8.5　Concrete Plug

8.5.1　A shaft concrete plug shall be placed between the shaft and cavern.A concrete plug shall be placed between the access tunnel and cavern.

8.5.2　The concrete plug shall have no movement or leakage under a load.

8.5.3　The shaft concrete plug's structural calculation shall involve the following parameters:

1）Atmospheric pressure；

2）Water inflow pressure；

3）Anti-seepage layer pressure:

4）Weights of pipelines，casings，and equipment；

5）Self-weight of the concrete plug；

6）Earthquake load and internal explosion load.

8.5.4　The access tunnel concrete plug's structural calculation shall involve the following parameters:

1）Water inflow pressure；

2）Atmospheric pressure；

3）Earthquake load and internal explosion load.

8.5.5　The concrete plug thickness design shall comply with the requirements as follows:

1　The concrete plug shall not have any movement relative to the surrounding rock or any leakage under combined loads.

2　The concrete plug's design thickness shall meet the requirements of the checking calculations of the shear stress at the concrete-surrounding rock interface and the concrete's compressive strength，leakage impedance paths，and allowed pressure gradient variation.

3　If possible，it is advisable to carry out the engineering design according to the field test data.

8.5.6　The concrete plug's structural design shall comply with the requirements as follows:

1　The concrete plug's location shall be determined according to the local gedogical and hydrological conditions.It shall not be placed in a weathered zone，a fault zone，a zone with high permeability，or a zone with an adverse joint dip direction.For the key slots of the concrete plug，a reasonable blasting technology shall be selected to reduce the rock mass disturbance.

2　The concrete plug should be based on a structure composed of plain concrete，on the upper and lower surfaces of which double-layered two-way crack-limiting steel bars should be symmetrically set up.The crack widths therein should not be more than 0.2mm.

3　The concrete plug's concrete grade should be C20 to C35.The diameters of the steel bars on the upper and lower surfaces shall be at least 14mm，their spacing should be 200mm or below，and the protective concrete layer should be at most 50mm thick.

4　The pipelines and casings through the concrete plug shall be placed at its center.Reinforcing steel bars shall be added where the pipelines and casings pass through it.For the reinforcing steel bars，stress check calculation shall be performed using a finite element numerical model.

5　A water-cooled heat sinking pipeline should be placed within the concrete plug concrete.

6　The depths of the concrete plug's key slots should not be less than 1000mm.

7　The rock mass around the concrete plug's key slots shall be supported by bolts or grouted for sealing.

8.5.7　The shaft concrete plug shall be firmly connected with the penetrating pipelines or casings in a watertight way.

8.5.8　Support shall be given for the following locations:

1　The key slots around each concrete plug；

2　Any access tunnel or shaft within 10m away from the center of the concrete plug on both sides；

3　The cavern top more than 5m outside the shaft along the cavern axis.

8.5.9　After pouring，the concrete of the concrete plug on the edge shall be subjected to grout sealing.

8.5.10　In the top of the shaft concrete plug，an at least 10m-thick anti-seepage layer shall be placed.

8.5.11　The concrete plug between the cavern and access tunnel shall be provided with a manhole，which shall be closed before the access tunnel has water inflow.

8.5.12　After the part of the access tunnel or shaft beyond the concrete plug is complete，it is advisable to fill fresh water to a level that is not lower than the designed stable ground water table.