Concrete Roof Support at Waihi Gold. Bruce Cloughley¹. Introduction

Transcription

1 Concrete Roof Support at Waihi Gold Bruce Cloughley¹ Introduction Gold Mining at Waihi on the edge of the Coromandel is entering a new phase in mining as it moves from open pit to underground ore recovery. This involves the construction of a decline roadway to access the ore body using in cycle mining techniques and sprayed concrete roof support systems on a 24 hour 7day week basis. The construction programme is 18 months into a 4.5 year programme to develop and build the decline. Description of Project. The planned length of the Favona Decline is 11kms with a 5.5m diameter tunnel. Depth is 170 metres at the end of June 2006 and will be 370 metres in June At the end of June 2006 the length of tunnel complete was 1.2 kms. Roof support is 50mm of sprayed shotcrete, with steel fibres or pinned mesh and roof bolts. The concrete is batched in the local readymix plant and hauled to the mine portal which is a distance of 4kms. Outline of Concrete Requirements. The first specification was performance based for wet or dry fibre reinforced sprayed concrete. site trials 3 day in accordance with BS 1881 site trials 7 day in accordance with BS 1881 site trials 28 day in accordance with BS 1881 Flexural Strength at 28 days from site trials Bond strength to rock Setting time 15 MPa 28 MPa 40 MPa 4.6 MPa 0.5 MPa 9 min 60 min Final set Minimum cement content 400 kg/m³ Maximum W/C ratio including liquid volume of admixtures 0.5 Fibres - Steel Length range Minimum content Minimum tensile strength 25-40mm 40kg /m³ 800 MPa Accelerators to be high performance alkali free additives applied at the nozzle and limited in dose to 3 to 10% of binder weight. The mix design to achieve final specification is: 14 7mm aggregates 250kg Sand 710kg Pap 850kg Cement 400kg Micro Silica 25kg Water total 200L Superplasticiser 3.5L Water Reducer 1.2L Steel Fibres 40kg Typical strengths - 28 day (Cylinders) 42 MPa Reinforcement. Options for reinforcement in roof support at this time are fibres of drawn wire or polypropylene and welded wire mesh. The trend is away from welded wire mesh and fibres are increasingly being specified due to; Durability is improved Ductility is increased Impact resistance is increased Rebound is reduced Application productivity and safety is improved Logistics are simplified No shadowing and deep backs behind mesh ¹Bruce Cloughley, Business Manager BASF Construction Chemicals

2 The most important parameters for the fibres are: The aspect ratio (overall ratio of the fibre length to it s diameter) The tensile strength The shape The numbers of fibres per unit mass An ideal fibre should have the following: A length such that it can overlap and bridge at least 2 of the largest aggregate particles used in the mix (typically a length between 25 and 40mm) A high aspect ratio (i.e. thin) High tensile strength A shape that results in a good anchor particularly at the fibre ends in the shotcrete. Figure 1. Comparison between the load bearing capacity of plain, mesh and steel fibre Reinforced Shotcrete. Load in kn Deformation in mm Plain shotcrete Fibre reinforced shotcrete Fibre reinforced shotcrete (at a higher dose) Wire mesh reinforcement 40 (after Vandewalle) Where steel fibre is used a dosage rate of between 30 and 60kg / m³ (0.40 to 0.75% by volume) is generally used. The use of polypropylene fibres in mining applications has increased dramatically recently with the development of high performance polymer fibres. The high tensile strength and crimped shapes have resulted in performances very similar to that obtained 50 with steel. Dosage rates from 6 to 12kg / m³ (0.75 to 1.5% by volume) are typical. Typically high performance steel fibres have higher residual loads immediately after failure but less load at high deformations than high performance synthetic fibres. The key to the cost effective use of fibres is to consider the Cost / J of energy. Concrete Management. Management of the concrete differs considerably to a construction site and the concrete supplier has a training programme for staff involved in the batching and delivery of concrete for shotcreting. At Waihi the haulage distant is short so a local concrete plant was able to supply therefore a mobile batching plant was not required on site. The concrete is produced with a standard dose of Hydration Control Stabiliser which will postpone hydration for 2-4 hours. This caters for contingencies that delay the spraying of the concrete. If delays continue then redosing is required before hydration commences (i.e. 3 hours) When shotcrete spray is required on night shift then the batching is completed before 7.00PM and the Hydration Control Stabiliser prevents hydration commencing until 6.00AM the following morning on occasions. Slump loss is a possibility over long periods of delay and this is corrected with another dose of Superplasticiser on site at the portal. The concrete truck is driven down the decline roadway until it reaches the face with a 4-5.5m³ load. At the face, the Robotic Shotcrete Spray Rig is placed so as to access the roof and sides of the tunnel that the blast has progressed. The concrete is discharged into the Spray Rig at 130 to 160mm slump. Spray Management. Prior to concrete spray substrate preparation is a key element in the successful application of shotcrete. The rock substrate should be free from loose materials, dust and oils. Scaling with the Jumbo will start this process and then cleaning with water and compressed air jet with the cleaning starting at the high point of the roof and work systematically to the foot of the sidewall (ribs). Timing of this procedure needs to be just prior to application of shotcrete commencing.

