How Big a Water Bowser for a Site? Cubic Metres, Trips, and Dust vs Compaction

"How many cubic metres?" is the first thing a lessor asks when a site calls for a water tanker, and the answer most site engineers give is a guess: "a 20, the biggest you have." Sometimes that is right. Often it is not, because how big a water bowser a site needs is decided less by the tank than by two other numbers: how much water the site uses in a day, and how many trips the tanker can actually make in a shift. A 20 m³ tanker filling at a slow tap 20 km away delivers less water than a 12 m³ tanker filling on site.

This guide works through that arithmetic with round, worked numbers, and separates the two jobs site water does: keeping dust down and bringing fill to compaction moisture. They use water differently, and mixing them up is how a site ends up both dusty and failing density tests. The general case for a water bowser, water sources and day-to-day running are covered in water bowser for dust control; this post is the sizing sequel.

The three tanker classes

In Egypt the trade talks about tankers by capacity in cubic metres — a "12 metre" or a "20 metre" tanker means 12 or 20 m³. Three classes cover most site work:

Most sizing questions are really "which rigid tanker, and how many", with a towed tank or a storage tank as supporting players. The exact capacity of any unit is on its spec sheet, not in its nickname — ask for the actual machine's figure.

Step 1: dust water — driven by area and time

Dust water does not depend on progress. The road needs wetting whether you placed a thousand cubic metres of fill today or none, and it needs it again every time the sun and the trucks dry it out.

Daily dust water = wetted area × litres per m² per pass × passes per day.

A worked example: a haul road 1.5 km long and 8 m wide is 12,000 m². Take a light pass of about 1 L/m² — enough to darken the surface without pooling; check it by eye on the first day and adjust. In a Cairo summer, with trucks running all day, assume four passes. That is:

12,000 m² × 1 L/m² × 4 = 48,000 L = 48 m³ a day.

In winter, or on a road with a crushed-stone surface, the same road might need two passes and half the water. The number of passes is the figure to argue about, because it moves the total more than anything else. Four is a starting point for a busy sand road in the hot months, not a rule.

Step 2: compaction water — driven by volume placed

Compaction water is the opposite: it follows the earthworks programme, not the clock. Sand, marl and gravel fills only reach specified density near their optimum moisture content, which the lab gives you from the Proctor test. The water to add is the gap between the fill's current moisture and that target.

Compaction water = compacted volume × dry density × (target moisture − current moisture).

A worked example with illustrative figures: the site places 400 m³ of compacted fill a day. The lab gives a dry density around 1.8 t/m³, and the borrow material arrives about 4 percentage points drier than optimum. Then:

400 m³ × 1.8 t/m³ × 0.04 = 28.8 t of water ≈ 29 m³ a day.

Two things make this higher on a real site. First, evaporation: water sprayed on a loose layer at noon in August loses part of itself before the roller arrives, so the earthworks engineer adds an allowance. Second, borrow material varies: a load from deeper in the pit may be wetter or drier than the last. Use the lab's figures for your material, not the illustration above, and re-check when the source changes.

Compaction water also has to go on evenly: spray-bar passes over the layer, mixed with a grader or disc harrow where the fill is thick, and placed just ahead of the roller. The sequence is covered in land levelling equipment.

Step 3: add them up, then add the extras

For the example site: 48 m³ of dust water plus 29 m³ of compaction water is 77 m³ a day, before curing and wheel washing. Round up for those — a site with a gate wash and slabs to cure can add several cubic metres a day on top.

Write the total on one line in the daily report. It is the number that decides how many tankers you need.

Step 4: count the trips a tanker can really make

A tanker's daily output is its capacity × the trips it completes. Each trip is:

Cycle time = fill time + travel to and from the fill point + spraying time.

Fill time is tank size divided by the tap's flow. Measure the tap; do not trust anyone's description of it. Spraying time follows from the pump and spray bar. Travel depends on the distance and the road.

Here is the same fill point (about 0.6 m³ a minute), the same average speed (about 24 km/h, loaded and empty, site and public road) and 8 productive hours a shift, applied to two tanker sizes at three distances:

Three lessons fall out of the table:

1. With water on site, size hardly matters. The smaller tanker delivers the same volume and is lighter on soft ground, easier to turn and quicker to respond.
2. At a middle distance, the tap is the bottleneck. The 20 m³ tanker spends its extra capacity waiting to fill. A faster fill point would help more than a bigger tank.
3. Far from water, a bigger tank wins — and so does storage. When travel dominates the cycle, every trip should carry as much as the road can bear.

Step 5: match output to demand

Back to the example site, which needs 77 m³ a day:

• Water on site or 5 km away: one tanker of either size covers it, with a modest margin. One slow fill or one flat tyre and the margin is gone, so the site should know what happens when the tanker stops.
• Water 20 km away: one tanker delivers 36–60 m³. The options are two 20 m³ tankers (two 12 m³ units fall just short), or a storage tank on site kept topped up by a semi-trailer delivery while a single rigid tanker sprays from it — which turns the 20 km case back into the "on site" row of the table.

Build in a margin. A plan that needs every trip of every day to succeed will fail in the first week.

Dust vs compaction: when one tanker cannot do both

The totals can work on paper while the day does not. Dust water is needed early morning and before the afternoon traffic peak; compaction water is needed right ahead of the roller, which on most sites means mid-morning onwards. A single tanker spraying the haul road at 10 am is not wetting the layer the roller is waiting for, and vice versa.

Signs you need to split the jobs:

• The roller waits for water more than once a day.
• Density tests fail on "too dry" while the haul road looks fine — the tanker is serving dust first because that is what people complain about.
• Neighbours complain about dust in the afternoon because the tanker was on the fill all morning.

The usual fix is two units with clear roles: a compaction tanker that follows the roller and nothing else, and a dust tanker — often smaller — on the haul roads. On a smaller site, one tanker with a written priority (compaction first until the layer is done, then roads) is enough, provided the driver and the foreman both know it.

Water quality matters differently too. Brackish or canal water is usually acceptable for dust; for compaction layers under a structure or a road, ask the engineer, because salts left in the fill can cause problems later.

What to send when you request a tanker

A request that includes these lines gets the right tank size on the first reply:

1. Daily volume you worked out, split into dust and compaction.
2. Fill point: distance from site, and the tap's measured flow if you have it.
3. Haul road length, width and surface.
4. Ground: can a loaded tanker drive on your fill and access roads?
5. Hours: shifts per day and days per week.
6. Driver included or not, and who supplies and pays for the water — the split is explained in rental with or without an operator.

The short version

Work out dust water from area and passes, compaction water from volume placed and the lab's moisture gap, and add them. Then measure the fill point and count the trips a tanker can make in a real shift. If the fill point is close, a smaller tanker is as good as a big one; if it is far, go bigger or put storage on site; if the roller keeps waiting, split dust and compaction between two units.

To compare tank sizes and current rates, browse water bowser hire — the page lists the tankers and their list rates.