Why the surprise about the mud slide in BC?

I was surprised that a house was built in the path of the mud slide in British Columbia early this month, July 4th.  Built right in a mud channel.  Particularly in view of the fact that the Ministry of Transport reported that more slides were possible – they knew. (Ref. 1) Why were houses approved for construction in such an unstable area?  Who is liable?

Assessing the potential for a mud slide in an area and quantifying the risk for residents is not rocket science.  Someone had done sufficient assessment to warrant the report of the Ministry.

You don’t see the messy stuff mentioned as such in engineering books but it’s there.  It’s classified as very soft clay and silt with a little sand and gravel and a few cobbles.  The analytical procedures are in the books too – reliable slope stability methods of analysis understood by experienced engineers.

So, what’s the big surprise about the mess in BC and why were people allowed to build there?  We’re not talking something small here.  We’re talking about a slide that engulfed a house to a depth of several feet according to pictures on line.  And moving fast too as seen in one video of a fellow running out of the way just in time.

You don’t need a lot of data to do such an analysis either and the data is readily available in the public domain:

  • The history of mud slides in the area and the rain fall at the time
  • Topographic maps to give you the shape and slope of the ground, and the location of mud channels
  • The results of terrain analysis identifying features relevant to mud slides, and evidence of past events (screen grabs from video taken from low flying drones has had a big impact on the engineering analysis of terrain in an area)
  • Soil maps (surficial geology maps) to tell you that a mixture of clay and silt underlies the area

Engineers have done this type of assessment often, and everywhere throughout North American and around the world.  And based on the news and Transportation’s report, likely for this area as well.

So why the surprise?

Why would a person even mistakenly build a house in a mud channel?  I can’t help but think there would be local knowledge that would kick in even if the government was silent.  Would you build a house in a river channel or on a flood plain between wet seasons?

Reference

  1. Notes taken from a CBC News report online.  “Debris flood, a mix of mud, gravel and cobbles 200 km east of Prince George, BC in a flood prone area 1:30 Saturday morning July 4, 2020.  Waist deep.  Second one that day nearby.  Ministry of Transport said more likely in the area”

You could be excused for thinking that everything is falling down

We recently learned about the potential for failures and accidents in the built environment. (Ref. 1) There are 1,000s of different ways these can happen based on the great number of structures that are out there – at least 124. (Ref. 1)  But where are the failures?  Where’s the evidence this might be happening?

It would seem to be all around you as you drive and walk in your neighbourhood, your community, your town and your city.  And you can’t escape the evidence by taking a break in the country.  It’s there too.  (I saw a barn on its way to collapsing during a drive in the country in New Brunswick on September 26, 2020)

By way of bringing you up to speed, I describe a few failures and accidents below, in some cases with a typical cause noted.  I’ve classified them according to whether they were:

  • Small
  • Medium
  • Large and Catastrophic
  • Personal or,
  • Stupid
  • The Reality

A. Small Failures

1. Manholes and catch basins in the street are sometimes higher or lower than the road surface by a few inches.  They are design or construction failures.

Car drivers in trying to avoid the bump sometimes have accidents.

2. A narrow depression a few inches deep across a road – sort of like a hollow, upside down speed bump – is a failure.  You know them from the bump-bump as you drive across.  They are located above trenches where storm and sanitary sewers and water pipes have been installed.

The depressions result from poor compaction of the soil placed in the trench to fill it after the sewers and water pipes are installed – a construction failure.

They can also cause motor vehicle accidents.

3. Pot holes in roads are a design or construction failure.

They’re due to a weak pavement and subgrade or a poorly drained subgrade.

It’s interesting, pot holes can “grow” larger after water collects in them.  The water helps soil stick to the wheel of a vehicle as it drives in and out of the pot hole.

4. Broken pavement in public parking lots and private driveways are failures.

In the case of parking lots, the failure is due either to inadequate design or construction.  In the case of driveways, it’s due to inadequate construction.

Like pot holes, the pavement and subgrade are weak or the subgrade is poorly drained or both.

5. Sloping floors in houses or apartments are failures.

They’re due to inadequate construction – an 11″ slope from one end to the other in one house that I examined.  The mistake was found in time but not corrected.

6. Wet basements and leaking roofs are failures, of course.

B. Medium Failures

1. The floors of a multistory building, a high-rise, slope and sag at least an estimated 2 and 3 inches – a failure.

I know how high-rises are constructed, and in this case I also learned about the tight schedule the contractor was under to get it up.  This was probably a construction failure that unfolded as the floors went up.

There’s an increased risk of slip and fall accidents on sloping floors, particularly if they’re wet.

