Trench laying The size of the pipe trench is determined according to the cable layout drawings, ensuring that all pipes, chambers and possible protective structures can be properly installed and compaction and casting work properly carried out. The trench must be no wider than the necessary working space requires. The guideline trench dimensions are presented later in this section. The walls must be sufficiently sloped to prevent danger of collapse and any stones that may be hazardous if dislodged must be removed.
Cable and Telecom
The need for a bedding layer is determined according to the subsoil quality. If no foundation structures are needed, the pipes are laid, according to the subsoil quality, either on bedding or directly on the subsoil. The chosen method is specified site-specifically in the construction plan or site meetings. Direct ground laying is permitted on clay and on silty or sandy soils. Direct placement is also permitted on till soils with a maximum stone size of 16 mm. The trench must be cleared of roots and large stones, and the subsoil and embedment materials must not be frozen. The trench bottom must be dug suf-
ficiently level to minimise cable pulling friction caused by bumps and level variations. The trench bottom must also be loosened and raked to a depth of at least 15 cm to ensure a stoneless bedding. The bedding is then compacted by tamping or with a vibrating plate compactor. The duct pipes must be cleaned or carefully inspected before blowing the pull-cord and pulling the cable. If the soil meets the above requirements, the pipes can be placed directly on the levelled trench bottom. Otherwise, a bedding layer of sand, gravel or crushed aggregate must be laid to a depth of at least 150 mm below the pipe. The maximum grain size of the natural aggregate to be used for the bedding is determined according to the outside diameter of the pipe. Before laying the bedding, the ready trench bottom must be cleared of large stones, levelled and compacted. The soil must be free of stones and rock to a depth of 15 cm below the bedding. The bottom must be levelled to ensure that the installed pipes are evenly supported along their entire length by a firm, stoneless base. In winter conditions the trench must be excavated in shorter stretches so
Final backfill - As-dug material - Compacted
min. 15 cm 5-20 cm
Concrete, K25 Duct pipe
D 15 cm 5 cm
Bedding - Bottom levelled and compacted - Stoneless sand depth min. 15 cm
Figure 8.2.29. Concrete reinforced sand-embedded cable channel 396
Uponor CABLE DUCTING SYSTEM