Raft Foundations: Design and Analysis with a Practical Approach

140 Pages·2008·4.41 MB·English

by  Sharat Chandra Gupta

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 Hyderabad
.
Lukhnow Mumbai Pune
Copyright O 1997 New Age International (P) Limited, Publishers
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CONTENTS
Preface
i
t 1. INTRODUCTION
I
2. NEED OF RAFT FOUNDATION
3. TYPES OF RAFT FOUNDATION
4. SURVEY OF AVAILABLE LITERATURE
I
1 Foundation Engineering by Peck, Hansen and Thornburn
I 4.2 Foundation Design and Practice by Elwyn. E.S. Seelye
1 4.3 Foundation Design by Teng
4.4 Foundation of Structures by Dunham
4.5 Indian Standard Code of Practice for Design and Construction of Raft
i
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 31 8 - 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
~ e s i ~ 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 i
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
5. DESIGN APPROACH AND CONSIDERATIONS
5.1 Rigid Approach
5.2 Flexible Approach
5.3 Parameters for Raft Design
5.4 Pressure Distribution Under the Raft
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
6. STRUCTURAL DESIGNERS DILEMMA
7. STUDIES CARRIED OUT ON EFFECT OF VARIOUS PARAMETERS ON DESIGN OF RAFT 38
7.1 Study 1 40
7.1.1 Examples Selected 41
7.1.2 Raft Size 41
CONTENTS
5 7.1.3 Soil Investigation
!
7.1.4 Load Considered in Study
7.1.5 Analysis
7.1.6 Discussions of Results
: '
7.1.7 Conclusions
7.2 Study 2 -Effect of Horizontal Loads
7.2.1 Example Selected
7.2.2 Discussion of Results
7.2.3 Conclusion
7.3 Study 3: Comparison with Conventional Rigid Methods
7.3.1 Details of Conventional Method: Combined Footing Approach
7.3.2 Examples Selected
7.3.3 Discussion of Results
7.3.4 Inverted Floor Method
7.3.5 Conclusions
1 7.4 Study 4. Another Office Building
7.4.1 Example Details
t 7.4.2 Comparison of Results
7.4.3 Discussions of Results
t1 7.4.4 Conclusions
!
I
8. STUDIES CARRIED OUT ON ANALYSIS AND DESIGN OF PILED RAFTS
1
t
8.1 Design Procedures being Used
i
! 8.2 Example Selected
8.3 Soil Data
8.4 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
8.5 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
I
8.5.4 Effect of End Bearing and Friction Piles
I
8.5.5 Summary of Results
I 8.6 Discussions
8.7 Conclusions
9. JOINTS IN RAFl'S
10. SUMMARY OF STUDIES
CONTENTS
11. FACTORS AFFECTING CHOICE OF MET,HODO F 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.
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 structural fonn 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-quantifiablef actors 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 ... '
2 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 shally
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 I
footings on both sides. It is also stated that these effects are considerably reduced as length over breadth ratio
I
of the footings approaches unity. There are practically no such effects in the case of punching shear failure. 1
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 circumstan-
ces 11. I
I
2.3 Situations exist in practice whp 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, I
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,