25.4 Open-tube samplers

25.4.1 Principles of design

25.4.1.1 General

COMMENTARY ON 25.4.1.1

Open-tube samplers consist of a tube that is open and made sharp at one end and fitted at the other end with means for attachment to the drill rods or "down the hole" sliding hammer. A non-return valve allows the escape of air or water as the sample enters the tube, and assists in retaining the sample when the tool is withdrawn from the ground.

Figure 1 shows the basic details of an open-tube sampler, which has a single sample tube and detachable cutting shoe. The use of a core catcher is discussed in 25.4.4. Figure 2 shows an open-tube sampler where the cutting edge is machined on to the tube itself.

All open-tube samplers should conform to the general geometry (diameter, length and the ratio between them) criteria specified in BS EN ISO 22475-1:2006, 6.4.2.2. Open-tube (OS) samplers should be divided into two types in accordance with BS EN ISO 22475-1:2006 — thin-wall (type OS-T/W) and thick-wall (type OS-TK/W). To be deemed a thin-walled type, an open-tube sampler should also conform to the detailed geometry (cutting shoe/sharpened edge) criteria specified in BS EN ISO 22475-1:2006, 6.4.2.3.

NOTE 1 Ideally, tube samplers cause as little remoulding and disturbance as possible when forced into the ground. However, BS EN ISO 22475-1:2006, Table 2 indicates that quality Class 1 samples from an open-tube sampler are normally only achievable with a thin-walled type.

NOTE 2 The tube design controls the degree of disturbance. The sampling procedure (see 25.4.2) is also an important factor in controlling sample disturbance.

25.4.1.2 General geometry

The open-tube samplers should conform to BS EN ISO 22475-1.

NOTE 1 BS EN ISO 22475-1 requires a length to diameter ratio of less than 20; commonly used samplers in the UK fulfil this requirement. The most common samplers are about 450 mm long and nominally 100 mm in diameter. Smaller diameter samplers (about 50 mm or 75 mm and generally of the type where the cutting shoe is not detachable) are also used in the UK.

NOTE 2 The longest sample tube in general use in the UK is 1 m (for an hydraulic piston sampler, see 25.5). For a 100 mm diameter tube this is still comfortably within the BS EN ISO 22475-1 length to diameter ratio criterion.

25.4.1.3 Detailed geometry

The cutting shoe, if used, should be of a design similar to that shown in Figure 1 and BS EN ISO 22475-1:2006, Figure 4. The cutting shoe (or sharpened edge) should embody the following features (see BS EN ISO 22475-1:2006, 3.3.11, 3.3.12, Figure 1 and 6.4.2.3.2).

  • a) Edge taper angle. For a thin-walled sampler this should not exceed 5 degrees. However, close to the cutting edge the angle should be increased to 20 degrees or 30 degrees in order to avoid an easily damaged feather edge (see Hvorslev, 1948 [28] and some figures in BS EN ISO 22475-1:2006, Annex C).
  • b) Area ratio. The area ratio represents the volume of soil displaced by the sampler in proportion to the volume of the sample (see Figure 1). It should be kept as small as possible consistent with the strength requirements of the sample tube. For a thin-walled sampler it should be less than 15%.
  • c) Inside clearance. The internal diameter of the cutting shoe, D1, should be slightly less than that of the sample tube, D3, to allow for slight elastic expansion of the sample as it enters the tube, reducing frictional drag from the inside wall of the tube and helping to retain the sample. A large inside clearance should be avoided since it would permit the sample to expand, thereby increasing the disturbance. For a thin-walled sampler it should be less than 0,5%.

NOTE In addition to a) and b) above, BS EN ISO 22475-1:2006, 6.4.2.3.2 allows for taper angles of between 5 degrees and 15 degrees and an area ratio of up to 25% but only if it can be demonstrated that the sample quality class is not affected. It also notes that for area ratios exceeding 15% the angle of the cutting edge decreases as the wall thickness increases

Outside clearance is defined in BS EN ISO 22475-1:2006, 3.3.13 but no quantitative criteria are set. The outside diameter of the cutting shoe should be kept to a minimum, without being less than the outside diameter of the sampler.

