Cable percussion with a rotary follow-on, or one multipurpose rotary rig?

On a site with soils over rock, there are two ways to get the borehole down. The traditional route uses two rigs: a cable percussion rig to work through the soils, then a rotary rig brought in afterwards - the "follow-on" - to core the rock. Our way is one multipurpose rotary rig that does both in the same hole. For most investigations the single-rig approach is faster, cleaner and better value, and it is how we run the majority of our work.

The traditional approach: cable percussion, then a rotary follow-on

Cable percussion (shell and auger) is the long-established way of sinking a borehole through soils - raising and dropping tools on a wire rope to advance the hole, take SPTs and recover disturbed and U100 undisturbed samples. It is proven and well suited to soils, but it cannot core rock. So when the borehole reaches competent strata and the design needs to continue into it, a second, rotary rig has to follow on to the same position to core the rock.

That hand-over is where the time and cost creep in:

• Two separate mobilisations - two rigs, two crews, two lots of transport to and from site.
• Programme gaps while the first rig moves off and the rotary rig is scheduled in behind it.
• Re-establishing the hole - re-entering, and often re-casing, the top of the borehole before coring can start.
• More standing time, more site visits, and a borehole recorded by two different crews.

Our approach: one multipurpose rotary rig

Our Comacchio rotary rigs are multipurpose. A single rig carries out dynamic (window) sampling, hollow and solid stem augering, rotary open-hole drilling, conventional and wireline (Geobor-S) coring and rotary-percussive work, with rig-mounted SPTs and undisturbed sampling - and it switches between the hydraulic impact hammer and the high-torque rotary head in a matter of seconds.

So the same rig advances the made ground and soils, takes the SPTs and samples, and then carries straight on into the rock and cores it - all in one borehole, with one crew and one mobilisation. The rotary rig does not follow the cable percussion rig; it replaces the need for one.

What it means for your scheme

• Faster. No gap waiting for a follow-on rig to be scheduled and brought back to the hole.
• Better value. One rig, one crew and one set of transport, rather than two of each.
• A cleaner borehole. One continuous hole, cored straight off the soils - no re-entry or re-casing, and a single consistent record from top to bottom.
• Fewer site visits and less disruption, which matters on live, occupied and access-sensitive sites.

Is cable percussion ever the better call?

It still has its place - for some large-diameter work in soils, and on jobs where it is specifically preferred. But where a scheme needs soils characterised and rock cored in the same holes, a multipurpose rotary rig removes the follow-on and the gap it creates. Tell us the ground model and your objectives and we will recommend the most efficient way to get your boreholes down.