Hi, new poster here needing a little bit of advice.

I have just excavated a new pond in a trapezoid shape.

Dimensions are;

Longest Width - 3.6m
Shortest Width - 2.8m
Sides - 3.6m
Deepest - 70cm d x 2m l approx
Shallowest - 45cm x x 2m l approx

Liner is in situ on a 25mm bed of sand, old bedroom carpet on top of sand with dedicated pond liner over the base and sides. I have left approximately 50cm minimum of liner around the edges (some sides have more due to buying a 6mx6m liner (was going to have the depths of 1m and 50cm but decided against it due to back killing me!!!!).

My dilemma is the edging of the pond. I originally was going to just lay slabs around the edge to trap the liner and underlay in place, but was thinking of making the pond deeper by standing the 900mm x 600mm slabs on the long edge around the pond. My initial thought was to trench around the pond one spade width wide & 30cm deep, leaving a soil edge of 15cm wide from the edge of the hole to the trench, then to place the slabs on edge around the pond and to concrete in place. Would this be sufficient enough concrete to hold the slabs in situ, with 30cm of slab below ground level and 30cm above (giving another 30cm depth to the pond)?

The liner & underlay would then be pulled over the top of the slab and then tucked down the back side of the slab where it would be held in place by the excess soil that has been dug out of the pond (a 75cm high "fence" would be built around the outer edge of the upright slabs with a soil width & depth of 30cm and finished with a capping stone).

My other thought was to dig a trench, a spade deep & wide, around the pond leaving a 15cm gap from the edge, tucking the liner into this and covering the liner excess with concrete, and then laying the slabs on top of this concrete trench. This way would be easier, but would make the pond shallower.

What are your thoughts on which way I should go? I am erring towards the upright slabs, but an a little concerned that they may topple over time. The latter I am a little concerned that the slabs may come loose or the concrete break and end up with a slab in the pond tearing the liner (as I would like to have an overhang of the slab to hide the liner edge).