tunnel waterproofing methods

WATERPROOFING (WITH MEMBRANE) METHODOLOGY FOR TUNNEL

                

For underground structures, water proofing membrane, continuously installed to the outside of the final (inner) concrete lining, without including provisions for other elements like water stops in concrete, sealing of joints etc.

Water proofing for underground structures should be such that with water proofing they are watertight and in case of leakage provision shall be taken for those to be repaired.

The main purpose of membrane waterproofing for the underground structures is to prevent the leakage of ground water into the Tunnel, channelize the ground water to drainage system and to protect the final concrete lining against damaging chemical influences. Waterproofing shall be applied to crown and sidewalls above footing or invert arch level. The water proofing membrane shall always be located between primary shotcrete lining and the final concrete lining and will not be provided for tunnel inverts (Excluding the cases where heavy ingress of water observe from invert).

The waterproofing system for underground structure (Tunnel) consists of two layers

·         The first shall be consists of a Protective Felt (a continuous filament non-woven poly-propylene geo-textile of uniform thickness) fastened to the shotcrete surface.

·         The second one shall be the actual Waterproofing Membrane {Like ECB (ethylene-copolymer bitumen) Waterproofing Membrane or  PVC (poly-vinyl-chloride Waterproofing Membrane,  a thin sheeting of different colour, bonded to one side, which is intended to facilitate the detection of damages.} properly by some special arrangement as recommended by the manufacturer.

METHODOLOGY OR HOW TO EXECUTE WATERPROOFING WORK IN TUNNELING?

SURFACE PREPARATION:

All the surfaces to which waterproofing is to be applied shall be sufficiently clean, smooth, freer from debris and deleterious materials and projections that can cause damage for puncture to the PVC membrane liner.

The following procedure to be performed for leveling of the surface and specific preparation of the shotcrete surface shall be carried out prior to the installation of waterproofing –

a)      A minimum 50mm thickness of shotcrete cover to the rock surface is required to which the protective felt and the waterproofing membrane is to be fixed.

b)      The surface shall be leveled with a layer of fine graded shotcrete to restore gaps, to eliminate irregularities and possible protrusion of metal parts; manual demolition of harshness and protruding parts in the shotcrete. The ratio of the diameter to depth of irregularities shall be not less than 5:1. Rounding at rock bolts (where applicable) shall have a minimum radius of 0.3m.

c)      Transitions and intersections of tunnel profiles shall be rounded off with a minimum radius of 500 mm.

d)     The projecting head of steel bars, wires, spacers, pipes etc. and all the other possible residues (remains) of previous works such as cable trench, ventilation ducts etc. found on the surface to be waterproofed shall be eliminated unless treated with a layer of shotcrete.

e)      The iron bars of the armors of the reverse arch and of the wall shall be kept far from the liner in order to avoid occasional damage of it.

f)       Exposed steel parts such as rock bolts, if not intended to remain accessible, shall be covered with shotcrete.

g)      All shotcrete surfaces will finally be smoothened with fine-graded shotcrete (rounded aggregates, grain size 0-8mm), applied inlayer of 25-30 mm minimum thickness.

APPLICATION:

All the surfaces, on which the waterproofing shall be applied, shall be inspected and approved by the Engineer in-charge before the application of the waterproofing. The application procedure shall follow the written instruction of the manufacturer. The procedures are as follows –

·         Preparations:

At Tunnel intersections and for projections passing through the membrane special preparations will be required for waterproofing and shall be carried out according to the manufacturer’s recommendation.

· Fixing of Protective Felt (Geotextile):

§  On the prepared shotcrete surface a layer of protective felt (Geotextile), of minimum 4mm thickness, for the protection of waterproofing membrane and drainage, shall be attached using suitable fixings specified by the manufacturer.

§  The protective felt shall be anchored to the finished shotcrete surface by fixing elements like fitting washer (PVC roundels) fixed with nails/screws by rotary hammer drill machine or impact tool. Depending on the 2 to 4 nos. fixing elements shall be used per square meter.

§  The fixing of the Geotextile (protective) felt shall be carried out in dry conditions and the adjacent sections of the felt shall be overlapped by 100mm and joint by point welding or similar suitable method.

