To build a Rastra-block pool, excavate a hole just larger than the pool's dimensions to allow for ample workspace. Most people choose to construct a pool 5 feet deep. For the bottom, either pour a concrete slab or cover the bottom with a rubber liner. Then line the bottom with gravel. Make sure to install a drain and backflow preventer. Lay one section of Rastra block along the edges of the slab, securing the Rastra to the pad with rebar. Fill the Rastra blocks' cavities with concrete. As the concrete flows from block to block, the structure is tied together. An expanding foam sealant is used between courses and at all joints to hold the blocks in place. Knowles recommends waterproofing the blocks by troweling on two coats of stucco. Backfill the space between the sides of the pool and the Rastra block with soil. You can finish the perimeter with stones laid from the top of the blocks out into the surrounding area, or you can grow plants to the edge of the blocks.