INTRODUCTION This paper presents details about large diameter rotary drilling techniques typically employed in the civil engineering industry. These techniques have also been applied to the resources / mining industry where deep pre-sinks are sometimes required in soft ground prior to blind boring or raise boring for deep ventilation or access shafts. The traditional approach to excavating pre sinks for raise bored shafts was to blind sink with miners working on the floor of the shaft, with all the inherent risks. The methods described in this paper do not require any personnel to work inside the shaft. Piling Contractors was established in 1983 and is now part of the worldwide Keller Group, a UK based publically listed company. Piling Contractors operates in all Australian states offering large diameter drilling, piling, foundations, retaining walls and ground improvement work. The company has developed large diameter drilling techniques in soft ground with applications in the civil construction area such as : 1) Bridge construction where high bending moment capacity are required. 2) Man holes for sewers and stormwater drains 3) Sub surface pump stations. 4) Access shafts for tunnels. 5) Footings for structures such as cooling towers and power stations 6) Power pole foundations. Over recent years large diameter rotary drilling using the auger and bucket system has also been applied in the mining industry for excavating: 1) Shaft pre sinks in soft ground up to 7.5 m diameter and to depths of up to 100 m. 2) Access shafts. 3) Bulk sampling for iron ore, coal and diamonds. ROTARY DRILLING METHODS Development of large diameter augering and bucket drilling techniques In Australia up until the early 1990s, rotary drilling using the auger and bucket system was typically limited to 1800mm diameters by equipment capability and convention. Drills were either truck or crane mounted and their torque capacity ranged from 6 to 20 tonne metres. Most rigs were driven through conventional gear boxes and direct drive rotary tables. The drilling tools used were buckets and augers either fitted with steel flat teeth, tungsten tipped ripper teeth or tungsten tipped conical teeth. Page 1 of 12
Driven by market demand, diameters and rig capacities have increased rapidly. The common Euro rigs (self contained crawler mounted vertical mast rotary rig) are now manufactured with torque capacities 40 tonne metres or more and depth capability up to 90 metres using telescopic kelly bars. Rotation speeds are typically between 5 RPM and 25 RPM and crowd capacities vary, but are available up to 80 tonnes. Bucket diameters have also increased to in excess of 5m and with the availability of reaming arms, shaft diameters of up to 7.5m are now possible in suitable ground conditions (Figure 1 and Figure 6). Figure 1 Piling Contractors has a range of dedicated crane mounted rotary drill rigs designed and manufactured in Australia specifically for shaft drilling, with torque capacities up to 50 tonne metres and capable of drilling 7.5 m diameter shafts to depths of 100 metres (Figure 2). This equipment is suitable for drilling the clays, shales and weaker rocks that historically have proven unsuitable for raise bore equipment and other hard rock drilling systems. Figure 2 Page 2 of 12
Drilling method using Rotary Techniques The drill system of the augers and buckets comprises ground engaging tools equipped with teeth for cutting the formation by drag action. The auger drill bit, although many times larger, is similar to that of a carpenter s wood auger (Figure 3). As the formation is cut, the cuttings accumulate on the auger flights, and they are removed from the hole by withdrawing the auger to the surface and spinning it to dislodge the cuttings. Figure 3 The drilling bucket comprises a cylindrical container located immediately above a flat plate with the cutting bit. The formation cuttings are pushed into he bucket by the rotating action of the tool. Hinged metal flaps prevent cuttings from falling out as the container is withdrawn to the surface for spoil removal (Figure 4). Figure 4 Page 3 of 12
Rotary drilling can either remove spoil to the surface using conventional drill buckets or can allow spoil to bottom drop through pilot holes into adits or drives below for later removal. Pilot holes can be advantageous to either assist with accuracy or speed up drilling by bottom dumping. When pilot holes are used they are typically directionally drilled and the completed diameter of the pilot hole needs to be at least 600mm. The large diameter pilot hole is necessary to facilitate the cuttings from blocking the pilot hole and also to provide a strong guide for the drilling bucket. Figure 5 below shows a typical drilling tool with a guide stub which fits into the predrilled pilot hole. In situations where pilot holes are not available, the cuttings can be withdrawn in the drill bucket, efficiently, to the surface using the multistage telescopic kelly bars. There is no requirement to unscrew, remove and later replace drill rods, which is common place with other drilling systems. Figure 5 In order to drill to diameters of 4 m and greater, several passes are required by the drill tools and each successive pass utilises a slightly larger drilling tool. Techniques and drill tools vary with the geotechnical conditions but generally first passes would be carried out using a conventional drill bucket equipped with either steel or tungsten tipped teeth. A typical size on a first pass bucket would be 1500mm diameter to 2400mm diameter. Subsequent passes utilise reaming arms which fit to either conventional or specialist reaming buckets (Refer to Figure 6 and Figure 7). Typically a reamer blade cuts a larger diameter than the previous pass, Page 4 of 12
