Environmental Engineering Reference
In-Depth Information
slide across the road on tracks via a rack-and-pinion drive mechanism. Swing
gates are hinged so they can swing horizontally across the road. Installation
of the cantilever, linear, or swing gate designs involves the excavation of a
pit approximately 48 inches deep for both the hinge and receiver footings to
which the gates are attached. Again, due to the depth of excavation, the site
should be inspected for underground utilities before digging begins.
Bollards
are vertical barriers at least 3 feet tall and 1 to 2 feet in diameter that
are typically set 4 to 5 feet apart from each other so they block vehicles from
passing between them. Bollards can be fixed in place, removable, or retract-
able. Fixed and removable bollards are passive barriers that are typically used
along building perimeters or on sidewalks to prevent vehicle access while
still allowing pedestrians to pass through them. In contrast to passive bol-
lards, retractable bollards are active security barriers that can easily be raised
and lowered to allow vehicles to pass between them; thus, they can be used
in driveways or on roads to control vehicular access. When the bollards are
raised, they project above the road surface and block the roadway; when they
are lowered, they sit flush with the road surface and allow traffic to pass over
them. Retractable bollards are typically constructed from steel or other mate-
rials that have a low weight-to-volume ratio so they require low power to raise
and lower. Steel is also more resistant to breaking than is a more brittle mate-
rial such as concrete, and it is better able to withstand direct vehicular impact
without breaking apart. Retractable bollards are installed in a trench dug
across a roadway, typically at an entrance or a gate. Installing retractable bol-
lards requires preparing the road surface. Depending on the vendor, bollards
can be installed either in a continuous slab of concrete or in individual exca-
vations with concrete poured in place. The required excavation for a bollard is
typically slightly wider and slightly deeper than the bollard height when it is
extended above ground. The bottom of the excavation is typically lined with
gravel to allow drainage. The bollards are then connected to a control panel
that controls the raising and lowering of the bollards. Installation typically
requires mechanical, electrical, and concrete work; if utility personnel with
these skills are available, then the utility can install the bollards themselves.
Portable or removable barricades
, which can include removable crash beams
and wedge barriers, are mobile obstacles that can be moved in and out of posi-
tion on a roadway; for example, a crash beam may be completely removed
and stored off-site when it is not needed. An additional example would be
wedge barriers that are equipped with wheels that can be removed after the
barricade is towed into place.
When portable barricades are needed, they can be moved into position
rapidly. To provide them with added strength and stability, they are typically
anchored to buttress boxes that are located on either side of the road. These
buttress boxes, which may or may not be permanent, are usually filled with
sand, water, cement, gravel, or concrete to make them heavy and aid in sta-
bilizing the portable barrier. In addition, these buttresses can help dissipate
any impact energy from vehicles crashing into the barrier itself.
