Wednesday, February 6, 2013

Geotechnical investigation of a Toronto sewer incursion

Extra Reading

Statement of Issue:

Rocks have been found in the main west sewage treatment plant.  Some fieldwork is being done before a major camera survey.

Payment for Report

Nada, nothing!  I wouldn't take your money if you threw it at me.  My philosophy is "Never let stupid people take you seriously, for on that path lies madness."


Obviously we are looking for a small break, since somebody would have fallen into a large one.  As well, since all of these sewers are surrounded by concrete, we are looking for steel liner exposure.  Since there is only one small section that is visited by this author and his dog, we shall look there.

The Chapman Creek sewer was built in the 50's, at a time when you could rip open a forest with impunity.  They laid in along a rapidly eroding creek.  Little did they know that massive development would turn this into an open storm sewer, with horrendous rain floods.  In the last 20 years there has been metres of erosion through the Queenston shale, probably accentuated by a lack of limestone layers in the lower sections.  This picture shows the sewer exposed in one area.

Even worse is upstream, where this picture shows a possible nightmare scenario.

Here, we see no pipe, just a lot of concrete rubble.  Note the cap concrete has been completely ripped up and inverted.  The concrete has been like this for at least 20 years (time of observation, going through 1 7/8ths dogs*).  *2nd dog is getting quite old.


This upper section has had the concrete ripped off, and the steel has been exposed to direct fluvial scour for at least 20 years.  It is covered by rock rubble, which probably blocks significant water inflow.  During flood events the cover is shifted, causing rocks to wash in and move through the sewer system.

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