Geotech Engg. Ch#05 bearing capacity

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  • 1. GEOTECHNICAL ENGINEERING - II Engr. Nauman IjazBearing Capacity of the SoilChapter # 05 UNIVERSITY OF SOUTH ASIA

2. FOUNDATION It is the bottom most structural element of the sub structure which transmits the structural load including its own weight on to and / into the soil underneath/surrounding with out casing shear failure or bearing capacity failure (sudden collapse) and excessive settlement. 3. CONTACT PRESSURE The pressure generated by the structural loading and self weight of the member on to or into the soil immediately underneath is called Contact pressure (o).o = Q / A The contact pressure is independent of soil parameters; it depends only on the load and the x-sectional area of the element carrying the load. 4. Q = 1000KNo = Q / A = 1000/(0.5 0.5) = 4000 Kpa AA0.5m Fig # 01 0.5m Sec A-A 5. Super-Structure and Sub- Structure The part of the structure which is above the GSL and can be seen with naked eye is known as Super-Structure. That part of structure which is below the GSL and can not be seen with naked eyes is known as Sub-Structure. 6. Q = 1000KNSuper- Structure GSLSub- Structure DfBL (22.5 m)o = Q / A Fig # 02= 1000/(2 2.5) = 200 Kpa 7. Foundation Depth (Df) It is the depth below the lowest adjacent ground to the bottom of the foundation. Need or Purpose of a Foundation Foundation is needed to transfer the load to the underlying soil assuming safety against bearing capacity failure and excessive settlement. 8. This can be done by reducing the contact pressure such that it is either equal to or less than allowable bearing capacity (ABC) of soil. i.e o < qa. In Fig- 1, the contact pressure under the concrete column is 4000Kpa which is much less than 21MPa(crushing strength of concrete) but much greater than 200KPa (ABC) of soil and needed to be reduced prior to transfer it to the soil underneath the column. The reduction can be achieved by; 9. Lateral spreading of load using a large pad underneath the column (Fig # 02) o = 1000 /5 = 200Kpa = ABCof soil The larger pad is known as Spread footing.FLOATING FOUNDATION Balance Partly or completely the load added to the load removed due to excavation is known as Floating foundation.i.e Provide basements. 10. Types of Foundation Foundation may be characterized as being either Shallow or Deep.Shallow Foundation Are those located just below the lowest part of the super structure which they support ( and get support from the soil just beneath the footing) and a least width generally greater than their depth beneath the ground surface, i.e Df / B < 1 Df = 3 m (generally) 11. Deep Foundations Are those which extend considerably deeper into the earth ( and get supported from the side friction (skin friction) and / or bottom (end bearing) and generally with a foundation depth to width ratio (D/B) exceeding five. 12. TYPES OF FOUNDATION Shallow foundations may be classified in several ways as below;SPREAD FOOTING OR INDIVIDUAL FOOTING This type of foundation supports one column only as shown below. This footing is also known as Pad footing or isolated footing. It can be square or rectangular in shape. This type of footing is the easiest to design and construct and most economical therefore. 13. For this type of footing, length to breadth ratio (L/B) < 5.PLANGSLELEVATION 14. ISOLATED FOOTING 15. CONTINUOUS FOOTING If a footing is extended in one direction to support a long structure such as wall, it is called a continuous footing or a wall footing or a strip footing as shown below. Loads are usually expressed in force per unit length of the footing. For this type of footing , Length to Breadth ratio (L/B) > 5. 16. A strip footing is also provided for a row of columns which are closely spaced that their spread footings overlap or nearly touch each other. In such a case it is more economical to provide a strip footing than a number of spread footing in one line. 17. COMBINED FOOTING A combined footing is a larger footing supporting two or more columns in one row. This results in a more even load distribution in the underlying soil or rock, and consequently there is less chances of differential settlement to occur. While these footings are usually rectangular in shape, these can be trapezoidal ( to accommodate unequal column loading or close property lines) 18. STRAP FOOTING Two or more footings joined by a beam (called Strap) is called Strap Footing. This type is also known as a cantilever footing or pump-handle foundation. This form accommodates wide column spacing's or close property lines. Strap is designed as a rigid beam to with stand bending moments, shear stresses. The strap simply acts as a connecting beam and does not take any soil reaction. To make this sure, soil below is dug and made loose. 