adv.found.eng

7/25/2019 Adv.found.eng

1/30

Deep Foundations

Lecture Note - By Alemayehu Teferra. ( Prof.) Page 1

8.3 Laterally Loaded Piles

8.3.1 General

In the design of laterally loaded piles there are basically three approaches. These are

a. The classical earth pressure theory

b. The method of modulus of subgrade reaction

c. A method which combines the results of theoretical and experimental investigations.

The first approach is not used, since it assumes the mobilization of active and passive pressures

which as a matter of fact does not occur except at complete failure.

The second method, even if it does not give an exact solution, is used for preliminary design.Here a modulus of subgrade reaction which varies with depth is frequently used.

The third method gives a relatively reliable result. Tables and diagrams for the design of

laterally loaded piles are given. Their application is however limited to the conditions in which

they are tested.

In the following pages, the last two methods will be discussed in some detail.

8.3.2 Method of Modulus of Subgrade Reaction

In the design of laterally loaded piles, two types of piles are identified according to this method.

These are short piles and long piles.

8.3.2.1 Design of Short Piles

Short piles are the categories of piles in which l/d


2/30

Deep Foundations


zo

l

+

_

_

Qo Mo

z

KshH yoMo

a

y

Z

( i ) ( ii ) ( iii ) ( iV) ( V)

zo

zo

M

H

M

H

y = yo- mzm = tan a

( i ) Rigid pile rotation

( ii ) Modules of subgrade reaction

( iii ) Contact pressure distribution

( iV) Shear stress distribution

( V) Bending momentdistribution

Fixed - end Free - end

Fig. 8 . 26 : Analysis of laterally loaded piles according to

the method of modulus of subgrade reaction

C

C

a) - Short piles

a) Long piles

Ksh = C.z


3/30

Deep Foundations


Inserting the value of y in Eq. (8.45),

(8.47)IfKsh varies with depth according to the relationship Ksh = Cz(Fig. 8.26a), then the intensity at

depthzwould be

(8.48)

(8.49)

Noting thato

o

z

ym tan , one may write Eq. (8.49) as follows:

(8.50)

The variation of the forcep(z) is as indicated in the sketch of fig. 8.26a.

From Eq. (8.49), one would obtain the shear force Q(z) and momentM(z) at any depthz.

Hence

(8.51)

(8.52)

(8.52a)

For z = 0, Q(z) = Qo, hence C1 = Qo

(8.53)

(8.54)

(8.54a)

(8.54b)

Forz = 0, M(z) = Mo, hence C2 =Mo

(8.55)

dmzyKp osh )(

dzdCmzCzyo )( 2

1

32

32 C

dCmzzdCyo

oo Q

dCmzzdCyzQ

32)(

32

dzzQzM )()(

dzQdCmzzdCy

o

o

32

32

ooo MzQ

dCmzzdCyzM

126)(

43

dmzyCzzp o )()(

2)( dCmzdCzyzp o

2)( zzdCydCzyzp

o

oo

dzzpzQ )()(

2

43

126CzQ

dCmzzdCyo

o


4/30

Deep Foundations


The variations of Q(z) and M(z) are given in Fig. 8.26a.

For the boundary conditionz= l, Q = 0 andM= 0, Eq. (8.53) and Eq. (8.55) become

(8.56)

(8.57)

From the above two equations, one may determine the values of Cyo and Cm:

(8.58)

(8.58a)

Similarly,

(8.59)

(8.59a)

Representative ranges of horizontal modulus of subgrade reaction are given in Table 8.12.

Table 8.12: Representative Ranges of Horizontal Modulus of Subgrade Reaction[2]

SOIL K sh- MN/m

Dense sandy gravelMedium dense coarse sand

Medium sand

Fine or Silty sand

Stiff clay (wet)Stiff clay (saturated)Medium clay (wet)

Medium clay (saturated)

Soft clay

220 400157 300

110 280

80 200

60 22030 11039 140

10 80

2 40

032

32

oo Q

dCmlldCy

0126

43

ooo MlQ

dCmzzdCy

o

o

o Ql

M

ldCy

4

3242

o

o

o Ql

M

dCly

4

3242

lQMl

dCm oo 2312

4

lQMdCl

m oo 2312

4


5/30

Deep Foundations


8.3.2.2 Design of Long Piles

For long piles, the criterion l/d > 10 is used. Here the method suggested by Snitko[quoted in 40]may be used. The pile is considered to be elastic, infinite in length and embedded in an elastic

medium. Using the method of modulus of subgrade reaction one may determine the deflection,

shear and bending moments. Because of the complexity of the problem, a constant modulus of

subgrade reaction is assumed. The determination of bending moments and shear forces above

point C, point of zero deflection(Fig. 8.26b), is of importance. The magnitudes of the above may

be obtained from the following equations[36].

