We have a pond integrated into our patio, and we want to fill it in to recover the space. We’ve emptied out the water, cut out the liner and removed the sacking etc that was underneath the liner. Initially, we thought the pond was lined with clay, but having dug down in the bottom of the empty pond, it looks like the soil is in fact clay. The problem is, that because the clay is impervious, the pond still holds water and starts to fill up again when it rains. What is the best way to fill it in? I was thinking of buying in sufficient Type 1 ballast to fill the space because if I used hardcore, then there will be gaps for water to gather. Any guidance gratefully received. The photo shows the pond after a night of heavy rain.