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Green Light for Greener and Safer Slopes

Green Light for Greener and Safer Slopes
It's an arguable precept in the engineering world that what is aesthetically appealing may not necessarily be functional. But the bio-engineering approach to new slope design in Hong Kong is quickly proving that a sound engineering design, if done right, can also be pleasing to the eye.

Tasked with monitoring all sizeable manmade slopes and retaining walls in Hong Kong, the Geotechnical Engineering Office (GEO) of the Civil Engineering Department has not only managed to bring down the risk of landslips through its comprehensive Slope Safety System, its introduction of vegetation and bold landscaping in new slope designs while upgrading existing Government slopes has proven a hit with the public at large and also served its primary designed purpose in ensuring the safety of nearby residents and passersby.

Head of GEO, Mr Raymond Chan, said that : "By 2000, the risk of landslide from old manmade slopes has been reduced to less than 50 per cent of the 1977 level, the year GEO was established. And by 2010, the risk would be further reduced by another 50 per cent of the year 2000 level."

"To put things in their proper perspective, in the 50 years after 1947, more than 470 people were killed as a result of collapse of manmade slopes and retaining walls. But the introduction of the Slope Safety System has essentially cut the number of casualties down to one in the last four years. However, there is no room for complacency about landslide risk and the Government will continue to strive to further reduce risk in partnership with the community.

"The main features of the System comprised the cataloguing of all 54,000 sizeable man-made slopes and retaining walls in Hong Kong, data for which are made available on the Internet (http://hkss.ced.gov.hk); identifying the maintenance responsibility of all the catalogued slopes; setting up an extensive network of automatically recording rain gauges to provide real time rainfall data for the issue of public Landslip Warnings; operation of a 24-hour year-round emergency service by geotechnical engineers to protect public safety; auditing the design and supervision of construction of new slopes to ensure that they meet the latest safety standards; a $900 million per year under the 10-year Extended Landslip Preventive Measures (LPM) Programme to upgrade substandard Government slopes and safety-screen old private slopes from 2000 to 2010; the expenditure of $600 million each year to properly maintain all Government slopes; ensuring that private owners take responsibility for their own slopes through the issue of statutory orders requiring investigation and rectification; an ongoing programme to inspect squatter villages on steep hillsides for priority clearance on safety grounds; and undertaking extensive public education on personal safety during periods of intense rainfall when landslides are likely.

"Proceeding in parallel with this System is GEO's introduction of plant vegetation under the LPM's upgrading works. Our designers try to make the finished slopes look as natural as possible in order to minimise any negative visual impact while improving the environment.

"In the past three years, 767 slopes and retaining walls were landscaped under the LPM Programme. Amongst these, 470 were greened. The others were treated with landscape measures to enhance their appearance and, if possible, to mesh in with their immediate natural surroundings," added Mr Chan, who has had 25 years of experience trying to improve slope stability and appearance.

The Government had begun experimenting with planting of vegetation on slopes in the early 1980s and an integrated approach to landscaping and bio-engineering of slopes at design stage was started in 1999.

"We also attempt to preserve existing mature trees on slopes wherever possible. And unless there are severe site constraints, the gradient of the finished slopes is designed to be suitable for hydroseeding.

"About 80 per cent of some 340 fill slopes and about 45 per cent of some 1090 soil cut slopes upgraded up to July 2001 under the LPM Programme have a vegetated surface cover. We plan to increase these percentages to about 70 per cent of the slopes upgraded in 2001 and 2002," said Mr. Chan.

A survey commissioned by GEO in 2001 showed that about 60 per cent of those interviewed expressed satisfaction with the general appearance of slopes, but 82 per cent considered safety to be the highest priority. This corresponds with the Government's policy to green the slopes wherever it is practicable but to permit "shotcreting", or concrete spraying, where emergency repair involving high-risk slopes is involved.

Apart from being more visually appealing and ecologically beneficial and less heat reflective, vegetation on slopes depletes water in the soil by root intake and transpiration. And its roots bind soil particles together and can increase the near-surface soil strength through root reinforcement effects. However, vegetation takes time to establish and is less effective than shotcrete in preventing water infiltration and surface erosion, both of which have an adverse impact on slope stability.

To control the use of shotcrete, Vetting Committees chaired by senior professional officers have been set up in all Government departments responsible for slope upgrading and maintenance works. In most cases, shotcrete is allowed only in emergency repairs to landslips where there is a high risk of human casualty. Under such circumstances, shotcrete is used as a quick and secure method to remove the immediate danger posed to the public and to avoid prolonged closure, causing inconvenience to the occupants of affected buildings and users of busy roads.

GEO's efforts to popularise the use of vegetation cover on upgraded and new slopes have not only won the general approval of the public but also the recognition of professional bodies. Its publication issued in September 2000, containing the technical guidelines on the use of landscaping and bioengineering on slopes, has won the Grand Award in the "Outstanding Green Project Awards 2000" organised by the Leisure and Cultural Services Department in association with the Hong Kong Institute of Landscape Architects and the Society of Horticulture, Hong Kong.

