Septic Tank Riser Construction — Part 1


This all started in November, 2011, a few months after we purchased the property.

The septic tank lids were to be exposed for emptying. That was completed, however, to prevent a repeat of this exercise, a means of reaching the lids without archeology would be useful. I am an electronic engineer, not a civil engineer. Man has been building for millennia, electronics for slightly more than half a century, so some of this was done by intuition and as per the civil engineers, everything divided by six for a margin of safety. We used the best bricks were could scrounge, mixed the strongest concrete (3-2-1 rather than 6-4-1) and added in binding agents and waterproofing compounds. We even made sure the bricks were level, experimented with ferro-cement methods, added in brick force every four to five courses, and put concrete between bricks with reinforcement. When that was not good enough, we added in columns, and finally added on a reinforced roof.

If not for the thought of unpleasant odour, it might also double as a bomb shelter, being much stronger than the tinder and kindling construction methods common in the housing industry (where the first woodpecker will destroy society as we know it).

Widening the hole and view of tank end

To add a riser without knowing what the loading forces were for the tank, I decided to build along the edge of the tank. A foundation would be needed, but then there was a pipe exiting the end of the tank.

Between the inspection pipes, the lids are now covered by rubble that falls into the pit when climbing in and out. It was a continual nuisance and more than one set of steps was built, but the rubble was loose and crumbled easily. The gum pole was for climbing in and out of the pit with equipment or buckets of cement.

Where to dump the fill?

The lid exposure was unpleasant, but to make the hole big enough to build a riser along the edge of the tank without the walls of the excavation caving in, well over 150 wheelbarrows of fill were removed. This rubble was placed in the back garden. A little while before this I had dug over the back garden twice to remove several generations of weeds, as well as bury almost forty wheelbarrows of wood chips. The wood chips had started to decompose and we were ready for the big vege garden. Anyway, that would have to wait. The top soil was placed one side, and the fill piled up far away from the excavation. This was all done manually, as there was no access to the propety for large machinery due to the garage construction and access paths along the side of the house. No doubt something that was forgotten by the builder.

The soil on the plot was fairly thick clay. In summer, the 44°C temperatures baked it as hard as a brick, while in winter the drizzle (a few times a week) made the soil soft. When the surface was scratched, it was a horrid powdery mess, but add water, and then almost pure clay good enough for pottery. You would have to remove the rocks. So here it was, and during the dig, a respirator was worn to add to the discomfort.

Some of the excavated rubble
Another view of the excavated rubble


During the dig, the walls threatened to cave in, so the sides were made less steep. We dug through several tree roots, the most surprising being the pepper tree. It has a distinctive smell, just like the leaves, and it was almost twenty to thirty metres away. The soil was still soft along the edge of the tank, so we assume that a rather larger hole was dug in the building stage, then the tank lowered into the hole by crane. We would later find out that the hole was fairly close to the size of the tank along the length as there was a bit of a “land slide” during the dig along the right hand side length. The tank was not supported along the length as the empty space was never filled — possibly to avoid manual expensive labour.

Foundation for end of tank

The foundation was started by placing cement beams along the base. They had come out of previous earthworks to keep the footpath in place, so provided a decent base. A thick plastic construction membrane was placed on top of this. Reinforcement was placed inside the trench, a couple to “top hats” put under the reinforcement to keep it from sinking to the bottom, and then a concrete pour. The foundations were covered up to harden.

The foundation went under the pipe feeding into the tank. In our past observations of drain pipe usage, we noted that many gutter downpipes used these thin walled pipes, which would then breakup along the pavements. The wall would build around this pipe and allow for some subsidance. The reinforcement was bent up at the ends to attach to the columns later.

Foundation for end of tank

Just poured. Taken from the top of the tank, next photo is 180° offset as it was taken from the edge of the excavation.

Foundation for end of tank with covers

A day or so old. Covers removed for the photo. The formwork is still in place. The covers are against the side wall of the big dig. Tank is at the top of the photo.

Wall construction

The rubble did not look too stable, and although Adelaide does not get much flash flooding, we did not want to take a chance of having the wall collapse from side pressure with wet soil in winter. The first wall used quality clay bricks on the outside of the foundation with space for another course inside (closer to the tank). The side continuously ended up in the hole, as can be seen in the first photo on this page when the lids are covered by the landslides from climbing into and out of the pit. The cement mix was 3-1 mix rather than a 6-1 mix as sold in premixed bags from hardware stores (a small bag of cement in a huge bag of sand for the price of a 20kg bag of GP cement). The huge bag would of course be under the “bend your knees” 20 kg limit, and a ratio less than 6-1. Anyway, sand is cheap and mixing cement is not that hard.

We had watched a few YouTube videos on ferro-cement water tanks and reinforced structures with chopped strands. We coated the wall with various mesh layers. Cement was plastered onto the wire mesh and left to set to bind to the wall. Cement was plastered onto the mesh once it was held in position by the earlier plastering.

