Garden Renovation Primer: Building a Sunken Garden

Under the old mantra that you can’t make an omelet without breaking some eggs, we are about to break a few dozen in starting our new sunken garden.

Why, you may ask, build a sunken garden in the first place? Good question, to which the short answer is: drama. In the same way that architects use changes in level to define space indoors, we use the same theories outdoors, for the same reasons. Raising, or lowering a section of the landscape defines a space in a way that no other method, such as a fence or hedge could, allowing a clear delineation of purpose without blocking an area from view. Changes of level also provide for a much more fluid and dynamic experience as the user moves through the garden. Yes, I know, that sounds like a lot of architect-speak, and it is. In plain English, sunken gardens are visually effective and fun to be in. Here in our project, the main reason for sinking a section of the garden is to separate the small games lawn from the rest of a flower-filled, romantic landscape.

Unfortunately, for something so simple in principle, a sunken garden turns out to be a very complicated creature. Not only does it have to retain a vast amount of weight (the whole yard, in fact) but improperly constructed, it can quickly turn into an underwater garden with the first heavy downpour. (This was particularly true at this site, as the entire yard sits on a glacial clay deposit, with next to no natural drainage.)

The style of the wall construction is also extremely important. Retaining walls can be made from all sorts of materials: wood, cut stone, natural stone, even concrete. Which you choose will make a very large difference to the final aesthetic. For the Cambridge project, we’ve chosen thermaled (half-smoothed) granite. Essentially granite “tiles” twelve inches wide, 2″ thick, and of random lengths up to 4′, these pieces of stone were designed to mimic the antique granite blocks that form the foundation of our home. Historically, a garden wall like this might have been made out of solid stone blocks, but these days we rarely do that for cost reasons. The under-layment of our wall is double row of concrete block, set on a concrete footing, to which granite facia pieces and capping stones will be applied. It sounds a bit complicated, but it really isn’t, as you will see. (Think how kitchen or bathroom tiles are mounted, and you begin to get the general picture.)

What is true is that it’s a big mess to construct.

The fist stage of the process is to rough out the excavation and start digging:

Here’s another shot giving you a better idea of the extent of the excavation. You may be wondering what happens to all the soil. Well, had the soil been of a good quality, it might of been reused on site, or even sold. But unfortunately for us, despite it’s deep color, this is mostly clay. In fact, the entire yard sits atop a hill of clay. (In the Colonial era there was a brick kiln just down the street.) So for our purposes, everything that comes out of this hole needs to be hauled away, and that’s a lot of hauling: the walls themselves are two feet deep, and they sit on a concrete foundation that extends down a further several feet…

Below you can clearly see the structure of the walls: the footing has already been poured, on top of which sits a double concrete block wall, filled with aggregate, to which the 2″ thick granite pieces are attached with mortar. The white PVC forms the drainage system, which in our case leads to a large drywell on the far edge of the property. The large gaps in the wall you see are for the solid granite, oval stairs, which haven’t yet arrived from the quarry in Vermont.

Now, as the walls take shape, the men slowly work themselves out of the whole, completing the structure from front (house-side) to back (driveway side).

Ah, a change of plans! You may remember from my earlier articles that I had proposed a water feature for this garden, but that the clients nixed the idea on the initial plans. However, as the garden evolved they had a change of heart, and asked me how they could incorporate some water into the landscape. I had just the idea: we cancelled the far stair leading down into the games lawn, and instead I designed a fountain that would have the same shape as the stairs, but in negative space. I know that’s a little hard to envision, but you’ll see what I mean in a moment. Below, the guys are forming the walls of the oval basin in concrete. (We chose to pour a concrete basin, by the way, rather than using fiberglass or liner, for longevity’s sake – this is not the kind of thing you ever want to have to replace!

I suppose I should also say a word or two about design changes-on-the fly like this one. Unlike building a house, where one small alteration to the plans can send the whole project spiraling into a budget abyss, building a garden allows you a certain level of planning flexibility, if you move agilely enough. I like to work this way, if the clients are amenable: If you are a good designer, you gain a vast sensibility for a project as it progresses, and sometimes that means you (or the client) suddenly realize an element of the design can be improved upon.You have to be flexible however, and allow for the idea of the design maturing as the work progresses. If you’re the type of person who needs to have every “i” dotted and “t” crossed before you start, (or if you are on a very tight budget) then this kind of thing can be very nerve-wracking and isn’t for you. Work methods are important though, and I always discuss this process with my clients beforehand; some like the idea, some don’t, and we can work either way. (I do, however, think a flexible mind-set produces a better end-product.) In our case, altering the stair and inserting the pool was a cost negative proposition, and to my mind, really enhances the pleasure of the final design.

This shot gives you a good idea of how the stairs are constructed. They are quite complicated really, and had to be hand-cut from a template we supplied to the quarry. From top to bottom: a granite partial oval on ground level, then a solid half oval, upon which sits a solid granite full 2 x 4′ oval (you can see one on the left) and then finally, behind that, the solid capping stone with a half 2 x 4′ oval cut out of it. The two guys in the blue to the right are about to strap and secure the solid capping stone piece so the machine can lift the granite into place. Each section weighs well more that a thousand pounds.

Here’s a view of the finished stair, with the footings of the summer pavilion in the ground to the rear. Now you can clearly see the four elements that make up the stairs. You can also see how the 2′ wide granite capping stones give the appearance of a solid stone wall, at a fraction of the cost.

And so, a few days later, with the stonework completed, and after a special sand/compost mix is added to insure good drainage, we have the start of a games lawn! From here, you can get a better idea of how the oval pond mimics the exact shape of the stairs, except in reverse, with a concave rear wall.

A view looking back towards the house. From the first picture to last, five weeks have elapsed.

And finally, another view back towards the fountain, with a trial urn & fountainhead in place to measure scale. Next add the plantings, the classical summer house, and allow a year to go by, and….

This is what you get!

Cambridge garden



Garden Renovation Primer: Building a Sunken Garden — 5 Comments

  1. It is really interesting to see the sunken garden being created. The owners must be delighted to see the garden evolve to this stage. Keep us updated !

  2. I love watching the garden grow too. As a person with less than none artistic/spacial ability I’m always completely awed to see a design take shape. And completely thrilled that there are people out there that know how to do this. i.e. design and create.

  3. I already have a fairly big pond which is now too big for to maintain.
    Do you think if I drain all the water out and fill it with rubble etc. place a new liner into the area and make a sunken garden would be easier to look after.

  4. My husband is not able to care for our pond now and I am considering what to do with it, possible a sunken garden would be easier to manage.

    We have the filter station and a waterfall le to care for it.but it all needs attention
    and I don’t feel like spending a lot of money as I personally will be unable to maintain it

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