(8.60)

(8.61)

where

Ks = modulus of subgrade reaction

E = modulus of elasticity of pile

I = moment of inertia of pile

zo = may be calculated from the following equation:

(8.62)

To determine the location of the maximum moment, one may use the shear equation, i.e. Q(z) =

0, and obtain

(8.63)

The range of values of modulus of subgrade reaction Ks given in Table 8.13 may be used as a

guide.

o

oz

zzQzQ

32 231)(

43

2

1)( z

zzzQMzM

o

oo

322

2

oz

4/1

4

EI

dKs

0166

222

23

o

o

oo

o

o

oQ

Mzz

Q

Mz

02

1

2

3 23

oo

zzzz


6/30

Deep Foundations


Table 8.13: Range of Values of Modulus of Subgrade Reaction[3]

SOIL K S- MN/m

Loose sand

Medium sand

Dense sand

Clayey sand (medium)

Silty sand (medium)

Clayey soil:

aq 200 kPa

200 < aq 800 kPa

aq > 800 kPa

aq = allowable bearing pressure

4.8 16

9.6 80

64 128

32 80

24 48

12 - 24

24 - 48

> 48

8.3.3 Method of Broms

Assuming different failure mechanisms consisting of plastic hinges, Broms[4] gave charts for

determining the limiting loads for restrained and free-headed piles subjected to horizontal

loadings. His analysis is governed by the following criteria:

a) The lateral deflection at working loads should not impair the proper functioning of the pile

group.

b) Complete collapse of the foundation or of the supported structure should not occur.

Broms used over-loadand under-strength factors. Here the design load is multiplied by a factor

greater than one (usually 2.0), and the pertinent soil parameter, i.e. shearing strength, by a factor

less than one(usually 0.75).

The charts presented in Fig. 8.27 and 8.28 for the determination of the ultimate lateral resistance

are based on the concept that the ultimate lateral resistance at relatively small penetration depths

is governed by the passive resistance of the soil surrounding the loaded piles, and that the

ultimate lateral resistance at relatively large penetration depths is governed by the ultimate or

yield resistance of the pile section. Fig. 8.27 represents the ultimate lateral resistance for

relatively short piles, while Fig. 8.28 represents the ultimate lateral resistance for long piles. In

the figures, Pult equals Pdesign multiplied by an over-load factors , and the shear strength parameter

cu is multiplied by under-strength factor. Kp is the coefficient of passive resistance, and is theunit weight of the soil.


7/30

Deep Foundations

Lecture Note - By Alemayeh Teferra. ( Prof.) Page 7


8/30

Deep Foundations



9/30

Deep Foundations



10/30

Deep Foundations


Broms has also provided charts for determining the lateral deflections at working loads (i.e P ult/2

or Pult/3) at the ground surface, using the modulus of subgrade reaction for both cohesive and

cohesionless soils.

For cohesive soils, the modulus of subgrade reaction is taken to be constant with depth. In Fig.

8.29a a dimensionless lateral deflection, yoKDL/P, at ground surface is plotted as a function of

dimensionless length L, in which

(8.64)

Kis the appropriate coefficient of the modulus of subgrade reaction.

For cohesionless soils, the coefficient of lateral subgrade reaction is assumed to increase linearly

with depth according to

(8.65)

where

nh = coefficient of lateral subgrade reaction for a long pile with

a width of unity at a depth of unity

z = depth below the ground surface

D = diameter or side of the loaded area.

The dimensionless lateral deflection PLnEIy ho /])()[(5/25/3 is plotted as a function of the

dimensionless penetration depth L, where 5/1)/( EInh (Fig. 2.29b).

