Shed Base Advice (and flooding)

[StevenF]

Hello,

I am getting a 10x14' shed put in and I'm trying to build a level slab base for it to sit on. I have lifted the existing slabs which were uneven and only laid on sand. I dug down through the top layer of sand, removed a weed/water membrane and some chips which were bellow that. I'm trying to dig around 175mm but it's difficult because the garden is on different levels. Anyway, it rained all of Monday and I woke up to this:

Although the drainage in the garden is terrible, that section of slabbing never flooded before. Will this be a problem or something I'll have to fix before laying the hardcore?

The base now looks like this:

I was planning on laying 75mm of hardcore then a 50mm sand/cement mix (6/1) to lay the slabs on. It is worth building a frame around the hole to stop it collapsing or will it be fine as it is? Sorry for all the questions but it's my first time doing something like this.

Thank you,
Steven

 

[martinfd]

I had a similar issue when putting in a cement base for my shed. I live in a really wet area and if you dig a hole it'll soon fill up to the level of the previous surface. When it rains the water level is essentially ground level. For me it always drains away eventually if you get a few dry days.

The way I looked at it was, if all I've done is remove material, you're not impacting the drainage capacity of the material underneath, it remains the same. The water following on your garden is simply gathering in the point of least resistance (in this case, a hole puts up little resistance!) If you hadn't dug the hole, the same amount of water is still going to have fallen on your land and perhaps it would have soaked more evenly into the ground, but it's still ultimately being soaked up. So if you're going to fill it again with material that WILL allow drainage, and the final level is at least as high as the original ground level (ideally a little above), I can't see any problem - the water will just spread itself out again as it was doing before. Where I put a cement base down, I have had no particular issues with water drainage around it.

Some formwork/frame won't hurt, I'm not sure if it's necessary though.