2. The concrete block and brick walls of buildings sometimes crack – a failure in some cases

The large size and configuration of some cracks point to inadequate design or construction.  Tiny cracks are usually normal.

3. The foundations of all manner of structures sometimes fail.  The failures are marked by excessive foundation settlement or total, catastrophic collapse.

A little settlement is normal.  Excessive settlement damages the structure above.  Collapse destroys the structure.

These types of failures are often due to inadequate geo-investigation of the foundation soils but sometimes due to inadequate foundation design or construction.

4. Uneven sidewalks are due to inadequate design or construction of the subgrade support or to poor drainage.

The unevenness is quite noticeable when the sidewalk is constructed of concrete slabs and one slab is a little higher or lower than the next at a joint – and easy to trip over.

I’ve classified these as medium failures because of the increased risk of trip and fall accidents.

5. The three Edmonton bridge girders that bent sideways during construction on March 15, 2015 was a failure.  No one was injured because the construction workers went home due to a wind storm.  A crane was left standing with it’s cable and strap connected to the outside girder of the three.

Although I was not involved in the investigation of this failure, I did study photographs on line and in newspapers and conferred with structural engineers and bridge designers.  I also examined the bridge from a security fence while visiting my daughter in Edmonton.

I concluded – an initial hypothesis – the wind caused the crane’s boom to vibrate and the strap to repeatedly tug on the girder and in time bend it sideways.  The outer girder was connected to the other girders causing them to bend too.  This was a construction failure.

6. The St. John river in New Brunswick sometimes floods in the spring and causes damage downstream.  Some people wondered during the flood of 2019 if it could be due to operation of the Mactaquac dam and reservoir upstream of Fredericton.

The dam was constructed to generate electricity when water pressure on the dam’s turbines cause them to turn.  The greater the pressure the faster they turn the more electricity generated.  The greater the depth of water in the reservoir behind the dam the greater the water pressure.

There would be interest in the operation of the dam in maintaining as great a depth of water as possible.  But, too great a depth would threaten over-topping of the dam and collapse – a failure.  Not good.

Water is released from the reservoir to prevent this.  But some years melting snow and rain in the watershed would cause the depth of water in the reservoir to rise more quickly.  The need to release water would get quite pressing.  The reluctance to do this would still be there because water depth/pressure is hydro power.

I can’t help but think these conflicting interests would have something to do with flooding of the St. John river.

7. Rain water flooded the electrical service rooms of a medical practice.  My investigation included uncovering the PVC pipe carrying the power lines into the rooms.  This revealed water seeping in around the outside of the pipe where it passed through the exterior wall.  Further investigation found evidence of rain water inside the pipe.

Water wasn’t supposed to be there because the top of the pipe on the outside wall where the power lines entered from the street was covered by a canopy.  Falling rain was shed by the canopy.

This was fine except a driving rain storm out of the southeast has rain soaked up-gusts.  These gusts of wind carry water up under the canopy and into the top of the PVC pipe and down the pipe and into the electrical rooms – a canopy design failure.

It’s interesting that the inadequate design was recognized during construction.  Steps were taken to accommodate the defect and it was a good solution.  Except, another problem developed involving the electric lines that breached the good solution after it was implemented.

At the end of the day, definitely a canopy design failure aided and abetted by construction failure of what seemed like a good idea.  Explaining all these issues would make your eyes glaze over so I’ll stop here.

8. The slump of soil on a cut slope along a highwaya mini landslide – is a design failure.

You sometimes see these along our highways.  They are often due to excavating the slope too steeply for the natural angle of repose of the soil, or poor drainage of the slope.  These types of failures can be up there with a catastrophic failure.

C. Large and Catastrophic Failures

1. The debris flood that happened in British Columbia early Saturday morning July 4th was a failure.  A waist high mix of mud, gravel and cobbles slid off the mountain and covered a residential property.  Another slide also occurred in the area.  The Ministry of Transport reported the likelihood of additional slides.

The slide could be attributed to poor planning years ago in allowing houses to be built in a slide-prone area in the first place or poor maintenance in not monitoring conditions like rainfall that precipitate landslides.

2. The bridge linking Canso to Durell’s Island in Nova Scotia that collapsed Tuesday July 7th was a failure.  The bridge fell down as a truck drove over it hauling a flatbed trailer loaded with a crane.

The failure was likely due to either the live load of the truck, flatbed and crane exceeding the design live load of the bridge or maintenance of the bridge or a combination.

(A live load is the weight to which a structure is subjected periodically in addition to its own dead load/weight which is always there)

3. I investigated the cause of a bridge collapse in a residential area.  A woman was injured when she drove onto the bridge debris in the stream below.

