本书共8章，主要内容包括绪论、土的物理性质及工程分类、土中应力、土的压缩性与地基沉降、土的抗剪强度、土压力、土坡稳定性、地基承载力。本书结合我国土力学课程的教学特点与内容，根据全国高等学校土木工程专业指导委员会提出的本科专业建设、认证标准要求和时代发展的需要，将相关的*新**和行业标准条文融入其中，内容简明扼要，通俗易懂，图文并茂，**突出，强调对土力学基本原理的理解和应用。书中还配套了立体化数字资源，可供读者延伸知识学习。

0Introduction



1. Research contents of soil mechanics



Soil is the most abundant resource on earth，and it has many causes and uses.What is soil？What are the engineering properties of soil？How to study and apply them to serve engineering construction？This is the question requiring the answer by soil mechanics.Soil mechanics is a subject that studies the stress，deformation，strength，seepage and longterm stability of soil.Generalized soil mechanics includes soil formation，composition，physical and chemical properties and classification of soil.Soil mechanics is also a practical subject，which is a branch of civil engineering.It mainly studies the engineering properties of soil to solve the engineering problems.



In nature，there are lithosphere，hydrosphere and atmosphere distributed on the crust surface.Rock is an aggregate of one or more minerals，and its engineering properties mainly depend on its mineral composition，while soil is the product of rock weathering.Soil is a variety of sediments generated by rock experiencing physical，chemical，biological weathering，denudation，transport，sedimentation and other complex natural environment.Therefore，the types of soil and its physical and mechanical properties are different，but in the same geological age and similar sedimentary conditions，there are similar characteristics.Strongly weathered rock properties that are close to the soil，also belongs to the research category of soil science and soil mechanics.



The solid particles in soil are the detrital materials after rock weathering，referred to as soil particles for short.The soil particle aggregate constitutes the soil skeleton，and there are liquid water and gas in the pores of the soil skeleton.Therefore，soil is a threephase material composed of soil particles(solid phase)，water(liquid phase)in soil and gas(gas phase)in soil；When the pores in soil are filled with water，it is a twophase body composed of soil particles and water in soil.Soil mass has different pore characteristics from general continuous solid materials(such as steel，wood，concrete，masonry and other building materials).It is not a rigid porous medium，but a porous material with large deformation.The flow of water in the pores shows the permeability of the soil(water permeability)；the change of soil pore volume shows the compressibility，expansion and contraction of soil；the dislocation of soil particles in pores shows the shear strength characteristics of friction and cohesion in soil.The density，porosity and water content of soil are important factors affecting the mechanical properties of soil.The size of soil particles varies greatly.There are giant particle groups with a particle size larger than 60mm，fine particle groups with a particle size smaller than 0.075mm，and coarse particle groups with a particle size between 0.075 and 60mm.



Engineering soil is divided into general soil and special soil.The general soil which is distributed widely in earth can be divided into heavenly soil and organic soil.The original sedimentary inorganic soil can be roughly divided into four categories：gravel soil，sand soil，silty soil and clay soil.When the content of large and coarse grain groups in the soil exceeds 50% of the total weight，it is chipped in gravel soil or sand soil.On the contrary，it belongs to silty soil or clay soil.Gravel soil and sand soil are called noncohesive soil，which are generally characterized by high permeability and noncohesiveness.Cohesive soil has small water permeability，plasticity，wet limit，swelling and shrinkage and frost heave.The silty soil has the liquefiable of sand soil and the plasticity of clay soil.Special soils include collapsible soil(such as common collapsible loess)，swelling and shrinkage soil(commonly known as expansive soil)，frost heaving soil(commonly known as frozen soil)，red clay，soft soil，filled and mixed soil，salt feed soil，contaminated soil，weathered rock and residual soil，etc.



