Like avalanches, mass movements of soil can leave a trail of death and disaster. Peter Conradi reports on how scientists in China are trying to prevent them.In the past century, hundreds of thousands of Chinese have been killed by thick layers of loess, a type of soil, that suddenly rushed down hillsides. Scientists are trying to determine how these deposits become unstable, how to reduce the effects and how to predict loess failure.
It must have been a frightening but spectacular sight: a solitary man "surfing" down a mountainside, riding on a moving sea of soil and clutching a tree for dear life. It happened in 1983 in the Chinese province of Gansu. The loess deposits of the Sale Shan mountain suddenly started moving. In less than a minute, more than 50 million cubic metres of loess and underlying rock rushed into the valley, covering a distance of 1.5km. The "surfer" was the only survivor; 217 people were killed.
Five per cent of the earth's surface is covered with loess, a very fertile but at the same time highly erodible soil. In the United Kingdom, it is found mainly in small pockets or thin blankets in the Southeast, the brick earths of Kent. In China, somewhat larger deposits are encountered: some 600,000km2, about 2.5 times the area of the UK. The largest uninterrupted cover is found in the Chinese Loess Plateau, where thicknesses of 100m to more than 300m are common.
The Chinese loess is in essence dust from the Himalayan mountains. Glaciers grind the rocks of the Himalayas into large quantities of dust, which streams carry to the Tibetan Plateau. The weathering of exposed rocks also produces fine dust particles in this vast region. Carried by wind into China, the result is loess.
The Sale Shan disaster led the Chinese government to initiate widespread research into mass movements of loess deposits. Among others, international scientists supported by the European Union became involved. One is Tom Dijkstra of Coventry University. He carried out research mainly in the loess region of Lanzhou, capital city of Gansu province.
Dijkstra says: "A large number of loess mass movements occurs in the Lanzhou region. Sometimes these processes are triggered as a consequence of internal weakening of the loess. However, there are also important external triggers, such as earthquakes."
The 3 million inhabitants of Lanzhou, which is in a seismically active region, are in a perilous position. Dijkstra says: "In 1920, a large earthquake (8.6 on the Richter scale) took place near Haiyuan just northeast of Lanzhou. The seismic shocks initiated many slope failures: an area the size of Scotland became mobile. Estimates of the number of casualties are 200,000 to 400,000 people. It was one of the most devastating natural disasters in history." It appears that seismic activity in the region is cyclic, and analyses of written historical sources - some relating to events more than 2,000 years ago - indicate that an earthquake comparable to that of 1920 is due.
Dijkstra says that a fatalistic approach has been dominant until recently, but the Sale Shan disaster made the authorities more aware of the problem. This meant more support for loess research, and much greater insights into the processes that lead to loess slope instability.
To protect the city, a change in land use also needs to be considered. The landscape around Lanzhou City is intensively farmed. Even very steep slopes are being terraced to allow agriculture. However, rainfall is low, only 125mm a year. Irrigation is used to support agriculture and reforestation. But this produces excess water, which drains into the soils and gradually weakens the structure of the loess deposits, making the slopes more susceptible to movement.
Dijkstra's research focused on the factors that influence the stability of a loess slope. He investigated properties such as density, cohesion, friction, plasticity and void ratio, but also looked at environmental parameters including moisture content, slope angles and the influence of the underlying bedrock.
"Loess particles are 80 to 90 per cent quartz, connected by 'bridges' consisting mainly of calcium carbonate and clay," Dijkstra says. "Wetting this loess results in dissolution of the 'bridges'. This will cause the rapid collapse of the structure."
Dry loess can also collapse, and the danger to the human environment is then many times greater. When a dry loess slope collapses, air can get trapped underneath and - like an avalanche - the whole mass can rush down the slope. A dry loess mass movement can reach speeds exceeding 60km an hour and escape is impossible, whereas wet loess is bulkier and slower.
Dijkstra says that the research carried out on the plateau will have solid benefits for its inhabitants. "A natural process such as a loess mass movement is difficult to halt. It is possible, however, to use our knowledge to reduce the negative effects. We now know a lot more about the variations in strength of the Lanzhou loess, which allows a better insight into loess slope failures. It will be possible to design more appropriate constructions that should stabilise slopes more effectively. When these structures are too expensive or technically not feasible, it will be possible to determine the region most at risk and evacuation of communities should then be considered."