8 Parameters and Criteria for Foundation Design 8.1 8.1
Intr Introd oduc ucti tion on
The foundation, being an important interface between the superstructure and the soil, has to safely transfer the large loads and moments coming from the superstructure to the soil at site. Whil Whilee the the supe supers rstr truc uctu ture re load loadss depen depend d on the the needs needs of the the proj project ect,, the the soil soil capac capacit ities ies are are limi limite ted d to its natural properties at site though minor manipulations are possible using suitable but expensive ground improvement methods. Thus the foundation design needs a very judicial selection selection of parameters parameters and design design methods methods and acceptability acceptability criteria. Some of these aspects are discussed in this chapter while the specific considerations for shallow foundations and pile foundations are presented in Chapters 4–7 and 9–12.
8.2
Design Design Consid Considera eratio tions ns
There are many aspects to be considered for a proper design of foundations, as outlined in Chapters 1–3, besides the specific requirements of the particular type of foundation being designed, as discussed in subsequent chapters. These are broadly classified as follows: 1. 2. 3. 4. 5.
Requirements Requirements of the project and choice of superstructur superstructures. es. Loads and moments coming coming from the superstructures superstructures.. Selection Selection of suitable suitable site. Soil propertie propertiess at the chosen site. site. Bearing Bearing capacity capacity, settlement settlement and compressibi compressibility lity,, stress stress distributi distribution on and lateral lateral pressure pressure where necessary. 6. Choice of the foundations foundations based on items 2, 4, and 5, as follows: follows: a. Shallo Shallow w foundat foundation ions; s; spread spread footin footings, gs, combine combined d footin footings, gs, strip strip footin footings, gs, mat/ra mat/raft ft foundations. b. Deep foundations; foundations; piles and pile groups, piers (including (including large diameter piers), piers), well foundations, that is, caissons, pile–raft systems and others.
Foundation Design: Theory and Practice N. S. V. Kameswara Rao © 2011 John Wiley & Sons (Asia) Pte Ltd. ISBN: 978-0-470-82534-1
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7. 8. 9. 10. 11.
Foundation Design
c. Foundations subjected to vibratory/dynamic loads. In addition to the normal requirement for static loads, additional criteria regarding resonance, dynamic amplitudes, additional pressures/loads at interfaces, natural frequency, noise due to vibration and so on, have to be considered for these foundations. These are discussed in Chapter 11. Geotechnical aspects for the design of the selected type of foundations, that is, guided by items 2, 4, 5, 6 as per codes and practices. Structural design of the foundation based on items 6 and 7 as per standard codes and practices. Criteria for assessment as per codes, practices and assessment of the structure designed with respect to criteria based on item 8. Acceptability of the design if the foundation designed satisfies the criteria specified based on item 8. If the foundation does not satisfy the specified criteria, it has to be redesigned or the soil properties have to be improved to meet the requirements until the soil and foundation requirements are acceptable with specified factors of safety.
The items mentioned in 7, 8 and 9 are presented in the following sections while most of the other aspects are described in the respective chapters of this book.
8.3
Codes, Practices and Standards
All designs, whether foundations, soils or structures, have to meet prescribed codes, practices and standards. These are developed as per national, provincial, city and local requirements and have to be complied with for acceptable design and construction practices. Since these are country-specific, only a few of the most commonly adopted criteria for the design of foundations are described below.
8.4
Design Soil Pressure
For any foundation design, one of the basic parameters to be computed is the design soil pressure, that is, the safe pressure that can be borne by the soil when the foundations transmit the superstructure loads to the soil below. This depends on many foundation factors, such as shape, size, depth and type, as described in Chapter 3. Even the parameters for design depend on the method of analysis, that is, conventional or rational methods, as presented in Chapter 4. While the specific requirements have to be closely studied, broadly the design soil pressure can be obtained using either the bearing capacity criterion or settlement/differential settlement criterion. Settlements to be considered are the long term consolidation settlements and bearing capacity relates to shear/or punching shear failure, as discussed in Chapter 3. The design pressure also depends on breadth of the foundation, as explained in Section 3.1 (Figure 3.2). Thus, the design pressure is the lower of the allowable/safe bearing capacity (SBC) value or punching shear (based on shear failure with factor of safety of 3) and the allowable soil pressure (ASP) based on allowable maximum settlement or differential settlement (based on consolidation theory), as per prescribed criteria. The details of evaluation of these values are discussed in Chapters 2 and 3. The following sections present some more details to provide clarity for the determination of these values.
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Parameters and Criteria for Foundation Design
8.5
Gross and Net Values of the Safe Bearing Capacity and Allowable Soil Pressure
It needs to be noted that Terzaghi’s ultimate bearing capacity equations are developed based on gross soil pressure, (qult ¼ qu) which includes all loads above the foundation level. Thus it is the gross pressure that can be considered by the foundation including the overburden pressure and is based on shear strength of the soil at the foundation level. However, the settlements are caused only by net increase of effective pressure over the existing overburden pressure. Thus, the ASP is based on net pressure and is based on consolidation settlement/differential settlement considerations.
