When the ground does not match the specification
A technique that is right on paper can still be wrong on the day. Ground moisture, recent rainfall and the age of the asset in front of you do not always match what the specification assumed when it named a technique for that ground type or that asset class.
What can change the answer on site
- Ground that has dried out or become waterlogged since the survey was done
- An asset that is older, more corroded or more fragile than its category suggests
- Standing water or freezing conditions that rule out a technique that would otherwise be the right one
Where the technique named in the specification turns out not to suit what is actually in the ground, that is a decision for the site manager to make and record, not one to work around quietly. The reasoning belongs in the same document as the rest of the specification, alongside operator card level, banksman requirement and spoil route, on the page covering specifying it in a scope of works.
Suction, air-lance and water-lance side by side
The technique that suits the ground can still be the wrong one for the asset sitting in it, so read the ground column and the asset note together before choosing.
| Technique | Ground and asset fit | Spoil reusability | Water supply needed |
|---|---|---|---|
| Suction excavationCPA SAVESIG GPG1 v3 (January 2025) section 10.5 sets out where suction alone is the appropriate choice. | Works across a range of soil types and is the fallback where air or water tools are ruled out by the ground or the asset. | Dry spoil, more readily graded against EWC 17 05 04 and reused under the CL:AIRE Definition of Waste Code of Practice. | No water supply required. |
| Air-lance excavationHSG47 3rd edition (2014) paragraph 113 notes that old, fragile cables can be damaged by an air jet as well as by water. | Suits porous soils that break up under air pressure. CPA SAVESIG GPG1 v3 section 10.7 prohibits air tools on asbestos-reinforced pipe because of the dispersal risk. | Spoil stays dry, but check for asbestos-reinforced pipe on site before this technique is chosen. | No water supply required. |
| Water-lance excavationHSG47 3rd edition (2014) paragraph 113 applies here too: old, fragile cables can be damaged by a water jet. | Suits non-porous soils such as clay, where air tools struggle to break the ground. | Slurry arisings need a separate disposal route from dry spoil. | Needs a water supply and a way to manage the resulting slurry on site. |
Drawn from CPA SAVESIG GPG1 v3 (January 2025) sections 10.5 to 10.10 and HSE HSG47 3rd edition (2014) paragraph 113. This sets out what those documents say about ground and asset suitability, not a tested result, and it does not replace a site-specific risk assessment.
What CPA GPG1 says about soil type and nozzle material
The CPA SAVESIG GPG1 v3 (Guidance and Practice for Safe Excavation in the Ground, January 2025) sets out which excavation method matches which soil, and the answer is not the same document for every ground condition. Section 10.5 recommends water tools for non-porous soils such as clay. Clay does not break down under a water jet the way looser ground does, so the cutting action stays controlled and the arisings stay manageable.
Section 10.6 goes the other way. In porous ground, such as sands and made-up granular fill, air tools are the guidance's recommendation. Air breaks up loose material without adding water to it, which matters because water introduced into a porous soil travels further than the operator can see, and the spoil it produces is harder to handle and harder to reuse.
Asbestos-reinforced pipe: air tools are prohibited
CPA SAVESIG GPG1 v3 section 10.7 prohibits the use of air tools where the buried asset is an asbestos-reinforced pipe. The risk named is dispersal: an air jet can lift asbestos fibres into the air around the excavation. Water tools are the alternative the guidance points to on this asset type, because a water jet suppresses the material it cuts through.
Nozzle material: spark and static risk
The same section of CPA SAVESIG GPG1 v3 addresses nozzle material against spark and static risk. Where the ground or the asset carries a flammable-atmosphere risk, the nozzle material used for either air or water excavation has to be selected against that risk. This is a specification point worth confirming before the technique is agreed.
None of this replaces a check against the asset owner's own specification for the network in question. Where a governing clause names a technique directly, that clause takes precedence over the general guidance in CPA SAVESIG GPG1 v3. You can check which specification governs a given dig on the scope of works page, and follow up any question through contact.
HSG47 paragraph 113: cables that a jet can damage
HSG47 (Health and Safety Executive guidance, 3rd edition, 2014), paragraph 113, warns that old or deteriorated cables can be damaged by the water jet or air jet used in non-destructive excavation. The paragraph does not ban either technique near electrical assets. It flags a condition of the asset as the thing that decides whether a jet is safe to use.
That distinction matters for a site manager standing over an unknown cable. A modern, well-sheathed cable and a brittle, aged one carry different risk under the same water-lance or air-lance nozzle, and paragraph 113 is the line that puts the asset's condition into the decision.
What this means before you choose a technique
HSG47 is guidance, so paragraph 113 does not override whatever the relevant electricity distribution network operator's plant-protection specification requires on that dig. Where the two give different steers, the asset owner's specification is the one that binds the job. The HSG47 paragraph is useful for understanding why that specification says what it does, and for making the case to a contractor who wants to skip a step.
Where the cable's age or condition is not known, that is itself the fact to work from.