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23. Probing and Penetration Testing
23.1 General
Probing from the surface probably represents the oldest method of investigating the
depth to a hard layer where the overlying material is weak and not unduly thick. The simplest
probe is a sharpened steel rod which is pushed or driven into the soil until it meets resistance.
The method is still of use where other means of site investigation have disclosed relatively thin
layers of very soft soils overlying much harder soils. In such cases, the thickness of the soft
layer may be determined over a wide area very quickly and economically. The method has
many limitations, and a variety of more sophisticated apparatus has been developed, both in
an attempt to overcome these drawbacks and to extend the use beyond that of detecting a hard
layer, e.g. to give some measure of the allowable bearing capacity of the soils present. Two
distinct types of probe have been developed : one where the probe is driven into the soil by
means of some form of hammer blow; the other where the probe is forced into the soil by a
static load.
23.2 Dynamic Probing
The apparatus for dynamic probing comprises a sectional rod fitted at the end with a
cone whose base is of greater diameter than the rod.
It is driven into the ground by a constant
mass falling through a fixed distance. A device commonly used in Hong Kong is the GCO
Probe (Figure 36 and Plate 10A), which is essentially a larger and heavier version of the
Mackintosh Boring and Prospecting Tool. Probe results are very useful for assessing the depth
and degree of compaction of buried fill, making comparative qualitative assessments of ground
characteristics, and in supplementing the information obtained from trial pits and boreholes.
Probing has also been carried out in the base of hand-dug caissons (Evans et al, 1982). Probe
results are normally reported as the number of blows per 100 mm penetration, as shown in
Figure 37.
As additional rods are added for probing at depth, the driving energy provided to the tip
is attenuated by the additional mass of the rods. Correction of the probe values is sometimes
made to allow for this effect. The correction is negligible at the shallow depths at which many
probings terminate, and it is unnecessary to apply a correction if only qualitative comparisons
between probe results at similar depth are being undertaken.
The fact that the rod and couplers are somewhat smaller in diameter than the base of the
cone prevents, to some extent, shaft friction from influencing the results; however, at depth in
certain soils, this factor should also be considered.
The primary use of dynamic probing is to interpolate data between trial pits or boreholes
rapidly and cheaply. Therefore, probing should first be carried out adjacent to a trial pit or
borehole where ground conditions are known, and then extended to other areas of the site. As
with other types of penetrometers, probing may sometimes be unsuccessful in soils containing
corestones, cobbles or boulders. In fill or completely decomposed rock, the maximum depth
to which a GCO probe can be driven is about 15 m. In order to minimise damage to the
equipment, probing should terminate when the blow count reaches 100, or when the hammer
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bounces and insignificant penetration is achieved.
23.3 Static Probing or Cone Penetration Testing
23.3.1 General Description
Several types of static probing equipment have been developed and are in use throughout
the world (De Ruiter, 1982; Sanglerat, 1972). The basic principles of all systems are similar,
in that a rod is pushed into the ground and the resistance on the tip (cone resistance) is measured
by a mechanical, electrical or hydraulic system. The resistance on a segment of the rod shaft
(friction sleeve resistance) may also be measured. Static probing, or cone penetration testing,
is also known by a number of other descriptive terms, depending on the manufacturer or
operator of the particular device being used.
There is no British Standard for cone penetration testing, but suitable recommendations
are given by the ISSMFE (1977) and the ASTM (1985k). Both of these test standards
recognize a number of traditional types of penetrometers, and it is imperative that the actual
type of instrument used is fully documented, as the interpretation of the results depends on the
equipment used. Two common types of penetrometers, mechanical and electrical, are
described further in Sections 23.3.2 and 23.3.3 respectively.
The reaction required to achieve penetration of the cone may be obtained by screw
anchors, the weight of the thrusting machine, kentledge, or a combination of these. When
cone penetration testing is done in shallow water, the thrusting machine may be secured to a
jack-up platform (see Section 14.7).
23.3.2 Mechanical Cone Penetrometers
Two common mechanical cones, the Dutch mantle cone and the Dutch friction sleeve
cone, are shown in Figure 38 (see also Plate 10B). These cones were developed mostly at the
Delft Soil Mechanics Laboratory in the l930's. With either type, the cone is pushed into the
ground by a series of hollow push rods. With the mantle cone, the force on the cone is then
measured as the cone is pushed downward by means of inner rods inside the push rods. This
force is generally measured at the ground surface by a hydraulic load cell. With the friction
sleeve cone, the same initial measurement is made, and then a second measurement is taken
while the cone and friction sleeve are together pushed downward a further increment. The
friction is calculated by deducting the former reading from the latter. This procedure is
normally repeated at regular depth intervals of 0.2 or 0.25 m.
