Compaction is obtained by coating the surfaces of the
soil particles with a thin, molecular film of water to
provide lubrication during our compaction effort. This is
the only moisture we wish to measure with the tester.
Unfortunately, however, some soils have other forms of
water known as "bound water" or water of hydration,
which bind themselves into the mineral matrix and
become a part of the soil particle. This form of water
does not contribute to the compactive effort, but is
measured by the neutron channel in the soil tester.
A simple corrective technique can be used to cancel this
error when detected.
The soil tester will always indicate a higher than actual
moisture value when this error exists.
It will be advisable to make periodic checks with
conventional testing means to verify that the problem
continues to exist in a given embankment.
An infrequent form of error is occasionally observed in
desert countries where a high deposit of boron may
result in absorption of the moderated neutrons producing
a low moisture reading. This is highly unusual. A
compensating calibration curve must be prepared using
oven samples for this occurrence.
1-30. MOISTURE REFLECTION ERRORS
Known as "trench wall effect", this error is the result of
reflection of moderated neutrons from the surface of a
nearby object to the tester, usually a trench wall or a
Knowledge of how the tester is constructed will minimize
this error and a simple test can be conducted to
determine the magnitude of this error in the soil tester.
The moisture detector is a long, 2 inch diameter tube
lying along the long axis of the tester under the display
area. Its most sensitive side is the side presented to the
exterior of the tester on the display side.
The least sensitive area is the ends of the tube and the
side of the tester facing the operator as he views the
It is proper procedure to place the tester so the
broadside of the tube "looks down" the trench rather than
at the wall.
Conversely, the geiger tube used for counting is located
at the end of the tester furthest from the source and lies
across that end. It would be best that this end be placed
looking away from a wall for density tests.
Thus, good practice in a confined space would be to face
the tester longitudinally in the direction of the trench for
density testing and across the trench for moisture
The operator can make a simple experiment with his
nuclear tester to determine the effect of the wall
Set the tester in the center of a concrete floor, at least
ten feet from nearby objects. Determine a moisture
reading on that spot.
Fill a two cubic foot box with damp sand or soil and
slowly move this box towards the tester on each of the
four sides, about 6-inches at a time, taking readings
each time. Note where the box is when the readings
produce an error sufficient to cause an unacceptable
field density conclusion.
This distance from the tester will differ on the four sides,
and will differ between density and moisture.
In general, the tester can be used in a trench of not less
than 18-inches span, preferably about 24-inches wide,
for best results on moisture. The density readings can
be taken with the tester nosed directly into the wall, but
the geiger tube end must be kept at least six inches from
Conduct this experiment with your testers to determine
your own limits in your shop.
1-31. ASPHALT CONTENT DETERMINATION
The testers moisture channel is a hydrogen analyzer.