A couple of holes will fill up quickly with detritus and block. Suggest you
get the builders back to fix their error. If this isn't an option, I would
drill out enough slabs to install a small gully (150mm dia) use a fence hole
borer hired from hss to create a 2 to 3 foot deep hole, install drain with a
length of 110mm plastic pipe on bottom so water going into gully top has to
travel to bottom of pipe. May be worth filling pipe with 10mm pea shingle
to within about 100mm of surface.
BUT best option is to get them back to do it right.
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