Careful selection of investigation techniques is paramount in gathering good quality geotechnical information in the difficult working environments often encountered on railway projects, says Mike Newton.
The UK rail industry has the unenviable task of providing a 21st century service with an infrastructure that essentially dates from the late 19th century.
Recent events have focused attention on asset management and the need for an active risk assessment strategy.
A key factor in managing geotechnical risk in the rail industry is up-to-date and accurate geotechnical information on the materials forming cuttings, embankments and foundations for structures. Comprehensive geotechnical data is also a prerequisite for design of new and upgraded track and systems.
For the geotechnical engineer faced with obtaining such data from exploratory holes, the rail environment presents some challenging conditions. These include site access, working windows and health and safety considerations. Frequently conflicts arise between technical preferences, the time available and the cost of obtaining information.
Techniques to reduce conflicts are constantly being sought and frequently the answer is choosing appropriate equipment. A wide range of techniques is available, especially when dealing with restricted access alongside tracks.
The techniques described here are applicable to investigating deep (natural or man-made) materials beneath or affecting the track. Investigations of superficial track ballast and formation materials were discussed in last month's Ground Engineering (Meeting report, April 2001).
Typically, geotechnical problems on the railway arise in cuttings or embankments because of unstable slopes or excessive settlement.These are often overgrown sites with steep slopes, restricted access and confined working areas.
Information is also required for new works, which present a different set of working conditions, usually with easier access, but affect train movements and the public.
Because ground investigations are time-consuming and their duration is uncertain, they are generally not suited to the restricted time slots available during railway possessions. From an operational aspect, the preference is to keep away from the track and avoid operations that are affected by or can affect the running of the railway.
The technical requirements of a geotechnical problem should be of paramount importance.With detailed knowledge of available equipment, and the information that such equipment can provide, the geotechnical designer can be confident of obtaining the appropriate level of information at a cost compatible with the contract requirements.
As with other investigations, information for railway work comes from insitu measurements, sampling and long-term monitoring. Insitu measurements for railway work are generally restricted to basic penetration tests such as the Standard Penetration Test (SPT) and the dynamic probe or the borehole shear vane.More sophisticated insitu measurements can be made with pressuremeters, but these are not common in the UK.
Typical instrumentation used includes slip indicators, inclinometers, settlement gauges, standpipes and piezometers, including those recording positive and negative water pressures that can be downloaded.
Trial pits and trenches Hand-dug pits form a key part of railway investigations of shallow slip features because they are so flexible. They expose detailed soil structure and enable hand shear vane and pocket penetrometer testing of insitu material.While samples are often disturbed, high quality block samples can be taken.
Deeper pits of 2-3m can be dug using 'mini' or 'midi' tracked machines, but unless there is clear access and a large working area it is not often that a large tyred excavator can be used.
Small diameter boreholes in superficial materials Offering portability and flexibility, the window sampler has become one of the standard items of equipment used on railway embankments and cuttings.The technique is relatively quick and can be used during possessions, although pilot holes must be hand dug through railway ballast.
An almost complete soil profile to depths of 5-6m can be defined in most cohesive soils.With heavy duty equipment, hydraulic jacks and favourable ground conditions, it is possible to investigate to 12m depth.
However, the technique has limited use in very stiff soils, granular materials that collapse once the sampler is removed, large obstructions or granular made ground.
Window samplers use either a petrol driven hammer or, more commonly, a heavy duty hydraulic driven hammer.Hole diameters vary from 100mm to 25mm depending on depth of hole and nature of the ground.
Samples almost matching undisturbed U38 standards can be recovered.More commonly, window sampling provides a continuous soil profile, an opportunity to carry out hand vane or pocket penetrometer shear measurements and collection of small disturbed samples for moisture content, Atterberg limit or contamination testing.
Instrumentation can be installed in window sample holes but with a base diameter of about 50mm, these are often restricted to the upper portions of the hole. Slip indicators and basic piezometers can be installed with reasonable success.Where groundwater is a fundamental issue, continuous data logged piezometers have been developed specifically for window sample holes that will measure a positive head of water and suction pressure.
Tracked dynamic sampling rigs are now available, based on a small tracked 'power barrow'These have more power and use the same sampling equipment.The power barrow will gain access to many sites and can be tracked along and operate on sloping, soft or uneven ground.Fitted with a drop hammer, the dynamic sampling rig can drive window sample tubes to 8-10m depth and can carry out SPTs.