In summary，there are many kinds of soil.Engineering properties are very complex.There are differences in the properties of soil in different geological ages，different causes，different regions and even different locations，and some even vary greatly.Soil will also change in nature due to changes in the environment，resulting in soil stress deformation and stability factors change.For example，a substantial decline in the groundwater level will change the stress state of the foundation and produce the new additional deformation，and the occurrence of largescale land subsidence and uneven settlement，will result in the abnormal use and even destruction of buildings.For example，the pit excavation of foundation will affect the stress deformation and stability of soil and adjacent buildings.and the seepage of water in soil will affect the stability of foundation pit and slope.



In the long historical process，the history of engineering construction experienced by human production and life is the process of constantly dealing with rock and soil，and countless projects have been built.There are many industries involved in soil mechanics，such as water conservancy and hydropower，roads and bridges，mines，energy，ports and waterways，urban and rural construction and municipal engineering，national defense construction，etc.People may build projects in various places，and choose different foundation or structure forms for different projects and different geological conditions.They will build dams，roads and railways，factories，wharves and residences，excavate deep foundation pits and tunnels，build subways and underground projects，manage river banks and slopes，and complete tailings storage and garbage landfills.They may encounter various types of foundations and geological conditions or geological environments with complex soil properties.



From the perspective of the wide application of soil mechanics，the role of soil in engineering(generalized as rock and soil)can be divided into three categories.The first one is the foundation of various types of buildings such as houses，factories，wharfs，roads and bridges，and that is the bearing role of foundation.The second one is used as the earthrock dam，tailings dam，embankment and other filling materials or other applications of engineering materials，namely the role of materials.The third one is used as the environment of different engineering facilities，and the people’s production and living environment，such as municipal engineering，housing basement，subway，foundation pit and other soil as its environment.The industrial and living solid waste landfill(accumulation)field and tailings pond are the environment of people’s production and living，highway，railway，plant，residential area and other side of the slope，and even the barrier lake，and that is the role of engineering environment.



The construction of various projects and the prevention and control of geological disasters(landslide，debris flow，barrier lake，etc.)almost all involve soil mechanics.Correct application of soil mechanics knowledge and basic principles are one of the important factors to ensure reasonable planning，correct design，safety during construction，safety and normal use after completion.Although different projects and different soils have their own characteristics and even“personalities”，they are diverse and complex.However，after summarizing human longterm engineering practice，it will be found that the nature of soil and its impact on engineering can be summarized into common topics，namely，three basic topics related to mechanical properties in soil mechanics：soil stability，soil deformation and soil seepage.To solve these three basic problems，there are three basic theories：soil shear strength theory，soil compression and consolidation theory，and soil seepage theory.Three types of basic topics should be considered in any project.However，for different soils and projects，their emphasis or main contradiction may be different，but they are usual.If they are interrelated and interact with each other，they should be regarded as a whole system.Around the three basic topics and theories，the content of soil mechanics textbooks actually includes：the formation and composition of soil，the physical properties of soil，the stress calculation of soil，the settlement of foundation and the relationship between settlement and time，the preliminary application of the principle of soil stability and limit equilibrium，etc.



2. Evolution of Soil Mechanics



The European industrial revolution in the 1860s and the second industrial revolution in the middle of the 19th century promoted the development of social productive forces.Modern projects such as reservoirs，railways and docks appeared，and many geotechnical engineering problems to be solved were put forward，such as foundation bearing capacity，slope stability，stability of retaining structure and so on.At the same time，the emergence of construction machinery also provided material conditions for the development of modern geotechnical engineering.The accidents and problems prompted people to explore the theory of soil mechanics and the technological innovation of geotechnical engineering.Many classical theories of soil mechanics began to appear.This process has lasted for about 160 years，which has prepared the conditions for the formation of the Terzaghi soil mechanics system in the 20th century.



