Indian Standard Code IS 8763
1978 Edition

IS 8763 recommends two primary methods for relatively undisturbed sampling of uncemented sands: stationary piston sampling with drilling fluid circulation, where a piston sampler is pushed into the sand while drilling fluid circulates to stabilize the borehole and reduce disturbance, and compressed air sampling, which uses compressed air to maintain borehole stability by displacing groundwater. Both techniques aim to obtain samples with minimal disturbance suitable for site characterization and in situ density measurements, while rotary core drilling is suggested primarily for cemented sands.

The stationary piston sampling technique involves lowering a thin-walled sampler with an airtight piston into a borehole filled with drilling fluid such as bentonite slurry. The piston rod is fixed in position at the sampler shoe to prevent soil disturbance and entry of debris. As the sampler penetrates the virgin soil, a partial vacuum is created above the piston during retrieval, minimizing disturbance. Continuous circulation of drilling fluid coats the sampler shoe, maintaining sample integrity by preventing washing away of sand particles. The drilling fluid’s specific gravity is adjusted based on sand grain size, relative density, and groundwater conditions, for example around 1.05 for fine sands.

According to IS 8763, the drilling fluid should be a bentonite slurry that is continuously agitated to maintain uniform consistency. It circulates through the drill rod during sampling to support the borehole walls and protect the sample. The fluid’s specific gravity varies depending on the grain size of the sand, relative density, and groundwater conditions; for fine sand, a specific gravity near 1.05 is recommended. The fluid film coating the sampler shoe helps preserve the sample by preventing disturbance during withdrawal. A separation tank is used to recover sand particles from the fluid for recirculation.

Compressed air sampling involves pumping air continuously into a bell-shaped chamber at the bottom of the sampler to displace groundwater, thus preventing water ingress and sample disturbance. The sampler tube is withdrawn into this air-filled bell, isolating the sand sample from water. The air pressure is carefully regulated to match the hydrostatic pressure of the groundwater column in the borehole. This method is especially effective for collecting relatively undisturbed sand samples below the water table without contamination from drilling fluids.

To maintain sample integrity, IS 8763 advises rotating the sampler before withdrawal to shear the sample cleanly at the tube base, locking the piston to prevent downward movement during cutting, and carefully removing drill and piston rods in stages to avoid disturbance. For preservation, freezing or chemical impregnation techniques are used to solidify the sand samples, employing materials such as kerosene at subzero temperatures, alcohol combined with dry ice, emulsified asphalt, or grout mixtures. These methods stabilize the sample structure during extraction, transport, and storage, ensuring representative and undisturbed specimens for laboratory testing.