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

Chapter 5 INTERPRETATION OF SOIL PROPERTIES
The cone penetrometer can be considered a miniature pile foundation and the measured tip stress
(q T ) represented the actual end bearing resistance (q b ). In bearing capacity calculations, the pile end
bearing is obtained from limit plasticity theory that indicates: q b = N q. σ vo ' where N q is a bearing
capacity factor for surcharge and depends upon the friction angle. Thus, one popular method of
interpreting CPT results in sand is to invert the expression (N q = q T /σ vo ') to obtain the value of φ'
(e.g., Robertson & Campanella, 1983). One method for evaluating the peak φ’ of clean quartz sands
from normalized CPT tip stresses is presented in Fig. 5.11.
Figure 5.11 Peak Friction Angle Of Un-Aged Clean Quartz Sands From Normalized CPT Tip
Resistance. (Calibration Chamber Data Compiled By Robertson & Campanella, 1983).
7.4.2 Pre-consolidation Stress of Clays
The effective preconsolidation stress σ p ', is an important parameter that governs the strength,
stiffness, geostatic lateral stress state, and porewater pressure response of soils. It is best
determined from one-dimensional oedometer tests (consolidation tests) on high-quality tube samples
of the soil. Sampling disturbance, extrusion, and handling effects tend o reduce the magnitude of σ p '
from the actual in-place value. The normalised form is termed the overconsolidation ratio (OCR) and
defined by:
OCR = σ p ’/σ vo ’ (5.14)
Soils are often over-consolidated to some degree because they are old in geologic time scales and
have undergone many changes. Mechanisms causing over-consolidation include erosion, desiccation,
groundwater fluctuations, aging, freeze-thaw cycles, wet-dry cycles, glaciation, and cementation.
March 2009 5-13