Underground Cat Tunnel

A safe secure way to getting your cats outside, WITHOUT it being an EYESORE!

Yeah, That means you!

Also, my old guy is so damn fast, I can't get a clear shot of him with my phone and my camera gear is all packed away and wrapped in plastic right now, so deal with it!

Step 1: Apologize

I did not take proper photo documentation of this project.

The Instrctable was not on my mind until I had an overwhelming amount of people ask questions about it, both in person and online!

So, in this case. I'm going to hold your hand every step of the way like a long walk on the beach at sunset!

WARNING! My hands get sweaty.

So grab some gloves, cause we're gonna need them for this project!

FYI, I'm going to use stock photos and downloaded images to help fill in the blanks.

Step 2: Point a to Point B

Pick a location in your yard where you plan to build an enclosure for your furry kitty or kitties!

For this tunnel, we happen to have a basement window about 15 feet away.

For thoes of you across the pond 4.6 meters.

For you space nuts (I'm one as well), that's 1/84,480,000th from the Earth to the Moon.

Step 3: Junk in the Way

So, in my case, I had a chain link fence in the way.

Not to mention roots, sprinkler lines and for some reason an underground collection of coax cables that perhaps someone thought might be worth something some day.

Every yard will be different. You will have to adapt accordingly!

Step 4: Mark and Your Tool

First call your local utility company or city and have them mark your underground utilities.

Once marked, it's your turn!

Make like a Perry Mason episode and draw an outline.

(The first time an outline of a body was shown on a
television series was in 1958 on the Perry Mason television series in the episode "The Case Of The Perjured Parrot")

Bad reference. I know!...

Now, grab your favorite digging apparatus backhoe, shovel, trowel, kitchen spoon, screwdriver, stick or what ever suits your fancy.

My favorite digging tool is somebody else personally.

In this case, I was not that lucky so I opted for a shovel.

Step 5: Length Height and Width

We all know Length Height and Width are important. Even if they assure you it's not!

So, now that you have your outline, figure out the size of the materials needed.

Measure your kitty at standing height assuming it's an adult. We don't need to get the police involved here!

Our biggest cat is about 15" standing height.

And this is a tunnel, and cats can duck.

Just remember ducks can't cat! NEVER FORGET THAT!

So for this project we went with 12 x 12 x 1.5" concrete pavers for a whopping $1.82 each.

As this is 4 sided and 15' long with 12" pavers do some math.

15 foot tunnel needs 15 ,1 foot pavers x 4. If my math is correct that's $60.

Since we are on the subject of math don't let it intimidated you! Math is as easy as pi!

Another bad joke... Sorry. Was despratly attempting not to make a girth joke...

Step 6: Sticky Time

We need some cantilevered support! And bricks are just the anchor we need.

Since this is underground we need to prevent all of the soil from pushing in on the walls of the tunnel.

I spend some time manufacturing industrial concrete retaining walls in my day, so I'm stealing some Ideas from the anti-geological movement earth holding walls.

Bricks are overkill, so that should work.

So we grabbed 30, 7.75 in. x 4 in. x 1.75 inch bricks. Cause sides...

Take 30, 12" pavers and 30 bricks and use some good construction adhesive to bond them together. Brick in the center is best.

If you have means of mechanical fastening, then by all means do so!

Let them set as required as per the directions on the tube.

Step 7: Back to the Hole

So we now have a hole that's 1.5" (paver) + 7.75" so for some wiggle room we need the hole to be about 10 x 2 times larger than the paver.

So we have a hole that needs to be about 3 foot wide.

As for the depth, that's 12 inches + (1.25" x 2) plus some soil above for the patio base. So we need a 17" ish hole.

Step 8: Can You Dig It?

We have our dimensions, so get to it.

Be ready to encounter all sorts of randomness.

Get anything out of your way that needs to be removed. Re-route sprinkler lines and what ever else you need to do to make this project a success.

I dug my trench out and dug under the deck.

Luckily, my window well was framed with a small section of no structural wood that could easily be removed giving a large enough opening for the cats to access the tunnel.

Step 9: Nerdy Math and Water

It's a tunnel.

As water dosen't defy gravity, we need to do some prevention.

Pick a spot where you want the tunnel to be the lowest.

Dig a hole in that location.

FYI more math is involved.

I'm not going to get into all the math but Google can help you out with that and snow fall.

