Our Civil Quote Listed Cubic Metres. It Did Not Say How the Tank Would Be Poured.

The civil section of our quotation was correct. Excavation in cubic metres, concrete in cubic metres, reinforcement in kilogrammes, formwork in square metres, a 3,000 psi mix, a figure at the bottom. I had signed off dozens of them in that shape. Then a client's project manager read it, put it down, and asked me how we were going to build the tank. Not what it cost. How it was going to be built.

I did not have a document to hand him. I had an answer, because we have poured a lot of these and the sequence is settled in my own head, but nothing on the sheet said any of it. So from that job onward every civil quotation we issue carries a drawn pour sequence, stage by stage, with the hold points marked. This is not the layout argument — that one is about shared cross walls and footprint, and it is settled. It is not the question of who pours the concrete either. This is about how the thing is poured, in what order, and why that order ended up inside a commercial document rather than in a site file nobody reads.

The line I keep coming back to is that a water-retaining tank is won or lost on the joints, not on the cubic metres. The cubic metres are the price. The joints are whether it holds water in ten years.

What we started putting on the sheet

Six stages, drawn, in the order they happen. Excavate and blind. Base slab in one continuous pour, with the waterstop in and a 150 millimetre kicker standing on every wall line — hold point. Walls, first lift. Walls, second lift, with a waterstop at the lift joint. Cover slab. Then fill-and-mark every chamber, a drop of no more than about one five-hundredth of the depth in 24 hours, witnessed — hold point.

That is the whole document. It is one sheet. It took an afternoon to draw properly the first time and it is now a template we adapt per job, which means it costs us nothing to include and it changes the conversation completely. A client reading a bill of quantities is comparing numbers. A client reading a sequence with two hold points in it is reading a plan, and he can tell the difference between a plan and a price.

Excavate, blind, and stop

The blinding layer is the stage people skip in their heads. It is a thin lean-mix slab poured straight onto the formation, and it does two things. It gives the steel fixer a clean flat surface, so the bottom cover on the reinforcement is what the drawing says instead of whatever the mud allowed that morning. And it stops the ground drinking water out of the base pour from underneath.

We stop there deliberately and look at the hole before anything structural goes in. Ground conditions are the one thing on a sewage treatment plant that nobody can promise from a drawing. If the formation is softer than the soil report suggested, or there is water coming in faster than we expected, that is the cheapest possible moment to find out. Everything after this point is harder to change, and on an underground plant the ground has already charged you once for the privilege.

The base slab is one pour, and the kicker belongs to it

The base slab goes in as one continuous pour. Not in halves, not in bays, not finishing the far end after lunch tomorrow. On a plant like this the slab is the one element where a construction joint you did not design becomes a leak you cannot reach, because it sits under several hundred tonnes of water and concrete with no face you can ever get a sealant to.

Which means the pour has to be planned as a delivery problem before it is a concrete problem. How many cubic metres, how many trucks, what the batching plant can actually send in a morning, where the pump stands, what happens if it rains at the midpoint. On a tight city site in Cebu the limiting factor is almost never the mix design. It is how many mixers can get down the access road in sequence, and that is a question you want answered a week before the pour rather than on the morning.

The kicker is the part I argue about most. It is a 150 millimetre upstand cast monolithically with the slab on every wall line, with the wall starter bars already in it and the waterstop already through it. Nobody loves it, because it means the slab pour includes fiddly little forms around the whole perimeter and every internal wall line, and the carpenter would much rather pour a flat slab and set his wall forms on top of it afterwards.

But a wall form landed directly on a flat slab leaks grout at the bottom, and a wall that leaked grout at its base has a honeycombed strip right where the water pressure is highest. The kicker gives the form something to seat against and takes the joint up out of the corner. It is a small cost at the stage it happens and an unreachable one afterwards, which is the same shape as most of the decisions on these jobs — the drawing decides the money, not the build.

The lift joint is the one you choose

Walls go up in two lifts rather than one. That surprises people who assume fewer joints is always better. It is not: fewer unplanned joints is better.

A wall three or four metres deep poured in a single lift is a lot of head of wet concrete pushing on a form, a long drop for the mix to fall without segregating, and a vibration job that has to be perfect from top to bottom of the pour with nobody able to see the lower half. Two lifts give the gang a reachable face each time. The cost is one horizontal joint per wall, and because we chose where it goes, we detail it: waterstop cast through it, the surface prepared, the next lift poured against a known condition.

That is the whole philosophy in one detail. There will be joints. The ones we draw are the ones with a waterstop in them. The ones that appear because a truck did not arrive are the ones that weep three years later, usually into the ground where nobody will see it until the surrounding soil starts smelling of something it should not.

Fill it, mark it, and leave it alone for a day

Before the cover slab goes on and long before any equipment arrives, every chamber gets filled with water, the level gets marked on the wall, and the tank is left standing for 24 hours. The drop should be no more than about one five-hundredth of the depth. Somebody from the client's side is welcome to watch it being marked and welcome to come back and read it with us.

Two reasons it is written in as a hold point rather than done as a courtesy. The first is that this is the last moment the tank is empty, open and reachable. A weeping construction joint found now is a chase-and-patch job from inside with the tank dry. The same joint found after the cover slab is on, the pipework is in and the plant is running is a shutdown, a tanker to empty it, a confined-space entry and an argument about who pays.

