Why Do We CAPWAP?

Pile Dynamics, Inc. Representative Days

September 27 – 29, 2017

• Incomplete Resistance Activationsmall set per blow< 1/10 inch; 2.5 mm

• What to do?Bigger hammer;higher energy hammer

• Caution Watch stresses!

• No point exceeding 6 to 8 mm set per blow( 1/4 to 1/3 inch per blow )

CAPWAP Limitations

Underprediction

• Temporary Loss of Capacity during driving

• Increased pore water pressure?

• Arching?

• Other?

• What to do?1. Restrike after sufficiently long wait2. Use early, high energy blow!3. Use Radiation Damping model

(if low set/blow, if drilled shaft)4. Use superposition of EOD and BOR

(only if BOR set per blow small < 2 mm)

Underprediction

Loss of Setup – Increasing Energy

Energy

Capacity

Incomplete Activation

Reduced Capacity

Blow Number

Underprediction

Get bigger hammer?

Are stresses OK?

Apply full stroke earlier?

“Superposition” to avoid underprediction

• Shaft (BOR) + End_bearing (EOD or EOR)

• Only used at restrike refusal blow counts

( very small set/blow )

“The Use of Superposition for Evaluating Pile

Capacity”

Mohamad H. Hussein – GRL

Michael R. Sharp – URS

William “Bubba” Knight - PSI (FDOT District Geotechnical Engineer during the project work)

30 inch concrete

pile

Length 65 ft

Blow

Count,

Blows/Set

Max. Trans

Energy

kN-m

Static Capacity, kN

Skin

Friction

End

Bearing

Total

Capacity

End of Driving 130 /

300mm

65 890 3960 4850

Static Load Test 3217 4123 7340

Restrike 10 / 2.5 mm 38 3315 1025 4340

Superposition 3315 3960 7275

Summary of PDA/CAPWAP and Static Load Test Results

Hussein, M., Sharp, M., Knight, W., “The Use of Superposition for

Evaluating Pile Capacity”, Deep Foundations 2002, An International

Perspective on Theory, Design, Construction, and Performance,

Geotechnical Special Publication No. 116, American Society of Civil

Engineers: Orlando, February, 2002

-45

-40

-35

-30

-25

-20

-15

-10

-5

0

0 1000 2000 3000 4000 5000 6000 7000 8000 9000

SETT

LEM

ENT

(mm

)

LOAD (kN)

STATIC LOAD TEST CAPWAP

Comparison of Static Load Test and CAPWAP results

-1.77 inch

7000 kN = 1560 kips

610 mm PSC, Pier A

0

1000

2000

3000

4000

5000

6000

0 10 20 30 40 50

Displacement (mm)

Lo

ad

(k

N)

Top

Toe

Underprediction

• Loss of Setup – Increasing Energy

• Analyze several blows

• Superposition resistance envelope( only if refusal blow count )

Overprediction ?

Relaxation

1. Weathered Shales

2. Negative Porewater Pressure (saturated silts)

3. Heave

Overprediction?

• Relaxation• Weathered Shales

• Negative Porewater Pressure (saturated dense silts)

• Heave

• Solution: restrike after wait time (look at high energy early blow)

HP14x117, to shale

EOD

BOR

Does CAPWAP have aUnique Solution?

Basic

CAPWAP®

Model

All get similar answers

Get many different answers from different interpretations of “failure”

Static Load Test does not give a unique result !

0

50

100

150

200

250

0.0 0.5 1.0 1.5 2.0

Load

(to

ns)

Movement (inch)

Chin 1.30

Brinch-Hansen 80% 1.17

Mazurkiewicz 1.15

Van Der Veen 1.13

Fuller and Hoy 1.12

De Beer 1.03

Butler and Hoy 1.02

Davisson 1.00

D/20 0.90

D/30 + elastic 1.04

D/10 1.10

After Fellenius, 1990.“Guidelines for the Interpretation of static loading tests”

Failure criteria – no unique answer

Static Test evaluation- Not Unique !

• Duzceer & Saglamar, “Evaluation of Pile Load Test Results”

• DFI Conference - Nice France, 2002

Static Load Test

Evaluations

0 0.2 0.4 0.6 0.8 1 1.2

Davisson

Brinch Hansen 80%

Chin-Kondner

Shen-Nu

DeBeer

Housel

Mazurkiewicz

Fuller-Hoy

Brinch Hansen 90%

Corps of Engr.

