6.5　Installation Requirements for Underwater Optical Cables

6.5.1　Selection of underwater cables shall be in accordance with the following principles：

1　For regular rivers or seasonal rivers that have stable river beds and banks with width greater than 150m and regular water velocity，steel wire armored cables with short-term tensile strength at 20000N or above are adopted.

2　For primary navigable rivers or rivers with unstable river beds and banks and water velocity greater than 3m/s，steel wire armored cables with short-term tensile strength at 40000N or above are adopted.

3　For severely eroded large rivers with unstable river beds and banks and width greater than 500m，reinforced steel wire armored cables with special design are adopted.

4　Through static water areas such as reservoirs and lakes，underwater cables may be adopted according to various elements including navigation conditions，operation of water engineering，and hydrological and geological conditions.

5　Can ensure the safety and not influenced by future engineering on the water，the directly buried cables may be used to go across the rivers that have stable river beds，low velocity and common width.

6　If the bed soil and river width can meet requirements of the directional drilling equipment，directional drilling construction method may also be considered.Under such circumstances，directly buried or duct optical cables may be placed in drills to go across rivers.

6.5.2　Crossing positions of underwater cables shall choose river sections where benches are straight，velocity is acceptable，river is relatively narrow，soil is stable，riverbed is relatively flat without obvious erosion and slopes are moderate.The following areas should not lay underwater cables.

1　Areas where rivers turn，bend or confluence，where waterways frequently change or where there are dangerous shoals and sandbars nearby.

2　Areas where water flow is unstable with whirls，where river banks are steep and unstable，or where river banks suffer from severe erosions and collapses.

3　The hazardous sections for ice floods.

4　Places where widening and dredging of the river are being planned，or where river conditions may change from potential ripraps and broken dikes.

5　Places where the soil on the river bed is not suitable for laying and burying cables.

6　Water areas with corrosive sewage drainage.

7　Places where other underwater pipelines exist nearby or where ship wreckage，explosives and sediments are found.

8　Places that are close to wharfs，harbors，ferries，bridges，anchorage points，locks，shelters and water operation sections.

6.5.3　Cable joints in water shall be avoided for underwater cables.

6.5.4　For extra-large rivers and important navigation river routes，underwater optical cables for back-up use may be set up according to the importance of the optical cable trunks.The active and standby underwater optical cables shall be switched either may be switched manually through connector cabinets and branch closures，or automatically.To achieve this，underwater cable terminal rooms may be set up.

6.5.5　The depth of underwater optical cables shall be determined according to water depth，navigation conditions，soil type on the river bed and other factors.

1　The depth of the river bed with water shall comply with the following requirements.

1）For sections where water depth during dry seasons is less than 8m，optical cables should be buried no less than 1.5m below the river bottom where the river bed is unstable or the soil is soft，and shall be buried no less than 1.2m where the river bed is stable or the soil is hard.

2）For sections where water depth during dry seasons is greater than 8m，optical cables may be directly laid on the river bottom without being buried.

3）For eroded and extremely unstable sections such as wandering rivers，optical cables shall be buried below the change range.When this cannot be achieved due to certain obstacles，the burying depth on the river bottom shall be no less than 1.5m and optical cables shall be reserved based on the future needs.

4）For the dredging sections as part of a plan，optical cables shall be buried 1m below the planned depth，or temporarily according to commonly accepted depth.However，optical cables need to be reserved with appropriate length in order to be buried as required for future dredging.

5）For stony and semi rocky river beds，the burying depth shall be no less than 0.5m and protection measures shall be added.

2　The depth for river banks and beach shall meet the following requirements.

1）For relatively stable areas，optical cable depth shall be no less than 1.2m.

2）For unstable section with eroded and loose soil from seasonal floods，burying depth should be greater than usual，and landing slope of optical cables should be less than 30°.

3　For large rivers，when special requirements for burying depth apply in relation to navigation，water conservancy，dikes，maritime and other relevant activities，when anchorage，transportation，fishing，aquaculture and other similar activities may take effect，or when the burying depth as described above cannot guarantee the safety of optical cables，a comprehensive discussion and analysis shall be taken to determine the appropriate burying depth.

6.5.6　The laying length of underwater optical cables shall be in accordance with the following requirements.

1　For river with dikes，underwater cables shall extend the soil excavation area，by no less than 50m from the dikes.For rivers without dikes，the extending of underwater cables from the banks should be no less than 50m，based on the stability level of the river banks as well as the degree of scouring along the banks.

2　For rivers or banks may be planned for widening or change，underwater cables should extend by 50m from the embankment being planned.

3　For sections with loose soil and unstable banks，vulnerable to scour，optical cables should be properly reserved.

4　The lengths of the active and standby underwater cables should be the same.If length deviation is necessary，transmission requirements shall be satisfied.

6.5.7　The length of the underwater optical cables that go across the rivers may be estimated or calculated based on Table 6.5.7-1 or Equation 6.5.7 respectively，according to the river width and terrain conditions.The length of single drum underwater optical cables should be no less than 500m.

