Presentation on theme: "SHALLOW FOUNDATION NAME: INDRAJIT MITRA"— Presentation transcript:
1SHALLOW FOUNDATION NAME: INDRAJIT MITRA PAPER NAME AND CODE: SEMINAR-I AND CE 792institute: university institute of technology,THE BURDWAN UNIVERSITY
2INTRODUCTIONShallow foundations are those that transmit structural loads to the near surface soils.According to the Terzaghi, a foundation is shallow foundation if its depth is equal to or less than its width i.e d ≤ w.For most of the residential buildings or buildings with moderate height or multistoreyed building on soil with sufficient strength, shallow foundation is used from economical consideration.
3Major Requirements : Near surface soil should be strong enough Foundation structures should be able to sustain the applied loads without exceeding the safe bearing capacity of the soil.The settlement of the structure should be should be within the tolerable limits.
4When shallow foundation avoided : When the upper soil layer is highly compressible and too weakIn the case of Expansive soilsIn case of Bridge abutments and piers because of soil erosion at the ground surfaceSoils such as loess are collapsible in nature
5Types of Shallow Foundation: Spread footing: A spread footing is one which supports either one wall or one column.Spread footing may be of the following types –Strip footingPad footingFig: Pad Footing
6Types of Shallow Foundation(cont.): Combined footing: When a spread footing supports the load of more than one column or wall.Fig: Combined Footings
7Types of Shallow Foundation(CONT.): Strap footing: : A strap footing comprises of two or more footings of individual columns, connected by a beam, called a strap.Raft foundation: A raft foundation is a combined footing that covers the entire area beneath a structure and supports all the walls and columns.Fig: Strap Footings
8Types of Shallow Foundation(CONT.): Fig- Raft foundationsRequirements for the raft foundations:The allowable soil pressure is low, or the building loads are heavyUse of spread footings would cover more than one-half of the areaSoil is sufficiently erratic so that the differential settlement difficult to control
9FACTORS FOR DEPTH OF FOUNDATION: Bearing capacity of soilGround water tableDepth of frost actionDepth of volume change due to presence of expansive soilsLocal erosion of soil due to flowing waterUnderground defects such as root holes, cavities, mine shafts, etc.excavation, ditch, pond, water course, filled up ground
10PRESSURE DISTRIBUTION BELOW FOOTINGS The distribution of soil pressure under a footing is a function of the type of soil, the relative rigidity of the soil and the footing, and the depth of foundation at level of contact between footing and soil.
11GROUND WATER TABLE AND FOOTINGS A RISING WATER TABLE HAVE FOLLOWING ADVERSE EFFECTS :Appreciable reduction in the bearing capacityDevelopment of uplift pressurePossible ground heave due to the reduction of the effective stresses caused by the increasing pore water pressures.Expansion of the heavily compacted fills under the foundationAppreciable settlements of the poorly compacted fills
12SOIL STIFFNESS PARAMETER AND FOOTING Soil stiffness is generally measured in the terms of Modulus of sub- grade reaction (K-value).Where, p = load intensity corresponding to settlement of plate (30cm x 30cm) of cm.TABLE: K-VALUE CHANGES WITH SOIL CHARACTERISTICS
13SOIL STIFFNESS PARAMETER AND FOOTING (cont). Foundation Size Effect on Modulus of Sub grade Reaction in Clayey Soil :Foundation Size Effect on Modulus of Subgrade Reaction In Sandy Soils:
14BEARING CAPACITY and footing Factors influencing Bearing Capacity:Type of soil III. Unit weight of soilSurcharge load IV. Depth of foundationV. Mode of failure VI. Size of footingVII. Shape of footing VIII. Depth of water tableIX. Eccentricity in footing loadInclination of footing loadInclination of groundInclination of base of foundation
15MODES OF SHEAR FAILUREGeneral shear failure: Seen in dense and stiff soil.Fig: Fig: General shear failureLocal shear failure: Seen in relatively loose and soft soil.Fig: Fig: Local shear failure
16MODES OF SHEAR FAILURE (CONT.): Punching shear failure: Seen in loose , soft soil and at deeper elevations.Fig- punching shear failureTERZAGHI’S BEARING CAPACITY THEORY:According to Terzaghi the equation for ultimate bearing capacity for a strip footing is obtained as follows, ultimate bearing capacityqf = cNC + γDNq +0.5 γBNγ
17BEARING CAPACITY OF FOOTINGS (CONT.) Circular footing :qf = 1.3 cNc + γDNq +0.3 γBNγSquare footing:qf = 1.3 cNc + γDNq +0.4 γBNγRectangular footing:qf = (1+0.3 B/L)cNc + γDNq + (1-0.2 B/L)0.5γBNγUltimate bearing capacity with the effect of water table is given by,qf= cNC + γDNq RW1+0.5 γBNγ RW2Effect of Water Table fluctuation :qf = cNC + γDNq RW1+0.5 γBNγ RW2
18Effect of Water Table fluctuation :(cont.) CASE 1:Where, ZW1 is the depth of water table from ground level.CASE 2:Where, ZW2 is the depth of water table from foundation level.
