(This is not an East Coast ghost story)
(The following is one in a series of cases I have investigated that illustrate the different forensic engineering methods I use to investigate the cause of failures and accidents that result in civil litigation. Knowledge of simple frost heave was important in this case)
The investigation of the vibrating building is reported under the following main headings with several sub-headings:
- The case (A description of: 1. The building and the problem experienced by the owner; 2. The building’s foundations, and the problems with the building, 3. The legal/technical issues, and, 4. My client)
- Forensic engineering investigation of the problem and the methods used
- Findings of the investigation (conclusions with respect to the technical issues)
- Lessons learned
Description of the building and the problem
The building was a large, well appointed mobile home in the Halifax area that vibrated quite noticeably during the winter months. The vibration occurred when the owner and his family walked the length of their home from one room to another.
The owner also wanted to know why the interior partitions at some locations were separating from the ceiling.
The main issues were the cause of the vibration and the cause of the gaps at the tops of the partitions.
I was retained to investigate the problem by the company who placed the mobile home on the site.
Forensic engineering investigation
My forensic engineering investigation involved the following methods:
- Take a briefing on the problem from the owner.
- Visually examine the building and the site it was on.
- Examine and determine how the building was supported and the foundations constructed.
- Sample and determine the type of foundation soils underlying the building site and their physical properties.
- Analyse the data collected during these examinations.
Investigations and Findings
Briefing The owner was quite clear in describing how the building vibrated in winter in walking from one end of his home to the other. He also described the gaps at the top of the partitions. The building did not vibrate during the summer.
I wasn’t on site during the winter but saw and measured gaps of about 1/4 to 1/2 inches during my visit.
Visual examination: The home was on a sloping site with the length of the building aligned up the slope.
Examine foundations: I crawled under the building and established that the mobile home was supported on two continuous steel beams running the length of the mobile home. The beams were in turn supported by concrete block piers at regular intervals. The piers were supported on the sloping ground a few inches below the surface.
Because of the sloping ground, the height of the piers and the home above the ground gradually increased from 1.5 feet at the upslope end to 3.5 feet at the downslope end.
Test foundation soils: I took samples of the soils supporting the piers and had the samples tested in a laboratory. I also researched the soil geology of the area – the surficial geology.
The tests and research established that the foundation soils comprised a dense, silty glacial till typical of the many drumlins in the area.
Drumlins are teardrop shaped glacial soil deposits. The Citadel in Halifax is on a drumlin.
Analyse data: The fact that the mobile home vibrated in winter but not in summer was interesting, and took some reflection on my part.
The shallow depth of the pier foundations supporting the mobile home – a few inches, was not typical for foundations in this area.
We dig our foundations down typically about 3.5 to 4.0 feet in the Halifax area to get below the depth of frost penetration and the effects of frost heave.
A characteristic of the fine grained soils found beneath the piers is that they are very frost susceptible – water collects in the soils easily and freezes in winter. The mixture of water and soil expands on freezing – frost heave to everyone. The more soil freezes – the greater the depth of freezing, the greater the frost heave.
The pier foundations would have heaved in winter for certain considering they were only a few inches below the ground surface, not 3.5 to 4.0 feet..
The depth to which the soil freezes depends on the severity of the winter. Deeper in cold winters, shallower in warmer winters.
A source of heat from an external source other than the weather can also affect the depth of frost penetration in the ground and the amount of frost heave.
Regardless of how well we typically insulate our homes, heat is lost in winter to the surrounding air. The air is warmed in the process and in turn warms other surfaces in contact where it is protected from the wind.
That was the case at the upslope end of the mobile home where the building was closer to the ground – 1.5 feet. The depth of frost penetration and heave could be expected to be less at this end of the building than at the downslope end where the home was 3.5 feet above the ground. It was also exposed to the wind at this downslope location.
Frost was penetrating the ground to an increasing depth from the 1.5 foot end of the mobile home to the 3.5 foot end.
All the piers along the length of the home would heave due to frost action but not necessarily a proportionate amount. This is because conditions at each pier could be expected to vary a little: Foundation soil conditions could vary, also heat loss from the mobile home, protection from the wind, etc.
The steel beams could be expected to be lifted off the piers completely at some locations – and “suspended” between adjacent piers, because of the disproportionate amount of heave at the adjacent piers.
Steel beams deflect between piers. The greater the suspended distance between piers providing support to a mobile home the greater deflection. Walking along a floor supported on such beams causes the floor and the beam to deflect and vibrate. I think a good many of us have walked along wooden planks supported at each end and felt the deflection and vibration.
I concluded that the mobile home was vibrating as much as it was because it was not properly supported by the piers in the winter time. Because of the magnitude of the vibration, I believed that the mobile home was only supported by the piers at the ends of the two beams.
The gaps formed at the top of the partitions because the joints between the tops of partitions and the ceiling are relatively weak and would separate when the supporting beams deflected. I suspect that small gaps would have formed at the bottom of the partitions as well but went unnoticed.
I recommended digging and founding the piers deeper and below the depth of frost penetration and heave.
- Always look at the weather conditions along different parts of a foundation when unusual problems are occurring in the structure above.