1、Unit8SoilMechanicsTextSoils are aggregates of mineral particles,and together with air and/or water in the void spaces,they form three-phase systems.A large portion of the earths surface is covered by soils,and they are widely used as construction and foundation materials.Soil mechanics is the branch
2、 of engineering that deals with the engineering properties of soils and their behavior under stress.The primary mechanism of soil creation is the weathering of rock.All rock types(igneous rock,metamorphic rock and sedimentary rock)may be broken down into small particles to create soil.Weathering mec
3、hanisms are physical weathering,chemical weathering,and biological weathering.1 Human activities such as excavation,blasting,and waste disposal,may also create soil.Over geologic time,deeply buried soils may be altered by pressure and temperature to become metamorphic or sedimentary rock,and if melt
4、ed and solidified again,they would complete the geologic cycle by becoming igneous rock.Physical weathering includes temperature effects,freeze and thaw of water in cracks,rain,wind,impact and other mechanisms.Chemical weathering includes dissolution of matter composing a rock and precipitation in t
5、he form of another mineral.Clay minerals,for example can be formed by weathering of feldspar,which is the most common mineral present in igneous rock.TextA naturally occurring soil sample may have particles of various sizes.Over the years,various agencies have tried to develop the size limits of gra
6、vel,sand,silt,and clay.Some of these size limits are shown in Table 8.1.Referring to Table 8.1,it is important to note that some agencies classify clay as particles smaller than 0.005mm in size,and others classify it as particles smaller than 0.002mm in size.However,it needs to be realized that part
7、icles defined as clay on the basis of their size are not necessarily clay minerals.Clay particles possess,the tendency to develop plasticity when mixed with water;these are clay minerals.Kaolinite,illite,montmorillonite,vermiculite,and chlorite are examples of some clay minerals2.Table 8.1 Soilsepar
8、ate size limitsTextTextTextAn important function in the study of soil mechanics is to predict the stresses and strains imposed at a given point in a soil mass due to certain loading conditions.This is necessary to estimate settlement and to conduct stability analysis of earth and earth-retaining str
9、uctures,as well as to determine stress conditions on underground and earth-retaining structures.Estimation of the increase in stress at various points and associate displacement caused in a soil mass due to external loading using the theory of elasticity is an important component in the safe design
10、of the foundations of structures3.The ideal assumption of the theory of elasticity,namely that the medium is homogeneous,elastic,and isotropic,is not quite true for most natural soil profiles.It does,however,provide a close estimation for geotechnical engineers and,using proper safety factors,safe d
11、esigns can be developed.A knowledge of the increase of pore water pressure in soils due to various loading conditions without drainage is important in both theoretical and applied soil mechanics4.If a load is applied very slowly on a soil such that sufficient time is allowed for pore water to drain
12、out,there will be practically no increase of pore water pressure.However,when a soil is subjected to rapid loading and if the coefficient of permeability is small(e.g.,as in the case of clay),there will be insufficient time for drainage of pore water.This will lead to an increase of the excess hydro
13、static pressure.TextAny given mass of soil consists of solid particles of various sizes with interconnected void spaces5.The continuous void spaces in a soil permit water to flow from a point of high energy to a point of low energy.Permeability is defined as the property of a soil that allows the se
14、epage of fluids through its interconnected void spaces.When a soil layer is subjected to a compressive stress,such as during the construction of a structure,it will exhibit a certain amount of compression.This compression is achieved through a number of ways,including rearrangement of the soil solid
15、s or extrusion of the pore air and/or water.According to Terzaghi(1943),“a decrease of water content of a saturated soil without replacement of the water by air is called a process of consolidation.”When saturated clayey soilswhich have a low coefficient of permeabilityare subjected to a compressive
