Cone Penetration Test (CPT)The CPT test has become one of the most common and economical methods of

subsurface exploration. The cone penetrometer is pushed into the ground at a

standard velocity of 2 cm/s and data is recorded at regular intervals (typically 2 or 5

cm) during penetration. The results provide excellent stratigraphic detail and

repeatability provided proper care has been taken in calibration of the equipment

(transducers and electronics). The cone penetrometer is instrumented to record a

number of different parameters, with the most common being the force of the tip, the

force of the sleeve, and the pore pressure behind the tip. Cone penetrometers have

also been used to provide or measure electrical properties, shear wave velocities,

visual images of the soil, acoustic emissions, temperature, and water samples.

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This is a relatively small CPT testing rig. If the

truck weight is insufficient to push the cone to

the desired depth, the truck has anchors that

can be screwed into the ground to increase its

reaction force. A hydraulic ram (center of

truck's flatbed) is used to push the 1.4-cm

diameter cone into the ground beneath the

center of the truck.

The conical cone tip has been unscrewed from

the lead rod. This tip is equipped for pore

pressure measurements behind the tip.

Both the friction sleeve and conical tip have

been removed from the lead rod. This rod

contains instrumentation that measures tip

force, sleeve friction force, pore pressures

behind the tip, rod inclination and accelerations

(for shear wave velocity measurements).

CPT penetrating the ground beneath the truck.

Close-up of the hydraulic ram used to push the

cone. A 1- meter section is about to be added to

the top of a previous rod. The ram will then be

raised, grab the new rod and push it down at 2

cm/s. Note that the electronic cable is

pre-threaded through the insides of the rods.

A computer screen shows the data acquisition

results while advancing the cone.

This larger CPT rig has an enclosed cabin. The

pop-up section on the cabin roof provides room

for the hydraulic ram to operate. The assistant is

about to strike a base plate with a sledge

hammer. The hammer impact triggers a high

speed data acquisition system which records the

resulting accelerations at the hammer and the

cone tip. Shear wave velocities versus depth are

calculated using the recordings obtained with

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the cone tip at different depths (typically

1-meter intervals).

A completed hole is being grouted with a

bentonite-cement slurry in accordance with

local regulations. The instrumented cone had

been fully withdrawn and a separate set of rods

were re-inserted to the bottom of the hole. The

rods are withdrawn as grout is poured into the

top. This procedure ensures that the grout fills

the entire hole, avoiding any potential blockage

of the hole by caving soil.

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