ENCE 4610

Foundation Analysis and Design

Static Load Tests

Pile Settlement

Pile Groups

Methods of Evaluation of Deep Foundations

• Full-scale static load tests on test piles

• Analytic methods, based on soil properties from laboratory and/or in-situ tests

• Dynamic Methods, based on dynamics of pile driving or wave propagation

Concepts to Review • Shaft and End Bearing Piles

• Resistance to load

– Shaft resistance (Qs )

– Toe resistance (Qt)

• End-bearing piles, toe resistance predominates

• Shaft Friction Piles, shaft resistance predominates

• For tension piles, shaft resistance predominates (toe cannot be in tension)

Ultimate vs. Allowable Capacity Ultimate capacity is the load required to

cause failure, whether by excessive settlement or irreversible movement of the pile relative to the soil

For driven piles, one must also consider the resistance to driving, which can be different from the ultimate capacity

Allowable capacity is the ultimate capacity divided by a factor of safety (ASD)

Important to distinguish between the two

Prediction and Verification

• At what load will the pile fail? (Bearing Capacity)

• How much will pile deflect under service loads?

(Settlement)

Prediction on basis of site investigation

and laboratory testing

Verification by some method of

load testing

Static Load Tests The most precise – if not

always the most accurate – method of determining the ultimate upward or downward load capacity of a deep foundation

Static load tests, however, are time consuming and expensive; must be used judiciously

Object of the test is to develop a load-displacement curve, from which the load capacity can be determined

Dead Load Test

• Considered a reliable static test method

• Slow and expensive

• Dangerous when done unsafely

• No longer commonly used in the U.S.; used where labor costs are lower

Reaction Piles

Static Load Procedure (ASTM D1143)

Two categories of tests Controlled stress tests – the

most common method

Controlled strain tests

For driven piles, need

to delay static load

test to allow pile set-up Granular soils – 2 days

Cohesive soils – 30 days

Procedure Set up reaction stand

Apply a test load to the pile

Record the load-settlement

history for each load applied

Apply the next load

Loads are generally applied in

increments of 25, 50, 75, 100,

125, 150, 175 and 200% of

proposed design load

Static Load Procedure (ASTM D1143)

Load test increments in

time Slow test – maintains load until

pile movement is sufficiently

small

Quick test – each load

increment is held for a

predetermined length of time,

for 2.5 – 15 minutes

Generally requires 2-5 hours

to complete

May be best method for

most deep foundations

Reaction Pile Test

Advantages and Disadvantages • Advantages

• Can be installed with same

equipment as production piles

• Test can be done on

inclination (batter)

Disadvantages Reaction piles may pull out

If not done properly, reaction

pile capacity may result

Flexible system stores energy

during tests

STATIC LOAD TEST

Advantages Gives reference

capacity Relatively slow

loading minimises dynamic components

Can customise to include creep effects

Can be instrumented to yield static resistance distribution & end bearing

Disadvantages

Time consuming

Expensive

Done on specially

designated piles

Often done

carelessly or

inaccurately

Static Load Test

Interpretation

At what point of the measured pile top load vs deflection curve do we define

the failure load Ru?

Loading method

and curve interpretation can make significant

differences in result.

Davisson’s Method

• Advantages – Somewhat Conservative

– Matches Dynamic Analysis

Failure Criterion (Quake)

– Relatively Independent of

Judgement (E-Mod. for

concrete, timber; diameter)

• Disadvantage – Capacity/Settlement a

Function of Pile Properties

NAVFAC DM 7.02

p. 7.2-229

Example of Davisson's Method Formulas for round pile; substitute actual

area for other shapes

1.5 Uplift Test

Equivalent Curve from Osterberg Data

Osterberg Test

Advantages and Disadvantages • No reaction load

needed

• Requires jack load only half of test load

• Shaft is loaded upward

rather than in

downward direction

• Tensile vertical strains

near toe will cause

cracking in soil

• Maximum movement is

at the pile toe rather

than pile top

• Only for specially prepared piles

Other Topics in Pile Capacity

Upward Load Capacity Most deep foundations derive

their upward load capacity

from the shaft resistance

Exceptions include belled drilled shafts (shown at left) and bulb piles (Franki)

For driven piles, ultimate

upward load = shaft capacity, but factors of safety/load-resistance factors can be different from compression

Pile Group Effects

Pile Settlement

Settlement Most methods for designing

deep foundations for bearing capacity insure that settlement does not exceed ½" (13 mm)

There are certain situations where it is necessary to know the settlement of a deep foundation

Structures sensitive to settlement

Toe bearing predominates

Downdrag loads are present

Compressible strata are present

Need an equivalent spring for finite element analysis

• In the case of “critical” structures, settlement analysis will be performed using a “t-z” method computer program o Example of one is in the wave

equation analysis routine at vulcanhammer.info

• What we need are quick methods of making preliminary estimates

Vesić’s Method for Settlement

• Steps for Vesić’s method o Compute ultimate shaft and toe

capacities using methods shown earlier o Compute elastic settlement o Load on the shaft is the smaller of two

loads: • the load applied to the pile • the shaft resistance

o Load on the toe is the smaller of two loads: • (The load applied to the pile) –

(the shaft resistance) (= 0 if negative)

• The toe resistance o Settlement is computed by the

equation S = Sf + Ss + Sp • where

o S = total settlement o Sf = elastic settlement o Ss = shaft settlement o Sp = toe settlement

• Vesić’s method can give a single number or a load-settlement curve o Method given as described in EI

