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IS 6403 : 1981 (Reaffirmed 2002)
Edition 2.1 (1984-05)
Indian Standard CODE OF PRACTICE FOR DETERMINATION OF BEARING CAPACITY OF SHALLOW FOUNDATIONS
( First Revision ) (Incorporating Amendment No. 1)
UDC
624.151.5 : 624.131.52 : 006.76
© BIS 2006 BUREAU
OF
INDIAN
STANDARDS
MANAK BHAVAN , 9 BAHADUR SHAH ZAFAR MARG NEW DELHI 110002
Price Group 4
IS 6403 : 1981
Indian Standard CODE OF PRACTICE FOR DETERMINATION OF BEARING CAPACITY OF SHALLOW FOUNDATIONS
( First Revision ) Foundation Engineering Sectional Committee, BDC 43 Chairman PROF DINESH MOHAN
Representing Central Building Research Institute (CSIR), Roorkee
Members Central Building Research Institute (CSIR), Roorkee DR R. K. BHANDARI Calcutta Port Trust, Calcutta CHIEF ENGINEER SHRI S. GUHA ( Alternate ) SHRI M. G. DANDAVATE The Concrete Association of India, Bombay SHRI N. C. DUGGAL ( Alternate ) DR R. K. DAS GUPTA Simplex Concrete Piles (India) Pvt, Ltd, Calcutta ADDITIONAL CHIEF ENGINEER ( Alternate ) In personal capacity ( 5 Hungerford Road, 121 SHRI A. G. DASTIDAR Hungerford Street, Calcutta ) SHRI V. C. DESHPANDE The Pressure Piling Co (I) Pvt Ltd, Bombay DIRECTOR (CSMRS) Central Water Commission, New Delhi DEPUTY DIRECTOR (CSMRS) ( Alternate ) SHRI A. H. DIVANJI Asia Foundations and Construction Co Pvt Ltd, Bombay SHRI A. N. JANGLE ( Alternate ) Indian Geotechnic Society, New Delhi DR JAGDISH NARAIN PROF SWAMI SARAN ( Alternate ) SHRI G. S. JAIN G. S. Jain & Associates, Roorkee SHRI ASHOK KUMAR JAIN ( Alternate ) JOINT DIRECTOR (D) National Buildings Organisation, New Delhi SHRI SUNIL BERY ( Alternate ) JOINT DIRECTOR RESEARCH (SM) Ministry of Railways (RDSO) JOINT DIRECTOR RESEARCH (B & S) ( Alternate ) ( Continued on page 2 ) © BIS 2006 BUREAU OF INDIAN STANDARDS This publication is protected under the Indian Copyright Act (XIV of 1957) and reproduction in whole or in part by any means except with written permission of the publisher shall be deemed to be an infringement of copyright under the said Act.
IS 6403 : 1981 ( Continued from page 1 ) Members DR R. K. KATTI SHRI S. R. KULKARNI SHRI S. ROY ( Alternate ) SHRI O. P. MALHOTRA
Representing Indian Institute of Technology, Bombay M. N. Dastur & Co Pvt Ltd, Calcutta Public Works Department, Chandigarh Administration, Chandigarh Central Warehousing Corporation, New Delhi Machenzies Limited, Bombay Engineers India Limited, New Delhi
SHRI A. P. MATHUR SHRI V. B. MATHUR SHRI T. K. D. MUNSI SHRI M. IYENGAR ( Alternate ) BRIG OMBIR SINGH Engineer-in-Chief’s Branch, Army Headquarters, New Delhi MAJ H. K. BHUTANI ( Alternate ) The Hindustan Construction Company Limited, SHRI B. K. PANTHAKY Bombay SHRI V. M. MADGE ( Alternate ) SHRI M. R. PUNJA Cemindia Co Ltd, Bombay SHRI S. MUKHERJEE ( Alternate ) SHRI N. E. V. RAGHVAN The Braithwaite Burn & Jessop Construction Co Ltd, Calcutta Vijayanagar Steel Plant (Steel Authority of India), SHRI A. A. RAJU New Delhi University of Roorkee, Roorkee PROF GOPAL RANJAN SHRI T. N. SUBBA RAO Gammon