French drain in heavy clay soil?

Our lawn is on top of heavy clay soil, and gets very waterlogged after a bit of wet weather. It's almost unusable throughout the winter and into spring - walk over the same spot a few times and it's churning up into mud.

The lawn is only about 15 feet wide - the first 2-3ft is flat, with the remaining 12ft sloping downward by a total of approx 8 inches. All but the last 3-4 ft of lawn is very squishy under foot, with the flat area being (not surprisingly) the worst. It's not helped by a patio above the lawn, as rainwater runs off here down a couple of steps onto the lawn.

I was thinking of putting a french drain through the flat area of lawn and in front of the steps, before taking it "downhill", but I'm dubious about how effective this would be. As clay is impervious to water, won't this prevent water from making its way into the drain?

Or have I misunderstood the purpose of a french drain? I'd always assumed they were to drain the surrounding soil, but are they really just meant to "catch" surface water travelling over the top of the drain?

Last edited: 06 March 2017 13:51:03