Appendix G - GPR Scan Report and Findings_Redacted.pdf

APPENDIX G – GROUND

PENETRATING RADAR (GPR)

SCAN REPORT

U.S. DEPARTMENT OF VETERANS AFFAIRS

AUDIE L. MURPHY MEMORIAL VA HOSPITAL

SAN ANTONIO, TEXAS

Subsurface Investigation to Locate Underground

Utilities Prepared For:

Prepared By:

Sr. Project Manager-San Antonio, Texas

11/12/2019

November 12, 2019

Attn:

Site: 7400 Merton Minter – San Antonio, Texas (VA Hospital Loading Dock)

We appreciate the opportunity to provide this report for our work completed on 11/6/19 at the address above is San Antonio, Texas.

PURPOSE

The purpose of this project was to search for underground utilities within the 5,500 sq. ft. project boundaries provided by the client. The client plans to excavate within the loading dock area prior to building an expansion.

EQUIPMENT

400 MHz GPR Antenna. The antenna is mounted in a stroller frame which rolls over the surface. The surface needs to be reasonably smooth and unobstructed in order to obtain readable scans. Obstructions such as curbs, landscaping, and vegetation will limit the feasibility of GPR. The data is displayed on a screen and marked in the field in real time. GPR works by sending pulses of energy into a material and recording the strength and the time required for the return of the reflected signal. Reflections are produced when the energy pulses enter into a material with different electrical properties from the material it left. The strength of the reflection is determined by the contrast in signal speed between the two materials. The total depth achieved can be as much as 8’ or more with this antenna but can vary widely depending on the conductivity of the materials. Depths provided should always be treated as estimates as their accuracy can be affected by multiple factors. For more information, please visit: Link

Electromagnetic Pipe Locator. The EM locator can detect the electromagnetic fields from live power or radio frequency signals. It can also be used in conjunction with a transmitter to connect directly to accessible, metallic pipes, risers, or tracer wires.

A current is sent through the pipe or tracer wire at a specific frequency and the resulting EM field can then be detected by the receiver. The receiver is moved over the surface without coming in contact with the ground so it is not affected by terrain. Depths provided should always be treated as estimates as their accuracy can be affected by multiple factors. Depths achieved can be as much as 20’ depending on the type of signal being traced or methods used. For more information, please visit: Link

Traceable Rodder. The rodder has a copper wire encased in 5/16” of fiberglass. The line is pushed through accessible pipes before placing an electromagnetic signal on the wire which is then traced from the surface. The maximum traceable depth is 10’ depending on the soil conditions and the maximum distance is 300’. The line can be pushed through a pipe with direct access such as a conduit at a pull box or a storm drain catch basin. We will not be able to push through deeper pipes within manholes. The signal cannot be located through metallic pipes. For more information, please visit: Link

GPS. This handheld GPS unit offers accuracy down to 4 inches, however, the accuracy will depend on the satellite environment and obstructions and should not be considered to be survey-grade. Features can be collected as points, lines, or areas and then exported into Google Earth or overlaid on a CAD drawing. For more information, please visit: Link

PROCESS

Our process began with using the EM pipe locator to locate pipes or utilities throughout the scan area. We first sweep all areas with the receiver to detect live power or radio frequency signals followed by connecting to any visible risers or tracer wires that may be in the area provided that there is an exposed metallic surface. Locations and depths are painted or flagged on the surface. Depths cannot always be provided depending on the location method and can be prone to error.

Initial GPR scans were then collected in order to evaluate the data and calibrate the equipment. Based on these findings, a scanning strategy is formed, typically consisting of scanning the entire area in a grid with 5’-10’ scan spacing in order to locate any potential utilities that were not found with the pipe locator. The GPR data is https://www.gp-radar.com/GSSI-Antennas-Brochure.html https://www.gp-radar.com/RD-8100-Brochure.html https://www.gp-radar.com/Duct-Hunter-Traceable-Rodders.html https://www.gp-radar.com/Trimble-GeoXH-6000-Series-Data-Sheet.html interpreted in real time and anomalies in the data are located and marked on the surface along with their depths using spray paint, pin flags, etc. Depths are dependent on the dielectric of the materials being scanned so depth accuracy can vary throughout a site. Relevant scan examples were saved and will be provided in this report.

LIMITATIONS

Please keep in mind that there are limitations to any subsurface investigation. The equipment may not achieve maximum effectiveness due to soil conditions, above ground obstructions, reinforced concrete, and a variety of other factors. No subsurface investigation or equipment can provide a complete image of what lies below. Our results should always be used in conjunction with as many methods as possible including consulting existing plans and drawings, exploratory excavation or potholing, visual inspection of above ground features, and utilization of services such as One Call/811.

