【正文】 foundation on either soil or rock. This material may be under water as for certain bridge and marine structures, but more monly we will place the foundation on soil or rock near the ground surface. Soil, being a mass of irregularshaped particles of varying sizes, will consist of the particles (or solids), voids (pores or spaces) between particles, water in some of the voids, and air taking up the remaining void space. At temperatures below freezing the pore water may freeze, with resulting particle separation (volume increase).When the ice melts particles close up (volume decrease). If the ice is permanent, the icesoil mixture is termed permafrost It is evident that the pore water is a variable state quantity that may be in the form of water vapor, water, or ice。 the amount depends on climatic conditions, recency of rainfall, or soil location with respect to the GWT of Fig. 11. Soil is an aggregation of particles that may range very widely in size. It is the byproduct of mechanical and chemical weathering of rock. Some of these particles are given specific names according to their sizes, such as gravel, sand, silt, clay, etc., and are more pletely described in Sec. 27. Soil may be described as residual or transported. Residual soil is formed from weathering of parent rock at the present location. It usually contains angular rock fragments of varying sizes in the soilrock interface zone. Transported soils are those formed from rock weathered at one location and transported by wind, water, ice, or gravity to the present site. The terms residual and transported must be taken in the proper context, for many current residual soils are formed (or are being formed) from transported soil deposits of earlier geological periods, which indurated into rocks. Later uplifts have exposed these rocks to a new onset of weathering. Exposed limestone, sandstone, and shale are typical of indurated transported soil deposits of earlier geological eras that have been uplifted to undergo current weathering and deposition back to soil to repeat the geological cycle. Residual soils are usually preferred to support foundations as they tend to have better engineering properties. Soils that have been transported— particularly by wind or water— are often of poor quality. These are typified by small grain size, large amounts of pore space, potential for the presence of large amounts of pore water, and they often are highly pressible. Note, however, exceptions that produce poorquality residual soils and goodquality transported soil deposits monly exist. In general, each site must be examined on its own merits. MAJOR FACTORS THAT AFFECT THE ENGINEERING PROPERTIES OF SOILS Most factors that affect the engineering properties of soils involve geological processes acting over long time periods. Among the most important are the following. Natural Cementation and Aging All soils undergo a natural cementation at the particle contact points. The process of aging seems to increase the cementing effect by a variable amount. This effect was recognized very early in cohesive soils but is now deemed