In the analysis of Broms, it is assumed that for L less than 2.25, or for L less than 2.0, the pile

is considered to be infinitely stiff. Fog L greater than 2.25 or L greater than 4.0, the pile isconsidered to be infinitely long.

The coefficient nh may be taken as 2.2, 6.7 and 17.9 MN/m3

for loose, medium and dense sand

respectively, when the ground water table is located below the depth L of 2.0. For the casewhere the ground water table is located near or above the ground surface, 60% of the above

values should be used[4].

4/1

4

EI

DKh

D

znk hh


11/30

Deep Foundations



12/30

Deep Foundations


One may also determine the magnitude of nh from lateral loading tests by using the following

equations, provided that L is later than 4.0:

(for free pile) (8.66a)

(for fully restrained pile) (8.66b)

For a steel pile, for which ample test evidence is available, plastic hinges form when the stress at

the section of maximum bending moment reaches yield strength of the pile material.

The yield moments may be estimated from the following equations, assuming that the axial load

is of negligible importance:

a) For Circular Sections

(8.67)

b) For H-Sections

(8.68)

or

(8.69)

if the applied load is in the direction of the maximum or minimum moment of inertia of the pile

respectively.

In the above equations:

fy = yield strength of the pile material.

The design yield strength if 0.9 times the average measured yield strength.

Z = section modulus of the pile section.

5/2

5/3)(

)(

40.2EI

n

Py

h

o

3/2

5/3)(

)(

93.0EI

n

Py

h

o

ZfM yyield 3.1

max1.1 ZfM yyield

min5.1 ZfM yyield


13/30

Deep Foundations


8.4 Drilled Piers (Drilled Caissons)

8.4.1 General

Drilled piers or drilled caissons are deep foundations constructed by placing concrete in an

excavated (drilled) well for the purpose of transferring loads from the superstructure to load-

bearing strata in the ground. They are essentially large bored piles. They may be with or

without steel reinforcements or with or without enlarged bases. The excavation may be done by

hand or by machines. Since drilled piers have numerous advantages over other types of deepfoundations, they are popularly used as deep foundations. Some of the advantages of drilled

piers are the following:

a. Drilled piers can be carried through soils that prevent penetration of piles.

b. The sides and bearing surface of the soil can easily be inspected by sending an inspector

down the pier shaft.

c. The problems of heaving or vibration of the ground which normally occur during pile-

driving, are eliminated.

d. Since no volume of soil is displaced, the problem of shifting and lifting of piles is eliminated.

e. Concrete caps, as for the case of piles, are not necessary, since piers may be used directlybeneath a column.

f. One may easily provide increased bearing capacity or anchorage by belling or under-reaming

the base of the pier.

8.4.2 Types of Drilled Piers

Drilled piers are best classified according to their function in connection with the type of

supporting materials and the primary components of load resistance[40]. Fig. 8.30 shows the

three main types of piers:

a. Floating piers in homogeneous soils

b. End-bearing piers in soil

c. End-bearing piers in rock.

The materials of the pier may be of concrete, concrete in steel shell, or concrete plus steel core in

steel shell. For small jobs, plain concrete may be used. Depending on the soil and loading

conditions, the piers may be reinforced either partly (upper portion) or entirely (whole length),

Table 8.14 provides useful information which may be used as design basis.

2.8. Design of Drilled Piers and Caissons(Principle of Foundation Engineering, Alemayehu Teferra)


14/30

Deep Foundations


Fig. 8 . 30 : classification of drilled piers [ 40 ]

c) End - bearing in rock

ii) Socketed into rock : approximate range of

working load 3 - 70 MN

i) End bearing in rock : approximate range

of working load 2 - 70 MN

b) End - bearing pire in

soil

Rock ii) Belled or underreamed : approximate range

of

working load 1 - 30 MN

i) Straight : approximate range of working

load 0.5 - 2.5 MN

Hard or dense

soil

Relatively soft and

compressible

soil

a) Floating pier in homogeneous soil

ii) Belled or underreamed : approximate

range of working load 0.5 - 5.0 MNi) Straight : approximate range

of working

load 0.1 - 1.5 MN

L2

DL 1

R

P

P

R

LD

D

P

B

L 1

1

2

D

L2

L1

R

P

21

D L

B

PP

R

L D

Rock


15/30

Deep Foundations


Table 8.14: Design Bases for Various Classifications of Drilled Piers [40]