16、 stress due to a foundation loading,the pore water pressure will immediately increase;however,because of the low permeability of the soil,there will be a time lag between the application of load and the extrusion of the pore water and,thus,the settlement.This phenomenon is called consolidation.The s
17、hear strength of soils is an important aspect in many foundation engineering problems such as the bearing capacity of shallow foundations and piles,the stability of the slopes of dams and embankments,and lateral earth pressure on retaining walls.New Words and Phrasessilicate n.硅酸盐metamorphic a.变形的；变
18、质的；改变结构的sedimentary a.（岩石）沉积形成的dissolution v.解散；溶解；融化precipitation n.（雨雪）降落；降雨量plasticity n.粘性；成形性，柔软性strain n.拉紧，绷紧；拉力；张力；应变displacement 代替；取代；移位；转移；置换elasticity a.可伸缩的；灵活的，能适应的；有弹性的，有弹力的homogeneous a.同形态（固态、液态或气态）的；同种的,同类的,相似的geotechnical a.土工技术的；地质的，土工的permeability n.透过性；渗透性，渗透率hydrostatic a.静水力学
19、的，流体静力学的seepage （液体或气体的）渗出，渗漏extrusion 挤出；推出；喷出consolidation 巩固；强化；使坚强；使结实retaining walls n.拥壁；挡土墙；护岸geologic time n.地质时期igneous rock n.火成岩metamorphic rock n.变质岩sedimentary rock n.沉积岩compressive stress n.压应力Notes1.Weathering mechanisms are physical weathering,chemical weathering,and biological weath
20、ering.风化机理包括物理风化、化学风化和生物风化。2.Kaolinite,illite,montmorillonite,vermiculite,and chlorite are examples of some clay minerals.高岭土、伊利石、蒙脱石、蛭石和绿泥石是粘土矿物的一些示例。3.Estimation of the increase in stress at various points and associate displacement caused in a soil mass due to external loading using the theory of
21、 elasticity is an important component in the safe design of the foundations of structures.使用弹性理论评估各个位点压力的增加以及由外部负荷引起的土壤质量的附加位移是安全设计建筑物基础的重要保障。4.A knowledge of the increase of pore water pressure in soils due to various loading conditions without drainage is important in both theoretical and applied
22、soil mechanics.在不排水情况下，由各种负荷条件造成土壤中孔隙水压力的增加是理论土力学和应用土力学的重要内容。5.Any given mass of soil consists of solid particles of various sizes with interconnected void spaces.任何给定的土壤质量均由各种尺寸的固体颗粒（包括紧密连接的空隙空间）所组成。Exercise（Translate the following Chinese into English,or English into Chinese）1.因为可以通过碾压、夯实、捣振或其他方法压
23、实土以增加其密度和提高其承载强度，所以可压缩性是土的一个重要特性。2.通过测定震动波传到基岩和作为反射波回到地面所需的时间，可确定基岩的深度。3.The general term weathering embraces a number of natural surface processes which result from the single or combined actions of such agencies as wind,rain,frost,temperature change and gravity.4.All soils originate,directly or in
24、directly,from solid rocks and these are classified according to their mode of formation as follows:IGNEOUS ROCKS formed by cooling from hot molten material(magma)within or on the surface of the earths crust,e.g.granite,basalt,dolerite,andesite,gabbro,syenite,porphyry.SEDIMETARY ROCKS formed in layer
25、s from sediments settling in bodies of water,such as seas and lakes,e.g.limestone,sandstone,mudstone,shale,conglomerate.METAMORPHIC ROCKS formed by alteration of existing rocks due to:(a)extreme heat,e.g.marble,quartzite,or(b)extreme pressure,e.g.slate,schist.Exercise（Translate the following Chinese
26、 into English,or English into Chinese）5.The common problem areas in soil mechanics may be conveniently summarized as:(1)Excavation.The digging and removal of soil in order to prepare a site for construction and services.The problems here are closely related to those of support.(2)Support of soil.In
27、the case of both natural and built slopes(embankments),it is necessary to determine their intrinsic ability for self-support.Where excavations(e.g.trenches,basements,etc.)or other cuts(e.g.road cuttings)are to be made,it will be necessary to determine the need for,and the extent of,external support.
28、(3)Flow of water.The effects of water in soil masses must be considered thoroughly.Where a soil is permeable and water can flow through it,problems related to the quantity and effects of seepage arise.(4)Soil as a support medium.The mass of soil under and adjacent to a structure is part of the found
29、ation system and its behavior as a support medium must be investigated.Exercise（Translate the following Chinese into English,or English into Chinese）(5)Building with soils.Soils are used extensively as construction materials in the building of roads,airfields,dams,embankments and the like.As in the
30、case when using other building materials,such as concrete and steel,the properties of the materials need to be measured and evaluated before use and some measure of quality control introduced to ensure a good and sound product.(6)Description and classification.The starting point in most engineering problems is a good description of the material.This has to be meaningful in an engineering sense,in both qualitative and quantitative terms.It is therefore convenient to commence a detailed study of the engineering behavior of soils by considering how they might best be described and classified.
