vered; rock cuttings are returned to the surface during the drilling and this allows a rapid examination of the strata. This method is often appropriate to locate shallow coal seams or mine workings and allows a number of boreholes to be completed in a short space of time.
Continuous monitoring of gases such as carbon monoxide, methane and carbon dioxide can be carried out while drilling through old mine workings.
Dynamic (Window) Sampling / Dynamic Probing
Window sampling is a percussive method of creating small diameter boreholes. Mostly the technique is achieved using hand-held equipment but some small drilling rigs have the capacity for window-sampling and the power of the rig enables window samplers to be driven to, and extracted from, greater depths compared to the manual technique.
A window sampler is a high tensile steel tube with a hardened cutting shoe to penetrate hard materials. Each sampler is usually 1 m or 2 m long with a series of “windows” or slots cut in the wall of the tube through which to view or extract soil samples. Samplers are driven down into the ground using a percussive hammer. A full set of window samplers usually consists of around four samplers ranging in diameter from 80 mm down to 35 mm. These are used systematically, starting with the widest and subsequently at reducing diameter to the required depth or limit of the technique. The depth limit tends to be around 8 m to 10 m although the technique is usually used to a maximum depth of around 5 m. In practice, the limiting factor tends not to be how deep the samplers can be driven in but rather whether the ground conditions are such that they may then be pulled back out. Coarse dense gravels, in particular, can grip the samplers and make extraction extremely difficult. The driller will assess the conditions at each location individually.
The start diameter is dictated by the ground conditions i.e. the softer the conditions, the wider the sampler set that can be driven in. Typically, however, a 60 mm to 80 mm diameter start sampler will be used, reducing to around 35 mm by around 4 m to 5 m depth. The full samplers are either jacked out manually or pulled from the hole using a hydraulic jacking system.
A full sampler will reveal a complete or partial ground profile although guidance from the driller will be needed on any compression that may have occurred and resulted in, for example, a 1.5 m sample occupying a 1 m space in the sampler. With some window sampling systems, samples can be recovered in a thin walled transparent liner placed inside the steel tubes. The liner is removed for visual inspection or capped at each end for transporting to the laboratory. Whatever system is used, the quantity of soil within the sampler is small and, particularly where the analytical suite is extensive, a sample may have to be taken from a 0.5 m or so section to ensure sufficient quantity. The advantage, however, is that once soil is taken for samples and backfilling purposes, there is rarely any excess requiring disposal.
Narrow diameter groundwater/gas monitoring installations can be constructed in the borehole once all samplers are withdrawn. Whilst the diameter of the standpipe is ultimately dictated by the diameter of the smallest (and lowest positioned) sampler, in practice it is usually possible to install a 19 mm or 35 mm standpipe with a fine gravel pack surround. With some systems, it is possible to drive a water well in through unsampled ground using the hammer unit.
This method is not suitable where there is a need to drill through an aquiclude into an aquifer as there is no way of protecting the aquifer from downwards migration of any contaminants from the ground above the aquiclude during drilling.
The equipment is generally brought to site in a van or four-wheel drive vehicle and from then on, it can be carried by hand around the site. In many circumstances the fact that this is largely a manual method is its advantage over even the smallest drilling rigs. Window sampling causes minimal disturbance to the ground surface (e.g. useful for inv