Block Failure of Pile GroupsBlock Failure of Pile Groups

Block failure of pile groups is generally Block failure of pile groups is generally only a design consideration for pile groups only a design consideration for pile groups in soft cohesive soils or in cohesionless in soft cohesive soils or in cohesionless soils underlain by a weak cohesive layer.soils underlain by a weak cohesive layer.

Three Dimensional Pile GroupThree Dimensional Pile Group

Figure 9-33

Block Failure of Pile GroupsBlock Failure of Pile Groups

Qug = Ultimate group capacity against block failure.D = Embedded length of piles.B = Width of pile group.Z = Length of pile group.cu1= Weighted average of the undrained shear strength

over the depth of pile embedment for the cohesive soils along the pile group perimeter.

cu2= Average undrained shear strength of the cohesive soils at the base of the pile group to a depth of 2B below pile toe level.

Nc = Bearing capacity factor.

QQugug = 2D (B + Z) c= 2D (B + Z) cu1u1 + B Z c+ B Z cu2u2 NNcc

NNcc = 5 [ 1+D/5B ] [ 1+B/5Z ] = 5 [ 1+D/5B ] [ 1+B/5Z ] ≤≤ 99

The bearing capacity factor, The bearing capacity factor, NNcc, for a , for a rectangular pile group is generally 9. rectangular pile group is generally 9.

However, However, NNcc should be calculated for pile should be calculated for pile groups with small pile embedment depths groups with small pile embedment depths and/or large widthsand/or large widths

Block Failure of Pile GroupsBlock Failure of Pile Groups

Settlement of Pile Groups in Settlement of Pile Groups in Cohesive SoilsCohesive Soils

TerzaghiTerzaghi and Peck proposed that pile group and Peck proposed that pile group settlements in clays could be evaluated by an settlements in clays could be evaluated by an equivalent footing situated at a depth of 1/3 D equivalent footing situated at a depth of 1/3 D above the pile toe.above the pile toe.

Figure 9-34

ggUse consolidation analysis to evaluate Use consolidation analysis to evaluate longlong--term settlements term settlements –– See Chapter 8See Chapter 8

Figure 9-35a

Figure 9-35b

Figure 9-35c

Figure 9-35d

Settlement of Cohesionless LayersSettlement of Cohesionless Layers

ggUse Use Schmertmann’sSchmertmann’s method discussed method discussed in Chapter 8in Chapter 8

DowndragDowndrag(Negative Shaft Resistance)(Negative Shaft Resistance)

Negative Shaft ResistanceNegative Shaft ResistanceNegative shaft resistance, or “downdrag” loads, develop as a soil consolidates and load is transferred to the pile

The amount of relative soil/pile movement required to mobilize negative shaft resistance is only 0.4 to 0.5 in (10 to 12 mm)

Most common situation for dragload is when fill is placed over compressible soils prior to, or after, pile driving

Negative Shaft Resistance Negative Shaft Resistance Design CluesDesign Clues

1.1. Ground surface settlements larger than Ground surface settlements larger than 4 in (100 mm)4 in (100 mm)

2.2. Ground surface settlements after pile Ground surface settlements after pile driving > 0.4 in (10 mm)driving > 0.4 in (10 mm)

3.3. Embankment height > 6.5 ft (2 m)Embankment height > 6.5 ft (2 m)

4.4. Thickness of soft compressible soils > Thickness of soft compressible soils > 33 ft (10 m)33 ft (10 m)

5.5. Water table lowered by more than 13 ft Water table lowered by more than 13 ft (4 m)(4 m)

6. Piles longer than 82 ft (25 m)6. Piles longer than 82 ft (25 m)

Negative Shaft Resistance Negative Shaft Resistance Design CluesDesign Clues

NewFill

Bridge Deck

Soft Soil ConsolidatingDue to Fill Weight

Bearing Stratum

NewFill

NewFill

Bridge Deck

Soft Soil ConsolidatingDue to Fill Weight

Bearing Stratum

Figure 9-36a

Soft Soil ConsolidatingDue to Recent Placementor Ground Water Lowering

Bearing Stratum

Soft Soil ConsolidatingDue to Recent Placementor Ground Water Lowering

Bearing Stratum

Figure 9-36b

Negative Shaft Resistance Negative Shaft Resistance Design MethodsDesign Methods

Calculate shaft resistance from soil layers above consolidating soil and add this resistance as a drag load

Cohesive soils ?

Cohesionless soils ?

α - Method

Nordlund Method

Methods for Reducing Methods for Reducing Negative Shaft ResistanceNegative Shaft Resistance

a.a. Reduce soil settlementReduce soil settlement

b.b. Use lightweight fill materialUse lightweight fill material

- preload and consolidate soils priorprior to pile installation

- EPS (geofoam), foamed concrete, wood chips

EPS (EPS (GeofoamGeofoam) Blocks) Blocks

c.c. Use a friction reducerUse a friction reducer

d.d. Increase allowable pile stressIncrease allowable pile stress

e.e. Prevent direct contact between soil and Prevent direct contact between soil and pilepile

- bitumen (47-90%), plastic wrap (78-98%)

- 16.5 ksi (114 MPa) instead of 9 ksi (62 MPa)

- casings, bentonite slurry

Methods for Reducing Methods for Reducing Negative Shaft ResistanceNegative Shaft Resistance

Uncoated Uncoated Pile SectionPile Section

OverOver--sized sized CollarCollar

Bitumen Bitumen Coated Pile Coated Pile SectionSection

OverOver--sized Collarsized Collar

Learning OutcomesLearning Outcomes

ggAt the end of this session, the At the end of this session, the participant will be able to:participant will be able to:-- Describe types of driven piles and Describe types of driven piles and

applications for useapplications for use-- Compute static capacity for driven piles in Compute static capacity for driven piles in

granular and cohesive soilsgranular and cohesive soils-- Identify 2 of the design steps in pile Identify 2 of the design steps in pile

groupsgroups-- Discuss negative skin frictionDiscuss negative skin friction

Any Questions?Any Questions?

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