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UNIQUE GEOMEMBRANE LINING DESIGN AND INSTALLATION
TO THE SECONDARY EFFLUENT TREATMENT PLANT, MONDI, RICHARDS BAY
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R.G Drayton Pr Eng. BSc Eng (Civ.)
AQUATAN (PTY) LTD
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MONDI BUSINESS PAPER’S Richards Bay mill required a secondary treatment
process facility to be able to treat effluent for the purposes of re-use or for discharging back into the natural environment.
This project was unique by virtue
of the size and capacity of the tank required to meet the client’s needs. It was undertaken by a Design Construct Consortium comprising Grinaker-LTA Civil
Engineering (GLTA), the construction team: Aquatan (Pty) Ltd, the Design and Installation of the membrane liner to the floor and wall/floor joint: PDNA, Structural
Engineers on the main tank, measuring well and cooling tower supporting structure: Semane, Structural Engineers on the surrounding infrastructure and technical
building, while ARQ undertook the geotechnical investigation and task of designing the secondary tank founding structure.
The 123,3m diameter outer tank and 72m
diameter inner tank with support infrastructure directing flow in and out of the secondary treatment tank were designed and constructed in the remarkably short time of
12 months. Construction started on 1 September 2004 with completion on 15 September 2005. This is one of the largest plants in the world for the treatment
of pulp and paper wastewater.
The Secondary Effluent Treatment Tank Details
As stated above, the Effluent tank consists of two concrete circular
tanks, the outer tank wall of 123,3m ø and 10,5m high (The Aeration Basin) and the inner tank wall of 72m ø and 6,5m high (The Clarifier) with a central integral
concrete core of 12m ø.
1) The requirement for a Geomembrane Liner
A conventional concrete floor would not have been feasible due to the following constraints:
1.1) The walls and internal core were supported on a pile and pile cap arrangement where various vertical settlements were expected (Outer wall – 8mm, inner wall – 22mm and central core 16mm).
1.2) The floors of the tanks were also expected to have substantial differential settlements of
20mm to the outer aeration tank and 26mm to the inner clarifier tank.
1.3) The walls were designed
for various movements both inwards and outwards due to tensioning (elastic shortening), creep, shrinkage and temperature change. The net result was that the
outer wall could move a total distance of 120mm horizontally and the inner wall a total distance of 80mm horizontally! Certainly these movements could not be
ignored in a water retaining structure and bearing in mind that these movements would occur at various phases both during the construction stage and the service life
of the Effluent Tank.
1.4) In the Aeration basin (the outer tank) there was a requirement for 5 No.
division walls, which spanned between the two circular walls. These walls were stationary and therefore had to accommodate and not impede the circular wall
designed movements.
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2) The Challenge for a Suitable Geomembrane Liner
Considering all of the above complications, it was evident that the customary dam lining materials would not be suitable. It was decided to use
Hyperliner, an Ethylene Vinyl Acetate material as a Geomembrane liner. This material accommodates differential and multiaxial movements and has an extensive
elongation characteristic prior to yield and break stresses. Due to construction activities over the installed liner, a 2,0mm thickness was proposed. Also
due to the concrete overlay (described later), the lack of UV resistance of this material was not an inhibiting factor. An additional advantage with EVA is that
it is possible to manufacture an extruded water bar out of exactly the same formulated material. This advantage will be described below.
3) Basic Design
The floor substrate generally consisted of a substantially thick compacted sand
backfill layer (±4m) between the walls. This was followed by an 80mm no fines layer and leakage detection system with a 20mm thick mortar topping to provide a
smooth surface for the 2,0mm Hyperliner Membrane.An 80mm thick reinforced concrete slab was then cast on top of the Hyperliner as a protection layer to prevent any
future operational damage from pedestrial traffic or mechanical scraper arms etc.
A specially manufactured continuous Hyperliner rearguard waterbar strip with 4
protrusions was supplied to and cast into the walls by Grinaker/LTA approximately 600mm above the floor level.
