25.4.3 Thin-walled open-tube samplers (Type OS-T/W)
COMMENTARY ON 25.4.3
Thin-walled samplers consist of a steel tube whose lower end is shaped to form a cutting edge. A typical thin-walled sampler is illustrated in Figure 2. An alternative thin-walled sampler incorporates a separate cutting shoe (and an optional core catcher); this sampler, see Gosling and Baldwin, 2010 [29], has been developed from the U100 (see 25.4.4).
These samplers are usually only suitable for fine soils up to firm or stiff consistency, and free from coarse particles, although samples have been successfully obtained from very stiff soils. They can give Class 1 samples of all fine soils, including sensitive clays, provided that sinking the borehole has not disturbed the soil. Samples between 70 mm and 120 mm in diameter are usually obtained.
The detailed geometry of thin-walled samplers should conform to 25.4.1.3. They should be pushed into the soil by continuous static thrust. Alternatively, they may be driven in accordance with 25.4.2 but a lower quality of sample is likely to be recovered.
NOTE 1 Disturbance at the base of the borehole occurs in weak soil below a certain depth because of stress relief. Piston samples penetrating well below the base of the borehole are, therefore, preferable (see 25.5).
NOTE 2 A variant of the open-tube sampler was developed specifically for recovering samples of sand from below the water table which presents special problems because the sample tends to fall out of the sample tube. A compressed air sampler as described by Bishop, 1948 [30] enables the sample to be removed from the ground into an air chamber and then lifted to the surface without coming into contact with the water in the borehole. The sampler normally takes samples 60 mm in diameter. If the sampler is driven by dynamic means, the change in volume of the sand caused by the driving gives a sample quality no better than Class 3. If static thrust is used, however, Class 2 and sometimes Class 1 samples can usually be recovered. This type of sampler has largely fallen out of use in the UK.
25.4.4 Thick-walled open-tube samplers (Type OS-TK/W)
COMMENTARY ON 25.4.4
A thick-wailed open-tube sampler is by definition one that does not comply with the criteria for being thin-walled. In the UK the most common of these is the 100 mm diameter thick-walled open-tube sampler (U100) which has been the tube sampler for routine use with cable percussion boring. The sampler is illustrated in Figure 1 and Figure 3, and consists of a rigid steel or alloy sample barrel, about 450 mm in length, with a screw-on cutting shoe and drive head.
The thick-walled open-tube sampler can be used in all fine soils and in extremely weak to very weak rock, such as the Southern Province chalk and in the weathered mudstones of the Mercia Mudstone Group. In fine soils of stiff or lower consistency, the standard open-tube sampler (see Figure 1) might give Class 2 samples but, more often, Class 3 samples. In brittle or closely fissured materials, such as certain weaker rocks and very stiff clays, and also in fine grained soils, the sampler gives Class 3 samples; this is because sampling disturbs these materials, reducing the sample quality to Class 3, or Class 4 if water has been added to the borehole.
The area ratio of a U100 should not normally exceed 30% and inside clearance is typically about 1%. The use of a liner system, which increases the area ratio up to 50%, should be taken to reduce the sample qualities by one or more classes. The sampler should be driven into the ground, normally using a sliding hammer or drop weight in accordance with 25.4.2.
NOTE Soils such as silt and some sands might be recovered with the U100 but are prone to fall out when the sampler is withdrawn from the ground. Sample recovery can be improved by inserting a core catcher between the cutting edge and the sample barrel, as shown in Figure 1 and Figure 3.
Key
| 1 Connection to boring rods | 3 Screws attaching sample tube to drive head |
| 2 Non-return valve with ports having a cross-sectional area sufficient to allow exit of free water and air above sample | 4 Thin-walled sampler tube |
Key
| A Standard system | 4 Steel body tube (enclosing plastic liner) |
| B Plastic liner system | 5 Core catcher (optional) |
| 1 Sample tube (cadmium-plated steel or aluminium) | 6 Spacing ring |
| 2 Core catcher (optional) | 7 Cutting shoe (plain or serrated edge) |
| 3 Cutting shoe (plain or serrated edge) | 8 U100 drive head (bell housing) |
25.4.5 Split-barrel standard penetration test sampler (type S-SPT)
COMMENTARY ON 25.4.5
A split-barrel sampler is used in the standard penetration test. It takes samples 35 mm in diameter and has an area ratio of about 100%. It is used to recover small samples, particularly under conditions which prevent the use of the 100 mm sampler, and gives Class 4 samples.
The equipment and procedure should be in accordance with BS EN ISO 22476-3.