25.4.1.4 Wall friction

In addition to a suitable inside clearance (see 25.4.1.3), wall friction should be reduced by maintaining a clean, smooth finish to the inside of the tube. A lubricant may be used within the sampler but it should not change the character of or contaminate the sample or the ground.

Figure 1 Basic details of open-tube sampler
Basic details of open-tube sampler

Key

  • 1 Connection to boring rods or sliding hammer
  • 2 Non-return valve with ports having a cross-sectional area sufficient to allow free exit of water and air above sample
  • 3 Overdrive space
  • 4 Sample tube
  • 5 Screw socket
  • 6 Cutting shoe
  • 7 Core catcher
  • D1 ID cutting shoe
  • D2 OD cutting shoe
  • D3 ID sample tube
  • D4 OD sample tube

25.4.1.5 Non-return valve

The non-return valve should have a large orifice to allow air and water to escape quickly and easily when driving the sampler, and a seal to assist in retention of the sample when removing the sampler from the borehole.

25.4.2 Sampling procedure

COMMENTARY ON 25.4.2

BS EN ISO 22475-1:2006, 6.4.2.5 gives a procedure for sampling with an open-tube (and a piston) sampler.

Before a sample is taken, the bottom of the borehole or surface of the excavation or heading should be cleared of loose or disturbed material as far as possible.

NOTE 1 Some or all of any such loose or disturbed material that is left normally passes into the overdrive space of the sampler.

Below the water table, certain types of soils occurring below the bottom of the borehole or excavation might be disturbed if the natural water pressure exceeds the pressure imposed by the water within the borehole or excavation. To reduce this effect, the level of the borehole water should be maintained above the groundwater level appropriate to the location of the sample.

The sampler should be advanced into the ground by a continuous static thrust or by dynamic means (using a drop weight, sliding hammer or percussive head). With a static thrust, the sampler advance should be made in one continuous motion.

NOTE 2 Static thrust can be achieved by pushing with the rig (where it is equipped with a suitable hydraulic thrust mechanism), or by an independent hydraulic jack or block and tackle.

If the sampler is dynamically driven, the drop weight should preferably impinge directly on the sampler head and have a mass sufficient to achieve the required penetration by the minimum number of blows. The quality of the sample that can be achieved is affected by these sampling procedures and the maintenance of the sampler in the vertical position.

NOTE 3 In the UK, cable percussion boring customarily makes use of down the hole hammers, which are manufactured typically with a slide length of less than 700 mm and a mass of either 40 kg or 50 kg. However, the energy imparted is subject to the operator's judgement of actual drop height, whether the mass is supplemented with "sinker bars" coupled to the hammer and whether there is buoyancy effect due to water in the borehole.

NOTE 4 In the UK, dynamic sampling rigs generally have either percussive heads or frame mounted drop weights which are used for dynamic driving the sampler via a string of rods.

The distance that the tube is driven should be checked and recorded because, if driven too far, the soil is compressed and damaged in the sampler; the penetration should not exceed the effective sampler length, particularly where the sampler used does not include an overdrive space (such as in Figure 1). In such cases the drive length should be limited to allow some head space to prevent compressional damage. The thrust or driving effort for each sample may be recorded as a qualitative indication of the consistency of the ground.

NOTE 5 A sampling head with an overdrive space (see Figure 1) allows the sample tube to be completely filled without increased likelihood of damaging the sample.

After driving, the sampler should be kept in position for a few minutes and then, if possible, rotated or raised slowly.

NOTE 6 This allows adhesion to develop between the sample and the sampler tube and then the sample to be sheared off at the bottom edge of the sampler.

The sampler should be steadily withdrawn. The length of sample that is recovered should be recorded, compared with the distance that the tool was driven, and any discrepancy investigated.

NOTE 7 For example, if the length of the sample is less than the distance driven, the sample might have experienced some compression or, alternatively, the sample tool could have permitted some of the sample to slip out as the tool was being withdrawn.

BS 5930:2015 Code of practice for ground investigations