3 Depending on the sophistication or age of the Spray Rig you can either dial in electronically the % of accelerator to be dosed into the concrete at the spray nozzle, or if a dosing pump is used at a fixed pressure regulate the output to calibrate to the piston stroke of the concrete pump. Figure 2 Spraymec Rig The accelerator is dosed into the nozzle at generally 6-9 % depending on the formulation, ambient and rock temperatures and the speed of set required. The rig operator manipulates the nozzle so that it is at 90º to the face being sprayed and the nozzle is 1-1.5m off the face. This is the optimum to minimise rebound and ensure the shotcrete hangs up. Generally the roof is sprayed first concentrating on the backs (back of the profile) and building up where thickness over 80mm is required overhead. Concrete Spray is even and measured over the surface and the aim of the accelerator is to achieve a 9-10 minute set. This can be accessed with the thumb print in the surface and final set at 1 hour measured using a Meyco Needle Penetrometer or a more rudimentary test is a soil penetrometer. Spraying as soon as possible after the blast prevents ravelling of the rock face by filling cracks within the strata, minimises the period the rock face is exposed to oxygen, constrains shrinkage due to water loss, prevents the rock mass compressing into the space created, which has a beneficial effect on the overall rock mass by tending to stabilise the rock. A major influence in achieving successfully placed shotcrete is the nozzle man. Equipment Equipment for this task is robust and expensive. There are several specialist suppliers world wide with a wide range of machinery for both wet and dry application. Power can be electric, diesel electric or diesel. Output of the equipment in Waihi is 8 15m³ per hour depending on the number of set ups, concrete slump, the volume of water on the surface of the rock and the nature of the rock being soft and clay like or hard. A safety cage is framed over the driver compartment although the nozzle man operates the nozzle and boom remotely but taking care not to position himself under roof that is unsupported. (Roof that is not sprayed to the specified thickness with cured shotcrete) Testing All the standard tests are carried out by the concrete supplier to maintain quality in the mix and the primary test is the crushing of cylinders at 7 and 28 Days. However this will not be more than an indication of what strength and quality of concrete is achieved on the rock. The shotcrete accelerators used at the nozzle typically hinder strength growth 10 to 15% from concrete at 28 days. Then other factors which will influence to a lesser or greater degree are curing condition, thickness, moisture content of rock surface, ambient and rock temperature, age and quality of equipment and quality of nozzle man. To check these other variables the standard approach is to core the shotcrete insitu and then have the samples evaluated for bond to rock, compaction, aggregate distribution, fibre distribution and then crushed and results recorded. This is satisfactory as far as the concrete goes however a better performance assessment of the cured shotcrete is to conduct the Round Determinate Panel Test (RDP). The RDP is a test to determine toughness of a representative sample of Fibrecrete by placing three loads within a sprayed round plate. Then the Fibre Reinforced Shotcrete panels are used to assess the post cracking capacity of FRS for applications in which the ability of shotcrete to re-distribute stress after cracking has occurred. This value is measured in Joules

4 Figure 3. Panel Angled to side of wall prior to spraying Safety is a constant daily imperative in these dynamic environments. Pre shift briefing with management review progress on the last shift, production hold ups and choose a safety theme for that shift. Monthly safety meetings are conducted to evaluate safety improvements and maintain high awareness with reinforcement of safety along with reinvigoration of safety messages and tools being a major management focus especially relating to new mining personnel. Rock Bolting Rock bolts of many types are available to be used in conjunction with the Sprayed Shotcrete with options and design depending on ground conditions. At Waihi Split Sets 2.4m long are installed on a grid 1.2m x 1.2m into the roof. Mining Sequence The mining sequence is a cyclic process to maximise productivity without compromising safety. The sequence is: Drill the face to place charges. Blast. Muck out. Shotcrete. Bolts. This sequence will vary in different ground conditions so evaluation of the rock strata and ground conditions will influence the mining regime that is determined. At Waihi there are four mining regimes as follows; Capital Development. Bolt Mesh 50mm Shotcrete. Fibrecrete 50mm Shotcrete Bolt. Ore Drives. Bolt - Mesh PPE is very rigorous and everyone underground having to maintain a very high standard in order they do not endanger themselves or workmates. Transgressions initiate a Performance Improvement Notice which is issued and formally discussed. Another tool is a Job Inspection Notice which detail hazard identification. Gas monitoring in the length of the decline is 2 weekly event at this time. The focus and effort pays off with 560 days with no lost time injuries on the Favona Decline. Acknowledgements 1. Firth Industries 2. HWE Mining Leighton 3. K & H Construction Services References. 1. Spearing, S., Shotcrete in Mining Melbye, T., Sprayed Concrete for Rock Support Very poor ground conditions. 50mm Shotcrete Bolt Mesh 50mm Shotcrete. Safety Requirements

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