We are concerned with placing the foundation on either soil or rock. This material may be under water as for certain bridge and marine structures, but more commonly we will place the foundation on soil or rock near the ground surface. Soil, being a mass of irregular-shaped 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 ice-soil 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. 1-1. Soil is an aggregation of particles that may range very widely in size. It is the by-product 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 completely described in Sec. 2-7.

人們一般關(guān)心基礎(chǔ)是坐落在“土體”或是“巖體”之上。對(duì)于某些橋梁或海上的結(jié)構(gòu)，其基礎(chǔ)可能在水面之下，但更為常見(jiàn)的還是將基礎(chǔ)放在地表以下較淺的土體或巖石之上。
土體作為形狀不規(guī)則、顆粒大小不等的土顆粒的集合，包括土顆粒（或固體）和土顆粒之間的空隙（部分空隙體積被水填充，其余孔隙體積被氣體占據(jù)）。當(dāng)溫度低于冰點(diǎn)時(shí)，空隙中的水將凝結(jié)為冰，致使土顆粒分離（也就是體積膨脹），反之，當(dāng)冰融化為水時(shí)，土顆粒又相互靠緊（也就是體積縮小）。如果冰常年不融化，這種冰-土混合物就被稱(chēng)為“多年凍土”。顯而易見(jiàn)，孔隙水是一個(gè)隨機(jī)變化的東西，其狀態(tài)可以為水蒸氣、液態(tài)水或固態(tài)冰?？紫端康亩嗌偻鶗?huì)取決于氣候條件、近期的降水量或土體的位置（在地下水位之上還是以下），如圖1-1所示。