and the cuttings fall into an open topped bucket for removal. Figure 6 Figure 7 Page 5 of 12
For example, assuming a shaft of 4.6m diameter a typical drill pass sequence would be as follows :- o Initial pass using 2.2m bucket o Second pass reaming from 2.2m to 3.2m o Third pass reaming from 3.2m to 4m o Final pass reaming from 4m to 4.6m The rate of penetration for each pass is dependant on the geotechnical conditions, in particular rock or other strata strength, and hole stability. Other factors include the presence of and orientation of bedding planes, fracture spacing, faults and the like. Typical drilling advance rates in extremely weathered rock (assuming spoil falls into pilot holes) for the above reaming sequence will be in the order of: Initial pass Second pass Third pass Final pass 4 to 6 metres per hour 5 to 7 metres per hour 3 to5 metres per hour 2 to 4 metres per hour In medium strength rock (UCS 30MPa), the advance rates would reduce to values in the order of: Initial pass Second pass Third pass Final pass 0.8 to 1.5 metres per hour 1 to 2 metres per hour 1 to 2 metres per hour 0.5 to 1.5 metres per pass These examples should be treated as indicative only due to the many variables and generally the production rates will be reduced as rock strength increases; all other variables being equal. Production rates where the spoil is brought to the surface by the drilling bucket will be slower than quoted above. Buckets and reamer arms can be equipped with either flat steel, tungsten tipped ripper, tungsten tipped conical or tungsten roller bits to optimise the drilling progress. LIMITATIONS OF ROTARY SHAFT DRILLING 1) The physical capacity of the rotary drilling rigs limits drilled diameters to around 8.0m and depths of up to 100m with the telescopic Kelly bar. It is possible to use kelly bar Page 6 of 12
extensions on a project specific basis. 2) Rock strength is a major constraint on rotary drilling. The auger and bucket drill tools will generally become unproductive where rock strength exceeds 80MPa UCS although, it is possible to drill localised layers of stronger rock. Material up to 200MPa has been drilled with difficulty using conventional teeth. However, when roller bits are fitted, higher strength rock can be penetrated, but this is relatively slow and costly. ADVANTAGES OF ROTARY SHAFT DRILLING IN SOFT GROUND Ideal for use in low strength, weathered rock which is typically unsuitable for raise bore systems and other hard rock drilling methods. Much faster than conventional hand mining techniques so reducing the construction programme. Safer than conventional hand mining as there are no personnel required to enter the shaft. Capable of movement between multiple sites with little cost and time. Fast preparation and set up. All of the above can result in significant cost savings. RECENT LARGE DIAMETER ROTARY SHAFT DRILLING PROJECTS Piling Contractors has completed the following large diameter rotary shaft drilling projects in recent years: Eloise Mine Site ventilation shaft - depth to 57m and a diameter of 3.2m. Cannington Mine Site ventilation shafts (5 of.) from 4m to 5.5m diameter down to 50 m depths. Pajingo Mine Site, Queensland 4.2m diameter, with a depth of 42m Fosterville Mine Site, Victoria, 4.7m diameter, with a depth of 37m Mornabah Mine Site, Queensland, 6.0m diameter with a depth of 54m Utility Water, Victoria, numerous shafts to 6m diameter Ventilation and access shafts on the Transgrid Cable Tunnel project at Redfern in Sydney 4.5m diameter to RL-28m in Class 2 Sydney Sandstone. Piled foundations to 3.5m diameter and 40m deep on the Qantas Hanger at the Brisbane Airport. Manholes for the Inner City Bypass at Brisbane with diameters of up to 3.6m. Piles foundations in high strength rock to 2.2m diameter on the Wallaville Bridge at Wallaville. SPECIAL REQUIREMENTS FOR ROTARY SHAFT DRILLING Platform Preparation Large diameter rotary drill rigs are normally crawler based and very mobile, so terrain is Page 7 of 12
usually not an issue, but a firm, level self draining designed and certified platform is required at the drill site. The weight of the larger rigs can be in the order of 120 tonnes, plus tooling and spoil. It may also be necessary to elevate the drill rig to allow larger buckets to bottom open and also clear the upstand of the safety liner, Figure 8. Shaft Stability Figure 8 Dependant on near surface geotechnical conditions, the top section of shafts (0m to 10m) will most likely require some lining by either steel casings, secant piling or a sprayed concrete liner. Secant piling involves the installation of piles in a manner that they overlap (Figure 10). This is necessary to ensure long term stability of the shaft collar lining. In some cases, secant piles have been installed prior to drilling and then a steel liner has been installed after drilling down to 10 m depth. SHAFT WALL LINING One of the major considerations when rotary drilling large diameter shafts is the stability of the shaft itself. These shafts are often bored in weak to unstable ground. The ground types of most concern are granular materials such as sands or saturated clays that are inherently unstable. The most common techniques employed in the civil construction industry to stabilise these Page 8 of 12
