Page 199 - Geotechnical Engineering Soil and Foundation Principles and Practice

Source: GEOTECHNICAL ENGINEERING
10 Soil Water

10.1 THE HYDROLOGICAL CYCLE

The most obvious part of the hydrological cycle involves rainfall that runs
downhill and accumulates into streams and rivers that combine and eventually
reach the sea. Less evident is the evaporation that simultaneously takes place, rises
to form clouds, and returns to the ground surface as rain or snow to rejoin the
hydrological cycle. Even less evident is water that infiltrates into the soil where its
influences are manifest in springs, wells, and green lawns. These factors plus
the influence of transpiration from vegetation and recycling by animals complete
the hydrological cycle.
The percentage of precipitation that infiltrates into the ground is close to
100 percent in cavernous limestone areas or desert sands and decreases to 10 to
20 percent on impervious soils such as in shale or granite. Much depends on the
rate of rainfall and the moisture condition of the soil. The measurement
and prediction of rainfall and runoff are the realm of the hydrologist, and
measurement and flow of groundwater that of the groundwater hydrologist.

Runoff water is the main sculptor of landscape, and erosion patterns and the
resistance to erosion are important clues to the compositions of rocks and soils, as
discussed in earlier chapters. The management of infiltrating water is a major
concern for geotechnical engineers as it influences such diverse factors as soil
strength, reservoir levels, and soil conditions and erosion at construction sites.
Water can reduce the bearing capacity of soils and is a key factor in landslides.
Geotechnical engineers also are concerned with seepage of water through earth
dams, levees, and soil lining irrigation ditches, as well as flow into underdrains
and wells. It is important that an engineer has a working knowledge of the
principles governing the flow and retention of water in soil, and the effect of water
on strength and stability of this material.

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