Building a tufa cliff

[David Sellars]

Over the past ten years I have been converting a long clay bank in front of our house into a useable rock garden.  The basic strategy is to pry large boulders out of the bank and backfill with coarse sand so that the boulders are retaining sand not clay. One of the last sections of the bank not given this treatment, had very large heathers covering some big rocks.  My wife doesn't like the heather and I like looking at rocks so I started the renovation of this area last weekend.  We are experiencing quite a warm wet January so rock work is quite feasible this month.

I have some large tufa blocks left over from my last saxifrage project so I decided to try and build a tufa cliff.  However it is a tricky design problem to use stratified rock within a chaotic boulder talus slope - a stratified cliff would look out of place.  So my solution was to attempt to construct a steep "rock slide" with pieces of tufa at different angles within the granite rocks. As tufa is so precious I decided to use the tufa quite high on the slope and create a foundation of granite rocks as the first layer or so.

The first picture below shows the clay bank (about 2 m high) with large heathers at the top.  I found a good rock to form a foundation and set it up about a metre from the foot of the clay bank and backfilled with sand.  I levered up a very large white rock and decided to use it as the left side of the tufa cliff as it would be a reasonable colour match.  The series of photos show the formation of the granite rock foundations.  The tufa blocks will go on top, slightly set back for stability.  I will show the placement of the tufa in a subsequent post.

[cohan]

looking forward to seeing the continued project
i'm jealous at the sheer volume of stone on your property...lol

[Armin]

David,
respect! I get backpains just looking at the muscle work and sweat.
Did you move all rocks without the help of an excavator?

[Maggi Young]

Great to see this project, David.

Remember Readers: click on the pictures to enlarge them

[Lesley Cox]

There are of course, some great examples to follow. The late Roy Elliott (Editor for many years of the AGS Bulletin) built a wonderful tufa cliff under cover and grew superb plants there, and then Harry Jans has built what amounted to tufa chimneys in which he grew (grows?) Jankaea heldreichii to perfection, among other plants.

[David Sellars]

Armin:
The rocks have a bit of a story.  When we bought our acre 23 years ago it was a tree covered gradual slope.  We had most of the trees cleared so we could build a garden and also had the machine excavate a steep bank to create different slopes in the garden.  The clearing and excavation turned up hundreds of glacial boulders, some very large so I had the excavator push the rocks over the bank so they slid down to form a rough rock garden.  Over the next 12 years or so, we tried to plant on the slope but it was very challenging because of the heavy clay on the bank. For the past 10 years I have been renovating the bank to make it more suitable for a real rock garden.

I have not used an excavator for the rock garden reconstruction.  I find I can pry up most of the largest rocks using a long steel bar and even move them to different spots on the bank using a fulcrum to slide the rock sideways.  It helps that most of the rocks are half way up the bank already. I have dug up other rocks all over the garden and these I move to to the top of the bank with a rubber tyred dolly. I then roll or slide the rocks down the bank with final rock placement by man-handling the rock or using a pry bar.

[David Sellars]

I have seen pictures of Harry Jans' tufa wall but that method won't work for me.  I recall that Harry's tufa is jammed vertically between several concrete pillars and the pillars provide the structural support.  When building a cliff in the garden I have found you have to be very careful with stability.  I learned my lesson a couple of years ago when I had a slow failure of a cliff. I had to tear it down and start again.  I try and ensure stability of a rock garden cliff by the following strategies:

Use mostly "blocky" rocks that are naturally stable with the largest rocks at the base of the wall
Relatively thin rocks should not be placed near-vertical - they will eventually topple with the pressure behind the wall even with a well-drained backfill.
If the rock is relatively thin, slope it back into the wall.
When placing a rock above a rock, step it back at least the width of a crevice.  This has the advantage of creating a planting area.
Test the stability of the cliff as you build by climbing it and pulling the rocks outwards.  If it is difficult to move them, the cliff will be stable in the long term.  You will also be able to climb it later for planting and maintenance.

The end result from making sure that the cliff is stable is that it has an overall slope back and is not near-vertical. But if the rocks are large it will still have a cliff-like appearance

[David Sellars]

TUFA PLACEMENT

I had decided not to build a stratified cliff as it would like out of place in our chaotic rock garden but as tufa is a stratified rock it is hard to avoid ending up with a layered look.  My design concept was to try and build a collapsing rock cliff or a rock slide.  I slid the first piece of tufa up a wooden plank and placed it as if it had fallen from a cliff.  I then placed another three pieces above bringing them in on a dolly to a point above the cliff and sliding/rolling them down the slope.

The photos show that the cliff is really a bit too stratified for my liking, plus it is not as cliff-like as I would prefer.  So this will not do.  I will have to work on a redesign.

Although the photos show the garden looking a little drab in our wet January there is some colour around so I threw in a photo of flowers on the Viburnum bodnantense to cheer things up a bit.

[Armin]

David,
thanks for your interesting story. Over the years you must have moved tons of rocks
God bless your back!               

[David Sellars]

THE SOLUTION

As ever is to study the rocks.  I had a large tufa slab left over which I thought I could use at the top of the cliff with a lean back into the slope so it would be stable.  As I looked hard at the slab it occurred to me that I might be able to incorporate it into an overhang.  Having an overhang in the tufa cliff would provide perfect habitat below for Porphyrion saxifrages.  It is not just that the rain spoils the flowers in the early spring but they are also less floriferous in our garden unless provided with winter rain shelter. I have built lots of rain shelters which show up in some of the earlier photos but they are time consuming to erect in the fall and to take down in the spring.

The big worry about incorporating an overhang is making it look "natural".  Anyway I decided to give it a go and rebuilt the top of the tufa cliff.  The slab went in below the top layer with one of the heavy tufa blocks on top to hold it in place by weighing it down.  Anyway, the result is that the tufa cliff looks much more natural plus I have a nice overhang to plant beneath.  I also envisage planting Saxifraga longifolia right on the lip of the overhang.

For the design concept of a collapsing tufa cliff, the overhanging slab looks like the next piece to fall but it is actually very stable. The piece of tufa that has "already fallen" has some nice solution features as shown in the close-up photo below.

[David Sellars]

NEXT STEPS

I now have to decide whether to keep raising the cliff or to construct a path along the top with a rock bank above the path.  I first have to construct a rock wall in the dark area to the left of the tufa cliff shown in the photo below. More backfill will be required and the photo of the current top of the cliff shows the sand backfill supporting the tufa cliff very clearly.

Further work on the project will also require moving a couple of rhododendrons which are in the way of bringing stone to the site from above. I estimate that the project will not be complete for another month or so.  After that - planting!

[Lesley Cox]

You're only 22 years old David. Right?

[Carlo]

Hi David...nice to see you on the forums--and relive the photos of your project. Still looks great.......(hard to believe it's been almost a year since Portland...)

[David Sellars]

You're only 22 years old David. Right?

Well I feel only 22 years old when playing with rocks.  Does that count?
Actually I will be retiring in March to become a full-time rock gardener and mountain traveller. I can't wait.

[David Sellars]

Hi David...nice to see you on the forums--and relive the photos of your project. Still looks great.......(hard to believe it's been almost a year since Portland...)

Good to hear from you Carlo.  We are heading back to Portland in April. The first trip of a new career!