What cement-stabilised backfill is
Cement-stabilised backfill is sand or soil mixed with a small amount of cement, placed and compacted like normal fill, and left to gain strength. In Australia it's often called cement-stabilised sand, or CSS. It's used where a trench needs stronger or more durable backfill than the material alone gives, such as under roads. It's either supplied pre-mixed or mixed on site.
How the cement content is specified
Specifications usually give the cement as a percentage by mass, most often of the dry sand or soil, for example 3% or 5%. Some give kilograms of cement per cubic metre instead. Check which one your specification uses before you calculate anything, because the two aren't interchangeable.
The calculation
- Compacted volume: the volume of the zone being stabilised, from your trench calculation.
- Dry mass of the fill: compacted volume × dry density. Use the density for your material, from testing or your supplier.
- Cement mass: dry mass × cement percentage.
- Bags or bulk: divide by the bag size, or order the mass in bulk, and add an allowance for wastage.
Worked example
Say 10.0 m³ of trench backfill is to be stabilised with 5% cement by mass of dry sand, and the compacted dry density of the sand is 1.8 t/m³:
- Dry mass of sand: 10.0 × 1.8 = 18.0 t
- Cement: 18.0 × 0.05 = 0.9 t, or 900 kg
- In 20 kg bags: 900 ÷ 20 = 45 bags
- With a 10% wastage allowance: 990 kg, or 50 bags
The density and percentage here are assumptions for the example. Use the values from your own specification and material.
Water
The mix needs enough water to reach the moisture content it compacts best at, which comes from testing the material. Too dry and it won't compact properly. Too wet and it's hard to place and compact, and it can lose strength. On larger jobs the water usually goes on by watercart, so work out the volume and the number of loads ahead of time.
Strength
Stabilised fill is usually specified by a target unconfined compressive strength (UCS), often checked at 7 and 28 days. The strength you actually get depends on the material, the cement content, the compaction and the curing, so any estimate is a guide, not a design value. Laboratory tests on your material are what confirm the mix.
Working time and weather
Cement starts reacting as soon as it's mixed with water, so stabilised fill has to be placed and compacted within the working time in your specification. Hot, dry or windy weather dries the mix out, and rain changes its moisture, so check the forecast before a big day of placing.
Doing it on site
TrenchCalc works this out from the trench dimensions up: cement from 0 to 20 percent for each zone, the cement mass and water, watercart loads, a 7-day and 28-day strength estimate, and a weather advisory for the day. It's part of the Lifetime unlock. For the volume itself, see how to calculate trench volume.