19. MAT OR RAFT FOOTING A large slab supporting a number of columns not all of which are in a straight line is known as Mat or Raft or Mass foundation. These are usually considered where the base soil has a low bearing capacity and / or column loads are so large that the sum of areas of all individual or combined footings exceeds one half the total building area ( to economize on frame costs). 20. Furthermore, mat foundations are useful in reducing the differential settlements on individual columns. A particular advantage of mat for basement at or below ground water table is to provide a water barrier. 21. SELECTION OF FOUNDATION TYPE The selection of the type of foundation for a given structure-subsoil system is largely a matter of judgment/elimination based on both an analysis of scientific data and experience. It is not possible to establish rigorous regulations and detailed recommendations for the solution of all soil problems, as the planning and designing of foundations for structures is more of an art than a science. 22. 1.The type of foundation most appropriate for a given structure depends on several factors but commonly the principal factors are three which are as follow: The function of the structure and the loads it must carry. Purpose of the structure i.e residential, office, industrial, bridge etc Service life Loading number of stories, basement. Type i.e framed RCC, masonry, column spacing etc. Construction method and schedule. 23. 2.Sub-surface Condition. Thickness and sequence of soil strata with subsoil parameters. GWT position and function limits. Presence of any underground anomalies.3.The cost of foundation in comparison with the cost of the super structure i.e funds available for the construction and foundation. 24. COMPARISON OF SHALLOW AND DEEP FOUNDATIONS Sr/NoDESCRIPTIONSHALLOW FOUNDATIONDEEP FOUNDATION1DepthDf / B < 1Df / B > 4+2Load DistributionLateral SpreadLateral and/or Vertical spread. For end bearing lateral spread. For frictional vertical spread. Generally both.3ConstructionOpen pit construction. Easy control and the best QA/QC. Less skill labour is required. Min. Disturbance. During construction dewatering is required for shallow GWT.In hole or driven Difficult QA/QC. Very skilled labour is required. Max.disturbance. Dewatering may or may not be required. 25. Sr/NoDESCRIPTIONSHALLOW FOUNDATIONDEEP FOUNDATION4CostLess as compared with deep foundations.Usually 3 times or more costly than shallow.5Structural Design ConsiderationFlexural bendingAxial Compression6SettlementMore than that of deep foundation.Usually 50% of the shallow foundation for similar loading.7Environmental SuitabilityDoes not suit to all environments specially for off shores sites.Suitable for all environment including off shore. 26. CRITERIA FOR FOUNDATION DESIGN1.2.When designing foundation; there are two criteria which must be considered and satisfied separately. There must be accurate factor of safety against a bearing capacity failure in the soil i.e soil shouldnt fail in shear. The settlement and particularly the differential settlement must be kept within reasonable limits. 27. Causes of Deformation Deformation of an element of soil is a function of a change in effective stress (change in volume) not change in total stress. Various causes of deformation of a structure are listed as follow; 1. 2. 3. 4. 5.Application of structural loads. Lowering of the ground water table. Collapse of soil structure on wetting. Heave of swelling soils. Deterioration of the foundation ( Sulphate attack on concrete, corrosion of steel piles, decay of timber piles). 28. 6. Vibration of sandy soil. 7. Seasonal moisture movement. 8. The effect of frost action. 29. DEFINITIONS OF BEARING PRESSURE Gross Bearing Pressure (q gross): The intensity of vertical loading at the base of foundation due to all loads above that level. 2. Net Bearing Pressure: (q net): The difference between q gross and the total overburden pressure Po at foundation level (i.e q net = q gross Po). Usually q net is the increase in pressure on the soil at foundation level. 1. 30. 3. Gross Effective Burden Pressure (qgross): The difference between the qgross and the pore water pressure (u) at foundation level. (i.e qgross = qgross u). 4. Net Effective Bearing Pressure (qnet): The difference between qgross and the effective over burden pressure Po at foundation level. (i.e qnet = qgross Po). 5. Ultimate Bearing Pressure (qf): The value of bearing pressure at which the ground fails in shear. It may be expressed as gross or net or total effective pressure. 31. 6. Maximum Safe Bearing Pressure (qs): The value of bearing pressure at which the risk of shear failure is acceptably low; may be expressed as gross or net or effective pressure. 7. Allowable Bearing Pressure (qa): Takes account the tolerance of the structure to settlement and may be much less than qs. 8. Working Bearing Pressure (qw): Bearing Pressure under working load. May be expressed as gross or net or total or effective pressure. 32. FAILURE MODES A soil underneath any foundation may fail in one or a combi