ch5 bearing capcity

8/12/2019 CH5 Bearing Capcity

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The Isl amic university - Gaza

Faculty of Engineer ing

Civil Engineer ing Department

CHAPTER(5)

Inst ructor : Dr. Jehad Hamad

Allowable Bearing Capacity& Settlement


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oIt was mentioned in Chapter 3 that, in many cases, the allowable

settlement of a shallow foundation may control the allowable

bearing capacity. The allowable settlement itself may be controlled

by local building codes. Thus, the allowable bearing capacity will be

the smallerof the following two conditions:

Introduction


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oThe settlement is divided into two categorizes:

Elast ic (Immediate set t lement).

Consolidat ion set t lement.

oIn some calculations of settlement it is required to find the increase

in stress at any depth of soil mass, so we will discuss the calculation

of increase in stress.


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Vertical stress increase insoil mass

to concent rated load:Due-1

X, Y and Z are the coordinates of point

under consideration.


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to circularly loaded area load:Due-2

q o: Load per unit area on circle.

B: Diameter of circle.

r: Distance from center of circle to point

under consideration.

z: Depth of point under consideration.

Go to table ( 5.1 )

find o Ds / q by determining the terms:

r/(B/2)

z/(B/2)


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q o : Load per unit area on rectangle.

I: influence factor.

The point should be under corner of rectangle, if not we have to

divide the rectangle to sub rectangles for which the point is a corner

for each part .

find I, by determining the terms:

see table 5.2

rectangular loaded area:Below-3

Iq o=

Z

Bm=

Z

Ln=


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:loaded areaAverage vert ical increase in st ress due to rectangular-4

When we have a foundation on a layer of soil has depth from z=0 to

z=H, the increase of stress decrease as the depth of soil increase, so

to calculate the average stress increase in such layer use the following

equation:( )

1)1(

2)2(

12

)1(1)2(2

1/2

HHTake;

HHTake;

=

=

=

aHa

aHa

HaHa

oHHavg

II

II

HH

IHIHq


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Go to figure 5.7 to find a I by determining the terms:

H

Bm =2

Ln =2


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1-Elast ic s ett lement ov er saturated clay: ( )s

oavge

E

BqAAS 21=

Go to figure 5.14 to find A1 by determining the terms (H/B and L/B).

Go to figure 5.14 to find A2 by determining the term (Df/B).

Es = !. cu

See Table 5.7 to get a typical value of != f (OCR , PI)

Sett lement calculat ions:


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2- Calculat ion of elast ic set t lement b ased o n elast ic i ty theory:

see table 5.8,9,10


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In 1999, Mayne and Poulos presented an improved formula for calculating the

elastic settlement of foundations. The formula takes into account the rigidity of

the foundation, the depth of embedment of the foundation, the increase in the

modulus of elasticity of the soil with depth, and the location of rigid layers at a

limited depth. To use Mayne and Pouloss equation, one needs to determine the

equivalent diameter Beof a rectangular foundation, or

oImp roved Equ at ion for Elast ic Set t lement :

Where:

B = width of foundation

L = length of foundat ion

For circular foundat ions Be = B


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o Improved equation for

calculating elast ic sett lement:

general parameters


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3-Elast ic set t lement of sand y s oi l us ing strain inf luence factor :

( )=

=

=2

0

'

21

zz

z s

ze z

E

IqqCCS

qq

qC

=

'15.01C1 Correction factor for depth of emb edment :

+=

1.0

yearsinTimelog2.012CC2 Creep correct ion factor :

fD=

qStress at the level of foundat ion inclu ding external loads and soi l w eight.

q Effect ive vert ical overburd en pressure. In the absence of water table

.

Es: Modulus of elast ic i ty of so i l below the found at ion w hich is var iable.

Why Es i s v ari ou s?

Due to n onhom ogenu ity o f so il th e value o f Es is varying , w e can est im ate the

value o f Es from field tes ts as Standard Penetrat ion Number (N) or Cone

Penetrat ion Resistance( )cq


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( ) ( )

NIb

mKNftt

ftIbftUS

q

q

ftUSNmKNN

E

c

cs

=

4484.4

27854.952

220002

22

1

//1

//tons1

footingstripFor5.3

footingsquareorcircularFor5.2

tons/8or/766

zI : Strain inf lu ence factor, it is given as shown b elow:


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Z Iz

0 0.1

Z1=0.5B 0.5Z2=2B 0

For square or circular foundation

Z Iz

0 0.2

Z1=B 0.5Z2=4B 0

For foundation with L/B 10

For L/B between 1 and 10, we have to do interpolation for each depth.


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The procedure for calculat ing elast ic sett lement using Eq. (5.49) is given here

(Figure 5.22):

Step 1. Plot the foundation and the variation of Iz with depth to scale (Figure 5.22a).

