Some thoughts on tunneling
Tunneling into colditz is a great book on this, and there are a lot of notes on narco tunnels in mexico, Afghanistan and vietnam.
Illegal/wildcat mining also can teach you a lot, this is done in Africa, south east Asia
If you are tunneling it's safer to dig deeper and have the tunnel run uphill, simply use set lengths of string hanging from the roof to maintain direction/angle- better still a lazer. you need two kinds of support for a roof, one for weight and one to stop erosion- bed slats and floor boards are the gold standard, line them with plastic. cupboard doors, large kitchen tiles and the insulation from your roof can all be put behind supports to hold back more earth.
try to make an arch shape as you dig through clay, but straight walls and complete boarding may be required in soft soil.
If you make an exit ensure there is a large bend- ideally vertically. This stops blasts being channeled down the tunnel and allows the entrance/exit to be collapsed or defended.
Something to consider is changing the angle of the tunnel entirely at the point of exit to mislead anyone who finds is as to where it originated- if it's in a straight line to your house it's likely troops will just guess where you are.
Of course the best option is simply not to dig an exit, just close to the surface where you can break through later- although time is a key consideration.
If you dig a side/branch for storing things you can collapse/dig it out again as required- cacheing things underground is enormously difficult and an artform in itself- the US army has some decent guides and you will quickly realize there are all kinds of unexpected issues.
How to dig:
Find a digging tool and some way to cart earth. Hammers and axes make good picks, plant pots make good buckets but so do lamp shades and the shells of electrical appliances.
To remove earth use a bucket on a rope, rope can be made from twisting strips of bed sheets. Metal edging/guttering is invaluable to make a chute your bucket can travel down.
You can use a hose to wet the face you are digging through but be very aware water invites collapse.
Clay is a total motherfucker to dig through, I had to dig through several meters in a crawlspace and I pulled half the muscles in my body- the clayline is also where water flows underground and if you make a hole in the clay expect your tunnel to become an aquifer- to avoid this you can take the clay and actually line the outside of the tunnel in the meters preceding the clayline so water flows around your tunnel- it also means you can dig some more nice soft earth and avoid carting the clay to the end of the tunnel- but definitely line the shaft with clay because otherwise it will become a well.
Where to put the removed earth:
for small trenches you may get away with just mixing with water and throwing it out your window and raising the height of your back yard- but this will be visibly apparent until plants grow over it (if they do). Another option is just filling a room with dirt- while not immediately visible it's clear evidence of tunneling, some can always be used sandbagging. other options include trying to flush it into storm-water, hiding it under/inside a small shed or collapsing a structure onto a pile of dirt to conceal it. Tunneling is back breaking work so don't take it far from the entrance unless you want to spend years digging.
If you dig uphill water will flow to the start of the tunnel which makes it easy to remove, you need not be worried about being trapped by a flood unless the tunnel is very long/ low. If you use roof tiles as flooring your life becomes easier.
If someone finds a tunnel entrance one of the first things they will try is pouring flammable liquid into it. This is bad.
The tunnel being dug uphill stops flooding- but also allows fuel to flow to the source of the tunnel. Simply put if your tunnel is short and it's found you are already fucked- however if it's long or connects to a secure underground area it's lucky you dug up into it and didn't dig it directly under your house.
First dig a low escape trench, then a tunnel- if you can tunnel under an adjacent building with a cement slab you can quickly build a large and effective bunker, don't make an exit into this building- in fact you might even collapse the structure so nobody occupies it.
If you dig uphill you will have the option of following the terrain and getting softer soil- or digging straight and hitting clay faster.
Hitting underground structures:
If you hit a gas main just ignore it, if you hit a water pipe just ignore it. not worth the risk. If you hit mains power don't get any ideas unless you have a MA in Electrical engineering and want to arc-weld or make a jackhammer.
Stormdrains are likely to back up and have enough pressure to push water up into your whole system- but if the mains water is out they may be safer and you might clear a sewer pipe with them.