Design

Factor

Pier Classification According to FunctionFloating Piers in

Homogeneous Soil

End-Bearing Pier in Hard

Soil

End-Bearing Pier in Rock

Straightshaft

Enlargedbase

Socketed

into

bearingstratum

Enlargedbase

Bearing on

competent

rock

Socketed into

Competent

rock

Approximaterang ofworking

load

0.1 -1.5 MN 0.5 -5.0 MN 0.5 2.5MN 1.0 30.0MN 2.0 70 MN 3.0 70.0 MN

Usual limiting

designcriterion

Settlement Settlement

andbearing

capacity

Bearingcapacity

Bearingcapacity

Shaftconcrete

Bond between

concrete androck

or shaftconcrete

Majorcomponentof resistance

atworking load

Shaftadhesion

Shaftadhesionand end-

bearing

Shaftadhesion in

bearing

stratum

End-bearing End-bearingShaft adhesioninrock

Majorcomponentof resistanceat

ultimate load

Shaftadhesion

End-bearing Shaftadhesionand end

bearing

End-bearing End-bearingShaft adhesionand End-

bearing

Usual methodofobtainingdesign

working load

Analyticalconsideration ofshaft

adhesion

Presumptivebearingstressor analysis

ofultimate

bearing

capacity

Presumptive bearing stresssupplemented by analysis ofultimate bearing capacitywhen

possible, or qualitativeevaluation of

bearing material

Presumptive bearing stressplusqualitative evaluation of rockquality

Requirement

fordown the hole

inspection

On selected

piers Every pier Every pier

Every

pier,probe or

corebelowselected

piers

Ever pier, probe or core

belowselected piers


16/30

Deep Foundations


8.4.3 Bearing Capacity and Settlement

Drilled piers transfer loads from superstructures to lower soil strata by

a. skin friction (Fig. 8.30a)b. end bearing (Fig. 8.30b)

c. a combination of skin friction and end bearing.

The magnitudes of the bearing capacity and settlement depend naturally upon the type of the soil

in which the piers are embedded. It is fitting, therefore, to consider these quantities for each soil

type separately.

8.4.3.1 Cohesive Soils

One may express the ultimate bearing capacity as follows[40]:

(8.70)where

Pu = ultimate bearing capacity

tC = undrained shear strength of the soil below the base of the pier (should be taken as

the average in a zone from about 1 shaft diameter above, to 1.5 shaft or base

diameter below the base}

sC = undrained shear strength of the soil along the length of the pier

At = area of the base of the pier

As = area of the periphery of the pier shaft

, = coefficients determined from field tests. Form most situations, with the exception

of London Clay, = 1.

= 0.35 to 0.40 with the limit 2kN/m100s

A factor of safety greater than 2 should be used to obtain the allowable working load from the

ultimate bearing capacity. In selecting the factor of safety, one should take into consideration the

expected settlement of the pier.

Long-term settlements may be calculated from consolidation theory. The soil stresses may be

calculated using the Mindlin solution[quoted in 2]. For estimating the immediate settlement one

may use the equation suggested by Burland et al[5]:

(8.71)

where

b = least lateral dimension of footing

= contact pressure from load

u = ultimate bearing pressure

= settlement coefficient.

ssttu ACACP 9

u

bs


17/30

Deep Foundations


For deep circular plate on saturated clay, the value of is given by[2]:

(8.72)

where

= submerged unit weightL = embedment length of shaft

cu = undrained shear strength of the soil

Es = modulus of compressibility of the soil

8.4.3.2 Non-Cohesive Soil

For a drilled pier in non-cohesive soils (sand and gravel), the allowable bearing capacity may be

estimated, using the following equations.

A. Equation of Berenzantzev as quoted in [2]

(8.73)

where

allow = allowable bearing capacity

= unit weight of soil

B = base diameter of pier

B = coefficient dependent of L/B.

