Volume 6 – Geotechnical Manual, Site Investigation and Engineering ...

Chapter 5 INTERPRETATION OF SOIL PROPERTIES
Figure 5.19 Normalized Undrained Strengths for NC Clay under Different Loading Modes by
Constitutive Model (Ohta, et al., 1985)
Based on extensive experimental data (Ladd, 1991) and critical state soil mechanics (Wroth, 1984),
the ratio (s u /σ vo ') increases with over-consolidation ratio (OCR) according to:
(s u /σ vo ’) OC = (s u /σ vo ’) NC OCR A (5.16)
where A ≈ 1- C S /C C and generally taken to be about 0.8 for unstructured and uncemented soils.
Thus, if a particular shearing mode is required, it can be assessed using either Figs. 5.19 or 5.20 to
obtain the NC value and equation (5-16) to determine the undrained strength for over-consolidated
states. In many situations involving embankment stability analyses and bearing capacity calculations,
the simple shear mode may be considered an average and representative value of the undrained
strength characteristics, as shown by Fig. 5.21 and given by:
(s u /σ vo ’) DSS = ½ sin ’ OCR A (5.17)
March 2009 5-19