"We will continue to conduct research on new techniques on bioengineering and landscaping. Several bioengineering techniques which have been used successfully elsewhere are being assessed for use on steep cut slopes with due regard to safety, cost and long-term maintenance requirements. Not all the successful techniques are applicable here because slopes in Hong Kong are generally steep, and the climate and soil conditions may be quite different from that where the techniques have been used successfully. Our standards must be higher because of our population density and close proximity of slopes to buildings and roads," Mr Chan added.

Local condition would in general safely allow plant vegetation on soil slopes inclined up to an angle of about 35 degrees to the horizontal. On steeper soil slopes, vegetation can still be planted but the designer needs to consider carefully whether the resulting higher rates of rainwater infiltration and potential for increased surface erosion are acceptable. Erosion control mats are often used on slopes with slope angles steeper than 35 degrees.

If the inclination is greater than 55 degrees, it is generally more difficult and much more expensive to construct a safe stable vegetation cover. In Hong Kong, where there is often little space available to cut back old slopes, a shotcrete cover is sometimes necessary under emergency repair conditions.

"Another method which have seen increasing usage in strengthening slopes is the use of soil nailing. This techniques allows long steel rods to reinforce the compaction of subsurface materials while allowing the nail heads to blend in with other vegetation on slope surfaces over time. So far, soil nails have been used on about 1000 Government slopes as a means of stabilisation in the past four years," Mr Chan said.

Recalling the establishment of GEO, Mr Chan said this was prompted by Hong Kong's tragic record of fatal landslides. In 1972, a major landslip which occurred in a private construction site at Po Shan Road killed 67 people and another landslip at Sau Mau Ping killed 71 people. These tragedies led to the formation of a specialist section in the then Buildings Ordinance Office, predecessor of the Buildings Department, for the geotechnical control of site formation for private projects.

Unfortunately, another major landslip occurred in Sau Mau Ping Estate in 1976, killing 18 people. The Geotechnical Control Office was set up the following year to regulate the design, construction, maintenance and monitoring of slopes in Hong Kong. Its name was changed to Geotechnical Engineering Office in 1991 to more accurately reflect its expanded role and scope of responsibilities.

In contrast to its predecessor's functions, one of the GEO's current major work area is to audit the design of all new slopes to ensure that they meet the latest safety standards. This is essential to keep the size of the problem of substandard manmade slopes from growing. Because of its importance to the slope safety system, about one third of the GEO's 220 professional staff are devoted to geotechnical control.

Referring to GEO's Catalogue of Slopes, which was an inventory of some 54,000 sizeable manmade slopes, Mr Chan stressed "It is only an inventory of the 37,000 government slopes and 17,000 private slopes. It should not be mistaken to be a 'List of Dangerous Slopes'. The catalogue allowed us to systematically safety screen all of them. If a private slope is found to be substandard, a Dangerous Hillside (DH) Order will be served by the Buildings Department on the owners."

"The Order may require the owner to carry out investigation and, if necessary, upgrading works to the slope. Where Government-owned slopes are found to be substandard, they will be included in the LPM Programme for upgrading.

"Some 250 substandard Government slopes have been upgraded and over 300 private slopes have been safety screened in the past 12 months. The total number of government/private slopes expected to be dealt with upon completion of the 10-year LPM Programme by end of 2010 will total 5,500.

"In view of the large number of pre-GEO slopes, priority for safety screening follows the 'highest risk first' principle. This dictates that priority action be given to large steep slopes adjacent to public housing blocks, residential buildings, schools and hospitals," Mr Chan explained.

Mr Chan advised those served with a Dangerous Hillside Order, as well as residents concerned about their slope safety, to obtain a copy of the GEO's Layman's Guide to Slope Maintenance. The Guide offers advice on maintenance of manmade slopes and retaining walls. It also contained the Model Brief for Engineer Inspections for Maintenance. The Guide can be downloaded from the GEO's Hong Kong Slope Safety Wedsite (http://hkss.ced.gov.hk).

Another useful data source on slope safety is the Slope Maintenance Responsibility Information System managed by the Lands Department (http://

www.info.gov.hk/landsd). This System has reduced considerably what used to be a significant bone of contention among property owners, tenants and management companies with regard to their respective responsibility for slope upkeep.

Mr Chan noted that one of the major components of the Slope Safety System is the ongoing public education campaign and various community advisory services being rendered throughout the year.

"There is no question that the Government would need the community's full cooperation in our constant battle to make the slopes as safe as practicable. The GEO is always ready to offer advice on slope maintenance matters and respond instantaneously as the need arises. Our Community Advisory Unit can be reached on tel. 27605800," Mr Chan added.

End/Friday, April 5, 2002

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