Berenzantzeev proposed the following equation for drilled pier resting on sand, for a settlement

ratio =s/B = 0.20:

(8.74)

where

= angle of inernal friction

L = depth of embedment

s = settlement.

s

u

u E

c

c

L 295.09

BB allow

)tan)4/(2

)tan)4/(

cossin1

)cos(sincos

22

cos667.1

e

eB

LB


18/30

Deep Foundations


B. Empirical Equation of Terzaghi and Peck [36,37]

(8.75a)

(8.75b)

where

Pallow(1) = allowable bearing pressure in

psf= 1/3 the ultimate bearing pressure

Pallow(2) = allowable bearing pressure in

psf for a maximum settlement of one inch

N = number of blows of SPT

B = diameter of base of pier in ft

L = depth of pier in ft. IfL >B, use the value ofB in the calculation.

ww RR , = reduction factors(Fig. 8.31).

The settlements of piers founded on non-cohesive soils are mainly immediate, and their

magnitude is relatively small.

8.4.3.3 Piers in Rock

Piers on or socketed into bedrock are commonly used for supporting heavy structures like major

industrial facilities and bridges. There is no clearly established basis for design of piers on rock.

Presumptive bearing values for rocks are given in EBCS-7[14]. The difficulty in assigning a safe

bearing capacity to rock lies in the evaluation of the influence of non-homogeneity and the

microscopic geologic defects on the behaviour of rock under loading.

If a pier is socketed into rock mass the peripheral resistance is calculated from

(8.76)

where

Qs = total allowable load resisted by adhesion

fa = allowable shaft adhesion

As = peripheral area of the rock socket.

The allowable shaft adhesion is difficult to assess. In most cases it is controlled by the bond

strength between concrete and rock. Normally an allowable adhesion between cf03.0 to

cf05.0 is used[40], where cf is the ultimate concrete strength.

wWallow RLNBRNp )100(232 22

)1(

wallow RB

BNp

2

)2(2

1)3(1440

sas AfQ


19/30

Deep Foundations


Fig. 8 . 31 : Correction factor for position of water level [ 35 ]

Reduction

factorR'w

Reduction

factorRw 1.0

0.9

0.8

0.7

0.6

0.2 0.4 0.6 0.8 1.00.5

0.00.0

0.51.00.80.60.40.2

0.6

0.7

0.8

0.9

1.0

B

L

L b

L a

B

Dwater level?

water level?


20/30

Deep Foundations


8.4.4 Design of Drilled Piers

8.4.4.1 General Design Consideration

The actual design of drilled piers involvesa. decision on the depth of the pier

b. determination of the allowable bearing capacity of the system, including the adhesion and

shear resistance on the periphery of the shaft and socket for the given soil profile.

c. selection of the type and size of the pier

d. design of the elements of the pier.

8.4.4.2 Design of the Elements of the Pier

The elements of the pier are the shaft with or without a bell and the cap.

A. Pier Bells

In order to reduce the soil pressure, drilled piers are normally constructed with enlarged baseswhich are called bells. The slope of the bells should not be less than 60

oin order to avoid

caving-in of excavated soils. Due to practical problems, it is advisable that bell diameter should

not exceed 3 times the shaft diameter. The bell should be proportioned so that it does not break

out from the shaft.

For the convenience of design engineers, Teng[35] has presented a table listing different sizes of

bells with their capacity for different design bearing pressures.

B. Pier Shaft

Since pier shafts are laterally supported by the surrounding soil, they do not buckle. Hence they

are designed using the formulas of short columns.For plain concrete shafts, minimum reinforcement should be provided. In the absence of

building code requirements, a minimum of 1 percent steel with appropriate ties is provided. An

allowable compressive strength of cf18.0 ( cf being the ultimate strength of a concrete test

cylinder) may be used[35]. The total shaft load with steel core reinforcement with or without

shell may generally be expressed as follows:

(8.77)

whereP = total shaft load

fc = allowable concrete stress

Ac = total cross-sectional area of concrete

As = cross-sectional area of steel core

fs = allowable steel stress

Ar = cross-sectional area of steel pipe.

a reduction of 1.6m from pipe thickness should be made

when calculating the area.

rrsscC fAfAfAp


21/30

Deep Foundations


fr = allowable stress of shell.

