Facing up to the Task of Face Stability

There are a great deal of options open to quarry managers for ways to make sure that all slopes remain geotechnically stable. David Gibson, Ian Christie and Glyn Barnes summarise some of the latest techniques and materials available.

It is imperative for owners of existing and redundant quarries to ensure that these are geotechnically stable. Owners and managers are taking. or will have to take, measures to carry out risk assessments to ensure that faces and slopes are, and will remain, safe.

Some owners have in-house geotechnical experts to do these risk assessments, while others need to employ third-party consultants for this task.

Either way, when physical on-site work is required, usually specialist geotechnical and/or drilling and blasting contractors are employed to implement the remedial proposals needed. In some cases, it is advantageous to have the specialist contractor involved from an early stage, since the contractor often has extensive experience and knowledge of the latest techniques and materials. In addition, the contractor has the specialist personnel, plant and equipment to provide the most economically-efficient solutions.

Investigation
Prior to any work occurring, a good knowledge of the geology involved is essential. This is a significant aspect of any planning and execution of geotechnical operations, and this is even more important when working at height.

Modern technology, such as stereographic photography (usually from helicopters or distant observations) and analysis, can assist in designing the solutions required and determining the safe and economical methods of work. Laser profiling, which is used extensively for quarry blast design, can also be a useful tool in this process.

Drilling and Blasting
One of the problems created by modern blasting techniques and the quarry operator's desire to make primary blasting as economical as possible has been the increased use of bulk emulsions. This has led to larger patterns, heavier blast ratios and larger shots. This results in final faces being left in a less stable condition.

The mitigating factor that can be offered by drilling and blasting falls into two categories.

Firstly, where current blasting operations are on faces approaching their extraction limits, techniques can be employed to provide long-term stability during production blasting. The judicious use of pre-split and smooth-wall blasting will result in faces and slopes that are durable, dependent on geology, and require minimal additional support and ongoing inspection. This approach is undoubtedly the most cost effective, while also minimising the risk of remedial work.

Secondly, where the problem is historical in working or redundant quarries, drilling and blasting can offer some solutions. The techniques available are similar to those previously mentioned, but require detailed geotechnical input to design the final solution. New blasting techniques, such as the use of electronic detonators, allow quality work with improved control of throw, fragmentation and, if planning restrictions are onerous, better control of vibration levels.

Historically, the major problem encountered is generally the lack of available space between the face and boundary, where quarry operators have tried maximising reserves without considering the long-term face stability. The problem with this approach is that there will be occasions when drilling and blasting is not an option due to lack of burden, or where new activities, such as house building, are just outside the redundant boundary.

Rock Removal
Should blasting not be possible but removal of rock is needed, then some form of scaling is required. Men working from ropes are common for this activity, but the use of modem 'cherry pickers' and long-reach excavators provides increased safety during this hazardous operation.

The use of temporary blast netting will ensure that the scaled material is contained and cannot fall onto people or property below.

Support
The techniques employed by geotechnical contractors are wide and varied. In the case of rock slopes, rock fall meshes are frequently installed to ensure that weathered and loose material is contained when it falls.

Rock bolts, soil nails and anchors provide support to faces and individual features. Sprayed concrete allows concrete to be placed without requiring difficult formwork and placing equipment, which is much more involved at height. Dentition may be provided by brickwork, stone masonry or sprayed concrete.

Recent developments in technology have seen tensioned surface meshes being specified by engineers. These meshes, such as Geobrugg's TECCO mesh system, provide active support to soil or rock faces, and are usually used with rock bolts; where soil slopes are to be stabilised then soil nails may be used.

Rock bolts and soil nails can be made of plain, galvanised or stainless steel bar; self-drilling hollow bar, such as Dwyldag's MAl; or Ischebeck Titan bar. In recent years, glass-reinforced plastic (GRP) and carbon fibre bars have been used, especially where corrosion or residual stray electric currents are a problem. Full double-corrosion protected (DCP) bars and strands are used in critical rock bolting, soil nailing, and for active ground anchors.

Geobrugg has introduced the Spider rock-protection system that is suitable for rock slopes and overhangs, where there is an irregular surface or clearly defined sliding mechanisms. The Spider system comprises a factory manufactured spiral rope net and, like rockfall netting and TECCO mesh, it is attached to anchors or bolts at the top of the area to be treated, and then unrolled downwards and secured around the perimeter. Its high strength provides active support to the face or a feature, such as an overhang or isolated loose block.

