Trench filling

It is vital that the thermally resistivity of the soils and other material surrounding the cables is controlled. The details of the trench filling materials for this scoping study are as follows:

The backfill placed above the concrete ductbank shall be of the fluidised thermal backfill type or selected sand/gravel or crushed rock and shall have a dried out thermal resistivity no greater than 0.9 K.m/W. [For information, in the absence of specific local measurements, a value of 0.9 K.m/W has been taken for the backfill in all of the cable ampacity calculations, having been abstracted from a) IEC 60267^[80]for the typical values in Canada, there being no recognised national values and b) the value given in the technical specification^[81] for the recent 240 kV cable installation in Edmonton for native soil resistivity for depths of less than 3.5 m.]

 Where the route crosses agricultural land the top of the thermal backfill must be no less than 900 mm below the surface to avoid disturbance by agricultural equipment (this would require to be verified for Edmonton farming conditions).

 The thermal backfill placed above the concrete ductbank forms part of the thermal design of the circuit; the warning tapes or tiles placed above thermal backfill must extend the whole length and width of the route.

 This layer can be replaced with concrete if more economical.

Trench filling above the thermal backfill and to either side of the trench must have a thermal resistivity of not greater than 0.9 K.m/W^[80,81] and not greater than 3.0 K.m/W^[82] when fully dried out. (The latter values would need to be verified by taking measurements and samples in situ in trial holes dug along the route when selected).

 The indigenous soils must be tested for thermal resistivity and can only be used to fill the trench if it is found to have a thermal resistivity which meets the above requirement. If unsuitable imported material must be allowed for.

 Parts of the route may have very high thermal resistivity, specifically where there is sphagnum moss. In such cases it should be assumed that the ground approximately three metres both side of the trench will have to be removed and replaced with suitable imported material. Sphagnum moss is also unlikely to be mechanically suitable for the supporting of cable trenches.

Local regulations or landowners may have specific requirements for topsoil to be stripped and stored during the course of construction and replaced as part of the final reinstatement. There may be requirements as to the time of year when topsoil can be stripped and the methods of storage.

a a b

Figure 75: Preliminary direct burial arrangement

To achieve the required ampacity, the following dimensions have been calculated:

Dimension mm

a 450

b 175

c 100

d 1300

e 100

f 450 *

g 1000

* Dimension “f” is based on a nominal cable diameter of 150mm Dimensions for Figure 75

For the purpose of initial cost estimates:

Auxiliary ducts shall be 100 mm PVC conduit

(Any optical fibre cable must be suitable for operation at 50 – 70°C when installed in close proximity to power cables)

The trench may be of rectangular cross section as shown in which case shoring (usually timber) is generally required or may be battered (sloped) to avoid the need for support. An example of a trench with sloped sides is shown in Figure 76. With this type of arrangement it can be more difficult to keep the bottom of the trench clear of falling debris.

Figure 76: Trench with sloped sides

During cable installation cable rollers are required at close spacing. Pre-formed skid plates must be used at bends. The trench must be kept clean.

The thermal backfill placed as a bedding under, over and around the cables shall be of the cement bound sand or fluidised thermal backfill type and shall have a dried out thermal resistivity no greater than 0.9 K.m/W^[80,81]. Cement bound sand shall be installed dry and then compacted. It shall be free of sharp stones or flints which could damage the cables.

 Where the route crosses agricultural land the top of the warning tiles placed above the thermal backfill must be no less than 900 mm below the surface. (this would require to be verified for Edmonton farming conditions).

 The backfill placed above the cable forms part of the thermal design of the circuit;

the warning tiles placed above thermal backfill must extend the whole length and width of the route.

Trench filling above the thermal backfill and to either side of the trench must have suitable thermal characteristics. For this scoping study the limits are that the normal thermal resistivity should not exceed 0.9 K.m/W^[80,81], and also that it should not exceed 3.0 K.m/W^[82]when fully dried out. (The latter values would need to be verified by taking measurements and samples in situ in trial holes dug along the route when selected).

:

 The indigenous soils must be tested for thermal resistivity and can only be used to

three metres both side of the trench will have to be removed and replaced with suitable imported material. The trench fill must be carefully compacted.

Local regulations or landowners may have specific requirements for topsoil to be stripped and stored during the course of construction and replaced as part of the final reinstatement. There may be requirements as to the time of year when topsoil can be stripped and the methods of storage.

Where minor roads are to be crossed, a ducted arrangement will be required. For the purposes of this installation study this may be considered identical to the duct block proposed for the duct-manhole system except that the ducts must generally be filled with bentonite for thermomechanical compatibility.