The failure was due to corrosion of the steel in the bridge that was missed during inspection and maintenance.

4. The crane that collapsed onto a multistory building in Halifax in 2019 was a failure.  It came to rest draped over the front of the building, over the top and down the other side.  The crane broke/bent at several locations along it’s length during the failure.

I am not involved in the investigation of this failure but from a distance outside the security fence it was easy to imagine – an initial hypothesis – that the wind that night, a live load, was too great for the crane.  It looked like an older crane and steel corrosion might be suspect too.

5. High retaining walls that collapse and fall down are usually design failures.  The base of one that collapsed on the coast of Nova Scotia a few years ago was too narrow.

Low retaining walls typical of residential landscaping that lean too much are construction failures and often due to inadequate drainage.

6. A man climbed a step ladder to do some work above a hung ceiling in a building.  He fell, hit his head on the concrete floor and died instantly.  One of the ladder’s legs was bent.

I was retained to investigate the cause of the accident.  There were no witnesses to report on whether or not the workman leaned one way or the other while on the ladder nor how far he had climbed up the ladder.

The bent leg and a leaning workman near the top of the ladder were of course initially suspect.  I planned a re-enactment of the accident with a professional stuntman however my client resolved a dispute arising from the accident in another way.

7. Ice falling off a roof and hitting and severely injuring a person is a maintenance failure.

8. A landslide that takes a house down with it is a catastrophic failure.

I investigated the cause of one like this on the coast of New Brunswick.  It was due to erosion of the toe of the natural slope by the Bay of Fundy.

The landslide was not an act of God because it could have been foreseen and prevented, or avoided by building elsewhere.

9. I saw a catastrophic failure waiting to happen in a drive through the New Brunswick countryside on September 26, 2020 (Example added in a blog update September 29).  A barn with a sagging roof – maybe 10 feet in the middle; a lot.  I’m sure no longer in use considering the sag.  It was the magnitude of the sag that caught my eye.

You see lots of large and small buildings in the country with roofs that are sagging a little or a lot.  Many are abandoned, but not all.

You can see buildings in town, houses, with a little sag to the roof, a tiny, few inches, just enough to catch your eye from the street.

The large sags are design failures.  Many of the tiny sags are design failures too but some are due to lumber shrinking as it drys after construction is done.

D. Personal Accidents

1. I was cleaning snow off the back of my neighbour’s car in his sloping driveway two winters ago.  He gets up a bit late.  I was out doing some shoveling so I thought I might as well.

I started to slide sideways down the slope towards the street.  As it turns out on some black ice.  I did good until I got to the windrow of snow left by the snow plow, fetched up and fell hard.

I like to think to this day that if I had been on skis I would not have fallen considering that East Coast ski hills have some icy runs.

My accident alerted me to the accidents waiting to happen on sloping, paved driveways – surfaces, in general, used by people –  due to black ice, due to questionable design and construction.

I see in recent years steel plates with roughed surfaces being installed on sloping sidewalks at intersections.  Smart.

E. Stupid Accidents

1. Three months after investigating the step ladder fatality I was up a step ladder putting the finishing touches on construction of a storage shed on my property.  I was nailing the fascia board in place and leaned sideways on the ladder to drive that last nail at the end when down I went.

I was lucky and didn’t hit any of the cobbles exposed at the ground surface but I did hit the ground hard and lay there for a while.  My ladder was not defective – no component failure, no bent legs – just my use of the ladder.

***

The Reality

There’s a lot of things broken and not working as they should.  It’s important to know this and that it’s all around us, even out in the country.  Also, that it’s our fault, we designers, builders and operators.  But know too, that the great bulk of the built environment works just fine, thank you very much, and that’s due to us too, we folk who live in the built environment.  There is the potential for 1,000s of failures but they don’t happen because we get it right almost all the time. (Ref. 2)

References

  1. What’s in “…the built environment” and how many ways can it fail?  Posted July 8, 2020
  2. Petroski, Henry, To Engineer is Human, The Role of Failure in Successful Design, Vintage Books, Random House, Inc., New York 1992

(Updated September 29, 2020 by Eric E. Jorden, M.Sc., P.Eng., consulting professional engineer, forensic engineer, Geotechnology Ltd., Halifax, NS, Canada E: ejorden@eastlink.ca)

What’s in “…the built environment” and how many ways can it fail?

On occasion, when blogging about the nature and methods of forensic investigation, I’ve wondered, just how many different structures are there in the built environment?  And are some more difficult to design and build than others?  If so, are some more prone to accidents and failures than others?

I have answers of sorts in the following.  But, like me, you are unlikely to believe anything more than an estimate even if I were to try that.

