Investigation Methods and Procedures - Geotechnical Engineering Investigation - page 106

Rubber
seal

Bronze ball
Retaining pin
Head assembly

O-ring

30-in. brass tube Screws

FIGURE 2.63
Thin-wall “Shelby tube” sampler. (Courtesy of Sprague and Henwood, Inc.)

Soft soils are difficult to sample and retain because they have insufficient strength to
push the column of fluid in the tube past the ball check valve. In stiff to hard cohesive
soils, samples are often taken by driving heavy-gage tubes ( Figure 7.79 illustrates a driven
sample of glacial till).

Standard Stationary Piston Sampler
A thin-wall tube is attached to a head assembly. The tube contains a piston ( Figure 2.64),
which is connected to a rod passing through the drill rod to the surface. When at the bot-
tom of the tube, the piston prevents soil from entering the tube as it is lowered into the
hole and permits seating through soft cuttings. The rod connected to the piston is held
fixed at the surface, while the hydraulic system on the drilling machine presses the tube
past the piston into the soil. With light rigs, the reaction can be increased by using Earth
anchors. In properly fitted piston samplers, a strong vacuum is created to hold the sample
in the tubes during withdrawal from the hole. The stationary piston sampler is used to
retrieve very soft to firm cohesive soils.

Osterberg Hydraulic Piston Sampler
A thin-wall tube contains an actuating and a fixed piston ( Figure 2.65) . An opening in the
head assembly permits applying fluid pressure to the actuating piston at the top of the
tube. Fluid pressure is applied to the actuating piston, which presses the tube past the
fixed piston into the soil. The actuating piston eliminates the cumbersome rods of the
standard stationary piston as well as the possibility of overpushing. The sampler is com-
monly used for very soft to firm cohesive soils.

Shear-Pin Piston
This device is similar to the Osterberg sampler except that the tube is attached to the pis-
ton with shear pins, which permit the fluid pressure to build to a high value before it
“shoots” the piston when the pins shear. The sampler can be used in soft to stiff cohesive
soils and loose sands. In loose sands disturbance is unavoidable. The “apparent” density,
however, can be determined by measuring the weight of the total sample in the tube and
assuming the volume calculated from the tube diameter and the stroke length. With the
shear-pin piston, the sampling tube will almost always be fully extended because of the
high thrust obtained.

Double-Tube Soil Core Barrels
Purpose
Double-tube soil core barrels are used to obtain UD in stiff to hard cohesive soils, sapro-
lite, and soft rock.