Advances in the design and construction of reaming heads and rock tools has seen the technique of raise boring become increasingly effective to the point where it can now be used in very hard rock, in confined urban areas, and in deep underground locations, writes Goran Strand of Sandvik Mining and Construction
Raise boring, although doubtless as old a concept as underground mining and tunnelling itself, has only become an accepted method of mechanised shaft sinking during the past half-century.
The technique is used to excavate a circular hole between surface and tunnel, or between two underground levels, without the use of explosives. At its simplest, it involves miners surrounded by a protective cage, digging upwards; a process that is every bit as hazardous as it sounds. Mechanised, however, the process is transformed.
A raise boring machine (RBM) is set on a flat platform on the upper level and used to drill a pilot hole through to the cavity below. Once the drill has broken through, the bit is removed and a reamer head, of the required diameter of the shaft, is attached and pulled back upwards, rotating and cutting as it goes. The excavated material falls into the lower level and is transported out of the works.
There are several advantages to this system. One is that it is fast. Another is that without the need for explosives it can be used in urban areas or areas of geological sensitivity. It is safe, for once the reamer has been attached the only reason personnel need to be on the lower level is for removal of the spoil.
Further, this method is very cost effective. Only a small team is needed to operate the RBM, and other than the engineer in charge, they do not need to be specialist mining or tunnelling workers. Admittedly the work is generally carried out by a specialist contractor, but this is actually an advantage because the skills and experience will lead to a quality job done to a precise schedule.
So nowadays raise boring is increasingly used for the installation of production and ventilation shafts and ore passes for mines, on infrastructural projects for tunnel ventilation, and for surge shafts in hydroelectric power projects.
Working on numerous hydropower and drainage projects around the world, Sandvik has found that project designers are increasingly attracted to the idea of raise boring because of the quality of the resulting shaft, with its smooth walls and its easy progress through difficult ground. Requests for larger installations are becoming more frequent. In Portugal, for example, specialist raise boring contractor Drillcon Iberia has this year, 2012, taken delivery of a new Robbins 91R shaft boring rig equipped with a Sandvik reaming head designed for 5.5 m shafts at depths of 500 m, and anticipates using the rig on forthcoming hydropower schemes.
New generation to be launched
Sandvik has been working for some while now on the next generation of reamer head, with extensive prototype field trials taking place in the hard granite of Hong Kong and elsewhere to develop raise boring technology that ensures faster penetration rates and extended life.
Trials for a new CRH 12EXL reaming head are also nearing completion to meet the demand for the more powerful machines entering the market able to drill larger and deeper raises, with up to 8 m diameter and shaft depths of up to 1500m.
Prototype trials are also underway with promising results for a new cutter concept. Testing is scheduled to continue throughout 2012 to allow evaluation of different projects and rock conditions.
Increasing versatility of reaming heads
Raise boring can cut through soft ground and through the hardest rock, with up to 700MPa having been recorded. Bore sizes are becoming increasingly dramatic: the world record for largest diameter is 7.1 m, and the longest raise has been recorded at 1,260m.
RBMs have not changed that much in technology in recent years, as they are essentially a solid upwards-drilling unit that needs to provide consistently applied muscle. The reaming head technology has however become increasingly sophisticated, and this above all is why the technique is finding an ever-increasing market.
Sandvik’s raise boring system includes a wide range of reaming heads in a number of types, sizes and configurations for boring holes from 0.6 to 6 m in diameter. We have found over the years that simplicity is vital, and so all the basic components are bolted to each other, enabling fast and easy mounting, assembly and servicing.
Since the stem is bolted to the base-head, different sizes of stem can be fitted to suit different sizes of pilot hole. The saddles, too, are bolted to the head. The Sandvik system is based on standard components, including the rock drill bits, which contributes greatly to its flexibility and high availability.
Sandvik reaming heads are easy to adapt to different rock conditions by re-arranging the cutters. The effect is to change the spacing between the rows of cemented-carbide buttons that break the rock. To enable row spacing to be varied, only two types of cutter are needed on any one reaming head. Placing one or the other type of cutter in different positions on the head gives either wide or narrow button-row spacing.
Larger diameter Sandvik reaming heads are available in segmented versions, which enable transport dimensions and weight to be reduced for projects in which there are size or weight restrictions. We also offer a wide range of extendable reaming heads, which can be built to different diameters by fixing segments of different types and sizes to the base-head.
Building a reaming head system
Sandvik raise boring heads are based on a modular component system with reaming head set-ups covering raise sizes from 0.6 to 6.1m. A flat-head configuration together with heads designed to fit almost any diameter permit available thrust to be exploited to the full. The cutters are easily rearranged or transferred to other positions for different button-row spacing, and the same applies to the stems, heads, saddles and segments for various pilot holes and reaming dimensions.
The reaming heads are built up with a flexible system of components covering full range for any diameter required.
An RS510 integral reaming head is the first choice for any raise boring project in which there are no size or weight restrictions. The integral head is very strong and rigid, and the stem, saddles and cutters are bolted to the head and easy to service. All Sandvik integral reaming heads have a flat cutting profile for smooth rotation and low torque demands. Stem sizes from 228 to 381 mm can be used.
The RS520 segmented reaming heads are used for projects in which there are restrictions on transport weights or dimensions. The segmented design comprises a base-head and two removable segments, enabling the reamer to be transported to the collaring site via small cages and narrow drifts. The segments are then fixed to the base-head using standard tools. Stem sizes range from 228mm to 349mm.
