"Slip, sliding away..."

[Skook]

Part 4 - You’re building your subdivision on a gravel deposit? Totally bummer, dude!

On June 1, 2012 the ground gave way under Seawatch Lane in a subdivision under construction in West Porpoise Bay in the District of Sechelt (DOS). After taking pictures, DOS Inspectors had the sinkhole backfilled with loose gravel for safety and the Lane was closed to all traffic and it remains closed today.

This was the second ‘event’ to occur at Seawatch at the Shores in five days; on May 26th a fresh water spring had appeared on Lot 3. Due to the quick succession of these events and because of other problems in the past, the DOS retained its ‘go to’ engineering firm, Thurber Engineering Ltd. In Vancouver, and asked for a “complete review of the June 1st sinkhole and previously reported sinkholes, slope instability, springs and erosions within The Shores subdivision.”

Thurber sent the completed report to the DOS’s law firm on July 12, 2012 and that report can be found on this DOS webpage dedicated to the ‘Seawatch Subdivison.’ I highly recommend a look at that report if only for the pictures; however, it is an interesting read, too.

District of Sechelt - Thurber Engineering - Shores Subdivision - Geotechnical Evaluation - July 2012.pdf . (Note: this is a direct link and you will get a pop-up dialog box when you click on it.)

So, what exactly is going on here? Well, here is my layman’s opinion - we are looking at a Whidbey Island scenario.

Let’s go back and isolate this area from that ‘Sunshine Coast Aggregate Potential’ Map in the previous post - that image is on the left below. This area is referred to as the West Sechelt Agricultural Plateau and according to the map legend the gold colour represents ‘Secondary’ aggregate potential and the overlaying dots ‘Moderate’ pit potential. The green dots were testing drill holes performed at the time of the 2001 survey and the white triangles represent existing gravel pits. The ‘blue’ box is the site of an old quarry which Astar Minerals (Pan Pacific Aggregates) claims and where they were blasting and mucking about in 2006-8 and generally causing a lot of aggravation (interesting how aggregates and aggravation go hand-in-hand, isn’t it? The two words are also one after the other in my Funk & Wagnalls). The black arrow shows the exact location of the Seawatch subdivision which abuts the edge of the DOS at Snake Creek in Snake Bay.

The subdivision site map comes from the developer’s own website. In addition to showing lot layout, it has included the topographical contour lines and those lines represent a height change of 1 metre and the closer they are the steeper the terrain. That June 1st sinkhole occurred on the Lane (Laneway) adjacent to Lot 28 and the May 26th spring appearance occurred on Lot 3 (neither lot has a house).

Now, in the Thurber report are two excellent drawings that were both produced for Concordia Homes Ltd. (the parent developer) by Engineering Ltd., in April, 2006 and I have included them below. What they illustrate is a text book example of what occurred on the west coast as the glaciers retreated 10,000 -15,000 years ago.

The blue is marine silt so named because at the time of the glaciers it was under water and became exposed as the ocean levels retreated - at one time the lower mainland was 200 metres under water. The yellow is fine sand and lenses of silt (a ‘lens’ is a deposit that is thick in the middle and thin at the edges) and beige is glacial till and marine till. You can see in those illustrations how the subdivision straddles these three types of deposits.

The second illustration shows the elevation and depth levels of these three types of deposits. At the top, you can see the division between the current phase 1 and proposed phase 2 developments. Seawatch Lane is at this upper elevation and is where the sinkhole occurred; the main roadway (North Gale Rd) is at the L-100 level. Homes are planned below the roadway on the marine silt and according to the lower diagram those are water-bearing layers - and this where that May 26th spring appeared.

What the diagram doesn’t show is the elevation level for phase 2, but there is a picture in the Thurber report that gives you a good idea. Normally, building on gravel is ideal, however, this development and many others on the coast are on the lower sloping sides of plateaus as developers seek out lucrative waterfront properties. In the meantime, those developers building on the plateaus are removing all the forests - forests that would normally absorb the huge amount of rainwater that falls in our temperate rain-forest climate.

I went to Google Earth and looked at different timelines for this development and the result is combined below. You can see how much forest cover was removed before Concordia began; how much Concordia removed; and, how much has been removed in areas nearby the Seawatch development after Concordia started its project. You would think this has to have an impact on drainage and on the underground water travelling along those three major layers of deposit.

So, where does this situation at Seawatch at the Shores stand? Well, it is beginning to get nasty as the Coast Reporter notes:

District investigates Seawatch issues, December 1, 2012

Sechelt starts investigation, developer sues, November 16, 2012

I think there are going to be more of these issues as those with expensive waterfront properties discover their lots are now “wet at the bottom, then rises steeply up” (yeah, I follow Vancouver Price Drop…LOL).

Be forewarned all you who dream of moving to the Sunshine Coast. The SC is an incredible place to live and I still “pine for the fjords” and would move back in a minute if I had the financial resources to do so. However, it has undergone an incredible pace of development in the last decade and there will be issues and some will be major. It also finds itself caught in the middle - is it an overflow for lower mainland growth or is it a source for natural resources - can it successfully be both? Time will tell.

(Note: Concordia Homes Ltd is also behind a Maple Ridge development called ‘The Pointe’ and I have read the market out in the Fraser Valley is in a free-fall. What a nightmare on both fronts. As for its issues on the SC, well, after reading the Thurber report, I wonder if they can ever successfully solve them. I think they have tried which is why after six or years the development is still not finished - it’s a ‘hit-the-mole-in-the-hole’ situation - you plug one leak and another pops up somewhere else.)

Okay, the gravel stories are now depleted...fini...totally, dude!



P.S. If any of the links fail to work in the future, please send me a private message through the board. TIA.