The pavers have started to drop and the sleeper surrounds have rotted. After pulling away the sleepers I have found much of the dirt to have washed away! How do I go about back filling and reconstructing a stable foundation again please?
Welcome to the Bunnings Workshop community @Honkys_Bar. Many thanks for your questions, I'd be happy to kick-off the discussion.
To begin with, you should identify the cause of the undermining to the concrete slab. As you mentioned, it is potentially due to the soil being washed away and this drainage issue should be addressed with the water being redirected elsewhere. If this is not corrected first then this issue will reoccur in several years time. If the pavers have started to drop, then it indicates that the slab is not structurally sound anymore and should be repaired by restabilising it.
Since this is a paved slab my advice would be to remove courses of pavers from the outside edge until you reach the furthest point of the undermined slab. From the images, it appears that the damaged portion should only be one or two courses deep. You should then drill several holes through the slab and insert Whites 12 x 450mm Hot Dipped Rib Reinforcing Bar vertically down into the slab and out the bottom into the void every 30cm. The Reo bar can be glued in place with Ramset 380ml Chemset 101 Plus. You can then create formwork along the edge of the slab and fill the void with Lanko 20kg 702 Durabed Structural Grout. You might need to hire a concrete vibrator to ensure the mixture penetrates right underneath the slab and fills all voids.
Alternatively, you could remove the courses of pavers as above and cut the damaged portion of the slab away with a concrete saw. Insert Reo bar as above into the non-damaged section and pour a completely new slab on the edges.
Please let me know if you need further advice or have questions on the process.
Thick enough if it is on a solid road base or similar under the concrete which in this case looks to have disappeared. Torn between the idea of boarding up the sides and drilling holes in the top large enough to pour concrete into and then drive reo rod into it, I haven't joined the chemset generation yet or ripping it up and sorting the drainage problem and relaying on a solid base.
Patching may be the quicker cheaper option not sure just from photos shown if it is the best option.
Yes, there was a sleeper acting as the wall. The pic shows the amount of washout. Below the layer of concrete you have narrowed to is a void and then the ground.
Two out of four downpipes on that side of the house weren't plumbed so the water which ran out of the other two downpipes were travelling the first garden level, cascading over a one foot pine retaining wall and straight into the pool surround pavers. I wish I had been more savvy as over the years is moved the pavers, cracked the copping which then has eroded the pool pebblecrete (luckily above the water line). From there it's kept flowing downhill to the set of stairs pictured and continued to wash out the dirt foundations etc. The void looks to be 2 or 3 pavers deep.