Raft Foundations Design and Analysis with a Practical Approach SHARAT CHANDRA CUPTA NEW AGE INTERNATIONAL (P) LIMITED, PUBLISHERS PDF

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Raft Foundations Design and Analysis with a Practical Approach SHARAT CHANDRA CUPTA Advisor, Indian Buildings Congress, Former Chief Engineer Central Public Works Department PUBLISHING FOR ONE WORLD NEW AGE INTERNATIONAL (P) LIMITED, PUBLISHERS - New Delhi Bangalore Calcutta Chennai Guwahati Hyderab...

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Raft Foundations Design and Analysis with a Practical Approach

SHARAT CHANDRA CUPTA Advisor, Indian Buildings Congress, Former Chief Engineer Central Public Works Department

PUBLISHING FOR ONE WORLD

NEW AGE INTERNATIONAL (P) LIMITED, PUBLISHERS

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New Delhi Bangalore Calcutta Chennai Guwahati Hyderabad Lukhnow Mumbai Pune

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Copyright O 1997 New Age International (P) Limited, Publishers NEW A G E INTERNATIONAL (P) LIMITED, PUBLISHERS NEW DELHI BANGALORE

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CALCUTTA CHENNAI

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GUWAHATI HYDERABAD

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LUCKNOW MUMBAI

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PUNE

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4835124, Ansari Road, Daryaganj, New Delhi-110 002 35, Annapoorna Complex, South End Road, Basavangudy, Bangalore-560 004 4018, Ballygunge Circular Road, Calcutta-700 019 20, IInd Main Road Kasthuribai Nagar, Adyar, Chennai-600 020 Pan Bazar, Rani Bari, Guwahati-781 001 1-2-41219, Gaganmahal, Near A.V. College, Domalguda, Hyderabad-500 029 18, Madan Mohan Malviya Marg, Lucknow-226 001 1281A. Noorani Building, Block No. 3, First Floor. L.J. Road, Mahim, Mumbai-400 016. 44, Prashant Housing Society, Lane No. 6, Paud Road, Kothrud, Pune-4 1 1029.

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ISBN :81-224-1078-2

Published by H.S. Poplai for New Age International (P) Limited, 4835124, Ansari Road, Daryaganj, New Delhi- 110002. Typeset by EPTECH, and printed ai Ram Printograph, C-114, Okhla Industrial Area, Phase I, New Delhi-110020. Printed in India Production : M.I. Thomas

CONTENTS Preface i

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1.

INTRODUCTION

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NEED OF RAFT FOUNDATION

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TYPES OF RAFT FOUNDATION

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SURVEY OF AVAILABLE LITERATURE Foundation Engineering by Peck, Hansen and Thornburn 1 Foundation Design and Practice by Elwyn. E.S. Seelye 4.2 4.3 Foundation Design by Teng Foundation of Structures by Dunham 4.4 4.5 Indian Standard Code of Practice for Design and Construction of Raft Foundation - IS 2950-1965 Raft Foundation - The Soil Line Method of Design by A.L.L. Baker Indian Standard Code of Practice for Design and Construction of Raft Foundation 1.S : 2950 (Part-I) 1973 Foundation Engineering Handbook Edited by' Hans F. Winterkorn & Hsaiyang Fang Foundation Analysis and Design by Joseph. E. Bowels Building Code Requirements for Reinforced Concrete (ACI 318 - 77) Foundation Design and Construction by M.J. Tomlinson Design of Combined Footings and Mats ACI Committee 336 Pile Foundation Analysis and Design by H.G.Poulos and E.H. Davis 1980 Reinforced Concrete Designers Handbook by Charles E. Reynolds and James C. Steedman - 9th Edition 1981 IS 2950 (Part I) 1981 -Code for Design and Construction of Raft Foundation Part I ~esi~n Eleventh International Conference of Soil Mechanics and Foundation Engineering San Francisco, August 12 - 16,1985 Foundation Design and Construction by M.J. Tomlinson, 5th Edition, 1986

