6.2 Installation Requirements for Directly Buried Optical Cables
6.2.1 The installation of directly buried cables shall avoid locations where roads and houses can be built in the future or ground can be excavated for construction.Areas where water level is relatively high or soil is waterlogged for a long time should also be avoided.
6.2.2 Depth of buried cables shall comply with the requirements specified in Table 6.2.2.
Table 6.2.2 Depth standards for buried cables
Notes：1 Design depth of side ditches should be consistent with those required by highway or urban construction authorities.Depth on man-made slotted stone side ditches may be reduced to 0.4m.Furthermore，anti-scour materials such as cement mortar are used to seal the ditches.
2 In stone and half stone regions，100mm thick fine soil or sandy soil shall be laid above the trench bottom and above the optical cables.The depth of optical cables shall decrease correspondingly.
3 The depth requirements for tundra are not included in the above table，and its burying depth shall be analyzed separately during the engineering design.
6.2.3 Optical cables may be laid with other communication optical cables or copper cables in the same ditch，but shall not be overlapped or crossed.The parallel spacing between the optical cables should not be less than 100mm.
6.2.4 The laying of optical cable line mark stones shall meet the following requirements：
1 Optical cable mark stones are buried in the following places：
1）Optical cable splicing，turning points，and reservation places.
2）Silicon core plastic pipe break and connection points for airflow installation and buried man（hand）hole positions.
3）Places where obstacles need to be surpassed or where long straight sections continue.Areas where locating optical cables is difficult via two marks or other reference materials.
4）The starting and ending locations where thunder-proof lines are laid or where optical and copper cables are laid in the same ditch.Monitoring mark stones shall be set up for joints of buried cables instead of ordinary mark stones.
5）Other locations where mark stones need to be buried.
2 When fixed signs are used to mark the optical cable positions，mark stones cannot be buried.
3 The optical cable mark stones should be buried on top of the cables positions of which shall be in accordance with the following requirements：
1）Mark stones for joints are buried along the optical cable line routing；
2）Mark stones for turn points are buried at the intersection of optical cable line bends；
3）Mark stones are buried in places where moving is not easy or where public transportation and planting is not affected；
4）If burying positions are not easy to choose，auxiliary markers in the vicinity may be added and triangle calibration method can be used to determine positions for optical cables.
6.2.5 The directly buried optical cable splices shall be arranged and placed at the higher，relatively flat and geologically solid areas as opposed to the sites for ponds，canals，ditches，roads and bridges in addition to places that are not convenient for construction and maintenance or easily affected.Cement cover plates or other suitable protective measures against mechanical damages may be used to protect optical cable closures.
6.2.6 When crossing railways，light rail lines，roads with heavy traffic or excavation restricted roads，optical cable lines shall use steel pipes for protection or adopt directional drilling to lay underground pipes whilst ensuring safety of other existing pipelines underground.Steel pipes shall be extended 1m out of both sides of the subgrade drainage ditch and the depth of optical cables buried from the drainage ditch bottom shall not be less than 0.8m in addition to meeting requirements by relevant departments.Inner diameter of pipes shall meet the requirements of sub pipes installation，but shall not be less than 80mm.Plastic sub pipes shall be placed inside the steel pipes and the number of plastic sub not fewer than 2 in most cases，is determined by the actual need.
6.2.7 Optical cable lines shall use plastics or steel pipes for protection when going through roads or village streets where excavation is allowed.Bricks or cement cover shall be used for protection when crossing the tractors plough road ground breaking possible.
6.2.8 Optical cable line may use long plastic pipe，brick or cement cover to be protected when going through ground breaking possible locations such as villages etc.
6.2.9 Passing through the relatively small ditches and ponds where dredging and widening is being planned or where dredging is likely，optical cable lines shall be covered with cement plates or mortar bags or other optional measures for protection.
6.2.10 The"S"shaped laying should be adopted when optical cables are to be deployed on slopes with angle greater than 20 degrees and length greater than 30m.Optical cable trenches on slopes that are likely scoured by water current shall take blockage strengthening or shunting measures.When laying optical cables on slopes with angle greater than 30 degrees，special structure should be adopted.
6.2.11 When going through or along areas susceptible to water erosion，such as mountain springs and streams，optical cables shall be protected by building overflow dam，cement sealing ditches，water retaining walls or other measures per specific circumstances.
6.2.12 For cable laying in up and down terrains，the depth and curvature radius of the optical cables buried shall meet specific requirements.Optical cable trenches should take measures to prevent soil from erosion and ensure the safety of the cables according to local conditions.When the elevation reaches 0.8m or above，ridges or protection slopes shall be added for protect the cables.
6.2.13 Effects from mechanical damages，vibrations and temperature changes shall be considered when laying optical cables on bridges，and corresponding protection measures shall be taken as well.
6.2.14 The minimum clearance between directly-buried cables and other building facilities shall be in accordance with specifications described in Table 6.2.14.
Table 6.2.14 The minimum clearance between directly buried cables and other building facilities（m）
Notes：1 The clearance between directly buried optical（copper）cables with steel protection can be reduced to 0.15m crossing with water pipes，gas pipes and oil pipelines.
2 The protection against lightning and thundering shall be considered for poles，guy wires，isolated trees and tall buildings.
3 The diameter of big trees is generally 300mm or above.
4 When going through various underground pipelines depth of which is similar to that of the optical（copper）cables，optical cables should be laid below the pipelines and protective measures shall be taken.
5 If the clearance does not meet the requirements listed in the above table，effective protection measures shall be taken in consultation with relevant departments.