In geotechnical surveying situations, different situations call for different approaches, just like with everything in life. There are those drilling rigs which are truly huge in scale and their drilling capabilities are unparalleled but whose footprint is sizable. Then, there are those rigs which have a smaller footprint but diminished capability as a result.
Unsurprisingly, there are also a range of rigs and techniques that sit somewhere between on this wide-ranging spectrum. Window sampling is one such drilling methodology. Bigger than handheld drilling options and a rig in its own right, a window sample ground investigation provides an effective drilling solution whilst still displaying minimal disturbance when compared with its larger counterparts.
The team here at Borehole Solutions put together this blog to look further at window sampling; what it is, how it works and when to use it?
What Is Window Sampling?
First and foremost, let’s look at what exactly comprises window sampling and how does it work? This methodology involves using a hydraulic drop-hammer (a pneumatic hammer is sometimes used if the sampling is being carried out in a handheld manner) to insert cylindrical rods and sample tubes (of around one metre in length and between 50mm and 80mm in diameter) down into the ground to create the borehole. The sample tube has a square cut out in its side which resembles the shape of a window, and it is from this cut-out section that the method gets its name.
As the rods and sample tube are driven into the ground, the soil enters the opening in the sample tube. When the required sampling depth has been reached, the inserted rods and sample tube are then extracted. This is done using either a manual or hydraulic jack. Care must be taken in less consolidated formations to install casings to prevent borehole collapse. Some instances require that the cored sample be isolated, in these cases a uPVC liner system is used to extract the soil profile in its full 1m length, as opposed to the relatively small soil sampling conducted through the window of the sample tube in the window sampling method.
This is windowless sampling, a similarly effective and commonly used method. Once back at the surface, the core can be sampled from the window at the side of the tube. Following this, an initial analysis and review can be carried out on-site. Alternatively, the sample can be sent off to the laboratory for more in-depth analysis.
When To Use Window Sampling?
This method is ideal for smaller scale soil sampling and site investigations. This sampling technique tends to be used in sands and more cohesive formations for geotechnical and chemical investigations. Generally speaking, window sampling utilises rigs such as the Dando Terrier and the Archway Dart; these rigs can provide a range of testing, from SPTs to U38s and many more. Window sampling can also be used for the following situations: contaminant logging and the installation of monitoring wells (both water and gas).
The sampler itself is almost always mounted on rubber tracks and is highly manoeuvrable as a result. This, when combined with the relatively diminutive frames of the rigs (the rigs primarily used tend to only weigh in at around 150Kg – 200Kg), makes it ideal for more environmentally sensitive situations whereas minimal disturbance as possible is required.
The Benefits & Limitations Of Window Sampling
The main benefits of this sampling method are that it’s cost-effective whilst still providing high-quality, almost entirely undisturbed samples. It also requires very low maintenance whilst at the same time offering a high degree of reliability. There’s a reason, after all, why this methodology has become so popular amongst geotechnical drilling companies.
Whilst it’s certainly a popular sampling method, there are situations where window sampling isn’t suitable. For example, it isn’t designed for sampling or coring in harder formations and strata. Another limitation of this method is that it requires a minimum of two people in order to carry it out. Though in reality, this isn’t an issue for most larger geotechnical drilling companies. The only other limitation of window sampling is that it can only be used in shallow formations, with a maximum depth of around 4.5m generally accepted as being its limit.
This geotechnical sampling method provides a cost-effective solution for smaller scale investigations and those jobs where a more environmentally conscious approach is required. Whilst window sampling has its limitations, what sampling method doesn’t? In any case, these limitations are far outweighed by its overall capabilities as a sampling method. So, if you’d like to find out more about our window sampling services, then get in touch!
Contact Borehole Solutions today on 01733 200 501.