8.3　Support

8.3.1　The support design shall comply with the requirements as follows:

1　For GradeⅠsurrounding rock，it is not necessary to provide support if the cavern span is not more than 10m；if the cavern span is more than 10m，it is possible not to provide support in case the construction safety is not endangered，and it is possible to provide shotcrete-bolt support where the rock is unstable.For GradeⅡsurrounding rock，support should not be provided if the cavern span is not more than 5m；shotcrete support should be provided if the cavern span is 5m or above；provide bolt support where the rock is unstable.

2　For GradeⅢandⅣsurrounding rocks，provide combined supports such as bolt，shotcrete，mesh，or steel frame.For GradeⅤandⅥsurrounding rocks，provide supports according to their specific conditions.

3　The bolt-shotcrete support design shall be based on the project comparison method and corrected according to the monitoring and measurement results as well as theoretical calculation results.

4　The bolt-shotcrete support design developed at the pre-feasibility study stage may adopt support types and parameters specified in Appendix B.The surrounding rock classes，support types，and parameters identified in the support designs developed at the other stages shall be corrected according to the gedogical investigation results obtained at the stages.

5　The access tunnel portal shall be provided with reinforcement according to the gedogical conditions.

6　As a major support target，the shaft wall above the moderately weathered surrounding rock shall be supported by reinforced concrete and bolts；in the moderately weathered surrounding rock and below，reinforced bolts and shotcrete shall be used for providing support.

7　Shotcrete and bolts shall be used for supporting the operation tunnel's headings and wall.Reinforced support shall be adopted at the operation tunnel portal where there is weathered surrounding rock.

8.3.2　The shotcrete support design shall comply with the requirements as follows:

1　The strength grade of the shotcrete shall not be lower than C20.The bonding strength of the shotcrete layer and GradeⅠorⅡsurrounding rock should not be lower than 1.0MPa；the bonding strength of the shotcrete layer and GradeⅢsurrounding rock should not be lower than 0.8MPa.

2　The impermeability grade of the shotcrete shall not be lower than S6.Materials such as setting accelerator，water reducing agent，expansion agent，or compound additive，steel fiber and synthetic fiber should be mixed in the shotcrete.Their types and usages shall be determined by testing.

3　The thickness of the shotcrete layer may be determined according to Appendix B and corrected according to the monitoring results.Its thickness shall be 50mm at least and 200mm at most.

8.3.3　If a surrounding rock with considerable plastic deformation，high ground stress，or rock burst tendency is encountered during tunneling，it is advisable to use steel-fiber shotcrete for support.The steelfiber shotcrete support design shall comply with the following requirements as follows:

1　The design tensile strength of the common carbon steel fiber should not be lower than 380MPa.

2　The steel fiber shotcrete's 28-day mechanical properties should comply with the following requirements as follows:

1）The specific weight should be 23kN/m3；

2）The compressive strength should be at least 32MPa；

3）The bending strength should be at least 3MPa；

4）The tensile strength should be at least 2MPa.

3　The steel fiber diameter should range from 0.3mm to 0.5mm，the steel fiber length should range from 20mm to 25mm，and the steel fiber usage should be 3%～6% of the mixture.

4　The thickness of the steel-fiber shotcrete layer shall be determined according to the requirement for shotcrete layer thickness determination.Its surface shall be coated with a layer of ordinary concrete with the thickness being at least 30mm.

8.3.4　The bolt design shall comply with the requirements as follows:

1　If there is local fall-off，the bearing capacity of the bolt shall comply with the requirements as follows:

1）The resistance of a bolt above the arch side to the unstable rock mass is calculated using the

following formula:

Cementmortar bolt:

Prestressed bolt:

Where　S——The design value of the load combination；

R——The design value of the bolt resistance；

rG——The unstable rock mass's partial safety factor for action，which is 1.2；

Gk——The unstable rock mass's standardself-weight value，N；

n——The number of bolts；

Ax——The area of a single bolt's cross section，mm2；

fy——The design tensile strength of a single bolt，MPa；

σcon——The design tensile strength of a prestressed bolt，MPa.

2）The resistance of a bolt in the side wall below the arch side to the unstable rock mass is calculated using the following formula:

Cement mortar bolt:

Prestressed bolt:

Where　G1k、G2k——The standard values of the component forces of the unstable rock mass respectively in

the directions parallel and perpendicular to the slide plane，N；

As——The area of a single bolt's cross section，mm2；

A——The area of the rock mass's slide plane，mm2；

n——The number of bolts；

C——The bonding strength along the rock mass's slide plane，MPa；

fgv——The design shear strength of the bolt，MPa；

f——The friction coefficient on the slide plane；

Pt、Pn——The component forces of the total pressure of the prestressed anchor cable or bolt onto the unstable rock mass respectively in the direction of resisting to the slide and in the direction perpendicular to the slide direction，N；

rG1、rG2——The unstable rock mass's partial safety factors for action，each of which is 1.2.

2　The bolts above the arch sides should be arranged in the direction being favorable for them bearing the force and the bolts below the arch sides should be arranged against the unstable rock mass's slide direction.

3　For the surrounding rock with fissures，the bolts shall be perpendicular to the main structural plane across the cross section.If the main structural plane is not obvious，it may be perpendicular to the cavern perimeter's contour line.They should be installed to be quincunx-shaped in the surface of the surrounding rock.The spacing between bolts should not be more than half of their length.In the GradeⅣorⅤsurrounding rock，the inter-bolt spacing should be 0.5m～1m，yet not more than 1.25m.

8.3.5　If the rock mass is broken or has fissures，it is advisable to use the bolt-shotcrete-mesh support method.The bolt-shotcrete-mesh support design shall comply with the following requirements:

1　The layout of the steel bar mesh should comply with the following requirements:

1）The longitudinal and circumferential steel bars of the steel bar mesh should have the diameter ranging from 6mm to 12mm and the spacing ranging from 150mm to 200mm；

2）The steel bar mesh should be connected with bolts by using the welding method.Its intersection points shall be firmly connected.Welding and binding should be performed every two points.

2　The protective layer for the steel bar-meshed concrete should be at least 50mm thick.