Monthly Archives: April 2014

Heavy rain (Ref. 1) and a good water-problem-classification system helps engineers understand and explain forensic engineering problems

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A reader in the U.K., Len Threadgold, (see profile below) commented on last week’s blog. (Ref. 1)  Len speaks with some authority when he notes that the problems with water can be grouped under the following three headings.  He also notes that separating problems into neat categories is useful but sometimes the categories work in association with each other – see examples below.

This classification is good: A way of understanding natural events and environmental processes.  In a sense, a way of measuring things.  Engineers like that.  It helps us solve problems and explain causes and technical issues to the justice system.  As such, it’s a forensic engineering method:

1. The problems that the presence of water causes

We don’t like flooding or working under water – so this problem concerns water level.

Examples would be your flooded basement, also the water pooling in your back garden that has drained from the land above – sorry to bring these examples up.  And the recent flooding that occurred in Truro, N.S., and in the U.K., and forecast for the Saint John River, New Brunswick.  Water pressure would also be a factor in a wet basement; see #3 below.

2. The problems that the flow of water causes

These problems are about erosion, the removal of soil as a result of water flow, the speed of the water.

For example, coastal erosion such as that occurring around the entire coast line of Prince Edward Island and also at Red Head, on the Bay of Fundy near Saint John, New Brunswick.  The flowing water on the coast is wave action and long shore currents.

I investigated one landslide at Red Head some time ago that destroyed a home at the top of a sea cliff.  I learned two weeks ago that erosion of the bottom of the cliff continues to remove the buttressing effect of the soil there.

Rain triggered the landslide at Red Head that I investigated – after erosion of the toe set the stage.  Rain did this by increasing the pressure in the ground water behind the cliff; see #3 below.

3. The problems that the pressure of water causes

These problems arise because water pressure affects soil strength.  You know this when your boots sink into the mud – clay in engineering – after a rain storm.

The problems are bigger than sinking, muddy boots though.

Water pressure – what we call pore water pressure in engineering – is often a factor in landslides as it was for the landslide at Red Head.  At times water pressure is the principal cause in changing an adequately stable slope to an unstable one.  Its effect is to reduce the frictional strength of the soil.

Water pressure was certain to have been a factor in the recent landslide in Washington State, U.S.  This landslide took more than a dozen lives with others still missing.  I saw early reports that there was a known risk of a landslide at this location.  The assessment leading to that conclusion is certain to have considered pore water pressure and its potential to change.

Len has investigated the potential for landslide problems in Hong Kong.  These would  be slopes that were susceptible to increases in pore water pressure.  Simple drains often fixed the problems but sometimes more in-depth water interception was necessary.

Water pressure is a factor in our wet basements.  And, believe or not, in many slip and fall accidents.  It’s the reason signs in a swimming pool caution us not to run on the pool deck.

In some slip and fall accidents, a person’s weight is momentarily applied to water on the floor surface.  The frictional or skid resistance of water is much lower than the material forming the floor surface – almost negligible by comparison.  So low in fact that the person slips and falls.

Len’s classification is a good one, an aid to those of us investigating problems with water and needing to explain the cause of a problem to the justice system.

Many of the forensic engineering problems I investigate – not just a few, and not just the obvious drainage and flooding problems – can be traced back to water and fall under one or more of the headings Len has identified.

Reference

1. Image credits and why forensic engineers like wet weather, the heavier the rain the better, posted April 9, 2014 http://www.ericjorden.com/blog/2014/04/09/image-credits-and-why-forensic-engineers-like-wet-weather-the-heavier-the-rain-the-better/

Len Threadgold’s Profile

Len is a civil engineer in the U.K. specializing in soil, rock, and ground engineering – geotechnical engineering.  We were colleagues when I practiced there.  Len’s firm, Geotechnics Ltd, provides consulting services to an international clientele including dealing with slope stability problems in Hong Kong and the U.K. – problems that would fall under #3 above.  The U.K. had their share of severe flooding problems this past year, problems with the mere presence of water; #1 above.  Len read a draft of this blog because it’s his classification system and some of the examples and comments are his.