Final borehole diameters are similar to those produced with the window sampler and will accommodate similar instrumentation.
Dynamic probe equipment is hand portable and used with a small diameter solid cone to measure relative resistance/density. It is suitable for rapid profiling of soft materials and identifying rockhead or the position of structural foundations. It does not yield a sample and the hole can only be used for the most basic groundwater monitoring installation.
Larger diameter holes in superficial materials and weak bed rock Larger equipment is needed to drive U100 sample tubes and operate SPT trip hammers.
Heavy duty dynamic sample rigs (for example the Competitor 150) are available which are capable of driving casing and U100 samples to 3m or 4m depth before having to reduce to a smaller diameter. But if high quality samples are needed at significant depths, a cable tool boring or hybrid rig has to be used.
The downside of cable tool equipment is its relatively large size. It is difficult to manoeuvre and requires a large, reasonably level working area.The erected tripod height of more than 7m also precludes its use in many trackside locations.
Variations are available to bring the advantages of this technique to the confines of the railway.These include sectional diesel or air powered tripod winches capable of drilling to 20m and 15m depths respectively.Both of these can be dismantled and reconstructed on a small levelled working area or scaffold platform and can also operate with reduced height tripods essential when track or overhead line clearance is an issue.
Air winch units have less power than diesel units and therefore a shallower depth range.They also require a compressor unit to provide power which in many railway situations can present its own problems; however the sections are lighter and more portable.
Tracked multipurpose rigs are available that can sample and carry out insitu tests in drift materials as well as having the capability to core bedrock. These are ideal where drift thickness and depth of core required are small.
For sampling and instrumentation in deep or large diameter boreholes over 15m depth in superficial materials with difficult access, sectional diesel powered winch units are preferable.
Boreholes in hard rocks Where level ground and access are unrestricted, the most economic method of obtaining quality rock core is with a conventional lorry or road-going all-terrain truck mounted rig. These are capable of producing high quality up to 100mm diameter core to depths in excess of 100m.These boreholes are suitable for installation of high quality groundwater monitoring, slope stability instrumentation or down-hole rock surveying such as a borehole televiewer and geophysical methods.
However, this type of equipment is often unsuitable for railway investigations.The smallest rotary rigs available are based on narrow rubber tracked expanding bodies.
Designed to pass through a standard doorway with a remote control and separate power pack, these rigs are compact and can traverse rough ground and reach confined spaces. They will operate on uneven and sloping ground and with a double pivoted mast can drill at any angle to obtain cores up to 100mm diameter in structures or in bedrock to depths of more than 30m.
Mini tracked rigs weigh between 3t and 4t and can be used from scaffold platforms with appropriate cranage.
Short masts can also enable drilling next to overhead lines, where larger plant would not be permitted.Trailer mounted rigs offer similar coring performance but are relatively difficult to manoeuvre on site. They are, however, more readily available and can operate in a fairly restricted, albeit level, work space.
Larger tracked rigs offer improved performance.
Mounted on a steel-tracked chassis, these can track through rough terrain and will produce a vertical hole from sloping ground to depths of more than 50m.
Structural investigation Structural coring is frequently required where bridges, arches or retaining walls are to be subject to new loading or require structural appraisal.
Typically the requirement is to obtain core of small diameter (about 50mm) through the structure to determine its quality and thickness and gain some indication of any backfill materials. This can be readily performed with small high-speed coring units mounted to the structure surface using removable expanding bolts and can drill in most directions including upward into bridge or arch roofs.
No compromise In some cases technical requirements are paramount and in these cases extraordinary effort has to be made to obtain information. This can entail the building of large scaffold access ramps and working areas, craning equipment into position and occasionally mobilising specialist equipment and services.
A recent contract for the proposed Thameslink 2000 rail upgrade through the centre of London required riverbed samples from the Thames and structural cores through piers at Blackfriars Bridge.A mini rotary rig and a heavy duty cable tool boring rig were loaded on to jack-up drilling platforms and then towed up river to the site.The cost of this work was substantial but the information obtained was essential for the subsequent design.
Possession working There are times when possession working is unavoidable.
In these circumstances planning, timing and the choice of the right equipment is crucial. Mobilisation of multiple units, additional crews, on-site fitters and stand-by equipment must all be built into the contingency plan.
Conclusion In the often difficult operating conditions of the railway environment, careful selection of appropriate plant will enable the cost-effective maximisation of geotechnical information that can be obtained.
Mike Newton is business development manager at Norwest Holst Soil Engineering.