Dear Editor:
People have expressed concern about “horizontal movements” in the proposed Banks Crescent Development site in Bristow Valley.
In scientific terms this refers to shear strength (i.e. the strength to resist shear/slippage), directly applicable to the Bank Crescent site.
Even though the vertical force from building activity may or may not cause a problem to the aquifer if it is deep enough, there could be shear strength failure, the tendency for sediments to slip under force, because the sediments are sloped in the valley towards the lake.
This is worse if water is involved. That is why such buildings need deep pilings to secure their stability.
For Banks Crescent deep pilings would puncture the aquifer with potentially disastrous consequences, so they will not be used.
The buildings directly load the sediments. If the aquifer depth is less than Lark suggests, even close to or at the surface (the aquifer in the valley is known to be unpredictable), the risk for failure exacerbates.
An educational and eye opening video from John Burland illustrates the “effect of water on soil strength.”
Burland is a well known Professor of Imperial College, London. His demonstration is not new, but if you have not seen this, it is a must see for the discussion about the stability on the proposed building site. Please go to YouTube, then search for “The effect of water on soil strength.” It directly applies to Banks Crescent.
Bottom line: Slope and water can pose a major threat to the stability of the Bank Crescent sediments, in particular if we load them with five buildings of six stories high. In this regard we should all be very concerned about the fact that “the proposed development will come within 40 metres of the spring area.” (Golder 2018.)
That alone should be enough cause for alarm.
Last but not least: May I challenge anyone, who still feels confident that these five buildings of six stories plus parkade without pilings to bedrock will securely stand forever, to walk the Centennial Trail and break off a fresh piece out of the high cliffs.
This is the same material as is contained in Bristow Valley which was largely derived from these cliffs.
If you can keep it in one piece, check what material this is. Minimal pressure will break it or shear it, water will disintegrate it and flow it.
These largely unconsolidated sediments, are not stable on the long term. There is a reason why so many landslides happen.
Aart J. Dronkers
Structural Geologist
Summerland