INTERNATIONAL FEATURE: LANDFILL RESTORATION
Restoring Hong Kong's Landfills


by Russ Canning, Landscape Architect & Director of Scott Wilson

Russ Canning outlines the approach to the restoration of two of Hong Kong's closed landfills for the benefit of the people of Hong Kong

Hong Kong has 13 landfills that have been closed after having being filled to capacity. Three major 'new generation' strategic landfills opened between 1993 and 1995 and these should meet Hong Kong's needs for the next 10 to 15 years. The closed landfills contain municipal, commercial, industrial, hazardous, and construction wastes and occupy site areas varying between 2 and 68ha. They collectively cover a total area of about 300ha. A number of these closed landfills are located within short distances of high density urban areas.

Because of Hong Kong's limited area and extreme topography - land suitable for open space and active recreational use is in limited supply. These factors, together with the close proximity of the closed landfills to urban areas, contribute to making their re-development for public open space use a priority.

As a consequence of the construction of these 'old generation' sites, their restoration is phased into an initial 'restoration' contract followed by a secondary 'after use' contract.

A unique combination of initiatives by the Hong Kong Environmental Protection Department, the Hong Kong Jockey Club, and Hong Kong's Municipal Councils, is enabling the beneficial use of these landfills as public parks, sports centres, golf courses and ecology parks.

Shuen Wan Golf Centre
Shuen Wan Landfill occupies a site area of about 50ha and was formed by in-filling part of Tolo Harbour in the north east new territories. About 14 million tonnes of municipal, construction, commercial and industrial waste was received between 1973 and 1995. The depth of waste varies up to 40m and the landfill has a frontage of almost 1km overlooking Tolo Harbour. The proposed Golf Centre will occupy about 18ha on three platforms that form the top of the landfill at elevations of 30m, 38m, and 40m above datum.

The Golf Centre includes a nine hole par 3 golf course, practice greens, a golf driving range, two clubhouses and a maintenance depot. The course will operate during daylight hours whilst the main clubhouse and the floodlit driving range will operate day and night until 23:00 hours.

The main clubhouse is located on the highest point of the landfill. Overlooking the golf course, the restaurant and terrace will have panoramic views across Tolo Harbour. A mini driving range located on the 38m platform comprises a single level of driving booths with a 60m chipping range and a shorter 15m driving range with catch netting.

Design objectives
As the site has been used as a landfill for 22 years, the Golf Centre is a positive enhancement of the site; particularly as there is a high demand for public golfing facilities in Hong Kong. The particular design objectives for the Golf Centre were to:
  • establish environmental safety procedures to ensure the safety of all persons using the centre;
  • integrate the Golf Centre with the landfill restoration measures and aftercare;
  • ensure that the construction of the Golf Centre did not interfere with the integrity of the landfill restoration measures;
  • provide a relatively simple par 3 golf course suitable for inexperienced players;
  • provide a driving range for driving, chipping and putting practice;
  • maximise the views across Tolo Harbour;
  • use local plant species with shallow root systems suitable for growing on the landfill cap;
  • provide an environmentally benign course capable of being managed with low water usage and through biological and organic control methods to ensure chemical applications and maintenance are minimised;
  • provide attractive integrated site entrance, landscape and building architecture to enhance the appearance of the site; and
  • introduce new habitats to the landfill cap to enhance the ecological value of the site.
Restoration measures
The Golf Centre is constructed above the landfill capping system, and the leachate and landfill gas extraction infrastructure installed by the landfill restoration contractor. The capping system on the platform comprises:

  • an 850mm thick general cover layer;
  • a geosynthetic (geonet/geotextile combination) drainage layer over the geomembrane;
  • an LLDPE geomembrane;
  • a reinforcement/cushion non-woven geotextile; and
  • a 500mm minimum thickness of soil intermediate cover material over the waste.
The nominally 1 metre thick cover material to protect the capping system is to be placed over the whole of the Golf Centre area above the capping system. Greater depths of soil up to a maximum depth of 3m will be placed where necessary to accommodate utility services, drainage pipes, and irrigation pipes, to provide surface relief by mounding to accommodate tree planting, and for sculpturing the surface profiles of the golf course.

