1、FOUNDATION ANALYSIS AND DESIGN FOUNDATIONSUBSOILS 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 m
2、ass 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 separati
3、on (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 condit
4、ions, 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
5、 as gravel, sand, silt, clay, etc., and are more completely described in Sec. 2-7. 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 soil-rock interface
6、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
7、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 curre
8、nt weathering and decomposition 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
9、 small grain size, large amounts of pore space, potential for the presence of large amounts of pore water, and they often are highly compressible. Note, however, exceptions that produce poor-quality residual soils and good-quality transported soil deposits commonly exist. In general, each site must
10、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 un
11、dergo 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 of considerable importance in cohesionless deposits as well. The effect of cementati
12、on and aging in sand is not nearly so pronounced as for clay but still the effect as a statistical accumulation from a very large number of grain contacts can be of significance for designing a foundation. Care must be taken to ascertain the quantitative effects properly since sample disturbance and
13、 the small relative quantity of grains in a laboratory sample versus site amounts may provide difficulties in making a value measurement that is more than just an estimate. Field observations have well validated the concept of the cementation and aging process. Loess deposits, in particular, illustr
14、ate the beneficial effects of the cementation process where vertical banks are readily excavated. Overconsolidation A soil is said to be normally consolidated (nc) if the current overburden pressure(column of soil overlying the plane of consideration) is the largest to which the mass has ever been s
15、ubjected. It has been found by experience that prior stresses on a soil element produce an imprint or stress history that is retained by the soil structure until a new stress state exceeds the maximum previous one. The soil is said to be overconsolidated (or preconsolidated) if the stress history in
16、volves a stress state larger than the present overburden pressure. Overconsolidated cohesive soils have received considerable attention. Only more recentlyhas it been recognized that overconsolidation may be of some importance in cohesionless soils. A part of the problem, of course, is that it is re
17、latively easy to ascertain overconsolidation in cohesive soils but very difficult in cohesionless deposits. The behavior of overconsolidated soils under new loads is different from that of normally consolidated soils, so it is important particularly for cohesive soils to be able to recognize the occ
18、urrence. The overconsolidation ratio (OCR) is defined as the ratio of the past effective pressure pc to the present overburden pressure p o OCR = Pc / Po A normally consolidated soil has OCR = 1 and an overconsolidated soil has OCR 1. OCR values of 1-3 are obtained for lightly overconsolidated soils
19、. Heavily overconsolidated soils might have OCRs 6 to 8. An underconsolidated soil will have OCR 1. In this case the soil is still consolidating. Overor preconsolidation may be caused by a geologically deposited depth of overburden that has since partially eroded away.Of at leastequally common occur
20、rence are preconsoli-dation effects that result from shrinkage stresses produced by alternating wet and dry cycles. These readily occur in arid and semiarid regions but can occur in more moderate climates as well. Chemical actions from naturally occurring compounds may aid in producing an over- cons
21、olidated soil deposit. Where overconsolidation occurs from shrinkage, it is common for only the top 1 to 3 meters to be overconsolidated and the underlying material to be normally consolidated. The OCR grades from a high value at or near the ground surface to 1 at the normally consolidated interface
22、. Quality of the Clay The term clay is commonly used to describe any cohesive soil deposit with sufficient clay minerals present that drying produces shrinkage with the formation of cracks or fissures such that block slippage can occur. Where drying has produced shrinkage cracks in the deposit we ha
23、ve a fissured clay. This material can be troublesome for field sampling because the material may be very hard, and fissures make sample recovery difficult. In laboratory strength tests the fissures can define failure planes and produce fictitiously low strength predictions (alternatively, testing in
24、tact pieces produces too high a prediction) compared to in situ tests where size effects may either bridge or confine the discontinuity. A great potential for strength reduction exists during construction where opening an excavation reduces the overburden pressure so that expansion takes place along
25、 any fissures. Subsequent rainwater or even local humidity can enter the fissure so that interior as well as surface softening occurs. A clay without fissures is an intact clay and is usually normally consolidated or at least has not been overconsolidated from shrinkage stresses. Although these clays may
