1、FOUNDATION ANALYSIS AND DESIGN FOUNDATIONSUBSOILSWe 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 ma

2、ss 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 separatio

3、n (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 conditi

4、ons, 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 inthesoil-rock interface zon

6、e. 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 soi

7、l 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

8、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 transportedparticularly by wind or waterare often of poor quality. These are typified by smal

9、l 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 be ex

10、amined on its own merits. MAJOR FACTORS THAT AFFECTTHE ENGINEERING PROPERTIES OF SOILSMost 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

11、 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 cementation and

12、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 the sm

13、all 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, illustrate the

14、 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 subjecte

15、d. 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 involves

16、a stress state larger than the present overburden pressure. Overconsolidated cohesive soils have received considerable attention.Onlymore recentlyhasit been recognized that overconsolidation may be of some importance in cohesionless soils. A part of the problem, of course, is that it is relatively e

17、asy 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 importantparticularly for cohesive soilsto be able to recognize the occurrence. The

18、 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. Heavily ov

19、erconsolidated 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 occurrence are pr

20、econsoli-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- consolidated soi

21、l 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. Quality of

22、 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 have a fissure

23、d 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 intact pieces

24、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 any fissure

25、s. 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 expand

26、 from excavation of overburden, the subsequent access to free water is not so potentially disastrous as for fissured clay because the water effect is more nearly confined to the surface. Mode of Deposit Formation Soil deposits that have been transported, particularly via water, tend to be made up of

27、 small grain sizes and initially to be somewhat loose with large void ratios. They tend to be fairly uniform in composition but may be stratified with alternating very fine material and thin sand seams, the sand being transported and deposited during high-water periods when stream velocity can suppo

28、rt larger grain sizes. These deposits tend to stabilize and may become very compact (dense) over geological periods from subsequent overburden pressure as well as cementing and aging processes. Soil deposits developedwhere the transporting agent is a glacier tend to be more varied in composition. Th

29、ese deposits may contain large sand or clay lenses. It is not unusual for glacial deposits to contain considerable amounts of gravel and even suspended boulders. Glacial deposits may have specific names as found in geology textbooks such as moraines, eskers, etc.; however, for foundation work our pr

30、incipal interest is in the uniformity and quality of the deposit. Dense, uniform deposits are usually not troublesome. Deposits with an erratic composition may be satisfactory for use, but soil properties may be very difficult to obtain. Boulders and lenses of widely varying characteristics may caus

31、e construction difficulties. The principal consideration for residual soil deposits is the amount of rainfall that has occurred. Large amounts of surface water tend to leach materials from the upper zones to greater depths. A resulting stratum of fine particles at some depth can affect the strength

32、and settlement characteristics of the siteSoil Water Soil water may be a geological phenomenon; however, it can also be as recent as the latest rainfall or broken water pipe. An increase in water content tends to decrease the shear strength of cohesive soils. An increase in the pore pressure in any

33、soil will reduce the shear strength. A sufficient increase can reduce the shear strength to zerofor cohesionless soils the end result is a viscous fluid. A saturated sand in a loose state can, from a sudden shock, also become a viscous fluid. This phenomenon is termed liquefaction and is of consider

34、able importance when considering major structures (such as power plants) in earthquake-prone areas. When soil water just dampens sand, the surface tension produced will allow shallow excavations with vertical sides. If the water evaporates, the sides will collapse; however, construction vibrations c

35、an initiate a cave-in prior to complete drying. The sides of a vertical excavation in a cohesive soil may collapse from a combination of rainfall softening the clay together with excess water enteringsurface tension cracks to create hydrostatic water pressure. In any case, the shear strength of a co

36、hesive soil can be markedly influenced by water. Even without laboratory equipment, one has probably seen how cohesive soil strength can range from a fluid to a brick-like material as a mudhole alongside a road fills during a rain and subsequently dries. Ground cracks in the hole bottom after drying

37、 are shrinkage (or tension) cracks. Changes in the groundwater table (GWT) may produce undesirable effectsparticularly from its lowering. Since water has a buoyant effect on soil as for other materials, lowering the GWT removes this effect and effectively increases the soil weight by that amount. Th

38、is can produce settlements, for all the underlying soil sees is a stress increase from this weight increase. Very large settlements can be produced if the underlying soil has a large void ratio. Pumping water from wells in Mexico City has produced areal settlements of several meters. Pumping water (

39、and oil) in the vicinity of Houston, Texas, has produced areal settlements of more than 2 meters in places. Pumping to dewater a construction site can produce settle ments of 30 to 50 mm within short periods of time. If adjacent buildings cannot tolerate this additional settlement, legal problems ar

40、e certain to follow.地基分析與設(shè)計(jì) 地基土體 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?。?。如果冰常年不融化，這種冰-土混合物就被稱為“多年凍土”。顯而易見，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

41、、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。土體是土顆粒的集合，這些土顆粒粒徑的分布范圍可能非常之廣。它屬于巖石機(jī)械風(fēng)化和化學(xué)風(fēng)化的產(chǎn)物。根據(jù)粒徑的大小可對(duì)顆粒進(jìn)行命名，如礫石、砂、粉土、黏土等，這些都將在后面進(jìn)行詳細(xì)論述。土可以分為“殘積土”或“運(yùn)積土”。 殘積土由主巖在原地風(fēng)化而成的。在土層和巖層的結(jié)合處常含有不同尺寸的、大小不一的巖石碎塊。巖石在一個(gè)地方被風(fēng)化，風(fēng)化產(chǎn)生物再被風(fēng)、水、冰或搬運(yùn)至現(xiàn)在的位置所形成的土稱為運(yùn)積土?！皻埣餐痢被颉斑\(yùn)積土”這兩個(gè)概念須結(jié)合上下文進(jìn)行理解，因?yàn)楫?dāng)前的很多殘積土可能形成于（或

