Material: Recycled
Glass
Issue: Material handling and compaction
procedures usually represent a greater cost in a construction fill operation
than the fill material itself. Engineers,
permitting authorities, and contractors must be familiar with proper
handling methods and compaction characteristics of fill material so
that practical specifications can be written, permits can be issued,
costs can be estimated, and the fill operation can be performed. Because recycled glass is a relatively new fill material for construction
applications, it is especially important to understand its handling
characteristics.
Best
Practice: A
typical fill operation begins with the preparation of the subgrade on
which the fill will be placed and compacted.
The subgrade must be firm and dry so that the risk of subgrade
settlement is minimal and the fill can absorb the compaction energy.
The glass fill may consist of 100% glass cullet, or a cullet-soil or
cullet-gravel mixture. For 100% cullet, the apparent cohesion, which
results from the surface tension of moisture, is low.
This means that the material flows relatively freely.
Therefore, using hoppers for directing flow has been found to
be efficient and successful. However,
some cullet particles are sharp so inflated rubber tires can be punctured
and damaged. Experience has
indicated that solid rubber tires are a better alternative.
The dumped or transported material should be leveled
into a horizontal lift 4 to 12 inches thick.
Generally, the lift thickness should be 4 to 6 inches for manually
operated equipment, and 8 to 12 inches for automatic compaction equipment.
The leveling can be achieved using a bulldozer in open areas, and hand
shovels and rakes in confined areas.
For 100% cullet fill, the material can be moved about relatively
easily using hand tools because of the smooth particle surface and lack
of apparent cohesion.
Vibratory compactors are effective for cullet or cullet
mixtures. In open areas, vibratory
roller compactors weighing two to ten tons can be used. In confined areas, backhoe or trackhoe mounted
hoepacs are effective. Hand-held
equipment such as Jumping Jacks and “Walk-Behind” Rollers can also be
used. The compaction effort
of a plate or “Slick” compactor penetrates only the top two to three
inches, and has been generally found to be ineffective.
The shear strength of cullet fill is proportional to
its confining pressure. At the
top one to two feet of the fill where confining pressure is low, the
material can be rutted by the tires of construction equipment due to
the low shear strength. This
rutting may seem to imply an unstable material, but it should not be
alarming. The surface of cullet fill should always be
covered by asphalt, concrete, natural soil or natural aggregate. The cover will prevent direct skin contact
with glass, and provide stability to the surface. The thickness and type of cover depends on the loading conditions
planned in the fill areas. For
non-loading areas, a three-inch layer of crushed rock has been found
to be adequate in providing stability for foot traffic. The lack of confinement near the surface and the low apparent cohesion
of glass also mean that pieces of glass can fly up during material handling
and compaction. Therefore, personnel
working with the material should follow normal safety rules and precautions.
Under normal conditions, these rules include wearing long sleeve
shirts, gloves and eye protection.
The compaction of cullet fill is relatively insensitive
to moisture content. However,
if the material contains more than ten- percent fines (particles smaller
than No.200 sieve), retained moisture can prevent effective compaction.
If there is a substantial amount of fines in the glass, it is
a good practice to cover the stockpiles with a thick plastic sheet during
wet weather. In dry weather,
water can be applied to the stockpile for dust control.
It is generally not necessary to wet the material to assist in
compaction.
The quality of the compaction can be tested by field
density tests using a nuclear densometer.
The test frequency varies for different applications, but typically
involves one test per 2,500 square feet of fill but not less than one
test per lift. The material
is porous, therefore field density tests using the backscatter mode
will produce lower than actual values and should be avoided when possible.
The direct transmission mode with the source probe extending
the full depth of a lift is a preferred test mode.
For a complete discussion of the use of nuclear densometers with
glass aggregate, see the Density Testing of Glass Aggregate Using a Nuclear Densometer
Best Practice.
Implementation: Special seminars or discussions
with the permitting offices at the city, county, and state should be
held so that the material handling and compaction procedures are understood
and acceptable. Information
should also be disseminated among engineers and contractors so that
the use of cullet fill can be included in project specifications.
Benefits:
Handling
and compaction procedures for cullet fill are similar to those of soil,
gravel, and rock fill. Dissemination
of the best practice information presented here will help engineers,
contractors and permitting authorities to understand that: (1) cullet
fill has been successfully used on construction sites,
(2) cullet fill is a viable alternative to natural materials,
and (3) no special machinery or preparation is required.
Application
Sites: Construction
sites and testing laboratories.
Contact: For more information about this Best Practice, contact
CWC, mailto:info@cwc.org.
References: Case Studies for the Use of Post Consumer
Glass as a Construction Aggregate, CWC, 1997. Soil Mechanics in Engineering
Practice, 2nd ed. John Wiley and Sons, New York. . Shin, C. J., S&EE, Inc., Bellevue, WA
Issue Date / Update: November 1996
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