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

Chapter 3 IN-SITU GEOTECHNICAL TESTING
10
8
Cu/N (kN/m 2 )
6
4
2
0
0 10 20 30 40 50 60 70
PI %
Boulder clay
Laminated clay
Sunnybrook till
London clay
Bracklesham bods
Oxford clay
Kimmeridge clay
Woolwich and Reading clay
Upper Lias clay
Keuper marl
Flints
Figure 3.3 Ratio of Undrained Shear Strength (Cu) Determined On 100mm Diameter.
Specimens to SPT N, As a Function of Plasticity (Stroud 1974).
A comparison between advantages and disadvantages of SPT is summarised in Table 3.1 as follows:
Table 3.1 Comparison between Advantages and Disadvantages of SPT
Advantages
Simple and rugged
Suitable in many soil types
Can perform in weak rocks
Easily available
Disadvantages
Disturbed sample (index tests only)
Crude number for analysis
Not applicable in soft clays and silts
High variability and uncertainty
3.1.1 Correction Factors for Spt
In recent years, it has become a practice to adjust the N valule of SPT test by a hammer-energy
ratio or hammer efficiency of 60% and much attention has been given to N values because of its
use in liquefaction studies. Geotechnical foundation practice and engineering usage based on SPT
correlations have been developed on the basis of standard-of-practice corresponding to an average
ER = 60 %. Normally the correction factor used for SPT tests N values is
Where
(N 1 ) 60 = N.C N .C E (3.1)
(N 1 ) 60 = Corrected N Value
N =
SPT N count obtained from Testing
3-4 March 2009