Protection Systems
Traditionally, rock falls have been controlled by using rock-fall meshes. These have reduced in price in recent years and are readily available from several suppliers.

They are usually made from hexagonal twist-wire net and are often PET-coated. Although they are mostly grey in colour, other colours are available, but usually cost extra.

While much of the high geotechnical work aims to stabilise or secure faces and cliffs, sometimes this is too difficult or uneconomic. In these cases, rock-fall barriers can be installed either at the bottom of the slope, or higher up above the road, railway or structure that is at risk from falling material.

Rock-fall barrier technology has moved rapidly in recent years, with the Swiss being at the forefront - a fact that is hardly surprising considering their Alpine topography. Specially designed rock-fall barriers, such as Geobrugg's ROCCO system, are available with up to 3000kJ capacity to meet Swiss govemment guidelines. To highlight what a 3000kJ impact is like, it is equivalent to a 10-tonne block, free falling vertically 30 metres. Geobrugg also offers a 5000kJ barrier, certified to Swiss govemment standards, which is equivalent to a 16-tonne block free falling 32 metres.

Personnel, Plant and Equipment
Operative training is an aspect that all responsible employers take seriously because it assists with safe operations, and also ensures that the work is done correctly in the first place and maximises the financial return on expensive plant and equipment.

Some modem plant being deployed for drilling and blasting, as well as geotechnical work, is highly sophisticated. Rigs can be radio remote-controlled to ensure operator safety, and some even automatically send condition and performance reports back to the office via satellite and the internet.

Outputs are optimised through computer designed in-hole equipment, and the use of modern steels and other materials.

Environment
With more and more emphasis on the environment, plus the desire to turn redundant quarries into nature reserves and public amenities, the use of 'green' solutions is becoming increasingly popular. Retention of top soils and their reuse afterwards is laudable and expected. In addition, soil-retention geotextiles (either plastic or biodegradable such as jute or coir matting) combined with hydro-seeding can assist vegetation to grow in the most barren environments. The use of open-mesh supports, as opposed to sprayed concrete or other concrete structures, allows the development of all forms of flora to progress unhindered.

This active encouragement of green growth also encourages birds and other wildlife, and reduces the ongoing impact of the scar left on the landscape.

Working at Height
Specialist geotechnical contractors are often required to work high up to do their inspections and investigations, remove dangerous or unstable ground, secure weak and weathered faces, and install support systems and drainage. The areas where these services are required, include railway and road cuttings and embankments, cliff faces, old quarry faces and new excavations.
This type of work needs safe and flexible access for people involved in the work, and also for the workforce and public below.

The emphasis on adopting safe work processes has become increasingly important. Close liaison between consultant and contractor is fundamental to achieving safe solutions.

The innovative use of long-reach excavators, fitted with remotely controlled drilling feed masts, illustrates well the minimisation of a person's exposure to working at heights, and eliminates the potential consequences of vibration white-finger syndrome. It also maximises program benefits by using high-output plant. This technique contrasts with the traditional labour-intensive method of roped access, with technicians using heavy hand-held rock drills where all of the above-mentioned risks are heightened and work output is slow.

Scaffolding, mobile working platforms, cherry pickers, long-reach excavators, cranes, rope-supported working platforms and rope access techniques are all used. Drilling operations are often used in geotechnical work, and sometimes special lightweight drilling machines are used as well as hand drilling.

Providing access can often be a significant proportion of any project's costs, but rope-access techniques can replace traditional solutions and offer clients significant savings.

Rope systems are usually quick to install and dismantle, flexible, and allow work to commence quickly and finish promptly, avoiding lengthy scaffolding installing and dismantling procedures. By removing operational and time constraints, rope access services can often extend the 'time window' available to do the work.

Ensuring safety with this type of activity is paramount. This begins with the client's requirements, the preparation of the health and safety plan, followed by the contractor's project management plan that incorporates method statements and risk assessments.

In conclusion, the quarrying industry has a wealth of resources, techniques and experience available, to ensure geotechnically safe faces and slopes, either in the short term or as a long-term legacy.

David Gibson, Ian Christie and Glyn Barnes are respectively the business development manager, drilling and blasting manager and contracts manager, with UK geotechnical experts, Ritchies.