design of pile groups

Design of Pile Design of Pile GroupsGroups

Reference Manual Chapter 9

• Group Capacity (compression loads)Group Capacity (compression loads)

• Group SettlementGroup Settlement

• Group Capacity (uplift loads)Group Capacity (uplift loads)

• Group Capacity (lateral loads)Group Capacity (lateral loads)

Design of Pile GroupsDesign of Pile Groups

9-116

• Piles for highway structures are almost Piles for highway structures are almost

always installed in pile groups.always installed in pile groups.

• The design of a pile group must consider The design of a pile group must consider

the group’s axial compression capacity, the group’s axial compression capacity,

settlement, uplift resistance, and lateral settlement, uplift resistance, and lateral

load capacity.load capacity.


The efficiency of a pile group is the ratio of the The efficiency of a pile group is the ratio of the

ultimate capacity of the group to the sum of the ultimate capacity of the group to the sum of the

candidates of the individual piles.candidates of the individual piles.


u

ugg

nQ

Q

Where: g = Pile group efficiency.

Qug = Ultimate capacity of the pile group.

n = Number of piles in the pile group.

Qu = Ultimate capacity of each pile in the pile group. 9-1169-116

The group efficiency may be less than 1 for a The group efficiency may be less than 1 for a

pile group driven into a compressible cohesive pile group driven into a compressible cohesive

soil, or into a dense cohesionless soil underlain soil, or into a dense cohesionless soil underlain

by a weak cohesive deposit.by a weak cohesive deposit.

The group efficiency in cohesionless soils is The group efficiency in cohesionless soils is

generally greater than 1.generally greater than 1.


Vibratory densification Densification from displacement

9-1189-118

The settlement of a pile group is likely to be The settlement of a pile group is likely to be many times greater than that of a single pile many times greater than that of a single pile carrying the same load as each pile in the pile carrying the same load as each pile in the pile group.group.


9-1169-116

Stress Zones from a Single Stress Zones from a Single Pile and Pile GroupPile and Pile Group

9-1179-117

Overlap of Stress Zones Overlap of Stress Zones

9-119

Group Capacity in Cohesionless SoilsGroup Capacity in Cohesionless Soils

1.1. The ultimate axial compression capacity of a pile The ultimate axial compression capacity of a pile

group driven in a cohesionless soil may be taken group driven in a cohesionless soil may be taken

as the sum of the individual capacities, unless as the sum of the individual capacities, unless

underlain by a weak deposit, jetted, or predrilled.underlain by a weak deposit, jetted, or predrilled.

2.2. If underlain by a weak deposit, the ultimate group If underlain by a weak deposit, the ultimate group

capacity is the lesser of the 1) sum of the capacity is the lesser of the 1) sum of the

individual pile capacities, or 2) the group capacity individual pile capacities, or 2) the group capacity

against block failure.against block failure.

3.3. A minimum center-to-center pile spacing of 3 A minimum center-to-center pile spacing of 3

diameters is recommended.diameters is recommended. 9-118

1.1. For pile groups in clays with undrained For pile groups in clays with undrained

shear strengths less than 95 kPa (2 ksf), shear strengths less than 95 kPa (2 ksf),

and the cap not in firm contact with the and the cap not in firm contact with the

ground, use a group efficiency ranging ground, use a group efficiency ranging

from 0.7 for c-t-c spacings of 3 diameters, from 0.7 for c-t-c spacings of 3 diameters,

to 1.0 for c-t-c spacings of 6 diameters to 1.0 for c-t-c spacings of 6 diameters

(interpolate in between).(interpolate in between).

Group Capacity in Cohesive Group Capacity in Cohesive SoilsSoils

9-1209-120

2.2. For pile groups in clays with undrained shear For pile groups in clays with undrained shear

strengths less than 95 kPa (2 ksf), and the cap strengths less than 95 kPa (2 ksf), and the cap

in firm contact with the ground, a group in firm contact with the ground, a group

efficiency of 1.0 may be used.efficiency of 1.0 may be used.

3.3. For pile groups in clays with undrained shear For pile groups in clays with undrained shear

strengths greater than 95 kPa (2 ksf), strengths greater than 95 kPa (2 ksf),

regardless of pile cap/ground contact, use a regardless of pile cap/ground contact, use a

group efficiency of 1.0.group efficiency of 1.0.


9-1209-120

4.4. Calculate the ultimate pile group capacity Calculate the ultimate pile group capacity

against block failure, and use the lesser against block failure, and use the lesser

capacity.capacity.

5.5. A center-to-center spacing less than 3 A center-to-center spacing less than 3

diameters should not be used.diameters should not be used.


9-1209-120

Group Capacity in Cohesive SoilsGroup Capacity in Cohesive Soils

Short-term group efficiencies in cohesive soils 1 to 2 Short-term group efficiencies in cohesive soils 1 to 2 months after installation may be as low as 0.4 - 0.8 months after installation may be as low as 0.4 - 0.8 due to high driving-induced excess porewater due to high driving-induced excess porewater pressures (results in decreased effective stress).pressures (results in decreased effective stress).

Pile groups in clays which are loaded shortly after pile Pile groups in clays which are loaded shortly after pile installation should consider the reduced short-term installation should consider the reduced short-term group capacity.group capacity.

In critical cases, piezometers should be installed to In critical cases, piezometers should be installed to monitor porewater pressure dissipation with time.monitor porewater pressure dissipation with time.

9-1209-120

Dissipation of Excess Pore Dissipation of Excess Pore PressuresPressures

9-1219-121

+ x - single pile - 9 pile group - 13 pile group

- 25 pile groupM - 116 pile group - 230 pile group

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.

