Building Design and Construction Handbook - Merritt - Ventech!

SOIL MECHANICS AND FOUNDATIONS 6.17
Besides gravel and groundwater conditions described above, there are many
different testing factors that can influence the accuracy of the SPT readings. For
example, the measured SPT N value could be influenced by the hammer efficiency,
rate at which the blows are applied, borehole diameter, and rod lengths. The following
equation is used to compensate for these testing factors (A. W. Skempton,
‘‘Standard Penetration Test Procedures,’’ Geotechnique 36):
N � 1.67 ECCN (6.3)
60 m b r
where N 60 � SPT N value corrected for field testing procedures.
E m � hammer efficiency (for U.S. equipment, E m equals 0.6 for a safety
hammer and E m equals 0.45 for a donut hammer)
C b � borehole diameter correction (C b � 1.0 for boreholes of 65 to 115
mm (2.5 to 4.5 in) diameter, 1.05 for 150-mm diameter (5.9-in), and
1.15 for 200-mm (7.9-in) diameter hole)
C r � Rod length correction (C r � 0.75 for up to 4 m (13 ft) of drill rods,
0.85 for 4 to 6 m (13 to 20 ft) of drill rods, 0.95 for 6 to 10 m (20
to 33 ft) of drill rods, and 1.00 for drill rods in excess of 10 m (33
ft)
N � measured SPT N value
Even with the limitations and all of the corrections that must be applied to the
measured SPT N value, the Standard Penetration Test is probably the most widely
used field test in the United States. This is because it is relatively easy to use, the
test is economical as compared to other types of field testing, and the SPT equipment
can be quickly adapted and included as part of almost any type of drilling
rig.
Cone Penetration Test (CPT). The idea for the Cone Penetration Test (CPT) is
similar to that for the Standard Penetration Test, except that instead of a thickwalled
sampler being driven into the soil, a steel cone is pushed into the soil. There
are many different types of cone penetration devices, such as the mechanical cone,
mechanical-friction cone, electric cone, and piezocone. The simplest type of cone
is shown in Fig. 6.2. The cone is first pushed into the soil to the desired depth
(initial position) and then a force is applied to the inner rods that moves the cone
downward into the extended position. The force required to move the cone into the
extended position (Fig. 6.2) divided by the horizontally projected area of the cone
is defined as the cone resistance (q c). By continual repetition of the two-step process
shown in Fig. 6.2, the cone resistance data is obtained at increments of depth. A
continuous record of the cone resistance versus depth can be obtained by using the
electric cone, where the cone is pushed into the soil at a rate of 10 to 20 mm/sec
(2 to 4 ft/min). Figure 6.3 presents four simplified examples of cone resistance
(q c) versus depth profiles and the possible interpretation of the soil types and conditions.
A major advantage of the Cone Penetration Test is that by use of the electric
cone, a continuous subsurface record of the cone resistance (q c) can be obtained.
This is in contrast to the Standard Penetration Test, which obtains data at intervals
in the soil deposit. Disadvantages of the Cone Penetration Test are that soil samples
can not be recovered and special equipment is required to produce a steady and
slow penetration of the cone. Unlike the SPT, the ability to obtain a steady and
slow penetration of the cone is not included as part of conventional drilling rigs.
Because of these factors, in the United States the CPT is used less frequently than
the SPT.