Is it important to know that a lot of the built environment has not been engineered but built on experience? Built on the basis of personal and first hand knowledge, observation and practice – that is, experience. Rather than on the application of science and mathematics – engineered. It is as I show in the following.

Some examples:

When was the last time you thought about the depth of a telephone pole in the ground and how this was determined – so it wouldn’t get pulled over by the power lines tugging at the top? And related, how the anchors for the pole’s guy-wires are figured out and constructed?

Not big issues you say? But note how some poles lean next time you’re driving around.

A leaning pole near where I live had a new anchor and guy-wire installed in the last couple of days, alongside the old anchor that didn’t work. Looking up you see lots of wires near the top tugging on one side but none on the other side. Was the new anchor engineered or experienced? Obviously, the old one wasn’t engineered because the pole leaned.

How about the foundations for your house and all the others on the street? For that matter, foundations for all kinds of structures.

We’re absolutely certain the foundations for a multistory building are designed by an engineer but what about a single story commercial building in an industrial park?

How about the foundations for the piers and buttresses of a bridge? I’m sure the foundations for the piers at a river crossing are engineered. But, what about those simple abutments?

When was the last time you wondered who built the drip loops on the power supply to a commercial building – an engineer or an experienced person? Then wondered what happens during heavy rain and strong, gusting winds out of the southeast?

(Drip loops are loops in the power lines from the street to a building. They are located near where the cable enters the building. Rain water on the lines drip off at the loop down rather than run along the cables and into the building. Except they don’t work well in heavy rain and strong winds with up-gusts as I determined recently – they fail in engineering terms)

What about the retaining walls along our streets and highways? And the paved bike paths in our green belts?

Also the soil slopes along our holidays? For example, see the highway slope failure near Exit 10 on Hwy 104 in Nova Scotia. It’s subtle but it’s there. Is that an engineering failure or an experience failure?

What about those large steel or concrete culverts carrying roads and streams beneath our highways. Are they engineered or experienced?

And those tall, slender propane tanks at service stations? For certain, the steel tanks have been engineered but what about the foundations to resist overturning of the tanks in strong winds?

I can give more examples but that’s enough; I’m sure you get the picture.

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I thought of this as I was driving about recently – so much of what we see constructed is based on experience. There are no engineers involved.

Why take an interest if things are working most of the time, performing as they should? Regardless of whether they were engineered or built on experience.

How is this relevant to understanding forensic work, the objective of this blog site?

Here’s why it’s relevant and important to take an interest

Things do fail and also occasionally injure people.

For example, if a kid on a bike falls and is injured after hitting a pot hole on a bike path what happens during the forensic investigation? Lots of bike paths are built based on experience. But pot holes form because of well understood engineering principles.

Or an electric room in a commercial building floods in spite of the engineered or experienced drip loops? What happens during the forensic investigation?

Or a highway slope fails, a bridge collapses, or a telephone pole leans?

We can figure out why something failed – the easy part. But determining the standard of care and what a reasonable person would do is tougher when the thing was built based on experience.

Who should determine the standard? A planner, an architect, a design engineer, a construction engineer, another experienced person, the owner of the structure, someone else?

And if things work quite okay, should we, nevertheless, wonder if they’ve been engineered or experienced – particularly things used by the public!?

Things built on experience are sometimes overbuilt and cost more money than they should. And sometimes they’re under-built and cost more money when they fail and occasionally injure people.

It’s important to take an interest because lots of questions arise when something fails that was built on experience, not engineered.

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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)      

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 visual 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 and the depth of frost penetration for the area.
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 November 20, 2020 ejorden@eastlink.ca)      

(Updated March 18, 2021)