5/5/2017

1

Mike Batten, PEBeth Howey, LG, PE

Verify Post-Scour Resistance of Jetted and Driven Pile Foundations

April 11, 2017

Outer Banks of NC

5/5/2017

2

Existing Bonner Bridge

36” Pre-stressed Concrete PilesSubsurface Profile of Navigation Unit

(-40 ft, CSE)

(-84 ft, DSE)

(-62 ft)

5/5/2017

3

NAVIGATION UNIT PILES

AASHTO 10.7.3.3 states that scour resistance should not be factored soscour = 1.0

resistance should be in accordance with AASHTO or NCDOT policy

Required Nominal Driving Resistance (Rndr)

5/5/2017

4

Theoretical and Measured Skin Friction

0

10

20

30

40

50

60

70

80

90

100

0 1 2 3 4 5 6 7 8 9 10

Dep

th (ft)

Unit Skin Friction (ksf)

FHWA: No Upper Bounds

FHWA: With Upper Bounds

Field Measured

Rndr = 4,198 kips

Rndr = 2,473 kips

Rndr = 1,963 kips

Overview of Rndr Calculation Including Scour

-120

-100

-80

-60

-40

-20

0

0 200 400 600 800 1000

Elev

atio

n (ft

)

Skin Friction (kips)

Mud Line Elevation -33

Q Skin-Installation

5/5/2017

5

-120

-100

-80

-60

-40

-20

0

0 200 400 600 800 1000

Elev

atio

n (ft

)

Skin Friction (kips)

Mud Line Elevation -33

CSE -40

DSE -84

Overview of Rndr Calculation Including Scour

Q Skin-Installation

-120

-100

-80

-60

-40

-20

0

0 200 400 600 800 1000

Elev

atio

n (ft

)

Skin Friction (kips)

Mud Line Elevation -33

CSE -40

DSE -84

Overview of Rndr Calculation Including Scour

Q Skin-InstallationQ Skin-Long Term

5/5/2017

6

-120

-100

-80

-60

-40

-20

0

0 200 400 600 800 1000

Elev

atio

n (ft

)

Skin Friction (kips)

Mud Line Elevation -33

CSE -40

DSE -84

USR

Overview of Rndr Calculation Including Scour

Q Skin-InstallationQ Skin-Long Term

Reference NHI Course Nos. 132021 and 132022 Design and Construction of Driven Pile Foundations

Effective Stress Method to Determine Beta

Where:

Fs = Unit skin friction = Bjerrum Burland beta coefficient = Kstan’v = Average effective stress along the pile shaftKs = Earth stress coefficient = Interface friction angle between pile and soil

5/5/2017

7

is primarily a function of:oRelative density of soil

oPile soil/interface material

increases with densification from driving in pile groups

is NOT a function of effective or total stress!

Characteristics of Beta

Reference NHI Course Nos. 132021 and 132022 Design and Construction of Driven Pile Foundations

Range of Beta Coefficients

Soil Type ’ Clay 25 – 30 0.23 – 0.40Silt 28 – 34 0.27 – 0.50

Sand 32 – 40 0.30 – 0.60Gravel 35 – 45 0.35 – 0.80

Table 9-6 Approximate Range of Beta Coefficients(Fellenius, 1991)

5/5/2017

8

Calculate effective overburden stress at the midpoint of installed pile between the Design Scour Elevation (DSE) and the Pile Tip Elevation (PTE) using ground surface at the time of pile installation.

From CAPWAP data, determine the average measured unit skin friction for this zone.

qs = β x σ’v qskin-installation = β x σ’v-installation

Method to Determine Beta

Effective Stress at Time of Installation

-84’ (DSE)

-33’ (Existing Mud line)

-120’ (PTE)

’v-Installation = 69 ft x 57.6 pcf = 3,974 psf

69 ft to midpoint b/w DSE and PTE

5/5/2017

9

For this Pile, calculated Beta was 0.394CAPWAP Output

Theoretical and Measured Skin Friction

0

10

20

30

40

50

60

70

80

90

100

0 1 2 3 4 5 6 7 8 9 10

Dep

th (ft)

Unit Skin Friction (ksf)

FHWA: No Upper Bounds

FHWA: With Upper Bounds

Field Measured

Rndr = 4,198 kips

Rndr = 2,473 kips

Rndr = 1,963 kips

5/5/2017

10

JETTING 36” SQUARE PILE

Reference NHI Course No. 132014 Drilled Shafts: Construction Procedures and LRFD Design Methods

Modeled Long Term Scour using FHWA Drilled Shaft Methodology

-40’ (CSE)

-84’ (DSE)

-33’ (Existing Mud line)

PTE

Scour Prism

ys

Pile embedment after scour

Long Term Scour

C

B

A

Local scour

5/5/2017

11

Reference NHI Course No. 132014 Drilled Shafts: Construction Procedures and LRFD Design Methods

Modeled Long Term Scour using FHWA Drilled Shaft Methodology

-40’ (CSE)

-84’ (DSE)

-33’ (Existing Mud line)