CONTENTS

Handbook of Concrete Engineering -Mark Fintel - 2nd Edition, 1986 Reinforced Concrete Designer Handbook by Charles E. Reynolds and James Steedman, 10th Edition, 1988 Building Code Requirements in Reinforced Concrete - ACI - 3 18 - 1989 Foundation Engineering Hand book by Hsai-Yang-Fang 2nd Edition, 1991 Design of Combined Footings and Mats - ACI committee 336 2R - 88 Published in ACI Manual 1993 Foundation Analysis and Design by Bowles, 4th Edition, 1988 Proceedings of Indian Geo-Technical Conference 1992, Calcutta, December, 1992 Designs of Foundation Systems - Principles and Practices by Nainan P. Kwian, 1992 13th International Conference on Soil Mechanics and Foundation Engineering, New Delhi, January, 1994 Soil Structure Inter-action -The Real Behaviour of Structures, published by the Institution of Structural Engineers, U.K. The Institution of Civil Engineers, U.K. International Association for Bridge and Structural Engineering in March, 1989

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DESIGN APPROACH AND CONSIDERATIONS 5.1 Rigid Approach 5.2 Flexible Approach 5.3 Parameters for Raft Design Pressure Distribution Under the Raft 5.4 5.5 Rigidity Criteria 5.5.1 Proposed by IS : 2950 (Part I) 1981 5.5.2 ACI Committee, 336 5.5.3 Hetenyi's Criteria 5.6 Modulus of Sub-Grade Reaction 5.6.1 Recommended by Bowles 5.6.2 IS : 2950 Part I Indian Standard Code of Practice for Design and Construction of Raft Foundation 2950 - 1981 5.6.3 I.S. 9214-1979 - Method of Determination of Modulus of Subgrade Reaction (k value) of Soils in Field 5.6.4. IS 8009 - Part I - 1978. Code of Practice for Calculation of Settlements of Foundations - Part I - Shallow Foundations. Subjected to Sy_mmetrical Static Vertical Load. 5.6.5 Recommendation by Alpan and Prof. Alarn Singh 5.6.6 Summary

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STRUCTURALDESIGNERS DILEMMA

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STUDIES CARRIED OUT ON EFFECT OF VARIOUS PARAMETERSON DESIGN OF RAFT 38 7.1

Study 1 7.1.1 Examples Selected 7.1.2 Raft Size

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CONTENTS

7.1.3

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Soil Investigation

7.1.4 Load Considered in Study 7.1.5 Analysis 7.1.6 Discussions of Results 7.1.7 Conclusions Study 2 -Effect of Horizontal Loads 7.2.1 Example Selected 7.2.2 Discussion of Results 7.2.3 Conclusion Study 3: Comparison with Conventional Rigid Methods Details of Conventional Method: Combined Footing Approach 7.3.1 7.3.2 Examples Selected 7.3.3 Discussion of Results 7.3.4 Inverted Floor Method 7.3.5 Conclusions Study 4. Another Office Building 7.4.1 Example Details 7.4.2 Comparison of Results 7.4.3 Discussions of Results 7.4.4 Conclusions '

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STUDIES CARRIED OUT ON ANALYSIS AND DESIGN OF PILED RAFTS 8.1 8.2

Design Procedures being Used Example Selected

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Soil Data Methods of Analysis Studied 8.4.1 Conventional Rigid Method with Simplified Models 8.4.1.1 Combined footing approach 8.4.1.2 Continuous beam analogy :inverted floor 8.4.1.3 Comparison of results 8.4.2 Piled RafPAnalysis Based on Finite Element Approach Study of Parameters Influencing the Raft Behaviour 8.5.1 Effect of Raft Stiffness on the Pile Loads and Raft Moments 8.5.2 Effect of Superstructure and Retaining Walls on Foundation Stiffness 8.5.3 Effect of Earthquake Loads and Moments 8.5.4 Effect of End Bearing and Friction Piles 8.5.5 Summary of Results

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8.6 8.7

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Discussions Conclusions

JOINTS IN RAFl'S SUMMARY OF STUDIES

CONTENTS

11.

FACTORS AFFECTING CHOICE OF MET,HODOF ANALYSIS

12. GUIDELINES APENDM - ILLUSTRATIVE EXAMPLES A.l Conventional Rigid Method - Combined footing approach A.2 Flexible Raft - Beam on elastic foundation A.3 Piled Raft-Plate on elastic foundation

INTRODUCTION I i t

In 1957, when the author was a student of Civil Engineering at the Indian Institute of Technology, Kharagpur, the first institute of national importance, one of his professors of Civil Engineering at his first lecture in the class said: "Civil Engineering is 50% common sense but common sense is that sense which is quite uncommon. " After 34 years of experience in Civil Engineering construction and design, the author only wonders how true the statement of his Professor was and how much more it is true in case of foundation engineering.

