The guide
Cement-bound sand (CBS)
Cement-bound sand, or CBS, is cable sand mixed with a small amount of cement, usually 14 parts sand to 1 part cement. It is laid and compacted around power cables like plain cable sand, then cures into a firm surround that keeps carrying heat away from the cable even when it dries out. It is also called stabilised sand. Builders and groundworkers use it too, for bedding paving and kerbs, blinding and filling voids.
Last checked 3 October 2026. Sources at the bottom of the page.
Why CBS instead of plain sand
It keeps carrying heat when it dries out
Plain sand is good at carrying heat away from a cable while it is damp, and much worse once it dries out, which is exactly what happens next to a hot cable. The cement holds CBS together so it stays dense and keeps carrying heat away after it has cured and dried.
It holds its shape
Once cured, CBS does not wash out, slump or shift, so the cables keep a solid surround for the life of the circuit.
The spec often asks for it
Network operators commonly ask for it around higher voltage cables, typically 33 kV and above, where the cables run hottest.
When the spec asks for it
Your drawings or the network operator’s installation rules decide. ENA TS 97-1, the industry standard for backfill around power cables, lists cement-bound sand as one of its three traditional backfills, alongside selected sand and a gravel and sand mix.
Two examples of operators’ own rules. Northern Powergrid (NSP/002, 2021) asks for a 75 mm bed of selected sand or CBS at 33 kV and above, with the same over the top cable or duct. UK Power Networks’ rules (the 2015 version we have read) ask for a CBS bed at 33 kV and above unless the cable rating calculations show it is not needed, with ducts at those voltages also laid in CBS.
Other operators set their own rules, so check the ones for your area. National Grid’s transmission jobs follow National Grid’s own specification, NGTS 3.05.07. Most low voltage and 11 kV work only needs plain cable sand.
The spec in plain English
For cable work, ENA TS 97-1 sets four limits for cement-bound sand. The engineer can ask to see test results for all of them.
1.2 K·m/W or less
Thermal resistivity
How hard it is for heat to get through, measured once the CBS has cured and fully dried out. Lower is better. Plain selected sand is allowed up to 2.7, so CBS has to be more than twice as good.
95% under 5 mm
Grading of the sand
At least 95% of the sand by weight has to pass a 5 mm sieve, so there are no stones big enough to press on the cable.
0.54 or less
Void ratio of the sand
How much empty space there is between the grains. Fewer gaps means a denser surround that carries heat better.
1.6 Mg/m³ or more
Density in the trench
The average dry density of the CBS once it is compacted in the trench and cured. That is 1.6 tonnes per cubic metre. The test certificate for the sand may set a higher figure to aim for.
These are from Issue 2 of ENA TS 97-1, the issue we have read in full. The current issue is Issue 3 (2021). A CBS test certificate issued under Issue 3 that we have checked uses the same limits. Your contract or the network operator may ask for more.
Testing and the certificate
The sand that goes into CBS is tested before the job: a lab mixes it to the spec, compacts it, lets it cure, dries it right out and measures how well heat gets through. It also checks the grading and the void ratio. The results go on a test certificate, which the engineer wants to see before work starts.
Read the density on the certificate. The lab finds the density the CBS needs to reach to get under 1.2 K·m/W. For some sands that is well above the 1.6 Mg/m³ minimum, and that is the figure to compact to in the trench, because density is what gets checked as the job goes along.
You can ask for our test certificateto check the sand against your spec.
How we supply it
Sand to mix on site
We deliver the sand by tipper or in bulk bags, and you add the cement on site to the ratio your spec gives. You control the mix and only mix what you can lay.
Pre-mixed dry
If you would rather have it mixed, we supply CBS pre-mixed dry in 10:1, 14:1 or 20:1, loose or bagged. Pick it on the form and put the mix in the notes. Ask us how long it keeps.
Delivered anywhere in the UK, including Northern Ireland and the Scottish islands. Every job is quoted, because haulage is most of the price. You can ask for our test certificateto check the sand against your spec.
CBS for other jobs
Cement-bound sand is not only for cables. We supply it for groundworks and building jobs too. Tell us the mix the job or the engineer asks for.
Bedding paving and kerbs
A firm bed under slabs, setts and kerbs that does not wash out.
Blinding
A thin, level layer over hardcore or sub-base before the next layer goes on.
Filling voids
Old fuel tanks, disused drains, cellars and other voids that need filling solid.