The magnitude of the allowable stresses should be given from building codes.

In the absence of codes one may use an allowable concrete stress fc = 0.25 cf , and for steel,allowable stress of 0.35 to 0.40fy. fy is the yield stress of the steel in question.

8.4.5 Constructional Considerations of the Elements

The bell and the shaft of the pier are constructed monolithically, and concreting is stopped at

some distance below the top of the pier. The remaining portion is poured together with the cap,

in which one normally uses a higher grade of concrete.

The cap is usually larger than the shaft so as to allow the placement of dowels or anchor bolts

without problems. It is advisable to proportion the cap in the same manner as for the pedestal ofa footing. The height of the cap should be either at least twice the difference between the width

or diameter of the cap and the diameter of the shaft[35], or equal to the length of embedment of

the anchor bolts or column dowels. It should be borne in mind that it is through the cap that

superstructure loads are transferred to the pier. Hence the cap should be designed carefully.

8.5 Caissons

8.5.1 General

Caissons are box-type structural elements of a foundation, consisting of many cells built either of

timber, steel, concrete or a combination of them, which are wholly or partly constructed at higher

level (or at times in a different position) and sunk to their final position. They are used to

transmit large loads through water and poor material to firm strata. Major areas in which

caissons are deployed are in bridge piers, quay walls, shore protection structures, water-front

structures, etc.. Because of their construction costs, caissons are advantageously used over other

types of foundations when any or all of the following conditions exist[35]:

a. The soil contains large boulders which obstruct penetration of piles or drilled caissons.

b. A massive substructure is required to extend to or below a river bed to provide resistance

against destructive forces due to floating objects, scour, etc..

c. The foundation is subjected to large lateral forces.


22/30

Deep Foundations


Basically caissons must be designed to resist two types of loading, namely permanent and

temporary loads. The permanent loads are the maximum vertical and laterally forces acting on

the caisson after it is constructed and sunk in the designated position. The vertical loads includeloads from the superstructure, and the caissons own weight minus buoyance force. The lateral

loads include force due to wind, earthquake, earth and water pressure, traction forces from

traffic, pressure from current flow. The temporary loads are those loads which subject the

caisson to large stresses during the construction period[35].

8.5.2 Type of Caisson

Caissons may be divided into three categories according to their methods of construction:

These are

a) Open caisson (Fig. 8.32a)

b) Pneumatic caissons (Fig. 8.32b)

c) Box caissons or floating caisson (Fig. 8.32c.

8.5.2.1 Open Caissons

An open caisson essentially consists of a box, open at top and bottom (Fig. 8.32a). The material

is removed by dredging or grabbing from inside the caisson. The sinking of the caisson proceeds

under the caissons own weight by overcoming the skin friction, which is assisted by the cutting

edges of the walls. Depending on the size of the caisson, its interior may be subdivided into cells

by diaphragms. The shape may be varied as desired circular, square, rectangular, oblong, etc..

After the desired level of penetration has been arrived at, concrete is poured at the bottom to

serve as a seal. After the concrete has set, the water is pumped out and cells are then filled with

concrete.

Open caissons have a relatively low cost of construction, and may be extended to great depths.

However, since the concrete seal is placed under water, its function is not as satisfactory as one

would wish it to be. The other problem associated with this method is that the soil near the

cutting edges may require hand excavation by divers.

A. Bearing Capacity

Caissons are placed in compacted sand, gravel, hard clay or bedrock, and never in soft soil or

decomposed rocks. The ultimate bearing capacity may be determined by using those bearing

capacity relationships used for mat foundations. The following relationships may be used:


23/30

Deep Foundations


Diaphragms for

stability

Diaphragmsfor stability

Air lock

Air shafts

Cutting edge

a) - Open caisson

Cutting edge

Compressed air inworking chamber

Fill

b) - Pneumatic casson c) - Box casson ( Floating casson)

Fig.8 .32 : Types of caisson


24/30

Deep Foundations


a) Cohesive Soil[40]

(8.78)where

= undrained shear strength below the base

b) Granular Soil[31]

(8.79)

where

ult = ultimate bearing capacity in psf

N = number of blows of the SPT

B = width of caisson in ft

l = depth of caisson in ft

ww RR , = reduction factors (Fig. 8.31).