8.6
Presumptive Bearing Capacity
These are the design soil pressure values recommended by various national or local building codes and standards. These are somewhat arbitrary and are termed presumptive because it is presumed that the soil can support the load safely without shear failure or undue total/ differential settlements. These can be used only as a helpful guide for preliminary design of foundations and should be supplemented and verified by laboratory tests, field tests and analysis. Some typical values are given in Tables 8.1 and 8.2 (IS: 1904–1966, 1966; Ramiah and Chickanagappa, 1981; Bowles, 1996).
Table 8.1
Typical values of safe bearing capacity. Cohesionless soils
Description
Cohesive soils Safe bearing capacity (t/m2)
Description
Safe bearing capacity (t/m2)
1. Gravel, sand and gravel, compact and offering high resistance penetration when excavated by tools 2. Coarse sand, compact and dry
45
1. Soft shale, hard or stiff clay in deep bed, dry
45
45
25
3. Medium sand, compact and dry
25
4. Fine sand, silt (dry lumps easily pulverized by the fingers) 5. Loose gravel or sand gravel mixture; loose coarse to medium sand and dry 6. Fine sand, loose and dry
15
2. Medium clay readily indented with a thumb nail 3. Moist clay and sand clay mixture which can be indented with strong thumb pressure 4. Soft clay indented with moderate thumb pressure 5. Very soft clay which can be penetrated several inches with the thumb 6. Black cotton soil or other shrinkage or expansive clay in dry condition (50% saturation)
25
10
15
10 5
15
(Reproduced from B.K. Ramiah and L.S. Chickanagappa, Soil Mechanics and Foundation Engineering , p. 394 (Table 4.12), Oxford and IBH Publishing Co., New Delhi, India. Ó 1981.)
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Summary of presumptive/safe bearing capacities from some building codes (in kN/m2).
Table 8.2
Soil description Clay, soft Clay, medium stiff Clay, stiff Sand, compact and clean Sand, compact and silty Inorganic silt compact Sand, loose and fine Sand, loose and coarse, or sand–gravel mixture, or compact and fine Gravel, loose and compact coarse sand Sand–gravel, compact a
Chicago, 1995 National Board of Fire BOCA 1993a Uniform Building Underwriters, 1976 Code, 1991 75 175 210 240 100 125 — —
100 100 — 140–400 140–400 140–400 140–400 140–400
100 — 140 140 — — 140 240
100 100 — 200 — — 210 300
300
140–400
240
300
—
140–400
240
300
Building Officials and Code Administrators International, Inc.
8.6.1
Design Loads and Factors of Safety
While a factor of safety of 3 is used for safe bearing capacity with respect to all dead loads, the following factors of safety (Table 8.3) for various combinations of dead and live loads may be used (Ramiah and Chickanagappa, 1981; Bowles, 1996). Table 8.3
Design loads and factors of safety.
Design load K DDL þ K L.LL þ K W.WL þ K S.SL K D.DL þ K L.LL þ K W.WL þ HL K D.DL þ K L.LL þ K EE þ K SS
Factor of safety for safe bearing capacity þ
HL
3 2 2
whereK D, LL, K W, K S, K E are reduction factors specified by codes for particular combination of loads, and DL ¼ dead load, LL ¼ live load, WL ¼ wind load, SL ¼ Snow load, HL ¼ Hydrostatic load, E ¼ earthquake load.
However, the designers should be aware of the factor of safety adopted in foundation design and provisions of codes of practice and standards.
8.7
Settlements and Differential Settlements
The total settlement of a structure consists of three components as given in Equation (3.47) (Section 3.7), that is
S ¼ S i þ S c
þ
S s
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where S i ¼ immediate/elastic settlement S c ¼ settlement due to primary consolidation S s ¼ settlement due to secondary consolidation Out of these, usually the consolidation (primary) settlement S c is the most important part of the total settlement as discussed in Section 3.7. Though there may not be a collapse or shear failure of the soil due to large settlement, the structures and foundations may become unserviceable. Further tilting and cracking of beams and slabs may occur due to differential settlements. These are shown in Figure 8.1. If the foundation of structure settles uniformly as shown in Figure 8.1(a), there may not be any structural damage. However, if one part settles differentially with respect to other parts of the foundation, as shown in Figures 8.1(b) and (c), then the structure undergoes distortion and the connecting beams, slabs and interfaces may crack and the floors may become unusable. For example a building with rigid components undergoes a uniform settlement (Figure 8.1(a)). Figure 8.1(b) shows a uniform tilt, where the entire structure rotates. Figure 8.1(c) shows a common situation of non-uniform settlement, namely dishing. Non-uniform settlement results from: 1. uniform stress acting upon a homogeneous soil, or 2. non-uniform bearing stress, or 3. nonhomogeneous subsoil conditions. As shown in Figure 8.1 the differential settlement Ds between two points is the larger settlement minus the smaller one. Differential settlement is also d characterized by angular distortion L which is the differential settlement between two points divided by the horizontal distance between them and may be referred to as a ratio.
