GTE UNIT 5 PDF

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u tu tu tu tu st as as as as om om om om pl pl pl pl di di di di @ @ @ @ Bearing Capacity of Soils m m m m m oo oo oo oo oo yr r r r r Introduction dy dy dy dy ud tu tu tu tu st as as as as The most videsprend use of soil is as a substratum over which the foundation of a structure m m m m o o o o ultimately the structure rests. The soil at that depth must be strong enough to support pl pl pl pl and thus di di di di allthis load. The load of foundations is mainly compressive. But shear comes into play due @ @ @ @ ta lateral forces on soil. The strength for resisting the compressive stress is called the bearing capacity of soil. om om om om om ro ro ro ro ro y dy dy dy dy ud tu tu tu tu st as as as as m m m m lo lo lo lo p p p p di di di di @ @ @ @ om om om om om ro ro ro ro ro y dy dy dy dy ud tu tu tu tu st as as as as m m m m lo lo lo lo Bearing Capacity and its Importance p p p p 5.1 di di di di @ @ @ Q. Define Bearing Capacity. @ [2019 (Even)] m om om om om Bearing Capacity: oo ro ro ro ro yr dy dy dy dy It is defined as the maximum average intensity in of applied pressured that loaded area ud tu tu tu tu st as as as as can carry before it fails shear. om om om om The knowledge of bearing capacity plays an important role in the following aspects of civil pl pl pl pl di di di di @ @ @ @ engineering: 1 Foundation design. m om om om om 2. Consolidation and compaction of earth. oo ro ro ro ro yr 3. Earth and gravity dams. dy dy dy dy ud tu tu tu tu st 4. Retaining walls as as as as ze om om om om 1a 2trylun 1 2 37ieng Tunnels etc.it 10ie D pl pl pl pl tgn2oi di di di di 2ni31-3/uFGE io.s1 @ @ @ @ 1. Foundation design Entire load of any structure ultimately comes on the foundation. The foundation then om om om om om transmits this load on the soil in contact with the base of the foundation. Thus the area ro ro ro ro ro of base of foundation in contact with the soil ultimately decides the intensity of the load dy dy dy dy dy tu tu tu tu tu  u tu tu tu tu st as as as as om om om om pl pl pl pl di di di di @ @ @ @ Geo Technical Enginecring [5-21 Bearing Capacity intensity of Sgit per unit area. Any oil has some fixed capacity of toleratingthe the intensity unit area. This 'eapacity, mainly constitutes the bearing capacity m m m m m city of soil soil. oo oo oo oo oo not change but yr r r r r ln foundation design, the load of the structure does the the area dy dy dy dy r ud tu e ofba tu tu tu ot foundation required, sa that the load intensity is less than that the the soil cantolerala st as as as as m m m m can be calculated. This is done along with a factor of safety. Thus, bearino ing capacty o o o o pl pl pl pl is important in foundation design. di di di di @ @ @ @ 2. Consolidation and compaction of enrth : In many civil engineering situations, the properties of soil or earth either chane nge or wn asconsolidatin om om om om om modified and enhanced. If the properties change naturally, it is known consol ro ro ro ro ro and if we deliberately change and enhance the properties, it is known as compaction y dy dy dy dy comn- ud tu tu tu tu st In both cases, the density of soil changes. This change in density leads to incres as as as as toincrease in m m m m the shear strength, decrçase in permeability and increase in the.bearing ing capaci lo lo lo lo capacit p p p p di di di di Thus, bearing capcity is important in determining various factors and paramete @ @ @ @ hie undertaking the process of compaction or estimating the amount of consolidation. and hence the settlement of the structure which will occr in the future. om om om om om 3. Earth and gravity dams: ro ro ro ro ro y dy dy dy dy Earth dams are entirely made up of soil and hence require detailed knowledge ofso ud tu tu tu tu st as as as as to be used for its construction as well as, the soil on which it will stand. m m m m lo lo lo lo Earth dams are mainly