§  The felt shall be laid with sufficient slack to avoid overstress during concreting.

§  The protective (Geotextile) felt shall be set transversally to the longitudinal axis of the tunnel and, in order to avoid any possible contact between the waterproof liner and the shotcrete.

§  The protective felt shall extend sufficiently along the bottom of the tunnel side walls to cover the lateral drainages as per drawing and specification.

·         Fixing of Waterproofing Membrane:

§  The waterproofing membrane shall be installed to cover the protective felt by overlapping the adjacent membrane sheet edge of 80mm to 100mm by means of thermal welding or by hot air welding.

§  The waterproofing membrane shall be laid and attached over the felt which is previously fixed with the finished shotcrete surface. The membrane sheet shall be fixed with the fixing elements like fitting washer (PVC roundels) by manual hand held welding machine tool.

§   The water proofing membrane shall be laid with the single layer towards inside and with sufficient slacks to prevent the overstressing during concreting.

§  Adjacent sheets of waterproofing shall be joined by a double weld.

§  The waterproofing membrane shall extend sufficiently along the bottom of the tunnel side walls to cover the lateral drainages as per drawing and specification.

§  The connections to the waterproofing of the structures in open cuts shall be carried out according drawing and specification to be furnished by the supplier.

§  The welding of the membrane shall be made through the following:

i)        Manual thermo welding with hot air welding machine tool by Leister Triac/BAK Or similar equivalent hand held machine tool (applicable for welding over PVC rondells and membrane overlaps by minimum 50mm).

OR

ii) Through thermo welding with Leister Twinny/BAK or similar equivalent automatic welding machine with   double lanes. (Applicable for membrane overlaps by minimum 80-100mm)

Standard welding systems are:

i) Manual Welding:

 Equipment: Manual welding with hot air welder by Leister Triac/BAK or any other equivalent hand held welding machine

 Overlaps: Sheets shall be overlapped by minimum 50mm and spot welded beforehand (pre-weld) approximately every 20-40cm apart.

Welding temperature: Ranging from 470⁰C to 550^oC depends on environment conditions.

ii) Automatic double – Seam Welding

Equipment: Leister Twinny /BAK or any other Automatic hot air welding equipment. By this equipment hot surfaces of membrane are pressed together by wheels or rollers.

Overlaps: Sheets should be overlapped by 80mm to 100mm. Automatic double-seem welding is performed using “thermos-fusion”.  Total welding width is around 50 mm, including around 15mm wide central air test channel.

 Welding temperature: Ranging from 470⁰C to 550⁰C depends on environment   conditions.

·         Fixing of Protective Membrane:

The protective membrane, with ho joining procedure, is fixed to the waterproofing one by means of thermal welding or by gluing of Velcro and felt disks systems on the two membranes and connecting them.

TESTING OF WELDED SEAMS:

All welded seams shall be tested and records of those tests shall be submitted to the Engineer to the contractor.

The following are some procedure for testing the welded seams –

A.    Seam Test with Compressed Air

B.      Seam Test with Vacuum Equipment

A.      Seam Test with Compressed Air:

·         For the double welded seams, between adjacent sheets of waterproofing membrane, testing for tightness shall be carried out by means of compressed air pumped into the channel which is formed by the double welded joint.

·         The initial test pressure shall be 2 bars for test period of 5 minutes or 1.5 bars for a test period of 10 minutes.

·         The joints shall be considered waterproofed if the loss of air pressure is in both cases not more than 20% and all the welding has to be tested (100%).

B.      Seam Test with Vacuum Equipment:

·         For the testing of areas of membrane of limited size such as special configurations of joints or local repairs with patches, vacuum equipment shall be employed, as instructed by Engineer.

·         The equipment consists of a vacuum bell which, after being fitted tightly over the area to be tested, is evacuated by pumping in order to detect leaks in the membrane.

PROTECTIVE MEASURES:

During and after the installation of water proofing membrane care shall be taken not to damage the membrane. If damages occurred shall be, repaired, and tested before the casting of final concrete lining.