Mudslide Zone!

I’m sad at the thought of all those lives lost and people being flooded out in British Columbia. Also the estimated 2,000 farm animals drowning; they got feelings and get scared too. I saw fear in the face of one of my Golden Retrievers one time when attacked by a German Shepard. It was an eye opener.

The flooding and the mudslides didn’t have to be such a surprise.

The flooding at Sumos Lake in B.C. was predicted by the Indigenous people when their land was taken over by farmers in the 1940s. (Ref. 1) A prediction based on millennium knowledge of the land.

The location of potential mudslides could have been predicted too – if you had talked to a geotechnical engineer or a surficial geologist.

Surficial geologists map the different types of soils in an area as deposited by the glaciers many 1,000s of years ago. Eastern Canada has been mapped completely. I’m sure also much of Canada and British Colombia. The data is readily available and easy to understand.

The soils beneath and near the site of a proposed road or bridge – like those washed out – would normally be determined before design and construction. This is standard engineering practice for all structures in the built environment.

The geology maps tell the type of soils and the contours on topographic maps tell the steepness of slopes in the soils – in jargon free language. Maybe a little high school math is needed.

Geotechnical engineers analyse the physical properties of these soils and how strong and stable they are when used or found in different ways. This can include the stability of natural slopes alongside a highway.

The cause of mudslides is understood well enough that signs could be put up to alert a driver – like, “Mudslide Zone“. Signs similar to, “Construction Zone“.

The cause has everything to do with the type of soil, the natural slope of the soil surface, and water. Take out the water – the trigger – and the land and mud will stay put, as it has for 1,000 of years – unless shaken by an earthquake. Water increases the weight of the soil on the (mud) sliding surface and water decreases the frictional resistance of that surface – like in high school physics.

Geotechnical engineers and surficial geologists can tell you were to put the Mudslide Zone signs. Drivers don’t need to be surprised and some swept away. Farm animals don’t need to drown if we listen to Indigenous people.

References

  1. Hughes, John M., PhD, Vancouver, Personal communique, November 2021

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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 21, 2021. Updated November 24, 2021 ejorden@eastlink.ca)   

The tiny culprit in a flooded basement

I recently investigated the cause of a wet basement in a house. A 3/4 inch deep layer of water appeared on the floor at the rear of the basement. And kept appearing after vacuuming and mopping up day after day. At the end of the day, I was surprised at the cause of the flooding.

I took the right approach and looked at all possible causes based on the construction of the basement, and the appliances and equipment in each room. I followed a work breakdown structure (WBS) common in project management and not unlike differential diagnosis in medicine. (Ref. 1)

Basement Construction

The two story, 38 year old house has a completely finished basement. There is an open living area at the front 2/3 of the basement, a computer room and library at the right rear and a laundry room at the left rear. The laundry room is tiled, the other rooms carpeted. The concrete basement walls, footings and some of the pipework are all hidden from view.

The 3/4 inch depth of water was seen on most of the tiled floor in the laundry room. The carpet in the computer room and a short distance towards the front of the open living area was soaking wet.

The house is on sloping ground that drains surface water away from the basement so an easy call during my investigation and no problem there.

A complicated structure

My investigation wasn’t helped by the fact that a basement is one of the most complicated structures engineers must deal with and one of the least glamourous. (Ref. 2)

  • It’s a structure in it’s own right
  • It’s the support for the structure above
  • It’s reliant on the awkward ground beyond and beneath the basement for foundation support and a good life
  • It must deal with the fickle groundwater (the water table) surrounding and beneath the basement
  • It’s got a complicated drainage system
  • You design, construct and investigate basements based on experience and observation (empiricalism) – there’s not a lot in the engineering design and construction handbooks
  • You can’t see much; it’s all buried in the ground

Contractors are happy as clams when they get construction out of the ground. Engineers too. Owners wish there was no such thing as a basement because of the disproportionately higher cost.

Drainage of well built basements

Well built basements are designed and constructed with a layer of free draining soil beneath the concrete floor – a drainage blanket. The layer of soil drains to a pipe – weeping tile – that runs along and just outside the footing. The bottom of the pipe – the invert – is at the bottom of the footing.

The basement walls are backfilled with soil that also drains to the weeping tile. The surface of the backfill slopes and drains away from the basement walls. The weeping tile drains all the water off the property from the drainage blanket and the backfill.

Laundry room floors in well built basements slope towards a floor drain connected by a pipe to the weeping tile – in case water is spilled on laundry day. This is the reason the depth of the flood water in this basement actually varies from zero to 3/4 inches.

Floor drains have a back flow valve that closes after water on the floor drains. The valve prevents water in the ground flowing up the pipe and onto the floor. This can happen when the water table rises.

Why didn’t the water on the floor flow down the floor drain? A good question. Possibly because the floor drain needs some maintenance. Or, the water table is just below the underside of the basement floor – this happens at times.

In general, a complicated drainage system for a complicated structure that can’t be seen, and sometimes gets the short end of the stick during basement design and construction.

What did I investigate?