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1.1 Foundation engineering has been practised as an art, without help of science, since time immemorial upto 1920 when it had achieved a considerable amount of refinement. It was in the earlier 1920s that a concerted effort was made to study and undentand the physical laks governing the behaviour of sub surface materials, i.e.. soil from which foundations derived their support and on whose behaviour its own behaviour depends. This is the time when study of soil mechanics was started and it was in 1919 when Karl Terzaghi, popularly known as 'father of soil mechanics', made successful attempt to explain the phenomenon of settlement oti a scientific basis. Though study of soil mechanics has provided us with new techniques for selecting appropriate type of foundation and predicting the behaviour of completed structures, it has not been able to decrease the importance of the accumulated experience of the ages. Amount of uncertainty and degree of variation in the properties of soil and number of parameters on which performance of a foundation depends, make exact solution impractical, if not impossible. With so much of advancement in science and computer application, structural design is still defined as:I5 a creation of a structuralfonn to satisfy a number of requirements. It is a combination of art and science. As a rule, there is no direct procedure leading to the solution of a specific problem. An engineer uses all his resources of knowledge experience and imagination to produce a trial scheme. He then constructs a mathematical model of such a solution to assess its adequacy and ifnecessary, modifies the original concept in the light of analytical results. The process is repeated until the designer is satisfied with thefinalproduct, taking into account not only structural adequacy but also such non-quantifiablefactors as aesthetics, ease of construction and performance. The design process is characterised by a complex interaction of parameters and the need to arrive at decisions based on incomplete data Intuitive decisions which have to be taken, appear to be diametrically opposite to the logical nature of ... '

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RAFT FOUNDATIONS-DESIGN AND ANALYSIS

Foundation design and analysis is, at a stage behind structural analysis and design for superstructure, and even now continues to be practised more as an art and will probably continue to be done so, for many years to come.

1.2 Available textbooks, handbooks, various publications and papers give widely different approaches to design of raft foundations. A designer, when faced with a task of designing a raft foundation, finds himself in a precarious position where he has to balance the time available for design, the cost of design, the need of adequate safety and, above all, acceptance of the design by the client and the professional community in general and decide the method of design to be followed by him. Generally, it is not practical for any designer to go through the various approaches as available in engineering literature at a particular time, compare their merits and demerits and select the most suitable for his purpose. He, therefore, perforce selects a particular textbook and applies the same to his problem, quite often little realising that the theoretical problem dealt with in the textbook is widely different from his practical problem relating to an actual building. Resulting solution may not be as satisfactory as he feels. An effort has been made in the following chapters to explain the various approaches suggested in literature, give their comparative limitations, examine the implications of the so-called more sophisticated approaches and finally make recommendation for the method which can be followed by a designer till he accumulates enough experience so as to select his own method particularly applicable to his problem. Intention of this publication is not to hinder initiative of an individual in going deeper in any problem, but to give him a comparative idea of available approaches with sufficient number of references which he can study during the beginning of his profession and formulate his own opinion in due course but still continuing to design satisfactory raft foundations. This publication should, therefore, be studied in this background.

NEED OF RAFT FOUNDATION. Raft or Mat foundation is a combined footing that covers the entire area beneath a structure and supports all walls and columns. This raft or mat normally rests directly on soil or rock, but can also be supported on piles as well. Raft foundation is generally suggested in the following situations: (a) Whenever building loads are so heavy or the allowable pressure on soil so small that individual footings would cover more than floor area. (b) Whenever soil contains compressible lenses or the soil is sufficiently erratic and it is difficult to define and assess the extent of each of the weak pockets or cavities and, thus, estimate the overall and differential settlement. (c) When structures and equipment to be supported are very sensitive to differential settlement. (d) Where structures naturally lend themselves for the use of raft foundation such as silos, chimneys, water towers, elc. (e) Floating foundation cases wherein soil is having very poor bearing capacity and the weight of the super-structure is proposed to be balanced by the weight of the soil removed. (f) Buildings where basements are to be provided or pits located below ground water table. (g) Buildings where individual foundation, if provided, will be subjected to large widely varying bending moments which may result in differential rotation and differential settlement of individual footings causing distress in the building. Let us now examine each of the above situations in greater detail.

