Compacted earthen liners (insitu)

Only suitable if insitu soils exhibit properties that will result in reduced permeability under compaction.

Compaction of material on batters can be difficult.

Technique will be limited to channels of sufficient size to accommodate compaction equipment.

Compaction equipment and experienced contractors widely available.

Expected seepage reduction is in the range of 70% to 90%

Estimated to have an effective life of up to 30 years.

Greater durability can be achieved by covering with topsoil or crushed rock.

Annual maintenance to check on integrity of liner.

Mechanical desilting methods not suitable due to potential damage to lining.

Generally a low cost lining option.

Compacted earthen liners (imported)

Requires the availability of suitable borrow material close to the remediation site.

Clay lining is a standard and straightforward technique for channel remediation, in which water authorities and earthwork contractors have expertise and equipment.

Imported material can be blended with insitu material to reduce haulage.

Access for compaction equipment may be limited for small channels.

Clay liner can be covered to improve longevity.

Expected seepage reduction is in the range of 70% to 90%.

Estimated to have an effective life of up to 30 years.

Greater durability can be achieved by covering with topsoil or crushed rock.

Annual maintenance to check on integrity of liner.

Mechanical desilting methods not suitable due to potential damage to lining.

Generally a low to medium cost lining option, especially if suitable materials are found nearby.

(eg approx $15/m² in 2003)

Bank Remodelling

Mitigates lateral seepage only.

Is a common technique, with good availability of equipment and resources.

N/A

Remodelled/ reconstructed banks are estimated to last for 100 years

Inspections for erosion, piping due to dispersion and shrinkage cracks.

Prevention/removal of tree growth.

Low

Loose Earthen Liners

Placement of a loose and uncompacted layer of selected clay soils spread over the channel bed and batters up to 300mm thick.

Ongoing treatment may decrease available waterway area.

N/A - One trial reported seepage rates through a 150mm loose earth blanket to be 4 times the rate through a 600mm compacted liner using the same material.

Loose earth liners are expected to be effective for only a short time - up to 5 years.

Some longer term benefits may result were the subgrade soil has pores into which the fine-grained lining particles can penetrate and become entrapped.

Continuing placement of loose material may be required to maintain seepage reduction.

Low

Soil Modification

Requires application of additives to soil substrate to improve material properties. Additives include lime, cement, resins, swelling clays and chemicals.

Following modification the liner is constructed similar to a compacted earth liner.

Expected seepage reduction varies considerably with materials used.

It can be as high as 80% to 90% with concrete.

 A seepage rate reduction of 70% is expected with bentonite.

Modified soil mixtures are estimated to have an effective life of 30 years, similar to compacted earth liners depending on the additives used.

Annual maintenance to check on integrity of liner.

Mechanical desilting methods not suitable due to potential damage to lining.

Generally medium to high.

Soil modification is an additional cost to a compacted clay liner.

Soil Sealants

Soil sealants are generally applied either directly to the sub-grade or dispersed in flowing or standing water.

Materials include silts and clays, bentonite, resinous polymers, soda ash, and asphalt emulsions.

Seepage reductions vary from 65% to 90% but only for a short time after application.

Generally provide good seepage remediation during the first few seasons of service only.

Would require frequent reapplication to achieve long-term success.

Medium

Concrete Formwork

Will require extensive site preparation, access to skilled workers and a source of cement.

Successful installation is reliant on a properly prepared and stable subgrade. Contraction joints are required to minimise cracking.

Concrete susceptible to damage from groundwater pressure, expansive soils, and undermining from piping in dispersive soils.

Can expect better than 95% seepage reduction in a well functioning system.

Normally 90% seepage reduction expected with time.

Estimated to have an effective life of 40 to 60 years.

Failure has occurred due to adverse subgrade conditions (expansive clays and piping failure), excessive hydrostatic pressure, and damage from freezing and thawing.

Annual maintenance to repair fractures and seal cracks is vital to the long-term success of seepage management.

 Water leaking from cracks for an extended period can erode the subgrade and cause failure of whole sections.

Very high       (est $47 - $52/m² in 1998)

Grouted Fabric Mats

Requires minimal subgrade preparation, removing only large boulders.

Installation of the grouted mats requires skilled labour and specialist equipment.

Can be installed in operating channels

Seepage reduction has varied from 98% immediately after installation to 70% after 7 years service.

The geomembrane is expected to erode after several years leaving the concrete pillows exposed.

The liner should still be operational in this state and can be expected to last for up to 40 years.

Requires maintenance repairs, mainly to transverse cracks and to holes that develop in places where the grout mattress was not installed to the required thickness.

High

($25/m² in 1994)

Shotcrete

Requires some earthworks preparation to remove loose rocks and channel debris.

Installation of Shotcrete requires skilled labour and specialist equipment.

Uniform thickness is crucial to prevent weak spots.

Shotcrete susceptible to damage from groundwater pressure, settlement, expansive soils, and undermining from piping in dispersive soils.