***

A structure is something (such as a building) that is built by people. It’s also a place where accidents and failures can happen.

Tunnels, bridges, canals, retaining walls and towers are all structures.  Also cars, trucks, helicopters and trains.  And patios, decks and raised flower beds.

It might also be something in the natural environment that is used by people.  Like the foundation soils supporting a building or a tower.  Also the slopes off to the left or right along our highways.

It’s usually a cut slope if it rises from the highway – the natural soil has been cut into or excavated to form the slope.  It’s a fill slope if it drops away from the highway – excavated soil has been dumped there and forms a slope.  These slopes assume the angle of repose of the soil, which varies for different soils.

All this excavating and dumping to construct a highway – another structure – so it can get to where it’s going.

***

One source listed 124 different engineering projects and classified these according to their complexity from Least Complex (#1) to Most Complex (#4). (Ref. 1) The classification took into account the number of parts or stages making up a project, their interrelationship, and the effort involved in analysing, designing and constructing the project.  Generally, the more parts, the more complex.

Examples of Level #1 are simple commercial buildings and storm and sanitary sewers, and of Level #3, ferry terminals, grain silos and small dams.

Almost all engineering projects involve a structure as distinct from a system or process, like a computer network.

Some of the interrelated parts of a bridge include the foundations soils, the foundations, the abutments, the piers and the bridge deck.

If it’s a suspension bridge like across Halifax Harbour there are also the main cables, the towers above the piers supporting the main cables, the anchors on the bank or shore at the ends of the cables and the vertical, suspender cables that tie the bridge deck to the main cables. (Ref. 2)

***

The next time I was driving after drafting the above I noted still more structures not included in the count of 124.  Albeit smaller ones like traffic lights at an intersection, gates at railway crossings, tall propane storage tanks at service stations and armored stone on an eroding shore line.  But still structures that can fail in some way or result in an accident.

And failures or accidents can happen at any of the 10 stages in the life of a structure, not just during the construction stage or service stage. (Ref. 3)

***

If a building, a single structure, can fail in 209 ways, excluding what happens in the basement (Ref. 4, 5), how many ways can each of the 124 different structures in the built environment fail, during each of the 10 stages in the life of each structure?  What would the total look like?

Just think of those 124 structures each with several interrelated parts and each part with several components.  A component that doesn’t work properly or breaks completely is a failure.

The stairs in a building are a component of the building structure.  The chute at the bottom of a grain silo is a component of the silo.  The railing along the edge of a bridge deck that prevents you driving or falling into the river or harbour is a component of the bridge structure.

The National Research Council of Canada found in a study that the lowly, humble basement of a building can fail in hundreds of ways. (Ref. 6)  What does that tell you about the number of ways failure can happen in the built environment?

(It occurred to me on reading this 185 page report that the study procedures and processes are a guide for analysing the cause of failures and accidents in structures, in general, other than just basements)

***

I was tempted but refrained from trying to multiply some of these numbers together to get an estimate of the likely 1,000s of ways failure can occur in the built environment.  It boggles the mind.

Thank heaven we also have good engineering in the built environment and most of the 124+ structures and the many ways each can fail – at least 209 for a building – get through the 10 stages of their life without failure or accident.  We don’t want failures but they do occur and engineers learn from them. (Ref. 7)

References

  1. Guideline For Engagement of Consulting Engineering Services, CENS, Consulting Engineers of Nova Scotia, Halifax, NS
  2. Personal communication, Jamie Yates, Yates Consulting Engineering, Fall River, Nova Scotia, June, 2020
  3. Stages in the “life” of a structure helps communication between counsel, insurance claims manager and an engineering expert. Posted July 2, 2015
  4. How many ways can a building fail, and possibly result in civil litigation or an insurance claim? Posted July 10, 2014
  5. Nicastro, David H., ed., Failure Mechanisms in Building Construction, ASCE Press, American Society of Civil Engineers, Reston, Virginia 1997 (Readily available by interlibrary loan from Memorial University, Newfoundland)  (Note: This study did not include failure at the foundation or basement level)
  6. Swinton, Michael C., NRC-IRC and Kesik, Dr. Ted, University of Toronto,  Performance Guidelines for Basement Envelope Systems and Materials, 185 pg, Research Report 199, National Research Council, Canada October 2005
  7. Petroski, Henry, To Engineer is Human, The Role of Failure in Successful Design, 251 pg. Vintage Books, Random House, Inc., New York 1992

(Updated September 29, 2020 by Eric E. Jorden, M.Sc., P.Eng., consulting professional engineer, forensic engineer, Halifax, NS, Canada ejorden@eastlink.ca)