The first theory related to soil mechanics was the Mohr Coulomb strength theory of soil，which was established by French scientist C.A.Coulomb in 1773 and later developed by O.Mohr.It laid the foundation for the analysis of soil pressure，foundation bearing capacity and soil slope stability.In 1776，Coulomb published the earth pressure theory based on the analysis of equilibrium conditions of sliding soil wedges.In 1846，A.Collin conducted a systematic study on the stability of soil slope with the sliding surface of the curve，and published the theory of slope stability.In 1856，H.Darcy，a French engineer，established the theory of water infiltration in empty medium，namely the famous Darcy’s law，through indoor seepage experiment research.In 1857，the British scholar W.J.M.Rankine put forward the earth pressure theory based on the analysis of limit equilibrium conditions of soil mass.The theory of earth pressure，which is called classical earth pressure theory by later generations，still has important theoretical value and certain practical value.In 1869，the Russian scholar Kap ЛОВИЧ published the world’s first Foundation and Foundation tutorial.In 1885，the French scholar J.V.Boussinesq and 1892 W.Flamant put forward the theory of displacement and stress distribution on the surface of a homogeneous and isotropic semi infinite body under the action of vertical concentrated force and line load respectively，which is still the main method to calculate the stress in the foundation.In 1889，the Russian scholar Kudiyumov(K Удюмов)model test was used for the first time to study the displacement of soil particles in the foundation when the foundation was damaged and sinking.At the beginning of the 20th century，soil mechanics continued to make progress.In 1920，according to the principle of plastic equilibrium，L.Prandtl derived the famous ultimate bearing capacity formula.These early famous theories have laid the foundation of soil mechanics.



At the beginning of the 20th century，the theory and engineering application of geotechnical mechanics made good progress.At that time，major landslide and collapse accidents occurred in Sweden，Panama，the United States，Germany，etc.，which showed that some analysis methods at that time could not meet the requirements of handling accidents，so special committees were set up or experts were entrusted to investigate.For example，Sweden set up a geotechnical committee within the National Railway Commission to deal with the continuous collapse along the railway.The Panama Canal set up a special committee to deal with a section of river slope accident that may block the canal.The American Society of Civil Engineers established a special committee to study landslides.The Kiel Canal in Germany set up an investigation committee to deal with landslide accidents during construction；K.Krey of Germany began to conduct extensive investigation and research on the earth pressure on retaining walls and dams.In addition，due to the destruction of Stigbetg wharf，Sweden established a special port committee to the analysis of dynamic causes led to the famous Swedish circular arc sliding method.In 1920，the Geotechnical Committee of the Swedish National Railway Commission established a geotechnical laboratory，which may be the first geotechnical laboratory in the world.



Around 1913，when soil mechanics took a turning point，it was also the stage when K.Terzaghi explored and studied soil mechanics and formed a leap.During 19061912，the young Terzaghi saw many unexpected failures of foundation engineering in his structural engineering and hydropower station engineering work，and found that the understanding of the mechanical properties of soil at that time was far from solving practical engineering problems.He decided to conduct longterm experimental research on the mechanical properties of soil，and formed the concept of effective stress of soil mechanics and the theory of soil consolidation between 19211923.The year 1925 was a milestone in the development of soil mechanics.Terzaghi published his classic work，Soil Mechanics，which was published in German under the title named Erdbaumchanik auf Bodenhysikalischer Grundlage.Later，he published a series of articles with the title of“Principles of Soil Mechanics”in the journal Engineering News Record to briefly introduce his research and findings.These achievements finally established his position as the founder of soil mechanics and made him recognized as an authority in soil mechanics and foundation engineering.



In the middle of the 20th century，Terzaghi’s Theoretical Soil Mechanics and Terzaghi’s and Peck’s Engineering Practical Soil Mechanics were comprehensive summaries of soil mechanics，which provided a solid theoretical basis for geotechnical engineering technology and had a profound impact on the development of geotechnical engineering from sensibility to rationality.



