ГОДИШНИК НА МИННО-ГЕОЛОЖКИЯ УНИВЕРСИТЕТ “СВ. ИВАН РИЛСКИ”, Том 57, Св. II, Добив и преработка на минерални суровини, 2014ANNUAL OF THE UNIVERSITY OF MINING AND GEOLOGY “ST. IVAN RILSKI”, Vol. 57, Part ІI, Mining and Mineral processing, 2014

THE UNDERGROUND MINING INDUSTRY EMBRACES SYNTHETIC FIBRE REINFORCEMENT

Garry Martin

Elastoplastic Concrete EMEA Ltd. – Ireland

ABSTRACT. This paper examines the nature and usage of structural synthetic fibres in underground applications. Covered in the paper are references to areas of usage, fibre types and quality and performance testing options, as well as a discussion on issues surrounding different types of fibres and some of the health and safety aspects that have improved since the introduction of fibres into underground reinforcement of concrete.

ПОДЗЕМНИЯТ ДОБИВ В МИННАТА ИНДУСТРИЯ ПРЕГЪРНА ИДЕЯТА ЗА ИЗПОЛЗВАНЕ НА АРМИРОВКА ОТ СИНТЕТИЧНИ ФИБРИГари МартинЕластопластик конкрийт ЕМЕА ООД – Ирландия

РЕЗЮМЕ. Този доклад изследва същността и приложението на синтетичните структурни фибри в подземното строителство. В текста се цитират референции от използване на фибрите в практиката, резултати от изпитвания на качеството и експлоатационните им характеристики, както и дискусии по въпроси свързани с различните типове фибри, някои здравни аспекти и повишаване на безопасността на работа, резултат от въвеждането на синтетичните фибри за армиране на бетон използван в подземното строителство.

At the end of the 2011-12 financial year the Australian mining industry made up 6% of the country’s GDP, employed around 195,000 people, produced $218 billion in revenue and accounted for over 60% of the country’s total national exports. Australia has the world’s largest reserves of gold, iron ore, lead, nickel, silver, uranium and zinc and is a top 5 producer of many other mineral resources.

This vast production experience combined with high labour and production costs, strict operational health and safety regulations and extreme environmental pressures has resulted in the Australian mining industry becoming a world leader in underground development in part due to its openness to explore and adopt new technologies. One technology rapidly adopted by the Australian mining industry over 10 years ago, was the use of structural synthetic fibre for the reinforcement of shotcrete used in ground support applications.

Introduced in the mid -1990’s, after extensive research and trials, structural synthetic fibre reinforced shotcrete (SyFRS) quickly became the norm for ground support in underground mining and today over 90% of mines in Australia use SyFRS as part of their ground support programs. These include BHP’s Cannington and Olympic Dam mines, the world’s largest silver mine and the world’s largest uranium mine respectively, along with Newcrest’s Cadia Valley Operation, the world’s deepest block cave mine.

Increased Performance and DuctilityThe performance of a shotcrete lining can generally be

determined by measuring its toughness which is a measure of its post-crack load carrying capacity and therefore an indication of the reinforcement’s capability. Toughness is measured by a plate test which gives the energy absorption capacity of the shotcrete and as such it is possible to compare reinforcements such as wire welded fabric (wwf) and synthetic fibres.

Testing over many years and on various project sites between the different shotcrete reinforcements has shown that SyFRS outperforms both steel fibre and wwf (see Graph 1) particularly at increased crack widths. This is critical to mines which are prone to converging or squeezing ground. The increased ductility and post crack performance of SyFRS allows the mine to visually inspect ground support issues and take any necessary remedial action so that a brittle or sudden failure is significantly reduced.

The ASTM C1550 round panel test or RDP is now the standard test for toughness in the Australian mining industry due its cost effectiveness, repeatability and low variation in results.

Graph 1 shows comparative testing of steel mesh, steel fibre and synthetic fibre reinforced shotcrete.

Reinforcing Material ASTM C1550 / RDP

EFNARC

EPC BarChip60 @ 5 kg/m3 530 Joules 1325 Joules

Competitor 58 mm Fibre @ 5 kg/m3

362 Joules 905 Joules

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Steel Mesh (500 MPa 4 mm wire 100 mm centres)

475 Joules 1187 Joules

Steel Fibre @ 30 kg/m3 444 Joules 1110 Joules

The table above shows the comparative results between EPC’s BarChip fibre, wwf and steel fibre from RDP testing. It underlines the importance of using a high performing fibre in these applications to reduce the dosage rate and deliver long term performance. Structural synthetic fibres used in shotcrete reinforcement should have a minimum tensile strength of over 500 MPa and be fully embossed to ensure good bond to the concrete.

Safety

Since the 1950’s the main ground support in underground mining consisted of mesh and bolt reinforcement systems. This method however still required workers to operate under unreinforced ground during its installation. As a result many injuries and deaths occurred as a direct result of rock bursts and falls. The development of mechanised equipment in the 1980’s enabled workers to apply ground support from a distance and greatly improved worker safety. Around this time shotcrete started being used as a primary ground support reinforced by steel mesh.