of two types : homogeneous and non-homogeneous. In non. p p p p di di di di @ @ @ @ homogeneous type, the core is made up of highly compacted impervious soil. The compaction involves the knowledge of bearirig capacity. In case of gravity dams, the uplift pressure below the dam and the total load intensity m om om om om oo of the dam coming over the soil in contact with the foundation base necessitate the ro ro ro ro yr dy dy dy dy ud knowledge of bearing capacity of soil. tu tu tu tu st as as as as 4. Retaining walls om om om om pl pl pl pl Retaining walls form a special area of soil mechanics, as the design of retaining wall di di di di @ @ @ @ involves the mobilisation of soil strength under different conditions limiting equilibium of Thus active, passive and neutral earth pressure comes into picture, m om om om om These are lateral pressures whereas the bearing capacity is essentially a vertica oo ro ro ro ro yr compressive stress. Still, in the analysis of active, passive dy dy dy dy or neutral earth pressure ud tu tu tu tu st the bearing capacity is one of the important factors. as as as as om om om om In Rankine's theory of earth pressures also, the bearing nt pl pl pl pl capacity plays an impo di di di di role. @ @ @ @ it dr tusnta ni its21t betl afi. om om om om om ro ro ro ro ro dy dy dy dy dy tu tu tu tu tu  u tu tu tu tu st as as as as om om om om Engincering pl pl pl pl Technical [5-31 Bearing Capacity of Suils di di di di @ @ @ @ Geo etc: Tunnels 5..Tunnels, road bgrades, stability of open cuts, docks and harbour construction, in short m m m m m all aspects of civil engineering where cxcavation of soil, safcty and use of soil as the oo oo oo oo oo yr r r r r ltimate substratum supporting the load are required, benring capacity plays a very dy dy dy dy ud tu tu tu tu important role. st as as as as m m m m 51.1 Concept of. Bearing Capacity o o o o pl pl pl pl di di di di @ @ @ @ A foundation is required for distributing the loads of thie super structure on a large area. foundation should be designed such that the soil below does not fail in shear and the The vithin the safe limits. The pressure which the soil can safcly withstand is known setlement is wit om om om om om he allowable bearing pressure. The conventional method of foundation design is based on ro ro ro ro ro y dy dy dy dy ud Poncept of bearing capacity or allowable bearing pressure of the soil. The benring capacity tu tu tu tu the c st as as as as Hefined as the load or pressure developed under the foundation without introducing danaging m m m m lo lo lo lo vements in the foundation and in the super structure supported on the frundation. Since p p p p di di di di mo @ @ @ @ damaging movements may result from foundation failure ie. collapse of the structure and as uell as from excessive settlement, the following basic criterion must be adopted in the design of foundations. om om om om om an adequate 6) Determination of tihe ultimate bearing capacity of soil and the selection of ro ro ro ro ro y dy dy dy dy ud factor of safety against shear failure (collapse). tu tu tu tu st as as as as Determination of settlement under the expected load and selection of adequate margin m m m m (i) lo lo lo lo p p p p against excessive settlement (comparison with permissible settlement). di di di di @ @ @ @ The ultimate bearing capacity of soil may be found out by any of the following methods: (a) By carrying out load tests on prototype foundation (plate load test) m om om om om methods.G oo (b) By using theoretical ro ro ro ro yr dy dy dy dy ud By penetration tests. i lo rytgw ini tu tu tu tu c) st as as as as although accurate, can not be practically used due to high cost involved in om om om om Plate load test, pl pl pl pl di di di di carrying out such tests. @ @ @ @ Many theortical methods are available for computing the ultimate bearing capacity of soil. No single theory can be said to take into account all the