Sandvik RS530 extendable reaming heads consist of a base-head to which four or six segments can be fitted. This means the reaming head can be built to different diameters, suitable for any project in which there are both size and/or weight restrictions and high demands for hole-size flexibility. Stem sizes from 280 to 381mm can be used, and the system allows raises of several different diameters to be bored with one and the same reaming head, thus reducing capital cost and keeping inventories to a minimum
Wider applications of technique
Increasing development of reaming head technology along with ever widening experience in the technique has led to the evolution of raise boring into methods by which a shaft can be bored from below, or horizontally. We can therefore divide the technique into the following specialisations.
Pilot drilling Conventional raise boring begins with the drilling of a pilot hole, using a roller bit with sealed bearings, together with hollow drill pipes 1.5 m in length with an international standard thread for high–torque applications.
The cuttings are removed from the pilot hole with the aid of water flushing. Introduced through the centre of the drill string, the water flows out of the drill bit and up through the annulus between the drill pipes and the hole wall. If required, the pilot drilling can be controlled by using a directional drilling system.
Raise boring When the pilot hole breaks through into the lower level, the roller bit is removed and replaced with a reaming head. The reamer is rotated and pulled back toward the drilling unit. The cuttings fall by gravity into the chamber at the bottom of the hole, where they are mucked out using an LHD-type loader. Raises up to 6 m in diameter and up to 1000 m in length are not uncommon.
Blind boring When a raise is required but there is no access to the upper level, it has to be bored blind from below, usually without a pre-drilled pilot hole. A special type of head is required for blind boring to drill the pilot hole and ream out the raise at the same time. The head is rotated and pushed upward. The cuttings fall out of the hole by gravity. Normal blind raise diameters are from 0.6 to 1.8 m. Since the drill string is under compression during blind boring, special large-diameter stabilisers are needed to support the drill string.
Horizontal boring Horizontal boring is an excellent method in urban construction projects where drilling and blasting is restricted or forbidden and tunnel boring machines (TBMs) are too bulky. First, a horizontal pilot hole is drilled, with the aid of a directional drilling system if necessary.
When the pilot bit breaks through, it is removed and replaced with a reaming head. Because the hole is horizontal, the reamer must be equipped with a special cuttings removal system.
Typical diameters for horizontal reaming are from 0.6 to 4.5m, making this method suitable for drilling tunnels through stable rock for cables, escape routes, sewage or water supply without disturbing the environment.
Reaming heads for special projects
In addition to the wide range of standard reaming heads for conventional raise boring, Sandvik offers to tailor-make reaming systems for any application that requires special design features.
Blind-hole boring systems can be provided with water-flushing, different drill-pipe to reamer connections, and different stems to fit different sizes of pilot hole. Down-boring systems can be designed with V-shaped reamers for improved cuttings removal while following a pre-drilled pilot hole, as well as different drill-pipe to reamer connections and removable wear-pad stem sleeves.
Horizontal-boring systems can be designed with scrapers and water flushing for efficient cuttings removal, and an underwater reaming system can be equipped with pressure-compensated cutters and specially segmented components that are easy to handle.
Raise boring in action
The period 2011 to early 2012 saw a classic case of an application for Sandvik raise boring.
For years, people living and working in the northwest of Hong Kong Island have had to put up with serious flooding caused by surface water run-off during tropical downpours. To solve the problem, the Drainage Services Department of Hong Kong decided to build a 10,500-m drainage tunnel with 32 urban intake shafts to catch the run-off water and drain it into the sea.
Most of the intake shafts were bored using the raise boring method, for which Australian specialist contractor Macmahon was employed, with Sandvik supplying the rock tools.
Macmahon used four RBMs, each equipped with a Sandvik reaming head for boring 23 dropshafts through the hard granite: the remaining shafts were either very shallow or were sunk in soft ground, dug by conventional means.
Raise boring was selected due to the isolated shaft intakes on the hillsides across the Mid Levels on Hong Kong Island, with difficult access and the restricted weight loadings of the adjacent pathways. It was also considered to be the safest shaft option, with no people required in the shaft during boring, and ensured the least environmental disturbance.
Macmahon said it was one of the most logistically challenging jobs the company had worked on, as the majority of the raise bore sites are in highly urbanised areas with restrictions on noise, traffic and dust emissions, and sites that are extremely tight. Not only was there very little room to manoeuvre and operate the equipment, in many cases the sites were on very steep terrain with little normal road access.
The project does however illustrate the benefits of raise boring within an urban environment, because the disturbance to the public was minimal. There was virtually no dust, and the spoil was carried out of the tunnel far away from where the shaft was bored. Noise was kept to the minimum because the rigs are very quiet, and the sound of boring was contained underground.
Macmahon’s contract was for an 18-month project period. The contractor started each shaft with a 35cm pilot hole using Sandvik Pilot bits and 28.6cm drill pipes. Four Sandvik SR530 extendable reaming heads were then used to bore two sizes of shaft, one of 2.441m and the other of 3.154m.
The shafts varied in depth between 32 to 172m across varying degrees of ground quality and stability, with hard granites and volcanic tuff being encountered, some of which had a compressive strength of more than 250 MPa.
Nonetheless, Macmahon recorded an average penetration rate of between 0.6 and 0.8 m per hour.
Göran Strand, Product Line Manager – Raise Boring, Sandvik Mining and Construction