Geotechnical site investigations require specialist rigs and techniques dependent on the environment in question. For example, drilling teams can use a huge sonic rig in an open field, whereas a site with limited space may call for a dynamic sampling machine for minimal disturbance.
The team at Borehole Solutions have offered dynamic, also known as window, sampling for many years. As such, we are well-versed in this fundamental aspect of ground investigations. Let’s take a deeper look into what’s involved and when this method is used.
What is Dynamic Sampling?
This technique is ideal for restricted access sites and limited depth contamination investigations where disturbance must be minimal. Track-mounted percussive samplers are used but alternatively, in areas where access is extremely tight, hand-held jack hammer window sampling or air probe techniques can be used. The borehole is advanced by driving a steel sampling tube, with a plastic liner to retain the soil, downwards into the ground. Once the tube is lifted out, the liner is removed to enable logging by a qualified engineer.
Dynamic Probing Test (DPT)
Moreover, dynamic sampling rigs are also capable of dynamic probing tests (DPTs). This is a continuous soil testing procedure that determines the soil strength at various depths.
This involves driving a cylindrical sacrificial or fixed cone into the ground using a high frequency percussive hammer. The number of hammer blows per 100 mm of penetration is recorded until you’ve reached the required depth, or you cannot dig any deeper.
The results from DPTs are related to standard penetration tests (SPTs) and can be used to generate a visual representation of the different soil strata.
Benefits and Limitations
Dynamic sampling has considerably increased in popularity over recent years, and for good reason. It’s able to collect high-quality and almost completely undisturbed samples with ease, whilst at a cost-effective rate. What’s more, window sampling rigs require very little maintenance and are highly reliable, whilst causing minimal environmental disturbance.
However, there are a few drawbacks to this drilling method. It’s generally unsuitable when working with tougher strata and can only be used in shallow formations up to 6 metres in depth. Nevertheless, these limitations are far outweighed by the advantages.