Why there is a gap between the concrete deck and the ground?

That is just a little erosion. The concrete is fine. The dirt under the concrete likely hasn't moved except for the edges. Wind, rains, watering from a hose will pull that dirt away. If you were to stick your hand inside, it would only go in a few inches as the dirt is protected by the concrete. If it worries you, you can always put dirt or gravel in there to prevent more erosion but it really shouldn't get too much worse. Now if you are going to put in a flower bed or soil for grass, the dirt isn't going to pull away anymore at all because it has no where to go. Just remember that if you fill the soil up to the concrete it's going to settle and look like it's disappearing too. Just fill it a bit higher than you want it to be. Or add more soil later on once its settled.

If the warranty is still good, I would call the company that poured the slab to inspect it. Just for your own piece of mind.

Probably not. You're not in Canada or even more north where it gets really cold. If it really bothers you or it does get worse, you *could* consider getting polyurethane foam in there. I think it's called "foam jacking". They put the foam in and it expands to support the concrete. I just don't know how much of a problem it would be for you where you live. I don't know how much rain and snow and frost you get.

When this pad was poured it should have had a footing at least 8 to 12 inches deep and at least 4 inches thick all the around the pad to keep your problem from happening. This is standard. A 3 to 4 inch flat pad should not be poured or your problem will occur.

Never heard of footings for concrete patios, walks, or even garage pads. They just dig down below grade, tamp down the soil, pour in a gravel substrate, tamp it down, then pour the concrete. Seen it done on many DIY shows.

Looks like erosion, you should be able to fill it.

The footing would prevent what is happening to this pad. The footing is not for support but to prevent loss of sand and erosion. If the pad is to be ground level, then dig the dirt out. If elevated, make a footing.

Sorry, Ljgordon! Never heard or seen anything done like what you say. Concrete patios, walks, garage pads are poured over a gravel substrate. I have even seen old cracked patios jack hammered into rubble and fresh concrete poured over the rubble. The rubble becomes the substrate rather than being removed. Footings are installed to support a structure above a pad and would be overkill.

It looks like the base gravel and sand was not extended beyond the pour and the surrounding ground is below the level of the base. An above grade slab like this is prone to erosion like this. Just fill this with base rock/sand (road mix is commonly used) or just fill this area with garden soil. It looks to me like this was a low bid job without any standards for the finish of the exposed surfaces.

If you have the room you could dig down the depth of an edging stone or cinder block, what ever will fit in the space between the concrete and the deck and another 2" of screening for drainage. Put the screening in 2" deep under the edging stone or cinder block and place them along the edge leaving a 1" gap between the stone and the concrete. Pour screening between the stone and the concrete to fill the gap. The stone should help re enforce the soil under the poured concrete and the screening will provide drainage so no more of your soil is eroded. If you allow too much of the soil to erode then yes it will eventually crack but it would have to be substantial for that to happen.

Maybe we are dealing with semantics. Don;t call it a footing. Call it a sand, gravel, and dirt retainer. Due to the slope of the land, I had to pour an elevated pad. As soon as I removed the forms, I could tell that I would lose sand, gravel and dirt on the downhill side. I reformed and poured a sand, gravel dirt retainer. Without the retainer, I would have a pad 3 inches thick on the uphill side and 7 inches thick on the down side with nothing to stop rain drainage from undercutting the pad. Since then, when I pour a pad, I pour a retainer/footing.

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It does not need a footing but proper soil bearing preparation such as 6-8 inches of crushed stone, two inches of quarry process then some rebar and pour the slab. However, in this case the easiest remedy is to foam fill the gap as previously mentioned, here we call it "mud jacking",

What you are posing is enough concrete and under layment to support 100,00 pounds. We are talking about a pad that people walk on. 3" with mesh is enough. What keeps the crushed stone and quarry and sand from being undercut by rain water?

Depending on the soil underneath you only need a sub-base of stone. Where I live it's generally 4-7 inches depending on if it's going to be solely foot traffic or if you're going to have any vehicles driving over it such as at a driveway entrance. Usually you'll have expansion joints (deliberate indentations) within the sidewalk or patio to allow for the shrinking/expansion of the concrete during the different seasons so if the concrete does crack it will crack at these locations where it's less noticeable and not through the middle of a clear section. Separation of the pad and dirt below is normal on the ends when they pull the forms off and don't backfill that area with soil or rock or something to keep it from eroding.

I agree with everyone here except Ljgordon. Ann just had an improper base installed and a bad concrete pour. Everyone has provided great similar solutions. Praises to you all!

thank you for the informative input!

I had a sidewalk done and I think they didn't do it right. They piled sand to level, but only poured the concrete on top, leaving a gap between the bottom of the concrete, sitting on approximately 4 inches of sand. I told them it floods and the sand will wash out. Shouldn't they have at least bordered it to the ground knowing that? The sand will wash out from underneath the concrete.