Simple Tool for Checking Trench Re-instatement & Soil Strength
The Clegg Impact Soil Tester type CIST/882
manufactured by SD Instrumentation (SDi) provides
a means for measuring and controlling soil strength
and consolidation levels during trench reinstatement.
It is also used to confirm uniform
compaction over wide areas of ground, identifying
poorly compacted areas and ineffective rolling of
materials. A special lightweight version of the
Tester is also available for checking sports turfs
whilst a heavy duty version is supplied for testing
harder materials and roadworks.
The New Roads and Street Works Act 1991 sets out a
Specification for the Reinstatement of Openings in
Highways, creating the need for a tool to check the
performance of trench filling operations and
compaction levels of materials, and imposing
penalties on undertakers who fail to meet the
specified standards. The Clegg Impact Soil Tester
provides a compact and reliable instrument for
monitoring and controlling the quality of
reinstatements while the work is in progress,
avoiding the need for re-excavation should later tests
reveal poor workmanship.
A simple test procedure was introduced by British
Gas following extensive trials of the Clegg Impact Soil Tester in the field and in their engineering
research laboratories at Killingworth. Several thousands of the instrument are now being used throughout
the United Kingdom and overseas, with operators employing these recommended test routines. Our
website explains the test routine.
The Tester consists of a 4.5 kg compaction hammer operating within a vertical guide
tube. When the hammer is released from a fixed height, it falls through the tube and
strikes the surface under test, decelerating at a rate determined by the stiffness of the
material within the region of impact. A precision accelerometer mounted on the
hammer generates an electrical charge which is fed by a cable to a hand-held digital
readout unit. The fast microcontroller in the readout scans and filters the signal and
registers the deceleration in units of Impact Value (IV). The Impact Value is an
indication of soil strength and shows good correlation with results from CBR tests.
Data from the Impact Soil Tester can therefore be used in a similar manner to results
from CBR tests performed in the laboratory and in the field. The Tester is powered by
a single 9V PP3 battery accessible in a pull-out drawer on the side of the readout unit.
The Tester has been approved by the American Society for Testing and Materials and
a Standard, under Designation No D 5874 was issued in 1995, entitled “Standard Test
Method for Determination of the Impact Value (IV) of a Soil”. A copy of this standard
is available on request.
CIST/882 HAND-HELD READOUT UNIT
The CIST/882 Readout Unit displays readings of Impact Value
(IV) and also shows the number of times that the hammer has
been dropped during each test operation.
To carry out a test, the ground surface is brushed lightly with
the foot to remove loose material and the guide tube is placed in
position. With the digital readout switched on and held in one
hand, the hammer is raised to a height of 450 mm as indicated
on the hammer head and then allowed to fall freely. This
procedure is repeated four times, the reading achieved on the
final drop being recorded as the Impact Value.
Tables of target IV for a range of backfill and subgrade
materials are given in the operating manual, enabling the
operator to check the compliance of reinstatements with the
20 Kg HEAVY CLEGG IMPACT SOIL TESTER
This model is designed for use on
harder materials such as those employed in roadworks.
Here the tester may be used in obtaining data for
estimating the traffic carrying potential of lightly
surfaced low traffic roads. Technical Note No 3 (Aug
97), available from SDi, reports the results of field tests
to evaluate the tester for
estimating Benkelman Beam
Deflection and the Surface
Modulus of flexible pavements
(Clegg Hammer Modulus, CHM).
The hammer has a mass of 20 Kg and a diameter of
130 mm. To simplify
carriage and handling, the
tester guide tube is fitted
with wheels, making it
into a pull-along cart, and
the readout is fixed to the