Non-woven geotextiles are fabrics that are used in civil works and finds application in water flow management and erosion mitigation, mechanical protection of foundation components, etc. This article looks at the commercial deployment of a new type of conductive geotextile, made possible by imgne® X3 coating, as part of Concept’s unique water storage tanks.
The Role of the Concept Tank
Concept’s solutions for fluid storage find various applications within the mining industry including at well heads and fluid processing facilities, for dust suppression water storage, irrigation water storage, construction water, grey water and potable water at camps. Concept’s storage solutions are also appropriate for wash down bays.
Concept has used Geotextile coated with imgne® X3 coating as part of its lining containment system to deliver considerable savings in construction time, design time, environmental impacts, and expenditure compared with conventional regulated dam construction. It is possible to establish Concept’s fluid storage tanks quickly and also to reuse and relocate it continuously.
To attain this process, containment tanks are designed with a primary and secondary membrane so that the fluids are safely contained from the local environment.
Development of the Tank Solution
Concept’s relocatable storage solutions have capacity ranging from 0.5 megaliters up to 40 megaliters. Concept’s tanks are designed to have a design life of 25 years. Most of the components in the system are reusable or recyclable. Products are manufactured in Australia to Australian Standards and can be deployed across the country.
In this particular project, the final customer was one of four major consortia that have rights to exploit CSG in Queensland. Concept Environmental Services was the company that built and installed the tanks.
This case study describes the process of commissioning and installing the containment tank.
Tank Construction (Concrete or Steel)
The Concept tank is a modular, high wall, open water storage tank. The tank has a low environmental impact which needs minimal excavations as soon as the tank pad has been cleared and compacted. The modular, open water storage tank does not need any concrete footings. A leak detection system and fluid recovery system connected to an external sump pit and pump are the options available for the tank. A satellite monitoring system gives notification of a potential liner breach.
Concrete: For wall construction, pre-stressed concrete panels with adjustable height are used by considering the location of the storage system. The pre-stressing of the panels leads to 20% less concrete - thus reducing CO2 emissions during production and resulting in minimal environmental impact. The concrete panels are subsequently post-tensioned using a unique cable system as part of the tank design that has been developed and patented by Concept.
Steel: Concept’s steel tanks are built from rolled and fabricated steel sections that are ready to be bolted together on site. The tanks’ unique design allows them to be robust and stable, and thus can survive harsh conditions.
The Tank Liner system is applied within the water storage tanks to retain water and to help manage potential leaks within the tank. The Tank Liner in most cases has four layers, each with a different purpose:
1) Primary Liner is the top layer, which retains water within the tank, but other layers are installed to manage the leaks in the event of damage.
2) Composite Net Layer (Geomembrane/Geonet) is an optional layer installed for separating the primary and leak detection layers, to assist in the flow of water.
3) Leak Detection Liner (Secondary Layer) is the layer installed to capture and drain any leaking water to the internal sump which is recirculated to the tank.
4) Geotextile Cushioning Layer (base layer) is employed as a buffer between the earth pad and leak detection layer and reduce the possibility of damage.
Leak Containment System
The Tank Leak Containment System is engineered to handle any leaks within a tank, transmit alerts, facilitate continuous monitoring and return the water to the tank. The operation of the system is:
- The Leak Detection Liner drains any water to the Internal Sump; then
- The water is directed to the External Sump Pit
- The Water Level Sensor records the water level of the External Sump Pit, and the Quad Pump is activated at a set level (for example. 400 mm) and deliver the water back to the tank; at the same time
- The recorded water levels are logged and transmitted to the Concept monitoring system (off-site); then
- An alert is transmitted to the designated persons
- The designated persons monitor the conditions of the tank and an optimum course of action is determined.
Bowen Basin Project
The installation details of the 12 megaliter Bowen Basin Tank are given below:
Geotextile Cushioning Layer
The walls and the ground were covered using Bidim® A34 geotextile. Bidim A34 is a ca. 280 g/m2 non-woven geotextile manufactured by Geofabrics Australasia Pty Ltd. Parallel copper wires were laid every 5 m to 10 m across the Bidim. This was to enable third party spark testing.
Leak Detection Liner
Layfield Enviro Liner 6040HD was used as leak detection layer (the secondary membrane)
Composite Net layer
Geofabrics Bidim-C an electrically conducting geotextile (ca. 240g/m2) made conductive with imgne® X3 coating from Imagine Intelligent Materials Pty Ltd. This was to enable third party spark testing.
Figure 1. General view of the lined 12 megaliter CSG process water tank.
Layfield Enviro Liner 6040HD was used as the primary liner (the primary membrane). It is necessary to note that in previous installations where Bidim-C (with imgne® X3) was unavailable to facilitate leak detection testing, the geotextile had to be wetted with water, adding cost and complexity to the construction. Here, in the first full-scale installation using Bidim-C, only the primary geotextile layer was replaced with conductive Bidim-C. The secondary layer of non-conductive geotextile and copper wires were retained as a reference.
Figure 2. Electrical inspection of the Primary Liner of 12 megaliter CSG process water tank.
Besides the cost of trucking in water, the traditional installation method using wetted non-conductive geotextile can give false reports regarding the presence of a leak – due to the presence of water in the geotextile. As Bidim-C does not need wetting, these false positives can be eliminated.
