Geotechnical

Specialist Geotechnical contractors are often required to work at height to carry out inspections, carry out 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 means of access, not only for the operatives involved, but also for the workforce and public below.

With the implementation of the Work at Height Regulations [WAHR] 2005, the emphasis placed on adopting safe method related work processes has never been greater. Close liaison between client, consultant and contractor is fundamental to achieving safe solutions.

The innovative use of long reach excavators fitted with remotely controlled drilling feed masts illustrates the types of method available to minimise an operative’s exposure to work at height and eliminates the potential consequences of vibration white finger syndrome whilst maximising programme benefits through the use of high output plant. This technique contrasts with the traditional labour intensive method of roped access technicians using heavy hand held rock drills where all of the above risks are heightened and programme output is slow. (click photo for more detail)

Scaffolding, mobile working platforms, cherry pickers, long reach excavators, cranes, rope supported working platforms and rope access techniques are all used. For drilling operations, which are often used in geotechnical work, special lightweight drilling machines are sometimes deployed along with hand drilling. (click photo for more detail)

In the case of rope access work, reputable contractors use IRATA (Industrial Rope Access Trade Association) trained and certificated operatives to ensure the highest safety standards (pdf document) . Since the provision of access can often be a significant proportion of any project's costs, rope access techniques can replace traditional solutions and offer clients significant savings.

Rope systems are usually quick to install and dismantle, are flexible and enable work to commence quickly and finish promptly avoiding lengthy scaffolding erection and dismantling programmes. By removing operational and time constraints, rope access services are often able to extend the 'time window' available to undertake work. (click photo on left for more detail)

Work programmes using rope access techniques that have been properly planned with full and checked risk assessments can enable significantly reduced road or railway possession times. This was demonstrated recently on a number of slope stabilisation contracts at Flax Bourton for Network Rail using rope access techniques. As a result the number of line possessions was greatly reduced. This, together with the ability to maintain normal line speeds meant that there was minimum disruption to train operations and hence the railway users.

Ensuring safety with this type of activity is paramount and the HSE campaign with regard to working at height in 2006 highlighted the importance placed upon robust working systems. With due regard to the CDM regulations this begins with the Client’s requirements, the preparation of the Health and Safety Plan following onto the contractor’s Project Management Plan incorporating method statements and risk assessments.

Good knowledge of the geology involved is a significant aspect of any planning and execution of geotechnical operations and this is even more important when working at height. Anchorages for rope supported personnel, plant and equipment should be properly designed and tested before use to ensure that they will take the loads with adequate safety factors.

Modern technology such as stereographic photography (sometimes from helicopters) and analysis can assist in designing the solutions required and determining the safe and economical methods of work. Laser profiling used extensively these days in quarries can also be a useful tool in this process.

Once survey and inspection is complete, remedial works on rock faces often commences with removal of loose and dangerous material. Use of temporary blast nettings can ensure that the scaled material is contained and cannot fall onto people or property below.

The techniques employed by geotechnical contractors are wide and varied. In the case of rock slopes, rockfall meshes such as hexagonal twist wire nettings are frequently installed to ensure that weathered and loose material is contained when it falls. Rockbolts, soil nails and anchors provide support to faces and individual features. Sprayed concrete enables concrete to be placed without the need for difficult formwork and placing equipment which is so much more involved at height. Dentition may be provided by brickwork, stone masonry or sprayed concrete. (click photo for more detail)

Recent developments in technology have seen tensioned surface meshes being specified by Engineers. These meshes with wire strengths of up to 12.5KN provide active support to soil or rock faces and are usually used in conjunction with soil nails.

Soil nails can be plain, galvanised or stainless steel bar, self-drilling hollow bar. In recent years Glass Reinforced Plastic (GRP) and Carbon Fibre bar has 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 soil nailing and for active ground anchors.

Recently a new high strength triple wire mesh has come onto the market and this system is well suited for rock slopes and overhangs where there is an irregular surface or clearly defined sliding mechanisms. This Swiss Rock Protection System comprises a factory manufactured spiral rope net, which like Rockfall nettings and high tensile mesh is attached to anchors or bolts at the top of the area to be treated and then unrolled downwards and secured. Its high strength enables it to provide active support to the face or feature, which may be an overhang or isolated loose block.

While much of the geotechnical work carried out at height is aimed at stabilising or securing faces and cliffs sometime this is too difficult or uneconomic. In these cases Rockfall barriers can be installed either at the bottom of the slope or higher up above the road, railway or structure at risk from falling material.

Rock fall barrier technology has moved on apace in recent years with the Swiss being in the forefront: a fact that is hardly surprising considering their Alpine topography. Specially designed rockfall barriers are now available up to 5000kJ capacity meeting Swiss government guidelines. To give some idea of what a 5000 kJ impact is like; it is equivalent to a 16 tonne block free falling vertically 30 metres. (click photo for more detail)

Currently the European Union is putting the finishing touches to the European Organisation for Technical Approvals document Guideline for European Technical Approval of Falling Rock Protection Kits. It is hoped that these guidelines will be published in 2007.

Of course a lot of work and cost can be avoided in the case of new excavations or where it is practical to reprofile by the use of modern blasting systems using pre-split or smooth wall blasting. This leaves the face with less instabilities and reduced fracturing so minimising the risk for falls in the first place.

Working at height on a natural slope or cliff that has suffered weathering and deterioration presents constant challenges to geotechnical engineers and contractors. However new techniques, safety systems, training and materials means ever reducing risks and more economic solutions. (click photo for more detail)

Author:
David Gibson B.Sc. (Hons), C. Eng., M.I.M.M.M. M.I.C.E