Creating an integrated plant bed and storage cupboard

Where there is water, there should be plants. To this end, the design includes a generous flower/plant bed at the shallow end of the pool. The design is such that the top 600mm or so is plant bed, while beneath that we have a large storage cupboard, accessible via a door from the lawn below. The final product should look something like this:

Plant bed 3d model
Computer model of the plant bed / cupboard design.

The design was quite easy. Working how to build it simply and safely was less so.

Dirt weighs a lot. Wet dirt more so

Earth weighs anywhere from ~1200 kg/m3 for loose dry earth to ~1800 kg/m3 for wet clay/mud. The soil in our plant bed will be <500 mm deep, so that gives us 900 kg/m3 at the top end. Add 100 kg for plants, and we have 1000 kg/m3 to support, or 9.8 kN/m3.

To support this weight, the base of the bed/ceiling of the cupboard will be made of compressed fiber cement sheets. These sheets are extremely strong. They are manufactured from Portland cement, finely ground silica, cellulose fibres and water. After forming they are compressed to a high density then cured in a highpressure steam autoclave. resulting in a durable, dimensionally stable product.

After reviewing the manufacturer's specs., we're using an 18mm product. At 400mm joist spacing, it will support uniform loading of 20.1 kN/m3, a comfortable margin over our expected soil load (plus few of people standing in the bed).

The walls are 190mm solid concrete with steel reinforcement on the rear side and 150mm on the front. Their strength is well in excess of what is required to retain the plant bed soil. Unlike the prior concrete work, we mixed the concrete for the garden bed walls onsite, given the smaller volumes required.

To ensure adequate drainage, 50mm weep holes were drilled through the rear wall at regular intervals.

Form work for garden bed
Form work for end wall and seating area prior to pouring concrete.
Garden bed wall under construction
Garden bed area prior to surrounding wall construction.
Drainage weep hole
Weep holes in rear wall for drainage.

To support the floor, 90x45mm ledgers were attached to the walls by way of threaded rod, which was in turn chemically anchored (i.e glued with crazy strong glue) into the walls. Several 90x45mm joists on the sheet edges complete the support framework.

Mounting the ledgers

Using sleepers to support the ledgers while marking the wall for drilling.

Floor ledgers and joists in place and walls partially waterproofed.

The cement sheets were then put in place and secured with galvanized nails. The sheets weigh 30 kg per square meter and so maneuvering them into place was somewhat challenging. Once secured, a polyurethane based sealant was applied to all joints and then the entire area was waterproofed with a bitumen based agent.

Garden bed
Completed garden bed prior to full waterproofing.
Cupboard
Completed garden bed cupboard from below.

The cupboard below utilizes the wall footings as a floor and provides a very long (~4m) area that can be used to store pool cleaning equipment.

Skipping ahead a little in the construction timeline, below is an image of the final plant bed, before planting.

Final plant bed
Completed plant bed after tiling work.
Final cupboard with door
Completed storage cupboard beneath the plant bed after door and tiling work.