INVESTIGATION
AUGER DRILLING
Auger drilling is the most common technique used for our site investigation drilling. Advantages of auger drilling over other methods are:
- ease of setting up;
- speed of auger drilling; and
- the provision of continuous disturbed sampling.
All of our drilling rigs are capable of auger drilling and have a standard set of augers enabling drilling to 20m or more. The type of augers most commonly used are of about 100mm diameter in lengths of 1.5m. Depths of about 30m may be achieved by larger rigs using augers in good soil conditions.
The auger strings are generally fitted with one of two types of bit; the 'V' bit or tungsten carbide (TC) bit. The TC bit is used more frequently and is able to drill in rock or to penetrate fill, concrete or boulders, etc, though damage to the bits can occur in these materials. When using a TC bit, rock quality and continuity can be assessed by variation in drilling resistance and from examination of recovered rock fragments; it must be recognised that this is an assessment of the rock strength and quality only, and cored boreholes may be necessary for cases where the rock strength or defects are critical.
During the drilling process, the augers are withdrawn from the borehole at intervals to allow sampling and insitu testing. Samples may be collected (after withdrawal) directly from the auger flights, but the samples can be very disturbed and layers mixed. Information from the auger sampling is of relatively lower reliability due to mixing or softening of samples by groundwater, or uncertainties as to the original depth of the samples; these uncertainties can be overcome by completing Standard Penetration Tests (SPT) which provide relatively undisturbed samples of soil strata, or by obtaining 'undisturbed' samples in specialised sampling tubes (U50).
Augering is a relatively economical means of drilling in clays and in sands above the water table but is of limited reliability below the water table, particularly in cohesionless soils.
WASH BORING
Wash boring refers to the method of advancing a borehole with a rotary bit where the cuttings are removed by the circulation of fluid. Drilling is effected by the cutting action of the rotating bit which is kept in firm contact with the face of the hole. The bit is carried on hollow jointed drill rods which are rotated by the drill head. The drilling fluid is pumped through the centre of the drill rods, and returns up the annulus of the borehole. Only major changes in stratification can be determined from the cuttings, together with some information from 'feel' and rate of penetration.
Unless the soils being drilled are cohesive and self-supporting, drilling mud is used as a circulating fluid to stabilise the borehole. The term 'mud' encompasses a range of products ranging from bentonite to polymers such as Quickmud or Lo-Loss. The mud tends to mask the cuttings and reliable identification is only possible from intermittent intact sampling (eg. from SPT and 'undisturbed' sample tubes) or from rock coring, etc.
Wash boring is usually required below the water table when it is necessary to obtain reliable test data from Standard Penetration Tests (SPT) or undisturbed samples. The borehole is normally started at the surface using augers and once the water table is encountered, the hole is cleaned and casing inserted to a short distance below the water table, prior to the commencement of wash boring.
Coring of bedrock is undertaken using drilling bits with small diamonds being either set into the surface of the bits or impregnated into the tip of the bit. The drilling bits are normally attached to triple tube (occasionally double tube) core barrels. During coring, drilling fluid, most commonly water, is passed through the drilling rods and core barrel to cool the coring bit, and to flush the cuttings from the borehole. The fluid and cuttings return around the outside of the core barrel and drilling rods to the surface.
The aim of diamond drilling for geotechnical investigation is to maximise core recovery as the zones of interest (clay seams, weathered seams, fractured seams, etc) are the first to be 'lost'.
Before commencing coring, it is normal to run casing into the upper surface of the rock to provide a seal to aid the return of flush water and cuttings to the surface. If the borehole has been drilled to rock using drilling mud and it is considered impractical to run casing down to the rock, then it is possible to continue to use drilling mud as the fluid for coring but there is some risk of the barrel becoming jammed and sometimes a whole drill string may have to be abandoned.
Coring is usually undertaken with truck or track mounted drilling rigs. Where there is no access for drilling rigs, such as due to narrow or steep access or headroom limitations, coring can also be undertaken using portable 'Melvelle' drilling equipment which comprises a petrol powered hydraulic pump and a hand-held drilling head. This equipment has significant limitations and can only be used when there is a limited depth of sandy soils (say less than 2m to 3m) due to the risk of drill strings becoming jammed. A double tube core barrel is usually used with the portable equipment, and hence there may be more drilling induced fracturing of the core and more core loss than would occur with triple tube barrels on track or truck mounted drilling rigs. Whilst relatively slow, it is possible to drill to depths of 20m with this equipment.
When there is no access for track or truck mounted investigation equipment, or where only limited subsurface information is required, hand augering can produce useful information. The information obtained by these techniques is often limited as refusal of the hand augers can occur on a variety of materials such as gravel, concrete brick or timber fragments in fill, or hard clay or gravel in natural soils, and not necessarily at rock level. Investigation with hand augers is usually limited to depths of the order of 2m in clayey soils or 4m to 5m in sandy soils. It is not possible to drill with hand augers below the water table in sands.
Hand auger drilling returns disturbed samples of the soils only, and gives very little information on the strength of the soils apart from assessment from the difficulty of drilling. Therefore, hand auger drilling is usually used in conjunction with Dynamic Cone Penetrometer (DCP) testing. The DCP test comprises a standard weight and drop slide hammer which is attached to 10mm diameter steel rods with a pointed tip. The DCP test rods are extendable in 1m lengths. The number of blows to drive the testing rods for each 100mm increment are recorded, and the strength of the soil is estimated by correlation with the DCP blow counts per 100mm. Whilst it is possible, in the right conditions, to complete DCP tests to depths of the order of 10m, testing is usually limited to about 4m to 5m.
These are normally excavated with a backhoe or a tracked excavator, allowing close examination of the insitu soils if it is safe to descend into the pit. Due to the nature of test pits, caution should be taken at all times when entering a test pit for logging purposes. WorkCover requires that shoring or other support is to be used for all vertical excavations deeper than 1.5m. The depth of penetration is limited to about 3m for a backhoe and up to 6m for an excavator. Limitations of test pits are the problems associated with disturbance and difficulty or reinstatement and the consequent effects on close-by structures. Care must be taken if construction is to be carried out near test pit locations to either properly recompact the backfill during construction or to design and construct the structure so as not to be adversely affected by poorly compacted backfill at the test pit locations. Test pits may be unsuitable in livestock paddocks as the settlement of poorly compacted test locations can cause a hazard to the livestock.
ELECTRIC FRICTION CONE PENETRATION (EFCP)
Cone penetration testing (sometimes referred to as a Dutch Cone) is carried out using an Electronic Friction Cone Penetrometer (EFCP).
In the tests, a 35mm diameter rod with a conical tip is pushed continuously into the soil, the reaction being provided by a specially designed truck or rig which is fitted with a hydraulic ram system. Measurements are made of the end bearing resistance on the cone and the frictional resistance on a sleeve just behind the cone. Transducers in the tip of the assembly are electrically connected by wires passing through the centre of the push rods to a computer-based data logging system in the truck.
Pore water pressures can be measured between the cone and friction sleeve, and dissipation testing can be used to determine consolidation characteristics.
The ratios of the sleeve resistance to cone resistance will vary with the type of soil encountered, with higher friction ratio in clays than in sands. Friction ratios of 1% to 2% are commonly encountered in sands and occasionally very soft clays, rising to 4% to 10% in stiff clays and peats.
Stratification can be inferred from the cone and friction traces and from experience and information from nearby boreholes, etc. Where shown, this information is presented for general guidance, but must be regarded as interpretive. The test method provides a continuous profile of engineering properties, but where precise information on soil classification is required, direct drilling and sampling may be preferable.