Effects of increasing normal effective stresses on the response
Effects of OCR on peak strength
and volume expansion.