Utility Openings for New Construction - Low- to High-Severity
Overview. Utility openings, manholes, tunnels, air conditioning
ducts, filters, or equipment access panels can provide intruders with an
attractive entrance or exit route. If possible, such openings should be
eliminated, or if necessary, kept below the man-passable size of 96 square
inches (0.06 sq m). The following briefly describes typical utility openings
and design options for hardening them when they can not be eliminated, or
must be larger than 96 square inches (0.06 sq m).
New Construction Design Considerations. Methods for hardening
electrical system conduits, mechanical system conduits for air conditioning,
heating, and venting systems, roof-mounted equipment, filter banks, manholes,
and other openings are discussed in this section.
Electrical and Mechanical Conduits. These consist typically
of sewers, manholes, pipe chases, and sleeves and trays.
a) Sewers/manholes. If possible, providing a cluster of
pipes, each less than 96 square inches (0.06 sq m) in cross section is more
desirable than one large pipe. If a large pipe is required, it is important
to ensure that structure walls, floors, or foundations which are accessible
from such underground routes provide the required penetration time against
penetration attempts. Furthermore, actual entry ports from the sewers to the
structure should, if possible, be constricted to make their expansion into a
man-passable opening very difficult and time-consuming. Obviously, an
effective intrusion detection system capable of detecting pre-intrusion
activities (e.g., a seismic system to detect digging) is very important for
i) Hardening against low- and medium-level threats.
Single (or multiple) fixed grills No. 4 (12.7 mm) or smaller bars and with
spacings of 3 inches (75 mm) or more can be installed in sewers to provide
the penetration times against hand-held tools as shown in Table 34. Figure
17 shows an example of a grill for concrete culvert pipe (see
MIL-HDBK-1013/10 for details). Although locked manhole covers may discourage
a less-dedicated intruder, typical fastening devices offer little penetration
delay times (<1 minute) against a dedicated threat. Manholes should not be
accessible to a potential intruder.
ii) Hardening against high-level threats. Single
(or multiple) fixed grills with at least No. 5 (15.9-mm) bars and with
spacings as shown in Table 34 provide the penetration times indicated.
b) Pipe chases. Pipe chases are horizontal or vertical
framed-in passageways that may be as small as 1-foot square (305-mm square)
to any desired size. They are typically constructed of studs and gypsum