Soil plugging in small diameter tube samplers in silty soils

One of the challenges in a geotechnical ground investigation is to obtain high quality samples, especially from coarser soils like sands and silts. Piston samplers are commonly used in the Nordic countries to take undisturbed soils samples. For coarser soils, one commonly reported issue is that the sample is disturbed particularly during withdrawal, causing even loss of sample. To avoid this, different types of catchers can be used. The paper presents some preliminary results of a study aimed to improve sampling quality for silty soils. ST1 and ST2 piston samplers with different setups are compared in a test site near Pori, Finland. One important observation of the study was that in some conditions the sampler was plugged, causing densification, disturbance and partly empty sample tubes. To avoid plugging, lubricant was used both inside the tube as well as on the outside surface. In this paper results from four different type of sampling setups will be presented, namely 1) ST1 with lubricant 2) ST2 with lubricant 3) ST1 with catcher, and 4) ST1 without lubricant. Type 4 presents the standard method used in Finland. The amount of soil plugging was measured after sampling. The sample quality is assessed by comparing unit weight and water content. The results show that plugging appears the most for ST1 sampler with the catcher, but to some extent plugging was an issue even without the catcher. The applied lubricant reduced the plugging effect quite effectively. Another important conclusion from the study is that plugging, due to its densification effect, may lead to results on the unsafe side. This has traditionally not necessarily been noted, as the lack of sample has been
interpreted as loss of sample rather than plugging.