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SHALLOW FOUNDATION

NAME: INDRAJIT MITRA PAPER NAME AND CODE: SEMINAR-I AND CE 792 institute: university institute of technology, THE BURDWAN UNIVERSITY

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INTRODUCTIONShallow 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.

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Major 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.

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When 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

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Types 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 footing Pad footing

Fig: Pad Footing

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Types of Shallow Foundation(cont.):Combined footing: When a spread footing supports the load of more than one column or wall.

Fig: Combined Footings

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Types 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

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Types of Shallow Foundation(CONT.):

Fig- Raft foundations

Requirements for the raft foundations:

The allowable soil pressure is low, or the building loads are heavy

Use of spread footings would cover more than one-half of the area

Soil is sufficiently erratic so that the differential settlement difficult to control

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FACTORS 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

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PRESSURE DISTRIBUTION BELOW FOOTINGSThe 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.

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GROUND WATER TABLE AND FOOTINGSA RISING WATER TABLE HAVE FOLLOWING ADVERSE EFFECTS :1) Appreciable reduction in the bearing capacity 2) Development of uplift pressure 3) Possible ground heave due to the reduction of

the effective stresses caused by the increasing pore water pressures.

4) Expansion of the heavily compacted fills under the foundation

5) Appreciable settlements of the poorly compacted fills

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SOIL 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 0.125 cm. TABLE: K-VALUE CHANGES WITH SOIL CHARACTERISTICS

 

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SOIL 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:

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BEARING CAPACITY and footingFactors influencing Bearing Capacity:

I. Type of soil III. Unit weight of soilII. Surcharge load IV. Depth of

foundationV. Mode of failure VI. Size of footingVII. Shape of footing VIII. Depth of water

tableIX. Eccentricity in footing loadX. Inclination of footing loadXI. Inclination of groundXII.Inclination of base of foundation

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MODES OF SHEAR FAILURE General shear failure: Seen in dense and stiff soil.

Fig: Fig: General shear failure Local shear failure: Seen in relatively loose and soft soil.

Fig: Fig: Local shear failure

MODES OF SHEAR FAILURE (CONT.): Punching shear failure: Seen in loose , soft soil and at deeper elevations.

Fig- punching shear failure

TERZAGHI’S BEARING CAPACITY THEORY:According to Terzaghi the equation for ultimate bearing capacity for a strip footing is obtained as follows, ultimate bearing capacity

qf = cNC + γDNq +0.5 γBNγ

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BEARING CAPACITY OF FOOTINGS (CONT.)Circular footing :

qf = 1.3 cNc + γDNq +0.3 γBNγ Square footing:

qf = 1.3 cNc + γDNq +0.4 γBNγRectangular footing:

qf = (1+0.3 B/L)cNc + γDNq + (1-0.2 B/L)0.5γBNγ

Ultimate bearing capacity with the effect of water table is given by,

qf= cNC + γDNq RW1+0.5 γBNγ RW2 qf = cNC + γDNq RW1+0.5 γBNγ RW2

Effect of Water Table fluctuation :

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Effect 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.

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General shear failure: qf = c Nc sc dc ic + q (Nq-1) sq dq iq + 0.5γ B Nγ sγ dγ iγ W

Local shear failure: qf = ⅔ c N'c sc dc ic + q (Nq-1) sq dq iq + 0.5γ B N'γ sγ dγ iγ W Shape factors for different shapes of footings:

BEARING CAPACITY ACCORDING TO INDIAN STANDARD CODES

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BEARING CAPACITY ACCORDING TO

INDIAN STANDARD CODES (cont.)Depth factors:

Inclination factor :

Values of W:1. Water table remain at or below a depth of (Df + B), then W=

1.2. Water table located at depth Df or likely to rise above the

base then, W= 0.53. If Df < Dw < (Df + B), then W be obtained by linear

interpolation

dc = 1 + 0.2 Df/B √Nφ dq = dγ = 1 for ф < 10° dq = dγ = 1 + 0.1 Df/B √Nφ for ф > 10°

ic = iq = (1- α /90)² iγ = (1- α /ф)²

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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 :

SETTLEMENTS OF SHALLOW FOUNDATION

S = Si + Sc +Ss

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Settlement s of shallow foundation(cont.)Primary Consolidation: The primary consolidation settlement Sc is given by the following formula: Sc =

Values of for different types of soil :𝜆

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Settlement 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)

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CORRECTION ON TOTAL SETTLEMENT FOR DEPTH AND RIGIDITY1) Effect of Depth of Foundation: Corrected settlement = Scorrected = Sc x Depth factor

Fig: Fox’s correction curves for settlements of flexibleRectangular footings of BxL at depth D

 

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CORRECTION ON TOTAL SETTLEMENT FOR DEPTH AND RIGIDITY2) Effect of the rigidity of foundation: Rigidity factor =

= 0.8 TABLE: Permissible uniform and differential settlement and tilt for footings

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PLATE LOAD TEST LOADING SYSTEMS: There are two loading set-up :

Fig: set up for gravity loading platform Fig: set up for reaction loading platform DETERMINATION OF SETTLEMENT:According to Terzaghi and Peck (1948):

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PLATE LOAD TEST(cont.) 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. 

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PLATE LOAD TEST(cont.)

Fig : Load- settlement curves o obtained from test

From 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.

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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.

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CONCLUSION

The 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. 

 

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REFERENCESIS 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 1080 - 1985 (Reaffirmed 1997): Code of practice for design and construction of shallow foundations in soils (other than raft, ring and shell).IS 2950 (Part1) -1981 (Reaffirmed 1998): Code of practice for design and construction of raft foundations - part 1 design.IS 8009 (Part 1) - 1976 (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) - 1980 (Re affirmed 1995): Code of practice for calculation of settlements of foundations part-2(deep foundations subjected to symmetrical static vertical loading).IS 9214 - 1979 (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. 

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