02C067, Design of Deep Foundations • Method described in Murthy

15.29 leaves out many steps and confuses shaft and elastic settlement

o Can be used with either drilled shafts or driven piles

• Drilled shafts can be analyzed with load-transfer curve methods o “Old drilled shaft method,” as

presented in book o “New drilled shaft method,”

which we will cover in LRFD

Vesić’s Method for Settlement

• Elastic Settlement Sf = (Qp+α*Qf)L/(A*Ep) o Qp = toe resistance = Q – Qs >

0

o Qf = ultimate shaft resistance (or working load if Q < Qf

o Q = applied or working load

o α = load distribution factor

o 0.5 < α < 0.7, generally assume 0.6

o L = pile length

o A = pile cross-sectional area

o Ep = elastic constant of pile material

• Toe Settlement

o B = pile toe size/diameter

o qpu = unit toe resistance

o Cw = (0.93+0.16*(L/B)1/2)Cs

o Assume zero if Qp < 0

pu

pw

pBq

QCS

Vesić’s Method for Settlement

• Shaft Settlement

Value of Cs shown in

table at the right for both driven and bored

piles

• If Q < Qf, use Q

Soil Driven Piles

Bored Piles

Sand (dense to loose)

0.02-0.04 0.09-0.18

Clay (stiff to soft)

0.02-0.03 0.03-0.06

Silt (dense to loose)

0.03-0.05 0.09-0.12

pu

fs

sLq

QCS

Settlement Example

Applied Load = 50 kips

Key Variables and Solution

Coefficients Use Cs = 0.03

Cw =

(0.93+0.16*(30/1.5)1/2)(0.03) =

0.049

α = 0.6

Solutions Sp = (0.049)(0)/((1.5)(7.529)) = 0

Sf =

(0+(0.6)(50))(30)/((0.191)(43200

00)) = 0.001’ = 0.013”

Ss = (0.03)(50)/((30)(7.529) =

0.007’ = 0.080”

• Results from Dennis and Olson Analysis

• Total Shaft Friction = 103.8 kips

• Total Toe Capacity = 13.3 kips

• Toe Unit Capacity qpu = 7.529 ksf

• Mobilized Shaft Friction Qf = 50 kips < 103.8 kips

• Mobilized Toe Capacity Qp = 0

• Other Key Variables • Pile diameter B = 1.5’

• Pile Length L = 30’

• Pile Cross-Sectional Area A = 0.191 sq. ft.

• Pile Modulus of Elasticity E = 4,320,000 ksf

Solution: S = 0” + 0.013” + 0.080” = 0.093”

Group Effects • Piles are generally used

in groups; drilled shafts are less frequently so

• Group capacity can be less than the sum of the individual capacities of the piles, depending upon a number of factors

• Group settlements can also be driven by different considerations than settlements of single piles

Stress Zones in

Supporting Soils

Basic Relationships in Group Capacity

• Cohesionless Soils o Centre-to-Centre spacing of the

piles/shafts should be > 3d

o Driven Piles

• Group capacity can be greater than the sum of the individual capacities, so Eg = 1

• Pile group should not be underlain by a weak deposit, in which case the settlement of the weak deposit will drive the performance of the group

• Jetting or predrilling should be avoided

o Drilled Shafts

• Use Eg = 2/3 for spacings = 3B; this increases linearly to Eg = 1 for spacing = 6B and is 1 above this

Basic relationship

Qgu

= allowable axial (down or up) capacity of group

Eg = group efficiency factor

ΣQu = allowable axial (down or

up) capacity of single pile

Considerations Pile Spacing

Drilled shafts vs. driven piles

Cohesive vs. cohesionless soils

Individual vs. block failure

u

gu

gQ

QE

Effect of Pile Spacing

Individual vs. Block Failure

Group Capacity for Cohesive Soils

Smallest of four options: Drilled shaft method for

cohesionless soils (always good for drilled shafts)

If undrained shear strength < 2 ksf (95 kpa) and the pile cap is in firm contact with the g round, Eg = 1

If undrained shear strength > 2 ksf (95 kpa), Eg = 1

Use block failure criterion to the left

As always, Centre-to-Centre spacing of the piles/shafts should be > 3d

95Z

15B

15

B+

D+=N*

c

Group Settlements Quick Methods Immediate

settlements – group settlement factor

Long-term consolidation – equivalent mat method

• Cohesionless Soils: Group Settlement Factor o Fg = Sg/S = (Hw/B)½

o Sg = group settlement

o Fg = group settlement factor o Hw = width of pile group o B = pile diameter o S = settlement of single pile

Pile group settlements can be treated in a similar manner to those of shallow foundations

Settlements can be divided into two types

Immediate settlements – those shortly after foundation loading, especially in sands

Consolidation settlements – in clays, same mechanism as with shallow foundations

Ultimately, for more accurate computation of group settlements, computer programs using the t-z methods should be employed

Cohesive Soils: Equivalent Mat Method

Replace group with a mat along the embedded pile length L; this depth is 2/3 of L for friction piles and L for end bearing piles

Distribute the load from the mat to the underlying soil by the 2:1 method

Calculate settlement of soil layers below the mat by one-dimensional consolidation theory; any soil above the mat is assumed to be incompressible

Cohesive Soils:

Equivalent Mat

Method

Group Settlement Example

Compute group settlement factor for sands (use sands for settlement calculations, since they are at the base of the group)

gf = (H

w/B)½ = (22.5/1.5)½ = 3.87

Compute group settlement

δg = (0.093”)(3.87) = 0.36"

This method is to be used with immediate settlements; for long term consolidation, use equivalent mat method with Terzaghi's consolidation theory

Find: Immediate settlement

of 3 x 3 pile group, Hw = 22.5'

Individual settlement at 50 kip load = 0.093"

Questions