4) Detailed Design and Solution
4.1) Waterbar
The entire
waterproofing of the lining system relied on the homogenous casting in of the waterbar into the walls. Special attention was provided to ensure continuity around
the protruding pre-stressing buttresses. This necessitated meticulous on site hot knife jointing methods.
As an added precaution an innovatively designed seal was extruded between the top of the waterbar and the
concrete and then a gum tape bandage was adhered between the top of the waterbar and the concrete wall. The floor membrane was then later extrusion welded to the horizontal waterbar. This extruded weld was
quality checked using the spark testing method.
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4.2) Differential Vertical Settlement between Wall and Floor
In order to prevent any shearing of the liner at the wall/floor joint, a system of 180mm thick circumferential jockey slabs were cast onto the
circumferential ring beam which supported the wall bearings.
The joints between each jockey slab had an aluminium strip nailed one
side, which accommodated any differential movement (both horizontal and vertical) between adjacent slabs.
4.3) Horizontal Wall Movements
It is felt that this particular detailed design ensured the complete integrity and success of the
water proofing system and hence the entire project!
In order to accommodate the substantial and varying wall movements, a triangular corner fillet
was cast at the wall/floor joint on top of the jockey slabs. This fillet was tied to the wall and a sliding joint was provided on top of the
jockey slab. This ensured that when wall movement occurred, the entire fillet moved with the wall by sliding on top of the jockey slab. Special
consideration had to be taken of what movements occurred first to ensure that the liner was not stressed (or “stretched”) at any stage.
In addition, one had to consider that the inward movement did not “pinch” or “squeeze” the liner against the 80mm concrete slab
overlay.
4.4) Stationary Internal Division Walls and Columns
As there was no horizontal movement to these internal walls and columns, it was not necessary to
make any provision for a movement joint. Where these walls joined the circular moving walls, they were supported vertically with a corbel on each
side and a sufficient gap was left between the dividing wall and the circular walls to ensure free movement.
4.5) Central Core
At the central core, in the Clarifier, a system of jockey
slabs was once again used to accommodate the vertical settlement.
The liner was mechanically fixed to the central core using a radiused 50
x 50 x 5mm Stainless Steel Angle, Gasketting and Bolts.
4.6) Pipe Support Columns
In order to eliminate any possible leakage at these structures, it was decided to line the entire columns. This operation incorporated some
intricate welding work around some of the interconnecting column tie beams.
5) Site Installation and Quality Control
5.1) Casting of 80mm Concrete Overlay on Top of Liner
Excellent cooperation
and liaison existed and was certainly necessary between the Grinaker/LTA and Aquatan site teams. Stringent conditions and controls were administered
to minimise pedestrian traffic over the exposed liner. Full cooperation was required on both sides with regard to the planning and programming of the
Geomembrane laying and the concrete casting operations. One welding operator was in full attendance during all concreting operations, both day and
night, to inspect and repair any accidental damage to the liner.
5.2) Waterbar Installation and Sealing
As steel shutters were used for the walls, Grinaker/LTA had to design a special fixing system to hold the waterbar in place and to form a recess
for the sealant at the top of the waterbar. The system worked well and the sealant and Gum tape bandage ensured an additional positive waterproofing
system.
5.3) Quality Assurance and Control
Due to the vulnerability of
the membrane on an active construction site, a stringent system of checking and rechecking was carried out. The subsurface was carefully checked by
both parties, signed off and all welds were checked using air pressure testing, spark testing and vacuum testing. The use of a clear Hyperliner
welding rod had the added advantage in that a visual check for off centre welding could be carried out.
Specially fabricated rubber tipped shovels
were used for the concreting operations to prevent any mechanical damage to the Geomembrane Liner.
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CONCLUSION
Considering all of the above implications, difficulties and the fast track programme and the end result in that NO LEAKAGE occurred, the Client can consider the Geomembrane Lining a complete success.
This was truly a phenomenal project, demanding the highest level of design ingenuity and careful supervision while continuously challenging conventional
Geomembrane installation and construction methods. This project is a testament to the dedication, commitment and skill of the on site installation team,
construction personnel, designers and the management of Aquatan and Grinaker/LTA.
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