Figure 8.1
Settlement and differential settlement.
The amount of settlement a structure can withstand is called the allowable settlement or permissible settlement. This depends on many factors, including the type, size, location and intended use of the structure and the pattern, rate, cause and source of settlement. Tables 8.4 and 8.5 give typical values of allowable settlements and differential settlements.
8.7.1
Total Settlement
Generally the amount of total settlement is not a problem of concern. But it is primarily a question of serviceability. However, there are situations where large total settlements can cause
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Foundation Design
Table 8.4
Permissible settlement as per Indian Standards.
Criterion
Permissible settlement (cm)
Angular distortion: office buildings, flats and factories
Differential settlement (as a ratio) should not exceed 1/500–1/1000
Maximum differential settlement : Clays Sands
4.0 2.5
Maximum total settlement: Isolated footings on clay Isolated footings on sand Raft foundations on clay Raft foundation on sand
6.5 4.0 6.5–10.0 4–6.5
(Reproduced from B.K. Ramiah and L.S. Chickanagappa, Soil Mechanics and Foundation Engineering , p. 406 (Table 4.27), Oxford and IBH Publishing Co., New Delhi, India. Ó 1981.)
serious problems, for example, a tank on soft clay near a waterfront can settle below the water level. The allowable total settlements are given in several building codes and the values specified by IS: 1904–1966 are illustrated in Table 8.4. The table also gives a range of values for permissible differential settlement.
8.7.2
Differential Settlement
It is usually the differential settlement (rather than the total settlement) that is important in the designing of a foundation as the consequences of differential settlement are more detrimental. The magnitude of differential settlement is affected greatly by the nonhomogeneity of natural soils and also by the ability of foundation to bridge over soft soil. Theoretical settlement should be computed for various points in a structure, such as center, corner, lightest and heaviest column locations, in order to compute differential settlement. The allowable angular distortions in buildings have been given in several codes and research reports (Ramiah and Chickanagappa, 1981; Bowles, 1996; Das, 2007). Some useful values given by Skempton (1956) are presented in Table 8.5. Table 8.5
Permissible maximum total and differential settlements of buildings (in cms).
Criterion
Isolated foundations
Rafts
Angular distortion
1/300
Greatest differential settlement (cm) Clays Sands
4.44 (3.81) 3.17 (2.54)
Maximum settlement (cm) Clays Sands
7.5 (6.35) 5.0 (3.81)
7.5–12.5 (6.35–10.0) 5.0–7.5 (3.81–6.35)
Note: The values in parenthesis take into account a safety factor of 1.25. (A.W. Skempton and D.H. MacDonald, “The allowable settlements of buildings,” Proceedings of the Institution of Civil Engineers , London, part III, vol. 5, pp. 759–760, Ó 1956, with permission from The Institution of Civil Engineers (ICE), Thomas Telford Ltd.)
Parameters and Criteria for Foundation Design
8.8
307
Cracks Due to Uneven Settlement
Uneven settlement creates cracks in connecting structural components such as beams and slabs. Even connecting walls and slabs develop cracks due to these uneven settlements. The cracks usually develop in the diagonal direction though vertical cracks are also possible. They may start from top if one end of the wall settles more than the next, as shown in Figures 8.2(a) and (b). If the middle part of the wall settles more than the ends, then cracks may start from the bottom, as shown in Figure 8.2(c). The cracks developed in walls due to differential settlement in nonhomogeneous soils are shown in Figure 8.3. The cracks in walls developed by causes other than settlements such as shrinkage are usually irregular or may terminate before reaching the edges of the wall, as shown in Figure 8.4.
Figure 8.2
Sketches showing the nature of differential settlement and cracks.
Figure 8.3
Differential settlement due to nonhomogeneous soils.
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Figure 8.4
8.9
Sketch showing wall cracks not caused by foundation settlement.
Suggestions to Reduce Large Differential Settlements
As pointed out in the above section, large differential settlements are more detrimental than the individual settlement to structures and foundations. To safeguard against large differential settlements, the following alternatives in design could be helpful 1. Use a raft foundation with or without stiffening beams in one or more directions. 2. Reduce the net pressure transmitted to the soil by providing deep basements. 3. Use piles, piers or basement slab foundations, pile–raft systems to transfer large loads from the superstructure to strong deeper soils with low compressibility. 4. Provide jacking pockets or brackets in columns to relevel the alignment of the superstructure when necessary. 5. Provide additional loads on lightly loaded parts of the structure if feasible.
