Building the pool retaining walls

The next step is to build the retaining walls that will enclose the pool on three sides. These walls are built upon the footings that were poured just before we broke for winter.

Walls and footings

The walls are built using hollow concrete blocks, which will be filled with concrete, per the design discussed in a previous post.

Not entirely level

The first challenge encountered (aside from moving ~600 ~20kg blocks from the front of the house to the site) was that despite best efforts during the pouring of the footings, a three meter section of the long footing for the lower wall ended up around 25mm higher than it should have. Although it's not essential that the block wall is 100% level (the top 100mm of it will be poured concrete that will be level of on top), the stickler in me demanded that it was.

Varying the thickness of the mortar under the first course (i.e. row) of blocks can provide some offset for this, but the difference is too great for mortar alone. So I manually cut varying amounts off the bottom of around 12 blocks to compensate.

Trimmed blocks awaiting mortar

Trimmed blocks awaiting mortar

Right-angle laser ensures the walls are straight and square

As well as being level horizontally, the walls must be straight and square at the corners
The first course is always the hardest

Although the blocks we're using are mortar-less above the first course, the first course must be mortared to provide a level base for the remaining courses. This is by far the most time consuming aspect of the wall construction.

In addition to ensuring consistent height, the walls must be perfectly straight, meet at the corners (in multiples of block length), and perhaps most importantly, be vertical. Since only the first course is mortared, it must be perfectly horizontal or the wall will will have a lean as the remaining courses are stacked on top of it. This can (and was) partially offset through the use of slivers of plastic inserted between the blocks, but the initial placement was crucial.

Mortaring the first course

Mortaring the first course.

To provide tensile strength to the walls, vertical and horizontal steel reinforcement is required. The starter bars from the footing are continued all the way past the top of the wall to where the bond beam will be. Horizontal N12 bars are placed every 400mm with an N16 bar around the top perimeter.

Retaining wall diagram

Reinforcement details for the two retaining wall types
Remaining courses

Once the first course is complete, the remaining courses can simply be stacked atop the course below. At least that's the theory. The reality involves the use of small plastic chips to account for slight manufacturing differences between the blocks as well as occasional drilling of the blocks to account for less than stellar positioning of some of the starter bars.

Corner of bottom wall

Corner of bottom wall. Top course yet to be laid

Bottom wall and privacy fence/garden bed wall

Bottom wall and privacy fence/garden bed wall. Top course yet to be laid.

Rear retaining wall

Rear retaining wall. Top course yet to be laid

Plastic wedges used for fine positioning

Small plastic wedges used for fine positioning.

The top course is left until later as we need room to backfill the gap between the wall and the pool shell. But before backfilling, we need to fill the walls with concrete.

Core filling

Core filling the walls with concrete is a relatively simple task when you have a concrete pump.

Ready mix truck and concrete pump

The readymix truck pours directly into the hopper on the concrete pump truck.

When filling walls, it is essential to ensure that the entire wall is filled with no gaps. With the aid of a high range water reducer (AKA superplasticizer), a very high slump can be achieved without compromizing strength, allowing the concrete to flow into the wall cavity without forming voids and without requiring vibration.

Our concrete is superplasticized and contains a relatively small coarse aggregate (7/10mm mix), giving it excellent flow characteristics.

After filling all of the pool retaining walls and the rear retaining wall, the walls were wrapped up to cure in preparation for backfilling.