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Site investigation: Cone confidence

Improved seismic cone penetration testing should bring even more benefits to rail site investigations, says Chris Dimelow, projects manager for Lankelma.

Seismic cone

The roadrail CPT rig can arrive by road and travel on track to the test location

Site investigations on the railway are often challenging, with tight timescales – mainly during night-time and weekend possessions – on sites with difficult access and tight working space. Then there is the challenge of data quality and timely access to results.

Cone Penetration Testing (CPT) is an ideal technique for site investigations on the railway because it is fast, quiet and clean, producing no spoil, and can often achieve deeper penetration than dynamic probing and window sampling.

A major advantage for railway investigations is CPT’s speed. Typically, a 15m deep CPT takes about 40 minutes, so between four and five tests (producing 7,500 data points) can be carried out in a sixhour night-time possession including the time for getting on and off site (depending on how far test positions are from the site access point).

CPT delivers high quality data which can be used to determine soil parameters, including soil type, relative density, insitu stress conditions and shear strength, for use in geotechnical design. Results are also available in real time, which allows engineers to take decisions on site during the investigation. This is particularly useful when identifying the best locations for further testing, sampling and monitoring.

THE ADVANTAGES OF CPT ON THE RAILWAY

  • Greater delineation of strata, as readings taken every 20mm
  • Repeatable, high quality and reliable results available in real time
  • Electronic data transfer means more manageable data handling
  • Test is quiet, produces no vibrations and creates minimal soil disturbance
  • High productivity (up to 150m tested per day)
  • Instant results allow on-site selection of the best locations for sampling, testing and monitoring
  • Huge range of cones for geotechnical investigations, contaminated land studies and unexploded ordnance detection
  • CPT platform can be used for obtaining high quality samples and installing instrumentation. 

Railway applications

The seismic cone is a particularly useful tool for railway site investigations. The test measures the insitu shear wave velocity, which allows the small strain shear modulus and the stiffness of the ground to be calculated. Small strain shear modulus is essential to understand the dynamic properties of the soil and can be used for calculating settlement and dynamic loading effects, such as those experienced by railway earth structures.

Lankelma recently refined the seismic cone test by developing a new seismic wave generator positioned next to the CPT rig (on the railway this can be a road rail vehicle (RRV) rig or one attached to a rail excavator arm).

Seismic beams are typically a wooden block with a metal face but this creates a disjointed source – the shear wave generated from the wood is not consistent and the integrity of the wood can deteriorate quickly after multiple strikes.

Clearer shear wave

The seismic wave generator, on the other hand, comprises a nylon block with ground anchors that produces a cleaner seismic shear wave. The nylon produces fewer harmonics, so dampening provides a clearer, repeatable shear wave signal. Anchors and teeth help secure to the ground, restricting horizontal movement. This means the force of the strike is better transmitted into a shear wave and not dispersed through surface movement.

The 1,500mm2 seismic cone comprises a piezocone unit with a seismometer above, which is pushed into the ground and stopped at 1m intervals to measure shear wave velocity and build up a seismic profile. The cone is an accelerometerbased seismometer, rather than a more conventional geophone system. This has a higher sensitivity, with a larger frequency response and less noise than a geophone system, delivering better quality data and to considerably greater depths.

Higher data quality ultimately means that interpretations are far more accurate, which in turn means design can be improved.

Because it can be used with most CPT equipment developed for the railway (see box), seismic cone testing is suitable for investigating a wide range of railway environments, including the track bed, embankments, tunnels and stations.

THE DEVELOPMENT OF RAIL-SPECIFIC CPT

Seismic cone

The seismic cone measures shear wave velocity at 1m intervals to build-up a seismic profile

As the use of CPT in railway investigations has grown, contractors have developed on-track plant and equipment designed specifically for investigations in the four main rail environments: track bed, embankments, tunnels and stations.

All of this equipment can be used with the full range of standard and specialist cones, can take samples and be used to install instrumentation. Provided there is sufficient reaction force and a power supply, CPTs can be carried out almost anywhere.

For example, in track bed investigations, CPT can be used to measure the thickness and quality of the ballast and sub-grade and can be used in conjunction with Ground Penetrating Radar to help correlate data.

Where CPT comes into its own is on embankment investigations because embankment materials – clay, sand and gravel – are particularly suitable. This has resulted in the development of highly mobile, lightweight plant able to test through the 1.2m ballast on an embankment; on the embankment or cutting slope itself; and at the toe.

Crawler rigs, for example, can cope with embankment slopes of up to 35° and carry out tests on slope angles of up to 15°. They can perform both vertical and inclined CPTs through embankments and some can screw themselves into the ground for additional reaction force. Minicrawlers can also travel up and down the cess with ease and, if needed, are small enough to work in positions with restricted height. Typically, once set up on position, a mini-crawler can carry out a 10m test in about 30-40 minutes.

Where high reaction forces are needed (up to 10t), a rail excavator-mounted CPT rig may be the answer. Mounted on the excavator arm, these run off the machine’s hydraulic and electrical systems, so are self contained, with the added advantage that testing can be carried out up to 6m from the track. They can also be used in tunnels, pushing vertically downward using a swivel hitch.

Investigations in tunnels can be a challenge, so where access is tight, lightweight hand-mobilised CPT rams have been developed. These are bolted into position on the tunnel walls to gain reaction and can test in multiple orientations as a result. The same equipment, which is electrically powered, and therefore quiet, is suitable for testing within open stations or at the end of platforms.

One of the biggest innovations in rail-site investigation in recent years has been the road-rail CPT unit, such as the one developed by Lankelma.

Arriving by road, the truck has a central turntable mechanism which lifts it into the air and allows it to be rotated through 360°, allowing it to gain access to rail tracks at most access points. It can travel easily between positions, can be driven backward and forward at the same speed and is self-contained, carrying two operators to position with all the testing equipment. 

 

 

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