10 January 2002
HEAT DISTRIBUTION SYSTEMS IN CONCRETE TRENCHES
4-1. GENERAL. The concrete shallow trench (CST) is a system, which allows insulated carrier pipes to
be routed underground but yet not have the piping in contact with the soil. The system also provides
comparatively easy access for maintenance and repair by means of removable concrete tops. These
exposed tops can be used as sidewalks since the system is installed at grade.
4-2. SYSTEM DESIGN. The designer is responsible for conducting a site investigation, designing the
distribution system plan and profiles, and designing the valve manholes, as described in chapter 3 of this
manual. In addition, the designer will use details and descriptions presented here to design a concrete
shallow trench system at a particular site.
a. Trench Systems - General.
(1) Piping and fittings. All carrier piping and pipe fittings will be carbon steel and will be designed
to satisfy the temperature and pressure requirements of the system. Materials will conform
requirements in the guide specification.
(2) Pipe supports. Pipe supports will typically consist of three types: free, guided, and anchor.
These supports are detailed in figure 4-1, figure 4-2, and figure 4-3. All of these pipe supports will be
mounted on channel supports mounted to the trench walls as detailed in figure 4-4. Supports may be
mounted by other means, such as on concrete pedestals, provided that paths for water flow are
maintained. Table 4-1 provides guidance for the sizing of the channel supports. Note that the channels
for anchor supports in the table are designed for more substantial loads than required for free and guided
supports. The anchor support channel in this table is adequate for approximately 1,000 pounds force
axially. The designer will determine axial force requirements at all anchor points and design the channels
to accommodate these forces.
\ Maximum distance between support channels shall be as listed in
table 4-2. /1/ These spacings are applicable to long, straight runs of piping only and must be reduced at
elbows, vertical risers, valving, and equipment. The actual spacings, in these instances, will be
determined by analyzing the pipe stresses with the pipe stress analysis, as described in chapter 3 of this
manual. The designer should note that there are other types of supports (such as roller type) that may
also be used. However, all supports must be capable of withstanding the thermal stresses and forces
exerted on them.
(3) Clearances. Clearances in the trench will be adequate to provide room for expansion,
movement, and a sufficient amount of access for cleaning and maintenance. There must also be
minimum of 4 inches clearance under the support channels to ensure ground water drainage along
trench floor. Figure 4-5 provides a trench cross-section, which corresponds to the table 4-3 trench
dimension schedule, which must be filled in by the designer.
(4) Insulation and jacketing. Insulation will be selected in accordance with insulation thickness
tables in the guide specification. These insulation thicknesses were developed using a life cycle cost
analysis. All insulations used have passed the Federal Agency Committee's boiling test
and are listed
in the guide specifications. All insulation in the trench will be covered with jacketing material in
conformance to the guide specification.
b. Trench System - Structural. The concrete trench will be field constructed of reinforced concrete
conforming to the current criteria. Trench walls and floors will be poured in place-they will not be precast.
Trench tops will be poured in place or precast. Walls, floors, and tops will be constructed with 4,000 psi
minimum compressive strength concrete. Reinforcing bars will conform to ASTM A 615, grade 60. Wall,