jacuzzi base prep

Hi all, im, well the builders are prepering a solid concrete base in my rear garden for a jacuzzi to be plased on in my eyes im unsure these guys no what their doing a brick square has been erceted but no plastic membraine has been layed on the inside ov the bricks, is this a must be for concrete is layed, and what and wher does the pea gravel go. Cheers all

Not sure what's being done there.

Step # 1 use spelchek