Moisture is a quantity, not an adjective
Soil moisture content is reported as a percentage of dry weight, which is the detail that trips people up: 13 percent moisture does not mean water is 13 percent of what you are looking at. It means that for every 100 lb of solids there are 13 lb of water, so water is 11.5 percent of the wet weight. Everything downstream follows from getting that right.
Work the default pad through. 2,000 sq ft brought up 24 in compacted is 4,000 cubic feet, 148.1 cubic yards in place. At 118 lb per cubic foot maximum dry density and 95 percent of it, the in-place dry density is 112.1 lb per cubic foot, so the fill holds 448,400 lb of dry solids — 224.2 tons. Take that from 9 percent moisture to 13 and the water goes from 40,356 lb to 58,292. The difference is 17,936 lb, which is 2,149 gallons and just under 9 tons.
Three lifts, and the water goes in three times
Twenty four inches at a maximum 8 in compacted lift is three lifts of exactly 8 in, and each one takes about 716 gallons across the full 2,000 sq ft — 358 gallons per 1,000 square feet. That is a 500 gallon tank filled five times over the course of the pad, and it is work that happens between the dumping and the rolling rather than as a separate operation.
The reason it matters is that compaction is not achieved by effort alone. Too dry and the particles will not slide past each other no matter how many passes the roller makes; too wet and the water carries the load instead of the soil skeleton. The proctor curve for your material has a peak, and the spec on your plan set names a band around it. Both of those numbers are somebody else and this page holds neither.
You are already buying water
| Line | Default figures |
|---|---|
| Dry solids in the fill | 448,400 lb — 224.2 tons |
| Material to buy at 9 percent moisture | 244.4 tons on the ticket |
| Water inside that | 20.2 tons, 8.3 percent of the ticket weight |
| At 18 dollars a ton | 4,398.80 total, of which 363.20 is water |
Fill sold by the ton is weighed wet, so a rainy week at the pit costs you money on every load and nobody sends a credit note. It is not usually worth arguing about, but it is worth knowing which way it runs when two quotes are close and one supplier stockpiles under cover.
What this cannot tell you
It gives a quantity of water. It does not give a method, and the difference is most of the job. Water poured on top of a lift wets the top couple of inches, runs to the low corner and leaves the bottom of the lift as dry as it arrived, which is why conditioning is done by mixing rather than sprinkling on any job where the result gets tested. Nor does it know how much evaporates between the tank and the roller, which on a hot windy day is not a rounding error.
Above all, it does not know whether your fill meets a specification. That is decided by a test on the compacted material, run by whoever the job says runs it, against the numbers on your plan set. If nobody has run a proctor on the material there is no target to condition toward, and if the material is already wetter than the target then the answer is not water at all — it is time, weather and a decision by the person holding the spec.
Questions people ask
How many gallons of water does a cubic yard of fill need?
It depends entirely on the two moisture contents and the density, which is why it is calculated rather than looked up. At the defaults here — 112.1 lb per cubic foot in-place dry density, 9 percent to 13 percent — a compacted cubic yard holds 3,027 lb of dry solids and needs 121 lb of water added, about 14.5 gallons. Halve the moisture gap and you halve the water. There is no per-yard constant because dry density and the moisture gap both vary by material and by the day.
What is optimum moisture content and where do I get the number?
It is the moisture at which a given compaction effort produces the highest dry density for that particular soil, and it comes off a proctor test run in a lab on a sample of your material. It is a property of the soil, not of the job, and it varies widely between materials. Your compaction spec normally requires a percentage of the maximum dry density and a moisture within a band of the optimum. Both numbers live on the lab report and the plan set. This page will not supply either and no honest page will.
Why does moisture content go above 100 percent for some soils?
Because it is measured against dry weight rather than total weight. A soil holding more water than solids — some organic soils and soft clays do — reports over 100 percent, which looks wrong and is not. The convention matters here because the arithmetic on this page works from the dry solids outward: dry weight times the percentage gives the water, at either end. Working from wet weight instead is the single most common error in moisture arithmetic and it always understates the water.
The material is wetter than the target. What now?
Then the calculator reports the pounds of water above the target and stops there, because drying soil is a weather-dependent process rather than an arithmetic one and it is not something to be given as a procedure. Whether the material can be placed at all at its current moisture, whether it can be dried, and what happens to the schedule are decisions for whoever holds the compaction spec and the testing on the job. This is also the main reason fill placement stops for days after heavy rain.
Does the water add weight I have to account for?
It does, and the page shows it. Adding 17,936 lb of water to the default pad is nearly 9 tons arriving on site in a tank rather than on a dump truck, and the finished in-place wet density comes out at 126.7 lb per cubic foot against 112.1 dry — 3,420 lb per compacted cubic yard. If you are buying material by the ton at its current moisture, part of every ticket is water you are paying to haul, which at 9 percent is about 165 lb in every ton.