19BEARING CAPACITY ACCORDING TO INDIAN STANDARD CODES General shear failure:qf = c Nc sc dc ic + q (Nq-1) sq dq iq + 0.5γ B Nγ sγ dγ iγ WLocal shear failure:qf = ⅔ c N'c sc dc ic + q (Nq-1) sq dq iq + 0.5γ B N'γ sγ dγ iγ WShape factors for different shapes of footings:
20BEARING CAPACITY ACCORDING TO INDIAN STANDARD CODES (cont.) Depth factors:Inclination factor :Values of W:Water table remain at or below a depth of (Df + B), then W= 1.Water table located at depth Df or likely to rise above the base then, W= 0.5If Df < Dw < (Df + B), then W be obtained by linear interpolationdc = Df/B √Nφdq = dγ = 1 for ф < 10°dq = dγ = Df/B √Nφ for ф > 10°ic = iq = (1- α /90)²iγ = (1- α /ф)²
21SETTLEMENTS OF SHALLOW FOUNDATION The total settlement of a footing in clay may be considered to three components (Skempton and Bjerrum, 1957)Immediate Settlement:Values for influence factors, If :S = Si + Sc +Ss
22Settlement s of shallow foundation(cont.) Primary Consolidation: The primary consolidation settlement Sc is given by the following formula:Sc =Values of 𝜆 for different types of soil :
23Settlement s of shallow foundation(cont.) Secondary consolidation: Secondary consolidation settlement is more important in the case of organic and highly-compressible inorganic clays which is given by,Ss =Cα = Secondary Compression Index =Fig: void ratio vs. time (log scale)
24CORRECTION ON TOTAL SETTLEMENT FOR DEPTH AND RIGIDITY Effect of Depth of Foundation:Corrected settlement = Scorrected = Sc x Depth factorFig: Fox’s correction curves for settlements of flexibleRectangular footings of BxL at depth D
25CORRECTION ON TOTAL SETTLEMENT FOR DEPTH AND RIGIDITY 2) Effect of the rigidity of foundation:Rigidity factor == 0.8TABLE: Permissible uniform and differential settlement and tilt for footings
26PLATE LOAD TEST DETERMINATION OF SETTLEMENT: LOADING SYSTEMS: There are two loading set-up :Fig: set up for gravity loading platform Fig: set up for reaction loading platformDETERMINATION OF SETTLEMENT:According to Terzaghi and Peck (1948):
27PLATE LOAD TEST(cont.) Table: Values of index n for different soils: According to Bond (1961):Table: Values of index n for different soils:DETERMINATION OF BEARING CAPACITY:Bearing capacity can be obtained from the load settlement curve that can be plotted from settlement data.
28Fig : Load- settlement curves PLATE LOAD TEST(cont.)Fig : Load- settlement curveso obtained from testFrom the corrected load settlement curves (given below)the ultimate bearing capacity in case of dense cohesionless soils or cohesive soils can be obtained without difficulty (curves D and B ) as the failure is well defined.
29Fig : Corrected Load–Settlement curve (in log-log scale) PLATE LOAD TEST(cont.)Fig : Corrected Load–Settlement curve (in log-log scale)The bearing capacity of sands and gravels increases with the size of footings.
30The following conclusions can be drawn , they are - Shallow foundations are used when the soil has sufficient strength within a short depth below the ground level.Terzaghi’s equation is generally used for computation of bearing capacity of soil.For design purpose, it is usually necessary to investigate both the bearing capacity of soil and the settlement of a footing.Plate load test is used to determine the ultimate bearing capacity and settlement of a footing in field.There are another tests like S.P.T and C.P.T also used to determine ultimate bearing capacity.
31REFERENCESIS 6403: 1981 (Reaffirmed 2002): Code of practice for determination of breaking capacity of shallow foundationsIS:1888:1982 (Reaffirmed 1995) : Method of load test on soilsIS (Reaffirmed 1997): Code of practice for design and construction of shallow foundations in soils (other than raft, ring and shell).IS 2950 (Part1) (Reaffirmed 1998): Code of practice for design and construction of raft foundations - part 1 design.IS 8009 (Part 1) (Re affirmed 1998): Code of practice for calculation of settlements of foundations part-1(swallow foundations subjected to symmetrical static vertical loads).IS 8009 (Part 2) (Re affirmed 1995): Code of practice for calculation of settlements of foundations part-2(deep foundations subjected to symmetrical static vertical loading).IS (Re affirmed 1997): Method of determination of modulus of subgrade reaction (k-value) of soils in field. Soil mechanics and foundation: Punmia, Jain and Jain.NPTEL – Advanced foundation engineering.