It has been noted that the occurrence of false positives has previously resulted in the perception of presence of leaks, which in turn resulted in the need to drain the tank (or dam) and retest. The draining, transportation, and storage of fluid from a tank under these situations is extremely costly and hence there is a high motivation to avoid this from occurring. While the engineers who undertake the installation might state that the tank is in fact leak free, they have no option but to respond to their customer’s requirement to re-check for leaks.
Bowen Basin Tank Specifications
Bowen Basin Tank (#1 installation)
- 59 m across
- 60 panels
- 2.5 m high walls
- 6.5 megaliters volume
Bowen Basin Tank (#2 installation)
- 79 m diameter
- 102 panels
- 2.5 m high walls
- 12 megaliters volume
Details of Tank Construction
- Earthworks team prepares the ground at the site according to geotechnical engineering requirements and requirements for leak detection system requirements and all round tank access.
- Leak detection external and internal sumps are installed.
- Tank construction begins with the placement of the steel posts on the ground. The posts are free standing and DO NOT require concrete or anchoring to the ground. The pre-stressed concrete panels are placed between the steel posts.
- Steel cable is then run around the tank at pre-determined height and is subsequently tensioned. This enables the tank to retain the required forces as appropriate to the fluid being stored.
- A lining team then installs the A34 Bidim® base layer of the lining system. This is applied to the ground surface inside the tank and to the inner walls (secondary geotextile) with an aim to protect the secondary liner that is subsequently installed on top.
- The lining team lays a copper wire along a diagonal of the tank, followed by laying parallel wires to the initial wire at a spacing of one every 5 m. The copper wires are connected to the steel tensioning cables for common grounding. These improve the conductivity of the A34 geotextile for testing the liner. Using a spark tester, the conductivity between copper wires is tested before laying liner.
- At this stage, further conductivity can be offered by wetting the A34 geotextile to provide conductivity between the copper wires.
- The lining team then lays the secondary liner, which is supplied in large, pre-welded pieces. The pieces themselves are made of rolls of membrane that have been previously welded together at the factory.
- The lining team performs reinforcing welds and patches on the joins in the pre-welded pieces. These reinforcing welds and patches are cataloged and tracked.
- Using a Sensor spark system, the liner testing organization tests the secondary membrane system by generating a high voltage between a handheld probe and the topmost of the steel tensioning cables which acts as an electrical ground attachment point.
- The liner team catalogs and repairs any holes that have been found and then the repairs are spark tested.
- Bidim-C from 3 m wide rolls is then installed as the primary geotextile with conductive side facing into the tank with adjacent lanes welded together:
- Along the floor of the tank
- in parallel lanes with 200 mm overlap
- conductive side up
- Along the wall of the tank
- by being unrolled sideways along the inner face of the wall
- with the lane of Bidim’s upper edge tied to the topmost steel cable for the purpose of:
- temporary support
- electrical connection to ground
- It’s welded
- Along the overlaps between lanes and also where the lane hung along the wall drapes onto the tank floor
- So it doesn’t get shifted around when the membrane is being laid on top of it (which would cause holidays in the coverage)
- The geotextile welds are tested for mechanical strength and repaired, if needed.
- The primary membrane is installed using the similar process as the secondary.
- The primary membrane is tested with a Sensor spark tester using the conductive Bidim-C as the conductive substrate.
The time taken for tank Installation and lining depends on the size of the tank.
- 1.9 megaliters – 2 days for tank construction, 2 days for liner system installation
- 12 megaliters – 3-4 days for tank construction, 10 days for liner system installation
- 25 megaliters – 8 days for tank construction, 14 days for liner system installation
These are only indicative times and can change depending on site conditions, and lining system required.
The 12 megaliter tank in this case study required 3 days for tank construction and 12 days for liner system installation due to weather delays.
- Concept’s alternative to Bidim-C is to lay geotextile and copper wire and to then wet the Bidim. This has the following challenges:
- It needs a large quantity of water to wet
- If it stays there for a long time before testing, the water evaporates.
- Copper wires are a trip hazard
- Once the geotextile is wet and the liner goes on top, the leak detection system (which the aforementioned sump is part of) can give a false reading.
- Using a conductive Bidim-C underneath the secondary liner would provide the same benefits for testing the secondary as it does for the primary.
- Geotest estimates the main reasons of holes found in the liner:
- 90% are due to crease damage caused by handling or wrinkles after it's been laid
- 10% are due to the T welds and patches that are done as a matter of course following assembling and welding the large pieces that come from the factory. The installers do ‘T’ welds and patches at the end to resist fraying because even the large pieces that come from the factory are themselves made of lanes of liner.
Bidim-C can Reduce Installation Costs:
- The amount of water needed to wet geotextile to make it adequately conductive for testing is quite large. If the site is remote, then the cost to transport can be very high
- When geotextile is wetted down, testing must be done quickly otherwise the water will evaporate, especially in a hot and dry climate such as is the case in Queensland where this installation took place
- Water used to wet geotextile between the primary and secondary can result in false leak reports in leak detection systems which can necessitate the draining, testing, and refilling of the tank in question. This can be very expensive if the tank is remote.
This information has been sourced, reviewed and adapted from materials provided by Imagine Intelligent Materials.
For more information on this source, please visit Imagine Intelligent Materials.