unstable ground types include: 1. Concrete or steel liners that can be progressively sunk with the excavation. 2. Drilling fluid with polymer or bentonite. This may also be used in conjunction with liners. 3. Progressive shotcreting using remotely operated shotcrete equipment (this system was used extensively at both Cannington, Pajingo, Moranbah, Fosterville and Eloise Mine Sites). 4. Contiguous or secant pile walls. 5. Diaphragm walls. 6. Jet grouting. 7. Pressure grouting 8. Slurry wall construction. Two stabilisation methods that may be suitable for the mining sector, but to date have not been used extensively are the contiguous/secant pile wall and diaphragm wall processes. These two types of stabilisation methods are outlined below. Contiguous And Secant Pile Construction This technique is commonly used in the building and civil construction industry for retaining walls and was used at Cannington Mine Site to provide shaft collar stability on several vent shafts.. The prime uses for this type of retention system are in basement construction for multi storey buildings and retaining wall construction for roadworks and tunnels.figure 9 Figure 9 Page 9 of 12
Contiguous piles comprise of a series of small to medium diameter piles drilled almost side by side (contiguous) to form a retaining wall (Refer to Figure 10). This type of wall does not provide a barrier to groundwater inflow or loose sands which can flow between the piles, causing adjacent ground surface settlement. All Piles, strong (40MPa) mix concrete and structurally reinforced. TYPICAL PLAN VIEW OF A CONTIGUOUS PILE WALL Figure 10 A more sophisticated version is the secant pile wall where adjacent piles overlap and cut into each other (Refer to Figure 11). This type of wall provides a better cut-off for groundwater and loose sands, than the contiguous wall. The secant wall is usually constructed by using the hard/soft method where primary piles are constructed of weaker concrete and secondary piles which cut into the primary piles are cast from stronger concrete. If required, secondary piles can be steel reinforced. Secant pile walls are formed by one of these methods:- a) Conventional piles drilled with an auger where the ground is stable and does not require a temporary liner for support. b) Standard secant pile drilling techniques using a temporary hole liner for stability, and placing the concrete while withdrawing the liner. Segmental casings can also be used c) Continuous flight auger drilling using a hollow stem auger and pumping concrete down through the centre of the auger. Secant and contiguous walls are normally limited to depths of about 15m metres due to accuracy of installation. For shaft construction a circular wall can be formed using the structural strength of a hoop effect. Page 10 of 12
Secondary Pile, generally strong (40MPA) concrete and structurally reinforced Primary Pile, generally weak mix concrete and no structural reinforcement TYPICAL PLAN VIEW OF A SECANT PILE Figure 11 Diaphragm Walls Diaphragm walls are used for retaining significant depths of soil. Unlike the secant pile solution, verticality is relatively accurate due to the self plumbing system of the equipment, and the finished surface of the retaining wall is smooth. The wall is made of concrete with reinforcement as required, depending on the availability of hoop stresses, Figure 12. Construction of the wall is carried out using a clam shell, which can be either rope or kelly suspended and operated mechanically or hydraulically. Hydraulic cutters can be used to excavate significant depths into hard rocks. The walls comprise panels typically 5m wide, 600mm to 1200mm thick and as deep as required, although generally the depths are limited to 50m or to top of rock. The top 30 m of the vent shaft on the M5 road tunnel in Sydney was excavated inside a diaphragm wall as was the top 50 m of the access shaft for the gas caverns at Botany Bay. Similar Diaphragm wall shafts, 13 m in internal diameter were constructed for the Tugin Desalination Plant on the Gold Coast. Figure 12 Page 11 of 12
CONCLUSION The conventional system of auger and bucket drilling has rapidly expanded over the past few years to allow the economical drilling of large shafts with diameters up to 8.0m and depths of 100m. The auger and bucket system is ideal in weaker ground where raise bore systems and other hard rock cutting methods bog down. The system is safer than hand mining due to the absence of personnel in the shaft. Localised layers of rock with strengths of up to 200MPa can be drilled and the nature of the jointing and discontinuities in the rock will influence the speed of drilling. The conventional tungsten carbide drilling teeth can be replaced with roller cutters in high strength rock. Man access is not required for the auger and bucket drilling system. The drill cuttings can be raised quickly to the surface using the multi stage telescopic kelly bar, or alternatively, the cuttings can be allowed to fall down a predrilled pilot hole. The drilling rigs are crawler mounted and capable of moving between drill sites with little cost and time. The system has been proven on numerous sites already including the Cannington, Pajingo, Fostervile, Moranbah and Eloise Mine Sites. Stabilisation of the loose soil profile using systems other than steel liners is possible and these stabilisation methods include contiguous/secant piled walls and diaphragm walls. Page 12 of 12