Trench reinstatement
Backfilling service trenches where the spec or the road owner asks for a bound fill.
Laying it
CBS goes in the same places as plain cable sand: a compacted bed along the bottom of the trench, then blinding over and around the cables or ducts once they are laid, packed in by hand around the cables and joints. It then cures into a firm surround.
Mix only what you can lay and compact, because cement starts to set once it is wet. The engineer may test it in the trench, so keep to the mix and compaction your spec sets.
How much cement. The spec puts 14:1 by volume at about 18 to 20 parts sand to 1 cement by weight. By our arithmetic that is about 50 to 55 kg of cement per tonne of sand, roughly two 25 kg bags. Your specification decides the exact mix.
Questions
What is the mix for cement-bound sand?
For cable work, usually 14 parts sand to 1 part cement by volume, which is why you see it written as 14:1 CBS. Other jobs use other mixes, such as 10:1 or 20:1. Always mix to the ratio, cement type and water your specification gives, because the engineer can test it.
Do you supply cement-bound sand ready mixed?
Either way. We supply the sand for you to add the cement on site, delivered by tipper or in bulk bags, or we can supply it pre-mixed dry in 10:1, 14:1 or 20:1, loose or bagged. Pick which you want on the form and put the mix in the notes.
Is stabilised sand the same as cement-bound sand?
Yes. Stabilised sand, stabilised backfill and cement stabilised backfill all mean sand bound with cement. In the UK it is usually called cement-bound sand or CBS.
Can I use CBS around low voltage cables?
You can if the spec asks for it, but most low voltage and 11 kV work only needs plain cable sand. CBS is mainly asked for at higher voltages. Your drawings or the network operator’s rules decide.
Does the sand in CBS have to be tested?
On network operator work, yes. ENA TS 97-1 sets tests for the sand that goes into CBS, and the results are part of the paperwork the engineer expects before work starts. Ask for our test certificate to check it against your spec.
What is the thermal resistivity of cement-bound sand?
For cable work to ENA TS 97-1, CBS must be 1.2 K·m/W or less once it has cured and fully dried out. Plain selected sand is allowed up to 2.7. Lower means heat gets away from the cable more easily.
What density should CBS be compacted to?
ENA TS 97-1 asks for an average dry density of at least 1.6 Mg/m³ in the trench once cured. The test certificate for the sand may give a higher density needed to reach the 1.2 K·m/W limit, and that is the figure to compact to.
How strong is cement-bound sand?
Strong enough to bear load, but much weaker than concrete, because there is far less cement in it. For cable work, ENA TS 97-1 does not give a strength figure. It asks for backfill that is load bearing and stands up to settlement, washing out, knocks and frost heave, and it sets limits for heat, grading and density. If your job specifies a strength, tell us when you ask for a price.
How much cement do I add to a tonne of sand?
For 14:1 by volume, about 50 to 55 kg of cement per tonne of sand, roughly two 25 kg bags. That is our own arithmetic from the spec, which puts 14:1 by volume at about 18 to 20 parts by weight. Your specification decides.
How long does pre-mixed dry CBS keep?
Ask us when you order. It depends on the mix, how it is bagged and how it is stored on site, so we will tell you for your delivery.
How much does cement-bound sand cost?
Every job is priced to the site, because haulage is most of the cost of anything this heavy. Send the form with the mix, how much and the postcode, and we come back with a price.
Sources
- Energy Networks Association, ENA TS 97-1 Issue 2, February 2016, Special backfill material for cable installations. The industry standard, and the issue we have read in full. The CBS limits and the 14:1 mix on this page come from it.
- Energy Networks Association document catalogue, ENA TS 97-1. Shows Issue 2 has been replaced by Issue 3 (2021). A CBS test certificate issued under Issue 3 that we have checked uses the same limits.
- Northern Powergrid, NSP/002 Policy for the Installation of Distribution Power Cables, version 5.0, October 2021. Selected sand or CBS bedding and blinding at 33 kV and above.
- UK Power Networks, ECS 02-0019 Installation of Underground Cables, LV to 132 kV, version 10.0, June 2015. The version we have read. UK Power Networks may have updated it since, so check the current one.
Get a price
Sand for CBS, delivered to your site
Pick “Sand for cement-bound sand” or “pre-mixed dry” and tell us how much, where and when. We price it to your site and come back to you.