lesson 09-chapter 9 deep foundations - part 1b (piles)

8/8/2019 Lesson 09-Chapter 9 Deep Foundations - Part 1B (Piles)

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NordlundNordlundMethod ProcedureMethod Procedure

STEP 10 Compute the ultimate capacity, Qu.

Qu = Rs + Rt

STEP 11 Compute the allowable geotechnical pile load, Qa.

SafetyofFactorQ=Q ua


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Example 9Example 9--22


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Tomlinson orTomlinson or--MethodMethod

Unit Shaft Resistance, fs

:

fs = ca = cu

Where:

ca = adhesion (Figure 9-14)

= empirical adhesion factor (Figure 9-15)


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Tomlinson orTomlinson or

--MethodMethod

Shaft Resistance, Rs:

Rs = fs As

Where:As = pile surface area in layer

(pile perimeter x length)


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Concrete, Timber, Corrugated Steel Piles

Smooth Steel Pilesb = Pile Diameter

D = distance from ground surface to bottom ofclay layer or pile toe, whichever is less

Tomlinson orTomlinson or--Method (US)Method (US)

Figure 9-14


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Sand or

Sandy Gravels

Stiff ClayDb


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b = Pile Diameter

D = distance into stiff clay layer

Figure 9-15a


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Db

Soft Clay

Stiff Clay


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b = Pile DiameterD = distance into stiff clay layer

Figure 9-15b


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D

b

Stiff Clay


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b = Pile DiameterD = distance into clay layer

Figure 9-15c


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HIGHLY OVERCONSOLIDATED CLAYSHIGHLY OVERCONSOLIDATED CLAYS

For 1.0, = 0.5 -0.5

For > 1.0, = 0.5 -0.25

In highly overconsolidated clays, the undrained shear strength

may exceed the upper limits of Figures 9-14 and 9-15.

In these cases, the adhesion factor should be calculated

according to API procedures based on the ratio of the

undrained shear strength of the soil, cu, divided by the

effective overburden pressure, po. The ratio of cu / po is .


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Unit Toe Resistance, qt:

qt = cu Nc

Where:

cu = undrained shear strength of the soil at pile toe

Nc = dimensionless bearing capacity factor

(9 for deep foundations)


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Toe Resistance, Rt:

Rt = qt At

The toe resistance in cohesive soils is sometimes ignoredsince the movement required to mobilize the toe resistance

is several times greater than the movement required to

mobilize the shaft resistance.


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Qu = RS + RT

Qa = QU / FS

and



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Example 9Example 9--33

Whi h il h th hi h t tWhich pile has the highest toe


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Which pile has the highest toeWhich pile has the highest toeresistance ?resistance ?


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Plugging of Open Pile SectionsPlugging of Open Pile Sections

(a) Open Toe Condition

b

qt

qt

fso fsi

(b) Plugged Toe Condition

qt

D

fso

(a) Open Toe Condition

b

qt

qt

fso fsi

(b) Plugged Toe Condition

qt

D

fso

Figure 9-18


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Plugging of HPlugging of H--Pile SectionsPile Sections

Figure 9-19


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The DRIVEN Computer ProgramThe DRIVEN Computer Program

ggDeveloped by FHWA in 1998Developed by FHWA in 1998

ggUse for calculation of static pileUse for calculation of static pilecapacitycapacity

ggDemonstration of the DRIVEN computerDemonstration of the DRIVEN computer

programprogram


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Piles Driven to RockPiles Driven to Rock

The capacity of piles driven to rock should be based on driving

observations, local experience, and load test results.

RQD values from NX size rock cores can provide a qualitative

assessment of rock mass quality.

RQDRQD Rock Mass QualityRock Mass Quality

9090

100100

ExcellentExcellent

7575 9090 GoodGood

5050 7575 FairFair

2525 5050 PoorPoor00--2525 Very PoorVery Poor

What is RQD? See Chapter 3


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Piles Driven to RockPiles Driven to Rock

Except for piles driven to soft rock, the structural capacity of

the pile will be lower than the geotechnical capacity of the rock

to support a toe bearing pile. (Fair to excellent quality rock).

The structural capacity of the pile then governs the pile

capacity.

lesson 09-chapter 9 deep foundations - part 1b (piles)

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