The sub-soil will be compacted to between 90% and 95% relative density under the golf course and landscaped areas, and to at least 95% relative density under buildings, pavements and utility services corridors, where settlements need to be minimised. The gradient of the finished surfaces of the golf course could be as flat as 2% and the subsoil will be covered by various types of growing media as described above.

Flexible pavements will have a surface wearing course of segmental concrete pavers overlying a compacted base, sub-base, and subgrade material. Pavement material thicknesses have been determined assuming that the California Bearing Ratio (CBR) of the underlying waste is less than 2%. The design will allow sections of pavement to be easily regraded if differential settlements over time results in unacceptable pavement surface profiles.

The careful use of pesticides, herbicides and fertilisers in the management of the golf course turf grass and planting is a key issue in ensuring that the water quality discharge standards are met. These aspects have been addressed by integrating the Landscape Management Plan with the Environmental Management Plan. Computer modelling to estimate pesticide concentrations in the runoff was carried out using a dedicated model - PESTRUN.

Drainage on the Golf Course area will merge and integrate inconspicuously with the finished landscape, and at the same time minimise opportunities for drains to claim misguided golf balls. The surface water drainage system is designed for a 1 in 200 year return period rainfall event to cater for the sub-tropical downpours that occur during Hong Kong's wet season and during typhoons. Typical design rainfall intensities are of the order of 325 to 375 mm/hr.

To minimise impacts arising from the consolidation and decomposition of the underlying waste, the following measures were considered in relation to the buildings construction:
  • structures of lightweight steel construction with floors elevated above the landfill cap and with provision for re-levelling are being used;
  • buildings are modular;
  • foundations consist of a floating raft type sited on top of the restored landfill cap (i.e. no excavation into the cap) and are designed to cater for loss of support due to localised ground settlements; and
  • flexible connections are used in building services where they pass into the building.
The possibility of migration of landfill gas into the buildings is considered to be low due to the capping membrane system. However, in case of a failure of the capping membrane, a 12 point programme of measures are being provided as a second line of defence to prevent landfill gas migration.

Jordan Valley Ecology Park

Jordan Valley Landfill occupies the site of a small, high level, disused water storage dam having a site area of about 7ha on the northern edge of Kowloon, overlooking Hong Kong Victoria Harbour.

1.5 million tonnes of domestic, construction, commercial and industrial waste was received between 1986 and 1990. The depth of waste varies up to 50m. The Ecology Park will occupy about 4.3ha comprising the top platform of the landfill at an elevation of 120m above datum. The landfill is bounded by steeply wooded slopes rising from the northern and western boundaries whilst the southern and eastern sides slope away from the platform to the valley and urban Kowloon below.

The development concept is 'to promote the green campaign message and to educate visitors on the concept of ecology and the importance of environmental protection'. However, the ecology park includes, inter alia, a pedestrian plaza with shaded seating, a radio controlled model car racing circuit, gateball pitches, an environmental education centre, and management offices. The park is divided into an active zone, which contains the more formal and noisy or intrusive activities on the northern part of the site next to the busy Clear Water Bay Road, and a passive zone, providing an informal or natural environment away from sources of disturbance to allow quiet enjoyment of the ecology park.

