Humans have been probing the ground since they could hold sticks, but only in more recent times have their probes been scientific. Researchers interested in studying soil stratification probed the ground with pointed rods during the early 1900s. A more refined instrument called a cone penetrometer was invented in the Netherlands during the mid 1930s to measure soil strength.

Cone penetrometers measure soil penetration resistance or soil strength encountered at various depths as a coneshaped object is pushed steadily into the ground. Advances in electronics and software have expanded penetrometer functions to include such features as moisture and temperature sensors, soil structure analysis, video, and soil composition analysis. These high-tech penetrometers are used mainly in environmental and geotechnical site investigation. They are large instruments, usually mounted on heavy-duty truck frames or tracked vehicles (figure 1). The cones are driven into the ground at consistent speeds to great depths.

Figure 1—This track-mounted, hydraulically driven soil
penetrometer, manufactured by AMS, Inc., is designed
for environmental and geotechnical site-investigation studies.
(Photo courtesy of AMS, Inc.)

Because these machines are too large and expensive for simple field applications, the Missoula Technology and Development Center (MTDC) evaluated three handheld electronic cone penetrometers. Until recently, similar penetrometers were comprised of a dial indicator with a stress ring connected to a round rod with scaled depth markings. The rod has a cone-shaped tip at one end and a handle for pushing the cone into the ground on the other end (figure 2). Using these penetrometers was a cumbersome operation best completed by two people; one to push the penetrometer into the ground and read the force on the dial indicator, and the other to record probe depth and force.

Figure 2—Mechanical cone penetrometers have
been used for decades. A dial indicator mounted
inside a metal stress ring shows the force exerted
on the handle of this vintage penetrometer.

The hand-held electronic cone penetrometers evaluated by MTDC (figure 3) can be operated by a single person. Each model electronically records the force required to push the probe into the ground and depth reading for computer download and analysis. As the probe is pushed into the ground, the force recorded by the electronic load cell is used to calculate the cone index, a number derived from the frictional forces on the cone's surface as it is pushed into the ground. The cone index is a relative indicator of the soil's strength, typically recorded in kilopascals or pounds per square inch.

Figure 3—The three hand-held electronic cone penetrometers tested by MTDC were
the Rimik CP40 (left); the Eijkelkamp Penetrologger (center); and the Spectrum
Field Scout SC–900 (right).

The penetrometer is able to calculate the probe's depth by determining the time it takes to bounce a signal from an ultrasonic transducer off a metal target on the ground, and back to the transducer (figure 4). A datalogger records the soil's strength and the probe's depth. For accurate readings, the penetrometer must be inserted into the ground at a steady speed of about 30 mm/s.

Figures 4—The operating components of a typical hand-held
cone penetrometer. The metal target bounces a signal back to
the transducer, allowing the penetrometer to calculate the
probe's depth. These photos show the Rimik CP40.