It would be stated at this point that the bearing pressure of caissons on bedrock should not

exceed that of the concrete seal. The bearing pressure for the seal is limited to about 350

N/cm2[35].

B. Concrete Seal

Concrete seals are placed to plug the bottom of the caisson during construction, and later they

serve as permanent base of the foundation. They may be designed as thick plates subjected to aunit bearing pressure[2]..

a) Circular Caissons[2]

(8.80)

where

t = thickness of seal

o = contact soil pressure or hydrostatic pressure

R = radius of caisson

fc = allowable concrete stress (0.1 to cf2.0

).b) Rectangular Caissons[2]

(8.81)

where

b = width or short side of caisson

= length or long side of caisson

coefficient depending upon /b.

tult C9

wwult RlNBRN )100(12422

c

o

f

Rt

2

09.1

c

o

f

bt

26

tC


25/30

Deep Foundations


/b 1 1.2 1.4 1.6 1.8 2.0 3

0.048 0.063 0.075 0.086 0.095 0.102 0.119 0.125

C. General Design Criteria

The external walls of concrete caissons should be designed to withstand both the lateral and

vertical loads. The internal walls should be so designed as to share the vertical load with the

external walls. The construction joints should be placed at sufficient heights so that the weight

of the caissons will be sufficient to overcome the skin friction, and can sink without additional

loading.

These general design criteria also apply for steel caissons. Steel caissons consist of outer and

inner shells made of skin plates braced together with frames which provide rigidly to the walls.The space between the shells is filled with concrete.

8.5.2.2 Pneumatic Caissons

The essential difference between open caissons and pneumatic caissons is in the provision of a

working chamber filled with compressed air (Fig. 8.32b). Here the top of the caisson is closed,

and compressed air is introduced to prevent water from entering the working chamber. The

excavation is done in dry conditions, thus giving the workers a better chance to have control over

the construction work. Placement of the seal will also be carried out in dry conditions, thus

giving a reliable quality.

It should however be clearly stated that pneumatic caissons should be deployed as a last-resort

solution, because of their very high cost. A health hazard is also associated with this method of

construction: workers can stay in the chamber for only a limited period of time. The depth of

penetration of the caisson below water is limited to a chamber pressure of about 35 N/cm2, since

the human body cannot endure higher pressure. Much of the labour time is consumed in the

compression and decompression cycles which are necessary to avoid the formation of gas

bubbles in the bloodstream of the workers, which may cause death or permanent disability.

The bearing capacity and seal thickness may be determined from equations given for open

caissons.

The following should also be considered in the overall design, in addition to what is stated for

open caissons:

i) design of the roof of the excavation chamber

ii) location of air locks and material locks

iii) provision of decompression chamber

iv) provision of means of removing excavated material.


26/30

Deep Foundations


8.5.2.3 Box Caissons (Floating Caissons)

A box caisson, as its name implies, is basically a box with a bottom or base. Box caissons areusually cast on land and towed to the site, and then sunk onto a previously leveled soil base (Fig.

8.32c). They may be constructed from reinforced concrete, steel or a combination of both. Such

caissons are used where the construction of an open caisson is costly or not feasible. They are

advantageously used in a site where the bearing stratum is near the ground surface. Since such

caissons are simple placed on the ground, the ground must be leveled or excavated so as to

acquire a level surface. It must also be protected against scouring.

Since box caissons are towed to the site, one should make flotation stability analysis, so that the

caisson does not overturn due to wave action.

The bearing capacity may be estimated from the equations given for open caissons. However,

sufficient soil borings should be made at the site to reliably establish the design depth. Since the

bottom of the caisson is an integral part of the caisson, one does not need a concrete seal. After

the caisson is sunk to the desired level, it is filled with sand, concrete, gravel, etc., as found

appropriate.

The general criteria that apply for the design of the walls of open caissons also apply for box

caissons.


27/30

Deep Foundations


8.6 Piled Raft Foundation

8.6.1 Concept of Piled Raft Foundation

At the end of this chapter it would be appropriate to briefly discuss a foundation type, which has

been introduced in foundation design practice since the last 30 years, known as piled raft

foundation.

