Land drainage

The usual suggestion here is to start here:

Lots and lots of information. Very experienced author. There are specific sections on drainage which should at least get you going.

I just dug a trench with a gentle slope down towards the drain, then installed some 40mm plastic pipe with plenty of holes drilled in it. Around this I put lots of large stones, added smaller ones on top then finally course sand with soil on top of that. The pipe must also slope down towards the drain. Works well here.

Installing a french drain on the uphill side of the retaining wall should help. This is a trench that you line with filter fabric and fill with free-draining material such as single size gravel or crushed stone. Near the bottom of the trench is a porous pipe that takes away the water.

The filter fabric prevents the fine soil being washed into the french drain and clogging the free-draining material, or blocking the pipe.

The pipe should be laid to a gentle fall and connected to a nearby drainage manhole with a non-porous pipe.

There are more sophisticated options with special filter fabrics that are laid against the back face of a retaining structure, but a simple french drain is probably all you will need. If done properly, the system will be virtually maintenance free.

Google on "french drain".

I found that filling a one foot deep ditch with gravel did the job without the need for pipes or fabric. It worked fine for 18 years until we moved.

Jonathan

The traditional french drain had no pipe in it. The first french drains I did, back in the 1970s, were as you say.

It will solve the ponding problem, but then the problem is, where does the water go? The flow will be concentrated wherever the french drain ends, and if it has nowhere to go, it will pond. All you have done is solved one problem and caused another.

The purpose of the pipe is to collect the water from the french drain and put it into the drainage system.

As for the fabric, in some types of ground, it may not be necessary. However, in many types of ground, it is the only way to get a french drain to work. And even where it isn't necessary, it may be desirable to prevent leaching of fine material which alters, among other things, the ability of the ground to retain nutrients.

Mine was in heavy clay but it did not appear to pond. I assume the drain held enough water for it to be able to drain into the soil over time.

No need to drill pipe, a farmers' shop will have flexible corrugated hose with holes in, specifically for land drains like this. Over- sizing it isn't a problem unless it's shallow and heavy-traffic. Narrow stuff is available too, if it's an area that seels to market gardeners with greenhouses.

Thanks folks for all this helpful info. It had never occured to me to lay the drain behind the retaining wall, but that makes perfect sense.

Keith