Humboldt H-4140 GeoGauge - Manual (Page 4)

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1.2: How The GeoGauge ® Works
The GeoGauge measures a material’s
mechanical impedance at the surface of
the ground. In other words, it measures
the force imparted to the soil and the
resulting surface deflection as a function of
frequency. Stiffness, force over deflection,
follows directly from the impedance. The
GeoGauge imparts very small displacements
to the ground (< 1.27 x 10 -6 m or <.00005")
at 25 steady state frequencies between
100 and 196 Hz. Stiffness is determined at
each frequency and the average from 25
frequencies is displayed. The entire process
takes about one minute. At these low
frequencies, the impedance at the surface
is stiffness controlled and is proportional to the shear modulus of the soil. With
Poisson's ratio, shear and modulus can be derived.
The GeoGauge weighs about 10 kg (~ 22 lb), is 28 cm (~ 11”) in diameter,
25.4 cm (~ 10”) tall and rests on the soil surface via a ring–shaped foot (Fig.
2). The foot bears directly on the ground without penetrating it and supports
the weight of the GeoGauge via rubber isolators. Also attached to the foot
are the shaker that drives the foot and sensors that measure the force and
displacement-frequency history
of the foot.
The GeoGauge is placed on the ground
to make a measurement with little or no
preparation of surface. Typically, a slight
rotation of the GeoGauge is needed
obtain the desired 100% contact
between foot and ground (Fig. 3). On
particularly hard or rough surfaces,
seating of the foot is assisted by the use
of less than 10 mm (1/4") of moist/wet
material such as mortar sand. Common
field practice is as applicable to the
GeoGauge as it is to most current field
measurement of material performance. The detailed procedure for using the
GeoGauge and preparing the ground is described later in this guide.
The GeoGauge displays and logs the data in memory with sufficient capacity
for a full day of data gathering (100s of measurements). The data may be
downloaded to a PC for archiving and further analysis. It is powered by 6
disposable and common D-cell batteries. Measurements can be performed as
close to operating construction equipment as operator safety will allow.