Won’t really matter if the wood is hard or soft, the size Of the lumber/timber matters more, as long as it is built correctly it will hold the weight of water back. Here in the u.s., railroad ties/ railway sleepers are made of pine(softwood), but are treated with creosote(oil based treatment), stuff lasts a loooong time with that treatment. I have built raised ponds out of CCA treated pine 6x6’s, and those have never budged much less failed.
In my younger days, I had to work with new creosote treated lumber...hate that stuff, if you got it on your skin on a hot summer day..... Yiiikes, Would feel like your skin is literally on fire, don’t even like looking at the stuff gives me a twitchy eye...but I digress.
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The sleepers I’ve got are 8 x 4’s and treated with tanatone 3950. Thanks for the feedback, really appreciate guys