Rail Ballast & Formation Condition Assessment using PANDOSCOPE®


PANDOSCOPE® for Rail Ballast & Formation Condition Assessment

  • Non-destructive rail track ballast and formation condition assessment (ballast fouling)
  • PANDOSCOPE® Coupling of tip resistance vs depth profile with down the hole imagery
  • Used for planning rail track maintenance and renewal


In rail applications, the PANDOSCOPE® measures geotechnical aspects of the track bed and can provide the following outcomes:

  • Layer characterisation for ballast and formation (identification, thickness, water content (qualitative), estimation of the soil grain size distribution and ballast condition (ballast fouling) assessment
  • Mechanical information: cone resistance (direct measurement) or CBR or other parameters with correlations

The PANDOSCOPE® data helps rail asset managers / infrastructure managers optimise the track maintenance and track renewal strategy by prioritising and allocating rehabilitation efforts and funding only to the sections that require priority attention whilst minimizing track downtime. Their objectives maybe:

  • Maintain the current usage requirements
  • Accommodate increases in safety, train frequency, speed and load

The PANDOSCOPE® provides engineering services with reliable geotechnical data for track design purposes. The knowledge of mechanical and physical properties of existing formation (subgrade and sub ballast layers) is very important for the future track design.

Sometimes, PANDOSCOPE® testing is combined with network Ground Penetrating Radar (GPR) data to provide more information in problem areas.

Here are some of the research papers on the PANDOSCOPE® technology rail applications.


The PANDOSCOPE® overcomes the limitations of the majority of classical geotechnical tests (drilling rigs, pot holing). Benefits include:

  • Proven approach
    • Tried and tested investigation strategy for track maintenance and renewal (localised or cross network)
    • Stand alone PANDOSCOPY or the coupled use of PANDOSCOPY and GPR
    • Several thousand kilometres of investigations over 50,000 tests
    • Passenger and heavy haul freight networks
    • Several countries including France (SNCF), Belgium, USA, Canada, Singapore and Australia
  • No destabilisation / disturbance of the track
    • Important if renewal works are delayed or do not proceed
  • Better informed decision making
  • Speed and versatility
    • Limited track possession time required (e.g. can test between trains working under lookout protection)
    • Light weight portable equipment enables track access onto embankments and into cuttings
    • Testing not limited by the height of equipment e.g. due to overhead electrification
  • Cost effective methodology
    • Reduced cost per hole compared to conventional approaches like potholing
    • Reduced head count on site
    • Less flights/accommodation required for remote site working

The PANDA® Instrumented Dyamic Cone Penetrometer (DCP) involves driving a variable energy cone penetration device into the rail track substructure to collect the strength (and modulus by correlation) profile with depth.

Condition monitoring of the rail track substructure layers is accomplished through insertion of a camera into the same hole, also called Geoendoscopy. The combined system is referred to as the PANDOSCOPE®. The PANDA®, PANDOSCOPE® and Geoendoscopy are all systems developed by Sol Solution.

Once the PANDOSCOPE® data has been processed, the results can be presented.


How it Works

The PANDOSCOPE® is a coupling of the PANDA® Instrumented Dynamic Cone Penetrometer (DCP) (tip resistance vs depth profile) and Geoendoscopy (imagery from down boreholes) combined with sophisticated data presentation and analysis software.

The PANDA® Instrumented Dyamic Cone Penetrometer (DCP) involves driving a variable energy cone penetration device into the rail track substructure to collect the strength (and modulus by correlation) profile with depth. Condition monitoring of the rail track substructure layers is accomplished through insertion of a camera into the same hole, also called Geoendoscopy. The combined system is referred to as the PANDOSCOPE®. The PANDA®, PANDOSCOPE® and Geoendoscopy are all systems developed by Sol Solution.

PANDOSCOPE S_27-03-16_XXXXXX_1_006_P_v2

Once the PANDOSCOPE® data has been processed, the results can be presented.

