Making a pond which is strong enough to defy the heavy moving dirt

Hi when excavating a soil area pond Hole the edges of the soil will have a tendency to collapse depending on the soil structure if not correctly installed. When the hole is dug make sure the floor and sides is clear of any sharp edges that will pierce the plastic liner. When fitting the liner into the hole fill the bottom of the pond with 1 foot of water and then use your hand to compact the soil down under the ledges of the liner. Then fill the liner half way with water and compact more soil with your hands pushing and compacting the soil Then fill to the top with water and repeat Pushing soil down. If this hasn't been done the pond will fold in but you don't need to use a retaining side if this has been done as I've said hope this helps.

It partly depends on the quality of the tub and probably best to use a good quality liner for most ponds. This way the pressure of the water pushes against the surrounding soil. In all cases some method of securing the rim of the liner or tub is necessary. My guess is that the hole was dug too big for your tub which, by the sound of it, is pretty flimsy and buckled as the pressure of the water pressed against the void beneath the tub.

Yes that's true they should of put lining down and around it to stop the mud from moving

Yes They had should strengthen the edges by concrete.