However due to the installation time, safety factors, difficulty spraying over mesh creating high levels of rebound steel mesh was quickly replaced with fibres as the primary means of shotcrete reinforcement. Corrosion problems with steel fibres combined with the greater performance and ductility of structural synthetic fibres resulted in SyFRS becoming the standard for ground support.

SyFRS is now widely recognised as the most effective means of rock support due to its speed of delivery, economy, safety and durability. Figure 1 shows the reduction in deaths, injuries and working days lost on site in Australia since fibre reinforced shotcrete was introduced in the early 1990’s.

Fig. 1. Deaths, injuries and days lost per 100 employees underground (Source: Western Australian School of Mines, 2008)

Corrosion

Underground mining environments can be highly corrosive as a result of high humidity and aggressive ground waters with high chloride and sulphate contents all of which can influence and promote corrosion of steel reinforcement or support. Ore bodies such as gold and copper contain massive sulphides that have the potential to destroy steel reinforcement in a matter of months, leading to sudden and unexpected failures in rock support.

Synthetic fibre reinforcement is not affected by these aggressive conditions. In experimental testing the durability of SyFRS reinforced with structural synthetic fibres was found to be excellent in these environments. In contrast, corrosion and loss of performance of FRS reinforced with steel fibres was substantial, even after only seven months exposure (Bernard, 2004).

The results of research undertaken comparing the two reinforcement types in an aggressive environment is shown in Table 1 and shows :

a) the total energy absorption, tested at 28 days, andb) the total energy absorption of pre-cracked specimens,

tested after 1 year of exposure

Results show that BarChip fibre reinforced concrete maintains 99.8% of its load carrying capacity after one year of exposure, compared to only 54% for steel fibre alternatives.

Table 1. Summary Results Exposure Testing (Bernard, E.S., 2004)

Product 28 Days 1 Year Exposure % Difference

BarChip Synthetic Fibre

549 Joules 548 Joules .2

Hooked End Steel Fibre 596 Joules 324

Joules 45.63

Embrittlement

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Additional to corrosion, embrittlement is another process that severely reduces the durability (and ductility) of steel reinforcement but has no effect on synthetic fibre. As shotcrete strength increases with age, steel fibre anchorage can become too effective, leading to a rupture based failure instead of a pull-out failure. This process of embrittlement greatly increases the risk of sudden and unexpected rock fall as a result of

deteriorating ground conditions, inappropriate ground support and seismicity.

Testing has shown that toughness of steel FRS increases over the first 28 days and then slowly decreases in toughness over the next 150 days as compressive strength moves from 35 MPa up to 65 MPa (DiNoia & Rieder, 2004).

Fig. 2a. Late age embrittlement of steel fibre Fig. 2b: No late age embrittlement of synthetic fibre

Speed of ConstructionA key advantage of SyFRS to the mining industry is reduced

cycle times. Mechanised equipment and improvements to bolting, shotcrete and reinforcement technology led to the development of in-cycle shotcreting in the early 2000’s. In-cycle refers to the method of spraying a primary layer of shotcrete directly onto the exposed rock face followed by bolting. In-cycle shotcrete with post bolting has now become the standard practice for mines in Australia, America and many European countries and was made possible through the replacement of steel mesh with fibre reinforcement.

In-Cycle shotcrete is 7.5 times faster than regular development using SyFRS at 50mm and reduces costs by over 11% (Methvin, 2004).

ConclusionsFibre reinforced shotcrete has been one of the most

influential advancements in mining technology of the past 20 years. Embraced fully by the Australian mining industry, SyFRS is now being adopted by mining industries in America, Europe, Asia and also in many civil underground construction applications.

Structural synthetic fibre dramatically improves concrete toughness and ductility but unlike steel reinforcement is immune to the effects of corrosion and embrittlement that significantly reduces its effectiveness and when combined with

in-cycle production SyFRS enables rapid mine development and reduced costs.

As mining operations continue deeper underground the associated ground deformations require even more demanding reinforcement systems. Structural synthetic fibre reinforcement such as Elasto Plastic Concrete’s BarChip fibre has proven itself as the best choice for these tough environments.

ReferencesMethven. G., 2004 In-cycle application of fibre reinforced

shotcrete, Shotcrete: More Engineering Developments, Bernard (ed), Taylor & Francis Group, London.

Bernard, E.S., 2004 Durability of cracked fibre reinforced shotcrete, Shotcrete: More Engineering Developments, Bernard (ed), Taylor & Francis Group, London.

DiNoia, T.P. & Rieder, K.A., Toughness of fibre-reinforced shotcrete as a function of time, strength development and fibre type according to ASTM C1550-02, Shotcrete: More Engineering Developments, Bernard (ed), Taylor & Francis Group, London.

ASTM Standard C1550. 2012. “Standard Test Method for Flexural Toughness of Fiber Reinforced Concrete (Using Centrally Loaded Round Panel)”, ASTM International, West Conshohocken, PA.

Recommended for publication by Editorial board.

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