2.1 In case of soil having low bearing pressure, use of raft foundation gives three-fold advantage: (a) Ultimate bearing capacity increases with increasing width of the foundation bringing deeper soil layers in the effective zone. (b) Settlement decreases with increased depth. (c) Raft foundation equalises the differential settlement and bridges over the cavities. Every structure has a limiting differential settlement which it can undergo without damage. The amount of differential settlement between various parts of a structure supported on a mat foundation is much lower than that if the sarne.structure was supported on individual footings and had undergone the same amount of maximum settlement. With these considerations, maximum total settlement which

RAFT FOUNDATIONS-DESIGN AND ANALYSIS

can be allowed for a particular structure on mat foundation is more than what is permitted when the structure is resting on individual footings. This, therefore, allows a higher bearing capacity for such situations. It may, however, be noted that if in a case deeper layers of soil are of very poor quality, increase in width of the foundation may not always lead to higher bearing capacity. In situation where comparatively s h a l l y top layers of soil are underlain with deeper layers of much poorer soils, it may be advantageous to provide individual footings so that the zone of influence of the footings remains within the top stronger layer. In such a situation, provision of a mat foundation may be disadvantageous.

2.2 Some designers work on the rule that if more than 50%of the area of the structure is occupied by individual footings, it is necessary to provide an overall raft. This is not true and quite often, the quantity of reinforcing steel and concrete required to avoid excessive deflection and cracking of a raft carrying unequal column loads, necessitating carry-over of stresses from one part of the raft to the other part, may be large and may make raft foundation uneconomical. In such situations, it may be more economical to excavate the entire site to a level formation, construct individual closed space footings (sometimes touching each other) and then backfill around them. In these cases, however, one must weigh form work costs against the extra footing material required by using mat foundation. It should be considered that it is possible to construct alternate footings by using spacer pads against already laid footings and thus save form work cost. Quite often, doubt exists about the structural behaviour of individual footings touching each other. This problem of interaction of footings has been studied by many researchers. It has been reported that the effect of adjacent footings may vary considerably with the angle of shearing resistance. For low values, they are negligible though for high values they appear to be significant, particularly if a footing is surrounded by other footings on both sides. It is also stated that these effects are considerably reduced as length over breadth ratio of the footings approaches unity. There are practically no such effects in the case of punching shear failure. For these and other reasons, it has been recommended that interference effects need not be considered in designs. Adesigner should, however, be aware of the possibility of their existence in some special circumstances 11. 2.3 Situations exist in practice w h p a soil stratum contains compressible lenses or the soils have a formation where individual layers of soil are neither parallel nor can be reasonably stratified into different layers of known properties to enable calculations of settlement to a reasonable accuracy. In such situations, individual footings, if provided, would undergo widely varying settlements resulting in large differential settlement which cannot be tolerated by the structure. 2.4 Situations, as mentioned in (c) and (d) above, are explicit and do not require further explanation. These are special cases, and adoption of raft foundation is more or less necessary by the particular nature of the problem involved. 2.5 In cases where soil is very soft and highly compressible and the buildings cannot be founded on such soils in normal circumstances, it may be possible to provide the building with a basement in such a manner that weight of the structure is equal to the weight of the soil removed and, thus, there being no change in the stresses in the soil beneath the basement and, therefore, little settlement. However, in practice it is rarely possible to balance the loading so that no additional pressure comes on the soil. However, in such cases still, it is only a part of the total load which comes on the bottom soil and, thus, it is possible to construct a building inducing a much larger load than the soil would have otherwise supported. The basement provided, gives additional space in the building for the owner and can be made use of. However while constructing such foundations,

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NEED OF RAFT FOUNDATION

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reconsolidation of the soil, which has swelled as a result of removal of over burden pressure in excavating for the sub-structure, should always be considered and necessary steps be taken to prevent detrimental effects.

2.6 Basements located below ground water table should use a mat as their base to provide water tight

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c.onstruction.The alternative of having individual columns footings connected by thin slabs has not proved to be successful in most of the cases; presents difficulties in water proofing; causes concentration of stresses at the junction of the thin slabs and footings and also at the junction of basement walls and raft causing cracks to develop. This arrangement, therefore, should not be resorted to unless the economy is of such a magnitude as to outweigh all other considerations. Even in cases where sub-soil water level is low and basement does not extend below ground water table, long-term built up of surface water accumulating against basement walls and bottom should be allowed for. This is particularly so in case of impermeable soils (permeability co-efficient below 0.1 mm per second) or of large surface areas draining towards the building. i.e., areas on sloping ground near hillocks. The basement walls ...