Window sampling, cable percussion or rotary: choosing a method

There is no single best method - the right one depends on the ground, the depth, the access and what you need from the samples. We run all three across a 16-rig fleet, so we specify the method to the site rather than to the rig we happen to own. Here is how they compare.

Window (windowless) sampling

A drop weight percussive hammer drives sample tubes into the ground to recover a near-continuous soil column. It is quick, low-disturbance and, on our light tracked Competitor Dart and Premier 110 rigs, gets into restricted sites. Typical reach is windowless sampling to a standard 5-6m, or up to around 10m in suitable ground conditions, and dynamic probing to around 20m, with monitoring wells installed on completion.

• Best for: shallow soils and made ground, environmental sampling, restricted access, monitoring wells.
• Limit: shallow, and it will not penetrate rock.

Cable percussion (shell and auger)

The UK's most common form of site investigation drilling: tools raised and dropped on a wire rope to advance the hole and recover samples. Our Dando rigs work 150mm to 300mm casing and tooling, handling deeper holes in soils - including hard clays, silts and sands - and supporting SPTs, U100 undisturbed sampling and monitoring or remediation well installation.

• Best for: deeper soils, undisturbed sampling, larger-diameter holes and installations.
• Limit: not a rock-coring method.

Rotary drilling

Rotary turns a bit to cut through soils and rock, and it is the method for coring rock, recovering continuous core or reaching depth. Our Comacchio GEO205, GEO300 and GEO305 rigs drill using rotary augers (solid or hollow stem), rotary through rock (open hole or coring), rotary percussive drilling with Overburden Drilling Systems, and window and dynamic sampling. A common approach is dynamic sampling with hollow stem augers or rotary casing - rapid, high-quality soil sampling while the hole is cased as sampling proceeds, with wells then installed through the centre of the hollow stem or within the casing.

• Best for: rock and solid geology, continuous core, and depth.
• Limit: more set-up than percussive methods for a simple shallow soil hole.

Choosing - in short

• Shallow soils or tight access? Window sampling.
• Deeper soils, undisturbed samples, larger casing? Cable percussion.
• Rock, continuous core or real depth? Rotary.
• A mixed profile across a site? A combination, planned together.

Send us the ground model and your objectives and we will recommend the right method, or mix of methods, and get the rigs and crews to site across the UK on our own transport.