From RODALCO:

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Top one is the pre warning strip, just thin plastic, 4 inches wide ( 10 cm ).
( put in trench at around 30 cm depth )
Bottom one is the magslab. it is 6 or 8 inches ( 15 or 20 cm ) wide and ¼ inch ( 6mm ) thick.
This will be on top of the sand in which the cable is laid. ( 60 cm or deeper )

Comes in 25 metre rolls or pre cut 1 metre sections to be put over buried cables.

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A lot of new cables now these days go in pre drilled heavy duty conduit.
These will have no warning strip above it, If a digger scrapes the ground it will possibly not break the conduit and therefore not damage the cable.
These methods are used extensively in Auckland now in OHUG programmes and for 11 and 33 kV feeder upgrades.
These feeders are laid in the middle of the road and manholes provided for termination joining pits.