The Challenge:
The primary challenge was the misidentification of the slab type. The client assumed the slab was slab-on-grade, but the unit was, in fact, sitting above a parking structure, making it a suspended slab. This misunderstanding posed risks to the structural integrity of the building if trenching were to proceed under the wrong assumptions.
Failing to accurately identify the slab type could result in structural failure, increased project costs, and significant safety risks for the crew on site. Without a proper solution, the client faced potential delays and costly rework.
Scope of Work:
The scope involved performing a comprehensive GPR scan to identify the position of rebar, conduits, and other subsurface features, including an analysis to determine the slab type. The GPR scanning covered several trenches laid out for plumbing and was aimed at ensuring that no critical elements (rebar, electrical conduits, or structural beams) would be compromised during the upcoming excavation work.
Upon recognizing inconsistencies in the radar data, our team immediately flagged the need for further analysis. Xradar was able to interpret the radargram accurately, identifying the slab as suspended rather than on grade. The ability to differentiate between air gaps (suspended slab) and soil beneath the slab (slab-on-grade) allowed us to provide the client with the correct information.
Our team's proactive approach to verifying site conditions ensured that no critical errors occurred, and a further visual inspection confirmed the presence of a lower parking level beneath the unit.
Methodology:
Our team used Ground Penetrating Radar (GPR) technology to scan the slab’s subsurface. We relied on Xradar™, which provides accurate readings of subsurface elements and slab conditions. Key steps included:
- Performing a site walkthrough to visually inspect the surroundings.
- Interpreting the radargram to detect anomalies such as air gaps below the slab, which indicated the presence of a suspended slab.
- Re-assessing data when inconsistencies between the expected slab type and radar signals were detected, followed by a secondary inspection of the area.
Findings/Results:
Our scans revealed that the slab was suspended, with clear indications of an air gap beneath the slab, as opposed to soil or aggregate expected in a slab-on-grade. Additional findings included:
- Two layers of rebar, which is more typical of suspended slabs.
- Electrical conduits were safely marked, preventing any accidental damage.
- The data collected helped confirm the exact slab thickness and structural reinforcements in place.
Conclusion:
By correctly identifying the slab type and providing accurate subsurface mapping, we prevented a potential structural disaster. Our proactive approach to scanning and the use of advanced GPR technology allowed the project to proceed safely, avoiding errors and delays.
Below are a few photos from the scanning company that were included in their job summary report:
