⸺Thameside Groundworks⸺
Window / Windowless Sampling
Window/windowless sampling is an intrusive site investigation technique which is used to obtain samples of soil, typically up to 6m below ground level, by driving hollow steel tubes into the ground to create a borehole.
Window/
windowless
sampling
Window/windowless sampling is an intrusive site investigation technique which is used to obtain samples of soil, typically up to 6m below ground level, by driving hollow steel tubes into the ground to create a borehole. These tubes are either open sided “windowed” allowing the sample to be removed using a trowel or in cases of much harder material a hammer and chisel, or “windowless” with a plastic liner inside which can be removed by unscrewing the end of the tube. These techniques allow the different strata within the soil to be logged effectively and for samples to be taken for either geotechnical or environmental laboratory analysis. Steel casing can slow be driven into the ground during the window sampling process, this casing keeps the borehole open to allow for further drilling when soft or wet ground collapsing into the borehole would make this very difficult or impossible.
Window / Windowless sampling process
When window/windowless sampling is carried out by a purpose built drilling rig additional insitu SPT testing can be completed during the drilling process, this will gain additional geotechnical information which cannot be gained through laboratory analysis. This information can be used to determine the bearing capacity of soils at given depths which can be used in determining the suitability and design of foundations.
After the window/windowless sampling has been completed the borehole can either be backfilled and reinstated or it can have an installation added. These installations allow for longer term monitoring of groundwater or gas present by inserting plastic pipework that contain slots to allow water and gas to pass into the pipe and have a rubber bung at surface to stop gasses escaping. This pipe is then surrounded by a shingle material to allow the water and gas to flow into the pipe and sealed at the surface using bentonite, a clay pellet or powder which swells and creates a watertight seal at the top of the hole. These installations typically have a raised or flush cover to protect the pipework from accidental damage and allow the area to be used as normal.
How we carry out the Window / Windowless sampling
Our window sampling is usually carried out using a tracked Premier 110 window sampling rig which weighs just under 1 ton and can fit through gaps >80cm wide allowing holes to be drilled in areas, such as back gardens, which cannot be accessed by much larger rigs.
This Premier 110 rig can also carry out heavy and super heavy dynamic probing. This is an intrusive technique which hammers a sacrificial steel cone into the ground, similar to SPT testing, allowing continuous strength information to be obtained. Dynamic probing can be carried out up to depths of around 30m below ground level, depths which would be unachievable through window sampling. This technique can be used very effectively to determine the size and depth of areas of backfill or very soft ground caused by solution features for example.
We also have a small Geotool window sampling rig, which can also carry out heavy dynamic probing, allowing us to complete an intrusive investigation and insitu testing in areas the Premier 110 rig cannot access. This rig is much smaller and lighter than the Premier 110 allowing it to be dismantled and carried into hard to reach places such as basements or areas with restricted access.