Retaining Walls in Waukesha County

Retaining walls that drain from behind.

A retaining wall is a drainage structure faced with block. Almost every failed wall failed because it was built the other way round.

Walk-out basements, graded subdivisions and lots that fall away at the back mean this county has a great deal of retained ground. Some of it is holding well after twenty years. Some of it is leaning after six, and the difference is behind the face where nobody looks.

Water is what pushes a wall over

Soil behind a wall gets wet. On clay it stays wet. Wet soil is heavier than dry soil and exerts far more pressure, and when it freezes it expands and levers the wall forward. A wall with a drainage path behind it sheds that water and never develops the pressure. A wall without one is being pushed every winter.

That is the whole mechanism. Everything below follows from it.

What belongs behind the face

• Free-draining aggregate against the back of the wall, not the clay that came out of the excavation put straight back in.
• A drain pipe at the base, with a fall, and - the part most often left out - somewhere for it to discharge to. A pipe that drains into the same wet ground achieves nothing.
• Separation so fine soil does not migrate into the aggregate and clog it.
• Compaction in lifts as the backfill goes in, not one pass at the end.
• Geogrid reinforcement once height passes the point where the blocks alone cannot hold - and that point arrives lower than most people expect.

Consider splitting the height

If there is room to do it, two lower walls with a planted terrace between them will usually serve better than one tall one. Each carries less load, the engineering threshold may never be crossed, and the result looks like part of the garden instead of a piece of civil works. The bench between also gives you planting, which a single high wall simply does not offer. A low wall at seat height doubles up in the same way, which is why it usually gets drawn together with a fire feature rather than after one.

Near water, the county has rules, and they are specific

If your parcel falls under the Waukesha County shoreland ordinance - around Pewaukee Lake, say - retaining walls are treated differently from other structures, and differently from how neighbouring counties treat them. Three points matter:

• The general shore setback is 75 feet from the ordinary high water mark - or the floodplain or conservancy district boundary, whichever is greater.
• A wall closer than that may be permitted only where it is necessary to abate a known and identified soil erosion and sedimentation problem. It is a remedial provision, not a design option.
• Setback averaging cannot be used to place a retaining wall closer, and a retaining wall cannot be used as the reference structure to average anything else closer - including another retaining wall.

Patios and decks are treated more permissively than walls in this respect, which surprises people. The detail is in the shoreland rules piece, and Delafield has its own overlay requiring 150 feet.

A wall that has already moved

A lean, a bulge, or courses stepping out of alignment all say the same thing: trapped water. Take the face down and rebuild it without addressing that and the new wall follows the old one's timetable exactly. What we quote on is the cause, established first - see repair and rebuild.