The second is that it settles a question that otherwise has no answer. A tank either held water or it did not. There is no judgement in it, no opinion, nothing to negotiate. We have had jobs where the only reason a late dispute did not turn into a long one was a marked level and a date, and that is worth the day it costs. It is the same instinct as writing the sampling date into the contract rather than treating commissioning as a day the power was switched on — a result is a fact, and a date is only a date.

Why the method belongs in the quotation and not in a folder

I will be honest about the commercial side of this, because it is the real reason it survived past the first job.

When two civil quotations sit side by side and both of them say nothing but quantities and a total, the only thing a reasonable owner can do is pick the smaller number. We have lost work that way and we have won work that way, and neither felt like it had much to do with engineering. Once the sequence is on the sheet, the comparison changes shape. It is not that we look more thorough. It is that the owner now has something specific to ask the other bidder, and the answer tells him something real. There are four things that have to match before two quotations are comparable at all, and the build method turned out to be one we had been leaving off our own page.

There is a cost to us as well, and I would rather state it than pretend it is pure upside. A drawn sequence with hold points in it is a commitment. If we have written that the base slab is one continuous pour and the gang pours it in two, that is now a visible departure from our own document, and somebody is entitled to ask about it. We decided we were comfortable with that, because we were going to build it that way regardless. A method statement is only uncomfortable if you were not planning to follow it.

It also sits oddly with something I have said for years, which is that designing and building the same plant creates a conflict that the client should be told about. The honest reading is that this cuts the other way: a build method published in our own commercial document is one of the few things that makes that conflict checkable by somebody outside it.

What we give the client's own contractor instead

This is where I have to draw a line that looks inconsistent and is not.

When the client's own contractor is building the tank to our design, he gets drawings. Dimensions, levels, reinforcement, the 3,000 psi mix, the waterstop and kicker details, and the fill-and-mark test as an acceptance criterion. He does not get our construction methodology, no quantities and no prices. Those are what he has to achieve, not how he has to achieve it.

Part of that is commercial — our sequence is our own work and I am not handing it to somebody who is quoting against us next month. But most of it is about liability, and I think it is the right way round for the client too. If I write a contractor's method statement for him, I have taken over his means and methods while he keeps the contract to pour it. If the pour then goes wrong, nobody can say cleanly whose fault it was. Dimensions and a watertightness test leave it clean: he chose how, the tank either passed or it did not. Where we are supervising rather than building, the same split applies — we witness, he pours, and the boundary between what we give away and what we are paid for stays where it was.

What showing the sequence does not fix

A pour sequence is not a quality system. The two hold points catch two specific things: the condition of the hole before anything structural is committed, and whether the finished chambers hold water. Everything between them still depends on a steel fixer who places bars where the drawing says and a gang who vibrate properly in the heat at two in the afternoon. I have never seen a document fix either of those. Standing there occasionally helps more.

It does not make the tank cheaper, either. Our working rate is still in the order of ₱14,000 per cubic metre of tank capacity at 3,000 psi on our own jobs in Cebu — a figure I trust only because we have paid it repeatedly rather than estimated it — and a kicker and two waterstops do not move that figure much in either direction; your own site, ground and filtration level will move it far more than the sequence does. What the method changes is the probability of a leak, which is not a line item anywhere and shows up as its absence.

And it does nothing at all about the numbers the tank was sized from. A beautifully poured chamber of the wrong volume is still the wrong chamber, and the flow and strength on the drawing are still the thing most owners cannot give us — almost nobody has an influent result, and the tank that treats nothing is usually the first one somebody asks to shrink. If you want rough figures to argue with before any of this, the STP price estimator and the design generator take a flow and a building type, and the discharge standards page sets out what the plant has to achieve in the first place. What your industry actually has to remove is on the significant parameters by industry page.

Where this does not transfer

The sequence above is for a sunk, water-retaining reinforced concrete monolith on ground we can dig, built by a local gang with a mixer truck that can reach the site. Change any of those and parts of it stop applying.

On a small shallow tank — a chamber a metre and a half deep — the two-lift argument disappears. One lift is reachable, and adding a lift joint to a wall that did not need one is adding a weakness for no reason. Below about that depth I would pour the walls in one go and keep the kicker.

On an above-ground plant on a plain slab, most of this relaxes. There is no external water pressure trying to get in, every face is visible for the life of the tank, and a joint that weeps is something you can see and seal on a Tuesday. The fill-and-mark test still earns its place, because a leak you can see is still a leak.

If the site is genuinely remote and concrete arrives by small batches rather than by truck, a single continuous base pour may not be physically available. Then the honest answer is to design the joint rather than pretend it will not exist: a planned construction joint with a waterstop and a shear key, in a position we chose, is a world away from the one that happens when the trucks stop coming.

And the 24-hour test figure is ours. It is a tolerance we are comfortable with on our own plants and it has served us well. A larger structure, a different code, or a consultant with a stricter specification may call for something tighter or a longer stand. I would not argue with a stricter one.

What I think travels is smaller than the sequence itself: the joints decide whether a tank works, you will have joints either way, and the only question is whether you chose where they are. Every site we walk onto is different, and the ground has surprised us more than once, so take the shape of this rather than the numbers. We have built these across Cebu and further out, and the completed projects page is the honest record of it. If the operation rather than the build is what concerns you, that sits with maintenance and operation and our support arrangements, and the wider picture of what we do is on the wastewater services page. If you already know your scope, tell us what you need.

If you have a civil quotation in front of you for a sunk tank and it does not say how the tank will be proven watertight, that is the question I would ask before the price. Send it over and book a free 15-minute consultation — fifteen minutes is usually enough to tell whether a method exists and simply was not written down, or whether it was never thought about.