India Limited, Bombay SHRI S. A. REDDI ( Alternate ) Bokaro Steel Plant (Steel Authority of India), Bokaro SHRI Y. V. NARASIMHA RAO DR V. V. S. RAO Nagadi Consultants Pvt Ltd, New Delhi SHRI ARJUN RIJHSINGHANI Cement Corporation of India, New Delhi SHRI O. S. SRIVASTAVA ( Alternate ) DR A. SARGUNAN College of Engineering, Guindy SHRI S. BOOMINATHAN ( Alternate ) Public Works Department, Government of Andhra SHRI K. R. SAXENA Pradesh, Hyderabad United Technical Consultants Pvt Ltd, New Delhi DR S. P. SHRIVASTAVA DR R. KAPUR ( Alternate ) Ministry of Shipping and Transport, New Delhi SHRI N. SIVAGURU SHRI D. V. SIKKA ( Alternate ) Central Public Works Department, New Delhi SUPERINTENDING ENGINEER (DESIGNS) EXECUTIVE ENGINEER (DESIGNS) V ( Alternate ) SHRI M. D. TAMBEKAR Bombay Port Trust, Bombay DR A. VARADARAJAN Indian Institute of Technology, New Delhi DR R. KANIRAJ ( Alternate ) SHRI G. RAMAN, Director General, ISI ( Ex-officio Member ) Director (Civ Engg) Secretary SHRI K. M. MATHUR Deputy Director (Civ Engg), ISI ( Continued on page 16 )
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IS 6403 : 1981
Indian Standard CODE OF PRACTICE FOR DETERMINATION OF BEARING CAPACITY OF SHALLOW FOUNDATIONS
( First Revision ) 0. F O R E W O R D 0.1 This Indian Standard (First Revision) was adopted by the Indian Standards Institution on 30 January 1981, after the draft finalized by the Foundation Engineering Sectional Committee had been approved by the Civil Engineering Division Council. 0.2 The bearing capacity of foundations is needed for dimensioning the foundation for any structure. Several methods are available for the determination of bearing capacity of shallow foundations and this standard gives some of the methods which are commonly used for the purpose. Comparison of the results shows that when each of the various methods is applied to different problems no one method consistently gives higher or lower values of allowable bearing pressure. The designer must therefore regard the methods as aids to design which cannot replace the critical role of engineering judgement. 0.2.1 This standard was first published in 1971. The principal modifications made in this revision are: (a) keeping its terminology in line with that of other related Indian Standards, (b) giving generalized equations for calculation of ultimate bearing capacity, (c) including cone penetration methods and (d) deleting adjustment for fine sand and silt. 0.3 This edition 2.1 incorporates Amendment No. 1 (May 1984). Side bar indicates modification of the text as the result of incorporation of the amendment. 0.4 For the purpose of deciding whether a particular requirement of this standard is complied with, the final value, observed or calculated, expressing the result of a test or analysis, shall be rounded off in accordance with IS : 2-1960*. The number of significant places retained in the rounded off value should be the same as that of the specified value in this standard. *Rules for rounding off numerical values ( revised ).