42、正在形成于）很早歷史時(shí)期的巖石，而這些巖石又由運(yùn)積土堆結(jié)而成。后來(lái)地殼的抬升使得這些巖石暴露在外面，成為風(fēng)化作用的新對(duì)象。這些暴露的石灰?guī)r、砂巖或頁(yè)巖是很早時(shí)期地質(zhì)運(yùn)積土沉積物的典型堆積結(jié)產(chǎn)生物。這些巖石被抬升后，在近期的風(fēng)化和分解作用下再變成土，然后繼續(xù)新一輪的地質(zhì)循環(huán)。比較以上的兩種土，發(fā)現(xiàn)人們更喜歡采用殘疾土形成的土層來(lái)作為支撐基礎(chǔ)的地基，因?yàn)樗麄円话憔哂休^好的工程性質(zhì)。被搬運(yùn)過的土，特別是被風(fēng)或水搬運(yùn)過的土，往往性能較差。這些運(yùn)積土的典型特征為顆粒直徑較小，孔隙體積大，有可能存在大量的孔隙水，通常具有高壓縮性。然而，以上的結(jié)論并不是絕對(duì)的，一般也可能存在性能比較差的殘積土以及性能好的運(yùn)

43、積土，總之，必須根據(jù)每個(gè)場(chǎng)地的特點(diǎn)來(lái)進(jìn)行評(píng)價(jià)。影響土體工程性質(zhì)的主要因素 影響土體工程性質(zhì)的大多數(shù)因素包括長(zhǎng)期的地質(zhì)作用，其中最重要的因素是：自然膠結(jié)與時(shí)效 所有土體自然的膠結(jié)過程都與土顆粒之間的接觸面有關(guān)，而時(shí)效過程似乎對(duì)膠結(jié)效果有著不同的影響。這種作用在很早以前就已經(jīng)被人們從黏性土中發(fā)現(xiàn)了。而現(xiàn)在認(rèn)為它在無(wú)黏性沉積土中也同樣起著相當(dāng)重要的作用。盡管沙土中的膠結(jié)于時(shí)效作用不如在黏性土中那么顯著，但是還會(huì)對(duì)基礎(chǔ)的設(shè)計(jì)產(chǎn)生重要影響。在準(zhǔn)確的定量確定這種作用時(shí)必須特別注意，因?yàn)樵嚇拥淖兓约笆覂?nèi)試樣與現(xiàn)場(chǎng)場(chǎng)地相比較缺少某些東西等因素都會(huì)對(duì)數(shù)據(jù)的測(cè)量有一定的影響，所以定量分析要比僅僅進(jìn)行估算困難的多

44、?，F(xiàn)場(chǎng)觀測(cè)已經(jīng)充分證實(shí)膠結(jié)與時(shí)效作用的存在。特別是沉積黃土，垂直岸坡的易于開挖體現(xiàn)了膠結(jié)的有利作用。超固結(jié)如果土體現(xiàn)有上面土層覆蓋的壓力就是它曾經(jīng)經(jīng)受過的最大壓力，則稱這種土為“正常固結(jié)”的。在經(jīng)驗(yàn)中發(fā)現(xiàn)，作用于土體單元的先前應(yīng)力會(huì)留下一個(gè)記號(hào)或稱為歷史應(yīng)力，它會(huì)一直保留在土體結(jié)構(gòu)中，直到有超過先前最大應(yīng)力的新的應(yīng)力力狀態(tài)出現(xiàn)。如果土體在應(yīng)力歷史上曾經(jīng)承受過比現(xiàn)有上面土層覆蓋壓力更大的壓力時(shí)，則稱這種土體為“超固結(jié)”。人們對(duì)超固結(jié)狀態(tài)的黏性土已經(jīng)相當(dāng)?shù)闹匾暳恕５旁谧罱藗儾耪J(rèn)識(shí)到超固結(jié)對(duì)黏性土也具有一定的重要性。當(dāng)然，造成此種情況的部分原因在于確定黏性土的超固結(jié)狀態(tài)相對(duì)較為容易，但對(duì)于無(wú)黏性

45、的沉積土來(lái)說就非常困難。超固結(jié)狀態(tài)的土體在附加荷載的作用下，其性狀不比正常固結(jié)狀態(tài)的土體。因此，了解土體以前的應(yīng)力歷史是相當(dāng)重要的（特別是對(duì)于黏性土）?！俺探Y(jié)比”（OCR）的定義為先期有效壓力Pc與現(xiàn)有上面土層覆蓋壓力Po之比 OCR= Pc/ Po “黏土”一詞通常用于描述那些含有足夠黏土礦物的黏性沉積土。這種土在干燥時(shí)會(huì)收縮并形成裂縫，因此會(huì)導(dǎo)致發(fā)生塊體滑移。當(dāng)沉積土?xí)蚋稍锒a(chǎn)生收縮裂縫，就稱這種土為“裂縫黏土”。這種土的現(xiàn)場(chǎng)取樣較為麻煩，因?yàn)樗赡芊浅?jiān)硬，而且列裂隙的存在會(huì)使的試樣難于制備。于原位測(cè)試（尺寸效應(yīng)可使得由裂隙造成的不連續(xù)性受到側(cè)向限制或?yàn)榧雍砂逅茉剑┫啾容^，在室內(nèi)強(qiáng)度試驗(yàn)中，裂隙面可成為潛在的