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Re: [Shop-talk] plumping question and sandrock sewers

To: "David Scheidt" <dmscheidt@gmail.com>
Subject: Re: [Shop-talk] plumping question and sandrock sewers
From: pethier@comcast.net
Date: Mon, 29 Oct 2007 18:48:24 +0000
From: "David Scheidt" <dmscheidt@gmail.com>
> Not bored, but interested.  Sewers are cool.  I'm perfectly happy to
> let others keep them working though.

So am I.  That's why I work in an office.

Here is something I wrote a few years ago for a fellow bureaucrat at the City 
of Pismo Beach, California:

====================
Sandrock, or sandstone, runs generally below a layer of limestone.  This 
stuff is soft enough that it can be penetrated by hydro-tunneling, but 
most of the tunnels hereabouts were done by hand-excavating a century ago 
when labor was cheap.  Since this stuff is soft, the tunnels tend to be 
Gothic-arch in shape.  Some areas are reinforced with brick or 
concrete.  Some areas have a half-pipe in the invert to prevent 
erosion.  In neighborhoods where you find sandrock sewers, the limestone is 
often only covered by a couple of feet of dirt, making open-cut sewer 
construction impractical in the old days.  In fact, there are houses in 
some areas here which have as a basement floor actual original rock.

The method a private property owner uses to hook up to a sandrock sewer:

The old way is to hire a contractor to go down in the tunnel and dig a 
lateral tunnel called a "drift" from the main tunnel to a spot under the 
property.  Then a well-drilling rig is used to drill a hole through the 
limestone,if any, and through the sandrock into the top of the 
drift.  Sewer pipe is installed in the hole at least far enough to get into 
the rock.  In the drift, an angled splash block is installed to direct 
flow towards the tunnel.  Obviously, this a pretty shaky deal over the long 
haul.  Loose sandrock pieces can block the drillhole.  I have actually 
heard stories of sewer-cleaners using firearms to break such pieces.

The currently-approved way is better.  If an old drift does not exist, a 
new drift must be dug.  A larger drillhole is required.  A PVC pipe is 
inserted the full height of the drillhole and a sweep elbow installed at 
the bottom.  PVC extends on the invert of the drift all the way to the main 
tunnel.  Then the pipe in the invert is covered with concrete.  The 
vertical pipe in the drillhole is grouted in with concrete.  The final 
touch is to block up the drift at the tunnel.  Since Minnesota does not
have discernable earthquakes, this setup should last for centuries.

Needless to say, this procedure is very expensive, especially since all 
material from the drift-digging must be carted to the nearest dropshaft, 
hoisted up the dropshaft, and removed in a truck.  It behooves prospective 
buyers of lots to check sewer availability before they buy the lot, in case 
sandrock is the only available sewer type.  The sudden shock of a $40,000 
sewer bid can't be good for the heart.

I have mentioned that Saint Paul is a river-bluff town.  Our city datum for 
elevations is USGS sea level minus 694.10 feet.  This is based on some 
point at some steamboat landing circa 1850.  I'm in an office downtown in a 
building on top of a bluff.  The city datum at the front door is about 95 
feet.  A nearby storm sewer is essentially down to river level, so the 
dropshafts here are about 90 feet deep.  The 150-foot-deep tunnels are out 
in the Midway district, carrying storm water to outfalls in the Mississippi 
on the west edge of town.

There are also some deep sanitary-interceptor tunnels.  The metro-wide 
treatment plant is located in Saint Paul, so sanitary flows are conveyed by 
a combination of tunnels and conventional pipes from other cities such as 
Minneapolis through Saint Paul to the plant.
============



--
Phil Ethier  West Side  Saint Paul Minnesota  USA
1962 Triumph TR4 CT2846L, 1992 Saturn SL2, 1993 Suburban,
1994 Miata C package
pethier [at] comcast [dot] net    http://forum.mnautox.com/forums/
http://www.flickr.com/photos/pethier
I decry the textmessagization of the American-English language.
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