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IS 6403 : 1981 1. SCOPE 1.1 This standard covers the procedure for determining the ultimate bearing capacity and allowable bearing pressure of shallow foundations based on shear and allowable settlement criteria. 2. TERMINOLOGY 2.0 For the purpose of this standard, the following definitions shall apply. 2.1 Terms Relating to Bearing Capacity 2.1.1 Net Loading Intensity — The net loading intensity on the foundation is the gross intensity of loading minus the weight of displaced soil above the foundation base. 2.1.2 Ultimate Bearing Capacity — The intensity of loading at the base of the foundation which would cause shear failure of the soil support. 2.1.3 Safe Bearing Capacity — Maximum intensity of loading that the foundation will safely carry without the risk of shear failure of soil irrespective of any settlement that may occur. 2.1.4 Safe Bearing Pressure or Net Soil Pressure for Specified Settlement — The intensity of loading that will cause a permissible settlement or specified settlement of the structure. 2.1.5 Allowable Bearing Capacity — The net intensity of loading which the foundation will carry without undergoing settlement in excess of the permissible value for the structure under consideration but not exceeding net safe bearing capacity. 2.2 General Terms 2.2.1 Density Index ( Relative Density ) — The ratio of the difference between the void ratios of cohesionless soil in the loosest state and any given state to the difference between its void ratios at the loosest and densest states. 2.2.2 Effective Surcharge at the Base Level of Foundation — The intensity of vertical pressure at the base level of foundation, computed assuming total unit weight for the portion of soil above the water table and submerged unit weight for the portion below the water table. 2.2.3 Footing — A structure constructed in brickwork, masonry or concrete under the base of a wall or column for the purpose of distributing the load over a larger area. 2.2.4 Foundation — That part of a structure which is in direct contact with soil and transmits loads into it. 4
IS 6403 : 1981 2.2.5 Shallow Foundation — A foundation whose width is greater than its depth. The shearing resistance of the soil in the sides of the foundation is generally neglected. 3. SYMBOLS 3.1 For the purpose of this code and unless otherwise defined in the text, the following letter symbols shall have the meaning indicated against each: A = Area of footing in cm2 A' = Effective area of footing in cm2 B = Width of strip footing, width of footing, side of square footing, diameter of circular footing in cm B' = Effective width of footing in cm b = Half of B c = Cohesion in kgf/cm2 c1 = Undrained cohesion of the top layer in kgf/cm2 c2 = Undrained cohesion of the lower clay layer in kgf/cm2 Df = Depth of foundation in cm Dw = Depth to water table in cm d = Depth of top clay layer with undrained cohesion c1 = Depth factors dc, dq, dγ e = Eccentricity of loading in cm eB = Eccentricity of loading along the width in cm eL = Eccentricity of loading along the length in cm H = Horizontal component of loading in kgf ic, iq, iγ = Inclination factors Kd = Depth factor (varies linearly from 1 for depth Df = 0 to 1.33 for depth Df = B ) L = Length of footing in cm L' = Effective length of footing in cm N = Corrected standard penetration value ' ' Nc, N c, Nq, N q, Nγ, N'γ = Bearing capacity factors 5
IS 6403 : 1981 = tan2 (π/4 + /2) q = Effective surcharge at the base level of foundation in kgf/cm2 qa = Net soil pressure for a specified settlement of 25 mm in kgf/cm2 qc = Static cone penetration resistance in kgf/ cm2 qd = Net ultimate bearing capacity based on general shear failure in kgf/cm2 q' d = Net ultimate bearing capacity based on local shear failure in kgf/cm2 R = Relative density of soil W' = Correction factor for location of water table sc, sq, sγ = Shape factors α = Inclination of the load to the vertical in degrees = Angle of shearing resistance of soil in degrees γ = Bulk unit weight of foundation soil kgf/cm3 N
4. GENERAL 4.1 Sufficient number of undisturbed samples, about 40 to 100 mm in diameter or more or block samples should be obtained, where possible. These samples are for the determination of field density of soil and conducting tests for determining the relevant shear and consolidation parameters of the soil. Tests on soils should be conducted in accordance with relevant parts of IS : 2720*. 4.2 Position and fluctuation of water table should be ascertained. Reference may be made to IS : 1892-1979† and IS : 2132-1972‡ for guidance regarding investigations and collection of data. 5. ULTIMATE NET BEARING CAPACITY 5.0 General — Three types of failure of soil support beneath the foundations have been recognized, depending upon the deformations associated with the load and the extent of development of failure surface. *Methods of test for soils. †Code of practice for subsurface investigations for foundations ( first revision ). ‡Code of practice for thin-walled tube sampling of soils ( first revision ).
