Pile Design and Construction Practice, Fifth edition

504 Ground investigations, contracts and pile testing

experience gained in these applications the test is particularly useful for piling investigations.
A correlation has been established between the static cone resistance and the angle of shearing
resistance of coarse-grained soils (see Figure 4.11). It also gives information on the resist-
ance to the driving of piles over the full depth to the design penetration level. The standard
mechanical (‘Dutch’) cone as developed by the Soil Mechanics Laboratory at Delft is shown
in Figure 11.3a. This cone is generally used in conjunction with mechanically operated
penetration devices which measure the thrust on the cone and on the sleeve separately by
means of hydraulic cylinders mounted on the machines. The electric cone developed
by Fugro shown in Figure 11.3b has electrical-resistance strain gauges mounted behind the
cone and inside the sleeve, giving continuous readings of penetration resistance by means
of electrical signals recorded on data loggers at the surface. Research by Meigh(5.20), and
modified by Fugro Engineering Services Ltd for UK soils, shows that the ratio of the local
side friction to cone-end resistance qc(the ‘friction ratio’) can be useful in identifying the
soil type. The cone shown in Figure 11.3b can be used in deep-sea ground investigations. In
this case, the cone together with rods and a drive unit are lowered together to the bottom of
the borehole and take the reaction from the drill string through latches bearing against the
core barrel. The signals from the electrical strain gauges are transmitted by cable to the
recording unit on the drilling vessel. In a comparison of the penetration resistance readings
reported by Joustra(11.3)the cone resistance by the electric cone was 3.3% less than that of
the mechanical cone in sands in Amsterdam.
The continuous dynamic cone penetrationtest is a useful and much neglected method of
logging the stratification of layered soils such as interbedded sands, silts, and clays. The
Borros penetrometer employs a 63 kg hammer impacting on a 50.5 mm cone at a rate of
20 blows per minute. The number of blows required for a penetration of 100 mm is denoted
as n.The torque on the cone is measured to provide an additional means of interpreting the
data. There is very little published information in the UK on correlation between nvalues
and the SPT N-value or qc values from static cone penetration tests. Cearns and
McKenzie(11.4)have published relationships between nand the SPT Nfor sands, gravels, and
chalk as shown in Figure 11.4.
Approximate determinations of the deformation modulus of soil and rocks can be made by
expanding a cylindrical rubber membrane against the walls of the borehole and measuring

Figure 11.3Types of cone for static cone penetration test (a) Mechanical cone (b) Electric cone.

Sleeve

containing

strain

gauges

Mantle

Cone with

base area

of 1000 mm^2

60 °

35.7 mm 35.7 mm

(a) (b)