China today rushed more troops and air-dropped relief supplies in areas cut off near the epicentre of the devastating earthquake in southwest Sichuan province as the toll mounted to nearly 20,000 with thousands of people still buried, trapped or missing. As troops reached the quake-battered areas, the state media reported that the death toll in Mianyang city alone rose from 3,629 to 5,540 with 18,486 more buried and 1,396 missing. In Yingxiu of Wenchuan County, only 2,300 persons were believed to have survived out of 10,000 residents, official Xinhua news agency said. The toll was expected to rise further once rescuers reach other towns in Wenchuan. Some towns near the epicentre have been "razed to the ground" with no houses left standing, a police official was quoted by the state media as saying. Damage tied to corruption: expert Corruption and lax enforcement of stringent building codes could be important factors behind many of the collapsed buildings in China's worst earthquake in decades, an expert said today. Monday's tremor levelled factories, homes, schools and hospitals across China's southwest Sichuan province. "Enforcement costs money and local officials at many levels are involved,' Ashley Howlett, a partner with Jones Day, who heads the Greater China construction practice, said. "China's building codes are very clear,' he said. "If a similar earthquake hit near Beijing, I don't think you would see this kind of damage.' His comments echo similar complaints aimed at China in the wake of recent product safety and healthcare scares that alarmed consumers around the world. Oldest irrigation system saved An over 2,000-year old irrigation system in Sichuan province has escaped any major damage to its structure in the earthquake, state media has reported. The quake caused cracks in the V-shaped dike of the Dujiangyan, the world's oldest operating water project, with collapses in a sluice control room and a standby power generation room, but the operating system was "not compromised,' the ministry of water resources said. Other major water projects in Sichuan such as the south-to-north water diversion project and the Three Gorges Dam, the world's largest hydroelectric project, too, have suffered no impact from the earthquake, Xinhua reported. The irrigation infrastructure built in 256 BC during