24.5 Headings (adits)
COMMENTARY ON 24.5
Headings are driven from the bottom of shafts or laterally into sloping ground and can be used for the in-situ examination of the strata, existing foundations and other underground constructions, as well as for carrying out special sampling and in-situ testing. One important use of headings is in the inspection of abutments of dams.
In soil and many types of rock, the sides and roof of the heading should be supported.
NOTE Driving a heading below the water table presents a major construction difficulty in strata that are not naturally self-supporting during the period required to erect support elements. Below the water table, it is probable that headings are only an economical means of ground investigation in rock and stiff clay.
24.6 Hand auger boring
COMMENTARY ON 24.6
The hand auger boring method uses light hand-operated equipment and generally no borehole casing is used. The auger and drill rods are usually lifted out of the borehole without the aid of a tripod. Boreholes up to 200 mm diameter to a depth of about 5 m can be made in suitable ground conditions (self-supporting strata without hard obstructions or gravel-size particles).
The boring should be advanced using a hand-operated auger. Hand auger boreholes may be used to obtain disturbed samples, small open-tube samples and for groundwater observations. Disturbed samples obtained from such borings are likely to be classified as Class 4 at best due to the mixing of soil that takes place as the tools are advanced.
NOTE Hand augers can also be used to provide access for in-situ tests, such as the field vane.
24.7 Dynamic sampling
COMMENTARY ON 24.7
Dynamic sampling is divided into two generic types, commonly referred to as window sampling and windowless sampling, depending on the downhole equipment used. For both types, the sampling tool essentially comprises a hollow steel tube, fitted with a screw-on cutting shoe, to which extension rods can be coupled as necessary as the sampling hole gets deeper. The differences in the sampling tubes are:
- Window samplers have longitudinal slots or "windows" cut in the wall on one side of the tube. The soil recovered in the sampler can be examined in the steel tube via the window and disturbed samples can be taken through it. This is the sort of equipment thought to be envisaged by the term "window sampling" in BS EN 1997-2.
- Windowless sampling tubes do not have slots, a plastic liner being inserted into the steel tube. The soil recovered can be retained as a sample in the plastic liner or as disturbed samples taken from it Sample catchers can be included between the cutting shoe and sample liner to retain soil.
The method has become popular for shallow geotechnical and, especially, geoenvironmental sampling owing to the recovery of a reasonably complete soil profile, a general absence of spoil, the relatively small size and light weight of the equipment which makes for easier access on restricted sites, smaller transport requirements and lower cost. It works most efficiently if the hole stays open unsupported with only limited squeezing; hence it is best suited to dry cohesive soils. Systems are available for samplers, whereby casings are driven simultaneously with the sampler. Dynamic sampling tubes are generally between 45 mm and 100 mm diameter and in favourable circumstances it is possible to take a sequence of samples down to a maximum depth of about 10 m.
The sampler should be driven into the ground by percussion using a hand- or rig-operated hammer. The depth interval of drive should not exceed the length of the hollow sampler tube and the equipment is then extracted either manually with a jack, or using the rig hydraulics. The process continues by inserting the sampler, or one of reduced diameter, down the open hole left by the first extraction and driving again. By repeating this process with successive reductions in sampler diameter a useful depth of investigation can frequently be achieved. (The quality classes of the samples is discussed in 25.8.)
NOTE 1 There are three generic types of dynamic sampling rigs:
- Hand held "rigs" comprise electric, hydraulic, pneumatic and petrol percussive hammers commonly used in the construction industry. Sampling depth achievable is limited due to the power and weight of the equipment, which is more suited to window than windowless sampling.
- Feed frame mounted, drop weight type rigs mounted typically on a rubber-tracked chassis, a small trailer or as a stand-alone wheeled unit. The larger units of this type can also be capable of carrying out greater diameter tube sampling operations and be able to mount a small rotary head for coring operations. This type of rig is also used to perform dynamic probing (see 38.1).
- Feed frame mounted, percussive hammer type rigs. These machines have a percussive hammer attached to a sliding carriage, which, in turn, is mounted on to a feed frame. Typically the sliding carriage is connected to the base machine's integral feed system and allows thrust to be added to the downward force of the hammer thus increasing penetration rates. Many of these percussive hammer types are combination rigs where the base machine is a rotary rig carrying a rotary head and percussive hammer both mounted on a sideways moveable plate attached to the sliding carriage.
NOTE 2 An earlier type of dynamic sample, the "flow through sampler", has largely become obsolete.