Specification for Shotcrete in Tunnel Construction

Shotcrete SpecificationThis Specification describes the requirements for shotcrete lining in a tunnel project. Applied shortly after excavation, the shotcrete lining shall rapidly medigate any disintegration or detrimental loosing of the surrounding rock.

1. Shotcrete shall be comprised of Portland Cement, coarse and fine aggregate, water and admixture(s), properly mixed to provide a consistency to allow spray onto a construction face by means of compressed air using pneumatic pumps and special nozzles.

2. Shotcrete shall be applied on a tunnel face, by itself or in conjunction with welded wire fabric and structural steel (in accordance with drawing requirements) as a structural tunnel lining. Shotcrete shall also be used as sealing along the tunnel and in structural concrete type applications in accordance with drawing requirements and/or as may be required by the Engineer.

3. Shotcrete may be applied using either the “Dry Method” or the “Wet Method” as described herein:

  • Dry method

The dry-mix process means thoroughly mixing the solid materials, feeding these materials into a special mechanical feeder or gun, carrying the materials by compressed air into a special nozzle, introducing the water and intimately mixing it with other ingredients at the nozzle. The mixture is the jetted from the nozzle at high velocity onto the surface to receive the shotcrete.

  • Wet Method

The wet-mix process, means thoroughly mixing all the ingredients except the accelerator, but including the mixing water, introducing the mixture into the delivery equipment end delivering it by positive displacement or compressed air to the nozzle. The mixed shotcrete is the air-jetted from the nozzle at high velocity onto the surface in the same manner as for the dry-mix process. The accelerator for the wet-mix process shall be added to the shotcrete mixture in such a way that the quantity can be properly regulated and the material uniformly dispersed throughout the shotcrete when it is applied.

4. Shotcrete, regardless of application methods shall comply with the requirement of this Specification.

5. Time is of the essence for work associated with this Specification Section. Therefore laboratory and field-testing shall commence as soon as possible after the Notice to Proceed in order to obtain an approved shotcrete mix before it is necessary to apply shotcrete to the permanent work.