Ground Penetrating Radar (GPR) By Dr. Eng. Zubair Ahmed

Transcription

1 Ground Penetrating Radar (GPR) By Dr. Eng. Zubair Ahmed

2 Acknowledgement Golder Associates, Whitby, Ontario Stantec Consulting, Kitchener, Ontario Infrasense Inc. USA Geophysical Survey Systems Inc. (GSSI), USA Gulf Engineering House, Riyadh

3 Presentation Outline GPR Overview GPR Principles GPR Equipment Data Collection and Processing Technology Evaluation Pavement Applications Q/A

4 GPR - Overview Non-destructive inspection of the Earth and its Infrastructure Commercialized use since early 1970 s Average radiated power is approximately 1% of cell phone transmitted power Safe to use Has many practical applications From 0.1 m to 300 m depths

5 GPR - Overview Pavement layer identification Concrete inspection (locate rebar, tension cables, etc) Utility detection (pipes, tanks, etc) Bridge deck condition assessment Rail-bed condition assessment Geology, mining Archeology Crime Scene Investigations (CSI) and Forensics Military/Espionage

6 GPR - Overview All GPR systems sold in the U.S. after July 15, 2002 must comply with FCC regulations. The radar collection unit is limited to a transmission rate of 100 KHz (U.S. versions only). Using 512 samples per scan, this is about 153 scans per second. Fortunately, for Pavement applications, this is not a problem. With 153 scans per second, 512 samples per scan, you can collect 3 scans/meter at approximately 110 KPH. The antenna must pass an FCC emissions test.

7 GPR. - Principles GPR uses high frequency electromagnetic pulses to image features within the subsurface Direct Coupling Antenna Reflections are generated from material interfaces in the subsurface Each material has different electrical conductivity Ground Coupling Layer Interface Layer Interface Layer Interface

8 GPR - Principles Electrical Conductivity The ability of a material to conduct an electric current Higher conductivities make GPR signal penetration difficult Use published sources or take readings from field Good Conductivity Air, Concrete, Asphalt Medium Conductivity Ice, snow, sand, silt, dry clay Poor Conductivity Wet Clay, Wet Shale, Seawater

9 GPR - Principles GPR sees changes in electrical conduction properties of a material It does not read changes in colour, temperature, or density Best at seeing sharp contrasts Metal targets stop 100% of signal

10 GPR - Principles Dielectric Constant Unit-less measure of a material s ability to hold and pass a charge Values range from 1 (air) to 81 (water) Lower the number, higher the velocity Referenced published values Asphalt Concrete values range from 3 to 5 PCC values range from 6 to 11 Cement Treated Base = 2 to 5

11 GPR - Principles We record the two way travel time and the amplitude of the reflection

12 GPR - Principles

13 Travel Time GPR - Principles 1. GPR Transmitter Transmits Electromagnetic Pulse into Ground Tx Rx Distance Asphalt Concrete

14 Travel Time GPR - Principles 2. Electromagnetic Energy Travels Down Into The Ground Tx Rx Distance Asphalt Concrete

15 Travel Time GPR - Principles 3. Reflections From Layer Interfaces are Reflected Back to Surface and Measured by the GPR Receiver Tx Rx Distance Asphalt Concrete

16 Travel Time GPR - Principles 4. GPR Transmitter/Receiver Are Moved and Testing is Repeated Tx Rx Distance Asphalt Concrete

17 Travel Time GPR - Principles 5. Testing is Completed over Entire Area To Be Tested Distance Pavement Surface Base of Asphalt Base of Concrete

18 GPR - Equipment System includes: GPR data collection system Air launched horn antenna (1 GHz or 2 GHz) or Ground Coupled antenna (200 MHz to 1 GHz) Antenna mounting hardware for vehicle Antenna cable Wheel mounted Distance Measuring Instrument (DMI) Data Processing software Road structure assessment module

19 Distance Measuring Instrument (DMI) GPR - Equipment Air Launched

20 GPR - Equipment Horn Antenna Mounting Adjustable straps Transducer cable fastened to rail 40 (1 meter) (46-51 cm) Antenna level

21 Trimble DGPS Receiver on Roof GPR - Equipment Ground Coupled 1000 MHz Radar System 500 MHz Radar System

22 Display Units to See Results in Real Time GPR - Equipment Real Time Data Acquisition

23 GPR - Data Collection Antenna Calibration An antenna calibration file is collected using a metal plate as a perfect reflector

24 GPR - Data Collection Raw Data

25 GPR - Data Collection Examples of Various Scan Rates Example at 10 Scans/m 5 Scans/m 2.5 1

26 GPR - Data Collection Field Guidelines Record any features that may be located within 2 m of the antenna during data collection. For example, note if any cars pass near the antenna. Do not use Cell phone while collecting GPR data. Interference Should have someone taking notes and writing down key observations Milepost signs Cracks Culverts, pipes Guard rails located near the antenna could also cause problems, especially if the antenna is mounted on the passenger side of the vehicle. The reflection from the rails may erroneously be interpreted as a layer interface

27 GPR - Data Processing Ground Truth Use core data to enter ground truth data

28 Time (ns) Depth (m, v=0.110m/ns) GPR - Processing C:\QEW Niagara GPR\Final Data\L7_1000 Human Interpretation Asphalt Concrete Position (m)

