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A major infrastructure project, the South-to-North Water Transfer Project, has been proposed to carry potable water from the Yangtze River region in the south to many arid and semi-arid areas in the northern regions of China, including Beijing. The proposed 1200 km ‘middle route’ of the project is likely to be an open channel with a trapezoidal cross-section. At least 180 km of the proposed excavated canal will pass through areas of unsaturated expansive soils. An 11 m high cut slope in a typical expansive clay with medium plasticity in Zaoyang, close to the ‘middle route’ in Hubei, was selected for a comprehensive and well-instrumented field study of the effects of rainfall infiltration. Two artificial rainfall events were created during a month of field investigation and monitoring. Prior to the rainfall events, high soil suction was measured within the top 1 m of soil. This high initial soil suction created an upward flux of water and moisture. The abundant cracks and fissures near the ground surface played a key role in the soil suction–water interaction. After the rainfall, there was a delayed response of about 2 days before any significant changes of pore water pressure, earth pressure and ground movement were measured. A perched water table was deduced at a depth of about 1·5 m below the slope surface. Two distinct modes of down-slope deformations were observed: a cantilever deformation within the top 1·5 m at the midslope, and a deep-seated down-slope displacement at the toe of the slope. The observed continual soil swelling after the start of the rainfall appears to be governed by the low coefficient of permeability to water and the expansive nature of the soil. After heavy and prolonged rainfalls, earth pressure measurements suggest that passive failures may take place within the softened clay at shallow depths.

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