An alternative quick continuous method of penetration is sometimes used with the
mantle cone. In this method, the cone and push rods are pushed into the ground with the cone
permanently extended and connected to the load cell. Accuracy is reduced in this operation,
however, and the free movement of the cone should be checked at frequent intervals.
For accurate work, the weight of the inner rods should be taken into account in
calculations. In very soft soils when soundings are carried to a significant depth, the weight
of the inner rods may exceed the force on the cone or cone plus jacket; in these circumstances,
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it is impossible to obtain readings. This effect can be reduced by the use of aluminium inner
rods. The inner rods should be free to slide inside the push rods, and the cone, and friction
jacket where used, should be checked for free sliding both at the start and at the end of each
penetration test. All push rods and inner rods should be straight, clean and well-oiled
internally. The accuracy of the load and pressure gauges should be checked periodically by
calibration.
23.3.3 Electrical Cone Penetrometers
A number of types of electrically-operated cone are in use, and these generally
incorporate vibrating wire or impedance strain gauges for measuring the force on the cone and
friction jacket. In use, the cone is advanced at a uniform rate of penetration by pressure on
the top of the push rods, and signals from the load-measuring devices are carried to the surface
by cable threaded through the push rods. Forces on the cone, and friction jacket, can either
be displayed on a readout at the surface or recorded automatically on a chart recorder, punched
tape or magnetic tape. Exclusive recording on punched tape or magnetic tape which does not
allow direct access to the data during or immediately after sounding is not recommended.
Provision should be made for calibration of the force-measuring system at regular intervals,
preferably on site. An inclinometer built into the cone is available with some equipment.
The cones are generally parallel-sided, and the friction jacket, where fitted, is
immediately behind the point, as shown in Figure 39a (see also Plate 10B). However, parallelsided electrical cones do not give exactly the same results as those obtained with the mechanical
cone penetrometer, although the difference is usually of little importance. Electrical cones
with a profile modified to give better agreement with the mechanical cone are also available
(Figure 39b and c).
One particular type of electrical cone penetrometer is the "Brecone", which has a
combined 5 kN and 50 kN force measurement range (Rigden et al, 1982). It has the advantage
of being able to measure cone resistances in clays containing dense sand layers without
suffering damage to the more sensitive load cell.
The recently developed "piezocone", which incorporates a pore pressure transducer
within an electrical cone, has also found application in some Hong Kong marine investigations
(Blacker & Seaman, 1985; Fung et al, 1984; Koutsoftas et al, 1987).
23.3.4 General Recommendations
The following general recommendations apply to cone penetration testing, whether
undertaken with mechanical or electrical cone penetrometers :
(a) The cone cross-sectional area should be 1 000 mm2, and the
cone apex angle should be 60°.
(b) The friction sleeve, if present, should have a surface area of
15 000 mm2.
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(c) The rate of penetration should be 20 ± 5 mm/s.
(d) Force measurements should be accurate to within ±5% of the
maximum force reached in the test.
23.3.5 Uses and Limitations of the Test
The cone penetrometer test is relatively quick to carry out, and inexpensive in
comparison with boring, sampling and laboratory testing. It has traditionally been used to
predict driving resistance, skin friction, and the end bearing capacity of driven piles in granular
soils. In recent years, the cone penetrometer test has also been used to give an indication of
the continuous soil profile by interpretation of the ratio of friction sleeve and cone resistances.
In addition, there is substantial published information relating cone resistance value with other
soil parameters.
The cone penetrometer test is also the preferred substitute for the standard penetration
test in soil conditions where results of the latter test are suspect, and where hard driving is not
anticipated. The test is also commonly used as a rapid and economical means of interpolating
between boreholes. Although it may be possible to estimate the type of soil through which the
cone is passing as described above, it is preferable to carry out the test in conjunction with
some other means of determining the nature of the soil present.
Cone penetration is limited by both the safe load that can be carried by the cone, and the
thrust available for pushing it into the ground. It is also limited by the compressive strength
of the inner rods; some machines are capable of crushing the inner rods before the rated capacity
of the machine is reached. Because of limited cone capacity, penetration normally has to be
terminated where dense sand or gravel, highly to moderately decomposed rock, or cobbles are
encountered. For this reason, cone penetration testing in Hong Kong has been limited to the
Recent alluvial and marine sediments.
23.3.6 Presentation of Results
Results are normally presented graphically with cone resistance (and local skin friction
where a friction jacket cone is used) plotted against depth. The friction ratio, defined as
(friction resistance/cone resistance) x l00, may also be plotted against depth. This ratio is
used to assist in interpreting the soil type penetrated. Suitable scales for plotting the results
are given in ISSMFE (1977).
23.4 Static-dynamic Probing
The standard penetration test is rather insensitive in loose materials and is not truly
relevant to cohesive soils. On the other hand, the cone penetrometer is of limited use when
dense or stiff layers are encountered. The static-dynamic test combines the two methods
(Sherwood & Child, 1974).
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