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IS 6403 : 1981 They are: a) general shear failure, b) local shear failure and c) punching shear. The choice of which method of analysis is best suited in a given situation is difficult to make, because only limited test data are available on full sized foundations to verify the reliability of the computed bearing capacity. However, guidelines given in relevant clauses may be used for guidance. Wherever possible bearing capacity calculations shall be made on the basis of shear strength parameters and c obtained from appropriate shear tests [ see IS : 2720 (Parts XI and XIII)* ] or from plate load test results as given in IS : 1888-1981† or from static cone penetration resistance qc obtained from static cone penetration test as given in IS : 4968 (Part III)-1976‡. 5.0.1 Effect of Eccentricity a) Single Eccentricity — If the load has an eccentricity e, with respect to the centroid of the foundation in only one direction, then the dimension of the footing in the direction of eccentricity shall be reduced by a length equal to 2 e. The modified dimension shall be used in the bearing capacity equation and in determining the effective area of the footing in resisting the load. b) Double Eccentricity — If the load has double eccentricity ( eL and eB ) with respect to the centroid of the footing then the effective dimensions of the footing to be used in determining the bearing capacity as well as in computing the effective area of the footing in resisting the load shall be determined as given below: L' = L – 2 eL B' = B – 2 eB A' = L' × B' 5.1 Soils with Cohesion and Angle of Shearing Resistance 5.1.1 The following formulæ shall be used for calculating ultimate net bearing capacity in the case of strip footings: a) In case of general shear failure qd = cNc + q ( Nq – 1 ) + 1 --2
b) In case of local shear failure q' d =
--23 1 --2
Bγ Nγ
cN' c + q ( N'q – 1 ) + BγN'γ
The values of Nc, N'c, Nq, N 'q, Nγ and N'γ may be obtained from Table 1. *Methods of test for soils: Part XI Determination of shear strength parameters of a specimen tested in unconsolidated undrained triaxial compression without the measurement of pore water pressure, and Part XIII Direct shear test ( first revision ). †Method of load test on soils ( second revision ). ‡Method for subsurface sounding for soils: Part III Static cone penetration test ( first revision ).
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IS 6403 : 1981 TABLE 1
BEARING CAPACITY FACTORS ( Clause 5.1.1 )
                              
BEARING CAPACITY FACTORS (Degrees) 0
Nc
Nq
Nγ 0.00
5.14
1.00
5
6.49
1.57
0.45
10
8.35
2.47
1.22
15
10.98
3.94
2.65
20
14.83
6.40
5.39
25
20.72
10.66
10.88
30
30.14
18.40
22.40
35
46.12
33.30
48.03
40
75.31
64.20
109.41
45
138.88
134.88
271.76
50
266.89
319.07
762.89 –1 NOTE — For obtaining values of N'c, N'q and N'γ, calculate ' = tan (0 67 tan Read Nc, Nq, and Nγ , from the Table corresponding to the value of ' instead of which are values of N'c, N'q, N'γ respectively.
).
5.1.2 The ultimate net bearing capacity obtained in 5.1.1 for strip footing shall be modified to take into account, the shape of the footing, inclination of loading, depth of embedment and effect of water table. The modified bearing capacity formulæ are given as under: a) In case of general shear cNc scdcic + q( Nq – 1 ) sqdqiq   = 1 --- Bγ Nγsγdγiγ W' failure qd +  2 b) In case of local shear failure q'd
  
=
2 --3
cN'c scdcic + q( N'q – 1 ) sqdqiq + BγN' γsγdγiγ W ' 1 --2
5.1.2.1 The shape factors shall be as given in Table 2. SHAPE FACTORS SHAPE FACTOR
                  
TABLE 2 SL SHAPE OF BASE NO. i) Continuous strip ii) Rectangle
sc
sq
sγ
1.00
1.00
1.00 1 – 0.4 B/L
1 + 0.2 B/L
1 + 0.2 B/L
iii) Square
1.3
1.2
0.8
iv) Circle
1.3
1.2
0.6
Use B as the diameter in the bearing capacity formula.
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IS 6403 : 1981 5.1.2.2 The depth factors shall be as under: dc = 1 + 0.2 Df/B Nφ < 10° dq = dγ = 1 for dq = dγ = 1 + 0.1 Df/B Nφ for
> 10°
NOTE — The correction is to be applied only when back filling is done with proper compaction.