A long-term seepage reduction of 60 to 70% is possible for a well designed and installed system.

Estimated to have an effective life of up to 40 years.

Annual maintenance to repair fractures and holes in lining.

High

($30/m² in 1998)

Pipelining

Recognised as being more suitable for channels of small capacity.

Requires less land (as it is buried).

Provides savings in O&M.

High initial cost.

Expected to reduce seepage by

95 to 100%.

Effective life of more than 60 years expected.

Regular inspection and maintenance required.

High to very high depending on pipe size.

Covered

Requires extensive site preparation including over excavation and forming and smoothing of the channel profile.

Flatter Batters are required for cover material stability.

Durability and thickness requirements of material and hence cost is reduced due to protective cover of backfill.

 Protection reduces degradation of liner due to UV, thermal expansion, direct puncture damage.

Liner is susceptible to damage during placement and backfill.

Wet and windy conditions can make installation difficult.

Liner delivered in rolls to site and joined in the field after careful laying using specialised equipment. Joins are the weakest points and are therefore critical.

Cost of over-excavation and cover with earth or concrete needs to be considered.

Many liner options available and varying prices including PVC, HDPE, VLDPE, LLDPE, and GCL.

Seepage reductions in excess of 95% for all of the case studies reviewed.

The type thickness of the liner will also influence the seepage rate.

Experience suggests this varies considerably.

Life expectancy is generally 40 years with an earth cover.

This is extended to 40 to 60 years with a concrete cover.

Difficult to inspect and locate and repair damage.

Annual maintenance to check the integrity of cover, liner and anchor system (trenches and metal strips).

Troubleshooting to fix tears and split seams as required.

May require specialist contractors.

Low to medium

Highly variable depending on subgrade, liner type and thickness, and cover material type and thickness.

Exposed

Will require extensive site preparation including forming and smoothing of the channel profile, but less excavation than covered membranes.

Steeper batters can be used to limit width of channel.

Liner delivered in rolls to site and joined in the field after careful laying using specialised equipment.

Joins are the weakest points and are therefore critical.

Some materials can be prefabricated into sheets and unfolded quickly into place.

Wet and windy conditions can make installation difficult.

Exposed liners susceptible to groundwater uplift, wind, UV damage, thermal expansion stresses and damage from vehicle and animal traffic.

Materials include polyethylene, rubber, asphalt and geofabric products.

Seepage reductions in excess of 85% for all of the case studies reviewed.

The type of plastic and lining thickness will also influence the seepage rate.

Experience suggests this varies considerably.

Life expectancy is generally between 20 and 40 years with the latest materials, although previous installations have failed after only 4-7 years.

Failure generally occurs on the batters due to exposure to elements and damage, changes in temperature, and creep of material down batters.

Critical factor is UV performance.

As above, but inspection and repair of the liner is much easier in exposed conditions, however, it is also more susceptible to exposure and damage from thermal expansion, creep vehicles, animals and vandals.

USA research has shown regular repairs of leaks and tears are required to achieve the documented life expectancy.

Mechanical desilting will not be appropriate with exposed flexible membrane liners.

Medium

Reduced subgrade preparation and cover costs.

Material costs are generally higher than covered applications.

Material costs vary.

Cut Off Trench

No works required in channel profile.

Trenching and drainage works undertaken on one or both banks depending on extent and location of seepage.

Numerous low permeability materials can be installed as the core such as clay, bentonite, concrete or flexible membranes.

May be undertaken while channel is operational.

Its success is site dependent, it works if the trench intersects a relatively impermeable barrier.

It is estimated to reduce seepage by < 50%.

Estimated life expectancy of 25 years.

However new seepage paths around or under the cut-off have been known to occur after one or two seasons.

Difficult to maintain.

Medium

But highly dependent on depth of cut-off and core material.

Ground-water Pumping

Interception of seepage by pumping from shallow bores adjacent to channel.

 Consideration needs to be given to disposal of water.

Manage-ment option to intercept seepage to reduce its impact on surrounding areas.

Dependent on type of pumping system installed.

Requires ongoing operation and maintenance of pumping infrastructure.

Medium to high

Trees

Selection and planting of appropriate species with the aim of managing the impacts of seepage, not to reduce seepage.

Manage-ment option to prevent impacts of seepage through reducing the groundwater level.

Has been successful at low seepage sites.

Dependent on species and site conditions.

Little to no maintenance however, measurement of groundwater levels should be conducted to ascertain success.

Low

Tile Drains

Requires trench construction and/or pipe laying, as well as collection dams and mechanisms for disposal or reintroduction of the collected water into the system.

Manage-ment option to prevent impacts of seepage through reducing the groundwater level.

Requires ongoing maintenance of drains and disposal options for desired life of project.

Requires maintenance of pumping operations.

Measurement of groundwater levels should be conducted to ascertain success.

Medium