Interactions between the atmosphere and the Earth

Interactions between the atmosphere and the Earth have always been important to civil engineers, and particularly to geotechnical engineers. Certain of these interactions can be rapid and catastrophic, resulting in the failure of structures such as dams and bridges, in loss of life and in widespread environmental devastation. Other interactions can be slow and insidious, but still destructive and costly in the long term, e.g. the effects that swelling or shrinking clays have on buildings on shallow foundations. After giving two examples of catastrophic interactions between the atmosphere and Earth structures, the paper will concentrate on examining the interaction between the atmosphere and the unsaturated soil zone above the water table. The behaviour and properties of the soils that comprise the unsaturated zone between the soil surface and the water table are greatly dependent on the transfer of water and energy between the atmosphere and the soil, through the soil surface. While agronomists, soil scientists and hydrologists have paid a great deal of attention to this interaction between the atmosphere and the Earth, geotechnical engineers have tended to ignore it. The geotechnical engineer has tended to accept the water table depth, water content profile, strength profile, etc., as he has found them, and has not concerned himself with how these conditions arose, or how they could advantageously be modified, except by short-term construction measures, e.g. dewatering or preloading. Many aspects of the behaviour of soils in the unsaturated zone are closely linked to the water balance between the atmosphere and the Earth, which determines such things as the water table depth, the average and seasonal water content profile, and seasonal swelling or shrinking. This paper will examine the water balance in detail and show how its various components can be measured. The water balance depends on the energy balance at the soil surface, and this will also be examined, its components analysed and information given on how to measure them. Examples will then be given of how the water balance principle and deliberate adjustments to the water balance can be used to solve practical problems in the geotechnical engineering of the unsaturated soil zone between the ground surface and the water table.