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Small excavation with concrete slab and retaining wall

I need small exacvation in the garage under my house completed. There is an existing concrete slab and I need to extend it by 1.2m on one side of the garage for extra storage. I have excataved half of this already (ground is clay), but do not have time to complete it myself. The spoil can be kept on site. The size of the extra slab is approx 2.5m wide x 1.2m deep, 60mm thick. At the back of the slab I will need a retaining wall built approx 1.35m high x 2.5 wide, this will also need sides, but these would only need to be 1.35m high x 1.2m wide approx. The ground where the excavation is, slopes up under the house, which is on poles. The excavated area is not close to any supporitng piles or foundations.