SPT-CPT CORRELATIONS

In the United States, the Standard Penetration Test (SPT) is used extensively. On the other hand, the Cone Penetration Test (CPT) is popular in Europe. A standard cone has a base area of 10sq cm and an apex angle of 60 ~ . European countries have developed many geotechnical engineering correlations using CPT data. Fig. 1.25 shows a standard CPT device.

Standard CPT device

The correlation between SPT and CPT shown in Table can be used to convert CPT values to SPT numbers or vice versa. Qc = CPT value measured in bars.

where

1 bar = 100 kPa

N = SPT value

D50 = size of the sieve that would pass 50% of the soil

Design Example 

SPT tests were done on a sandy silt with a D50 value of 0.05 mm. The average SPT (N) value for this soil is 12. Find the CPT value.

Solution

From Table 1.4, for sandy silt with a D50 value of 0.05 mm

Qc/N =3.0

N=12

Hance

Qc = 3 x 12 = 36 bars = 3,600 kPa