New drilling rigs redefine how to approach slope stability investigations.

Slope failures in the railway environment are very common, and scores of site investigations are undertaken every year to provide data about the properties and behavior of the soil and/or rock that makes up or underlies these slopes, enabling geotechnical engineers to model failure mechanisms and design remediation measures.

Traditionally, investigation of these sites either entailed setting up a scaffold platform to carry a cable percussion or rotary rig, or accepting a severe downgrade in the data that could be recovered by using man-portable window sampling equipment. As the sites are often remote from authorised access points, then RRV’s and the collateral expense of possession working is often the only way to move access and drilling equipment on and off site.

Geotechnical Engineering Ltd. (GEL) have a long history of providing innovative solutions to the ground investigation industry, and have, over the past year or so, been at the forefront of developing a unique range of drilling rigs that are able to access these sites and provide the good geotechnical data required by their clients’ engineers, without resorting to possessions or RRV movements.

Access to the NRI for large items of plant is often difficult, as most Rail Technology Magazine readers will know, and requires haul roads to be constructed, access routes and compensation agreed with landowners, large scale devegetation of embankments or cuttings and long runs of fencing to be removed and subsequently reinstated.

If an ‘overland’ route is unavailable or unfeasible for investigations at this scale, then the alternative is the possession and RRV route. Providing a safe working platform for the drilling equipment and personnel on the slope is then a factor. At present this is likely to be either a platform cut into the slope using an excavator or a scaffold platform erected on suitable foundations. To create a platform requires getting an excavator to the site and then cutting into an already potentially unstable slope. To erect a scaffold platform requires carrying in a few tonnes of scaffold and erecting it on a steep slope, which leads us to catch 22-how is this access equipment/plant/material to access the site prior to the rig being mobilised?

Given the factors outlined above, it is hardly surprising that a good percentage of ground investigations on railway slopes end up being carried out using handheld window sampling equipment, which at least can be carried in to the site by manpower, and set up and operated in a fenced green zone. This is a relatively inexpensive option.

However, you get what you pay for-from a geotechnical standpoint, this drilling method gives the worst results. The client saves on investigation costs, but the remedial measures cost more and they are the expensive part of the project!

GEL have approached this problem by firstly asking clients what information they really wanted from an investigation, and then applying their knowledge and experience of current drilling technology and ‘difficult access’ to design rigs that were first and foremost true ‘geotechnical’ rigs.

The result has been the development and subsequent introduction to the market of two completely new drilling rigs, the ‘P45’ and ‘P60’ slope climbing rigs.

The ‘P45’ is a track-mounted soil sampling rig on a specially designed chassis to enable it to safely and efficiently climb up, and operate on, slopes of up to 45°, and it can traverse slopes of up to 22°. In the right ground conditions, the ‘P45’ is capable of forming boreholes to 15 metres depth (of which the first eight metres can be cased), dynamic sampling of soils to obtain near-undisturbed continuous samples retained in a plastic liner, U70 sampling, standard penetration tests (SPT), installing standpipes, monitoring wells and biaxial inclinometer tubing, as well as dynamic probing.

And the recently completed ‘P60’ is the ‘big brother’ of the ‘P45’, which was developed following the success of the first rig, to satisfy clients who were impressed by the ease of access, superb data capture and good instrument installation capabilities but wanted deeper holes than are possible with the ‘P45’. GEL have responded to these requests by redesigning the drilling equipment to incorporate the facility to undertake rotary drilling, be it rock coring, turning in longer casing strings or open holing so that the new rig has ended up with a depth capability in excess of 80 metres.

No scaffolding, no possessions. ‘Go figure,’ as our American cousins would say.

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