WATERPROOFING (WITH MEMBRANE) METHODOLOGY FOR TUNNEL
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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 550oC
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.
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