5.1.2.3 The inclination factor shall be as under: α 2 ic = iq =  1 – -----90  α 2 iγ =  1 – φ---  5.1.2.4 Effect of water table a) If the water table is likely to permanently remain at or below a depth of ( Df + B ) beneath the ground level surrounding the footing then W ' = 1. b) If the water table is located at a depth Df or likely to rise to the base of the footing or above then the value of W ' shall be taken as 0.5. c) If the water table is likely to permanently got located at depth Df < Dw < ( Df + B ), then the value of W ' be obtained by linear interpolation. 5.2 Cohesionless Soil ( c = 0 ) — The ultimate net bearing capacity shall be calculated as given in 5.2.1 and 5.2.2. 5.2.1 Based on Relative Density — The formulæ given in 5.1.1 and 5.1.2 shall be used, together with relevant shear strength parameter. 5.2.1.1 The relative density as given in Table 3 shall be used as a guide to determine the method of analysis. TABLE 3
METHOD OF ANALYSIS BASED ON RELATIVE DENSITY
RELATIVE DENSITY SL NO. (DENSITY INDEX) i) Greater than 70 percent ii) Less than 20 percent iii) 20 to 70 percent
VOID RATIO Less than 0.55 Greater than 0.75 0.55 to 0.75
CONDITION
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METHOD OF ANALYSIS
Dense
General shear
Loose
Local shear (as punching shear)
Medium
Interpolate i) and ii)
well
as
b e t w e e n
IS 6403 : 1981 5.2.2 Based on Standard Penetration Resistance Value — The standard penetration resistance shall be determined as per IS : 2131-1981* at a number of selected points at intervals of 75 cm in the vertical direction or change of strata if it takes place earlier and the average value beneath each point shall be determined between the level of the base of the footing and the depth equal to 1.5 to 2 times the width of foundation. In computing the value, any individual value more than 50 percent of the average calculated shall be neglected and average re-calculated (the values for all loose seams shall however be included). 5.2.2.1 The ultimate net bearing capacity shall be calculated from following formula (covering effect of other factors as mentioned in 5.1.2): qd = q( Nq – 1 ) sqdqiq +
--12
BγNγsγdγiγ W '
Where may be read from Fig. 1, Nγ, Nq may be read from Table 1, Sq, dq, iq, sγ, dγ, iγ, and W ' may be obtained as in 5.1. 5.2.3 Method Based on Static Cone Penetration Test — The static cone point resistance ‘qc’ shall be determined as per IS : 4968 (Part III)-1976† at number of selected points at intervals of 10 to 15 cm. The observed values shall be corrected for the dead weight of sounding rods. Then the average value at each one of the location shall be determined between the level of the base of the footing and the depth equal to 1 1--2- to 2 times the width of the footing. The average of the static cone point resistance values shall be determined for each one of the location and minimum of the average values shall be used in the design. The ultimate bearing capacity of shallow strip footings on cohesionless soil deposits shall be determined from Fig. 2. 5.3 Cohesive Soil (when = 0) 5.3.1 Homogeneous Layer 5.3.1.1 The net ultimate bearing capacity immediately after construction on fairly saturated homogeneous cohesive soils shall be calculated from following formula: qd = cNc sc dc ic where Nc = 5.14. *Method for standard penetration test for soils ( first revision ). †Method for subsurface sounding for soils: Part III Static cone penetration test ( first revision ).
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IS 6403 : 1981
FIG. 1
RELATIONSHIP BETWEEN
11
AND
N
IS 6403 : 1981
FIG. 2 CHART FOR STATIC CONE TEST The value of c shall be obtained from unconfined compressive strength test. Alternatively, it can also be derived from static cone test ( see 5.3.1.2 ). The values of sc, dc and ic may be obtained as in 5.1. If the shear strength for a depth of 2--3- B beneath the foundation does not depart from the average by more than 50 percent, the average may be used in the calculation.
5.3.1.2 Alternately, cohesion c shall be determined from the static cone point resistance qc using the empirical relationship shown below: Soil Type
Point Resistance Values ( qc ) kgf/cm2 Normally consoliqc < 20 dated clays Over consolidated qc > 20 clays
Range of Undrained Cohesion (kgf/cm2) qc/18 to qc/15 q   q ------c-  to  -----c26   22
5.3.2 Two Layered System — In the case of two layered cohesive soil system which do not exhibit marked anisotropy the ultimate net bearing capacity of a strip footing can be calculated by using the formula given below: qd = c1 Nc where Nc may be obtained from Fig. 3. 12
IS 6403 : 1981 5.3.3 Desiccated Soil — In the case of desiccated cohesive soils, the undrained cohesion is likely to decrease along with depth and is likely to get stabilized at some depth below ground level, around 3.5 m, if other factors do not influence. If a plot of undrained cohesion, values as shown in Fig. 4 is obtained, and where the pressure bulb falls within the desiccated top soil the ultimate net bearing capacity shall be obtained with the assumption of cylindrical failure surface from Table 4.