At this site, our scans were limited by equipment lying on the ground in the scanning area. The GPR had limited data due in some areas due to the heavy reinforced concrete.

FINDINGS

We found that the soil allowed for maximum GPR depth penetration of 4’ in most areas. We were able to locate the sewer lines by opening the man-hole covers and inducing a signal with the tracer rodder equipment. We found that all sewer lines were approx. 6’-8’ deep below the surface. All findings were marked directly on the surface with spray paint. The scanning perimeter was marked with an arrow labeling GPRS. We suggest a 2’ buffer from our marks when excavating.

The following pages will provide photos and further explanation of our findings.

7400 Merton Minter Site Map of GPR scanning area an findings

7400 Merton Minter GPR Data Screenshots and Photos

GPR data screenshot of Location #1. The depth scale is on the left and the distance of the scan is across the top, forming a cross section view of the subsurface.

GPR data screenshot from Location #3. The data file above shows no anomalies within the 4’ GPR max depth.

Photo of Location #2. This was a general area scanned with no utilities found.

Photo of Location #3. We opened multiple man-hole covers and discovered the sewer lines were approx. 6-8’ deep.

Photo of Location #1. In this area we located a line running from the salt storage vault to the building at 2’ deep. (Circled in green on the left side GPR data file)

GPR data screenshot from Location #2.

Heavy reinforced concrete with rebar

7400 Merton Minter Additional Site Photos

Photo of Location #4. The photo above shows materials stacked on the ground in the scanning area. We were unable to make good data passes inside that area.

Photo of Locations #8. GPR findings marked in green spray paint of the sewer lines located on site.

Photo of Location #7. We were unable to scan inside the grass area due to piled up scrap metal.

Photo of Location #5. The perimeter of the areas scanned were marked with a white GPRS arrow.

Photo of Location #6. The perimeter of the areas scanned were marked with a white GPRS arrow.

CLOSING

has been in business since 2001, specializing in underground storage tank location, concrete scanning, utility locating, and shallow void detection for projects throughout the United States. I encourage you to visit our website ( and contact any of the numerous references listed.

With the data collected on site, we can conclude the sewer lines were the only utilities found within the scanning area. All lines were collected with the GPS device and will converted to a google earth .kmz and CAD file.

GPRS appreciates the opportunity to offer our services, and we look forward to continuing to work with you on future projects. Please feel free to contact us for additional information or with any questions you may have regarding this report.

Signed, Sr. Project Manager – San Antonio, Texas

1.

2.

3.

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Know what'sbelow.

before you dig.Call

DATE

DRAWING NO. REV. 0

PREPARED FOR:

LOCATION:

GPRS UTILITY

FINDINGS MAP

FOR INFORMATION

ONLY

SUBMITTED VIA EMAIL FOLLOWING COMPLETION OF FIELD WORK.

EQUIPMENT USED, SEE THE "JOB SUMMARY REPORT" THAT WAS

FOR ADDITIONAL INFORMATION REGARDING FINDINGS AND

PRIOR TO ANY AND ALL EXCAVATION ACTIVITIES.

CALL/811 SERVICES. STATE LAW REQUIRES 811 TO BE CALLED

PRIVATE UTILITY LOCATING IS NEVER A REPLACEMENT FOR ONE

OWN. SEE NOTE #4 ABOVE.

THE SERVICE IS USED ON EVERY PROJECT IN ADDITION TO OUR

GPRS IS NOT AFFILIATED WITH 811 BUT DOES RECOMMEND THAT

PROJECT MANAGER:

0 20'10'5'

D= DEPTH

https://m.gp-radar.com/terms-conditions

CONDITIONS THAT CAN BE VIEWED WITH THE LINK BELOW:

SITE. ALL FINDINGS ARE SUBJECT TO THE TERMS AND

TO LOCATE ALL UTILITIES OR OTHER SUBSURFACE FEATURES ON

SYSTEMS (GPRS). GPRS CANNOT GUARANTEE IT WILL BE ABLE

COMMENTS COLLECTED BY GROUND PENETRATING RADAR

THIS DRAWING IS INTENDED TO DOCUMENT MARKINGS AND

SURVEYING, OR ENGINEERING SERVICES.

GPRS DOES NOT PROVIDE GEOPHYSICAL, GEOLOGICAL, LAND

STORM MH

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11/13/2019

O'BRIEN ENGINEERING

VA HOSPITAL

7400 MERTON MINTER

SAN ANTONIA, TX 78229

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DOUG.BEVANDRAWN BY:

SCAN AREA

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LEGEND

TERMS AND CONDITIONS

D=4'+

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D=2'

D=2'

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