29 GPR - Data Processing Interpretation Software

30 GPR - Data Processing Interpretation Software

31 GPR - Data Processing Result The.LAY file is a generic comma delimited ASCII file that is easily imported into Excel and a variety of other software programs

32 GPR - Visual Image Flexible Pavement Bottom of asphalt layer Bottom of aggregate layer I-676 N at Milepost 0.39

33 GPR - Visual Image Rigid Pavement Bottom of concrete layer Bottom of aggregate layer

34 GPR - Visual Image Composite Pavement Bottom of asphalt layer Bottom of concrete layer Bottom of aggregate layer

35 GPR Example Used 2 Ground Coupled Systems: 1000 MHz and 500 MHz Data Collected at 0.1 m Increments 2 Passes Per Lane 30 km/h Acquisition Speed Location using Wheel Odometer Tied to Chainage Markers and GPS Total GPR Lane Distance = 125,000 m Total Survey Time = 9 hours 1.5 m 1.5 m

36 Time (ns) Depth (m, v=0.110m/ns) C:\QEW Niagara GPR\Final GPR Data\Edit1\L6_ Example Asphalt Concrete Dowel Pins Asphalt/Concrete Pavement to ~ Position (m) Westbound Median Lane Right Side

37 Time (ns) Depth (m, v=0.11m/ns) GPR - Example C:\QEW Niagara GPR\Raw Data\1000 MHz\Position\LINE1.dt Pavement Surface Asphalt Repair Of Failed Concrete Slab Asphalt Concrete ~ 80 x Vertical Exaggeration! Position (m)

38 GPR Technology Evaluation Direct coupling Ground coupling Metal strips buried at sub-base and sub-grade interfaces

39 GPR Technology Evaluation

40 GPR Technology Evaluation I -195 SPS 5 Section GPR AS-BUILT Section From MP To MP Surface Base Subbase Surface Base Subbase

41 GPR Technology Evaluation Average % Difference in Layer Thickness 75th Percentile % Difference in Layer Thickness Pavement Type AC PCC Aggregate AC PCC Aggregate Flexible Rigid Composite (flexible over rigid)

42 GPR Technology Evaluation 1 GHz Horn Antenna 2 GHz Horn Antenna

43 GPR Technology Evaluation Wearing Course 2 GHz Data Pavement Bottom 1 GHz Data

44 GPR Technology Evaluation 4.7 cm Wearing Course Thickness GPR Calculated Wearing Course Thickness = 4.8 cm

45 Pavement Applications

46 Pavement Conventional Investigation Methods Boreholes Visual Survey FWD Cores

47 Time (ns) Depth (m, v=0.110m/ns) Pavement Integration of Visual Survey with GPR Possible Full Depth Asphalt Repair Identified and Interpreted Based on GPR and Visual Survey Results 0.0 C:\Unsorted Data\QEW Niagara GPR\Final Data\Edit1\L1_ Position (m) 1.1

48 Pavement Integration of Borehole Data with GPR

49 Accurately Identify Pavement Changes Pavement Integration of FWD Results with GPR

50 Depth of Pavement (in) Normalized Maximum Deflection (mils) Pavement Integration of FWD and Core with GPR 0 Comparison of GPR, Cores, and Deflection Data on I-287 North Legend AC - GPR PCC - GPR Base - GPR AC - Core PCC - Core Base - Core Normalized Deflection Mile Post 2 0

51 Pavement Management System As-Built, Core/Bore, GPR, FWD, and GPS Databases Airport PMS Photo Courtesy Stantec Consulting

52 PMS Inventory Integration

53 PMS Life Cycle Cost Analysis Additional Cost for Overlay Thickness (in) Overdesigned Section Difference Sections (per 2-lane Number Non-GPR GPR (in) Status mile) Overdesign $316, Underdesign Underdesign Underdesign Underdesign Underdesign Underdesign Underdesign Underdesign Underdesign Underdesign Underdesign Underdesign Underdesign Overdesign $105, Overdesign $105,600

54 Assuming three-mile project sections PMS Life Cycle Cost Analysis Additional cost for the three over-designed sections is about $1.6 million per 2-lane mile PWV of the additional cost for an under-designed section is around $85,000 per lane mile Additional cost for the thirteen (13) underdesigned sections is about $ 6.6 million per 2-lane mile In addition, a significant user delay cost is expected for the under-designed sections

55 GPR Conclusions A non-destructive technology Continuous Profiles obtained compared to point information provided by cores, bores and sawcuts Reduces the amount of intrusive work needed, while raising the level of confidence in structural assessments Minimized traffic control costs and disruptions to traffic flow in comparison to intrusive methods Integration of GPR with visual, destructive and non-destructive methods present a complete picture of the pavement condition and help in determining the most appropriate rehabilitation strategy for each section within the project limits

56 GPR Conclusions GPR can be used as a supplementary tool to ground truths for acquisition and development of database for network and project level PMS applications Incorporation of GPR data minimizes the gaps and deficiencies in the PMS database and enhances the PMS through improved and effective M&R and optimization analyses Overall, GPR is a useful tool for pavement engineers, providing them the information they need to make improved and sound engineering decisions

57 Thank You. Questions??