Piled raft foundation is a geotechnical composite structure in which pile and raft work in unisonto support loads imposed on them and transmit the loading to the soil. The soil structure inter

action involved is indicated in Fig. 8.33 as proposed by Hanisch et al [16].

Depending on its stiffness, the raft distributes the total structural load partly as contact pressure

to the immediate ground and the rest to the piles, generally represented by the sum of pile

resistance.

The total resistance of the piled raft system, which is a function of settlement, is made up of the

sum of the pile resistance and contact pressure of the raft.In general

(8.82)

2.9. Piled Raft Foundation (Principle of Foundation Engineering, Alemayehu Teferra)

m

j

kraftjkpilektot sRsRsR1

,,,, )()()(


28/30

Deep Foundations


1 j

t

( x , y)

qs,1

( z )

qb , 1

Interaction between

piled raft and soil

Interaction effects:

1 Soil - pile interaction

2 Pile - pile interaction

3 Soil - raft interaction

4 Pile - raft interaction

s s

qb , j

s , jq (z)

34

2

1

1

Fig . 8.33 : Soil - structure interaction effects for piled raft foundations [ 16 ]

l

tot,k

( x , y)Rpile, k,1 Rpile, k , j


29/30

Deep Foundations


The pile resistance of an individual pile j, made up of the sum of the point resistance and skin

friction.

(8.83)

with

AqR jkbjkb ,,,, (point bearing)

where

jkbq ,, = bearing pressure

A = cross-sectional area of the pile

SzsqR jksjks ),(..,, (skin friction)

wherejksq ,, = skin friction

S = surface area of the shaft

For circular piles with diameterD.

(8.84)

(8.85)

The load carrying mechanism of a piled raft is defined by the factor kpp , which indicates the

position of the settlement/resistance out of the total settlement/resistance taken up by the pile.

(8.86)

The value of kpp varies from 0 for the case where the load is totally supported by the raft to 1,

for the case where the load is totally supported by the piles.

Piled raft foundations have been used in a considerable number of high-rise buildings for the last

thirty years[19]. The numerous buildings founded on piled raft have been monitored with regard

to settlement and sharing of loads between raft and pile. Systematic stress measurements on

piles and rafts of many piled raft foundation indicated that the value of kpp varied from 0.3 to

0.8[16].

8.6.2 Advantages of Piled raft Foundations

The advantages derived from piled raft foundations include the following [19]

(i) In Comparison to actually piled foundation, a significant reduction in pile length is

achieved.

(ii) Maximum total settlements and differential settlements are reduced.

4

2

,,,,

DqR kjbjkb

)()()( ,,,,,, sRsRsR jksjkbjkpile

dzDzsqR jksjks ),(,,,,

)(

)()(

,

,,

sR

sRs

ktot

jkpile

kpp


30/30

Deep Foundations

Lecture Note By Alemayehu Teferra ( Prof ) Page 30

(iii) Internal stresses and bending moments in the raft are reduced by an optimal arrangement of

piles beneath the raft.

(iv) Due to the load sharing between the pile and raft the bearing capacity of shallowfoundation is improved.

(v) For eccentrically loaded rafts, centralization of the resistance of the foundation is achieved

by concentrating piles under the eccentrically loaded area of the raft.

8.6.3 Limits on Applicability of Piled Rafts

Piled rafts are general used in homogeneous clays, silt and sand. However one may cautiously

use piled rafts for other situations by considering the following points.

(i) If the soil is stratified and if the first layer immediately below the raft has a lower modulus

of Elasticity(E1) in comparison to the second layer{E2), the method is not add admissible.

(ii) The German Code specifically suggest that if10

1/ 21 EE , one should not use the method.

(iii) The method is not recommended for the case where kpp > 0.9.

8.6.4 Calculation Approaches

As could be observed in Fig. 8.33, the soils structure interaction mechanism involved in

complicated. Over the last decades a great number of different calculation methods have been

developed to analyze the load bearing behaviour of piled rafts. The accuracy of the methods

depend on the constitutive laws used, assumptions made and the calculation methods. In recent

years software have been developed which use the finite element method. However, because of

their high cost, they are not yet readily assessable in consulting offices.

===========//==========

adv.found.eng

Documents