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Measuring success

Design by measurement for underground roadways supported with rockbolts in UK coal mines by P Cartwright, G Watson and P Altounyan

Introduction

Rockbolting is now the main system of support in UK coal mine roadways and was introduced using a systematic approach. Its successful introduction was dependent on two key technical factors:

The use of a high strength bolting system

The use of a systematic design and monitoring approach based on detailed measurement of strata movement, bolt loading and routine monitoring of roof behaviour (Altounyan & Hurt, 1998).

It is considered that the approach adopted was the major factor behind the successful introduction of rockbolting into UK coal mines and the high level of safety achieved. This article describes the geotechnical investigations and verification systems adopted when using rockbolt support in coal mines.

UK coal mining conditions

Coal mining in Britain takes place at depths from 100m to 1,200m, the typical depth being 800m. The strata above coal seams is variable, ranging from weak mudstones with compressive strengths of 30MPa, grading up to siltstones and sandstones whose compressive strengths can exceed 60MPa.

The stress field is a major factor in dictating mining conditions. In the UK, as in many other parts of the world, it has been found that the vertical stress is proportional to depth of cover with a non-uniform horizontal stress regime. The magnitude and direction of the major horizontal stress component can have a major influence on roadway failure (Altounyan & Hurt, 1998).

Under high horizontal stress and weak roof conditions, a roof could fail to a height of between 2m and 5m. In addition, at some mines a number of different seams have been worked resulting in problems with complex interaction being encountered.

The two main approaches to rock stabilisation are roadway support and roadway reinforcement. Roadway support techniques are often based on the premise of supporting the weight of a zone of failed rock, generally by the use of steel H-section colliery arch supports. Under low stress conditions the supports are used to maintain stability against local gravity falls of weak rock. Under high stress conditions the steel frame supports provide some resistance to strata movement in order to maintain opening shape.

In roadway reinforcement, rockbolts are generally employed to develop forces within the rock to create a self-supporting structure. When properly applied, rockbolting restricts roof failure and greatly improves roadway stability. In the UK, the approach has been to use the concept of rock strengthening described by Siddall and Gale, 1992.

At the time rockbolting was reintroduced in the late 1980s there were some 60 large coal mines in Britain, all using steel frame or arch supported roadways. Today there are only 20 large mines operating and rockbolts are the main system of support.

Overview of current roofbolt support design

The design approach applied to rockbolting in UK coal mines is based on the observation that horizontal stresses in the roof cause shear failure of the strata. The UK high strength bolt system is intended to provide high bond strength and a high resistance to shear. The aim is to minimise stress redistribution and confine roof shear to within the bolted height. The bolts work by directly resisting roof shear and by providing high confinement to the roof so that the shear strength of the roof rock is greatly increased. This is demonstrated in Figure 1.

The rockbolting system used in UK coal mines utilises standardised consumables which produce a high strength high stiffness system. With only one specification for these rockbolting materials there can be no confusion as to which materials can be installed. The consumables must meet the criteria set out in the British Standard BS7861 part 1 for strata reinforcement support system components used in coal mines (British Standards Institution, 1997).

The Advanced Technology (AT) system specifies 22mm diameter bolts with a special ribbed profile and a yield strength of 25t, installed into holes with a nominal diameter of 28mm. A high strength polyester resin is used to encapsulate the rockbolts, supplied in easily handled plastic capsules for rapid installation. The small annulus results in a very high bond strength system.

The behaviour of coal mine roadways is complex, highly site specific and often poorly understood. The general design is based on sound rock mechanics logic. An initial design is arrived at based on an assessment of the available data obtained from site investigations in the form of a geotechnical appraisal. Where this indicates that the strata is suitable for rockbolts a support system design needs to be prepared. If an existing design has been found to be suitable, and the geology and conditions are the same, then the existing design can be used. Design takes account of:

roadway profile

rock bolt length (minimum 1.8m)

density of rockbolts (minimum 1 bolt/m2)

location of rock bolts

type of rock bolt

location and type of drilled holes

system of support for the road sides.

The design is installed underground and the important parameters measured to confirm strata and support behaviour. These include extensometry and the loading developed in the rockbolts using strain gauged rockbolts. It is important to measure these parameters accurately and in sufficient detail to enable a worthwhile analysis to be carried out.

If the analysis indicates that the rockbolt system is fit for the purpose then no change is required, otherwise the design is altered and remeasured.

