parameters for foundation design

8

Parameters and Criteriafor Foundation Design

8.1 Introduction

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.

While the superstructure loads depend on the needs of the project, the soil capacities are limited

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 of parameters and design methods and 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 Considerations

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. Requirements of the project and choice of superstructures.

2. Loads and moments coming from the superstructures.

3. Selection of suitable site.

4. Soil properties at the chosen site.

5. Bearing capacity, settlement and compressibility, stress distribution and lateral pressure

where necessary.

6. Choice of the foundations based on items 2, 4, and 5, as follows:

a. Shallow foundations; spread footings, combined footings, strip footings, mat/raft

foundations.

b. Deep foundations; piles and pile groups, piers (including large diameter 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

c. Foundations subjected to vibratory/dynamic loads. In addition to the normal require-

ment 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.

7. 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.

8. Structural design of the foundation based on items 6 and 7 as per standard codes and

practices.

9. Criteria for assessment as per codes, practices and assessment of the structure designed

with respect to criteria based on item 8.

10. Acceptability of the design if the foundation designed satisfies the criteria specified based

on item 8.

11. 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.

302 Foundation Design

8.5 Gross and Net Values of the Safe Bearing Capacityand 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 Cohesive soils

Description 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

45 1. Soft shale, hard or stiff clay in

deep bed, dry

45

2. Coarse sand, compact and dry 45 2. Medium clay readily indented

with a thumb nail

25

3.Medium sand, compact and dry 25 3. Moist clay and sand clay

mixture which can be indented

with strong thumb pressure

15

4. Fine sand, silt (dry lumps easily

pulverized by the fingers)

15 4. Soft clay indented with

moderate thumb pressure

10

5. Loose gravel or sand gravel

mixture; loose coarse to

medium sand and dry

25 5. Very soft clay which can be

penetrated several inches with

the thumb

5

6. Fine sand, loose and dry 10 6. Black cotton soil or other

shrinkage or expansive clay in

dry condition (50% saturation)

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. � 1981.)

Parameters and Criteria for Foundation Design 303

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).

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 Si þ Sc þ Ss

Table 8.2 Summary of presumptive/safe bearing capacities from some building codes (in kN/m2).

Soil description Chicago, 1995 National Board of Fire

Underwriters, 1976

BOCA 1993a Uniform Building

Code, 1991

Clay, soft 75 100 100 100

Clay, medium stiff 175 100 — 100

Clay, stiff 210 — 140 —

Sand, compact and clean 240 140–400 140 200

Sand, compact and silty 100 140–400 — —

Inorganic silt compact 125 140–400 — —

Sand, loose and fine — 140–400 140 210

Sand, loose and coarse, or

sand–gravel mixture, or

compact and fine

— 140–400 240 300

Gravel, loose and compact

coarse sand

300 140–400 240 300

Sand–gravel, compact — 140–400 240 300

aBuilding Officials and Code Administrators International, Inc.

Table 8.3 Design loads and factors of safety.

Design load Factor of safety for safe bearing capacity

KDDL þ KL.LL þ KW.WL þ KS.SL þ HL 3

KD.DL þ KL.LL þ KW.WL þ HL 2

KD.DL þ KL.LL þ KEE þ KSS 2

whereKD, LL,KW,KS,KE 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 ¼earthquake load.


where

Si ¼ immediate/elastic settlement

Sc ¼ settlement due to primary consolidation

Ss ¼ settlement due to secondary consolidation

Out of these, usually the consolidation (primary) settlement Sc is the most important part of the

total settlement as discussed in Section 3.7. Though theremay not be a collapse or shear failure of

the soil due to large settlement, the structures and foundationsmay become unserviceable. Further

tilting and cracking of beams and slabsmay 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), theremay

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 Dsbetween two points is the larger settlement minus the smaller one. Differential settlement is also

characterized by angular distortion dLwhich is the differential settlement between two points

divided by the horizontal distance between them and may be referred to as a ratio.

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 situationswhere large total settlements can cause

Figure 8.1 Settlement and differential settlement.


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 distor-

tions 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 8.4 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 4.0

Sands 2.5

Maximum total settlement:

Isolated footings on clay 6.5

Isolated footings on sand 4.0

Raft foundations on clay 6.5–10.0

Raft foundation on sand 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. � 1981.)

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 4.44 (3.81)

Sands 3.17 (2.54)

Maximum settlement (cm)

Clays 7.5 (6.35) 7.5–12.5 (6.35–10.0)

Sands 5.0 (3.81) 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, � 1956, with permission from The

Institution of Civil Engineers (ICE), Thomas Telford Ltd.)


8.8 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.


8.9 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 superstruc-

ture when necessary.

5. Provide additional loads on lightly loaded parts of the structure if feasible.

Figure 8.4 Sketch showing wall cracks not caused by foundation settlement.


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