Window sampling vs rotary drilling: which method do you need?

It comes down to four things: the depth you need to reach, the ground you expect to find, how you can get on site, and the samples and testing the scheme calls for. As a rule of thumb, window sampling handles the shallow soils and the awkward access; rotary takes over for rock, continuous core and real depth. On the majority of our sites we run both, and plan them together.

Window sampling - fast, clean and into tight spots

Windowless sampling barrels, between 86mm and 116mm diameter, are driven into the ground with a hydraulic jackhammer, recovering a near-continuous soil column in clear plastic liners for excellent sample presentation. It is quick, relatively non-intrusive and reinstates cleanly, which is why it suits environmental sampling, in-situ testing of the shallow soils and the installation of monitoring wells. We can carry out Standard Penetration Tests (SPT) and Dynamic Probe Tests (DPT) through the hole, take U100 and U70 undisturbed samples, and drill in sufficient diameter for 50/63mm monitoring wells or 19/25mm piezometers.

Depths are typically in the order of 5-8m, dependant on strata, and where a wheeled or tracked rig still cannot get in, handheld window sampling reaches anywhere a driller can get to on foot. What window sampling will not do is go deep or penetrate rock - that is where rotary comes in.

Rotary drilling - for rock, core and depth

Our crews deploy a range of rotary techniques through modern, multi-purpose rigs to achieve good core recovery in any geological formation. Boreholes are progressed by a combination of dynamic sampling and rotary coring, with rotary casing or hollow stem augers used to get through the geology encountered, to depths in the order of 200m. Wireline coring is available as an alternative to conventional coring - more efficient at greater depths, and good for class 1 recovery in softer formations. Both air and water flush can be used, and undisturbed sampling and geotechnical testing can be carried out at any stage in the hole. For deep open-hole work we drill in excess of 300m and in diameters up to 300mm.

Choosing between them

• Shallow soils, restricted access, environmental sampling or monitoring wells? Window sampling.
• Rock, continuous core, or depth beyond the reach of percussive methods? Rotary.
• A mixed profile - soils over rock, across a large site? Both, planned as one scheme.

In practice it is rarely either/or

Most investigations use window sampling for the shallow and awkward positions and rotary where holes need depth or rock - and more often than not, both happen in the same borehole. Our rotary rigs are multipurpose: a single Comacchio rig switches seamlessly between dynamic (window) sampling and rotary drilling, moving from the hydraulic impact hammer to the high-torque rotary head in a matter of seconds. That means we can sample the made ground and soils and then carry straight on into rock in one hole - no changing rigs, no demobilising.

Because we own and operate the full range - dedicated window samplers alongside those multipurpose rotary rigs - and we are one of the few UK contractors to run our own transport to move them, we can specify the method to the ground rather than to the rig we happen to have free. Send us the ground model and your objectives and we will recommend the most efficient combination before you finalise the specification.