Sampling forms an essential part of the geotechnical investigation or structural inspection process and good sampling is essential for proper laboratory testing of samples for determining strength and compressibility characteristics of soil. The following sampling techniques are employed for different jobs:
Disturbed samples are collected through standard penetration test in split spoon sampler. These samples are stored in plastic bags immediately after collection. The storage of split-spoon samples in plastic bags ensure retention of natural moisture of the samples which is later determined in the laboratory.
Undisturbed sampling is carried out in soft to medium stiff clay. Undisturbed sampling is done with the help of a thin walled Shelby tube. The Shelby tube consists of an open tube with a one sharp end with the other connecting to a head which acts as an attachment to the drilling rods. The Shelby tube sampling is done by pushing the rods by exerting hydraulic pressure. The tube is lifted from the borehole after sampling and is preserved by waxing the open ends of the tube.
Bulk samples are recovered from shallow test pits. These samples are collected in bags in bulk quantity (>30kg). The samples are transported to lab for determination of index properties, moisture – density relationship, and maximum dry density through Proctor’s Test.
Groundwater samples are collected from water well / boreholes from designated depth using groundwater sampler. The sampler consist of a tube closed at one end and a lid at upper end. The sampler is lowered to the desired depth, and the lid is opened. After filling of sampler, the lid is closed once the sample is collected.
Block sampling is done in situations where large-sized intact samples are required for laboratory testing.
Pavement Core Sampling
For determination of layer thickness of asphalt wearing course in a pavement, it is a practice to extract a core sample and measure the thickness of layers. Pavement core samples are extracted using a core cutting machine powered by electrical motor or gasoline engine, capable of applying enough effective rotational velocity to secure a drilled specimen.
STS is capable of providing services for asphalt core extraction up to 6 inch diameter and up to 12 inch depth.
Concrete Core Sampling
In order to determine the in-situ compressive strength of concrete, core samples are extracted and tested in laboratory. Core samples are extracted using a core cutting machine.