Step 2. Using the correlation from standard penetration resistance (N60) or cone

penetration resistance (qc), plot the actual variation of Es with depth (Figure 5.22b).

Step 3. Approximate the actual variation of Es into a number of layers of soil having

a constante Es, such as Es(1), Es(2), . . . , Es(i), . . . Es(n) (Figure 5.22b).

Step 4. Divide the soil layer from Z = 0 to Z = Z2 into a number of layers by drawing

horizontal lines. The number of layers will depend on the break in continuity in the Iz and Es

diagrams.

Step 5. Prepare a table (such as Table 5.11) to obtain

Step 6. Calculate C1 and C2.

Step 7. Calculate Se from Eq. (5.49).

=

=

2

0

zz

z s

z zEI


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Procedure for calculation of Se using t he st rain inf luence factor


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Meyerhof (1956) proposed a correlation for the net bearing pressure for

foundations with the standard penetration resistance, N60. The net pressure has

been defined as

Sett lement of Foundat ion on Sand Based on Standard

Penet rat ion Resistance Meyerhof s Method

According to Meyerhof s theory, forAccording to Meyerhof s theory, for 2525 mm (mm (11 in) of est imated maximum set t lement,in) of est imated maximum set t lement,


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Since the time that Meyerhof proposed his original correlations, researchers have

observed that its results are rather conservative. Later, Meyerhof (1965) suggested

that the net allowable bearing pressure should be increased by about 50%. Bowles

(1977) proposed that the modified form of the bearing equations be expressed as


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Burland and Burbidge (1985) proposed a method of calculating the elastic settlement

of sandy soil using the field standard penetration number, N60 (See Chapter 2.)

The method can be summarized as follows:

1. Variation of Standard Penetration Number with Depth

Obtain the field penetration numbers N60 with depth at the location of the

foundation. The following adjustments of N60 may be necessary, depending on the

field conditions:

For gravel or sandy gravel,

oBurland and Burbidges Method

For fine sand or silty sand below the

15>groundwater table and N60


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2. Determination of Depth of Stress Influence (Z):

In determining the depth of stress influence, the following three cases may arise:

Case I. If N60 [or N60 (a) ] is approximately constant with depth, calculate from

Case II. If N60 [or N60 (a) ] is increasing with depth, use Eq. (5.65) to calculate Z

Case III. If N60 [or N60 (a) ] is decreasing with depth Z = 2B, or to the bottom of soft

soil layer measured from the bottom of the foundation (whichever is smaller).

3. Calculat ion of Elast ic Sett lement Se :

The elastic settlement of the foundation Se, can be calculated from


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As mentioned before, consolidation settlement occurs over time in saturated clayeysoils subjected to an increased load caused by construction of the foundation.

(See Figure 5.29.) On the basis of the one-dimensional consolidation settlement

equations given in Chapter 1, we write

o Primary Consolidation Settlement Relationships

(for normally consolidated clays)


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The consolidation settlement calculation presented in the preceding section is

based on Eqs. (1.61), (1.63), and (1.65). These equations, as shown in Chapter 1,

are in turn based on one-dimensional laboratory consolidation tests. The

underlying assumption is that the increase in pore water pressure u,

immediately after application of the load equals the increase in stress!, at any

depth. In this case,

oThree-Dimensional Effect on Primary

Consolidation Settlement


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Circular foundat ion on a clay layer

Kcir = set t lement rat io for circular

foundations.


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Sett lement rat ios for circular (Kcir)

And Cont inuous foundat ions

(Kstr)


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At the end of primary consolidation (i.e., after the complete dissipation of excess pore

water pressure) some settlement is observed that is due to the plastic adjustment of soil

fabrics. This stage of consolidation is called secondary consolidat ion.

o Settlement Due to Secondary Consolidation

Variation of ewith log t under a given

load increment , and definit ion of

secondary compression index


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The ultimate load-bearing capacity of a foundation, as well as the allowable

bearing capacity based on tolerable settlement considerations, can be effectively

determined from the field load test, generally referred to as the plate load test.

The plates that areused for tests in the field are usually made of steel and are

25 mm thick and 150 mm to 762 mm in diameter. Occasionally, square plates that

are 305 mm " 305 mm are also used.

o Field Load Test

( ) ( )pufu

qq =

( ) ( ) P

f

pufuB

B

qq =

( )

( )

plateofWidth:

foundationofWidth:

plateforcapacitybearingUltimate:

foundationforcapacitybearingUltimate:

p

f

pu

fu

B

B

q

qFor clay soi l :

For sand s oi l :


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Plate load test: (a) test arrangement;

(b) nature of load settlement curve


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p

f

pfB

B

SS =

2

2

+=

pf

f

pfBB

BSS

fS

pS

For clay:

For sand:

: Settlement of foundation

: Settlement of plate

Settlement relationships from plate load test:


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ch5 bearing capcity

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