7.2　Trench Excavation and Backfill

7.2.1　The pipeline construction execution plan shall be developed before pipeline construction operation，and shall include the construction method statement and safety measures to protect the existing pipelines and other facilities in the construction area.Construction operation procedures may be transacted only after the pipeline construction is reviewed and approved by relevant management authorities.In case there is discrepancy between construction plane layout and plant site general layout or the construction plane layout impacts the plant site general layout，the construction execution plan shall be revised.

7.2.2　Before trench excavation，survey and exploration shall be performed for geology，hydrology，tomb pit and sand belt within 5m of the excavated zone，and the present conditions of the existing above ground/underground pipelines or other facilities，permanent survey marks，geological and seismic observation holes shall be found out.

7.2.3　Before construction operation，the engineering and technical personnel shall clarify the construction method statement to the construction operators and implement the safety measures.

7.2.4　For trench excavation，trench excavation diagram shall be compiled per engineering document，construction execution plan，method statement and field condition，and the position of above ground/underground facilities，the cross-section size，routing and coordinate of inflection point for trench excavation shall be specified in that diagram.

7.2.5　The trench excavation method shall be decided based on the conditions of the existing above ground/underground pipelines or other facilities and geological conditions.Regardless of what method is used，the safety of the existing above ground/underground pipelines or other facilities in the surrounding shall be guaranteed.The trench excavation shall be done layer by layer，and the footing and penetrating excavation method shall not be used，and measures to prevent landslide and collapse shall be taken.Within the range of 3m at both sides of underground facilities，manual excavation should be used，and necessary protection shall be provided for the existing pipelines or other facilities dug out.

7.2.6　In the trench excavation，the machinery，excavation method，setting-out survey，excavation procedures and safety measures used shall be inspected and confirmed.The trench excavation size and side slope protection measures shall be key-examined and patrol shall be strengthened.

7.2.7　During construction in rainy season and debacling season，the trench side slope shall be timely inspected.When the crack or constant falling earth from the side slope and loose，distorted，broken supports are found，the operation shall be overhead immediately，and construction may only resume after taking reliable measures.

7.2.8　During construction，the contractor shall assign dedicated personnel for construction safety supervision on the existing above ground/underground pipelines or other facilities，the permanent marks for survey，geological and seismic observation holes，and shall take safe and effective protection measures，and strengthen observation to prevent displacement and settlement，and ensure the safety of constructors and the normal operation of existing pipelines and other facilities.

7.2.9　During construction，if the crossing of existing pipeline and other facilities is found，or unidentifiable object is found，the object shall not be knocked or moved.The operation shall be stopped immediately，and reported to the supervision and the project construction management department，the engineering contractor should be informed immediately.In case that the accident hazards of existing pipelines and other facilities were found out or accident occurred in the construction area，they should be timely reported to the relevant authorities as required，and an active cooperation with relative authorities is required for emergency repair.

7.2.10　Blasting excavation of trench should be completed before pipeline laying，and safety distance shall be decided before blast operation so as not to threat the safety of people nearby and the safety of existing above ground/underground pipelines and other facilities.

7.2.11　When the depth of pipeline or pipe trench is less than 5m，the excavation width of the trench bottom should be calculated by the following formula:

Where:

B1——Excavation width at the bottom of pipeline or pipe trench（mm）.

D1——Width of the external edge of pipeline or pipe trench structure（mm）.

b1——Width of working face at one side of the direct burial pipeline（mm），whose value shall comply with the specifications in Table 7.2.11.

b2——Support thickness at one side of the direct burial pipeline which may be 150mm～200mm.

b3——Thickness of formwork at one side of the pipe trench by field cast concrete or reinforced concrete（mm）.

7.2.12　In case of good geological conditions，even soil quality，groundwater level lower than the elevation of bottom of trench，and side slope not supported，the allowable depth for trench without side slope shall comply with the specifications in Table 7.2.12-1.When trench depth exceeds the values in Table 7.2.12-1，the steepest side slope with excavation depth within 5m shall comply with the specifications in Table 7.2.12-2.

7.2.13　Before trench excavation or removal of support，safety inspections shall be made for the nch wall and the architecture/structure，process facility，tank，road，railroad，existing pipeline at both sides of the trench or in crossing with the trench.Methods to prevent the settlement and deformation of aforementioned facilities shall be provided，as well as the detailed procedures and safety measures for removal of support.

7.2.14　The depth and width of trench for foundation of pipe sleeper and pipe rack shall comply with the specifications in engineering documents and the current applicable national standard GB 50202 The Code for Acceptance of Construction Quality of Building Foundation.

7.2.15　Earth/stonework balance for the trench excavation construction shall be made.

7.2.16　The trench excavation acceptance quality shall meet the following requirements:

1　The natural foundation shall not be disturbed or the foundation treatment meets the design requirements.

2　Trench wall shall be neat and flat，and side slope shall meet the requirements in construction plan.

3　The net width at each side of the trench centerline shall be no less than1/2 of the design excavation width of the pipeline or pipe trench.

4　The depth，width，sloping，inflection point，bottom treatment and trench wall support and protection shall meet the requirement in engineering document.

5　The straight trench shall not be curved，while the curved trench shall ensure the same radius of curvature as engineering requirements.

6　The trench centerline，trench bottom elevation and width，and gradient change point shall comply with the engineering document，and the trench allowable deviation shall comply with the specifications in Table 7.2.16.

7.2.17　After laying of direct burial pipeline and pipeline in pipe trench，acceptance and the pipeline asbuilt survey，a timely backfill shall be made and compacting shall be done layer by layer.In the trench backfill process，the moisture content of backfill material，loose paving thickness，compacting times and compaction degree shall be checked.

7.2.18　For the trench backfill earth in the greening area，the range of 50cm on the surface should not be compacted，but the surface may be leveled，and settlement should be reserved.

7.2.19　After trench backfill，marks shall be made as engineering requirements，and physiognomy shall be timely restored.

7.2.20　The trench excavation and backfill shall comply with the specifications in the current applicable standards GB 50330 Technical Code for Building Slope Engineering，GB 50202 The Code for Acceptance of Construction Quality of Building Foundation，JGJ 120 Technical Specification for Retaining and Protection of Building Foundation Excavations and SH 3533 Specification for Construction and Acceptance of Water and Sewerage Pipeline in Petrochemical Industry.