• Introduction

• HDPE lining membrane lends itself to an excellent lining system for a wide variety of applications in the area of water storage and conveyance. The combination of physical properties, large rolls without prefabrication, ease of high welding quality, and availability in large thickness are among the design considerations for selecting HDPE membrane liners.  In addition, its complete inertness makes it safe to carry drinking (Potable) water. HDPE is free of plasticizers , fillers, lubricants or any other toxic or tainting substances that could leak out during its lifetime.   HDPE liners were tested by the metropolitan water board (UK) and the NSF (USA) and was found safe for storing drinking water. HDPE has close molecular structure , thereby resisting the adhesion of algae and similar growths; snails or limpets may be scraped from the installed lining without risk of puncture .

• HDPE membrane lining for water reservoirs may take a number of different forms

• A complete impermeable lining of embankment and base without cover
• A complete lining with cover of earth or gravel for reinstatement offish or plant life .
•  Embankment lining to prevent erosion by wave action and rapid water level changes .
• An intermediate layer between soil binding and structural concrete to safeguard future cracking of the concrete.
  
  
• HDPE applications for water storage and conveyance include :

• Lining of water conservation reservoirs .
• Lining of irrigation canals. Sealing of earth and rock fill dams and dikes .
• Lining of potable and raw water reservoirs for water supply waste .
• Lining of rain harvest reservoirs
• Lining of balancing reservoirs for irrigation .
• Lining offish ponds .

• HDPE lining has offered great opportunities for the economic use of earth reservoirs. Earth reservoirs constructed either by excavation in the natural ground or enclosing a site with embankments have always been the choice of engineers for cost-effective storage facilities . However, this practice was restrained by the availability of sites with impermeable soils. Alternatively, various types of less effective lining were used to control seepage from the storage facility. HDPE is widely used as the optimal lining system for water conservation reservoirs .

The use of FMLs (HDPE) to achieve waterproofing in hydraulic structures such as reservoirs Canals and dams is now a well established practice. As for ground water protection applications, the excellent properties of HDPE sheet can provide secure water proofing solutions which meet the extremely long service life time requirements which arise in many hydraulic engineering applications .

• HDPE sheet has excellent resistance to degradation under the influence of ultraviolet light due to the incorporation of up to 2.5% Carbon black which acts as an inhibitor against the photochemical reactions which result in the breakdown of the molecular structure of the polyethylene.

• Stress crack corrosion, is the primary determinant of service lifetime in buried application, i.e. where the surface of the FML (HDPE) sheet is protected from radiation and other weathering effects. Stress Crack Corrosion is a physical phenomenon whereby the crystal structure of the polyethylene is disrupted by the introduction of other molecules. The ability of an FML to resist the stress crack corrosion is very much dependent on the following:

• Nature of the resin.
• The process by which the sheet is manufactured.

HDPE sheet should has outstanding capability to resist stress crack corrosion; under ASTM 1693 some products achieve a time to failure in excess of 2000 hours and some grades of HDPE exceed 5000 hours. iii.   Protection against weathering is not the only reason that protective materials are applied in the engineering of hydraulic Structures Containing FMLs. Also to be considered are factors such as wind load and wave action as well as protection against accidental damage and vandalism-particularly where public access may be involved

Protective Coverings may include

• Cast in situ concrete.
• Revetment blocks,
• Rip - Rap.
• In some cases simple soil coverings or gravel finishes may be adequate.
  

In all cases the protective cover requires careful engineering to ensure that it maintains its stability throughout its service lifetime.

• The biological properties of FMLs(HDPE)in Hydraulic applications are also important; not only resistance to biodegradation, as in the case of ground water protection applications, but also other factors such as:

• Resistance to root penetration.
• Attack by insects such as termites, and burrowing animals are vital to ensure the longevity of the lining system.

• In potable water storage, the excellent Properties of HDPE sheet already mentioned are supplement by its physiological inertness. The principle advantage of HDPE is that it Contains no materials such as plasticizers, which can be dissolved into the water, and the site jointing techniques are based on fusion of the parent materials and not on the introduction of additional components such as solvents, adhesives etc. HDPE has Proven to be the most versatile of FML materials in regards to possible water applications

• The following are typical ideal HDPE lining system applications in the area of water conveyance and storage.

Large area lakes are often the main element in the design of leisure parks for urban concentrations. The lakes are used for a variety of applications , and include protection areas for flora and fauna, swimming area , marinas, fishing, etc... The banks of these pleasure lakes are designed in accordance with their application- , e.e. a flat- sandy beach for a swimming area or a vertical, massive wall for marinas .

HDPE sheet offers the design engineer a wider range of solutions than do clay or asphalted products, and standard installation methods can be used to carry out pipe penetrations and connections to structures. HDPE liner has such a high resistance to puncture force that it can withstand usual operational stresses of these applications without an additional protection cover. The embankment areas can, however, be constructed in accordance with the requirements of the specific area, such as public access, boating, and root penetration. HDPE sheet is resistant to all types of root penetration and to attack by rodents .

Lining of swimming ponds

The inner lining of concrete tanks is often made necessary when zero permeation is required. Ideally, the lining system should be designed into the project, however, it is usually possible to the line existing tanks . HDPE sheet T-Grip concrete bonding system is particularly suitable for the inner lining of concrete tanks. This product offers long-term bonding of HDPE to concrete without any further jointing (except for extrusion welding of the individual panels). This system and bond is resistant to leaks through the concrete and will not fail even under extreme conditions, such as concrete swelling etc ... Structural sealing applications require particularly careful design and execution. These projects can be carried out with a single FML layer. An injection system can, in addition to the control design, be installed into the system to facilitate repair of the control area .