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

Chapter 3 IN-SITU GEOTECHNICAL TESTING
Table
3.3 Diagnostic Features of Soil Type
Soil type
Organic
soil
Normally consolidated clay
Sand
Gravel
Cone resistance
Low
Low
High
Very high
Friction ratio
Very high
High
Low
Low
Excess pore
pressure
Low
High
Zero
Zero
Useful relationships between angle
of shearing
resistance and cone resistance, qc, can be found in
Schmertmann (1978), and Durgunoglu and Mitchell (1975). A correlation between qc and SPT N,
based on
particle size, is shown in Fig. 3.11.
Figure
3.11 Ratio of (CPT Qc) (SPT N) As a Function Of D50 Particle Size Of The Soil (Thorburn,
1971).
Well-known methods of predicting
the settlement of shallow footings (de Beer and Martens 1957;
Schmertmann 1970; Schmertmann et al. 1978) use cone resistance directly. For example,
Schmertmann et al. ( 1978) use E = 2.5 q c . Such relationships, although of great practical value, are
known to
be of limited accuracy. This is to be expected, because the CPT
test involves the continual
failure of
soil around the cone, and cone resistance is a measure of the
strength of the soil, rather
than its compressibilit
ty.
It has been shown (Lambrechts and Leonards 1978) that while the compressibility of granular soil is
very significantly affected by over-consolidation, strength is
not. This shortcoming is
shared by the
SPT. However, settlements of spread footings predicted using the CPT tends to be considerably more
accurate than those using the SPT,
because there is no borehole disturbance.
3-14
March 2009