During this period，W.Fellenius proposed the famous Swedish arc method to analyze the stability of soil slopes，and A.Casagrande，who was once the most important assistant of Terzaghi，also made great contributions to soil mechanics.Kashagrand’s research achievements in soil classification，soil slope seepage，shear strength，sand liquefaction and other aspects have had an impact so far.For example，the“A line”in the plastic diagram of cohesive soil classification was named after Arthur.Kashagrand has trained famous soil mechanics talents including N.Janbu，who has made outstanding contributions to the development of soil mechanics in the research of soil compressibility，slope stability，etc.



Since then，Terzaghi，A.W.Skempton，G.G.Meyerhof，A.S.Vesic and B.Hansen have respectively repaired，supplemented and developed the foundation bearing capacity theory，and put forward various foundation bearing capacity formulas；D.W.Taylor and Jean Bu developed the theory of soil slope stability；A.M.Biot established the threedimensional consolidation theory of soil skeleton compression and seepage coupling.These achievements provide an important theoretical basis for the development of modern soil mechanics.



The concept of modern soil mechanics was firstly appeared in the early 1950s，and was mainly considered two basic characteristics of soilcompression and dilatancy.With the development of soil mechanics theory and the deepening of engineering practice，people was more and more dissatisfied with the simplified description of soil as an ideal elastic medium or an ideal rigid plastic medium.On the other hand，the vigorous development of modern electronic computing technology also made it possible to adopt complex computing models，thus creating objective conditions for the establishment of modern soil mechanics.In 1963，K.H.Roscoe published the famous Cambridge model，proposed the first mathematical model that can comprehensively consider the soil’s compressibility and dilatancy，and created critical state soil mechanics.His achievements marked the birth of modern soil mechanics.



3. Characteristics，learning methods and requirements of soil mechanics



Soil mechanics is an important professional basic course for civil engineering，water conservancy，transportation and other majors.Its main contents include the physical properties and classification of soil，the seepage theory of soil，the calculation of stress in soil，the calculation of foundation settlement，the theory of foundation consolidation，the calculation of foundation bearing capacity，the calculation of soil pressure，the stability analysis of soil slope，and the dynamic properties of soil.The contents of each part of the soil mechanics course are both independent and related to each other.When learning，you must clear up your ideas and form a system.Soil mechanics is the basis for many followup courses，relevant professional courses and further study，and is widely used to solve engineering problems，such as engineering investigation，foundation design，foundation pit design，support design，foundation treatment，onsite testing and analysis，and geological disaster prevention.Therefore，this course is a practical and theoretical course.In the whole teaching plan，it plays a bridge role in the transition from basic courses to professional courses，and is an important link before professional teaching.



Because of the complexity of soil，many calculation theories and formulas of soil mechanics are established under certain assumptions.For example，when calculating the stress in soil，it is often assumed that the foundation soil is an isotropic and homogeneous elastic body；When studying the permeability and deformation of soil，it is assumed that the soil is a continuous porous medium；When studying the strength of soil，it is assumed that the soil is an ideal rigid plastic body.In learning，we should pay attention to the simplified assumptions of different theories or methods，apply them flexibly，and do not apply them mechanically.When solving engineering problems based on basic theories，we often need to make some more practical simplified assumptions，but do not deviate from the original assumptions of the theory.



The beginners of soil mechanics often have many new terms and clues and there are fragmented，poor coherence feeling.It is not the case.Although each chapter of soil mechanics course is relatively independent，the relevance and comprehensiveness of the whole course content are very strong.To achieve integration and to learn from this and from the outside to the inside，it is recommended to take the learning path of the concepttheorymethodapplicationexpansion.To be combined with theoretical study to carry out a variety of physical and mechanical tests and through the test to develop skills and deepen the theoretical study，one should master the method and principle of determining the calculation parameters and focuse on the understanding of the basic concepts and knowledge.In addition，through a certain amount of examples and exercises，one should understand the relevant engineering geological knowledge，building structure and construction knowledge and its relationship with the followup courses.



The specific contents and learning requirements of this course are as follows：



Chapter 1—Engineering properties and engineering classification of soil mainly introduce the composition of soil，conversion of threephase proportion index and classification of soil by using geotechnical index.It is required to master the geological causes of soil；be able to use threephase diagram to master soil physical property index and index conversion and master the principles and methods of soil engineering classification.