In order to guarantee safety in UK single entry roadways, continuous monitoring of roof movement is part of the design process. The UK is the only country to undertake systematic design by measurement for rockbolting based on industry guidance (HSE Books, 1996).

Site investigation and assessment

The investigations include reference to:

Geology: including the strata section, rock properties, faults, cleat, parting planes, presence of water, borehole information, gradients etc (Figure 2).

Rock stress: the direction and magnitude of the stress field components for pre-mining, mining induced conditions and interaction (Figure 2).

Pillar design and effects: the assessment needs to take into account pillar loading and its affect on roadway stability.

Environmental effects: the effects of ambient temperature, mine water and its associated impurities.

Bond strength: measured by short encapsulation pull tests using proposed rockbolt materials and components. The tests need to be carried out for all major roof horizons within the length of the proposed rockbolt and prescribed minimum values will need to be obtained.

Stand up time: account is taken of roof rib stability during the period between excavation and support installation.

If there is a major change in the geotechnical environment then supplementary investigations are carried out.

Support rules

Coal mining legislation requires mine managers to prepare support rules. These set out the stage by stage cycle of operations which miners should take to ensure the design is safely achieved.

When the support rules are first implemented they should be checked to see:

they work in practice

whether the assumptions made about the risks to miners implementing the support system are correct.

Design verification monitoring

The initial design needs to be verified by comprehensive monitoring. The monitoring includes detailed measuring of roof and sides dilation and rockbolt loads. The measured parameters can be used to assess whether the assumptions made at the design stage were correct, or whether further action needs to be taken to secure the required degree of stability.

Design verification is typically undertaken using instrumented rockbolts and sonic extensometers. Extensometers are installed in the roof and sides of the roadway and measure dilation in the strata surrounding the roadway. Strain gauged rockbolts are installed in place of normal rockbolts to assess the level of load and bending generated within the bolts. A typical array of these instruments is shown in Figure 2.

Routine monitoring scheme

It is the duty of the mine manager to ensure that a routine monitoring scheme is prepared for roadways supported by rockbolts. A routine monitoring scheme includes details of the types and locations of devices to be used to monitor the longer term performance of the support system and details the maximum intervals between successive readings. The manager also appoints a competent person to implement, co-ordinate and audit the scheme.

Detailed sonic extensometer measurements allow roof movement 'action levels' to be derived for a site. These action levels are the amounts of roof displacement within and above the bolted height that can be safely allowed before action is required to prevent a fall. Typical action levels used in UK coal mines are 25mm either within the bolted height or in the monitored height above the bolts.

These action levels are by no means universal, being affected by stress conditions and local geology. Once action levels have been reached, remedial action is undertaken to secure the roof. Roof movement is routinely monitored using dual height tell tales, which provide a visual indication of roof displacement. They are positioned at regular intervals, normally every 20m, and indicate movement within and above the bolted height.

Safety

Rockbolting has been introduced progressively without prejudicing the safety of workmen. Although there have been accidents attributable to falls of ground in rockbolted roadways, the incidence is significantly less than that experienced in roadways supported by steel frame or arch supports.

Conclusions

Rockbolting has become the main support method in UK coal mines. This success should be viewed against the difficult and highly variable mining conditions found here. Improvements in safety have been realised and this can be attributed to the rigorous approach described above.

The introduction of rockbolting has also allowed improvements in productivity. The use of rockbolting in coal mines can be seen as an example of technical change allowing both productivity and safety to be increased.

Although mining conditions, methods and practices vary considerably around the world, the principles and general approach has been found to be valid in other countries.

References

Altounyan PFR and Hurt KG, 1998. The introduction of high-production longwalls, utilising rockbolting, to traditional mining industries, Coal International, vol 246, no 2, pp62-67.

Altounyan PFR and Hurt KG, 1998. Advanced rockbolting technology, World Coal, vol 7, no 5, pp30-36

British Standards Institution, 1997. BS7861, Strata reinforcement support system components used in coal mines, Part 1: Specification for rockbolting, BSI, p28.

Hayes AW and Altounyan PFR, 1995. Strata control - state of the art, mining technology, vol 77, no 892, pp354-358.

Health & Safety Executive 1996. Guidance on the use of rockbolts to support roadways in coal mines, Deep Coal Mines Advisory Committee, HSE Books.

Siddall RJ and Gale WJ, 1992. Strata control - a new science for an old problem, The Mining Engineer, vol 151, no 369, pp341-356.

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