An engineer responsible for a hotly disputed plan to put a wall along part of Old Peak Road says the planting of trees or shrubs is not a suitable alternative to stop people falling from the popular Hong Kong Island walking trail. Such plantings would require water pipes for irrigation, which could leak and damage the steep hillside's stability, senior Highways Department engineer Wong Chun-hung said yesterday - an explanation termed laughable by a leading campaigner against the wall. Wong was speaking to Central and Western district councillors and the public about three planned sections of wall adding up to 28 metres - down from four sections and 46 metres - which walkers and residents say are unnecessary and will spoil the view. Opponent Vivian Leung Tai Yuet-kam, a founder of the Lung Fu Shan Environmental Concern Group, called the explanation laughable. 'It tells us everything - the wall must be built.' Civic Party legislator Tanya Chan, one of the few district councillors who opposes the wall proposal, said walkers knew how to take care of their safety. She said warning signs should be enough to remind them to be vigilant.