The following is like a work breakdown structure (WBS) in project management. It’s an identification of the tasks that must be carried out to complete a project. The project in this case is determination of the cause of the 3/4 inch flood. (Ref. 3)

  1. Check that water valves and taps are shut off. The basement laundry room did not have main water shut off valves to the washing machine and wash tub so no problem there. The tap in the laundry tub was turned off, and was kept that way during my investigation.
  2. Note the location of the water on the floor. Part of the reason for locating the water in detail was to eliminate the cause of the flood as due to water getting in from the outside. This happens at the contact between the bottom of the concrete basement walls and concrete floor. Water was located on the tiled laundry room floor as far as the left, right and front walls. It stopped short of the rear concrete basement wall which was encouraging. The floor was damp near the electric water heater in the right rear corner of the laundry room. This was similar to the edge of the water everywhere in the laundry room. This threw me off at the beginning of my investigation. The water returned in the laundry room after it was vacuumed up different times over the next few days. The carpet in the computer room was soaking wet out to all the walls. The carpet in the rear one third of the living room at the front of the basement was soaking wet too. It was difficult to see in the computer room if water was getting in at the contact between the basement wall and the carpet covered floor, so I kept going.
  3. Examine exposed concrete basement wall and floor and the pipe work in the left, rear corner of the laundry room. I did that and everything was dry as a bone. Nice to see.
  4. Examine the electric water heater. I checked the water heater periodically during my investigation. The water on the floor below was similar each time to that elsewhere in the laundry room at the outer limit of the 3/4 inch depth of water. Sometimes I saw a drop of water at the drainage tap at the bottom of the heater – like condensation. Other times I didn’t. The drop of water didn’t raise any alarms.
  5. Remove the carpet to expose the floor and any pipes. I removed the carpet from the floors in the computer and living rooms and did not see any pipes at the bottom of the walls. I saw the water on the computer room floor with the edge extending near the front, rear and right walls, and under the left wall common with the laundry room. But no water on the floor in the living room. I concluded – an easy call – that the wet carpet in the living room was due to capillary action from the soaking wet carpet in the computer room.
  6. Monitor the water in the floor drain after fixing the back flow valve open. I noted that the level of water in the drain occasionally dropped to about 2 inches below the floor. So, the water on the floor from somewhere was slowly draining away through the floor drain. The 2 inch level below the floor was suggestive of a natural water table just below the floor.
  7. Examine the bottom of all walls in the laundry and computer rooms for leaks from outside the basement. I examined the bottom of the walls but did not see any leaks from outside the basement. So, the water on the floor was not getting in from outside the basement. An important finding.
  8. Vacuum the water off the floors after the carpet was removed. I vacuumed the water off the floors several times and noted the same area was covered when it returned after about a day, including near the water heater. And as noted above it was sometimes draining away through the floor drain. I checked the water heater again. A drop of water continued to be occasionally seen at the tap on the heater. I felt the water on the floor below the tap as I occasionally did. It was warm one time. Other times it was cool like the concrete floor.
  9. Collect the drops of water in a saucer set below the tap on the water heater. I did this a couple of times when I saw drops of water and noted that the saucer quickly filled with water. Hmmmm?
  10. Research operation and performance of water heaters I interviewed a home owner I knew who had an electric water heater and sales people in stores that sell electric water heaters. I learned that water heaters must be drained and cleaned on a regular basis – a few months to a few years depending on who you’re talking to, whether or not you’re on city or well water, and what’s going on in your neighbourhood, e.g., construction – and also that they sometimes leak. That was a surprise – a leaking, glass lined, steel encased electric water heater. But not a surprise for long because water heaters are drained through a pipe that passes through a hole cut in the steel, and water has corrosive elements that love cut steel edges and surfaces. Bingo!
  11. Conclusion: Recommend replacing the water heater I recommended replacing the water heater, including a drip pan beneath the heater – missing at this residence in the past – and monitored it’s installation. I advised draining the heater on a regular basis in future. The culprit – the rogue drop of water – hasn’t been seen for weeks and the basement floor is dry.
  12. Remove the damaged gyproc drywall from the bottom of the walls to better examine the condition of the inside of the walls The drywall softened and swelled for several feet above the floor when flood water was drawn up by capillary action from the floor. This engineering investigative task has been postponed pending monitoring the floor for flood water over time.
  13. Video the floor drain pipe I considered snaking a video camera down the drain pipe to see if there is anything in the pipe that could prevent water draining. This could consist of sediment that can collect over time or a collapsed pipe. The need for this task will be evaluated after the water level in the floor drain has been monitored over time.
  14. Replace the floor drain with one fitted with a suitable back flow valve. The back flow valve needs to be sensitive – open and close – under very low hydraulic heads, like a very few centimetres. The need for this task will be assessed pending the outcome of videoing the floor drain pipe and noting how well the drain works over time.
  15. Drill observation holes in the concrete floor to check for a water level beneath the floor. The need for this task will be assessed pending the outcome of videoing the floor drain pipe.

Lessons learned

  1. Suppress your expectations at what might be the cause of a problem, and what might not be a cause because its so tiny and inconsequential. Continue to go the project management, work-break-down route.
  2. If a drop of water can wear down a rock over time, or change rock into residual soil like in Australia, it can flood a basement.

References

  1. Differential diagnosis in medicine and forensic investigation, and soft, initial thoughts on cause. Posted December 20, 2019
  2. Swinton, Michael C., and Kesik, Ted, Performance Guidelines for Basement Envelope Systems and Materials, Research Report 199, October 2005 pp 185 NRC-IRC and the University of Toronto
  3. Kerzner, Harold, Ph.D, Project Management. A systems approach to planning, scheduling, and controlling. 8th edition, 2003, John Wiley & Sons, Inc., New Jersey

(Posted by Eric E. Jorden, M.Sc., P.Eng. Consulting Professional Engineer, Forensic Engineer, Geotechnology Ltd., Halifax, Nova Scotia, Canada November 15, 2021 ejorden@eastlink.ca)