Pavement Deflection Measurements Using the Geobeam

Steven Anderson

Auckland Regional Manager

Geotechnics Ltd.

The Geobeam

• The Geobeam is an

instrumented

Benkelman Beam

• Developed in the late

1980’s by Tonkin &

Taylor

• Measures deflection

bowls

• Slow static test

The Geobeam

• Cantilevered and an inductive

transducer - eliminate friction affects

Transducer

Geobeam

Upgraded Geobeam

• New wireless technology

Before 2005 After 2007

• Fifth wheel with rotary encoder

• Measures a deflection for every ~10mm of

travel

Upgraded Geobeam

Fifth Wheel

Distance Measurement

Field Program - Data Capture

• Data capture shows real time deflection bowls

Capture Software

Geobeam Report Processor

Geobeam Report

Verbose Raw Data

Smoothed Data

Geobeam Report

2.694m 1.410m

4.104m

dbowl

d

Deflection, d0

1. Without Bowl Correction

(Where (s-i) and (s-f) differ by less than 0.03mm)

d0 = ( s-f )

s = start reading at maximum deflection 0m (mm)

i = intermediate reading at 2.694m (mm)

f = final reading at ≥10m (mm)

2. With Bowl Correction

(Where (s-i) and (s-f) differ by more than 0.03mm)

d0 = (s-f) + 4.104 ( i-f ) = (s-f) + 2.91( i-f )

1.410

NOTE – Assumes rear foot is not in the bowl

s i f

d0

Calculation of Pavement Deflection, d0

Calculation of Bowl Radius and Curvature Function

Load

P

Distance, LL = 0.2m

Curvature Function, CF

CF = d0-d200

Bowl Radius, R

R2 = {R-(d0– dL)}2+L2

R = L 2

2(d0-dL)

R = L2

2δ

When L = 0.2m

R = 0.04

2δ

d0 dL

R

L

δ

R

Calibration of Equipment

Calibration Average Data

-6.00000

-5.00000

-4.00000

-3.00000

-2.00000

-1.00000

0.00000

1.00000

2.00000

3.00000

4.00000

0.00 2.00 4.00 6.00 8.00 10.00

Micrometer (mm)

Defl

ecti

on

Un

it (

V)

60mm D 6mm T Puck Offset 10mm

60mm D 6mm T Puck Offset Nil

60mm D 6mm T Perspex Offset Nil

60mm D 6mm T None Offset Nil

40mm D 10mm T Puck Offset 20mm

40mm D 10mm T Puck Offset 10mm

40mm D 10mm T Puck Offset Nil

40mm D 6mm T Perspex Offset 20mm

40mm D 6mm T Perspex Offset Nil

40mm D 6mm T None Offset Nil

Correlation with Benkleman Beam

Geobeam vs Benkelman

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190

Chainage

De

fle

cti

on

mm

Geobeam

Benkelman Beam

More Information

More data means more information

Excellent deflection bowls

Real time bowls indicate test problems

Accurate determination of curvature function

Where to from here?

More development

• More field work to bullet proof the electronic

system

• More validation, correlation and calibration

Interesting projects

• Consolidation effect in pavements

• Measuring subgrade and basecourse bowls

at the same time