Geotechnical Engineering

Introduction to Geotechnical Engineering

Geotechnical drilling is mainly used as part of the investigation process carried out on site prior to construction and is usually undertaken by specialist contractors who are qualified to operate the geotechnical machinery (HanjinNordmeyerBoart LongyearDandoPilconBonne Esperance and many more), in addition the drilling is supervised by a geotechnical engineer who oversees the process to ensure that the drilling meets the requirements of the project.

As part of the construction process, geotechnical drilling involves preparation for foundations, caissons, and various supports. The supervising engineer verifies the positioning of the drill and makes certain that the drilling is carried out correctly. Sinking a hole in the wrong spot or drilling incorrectly can cause problems.

Typical uses of geotechnical drilling are site analysis for project suitability. This involves drilling to collect rock and soil samples, as well as drilling to determine soil stability and other matters of interest.
Site analysis is crucial for very big structures. Unsafe rock or soil conditions could trigger a structural collapse or cause a hazardous scenario during an earthquake or flood.

To be successful in your geotechnical endeavour in addition to your geotechnical drilling rig you will need to have precise processes to follow to enable the safe and effective drilling through all ground conditions. Improper procedures and methods can impair the intended outcome and even worsen the ground properties. There is an array of techniques and technical aspects to take into consideration for each specific project.

The specialty geotechnical construction processes of grouting, anchoring, micro piling, soil nailing, and ground freezing all require the drilling of holes through overburden and/or rock. Such holes are typically 75 to 300 mm in diameter and are rarely more than 60 m deep. Holes may range in inclination from vertically upwards to vertically downwards, with most holes for grouting, micro piling, and freezing being within 30º of vertical, and most holes for anchoring and nailing being within 30º of horizontal. Although rock masses are naturally variable in terms of strength and structure, overburden – from the drilling viewpoint – usually poses far greater difficulties to the drilling contractor.

Overburden is rock or soil overlying a mineral deposit, archaeological site, or other underground feature and is regarded as any non-lithified material, either deposited / formed by nature or placed / created by man. Such material may range from soft and loose to hard and dense, and from dry to saturated. Overburden may contain alien and/or atypical inclusions or horizons which will be problematical to penetrate – for example boulders or deep foundations in soils.  Such conditions will challenge the drilling contractor who, for financial reasons, will always want to drill the holes as quickly as possible, with the minimum practical “footage” cost.

There are three methods of rock drilling for production holes:

1a           Rotary High rotational speed, low torque and thrust

Rotary High rotational speed (above 600 rpm), low torque and low thrust: relatively light geotechnical drill rigs can be used to extract core samples, when using a core barrel system, or can also be used simply to drill “footage,” using “blind” or “plug,” surface set or impregnated, diamond, or tungsten insert bits.   The method is typically used for holes up to 75 mm diameter to depths of 50 to 150 m.

Advantages of high speed rotary drilling rigs

• The same equipment can be used for both investigatory and production hole drilling.

• Continuous or intermittent exploration of the rock is possible over the entire length of the hole.

• Drilling can be done to relatively great depths (300 m).

• Relatively straight holes can be drilled with less deviation than top hammer rotary percussion.

• No or limited clogging of the rock fissures.

• It is possible to drill in all kinds of rock.

• It is possible to use most power alternatives to drive the equipment (i.e., air, electricity, diesel).

Rotary drill bits produce smooth hole walls which make subsequent packer installation easier for rock grouting.

• Good penetration speeds can be achieved in soft formations.

• No vibrations are imparted to the rock formation and adjacent structures.

Despite these advantages which are widely exploited in certain applications (e.g., deep mineral mines), the use of this drilling method is declining in geotechnical construction, largely on economic grounds under competition from rotary percussive drilling in particular. Rarely are coring methods used for production drilling, except in situations where heavily reinforced concrete must be first penetrated.

1b           Rotary Low rotational speed, high torque and thrust

Low rotational speed, high torque and high thrust: used with heavier and more powerful rigs to drill holes of greater diameter to considerable depths. The penetration rate depends largely on the amount of thrust applied to the bit. A variety of carbide tipped tricone roller, or finger bits are used, to penetrate via “grinding and shattering” mechanisms, rotary drills equipped with continuous flight augers…are commonly used to advance uncased holes in soft rocks or soils.

2             Rotary Percussive – Top hammer – Down-the-hole hammer

In general the percussive energy determines the penetration rate. With a top hammer rig, the drill rods are rotated and percussed by the drill head on the rig.

With a direct circulation down-the-hole hammer, the (larger diameter) drill rods are only rotated by the drill head and compressed air fed down the rods activates the percussive hammer mounted directly above the drill bit.

Top hammer drilling is performed at rotation speeds of approximately 80 to 160 rpm to provide hole diameters up to 102mm. Hole depth is limited to around 60m by power availability and by hole deviation concerns.  Due to the path by which the energy is transferred to the bit (i.e., via successive rod couplings), penetration rate decreases with depth.