PTE

Scour Prism

ys

W = 2ys

Pile embedment after scour

Long Term Scour

C

B

A

Local scour21

Reference NHI Course No. 132014 Drilled Shafts: Construction Procedures and LRFD Design Methods

Modeled Long Term Scour using FHWA Drilled Shaft Methodology

-40’ (CSE)

-84’ (DSE)

-33’ (Existing Mud line)

PTE

Scour Prism

ys

W = 2ys

Pile embedment after scour

Long Term Scour

C

B

A

Local scour

Vary stress from zero at C to stress controlled by streambed elevation B at depth = 1.5ys

1.5ys

Below this depth, compute stresses based on mud line elevation B

21

5/5/2017

12

‐160

‐140

‐120

‐100

‐80

‐60

‐40

‐20

0

0 1000 2000 3000 4000 5000 6000 7000

Elevation (ft)

Effective Stress (psf)

Reference NHI Course No. 132014 Drilled Shafts: Construction Procedures and LRFD Design Methods

Modeled Long Term Scour using FHWA Drilled Shaft Methodology

Mud Line Elevation -33

CSE -40

DSE -84

PTE -120

1.5ys = -150

No Local Scour

‐160

‐140

‐120

‐100

‐80

‐60

‐40

‐20

0

0 1000 2000 3000 4000 5000 6000 7000

Elevation (ft)

Effective Stress (psf)

Reference NHI Course No. 132014 Drilled Shafts: Construction Procedures and LRFD Design Methods

Modeled Long Term Scour using FHWA Drilled Shaft Methodology

Mud Line Elevation -33

CSE -40

DSE -84

PTE -120

1.5ys = -150 Local Scour

No Local Scour21

v= -2,246 psf

5/5/2017

13

Calculate the effective overburden stress after scour with procedure on previous slide with 2:1 scour hole from CSE to DSE.

Using calculated during installation, calculate long term skin friction.

qs = β x σ’v qskin-long term = β x σ’v-scour

is CONSTANT – it does not change after the scour event, only effective stress changes.

Calculate Long Term Skin Resistance

Unfactored Scour Resistance (USR) is the difference between long term post-scour skin friction and measured skin friction during pile installation.

USR = Qskin-installation - qskin-long term x A

Where:

qskin-long term = Unit skin friction after scour event

A = Surface area of pile between DSE and PTE

Qskin-installation = Skin friction resistance from CAPWAP

Calculate Unfactored Scour Resistance

5/5/2017

14

DRIVING 36” SQUARE PILE

Rndr and Measured Skin Friction

1,500

1,700

1,900

2,100

2,300

2,500

2,700

2,900

0 500 1000 1500 2000

Rnd

r(k

ips)

Skin Friction from CAPWAP (kips)

Navigation Unit Bents

EOIDRestrike

5/5/2017

15

Rndr and Measured % Skin Friction

1,500

1,700

1,900

2,100

2,300

2,500

2,700

2,900

0% 25% 50% 75% 100%

Rnd

r(k

ips)

% Skin Friction of Total Resistance from CAPWAP

Navigation Unit Bents

EOIDRestrike

Measured Skin Friction and

0

200

400

600

800

1,000

1,200

1,400

1,600

1,800

0.0 0.2 0.4 0.6 0.8

Ski

n F

rictio

n fr

om C

AP

WA

P (

kips

)

Average Measured below DSE

Navigation Unit Bents

EOIDRestrike

5/5/2017

16

Measured Skin Friction and

0

200

400

600

800

1,000

1,200

1,400

1,600

1,800

0.0 0.2 0.4 0.6 0.8

Ski

n F

rictio

n fr

om C

AP

WA

P (

kips

)

Average Measured below DSE

Navigation Unit Bents

EOIDRestrike

0.3 < < 0.6Per FHWA

Rndr and

1,500

1,700

1,900

2,100

2,300

2,500

2,700

2,900

0.0 0.2 0.4 0.6 0.8 1.0

Rnd

r(k

ips)

Average Measured Below DSE

Navigation Unit Bents

EOIDRestrike

5/5/2017

17

Rndr and USR

1,500

1,700

1,900

2,100

2,300

2,500

2,700

2,900

0 500 1,000 1,500

Rnd

r(k

ips)

Unfactored Scour Resistance (kips)

Navigation Unit Bents

EOIDRestrike

USR and Measured Skin Friction

0

200

400

600

800

1,000

1,200

1,400

0 500 1,000 1,500 2,000

US

R (

kips

)

Skin Friction from CAPWAP (kips)

Navigation Unit Bents

EOIDRestrike

5/5/2017

18

Rndr and Total Measured Resistance

1,000

1,500

2,000

2,500

3,000

1,000 1,500 2,000 2,500 3,000

Rnd

r(k

ips)

Total Measured Resistance (kips)

Navigation Unit Bents

EOIDRestrike

Rndr and Total Measured Resistance

1,000

1,500

2,000

2,500

3,000

1,000 1,500 2,000 2,500 3,000

Rnd

r(k

ips)

Total Measured Resistance (kips)

Navigation Unit Bents

EOIDRestrike

D/C > 1, not OK

D/C < 1, OK

5/5/2017

19

QUESTIONS?