 

 

 

 

Image credits, and why forensic engineers like wet weather, the heavier the rain the better

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The tranquil sea coast in the heading is deceptive.

Many forensic engineering problems are caused by water or water is a factor in the problem.  And – at the risk of annoying you, I must tell you that experienced forensic engineers like heavy rain when investigating a flooding or drainage problem.

They’ll also use somewhat unconventional methods to gather data to solve the problem – this blog site is partly about forensic methods.  And think outside the box when analysing the data.

There’s no shortage of flooding and drainage problems in eastern Canada in the spring.

We are almost always interested in how rain and melt water are flowing across a site and around, beneath, and into structures.  We can do detailed topographic and contour surveys – which can take days, and determine drainage patterns and watercourses the text-book way.

Or we can wait and hope for heavy rain – sorry, and note and measure the location of the water as it flows across the site and in the vicinity of the structures.  I often do the latter.  It’s very accurate.

Also, we note in some problems where it’s not flowing – this was relevant in one problem I investigated a few weeks ago.

We’re interested in discharge too.  I went to one site four times in 24 hours to measure the discharge in a drainage channel – I was interested in how quickly the site drained after a rain storm.  Quite quickly as turned out.

Noting where water was not flowing was relevant in another problem I looked at a couple of years ago.  Unfortunately, I was retained after $140,000 was spent in misguided investigation.  Getting out on a rainy day and noting where water was not flowing would have saved an awful lot of money.

Sadly, sometimes there’s no heavy rain when a drainage problem is being investigated.  Or greater flow is needed than can be expected from typical rainfalls for the season.  Fire hoses and water trucks can be used to generate flow.  I planned this for one investigation.

The scene in the heading is Prospect Bay, Nova Scotia, Canada, photographed by J. Knoll, Halifax.

Other days are not as nice as this scene along the 1,000s of kilometres of Atlantic sea coast – almost as long as Canada is wide.  Nor are the 1,000s of rivers and streams that flood – and they certainly have flooded this late winter, early spring, 2014.

The rivers and lakes are still flooding this morning after the heavy rain yesterday as the land sheds the rainfall and melt water in runoff to our watercourses.  And the lakes are overtopping their banks – there are more than 4,000 lakes in Nova Scotia.  That’s just the water you can see on the ground surface.

There is also the ground water – the water table, that you can’t see.  Water that is just below the ground surface or quite deep, and fluctuating up and down all the time – certain to be well up and high after the rain this year.

Ground water flows through the ground much like a river flows on the land; welling up behind obstacles buried in the ground – e.g., foundations and basement walls, like river water wells up behind a boulder in a stream.

Forensic engineers use monitoring wells to measure how ground water flows and drains beneath a site, and also measure ground water discharge.  They are looking for similar parameters to those for surface water.

It’s been said that if you could take the water out of the ground, out of the soils and rocks beneath our feet, you would reduce foundation and ground engineering problems to a fraction.

The camera operator in the heading is filming the re-enactment of a fatal MVA from a sea king helicopter during a forensic engineering investigation.  The sea and snow – frozen water, were factors in the fatal accident.  For this forensic investigation, I set up a full scale test site at Shearwater airbase in eastern Canada complete with:

  1. A traffic lane,
  2. An obstacle in the lane,
  3. A vehicle,
  4. Monitoring devices to track vehicle behaviour on striking the obstacle, and,
  5. Film crews to record the tests.

The surveyor in the heading is checking the adequacy of the underpinning of a structure during a forensic investigation.

Surveyors also carry out topographic surveys of sites with drainage problems.  But, noting and measuring where the water is draining during heavy rain can often give us all the data we need and much quicker and more accurately.