Why drainage depth changes your whole programme
Short answer
Drainage cost and programme do not scale linearly with depth. Past roughly three metres you add shoring, wider excavation, more arisings to handle and tighter confined-space control. Each of those changes the plant, the crew size and the daily output rate, so a small depth increase can reshape the whole sequence.
The short version
- Drainage cost rises in steps, not in a straight line, as depth increases.
- Shoring requirements change the excavation width and therefore the muckaway volume.
- Deeper runs mean fewer metres laid per day, which moves the programme more than the price.
- Groundwater, not depth alone, is what usually decides the method.
- Getting a depth-accurate survey early is cheaper than discovering it in the trench.
Somebody asks for a rate per metre on a drainage run. It is a reasonable question. It is also close to unanswerable.
Not because we are being difficult. Because depth does not behave the way the question assumes.
Cost does not rise in a straight line
Here is the intuition most people start with. Twice as deep, roughly twice the work.
That is not what happens. Drainage cost rises in steps. You sit in a band, nothing much changes, then you cross a threshold and several things change at once.
| Band | What changes | Effect on programme |
|---|---|---|
| Shallow | Open trench, battered sides, small excavator | Good daily output, simple sequence |
| Around 3m | Shoring becomes necessary, trench widens | Output drops, extra plant on site |
| Deeper still | Heavier support, confined space control, larger plant | Fewer metres per day, longer lead-in |
| Any depth, with water | Dewatering, bedding method changes | Can dominate everything above |
Each row is a different job, really. Same pipe. Different operation.
The arisings nobody counted
This is the quiet one.
Dig deeper and wider and you generate considerably more spoil. On a long run that is a lot of lorry movements, and lorry movements cost money and annoy neighbours.
It is also why we have reprocessed material on site since 2002 wherever the specification allows it. Reusing what is already there beats carting it off and buying it back in. You can see where that sits in our core services, and the same thinking runs through our wider groundworks capability.
Water beats depth
If we only get to ask one question about a drainage package, it is not how deep. It is whether there is water.
Groundwater decides whether an open trench is viable. It decides your dewatering. It decides how bedding goes in and whether it stays where you put it.
A shallow run in bad water can be harder than a deep run in dry ground. People are consistently surprised by that.
Depth sets the method. Water decides whether the method works.
Why this changes programme more than price
Here is the part that catches people on a main contract.
A deeper run might add a manageable amount to the package price. But if your daily output halves, the drainage sits on the critical path for twice as long.
Everything behind it moves. Sub-base, kerbs, surfacing, the lot. The programme cost lands on the main contractor, not the drainage line.
That is the argument for self-delivery, honestly. When the same contractor is doing the drainage and the roadworks that follow it, the sequence is one conversation rather than two companies protecting their own dates.
Sequencing with everything above it
Drainage goes in first and gets built over. That is the whole problem in one sentence.
Anything missed has to be dug back up through whatever has been laid on top. Sub-base, binder course, kerbs. The further the job has progressed, the more expensive the mistake becomes.
Which is why the drainage package deserves attention at programme stage rather than being treated as a preliminary. It is the thing everything else sits on, and it is the hardest thing to go back to. That logic carries straight into the adoptable highway works that follow it.
What we want before we price
Drawings with levels. A site investigation. Anything you know about water.
With those three we can price what is actually there. Without them we are guessing, and a guess priced low is worse for you than a fair price, because it will move.
Got a drainage package that needs pricing properly? Send us the information and we will tell you what we see in it.
Common questions
At what depth does drainage get significantly harder?
There is no single number, but around three metres most schemes change character. Shoring becomes non-negotiable, the excavation widens, and output per day drops. Groundwater can bring that threshold much shallower.
Does deeper always mean slower?
Almost always, yes. Not because digging is slower, but because the supporting activities multiply. Setting and striking shoring, handling more arisings and managing access all consume time that shallow work does not.
What size pipe can you install?
We install pipes of all diameters at all depths, including steel and pre-cast pipe up to 1800mm, and large attenuation structures. Large diameter changes the lifting plan more than it changes the excavation.
How much does groundwater change things?
More than depth does, often. Water dictates whether you can work in an open trench at all, what dewatering you need, and how the bedding is placed. It is the first thing we want to know from a site investigation.
Can you give a rate per metre?
Not usefully, no. A rate per metre with no depth, no ground conditions and no water table is a number that will change. Send drawings and a site investigation and we will price what is actually there.