Down-the-hole drilling is performed at approximately 10 to 60 rpm in hole diameters above 90 mm to depths of over 100 m. Since the percussive effect is applied immediately above the bit, regardless of depth of hole, penetration rate is constant with depth and other factors equal.

Advantages of percussion drilled grout holes include:

• higher and consistent penetration rates than rotary

• relatively small, light, and mobile drill rigs can be used

• low drilling costs

• down the hole drilling provides the potential for minimal hole deviation with production rates of 5 to 15 m/hour. There are currently four basic concepts in down the hole (DTH) drilling

• Direct circulation (DC) air-driven DTH hammers, with the returning air flush in contact with the sidewalls of the entire length of the drill hole.

• Reverse circulation (RC) DTH hammers utilise dual wall drill rods and can also use air, or air with a water mist. The flush is returned to the surface through the inner orifice and so it helps to increase hole cleanliness by protecting the hole from the drill cuttings and flushing medium. Care must be taken to ensure that plugging of the inner drill rod is always avoided.

• Dual Fluid Drilling Systems (DFS) is a new concept comprising a special air-activated DTH hammer, which incorporates a centre tube through the hammer body that allows water to be used as the sole flushing medium. The driving air is exhausted between the outer casing and the inner drill string and so never contacts the rock. This system has the lowest DTH penetration rate potential.

Water DTH Hammers (WH) use water at high pressures (about 20 MPa) to activate the hammer and flush the hole. A potential technical draw back is that the formation will be exposed to these very high pressures, resulting in the possibility of localized hydrofracture. In principle the prime technical controls over the choice of drilling method should ideally be the geology, and the hole depth and diameter.

Other considerations such as hole linearity and drill access restraints may also have significant impact on choice on any given project.

3              Rotary Vibratory (Sonic)

Rotary Vibratory (Sonic) This technique was developed in the late 1940s and is becoming increasingly popular where strong environmental restraints are in force. It is a dual cased system that uses high frequency mechanical vibration to provide continuous core samples, or simply to advance casings for other purposes, such as deep wells or freeze holes. The string is vibrated at continuously adjustable frequencies between 50 and 150 Hz and is rotated slowly in harder formations to evenly distribute energy and bit wear. The frequency is adjusted to achieve maximum penetration rate by coinciding with the natural resonate frequency of the drill string. Resonance provides extremely high energy to the bit, and in soil it also displaces laterally the particles, greatly facilitating penetration rate. Penetration is optimized by varying frequency and thrust parameters.

Advantages

• can provide continuous cores in soil (75- to 250-mm-diameter) without using flushing media, at very high penetration rates

• can readily penetrate obstructions (natural and artificial)

• has been used to depths of 150m

• can easily convert to other types of rock or overburden drilling

• requires no flush in overburden and only minor amounts in rock. Several major geotechnical construction-related applications have been recorded to date including projects through dam embankments. The rotosonic system has exceptional potential for rock and soil drilling in certain combinations of circumstances.

Drilling Equipment

In brief there are several key factors which should logically lead to the selection of the best rig for any project:

• Power: must allow the chosen drilling method to be used with maximum efficiency.

• Thrust/Pull back: must give appropriate thrust during drilling, and sufficient pull back from the maximum drilling depth.

• Manoeuvrability: must permit the drill hole to be drilled at every angle and azimuth with minimal delay between set ups.

• Stability: must ensure that the mast remains constant in orientation during drilling with minimal vibration or “drift”.

• Accessibility: must permit access to all hole locations, even if in low headroom, tight access locations or steep slopes.

• Noise emission: must satisfy regulatory requirements.

• User friendliness: must be readily and safely operable by personnel who may not be fully acquainted with the specific rig type.

There are a wide range of equipment sizes, ranging from very light frame-mounted/ hand-held gallery rigs through the “work horse” 100- to 150-Hp diesel hydraulic track rigs to large, almost wholly automatic rigs with all the rods and casings pre-mounted in magazines or carousels.

Even very powerful rigs can operate in low headroom conditions (less than 2.5 m) and by virtue of modular construction or “expandable” track bases, can reach drill locations through man entry openings (1 m wide). Rigs often can use a variety of rock and overburden drilling systems and all flush types. Drill rod/casing lengths as little as 1 m or as much as 20 m can be accommodated depending on project constraints.

Rigs can operate off air, diesel, or electric power sources, in open air locations, cliff sides, underground galleries or tunnels. Specially designed features such as swivelling drill heads and hydraulic rod and casing breakers reduce human effort, increase safety, and promote higher outputs. The ability of contractors to use cranes, rough terrain vehicles, trucks, and excavators as well as conventional crawler-mounted bases to mount drill systems upon, is a common benefit to all parties, and the project itself.

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