varying conditions in the field. The present trend is to make use of penetration test values such as the standard penetration test m om om om om oo ro ro ro ro or statfic cone penetration test values for computing ultimate bearing capacity of soil. The yr dy dy dy dy ud method of determining tu tu tu tu procedure though empirical is simple and useful. This is the only practicable st as as as as om om om om allowable bearing pressure of foundations on cohesionless soils pl pl pl pl Capacity and di di di di D.1.2 Significance of Ultimate Bcaring Capacity, Net Safe Bearing @ @ @ @ Allowable Bearing Pressure is transmitted to the soil, underneath, through foundations. A ne load of the structure om om om om om ground wvhich supports bund dation is, therefore, a connecting link betwveen the structure and the ro ro ro ro ro dy dy dy dy dy tu tu tu tu tu  u tu tu tu tu st as as as as Gco Technical Engineering om om om om Bearing 541 Capacity. pl pl pl pl di di di di it. A foundation is over. required for distributing the loads of the super structure ucture over @ @ @ @ The foundation should eralarye be designed such that ) the soil below does not fail in shear m m m m m oo oo oo oo oo (i) the settlement is in safe limits. yr r r r r dy dy dy dy ud tu tu tu tu For satisfying the above two conditions, st it is necessary to understand tand the signi as as as as signiid Kicance m m m m the following terms o o o o pl pl pl pl d di di di di may be defined as the ultimate or gross Ultimate @ @ @ @ Bearing Capacity: It rosspresure the base of the foundation at which the soil fails in shear. It is generally denoted denoted h by Value of ultimate bearing capacity depends upon the intensity of load oming (q om om om om om coming over Structure. This value of gross pressure must be avoided to make the soil safe th ro ro ro ro ro in shear e afe in y dy dy dy dy failur ud tu tu tu tu 2. Net Safe Bearing Capacity 2019 (Even) : It is the net soil pressure which st as as as as can De hasafehy m m m m applied to the soil considering only shear failure but excessiIve settlement is not tak lo lo lo lo p p p p consideration. It is generally expressed as Ins into di di di di @ @ @ @ Net safe bearing capacity can be obtained by dividing the net ultimate nate bearing capacityt capaciv a Factor of Safety (FOS). om om om om om Mathematically, ro ro ro ro ro y dy dy dy dy ud tu tu tu tu st m as as as as m m m m 9 F.O.S. lo lo lo lo p p p p di di di di @ @ @ @ where, Net safe bearing capacity R Net ultimate bearing capacity m om om om om -rD oo ro ro ro ro yr where, dy dy dy dy r = unit weight of foundation soil in kN/m ud tu tu tu tu st as as as as i tig miden igià s ah Lssr di Df Depth of foundation below ground level (in m) om om om om pl pl pl pl F.O.S Factor of safety generally taken as 3.0 = di di di di @ @ @ @ 3. Allowable Bearing Pressure: It may be defined as the net loading intensity at which neither the soil fails in shear nor there is excessive settlement deterimental to the structur m om om om om Allowable bearing pressure is also known as net allowable bearing pressure. The net oo ro ro ro ro yr allowable bearing pressure is the value which can be safely used for the design of foundation. dy dy dy dy ud tu tu tu tu st as as as as Thus allowable bearing pressure value must not be exceeded for the soil to be sare om om om om shear and excessive settlement. pl pl pl pl di di di di @ @ @ @ 5.2 Bearing Capacity from Building Codes3149799 is The table 5.1 gives the bearing capacity for various soils according to IS: 1904- 1 om om om om om ro ro ro ro ro dy dy dy dy dy tu tu tu tu tu  u tu tu tu tu st as as as as om om om om hnical Engincering Geo [5-51 pl pl pl pl Bearing Capacity of Soils di di di di Thble 5.1: Value of sase bearing @ @ @ @ capacity according to Indian Standard IS : 1904-1961. m m m m m s.No Cohcsionless Soils oo oo oo oo oo Cohesive Soils yr r r r r dy dy dy dy Safe ud tu tu tu tu