[time-nuts] Re: in-ground clock room

[Lux, Jim]

On 9/8/21 6:54 PM, Tom Van Baak wrote:
> I am considering a below ground "clock room" away from the house. This 
> will be for some low-drift quartz oscillators and also a couple of 
> precision pendulum clocks. The goal is long-term, unattended, and very 
> undisturbed operation.
>
> For scale, assume the room is 1 meter × 1 meter × 2 meters deep. So 
> that's vastly smaller than digging a basement, but much larger than 
> drilling a 8 inch round pipe. Digging down gives some natural 
> isolation and temperature regulation. A couple tons of concrete gives 
> high stability vertical walls for the pendulum clocks.
>
> If any of you have personal or professional experience with the design 
> or construction of this sort of thing, especially experience with 
> precast (utility) vaults or poured concrete, please let me know.
>
A few years back, we were looking at building a "test rubble pile" for 
search and rescue applications (actually for testing equipment that 
would be used for this).

For the depths you're talking about (few meters) the easiest approach is 
to get someone with a backhoe to dig a pit, then use the backhoe to 
carry the precast vault, drop it in, and you're done.  If you were going 
more than "backhoe depth" you're looking at something like an auger, and 
they can drill any diameter (up to about 4 feet) and any depth, as long 
as they can get the truck into position (same as the backhoe).

I went around looking at the precast concrete vendors - they're 
available in any size you want. You can also just sink a drain pipe of 
any size vertically, and fill the bottom with concrete. The vaults often 
come with "knockouts" - places where you can bring conduit in by 
knocking out the plug of concrete (it's cast with a thin ring, so you 
hit it with a sledge) - the conduit (or duct, or sewer pipe) to vault 
seal is done with a variety of goops. Some of the ones I've seen look 
like tar (the same stuff used to fix roads, roofs, shower pans), others 
use some sort of foam.

The prices for this stuff are fairly standardized - your local utility 
or roads department probably has a "standard bid list" for most of the 
more common items that they use for doing estimates. Caltrans certainly 
does (a 48" precast concrete manhole is about $1500). So that gives you 
a ballpark when you start calling vendors.  The big driver for most 
people in a "residential" environment would be access.  It's one thing 
to have a double flatbed show up with a bunch of concrete pipe on it at 
a business, totally another on a winding residential street.  We had 
stuff delivered on a flatbed truck with a small crane - but they could 
just pull up to the site, sling the vault, and lower it to the ground, 
and then the backhoe guy moved it.   Going up a driveway, around the 
corner, avoiding the fence, etc. would be different. Or, A *really big* 
crane - I've seen that done - 80 or 100 foot boom in the street - they 
pick the load up, lift it up and over the house and place it in the back 
yard.

https://precastconcretesales.com/manholes/

https://www.columbiaprecastproducts.com/products/precast-manholes/

https://www.gardenstateprecast.com/pdf/2019-price-list.pdf

Google "precast concrete manhole" and select accordingly.



You're looking at the "more than $1000 probably less than $10k" range, 
all done in a day, plus a day of prep and a day of cleanup.

Backhoes (in SoCal) are going for $300-500/day plus the operator 
($50-60/hr).. Or you can learn <grin> - it's hard to do precise work as 
a rookie, but digging a hole is pretty easy to learn on your own.  And I 
just looked it up, the smaller skid-steer bobcat type could probably dig 
a 2 meter deep hole, and you're less likely to do major damage like you 
can with a full sized unit.

Home Depot actually rents them.. 2-3 ton miniexcavator is $359/day , 
$1005/week

https://www.compactpowerrents.com/rental-equipment/mini-excavator/25-3-ton-mini-excavator/
You might find someone who's usually doing stuff like pools and spas, 
and will dig your hole for you. In any case, probably not more than $1000


One other source for design is "storm shelters" - FEMA has published 
pre-engineered designs for a variety of inground shelters (often, 
installed below a garage)


One final thing - even if it's waterproofed, it *will collect water* 
either by condensation or seepage. You might need to make your clock 
vault have some sort of dehumidifier and/or sump pump.


The other alternative is to "build a basement" - dig the hole by hand, 
pour a slab, and either pour walls or stack blocks and pour into the 
cavities.  1x1x2 meters is sort of doable.. figure standard 8x16" 
blocks, so the "hole" is going to be 16+39=55" square - figure 5 ft.  
That's big enough to stand in, but there is a significant cave-in 
hazard, If you're down 2 meters, and the side collapses on you, you'll 
probably die before they can dig you out. Or, your soil is sturdy 
enough, you manage the risk.
I don't think you could dig 2 meters down with one of those small bobcat 
backhoes or excavators, you'd have to dig a ramp, stack your blocks, 
then backfill.


> In case this gets too off-topic for time-nuts, off-list email to me is 
> fine (tvb at leapsecond.com).
>
> Thanks,
> /tvb
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