What can you get from a virtual visual site assessment about the cause of a leaning retaining wall?

What can you get from a virtual visual site assessment? That’s easy, pretty much the same as from a real visual site assessment. Except the expert doesn’t go to the failure or accident site and look at everything up close in assessing cause.

The expert relies on what s/he can see in photographs and video, after a briefing by the client and reading documents. He may also ask for additional information from someone at the site as I did recently.

A visual site assessment examines what is exposed at the surface of a site. It’s a preliminary stage in a comprehensive forensic investigation that also examines what is exposed below the surface after taking things apart or probing with some device.

The following is a good, easily understood example of what you can learn from pictures – the data you can get and the conclusions you can draw. The data is in italics in the following:

The wall in general

A party called me from Ottawa about a concrete retaining wall that was leaning. He wanted to know why and if it was dangerous.

He briefed me on the situation. I asked for photographs which he sent and I studied. He also sent some measurements of the lean. I then asked for aerial video from a kid’s drone and I got that. A little later a Google Earth shot of the scene was sent to me which was revealing.

I saw that the wall was low, maybe 2′ near the ends, and about 6′ at the highest point. Not a big wall but still a problem – it will fall down in time but when is difficult to predict.

I got this data based on the height of cars that I could see in the scene. Also knowing the standard height of 8′ from the floor of a building to the ceiling. This was evident on the outside of buildings in the area by the distance from the bottom of the brick work to the eaves.

The lean was greater at the higher sections of the wall. This was clear in photographs along the length of the wall. No thinking and figuring was necessary. But, was it significant?

There also appeared to be a building behind the wall near the higher section, and a smooth area too – pavement? These apparent features were just visible beyond grass, shrubs and trees in back of the wall.

Hmmmm, a budding conclusion? Was there some association between the the higher section of the wall, the greater lean and a possible building behind this section of wall?

The face of the wall:

I saw in close-up pictures at least seven vertical cracks in the wall from top to bottom. Concrete retaining walls are not supposed to crack.

The concrete was stained white along the length of the cracks. Water running down concrete can stain it.

The concrete wall on one side of some cracks leaned farther out. This would not happen if there was horizontal reinforcing steel in the wall as is often the case in well designed retaining walls.

Conclusion? Properly designed and constructed walls do not lean a lot, crack at several locations, drain water from the cracks, and lean farther out on one side of a crack.

I did not see any holes in the face of the wall – possibly one or two – typical of drain holes that are often designed and constructed in a retaining wall.

The base of the wall:

Concrete retaining walls are supported on a concrete base that extends to the rear of the wall, called the heel, and a little to the front, the toe.

Close up pictures of the bottom of the wall where the soil was removed at one location showed the toe. The toe was a few inches wide and the top a few inches deep.

The toe of the base of a properly designed retaining wall is usually much wider than this and deeper – at least below the depth of frost penetration for the area. The size and depth of the toe was evident from maple leaves in the picture.

The rear of the wall:

Video from the camera on the kid’s drone, and screen grabs from the Google Earth video, particularly the latter, were quite revealing too. I was surprised at the excellent resolution of the screen grabs.

A house, driveway and garage could be seen behind the retaining wall in the aerial photographs. The garage was opposite the section of wall that was leaning the most. The near side of the garage was about 9.0 feet from the retaining wall. I got the distance from a scale on the Google Earth photograph.

The driveway was paved and ran from the street to the garage, and over to the side of the house.

I asked my client to check the distance from the wall to the side of the garage. He did this and got 8.5 to 10 feet and confirmed the approximate measurements I took from the Google Earth photograph. He also noted the garage roof did not have an eaves trough.

Conclusion? Some of the rain water falling on the driveway and the garage roof would seep into the ground at the rear of the retaining wall.

Conclusion about why the wall is leaning:

I concluded based on the data from my virtual site assessment that the retaining wall was inadequately designed and constructed and this was the cause of the lean.

For example, the rear of the wall was not well drained. I’m sure this is a significant cause of the lean. Water in the soil behind the wall expands on freezing in the winter and pushes against the wall, moving it a little each year.

This is like the frozen water in soil pushing against the underside of a house footing that has not been constructed below the depth of frost penetration – frost heave.

Also, the wall did not have an adequate foundation base, certainly not at the toe. This would contribute to the lean. The soil at the back of a retaining wall applies pressure on the wall trying to push it over – make it lean. A suitably wide base at the heel and the toe prevents this. This kind of pressure is in addition to the push from frozen rain water that has seeped into the ground.

The shallow depth of the base means it is susceptible to frost heave which might also contribute to the wall leaning.

The wall did not have horizontal reinforcing steel. This type of reinforcement stiffens the wall and helps prevent leaning.

Future Forensic Engineering Investigation:

Future investigation would consist of:

  1. A real visual site assessment to confirm the findings of the virtual visual site assessment.
  2. Collecting data from the site for design of a new retaining wall. Data like the nature of the foundation soils at the location of the wall.
  3. But, particularly, data on how to collect and remove rain water shed by the nearby driveway and the garage roof – for that matter, data on how to drain water from behind the wall in general.

Summary

I got all of the above from a simple virtual visual site assessment of the cause of the retaining wall’s lean – by studying pictures. The assessment took about an hour and a quarter of my time.

This type of assessment is possible for all failures and accidents. For certain, the amount of data got will vary depending on the incident. But, also for certain, you will be surprised at what you can get from a simple virtual visual site assessment by an experienced forensic engineering expert.

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

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