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CPT: The ideal solution for rail ground investigations
Tuesday, August 20th, 2019
Chris Dimelow, Lankelma General Manager.
Ground 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.
In the past, most rail investigations have relied upon dynamic probing and window sampling over conventional boreholes. However, while these techniques are fast and relatively simple, they are limited in the subsurface materials they can investigate; often they cannot reach the required depths and the data they provide is of limited use.
CPT is ideal for rail investigations because it is fast, quiet and clean, producing no spoil, and can achieve deeper penetration than dynamic probing. The high quality data it produces can be used to determine soil parameters including soil type, soil density, insitu stress conditions and shear strength for use in geotechnical design. For instance, CPT results can be used directly for piled foundation design in sands and gravels.
You can find out more by reading our introduction to CPT blog.
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 can be carried out in a six hour night-time possession, including the time for getting on and off site (depending on how far test positions are from the site access point, of course).
CPT 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. This is a key consideration, as detection of potential problems is of paramount importance on the railway.
On-track CPT plant
As the use of CPT in railway investigations has grown, Lankelma has developed on-track plant and equipment designed specifically for investigations in the four main rail environments: trackbed, embankments, tunnels and stations.
All of this kit 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 trackbed 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, however, is on embankment investigations because embankment materials – clay, sand and gravel – are particularly suitable for CPT. This has resulted in the development of highly mobile, lightweight plant able to test through the 4-foot ballast on an embankment; on the embankment or cutting slope itself; and at the toe.
Our crawler rigs, for example, can cope with embankment slopes of up to 40°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. Mini-crawlers 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), our rail excavator-mounted CPT rig may be the answer. Mounted on the excavator arm, this runs off the machine’s hydraulic and electrical systems, so is self contained, with the added advantage that testing can be carried out up to 6m from the track. It can also be used in tunnels, pushing horizontally and even vertically downward using a swivel hitch.
Investigations in tunnels and stations
Investigations in tunnels can be a challenge, so we have developed lightweight hand-mobilised CPT rams for tight access positions. 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.
Lankelma’s road-rail CPT unit
One of the biggest innovations in rail-site investigation in recent years has been our road-rail CPT unit. 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.
CPT is an ideal choice for rail site investigations, as it is fast, safe and technically superior to conventional investigation techniques. Producing minimal soil disturbance and spoil, CPT can characterise subsurface materials insitu, produce continuous profiles immediately and its high quality repeatable results can be transferred digitally.
We’re here to help
We are always looking at new ways to help clients and partners, so please get in touch if you want to discuss how we can work together to share ideas and develop solutions to your geotechnical challenges.