In the equation：L——the underwater optical cable length（m）；

L1——the length measured between the two terminals of underwater optical cable（m）；

L2——the increased length of terminal fixing，crossing the embankment，"S"shaped laying，banks and joints etc（.m）；

L3——the increased length for various reservations between the two terminals（m）；

L4——the increased length for laying planar radian（m），which may be determined referring to Table 6.5.7-2；

L5——the increased length for underwater vertical radian which shall be calculated according to the river morphology and the section review of optical cables laid（m）；

L6——installation allowance，determined according to different construction technologies（m）.8%-10% of water surface width for tug laying，3%-5% of water surface width for anchor laying.It shall be calculated separately for plough laying according to actual situations，whilst allowance is generally unnecessary for artificial lifting laying；

a——The growth rate for natural bending，according to terrain conditions，adopts 1%-1.5%.

6.5.8　The best construction time and feasible installation method of underwater optical cables engineering design should be determined according to the on-site survey and analysis of hydrological data.The installation method shall be selected according to factors such as specifications of optical cables，hydrological and geological conditions，construction technologies，equipment and management capabilities as well as economic benefits.Manual or mechanical trench laying and laying with special equipment may be available methods.For rocky river beds，method of blasting trench may be adopted depending on the actual circumstances.

6.5.9　Installation positions of optical cables at the river bottom shall take the baseline measured as reference，laid by arc towards the upstream.Scope of the actuate laying should include the river banks which may be scoured during the flooding season.The vertex of arc shall be positioned at the mainstream of the rivers.The distance from the vertex of arc to the baseline shall be determined according to the stability of the rivers and the size of the arc chord，generally 10% of the chord length.This can be adjusted according to erosion conditions and the width of the water surface.The"S"shaped laying may be adopted if the arc laying is difficult.When two and above underwater optical cables are laid，or there are other optical cables or pipelines laid in the same region，sufficient distance shall be maintained to ensure safety.The connecting method of the steel sheath and armored wires at the underwater optical cable joints shall be able to ensure the electrical performance，airtight performance and the necessary mechanical strength as required of optical cables.

6.5.10　Underwater cables that are close to certain portions of river banks are likely to be affected or damaged by erosions，landslides，riprap revetment and ship landing，the following measures may be adopted under such circumstances.

1　Deepen the burying depth.

2　Covered with cement board.

3　Using the cable joint sleeves.

4　Building stone optical cable trench（measures shall be taken to prevent cables from wearing down）.

6.5.11　Laying optical cables through the levees and the related protective measures shall meet the following requirements.

1　The position for optical cables to go through the levees shall be above the highest flood level ever recorded in history.The position that flood level may reach during the lifetime of optical cables shall be considered for rivers with siltation issues.

2　For optical cables that go across the soil embankment，the climbing dam laying method should be adopted and the burying depth at the top of the embankment shall be no less than 1.2m whilst the depth on embankment slopes shall be no less than 1.0m.The corresponding protective measures shall be taken when the top part of the embankment are used as roads at the same time.Raising the embankment surface partially may be adopted if it is difficult to meet the requirements for burying depth.Under such circumstances the thickness of pad soil above the optical cables shall be no less than 0.8m.

3　Optical cables should not pass through the stone or concrete embankment.

6.5.12　The fixing method of the underwater cable terminal shall take the following measures according to different situations：

1　For regular rivers，the joints for land and underwater optical cables shall be set at the higher terrains and placed with stable soil，or may be directly buried under the ground.For the convenience of maintenance，joint hand holes may also be set up.Underwater optical cables at terminals shall adopt 1 to 2"S"curves as measures for anchorage and reservation.

2　For large rivers，with bank scouring and unstable soil conditions near the optical cable terminals，anchoring shall be implemented for underwater optical cables，in addition to measures above.

6.5.13　For navigation rivers where underwater optical cables are laid，signs shall be set up at the cross sections on the river banks or river levees.The number of signs and the setting of signs shall meet the requirements of the relevant maritime and navigation management authorities.The following requirements may apply if there are no other specific provisions.

1　For rivers with width less than 50m，signs may be set up both the upstream and downstream embankment on one side of the river；

2　For wide rivers，signs may be set up on both the upstream and downstream embankment on each side of the river；

3　For rivers that cover a long piece of bottomland or the main navigation channels deviate to one side of the river bed，signs may be set up near the channels；

4　For rivers that are busy with night-time navigation，lighting equipment may be arranged on the signs.

6.5.14　For navigation rivers where underwater optical cables are laid，prohibited areas for anchoring shall be designated，scope of which shall be in accordance with the requirements of the relevant shipping and navigation authorities.The following requirements may apply if there are no other specific provisions.

1　For rivers with width less than 500m，the distance between the upstream prohibited areas and the optical cable arc vertex is 50m-200m whilst the distance from the downstream prohibited areas to the optical cable routing baseline is 50m-100m.

2　For rivers with width more than 500m，the distance from the upstream prohibited areas to the optical cable arc vertex is 200m-400m whilst the distance from the downstream prohibited areas to the optical cable routing baseline is 100m-200m.

3　For extra-large rivers，the distance from the upstream prohibited areas to the optical cable arc vertex shall be greater than 500m whilst the distance from the downstream prohibited areas to the optical cable routing baseline shall be greater than 200m.