Butler-Hoy

Tangent

Q/Qavg

Range 0.73 (DeBeer) to 1.42 (Chin)

COV 18.4%

Guidelines for correlation of PDA with static testing

• must activate all resistance in dynamic test(minimum 2 to 3 mm set per blow)

• allow strength changes to occur (restrike test)(set-up increase on shaft, relaxation at toe)consider 3 dates: install, static and dynamic tests

• must have high quality static test(good measurements, test to failure)

• if either test not to failure, gives lower bound solution

Test Comparisons - cohesive soils

cap

acit

y

1 d

ay

10 d

ays

100 d

ays

1000 d

ays

log time

PDA Test @ 12 hours

Static test @ 14 days

2nd PDA test

@ 15 days

24-inch PSC+H Silty and Calcareous Sand

CAPWAP versus Static

Load Test

0

10,000

20,000

30,000

40,000

0 10,000 20,000 30,000 40,000

CA

PW

AP

[kN

]

Static Load Test [kN]

Unconservative(potentially unsafe)

Conservative (residual strength)

0

500

1000

1500

2000

2500

3000

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Long Pipe in Sensitive Clay

Series1 Average StdDev+ StdDev-

“Variation of CAPWAP Results as a Function of the Operator”

• by Fellenius, Third Int’l Conf. on Application of Stress-wave Theory to Piles, 1988

0

200

400

600

800

1000

1200

1400

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Pipe in SIlt

Capacity Estimate Average StdDev+ StdDev-

0

100

200

300

400

500

600

700

800

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

H-Pile in Sand

Capacity Estimate Average StdDev+ StdDev-

0

200

400

600

800

1000

1200

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Pipe in Weak Sandstone

Capacity Estimate Average StdDev+ StdDev-

Discuss in more detail

Question 1: is CAPWAP result unique?Question 2: is any variation significant?

Answer: results of experienced testers have little variation

0

500

1000

1500

2000

2500

3000

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Long Pipe in Sensitive Clay

Series1 Average StdDev+ StdDev-

COV13%

0

200

400

600

800

1000

1200

1400

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Pipe in SIlt

Capacity Estimate Average StdDev+ StdDev-

0

100

200

300

400

500

600

700

800

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

H-Pile in Sand

Capacity Estimate Average StdDev+ StdDev-

0

200

400

600

800

1000

1200

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Pipe in Weak Sandstone

Capacity Estimate Average StdDev+ StdDev-

COV14%

COV6%

COV5% “Variation of

CAPWAP Results as a Function of the Operator”

• by Fellenius, Third Int’l Conf. on Application of Stress-wave Theory to Piles, 1988

0

500

1000

1500

2000

B19 B21 B23 - B3 B12 B28 B30 B22 B23 B24 B25 B3 B6 B11 B15 B20

Ult

imat

e R

esis

tan

ce (

kN)

High Strain Dynamic Test Results (referenced from Table 20)

Rs (kN) Rb (kN) Case (kN) SLT SLT +20% SLT -20% avg DLT w/CW1 5432 6

International Prediction Event on the Behaviour of Bored, CFA and Driven Piles in CEFEUP/ISC’2 experimental site – 2003

SLT

+20%

-20%

A. Viana da Fonseca, President of ISC’2; Jaime Santos, Coordinator of the experimental site

0

500

1000

1500

2000

B19 B21 B23 - B3 B12 B28 B30 B22 B23 B24 B25 B3 B6 B11 B15 B20

Ult

imat

e R

esis

tan

ce (

kN)

High Strain Dynamic Test Results (referenced from Table 20)

Rs (kN) Rb (kN) Case (kN) SLT SLT +20% SLT -20% avg DLT w/CW1 5432 6

International Prediction Event on the Behaviour of Bored, CFA and Driven Piles in CEFEUP/ISC’2 experimental site – 2003

SLT

+20%

-20%

A. Viana da Fonseca, President of ISC’2; Jaime Santos, Coordinator of the experimental site

Unique solution?• Yes - if you pursue to get the absolute minimum error in

Match Quality.

But does it really matter?• No – since solutions are generally all similar (within

reasonable tolerance) and different blows may give slightly different answers

• Static test interpretation also is not unique!

All get similar answers

Get many different answers from different interpretations of “failure”

CAPWAP might be more unique than Static Load Test interpretation!

Dynamic Load Test result (CAPWAP) is generally conservative

• CAPWAP on average less than Davisson

• Davisson generally rather conservative

• Continued set-up on most piles after the DLT

• Group effects – densification during production

• Most piles driven harder than criteria

• DLT often used with slightly higher S.F.

• Better site coverage by more DLT

• Possibly less risk with DLT than with SLT

Parting Comments and Advice….

• Many codes require “signal matching” (e.g. AASHTO)

• CAPWAP is only signal matching software that has been compared with an extensive correlation database

• Failure to use CAPWAP is contrary to “state-of-practice”

• Do whatever it takes to become proficient and thereby obtain the best possible results and reduce legal exposure

• Seek second opinions for difficult cases