9-1229-122

Three Dimensional Pile Three Dimensional Pile GroupGroup

9-1239-123


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. 9-1229-122

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


NNcc = 5 [ 1+D/5B ] [ 1+B/5Z ] = 5 [ 1+D/5B ] [ 1+B/5Z ] ≤ 9≤ 9

The bearing capacity factor, Nc, for a The bearing capacity factor, Nc, for a

rectangular pile group is generally 9. rectangular pile group is generally 9.

However, NHowever, Ncc 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

9-1229-122

Lateral Capacity of Pile Groups

9-1509-150

LATERAL CAPACITY OF PILE GROUPSLATERAL CAPACITY OF PILE GROUPS

The lateral deflection of a pile group is typically The lateral deflection of a pile group is typically

2 to 3 times larger than the deflection of a 2 to 3 times larger than the deflection of a

single pile.single pile.

Piles in trailing rows of pile groups have Piles in trailing rows of pile groups have

significantly less lateral load resistance than significantly less lateral load resistance than

piles in the lead row.piles in the lead row.

Laterally loaded pile groups have a group Laterally loaded pile groups have a group

efficiency less than 1.efficiency less than 1.

9-1509-150


The lateral capacity of an individual pile in a The lateral capacity of an individual pile in a

group is a function of its position (row) in the group is a function of its position (row) in the

group, and the c-t-c pile spacing.group, and the c-t-c pile spacing.

A p-multiplier, is used to modify p-y curve

Laterally loaded pile groups have a group efficiency less than 1.

9-1509-150


The lateral capacity of an individual pile in a The lateral capacity of an individual pile in a

group is a function of its position (row) in the group is a function of its position (row) in the

group, and the c-t-c pile spacing.group, and the c-t-c pile spacing.

A p-multiplier: 0.8, 0.4, & 0.3 (thereafter)

TABLE 9-19 LATERALLY LOADED PILE GROUPS STUDIES

Soil Type

Test Type

Center to Center Pile

Spacing

Calculated p-Multipliers, Pm For

Rows 1, 2, & 3+

Deflectionin mm

(in)

Reference

Stiff Clay Field Study 3b .70, .50, .4051(2)

Brown et al,(1987)

Stiff Clay Field Study 3b .70, .60, .50,30

(1.2)Brown et al,

(1987)

MediumClay

Scale Model- Cyclic Load

3b .60, .45, .40600 at

50 cycles(2.4)

Moss(1997)

Clayey Silt Field Study 3b .60, .40, .4025-60

(1.0 - 2.4)Rollins et al,

(1998)

V. DenseSand

Field Study 3b .80, .40, .3025(1)

Brown et al,(1988)

M. DenseSand

Centrifuge Model 3b .80, .40, .3076(3)

McVay et al,(1995)

M. DenseSand

Centrifuge Model 5b 1.0, .85, .7076(3)

McVay et al,(1995)

LooseM. Sand

Centrifuge Model 3b .65, .45, .3576(3)

McVay et al,(1995)

LooseM. Sand

Centrifuge Model 5b 1.0, .85, .7076(3)

McVay et al,(1995)

LooseF. Sand

Field Study 3b .80, .70, .3025-75(1-3)

Ruesta et al, (1997)

Lateral Load

Front Row

Second Row

Third & Subsequent

Rows

Lateral Load

ps

Pm ps

Single Pile Model p-y Curves for Group9-151

STEP 1STEP 1 :: Obtain Lateral Loads.Obtain Lateral Loads.

STEP 2STEP 2 :: Develop p-y curves for single pile.Develop p-y curves for single pile.

a. Obtain site specific single pile p-y curves from instrumented lateral pile load test at site.

b. Use p-y curves based on published correlations with soil properties.

c. Develop site specific p-y curves based on in-situ test data.

STEP BY STEP DESIGN PROCEDURE STEP BY STEP DESIGN PROCEDURE FOR LATERALLY LOADED PILE GROUPSFOR LATERALLY LOADED PILE GROUPS

9-154

a. Perform LPILE analyses using the Pm value for each row position to develop load-deflection and load-moment data.

b. Based on current data, it is suggested that Pm values of 0.8 be used for the lead row, 0.4 for the second row, and 0.3 for the third and subsequent rows. These recommendations are considered reasonable for center to center pile spacing of 3b and pile deflections at the ground surface of .10 to .15b. For larger c-t-c spacings or smaller deflections, these Pm values should be conservative.

c. Determine shear load versus deflection behavior for piles in each row. Plot load versus pile head deflection results similar to as shown in Figure 9.69(a).

STEP 3STEP 3 :: Perform LPILE Analyses.Perform LPILE Analyses.

a. Average the load for a given deflection from all piles in the group (i.e., each of the four rows) to determine the average group response to a lateral load as shown in Figure 9.69(a).

b. Divide the lateral load to be resisted by the pile group by the number of piles in the group to determine the average lateral load resisted per pile.

c. Enter load-deflection graph similar to Figure 9.69(a) with the average load per pile to estimate group deflection using the group average load deflection curve.

STEP 4: Estimate group deflection under lateral load.STEP 4: Estimate group deflection under lateral load.

a. Plot the maximum bending moment determined from LPILE analyses versus deflection for each row of piles as illustrated in Figure 9.69(b).

b. Check the pile structural adequacy for each row of piles. Use the estimated group deflection under the lateral load per pile to determine the maximum bending moment for an individual pile in each row.

c. Determine maximum pile stress from LPILE output associated with the maximum bending moment.

d. Compare maximum pile stress with pile yield stress.

STEP 5:STEP 5: Evaluate pile structural acceptability.Evaluate pile structural acceptability.

STEP 6: STEP 6: Perform refined pile group evaluation that Perform refined pile group evaluation that considers superstructure substructure considers superstructure substructure

interactioninteraction..

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design of pile groups

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