Design objectives
The environmental education concept has been distilled into a design theme which stresses the historical and current importance of certain plant species and communities to Hong Kong, improves environmental awareness, reinforces the conservation message, and demonstrates how an old landfill can be transformed into an attractive and safe environment for public enjoyment. The plant species being established will demonstrate:
  • ecological value (as providers of forage, nesting and roosting habitats for wildlife);
  • scientific value (as maintaining biodiversity and Hong Kong's resource of rare,
  • endemic or endangered species;
  • economic value (as sources of commercial timber, incense, fruit);
  • medicinal value; and
  • as symbols of cultural or social value.
The plant species selected are suitable for growing on the restored landfill site without their root systems posing a threat to the capping system. They do not require intensive and costly management to sustain in the longer term. The woodland, tree and shrub planting will include a 'Fung Shui' woodland surrounding a water feature (located on natural ground), a camphor avenue, bamboo corner, mixed shrubs and herbaceous borders, and open clearings.

The Environmental Education Centre, located on natural ground, will include an exhibition hall, reception counter, refreshment kiosk, toilets, and a rooftop platform and viewing terrace overlooking Hong Kong harbour. An audio visual display will demonstrate to the viewing public the life cycle of both 'old' and 'new' generation landfills from initial site selection through site development, operation, landfilling processes and systems, aftercare and afteruse periods until the landfill is fully stabilised and poses no further threat to the environment. It will include features on options for utilising landfill gas and treating leachate. The display will also stress the need for improved environmental awareness, and encourage people to reduce waste generation to conserve landfill space.

The landfill restoration contractor has similar obligations to the Shuen Wan Landfill in respect of surface water drainage, leachate and landfill gas management, and landscape restoration. In the case of Jordan Valley, the existing soil capping material is at least 2.5m thick. Re-working of up to 1.5m depth of the upper part of the capping material is required to ensure a minimum compaction standard and a minimum surface gradient of 4%.

A new restoration capping system of geosynthetic components, including a LLDPE geomembrane liner, cushion geotextiles, and geodrains, with 850mm thickness of compacted soil cover material is being constructed. The geosynthetic capping system will segregate the afteruse from the underlying waste, and from migration of landfill gas.

The leachate and landfill gas extraction systems are installed beneath geomembrane liner. The drainage layer component of the capping system allows the collection of infiltration from the afteruse separately to the surface water runoff, and thereby separate collection and treatment, if any contamination from fertilisers, pesticides, and herbicides is detected. The capping system is designed to withstand the loading from an additional 3m depth of filling.

Soil depths for different types of planting are up to 2.5m for woodland, tree and shrub planting areas, 1.5m for herbaceous planting areas, and 0.5m for grassed areas and are achieved by placing additional fill above the 850mm deep soil cover of the capping system. The contours and slopes of the finished soil surface have been formed with consideration being given to:
  • providing easily graded walking pathways through the park; and
  • ensuring adequate and effective drainage of the site.
As with Shuen Wan Landfill, under the landfill restoration contract, drains consisting of concrete U channels have been provided around the perimeter of the platforms for permanent drainage, together with temporary drains on the platforms pending completion of the Ecology Park. The perimeter U channels discharge into a number of concrete step channels that convey the surface water down the side slopes of the landfill for discharge into downstream watercourses. Drainage of the park area will merge and integrate inconspicuously with the finished landscape to give the appearance of natural rocky watercourses that flow only in wet weather. The design standards and principles, and the rainfall intensity criteria, are the same as those adopted for the Shuen Wan Landfill Golf Centre.

A 'Fung Shui' water feature is incorporated into the design but it has been possible to locate this feature over natural ground to avoid potential problems with differential settlements and leaks causing unnecessary water infiltration into the capping system if it were located over waste. The Management Office building and Environmental Education Centre building are also located on natural ground and so no special foundation design was need to cater for differential settlement.

Conclusion
The restoration and after use development of the Shuen Wan Golf Centre and Jordan Valley Ecology Park demonstrate that with co-ordinated and innovative solutions to engineering and landscape issues, even recently closed sites can be transformed to safe and beneficial community use.

Acknowledgements

The authors would like to thank the Director of Environmental Protection, Hong Kong SAR Government, and the Construction Projects Controller of the Hong Kong Jockey Club for permission to publish this article.

Wastes Management April 1999
Pages 43-44