Making Artificial Landscape Rocks Using a Hole in Ground as a Mold

Written by david claerr
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Making Artificial Landscape Rocks Using a Hole in Ground as a Mold
(David A. Claerr)

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Artificial Landscape Rocks

Landscape elements for a decorative garden setting often include the use of rock formations to divide sections, create raised terraces or to serve as an artistic counterpoint to the blossoms and greenery. You can create your own landscaping rocks by using moulds created by excavating holes in the ground and then casting with a concrete mix. For large-scale rock formations, a method of hollow-casting, which saves on materials cost and results in a much lighter structure will be discussed in this article.


Dig a few small test holes to find an area suitable for your mould. The best soil for mould formation is either sandy soil or clay, but any soil that is fairly stable when dampened and compacted will suffice. Excavate a hole in the approximate diameter and depth of the rock you wish to create. A hole with an elliptical shape and rounded contours will result in a shape like an eroded rock. If possible, leave a gradual, rounded overlap around the top lip of your hole so that the cast rock will have a rounded base on the bottom edge.

Making Artificial Landscape Rocks Using a Hole in Ground as a Mold
Excavated Hole

Preparing Mold Surface

Dampen the soil in the hole with a fine misting spray and compact the surface by tamping down with the back of a shovel, trowel or the palms of your hands. (If the soil in your area is too loose or grainy to compact, line the hole with a thin, flexible plastic membrane, such as the bags that dry-cleaned clothes are delivered in.)

Mixing the Concrete

Mix your concrete ingredients with water. If you are using a ready-mix, follow the proportions listed by the manufacturer. To mix your own concrete from Portland cement, the proportions are as follows: 1 part water and 1 part cement to 2 parts sand and 3 parts gravel. For hollow-casting, substitute PVA or AR Glass fibres for the gravel. Depending on the air temperature and dryness, more water may be added to increase workability. A thinner mix with more water is desirable for pouring castings, and a stiffer mix is more desirable for hollow-casting. Colour pigments can also be added to the mix if desired.

Casting the Artificial Rock

For smaller pieces, pour in the mixed concrete and allow ample time for curing. For hollow-casting, the thick, fibre-reinforced mixture is applied to the surface of the mould using trowels and spatulas in a thickness of one-and-a-half to two inches to form a hollow shell. This is a labour-intensive technique, but one that can yield superior results in that large and intricate forms, such as waterfalls and terraces can be created that are strong yet relatively lightweight.

Start at the lowest point of the excavation and build upward. As the surface of the shell is filled in to the point where the slope reaches the vertical, add straw packing to support the shell wall. (The packing is removed after the concrete cures.) Continue working upward, adding to the shell wall and filling in with straw packing for about every four inches of wall height added.

Making Artificial Landscape Rocks Using a Hole in Ground as a Mold
Straw Packing for Support

Final Steps

Allow for sufficient curing time (overnight is recommended) and remove the piece from the ground. Rinse the casting with water and remove residue with a scrub-brush. Casting defects can be removed with a rough-grit sandpaper or disc-grinder. Your landscaping rock is ready for placement in its setting.

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