Sub-surface drainage put simply is under ground drainage that directs excess water away from below the ground. You need a trench that has 'fall' in it, a socket agiline, gravel and this is wrapped in Geofabric for silt retention.
Excavating around existing pools, in tight access areas, close to deep trenches or services like natural gas take a skilled operator and contractor with years of experience to assess the site and the conditions so no accidents happen.
Extreme Hand Digging
Some times when you cant get a machine in you have to go back to basics and do it by hand.
We had to dig down 1.4m deep for an engineer inspection and found rocks that had to be dug out.