Chapter 2—The stress in foundation mainly studies the change of soil stress state and its practical calculation method under load.It is required to understand the purpose and use of stress calculation，the concept and basic assumptions of semiinfinite space foundation.Moreover，it is required to skillfully master the calculation method of soil gravity stress，the influence of groundwater on gravity stress distribution，the basic solution of additional stress calculation in foundation，and the additional stress calculation method in space and plane problems of foundation additional stress calculation.Finally，it is required to understand the distribution characteristics of additional stress in foundation.



Chapter 3—The permeability of soil mainly studies the permeability characteristics and seepage analysis methods of soil.It is required to understand the purpose and significance of seepage research and master the laminar seepage law and permeability index of soil.It is required to be familiar with the determination method of permeability index and its influencing factors，the calculation of seepage volume during seepage，seepage failure and seepage control problems.It is also required to understand the concept and application of twodimensional seepage and flow network in soil.



Chapter 4—The compressibility of soil and settlement calculation of foundation mainly introduces the test method of compressibility index and the calculation method of foundation settlement.It is required to master the deformation and consolidation characteristics of soil；understand the stress history of soil through compression test and consolidation test；master Terzaghi’s onedimensional consolidation theory，effective stress principle and its application；grasp the foundation deformation and the relationship between deformation and time and master foundation settlement calculation and consolidation calculation.



Chapter 5—The shear strength of soil mainly discusses the limit equilibrium theory of soil，the test method of soil shear strength index and engineering application.It is required to deeply understand the concept of limit equilibrium and master the formula and application of soil shear strength；grasp the test methods for obtaining strength indexes such as direct shear and triaxial test；understand the shear behavior and strength characteristics of sand and clay under different drainage conditions，and realize the reasonable value and correct application of parameters in combination with the principle of effective stress.

Contents 0Introduction(1) Chapter 1Physical Properties and Engineering Classification of Soil(8) 1.1The formation of soil(8) 1.2Composition of soil(10) 1.3Structure and structure of soil(25) 1.4Physical property index of soil(28) 1.5Compaction mechanism and engineering control of soil(42) 1.6Engineering classification of soil(49) Chapter 2Stress in Foundation(57) 2.1General introduction(57) 2.2Selfweight stress in foundation(58) 2.3Base pressure(62) 2.4Additional stress in foundation(69) Chapter 3Soil Permeability and Permeability Stability(92) 3.1General introduction(92) 3.2Soil permeability(92) 3.3Application of twodimensional seepage and flow network(102) 3.4Permeation stability(106) 3.5Pore water pressure and effective stress in soil mass under different conditions (112) Chapter 4Compressibility of Soil and Foundation Settlement(117) 4.1General introduction(117) 4.2Compressibility of soil(117) 4.3Calculation of final settlement of foundation(128) 4.4One dimensional consolidation theory of saturated cohesive soil(157) Chapter 5Shear Strength of Soil(172) 5.1Introduction(172) 5.2Shear strength theory of soil(173) 5.3Shear strength test of soil(178) 5.4Shear strength of saturated cohesive soil(189) 5.5Shear strength of cohesionless soil(197) Chapter 6Earth Pressure(200) 6.1Introduction(200) 6.2Earth pressure on retaining wall side(200) 6.3Rankine earth pressure theory(203) 6.4Coulomb earth pressure theory(212) Chapter 7Stability Analysis of Soil Slope(228) 7.1Introduction(228) 7.2Stability analysis of cohesionless soil slope(230) 7.3Stability analysis of cohesive soil slope(232) 7.4Stability analysis of soil slope under special circumstances(248) Chapter 8Bearing Capacity of Foundation(251) 8.1Introduction(251) 8.2Determine the bearing capacity of foundation according to the depth of plastic deformation zone(252) 8.3Ultimate bearing capacity of foundation(258)