I chatted with a stranger sitting next to me at a concert recently. He told me about a concrete slab on his property that settled 10 cm in a few days after 10 years. The slab supported a small out-building. I don’t know the location of the property.

A vibratory roller was compacting layers of soil during construction of an eight (8) foot deep fill on the next lot. The chap attributed settlement of his concrete slab to the work on the adjacent property.

He filed a claim with his insurance company who had an engineer investigate the problem. I gather from the concert friend’s comments that the engineer’s investigation consisted of walking over the property and visually examining it. The insurance company denied the claim based on the engineer’s report.

I remember thinking at the time that examination of the building wall where it rests on the concrete slab would be helpful. If the slab was constructed level 10 years ago then recent settlement of 10 cm would distort the contact between the wall and the slab in some way and this would be visible.

If the slab was not constructed level years ago then the building wall would be constructed to rest on a sloping concrete slab which would also be evident during a visual examination. For example, the 2 by 4 wall studs would be longer at one end of the wall.

My concert companion also commented to suggest his lot was underlain by soft soils. Construction of a 8 foot deep fill next door suggests soft soils there as well.

Finally, vibratory rollers shake the ground, and not just directly below the roller but also beyond the roller to some extent. That’s how they compact soil and make it stronger and less compressible. Shaking the ground is accompanied by settlement of the soil surface.

So, what would application of the scientific method accomplish/do in this situation? We can go through the steps as referenced in a previous blog (Fig. 1) and listed below in the Appendix.

1. The problem in this situation is confirming that the concrete slab did settle 10 cm recently and, if this is the case, why – the cause?
2. Talk with the owner and learn about construction of the building. Talk with the adjacent lot owner and the contractor and learn about construction and compaction of the fill. Visually observe construction of the site, also the concrete slab and the building resting on it.
3. If observations confirm that the slab has settled recently and a vibratory roller operated next door then the roller causing the slab to settle is a reasonable hypothesis.
4. Survey and measure the layout of the site including the distance of the 8 foot deep fill from the building that settled. Measure construction of the building that settled and confirm the reported settlement. Check surficial geology maps for the type of soil underlying the site. Experiment: Excavate a test pit and confirm the type of soil. Re-enact construction activity at the site by measuring the vibration of the soil at different distances from a walk-behind plate compactor.
5. Collect data: Note the data collected from each of these measurements and experiments.
6. Analyse the data, the results: Note the cause of the building settlement as indicated by each measurement and experiment. Also where different data agree and reinforce cause and where data don’t agree. Identify additional measurements/experiments that could be carried out. For example, measure the vibration at distances from a vibratory roller similar to that used during construction and compaction of the 8 foot fill. Also measure the settlement of monitoring points at different distances from the vibratory roller. Analyse this data and how it agrees/disagrees with other data on cause.
7. Conclusion: Note the probable cause of the building settling 10 cm based on the analysis – the vibratory roller. Also note other possible causes – none – and why these were dismissed.
8. Form opinion: The vibration from the vibratory roller caused the building to settle 10 cm the same as it caused the 8 feet of soil to settle as it was being compacted.
9. Write a detailed report on the measurements and experiments done during the mini scientific investigation, the data collected, analysis of this data, the conclusion drawn and the opinion arrived at. Do this, for example, according to guidelines for experts in Nova Scotia and also the excellent expert report writing manuals available for sale.
10. Peer review: If a cost/benefit analysis justifies, get a peer review of your forensic investigation of whether or not compaction of the 8 foot fill caused the 10 cm settlement of the building. Get it done by a colleague rather than another. If you’re out on a limb because of an error – they sneak in at times – better that you learn from a colleague and get back off the limb, and fix things, rather than a stranger do the peer review and you fall to the ground.

References

1. The science of peer review in forensic investigation. Posted November 30, 2023

Appendix

Steps in the Scientific Method

1. Problem: The problem is determination of the cause of the failure or accident.
2. Observation: Get briefed on what is known about the failure or accident. Read documents. Walk over the site and visually examine where the accident or failure occurred.
3. Hypothesis: Note the possible cause of the incident based on the evidence from the briefing, reading the documents and visually examining the site of the failure or accident.
4. Experiment: Identify investigations suggested by the possible cause. Investigations like a) the layout of the site, b) the size of the site and its components, c) maintenance of the site, and, d) activity at the site when the accident or failure happened. Re-enact the accident.
5. Collect data: Note the data got from each of these investigations.
6. Analyze results: Note the cause of the failure or accident as indicated by each piece of data. Note where there is agreement and disagreement amongst the data on cause. Identify additional investigations that could be carried out. Carry out these investigations and analyze the data.
7. Conclusion: Note the probable cause of the accident or failure based on the analysis. Note other possible causes and the data supporting these, and why these causes were dismissed.
8. Form opinion: Form and state opinion on the cause of the accident or failure.
9. Report: Report in detail what was done during each step in the investigation according to the guidance of civil procedure rules like Rule 55 in Nova Scotia and manuals on expert report writing.
10. Peer review: Review what was done during each step of the investigation and it’s conformance to the standard of practice and what a reasonable person would do.

(Posted by Eric E. Jorden, M.Sc., P.Eng. Consulting Professional Engineer, Forensic Engineer, Geotechnology Ltd., Halifax, Nova Scotia, Canada December 31, 2023 ejorden@eastlink.ca)