Brick wall
Ferro-cement wall

Wire mesh and brick mesh

We chose to use several layers of different sized mesh (chicken wire, square for aviaries, etc). The bricks were covered with a brick force mesh every four to five courses. The wire was galvanised. The trench mesh was not galvanised, but was fairly new whenever we purchased it from Bunnings. As with most things, it was imported from China. The cement was local ABC with a 25 MPa rating. We had used Chinese cement in South Africa when they were busy with the world cup stadiums, and a shortage meant an import, but how that passed any testing, not sure. We were very happy with the ABC GP cement. The mesh was kept out of the rain before use.

Wire mesh for ferrocement covering
Wire mesh for ferrocement covering
Close-up of wire mesh for covering

A closeup of the mesh covering above the pipe outlet.

Wall at top of tank

The first wall was against the highest excavation (ground slope was towards the house). It was short and would be pinned on either side by columns and two longer walls. The ferrocement work was experimental and also used on some water tanks (that would later become raised beds). We decided to add a double wall with reinforcement between the two walls. Shown here is the inner wall with the brick mesh on every three to four courses, and the trench mesh placed sideways in between the walls. Concrete was added in as the wall was built, as it would be very difficult to pour it in later and reach all the gaps.

Gravel and rubble pile

Above ground, the gravel for the concrete and the ever increasing rubble mountain. Gum poles were used to secure formwork or place against the side of the pit so you could climb out if it collapsed.

Second inner wall with brick mesh and reinforcing
A few more courses of second inner wall

Some additional courses and detail of the concrete between the two brick walls.

Side wall

The left side wall was next. This side of the excavation was shallower than the right hand side, plus the storm water runoff from two sheds would end up in this side first. With winter rainy weather, pretty important. A long concrete beam from a garden path was placed along the side of the tank on top of construction membrane. Cement was poured on that, then another thick plastic construction membrane before placing trench mesh and top hats onto the membrane. A concrete pour is shown here with a level. The brick lines are drawn in.

In the second photo, in the lower left hand side is one of the concrete beams placed under the foundations. They were going to be taken to the dump, but we conveniently buried them and they provided a better base for the foundations. The access to the pit was at the bottom of the photo. On the right hand side, the tank was not filled in and there was a bit of a sink hole. The gushing noise was a surprise, but I climbed on top of the tank quickly and then pushed the rubble on the tank into the gaping crevasse.

Side wall foundation
Side wall and end wall

Right hand side wall

Right hand side foundation

As soon as the left hand side wall was high enough to keep the falling rubble out of the pit, we started on the right hand side. It would have the usual membrane, concrete beam, concrete, membrane, followed by trench mesh on top hats and another concrete pour.

The pit access is to the right in the above photo. The loose rubble was forever sliding into the pit.

Right hand side foundation
Right hand side foundation

Back to the left hand side

While the foundations set on the right hand side, the left hand wall managed to gain a few bricks.

Right hand side foundation
More on left hand wall

Wall reinforcement

The right hand side wall with reinforcement between layers and intersecting with the top wall horizontal reinforcing.

Right hand side wall reinforcing
Left hand side wall reinforcing

Left hand side with formwork for a column. Part of the column has just been poured. Between the brick walls is a gap for concrete reinforcement.

Right hand side with formwork for a column

Right hand side with formwork for a column.

Behind the wall at the bottom of the photo is some of the “ferrocement” wrapping just below the top two bricks.

More walls

Another angle of the left hand side wall (with a few more bricks).

Left hand side wall
Both walls

A view of both side walls with some additional work on foundations. The “steps” are at the bottom, with the continual rubble glacier.

Side walls almost done

This is a little after some later photos, but as taken vertically, it was easier to fit in here. The walls have been plastered, probably due to running out of bricks for the day and still having mixed cement to get rid of.

Some more reinforcing

Rebar was pushed into concrete between the two wall layers. The horizontal bars were tied to the vertical ones to stop them from sinking or tilting, and were removed once the concrete had set.

Rebar in wall

Similar to the side walls, rebar was inserted into the gap in the top walls.

Rebar in the top wall

The cover

We thought of using sheetmetal to cover the opening, but recall as a kid when a cow from one of the neighbours went through our vege garden and stepped over (then into) the septic tank. The poor animal left through the bottom gates and took them down the street for a few metres. That was fifty years ago, and the cover was sheetmetal (probably roofing corrugated sheet). The septic tank was a two stage pit called a French drain. Metal would ultimately rust, plus how to transport it? So concrete it was — again some experimentation.

Cover formwork

Formwork was plywood with pine temporary supports.

First pour for cover

First section of the roof cover being poured.

First pour for cover

Another view and clamps to hold the formwork, as simple nails and screws won't hold when it sets.