An English historical comparison: The Ha-Ha Wall

In a very different context, a type of retaining wall is used to form an invisible barrier. Found on many historic estates, they are a devices designed to avoid obtrusive fencing so when the gentry used to look out to the horizon, they couldn’t see where their land ended and where the countryside began. Furthermore, they keep livestock out of private grounds. In 1712 Architect John James translates from Dezallier d'Argenville's La theorie et la pratique du jardinage (1709):

"Grills of iron are very necessary ornaments in the lines of walks, to extend the view, and to show the country to advantage. At present we frequently make thorough views, called Ah, Ah, which are openings in the walls, without grills, to the very level of the walks, with a large and deep ditch at the foot of them, lined on both sides to sustain the earth, and prevent the getting over; which surprises the eye upon coming near it, and makes one laugh, Ha! Ha! from where it takes its name. This sort of opening is haha, on some occasions, to be preferred, for that it does not at all interrupt the prospect, as the bars of a grill do."

There are hundreds of examples of these walls in England, mostly built in 17th and 18th centuries. They are rumored to have been introduced from France but also link strongly to Chinese gardening ideas of concealing barriers with nature.

Long sectional diagram: view from house over garden and beyond to the countryside

In comparison, a more contemporary example can be found at the Washington Monument. This ha-ha wall though is designed to minimize the visual impact of security measures after 9-11. The one-sided ha-ha is made of granite that reach deep enough into the ground and overlap at just the right points to stop an explosive-laden Humvee. The wall doubles as a seating bench and incorporates lighting. The secondary functions have similarities to the new cor-ten walls at Seonyudo Park.

Although the ha-ha walls are set in different context e.g. topography, density of construction and scale, it is useful to understand how retaining walls are used to control views. It highlights the importance of a visual connection with the landscape.