PVC Sealing System

 

Requirements on the sealing system
The seal made of plastic membranes for tunnel construction fulfils the following requirements.


  1. It is watertight and has durable resistance of the soil/water mixture present on the surface, including the chemicals contained in it
  2. It can be adapted in a technically satisfactory manner to sealing carriers with an inconstant progression
  3. It bridges spontaneous crack formations without losing its functioning capacity
  4. It can be welded by thermal means
  5. It possesses low inflammability
  6. It can be laid on moist or wet substrates

  • Tightness and mechanical strength of the seams; can be safely and economically checked
  • Technically satisfactory; straightforward and safe to repair; if damage caused by third parties is detected before the inner shell is introduced
    The requirements on tunnel sealing membranes as listed here are not claimed to be capable or applicable in their order of itemization.


Layer structure of the sealing system

  • Rock
  • Shotcrete retention layer, sealing carrier at the same time
  • Protection and drainage geotextile
  • Fastening elements
  • Sealing membranes for the tunnel construction
  • Interior shell made of concrete, steel-reinforced concrete or shotcrete.
    Sealing membranes for tunnel construction are laid loosely and on discs which hold the protection or drainage geotextiles on the shotcrete, welded by means of hot air. One property of the discs is that the shearing areas located in them become separate from the fastening element above a certain tensile force and thanks to their elasticity adapt straightforwardly to changes in shape which may occur as a result of:
  • Silo pressure of the fresh concrete
  • Hydrostatic pressure
  • Compression between internal and external shells
  • Contraction, creep and temperature changes of the outer concrete casing
    Excessive stresses on the sealing membranes are thus avoided by automatic fixing.

    The straightforward rapid processing and immediate load-bearing capacity are further advantages of the sealing systems for tunnel construction. Experience and know-how, the latest state of the art, production and processing are further quality criteria.


Technical preconditions
The sealing work is carried out on the basis of observance of the relevant national and international safety provisions and the requirements of safety at work. Since the rock cavity is in some cases subject to harsher influences than other engineering works, it is advantageous to only start the sealing work when the tunnel structure to be driven has been broken through. If this cannot be observed, transportation to the installation site becomes more difficult, the safety precautions are tightened, the possibilities of emergency escapes are reduced, and the work is interrupted quite often.
In order to ensure a smooth working sequence and avoid contingent obstacles, a distance from the face of about 200 m should be observed in sealing work.

Requirements of the backing layer
Sealing Carrier
An important criterion for the functioning capacity of sealing membranes in tunnel construction is the shotcrete retaining layer which serves as a backing and for fixing the sealing system at the same time.
The shotcrete shell, approx. 5 - 15 cm thick, but also possible considerably thicker depending on the geological conditions, guarantees
a) the temporary dimensional stability of the rock
b) the covering of edges, burrs, excavation points, projecting bolt heads, extension clips and attachments
The following conditions must be fulfilled for a suitable sealing carrier:
  • thickness >= 5cm
  • maximum grain 16 mm, if possible graded into
  • base layer 0-16 mm
    covering layer 0-8 mm
  • ratio of diameter to height of unevenness, caused locally by excavation, in the sealing carrier when installation sealing membranes for tunneling: at least 5:1
  • in flexible sealing membranes: say, 2.0 mm thickness and at least 10:1
  • in less flexible sealing membranes: say 3.0 mm thickness
  • vertical curve: 20 cm
  • adequate dimensional stability.
  •  

    Sealing the vault
    Sealing the crown and side wall
    The sealing membranes for the crown and side are well laid radially to the structural axis from a mobile laying scaffold and welded to the axially laid drainage connection or to the axially laid floor sealing. The number of fixing points (discs) required in this instance, which are connected onto the shotcrete/geotextile sealing carrier by means of steel nails, amount to the following on average:

  • floor area 0,0 - 0,5 unit/m²
  • side wall area 1 unit/m²
  • roof area 2 - 3 units/m²

  • In the first work cycle, the protection and/or drainage geotextile, approx. 2 - 4 m wide, are fixed onto the sealing carrier or shotcrete with the aid of discs, and fixed to the points farthest from the axis (valleys) in the vault and side wall area. By means of this arrangement of fixing points, sufficient basic material is available for the sealing membrane and the possibility of overstressing valleys in the sealing carrier is eliminated in the event of straining movements. Subsequently the sealing membranes are laid with adequate side edge overlapping.


    Welding and inspection techniques
    Homogeneous welding of the sealing membranes is undertaken by means of automatic heated-wedge welding units which produce a hollow channel between two weld seams running in parallel. The double seam thereby produced is check-ed for tightness and mechanical strength by injection of compressed air, for the purpose of which a test pressure of approx. 2.0 bar must be held over a period of approx. 10 minutes. The pressure drop must not be greater than 10%. The outstanding quality of such a welding system can be achieved, as a function of the weld temperature between heated-wedge and sealing membrane, and by the speed of the infinitely variable drive of the automatic welding machine, as well as the constant subsequent pressure on the weld surfaces, which is generated by a back-pressure roller gear unit.

    Welding can be carried out irrespective of the state (roughness, corrugation) of the sealing carrier. In this process, the laying personnel merely guide by hand the light and handy welding unit between the overlapped sealing membranes being welded and ensure the prescribed overlapping width of the strips.

    Here it is advisable to use a welding machine with a display and recording device for temperature, contact pressure, welding speed and voltage check.
    In the event of a fault, the welding unit can be switched off immediately. It can be set at any desired end of the sealing membrane and is normally run to the other end. Double-seam welding is also undertaken if axially and radially laid sealing membranes have to be welded (T-joints):

    In the already welded overlapping area of the flat seam of the transverse strips, the seam edge is bevelled off.


     

    Scaffolding
    The whole assembly of the tunnel sealing system is undertaken from a self-propelled laying scaffold, on which easily accessible working platforms are installed at different levels. The scaffold must be arranged in such a way that the laying work on the seal is not hindered. The practical design of the scaffold plays a part which must not be underestimated in the work progress achievable later.
    Workplace safety measures include railings, lighting, trip-free storekeeping, equipment for fire-fighting, emergency respirator, emergency medicine box and helmets. Preventive measures against fire accidents have absolute priority in tunnel construction.

    When sealing structures in an open mode of construction with tunnel sealing membranes, the following points should be borne in mind:

    Substrate
    The substructure must be handed over to the sealer - by the general contractor - free of shrink holes, pockets, burrs and pointed objects.

    Laying/jointing technique
    The sealing membranes are laid loose. The material must be overlapped by at least 8 cm in the boundary are. Any T-joints occuring must be chamfered in the overlapping area.

    Closure at the foot
    At the foot, the sealing membrane must be fastened to the substructure in the longitudinal direction of the tunnel by means of fastening elements. Spacings of the fastening points approx. 2.0 m

    Boundary area/closures
    It is advisable to use the external joint ties, concreted at the building site, of the same material as the sealing membranes, to which uniform welding of the seal can be undertaken. Otherwise a process of securing by means of pressed flanges should be carried out.

    Penetration points
    The seals must be constructed according to DIN 18195 with loose and rigid flange constructions.

    Protective measures when backfilling
    After installing the seal, an immediate start must be made on the introduction of the necessary protection layers. With sealing membranes 4 mm thick, the seal can be covered directly with fine backfilling material, either 10 cm sand layer or 30 cm 16/32 gravel.