Mine will be covered, by an enclosure and the deck so I just need to calculate surface water and roof water/ snow.

Determine soild type and soil permeability (how fast the water drains through soil)

After you have all this done, determine the size of hole
Dig a hole according and calculate. For displacement from rock. This is known as a dry well.

I'm attaching a screen shot of missplumeau's Instructable https://www.instructables.com/id/55-gallon-barrel-dry-well/ . There is a good explanation in there about dry wells if you want to know more!

You can skip this step if your cats are certified and experienced cave divers!

Step 10: Dry Well

As I have super sandy soil my well was basically non existent.

A simple hole, followed by some weed mat filled with rocks and we where on our way.

Now, you need some tubing. I used PEX because I have a ton of extra in my garage.

Cut more than you need, push the tubing into the weed barrier and zip tie it to hold it firm. On the other end, use some more weed barrier to prevent soil from getting in the end.

Image us obvously an exageration.

Step 11: Grade and Set

Since we now have drainage in place, set your first paver. Ensure that it drains at least .5% grade to the low spot (your dry well).

I went with 2% to get the water moving quickly. Surface tension prevents water from draining until around 2%.

More math!

Calculate the slope. Each foot of elevation drop over a 100-foot length is 1 percent. Therefore, it takes 2 feet of elevation change over each 100-foot length of a swale to create a 2-percent slope. If the distance is 10 feet, you'll need a fall of 0.2 feet (roughly 2.5 inches) to create a 2-percent slope.

Step 12: Cube Work

Now that we have proper grade it's time to make each section in place.

Set your first bottom piece.

Tamp to make sure drainage and surfaces are at the right grade.

Next apply some more construction adhesive to the edges.

Grab two of thoes pavers with a brick stuck to it and ease it down onto the adhesive.

At this point I like to take another paver and drop it on top to help align the sides and keep it square! (Not glued)

Now continue this step for every single paver, ensuring liberal amounts of construction adhesive are applied wherever the pavers meet.

This means the next bottom paver is to be set on the ground and glued to the other bottom, the sides glued to the bottom and the sides glued to the sides we just made.

Remember to leave the top pieces unglued so we can access it later!

DON'T WORRY ABOUT GAPS! we will take care of this later.

Also, When you get to the center where your drain tube is you should have a small gap at the bottom of your pavers. Feed the drain tube I to this joint and seal around it.

Remember you want keep it low so it drains!

Step 13: Tunnel Ends

Now that you basically have your tunnel built it's time to think about ends.

Again pavers work just fine for this.

I personally used another paver with a brick on it for the ends and glued it to the sides of the tunnel closing the ends off.

This may be different for your application.

Step 14: Post Dry Time

Now that your adhesive in the tunnel has had plenty of time to cure. It's time to do more sealing.

Remove the tops of your tunnel and set them aside for later.

Scrape any excess adhesive flush with the pavers.

Use your construction adhesive to make a half inch bead where possible where any pavers meet.

Now take a 2" putty knife and spread it evenly on both the inside and the outside of any exposed joints.

Some of the tighter areas you may want to apply construction adhesive to your knife first for application.

Step 15: Gaps

Any large holes in your pavers due to angle or sweeping turns can easily be filled like this!

Simply use some concrete masking or duct tape to apply to one side. Use adhesive to fill liberally.

Allow to cure, remove the tape and do the same on the other side.

Don't do it like this guy in the picture!

Step 16: Set the Top

Now it's time to set the top just like the rest. Lots of adhesive on the top of your sidewalls and seat the top down into it to make a good seal!

Every time you place a top reach in and use your putty knife to seal any gaps between the union if the pavers, do this again on the outside.

Again, tape can be applied at this step to help close large gaps

Step 17: Dry and Fill

Let your construction adhesive cure fully.

Once cured, start to back fill!

Make sure to pack each layer of soil as you fill. Every 3"-6" is best.

Add water as needed to ensure the soil is where it should be and well packed!

Once filled spray water over the top to help further pack any loose soil.

Step 18: Fin

Now that your backfilled, level the soil with a good rake.

Feel free to seed over this or add pavers/concrete/rock/mulch.

And there you have it. A good old fashioned cat tunnel just like your great grandma use to make.

The only thing left to do is build the enclosure and let your cat explore the tunnel so they feel comfortable.

Congratulations! You are now officially a qualified applicant for the Boring Company. Elon Musk would be proud!