FIG. 3
BEARING CAPACITY FACTORS FOR LAYERED COHESIVE SOIL DEPOSITS 13
IS 6403 : 1981
FIG. 4
DECREASING COHESION WITH DEPTH IN CASE OF DESICCATED COHESIVE SOIL TABLE 4
DATA FOR DETERMINING ULTIMATE NET BEARING CAPACITY
4λ B ----------qd
qd ------c1
0.0 0.2 0.4 0.6 0.8 1.0
5.7 5.0 4.5 4.0 3.6 3.2
λ , c1 shall be obtained from the borehole data, and for a known value of the width of the strip footing ‘B’, by trial and error qd can be qd 4λ B -----estimated by matching -----------q d with c 1 . 6. ALLOWABLE BEARING CAPACITY 6.1 The allowable bearing capacity shall be taken as either of the following, whichever is less: a) Net ultimate bearing capacity as obtained in 5 divided by suitable factor of safety, that is, net safe bearing capacity. 14
IS 6403 : 1981 b) The net soil pressure ( see 6.1.1 ) that can be imposed on the base without the settlement exceeding the permissible values as given in IS : 1904-1978* to be determined for each structure and type of soil, that is, safe bearing pressure. 6.1.1 Safe Bearing Pressure — The permissible settlements for different types of soil formations are specified in IS : 1904-1978*. The methods for calculations of settlements for assumed pressure from standard penetration resistance are specified in IS : 8009 (Part I)-1976†; by calculating the settlements for two or three probable soil pressures and interpolating, the net soil pressure for permissible settlement may be estimated. This safe bearing pressure can also be calculated based on plate load test ( see IS : 1888-1982‡ ).
*Code of practice for structural safety of buildings: Shallow foundation ( second revision ). †Code of practice for calculation of settlement of foundations: Part I Shallow foundations subjected to symmetrical static vertical loads. ‡Method of load test on soils ( second revision ).
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IS 6403 : 1981 ( Continued from page 2 )
Bearing Capacity of Foundation Subcommittee, BDC 43 : 4 Convener SHRI S. GUHA
Representing Calcutta Port Trust, Calcutta
Members DEPUTY DIRECTOR STANDARDS (B&S), CB-I, RDSO
Ministry of Railways
EXECUTIVE E N G I N E E R (DESIGNS) V
Central Public Works Department, New Delhi
SHRI T. N. MUKHERJEE
Martin & Burns Co Ltd, Calcutta
SHRI B. G. RAO
Central Building Research Institute (CSIR), Roorkee
SHRI AMAR SINGH ( Alternate ) DR SWAMI SARAN
University of Roorkee, Roorkee
SHRI K. R. SAXENA
Engineering Research Laboratory, Hyderabad
SHRI O. S. SRIVASTAVA
Cement Corporation of India, New Delhi
SHRI S. K. CHATTERJEE ( Alternate )
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Bureau of Indian Standards BIS is a statutory institution established under the Bureau of Indian Standards Act, 1986 to promote harmonious development of the activities of standardization, marking and quality certification of goods and attending to connected matters in the country. Copyright BIS has the copyright of all its publications. No part of these publications may be reproduced in any form without the prior permission in writing of BIS. This does not preclude the free use, in the course of implementing the standard, of necessary details, such as symbols and sizes, type or grade designations. Enquiries relating to copyright be addressed to the Director (Publications), BIS. Review of Indian Standards Amendments are issued to standards as the need arises on the basis of comments. Standards are also reviewed periodically; a standard along with amendments is reaffirmed when such review indicates that no changes are needed; if the review indicates that changes are needed, it is taken up for revision. Users of Indian Standards should ascertain that they are in possession of the latest amendments or edition by referring to the latest issue of ‘BIS Catalogue’ and ‘Standards : Monthly Additions’. This Indian Standard has been developed by Technical Committee : BDC 43 Amendments Issued Since Publication Amend No. Amd. No. 1
Date of Issue May 1984
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