The right hand chart shows the Penetrogram of the PANDA® cone resistance according to depth. The left hand window shows the stratigraphy of the track layers (thickness, nature and hydrous state). The degree of ballast fouling is clearly visible, even if there if there is no distinct interface between clean and fouled ballast.


PANDA® Instrumented Dynamic Cone Penetrometer (DCP)

 PANDA Instrumented Dynamic Cone Penetrometer (DCP) Schematic

PANDOSCOPE Ballast & Formation Strength Profile from PANDA DCPThe PANDA® test is an instrumented variable energy dynamic penetration test. The tests consists of driving a set of steel rods equipped with a conical tip of 2cm2 cross-section through the material by hammering an Anvil with a standardised hammer. At each hammer blow, the energy is measured in the anvil with strain gauges.

Sensors measure simultaneously the settlement or vertical displacement of the cone. All the data is transmitted to the Central Acquisition Unit. The results are displayed immediately on the Dialogue Terminal as penetrograms, graphs that show the evolution of cone resistance according to depth.

For each blow, the depth of penetration and the driving variable energy are measured to calculate the dynamic cone resistance (qd) with the corresponding depth using the Dutch Formula, shown in this figure.

  • PANDA Instrumented DCP Dutch Formula A is the cross-sectional area of the cone
  • E is the kinetic energy fed into the system during the impact
  • e is the penetration per blow
  • P is the weight of the driven parts during impact (impact head/anvil, rods and tip)
  • M is the weight of the striking hammer

The PANDA® collects mechanical information including cone resistance (direct measurement) or CBR or other parameters with correlations. This is presented in graphical format, knows as a penetrogram.

The PANDA® is a versatile equipment with a total weight less than 20 Kg. Within the framework of the railway use, adaptations have been made to make it even lighter and tougher.


The Geoendoscopy test uses a tiny video camera (wired to a data logger with a soft cable) to observe the soil. The camera is introduced into the hole of a previously performed PANDA® DCP test (15 mm of diameter). The collection of imagery from down the hole allows a qualitative characterisation of soil.

In the railway application, it enables condition monitoring of the railway track substructure layers. The layer characterisation for ballast and subgrade includes layer identification, layer thickness, water content (qualitative), condition (ballast fouling) and even the estimation of the soil grain size distribution

Why is Rail Track Ballast Condition Important?

Rail tracks are positioned on railway ballast, a granular material, generally comprises large, angular particles of typical size ranging between 25 and 50 mm. The main functions of railway ballast are:

  • to provide high load bearing capacity which reduces pressure from the sleeper bearing area to acceptable levels at the surface of the subgrade soil
  • to provide rapid drainage

Rail ballast usually contains uniformly graded material creating a sufficiently large pore structure to facilitate rapid (free) drainage. When ballast is aged and degraded, fine particles accumulate within the voids (fouling) thus impeding drainage. The process of ballast fouling, when it becomes extreme, can also generate excess pore water pressure under fast moving trains (i.e., high cyclic loading), thereby reducing the track resiliency and stability (undrained).

The maintenance costs of ballasted tracks can be significantly reduced if an accurate estimation of the different types and degree of fouling materials can be related to track drainage.




Publication Date

Geotechnical investigation of a French conventional railway track-bed for maintenance purposes - Lamas-Lopez, Cui, Costa D'Aguiar, Calon – SNCF - Soils and Foundations - Japanese Geotechnical Society Dec 2015

Author: Lamas-Lopez, Cui, Costa D'Aguiar, Calon

Date: December 2015

A comparison of two methods of determining the stiffness of track-bed materials (dynamic penetration and dynamic plate load) comparing the PANDA and the Light Weight Deflectometer.

Unsaturated Railway Track-bed Materials – Yu-Jun Cui (2016)

Author: Yu-Jun Cui

Date: 2016

On-site Concrete Segregation Analysis using Image Analysis – Journal of Advanced Concrete Technology Vol 6 – P Bruel, J-M Geoffray & Y Haddani – Feb 2008

Author: Pierre Bruel, Jean-Marie Geoffray, Younes Haddani

Date: 2 February 2008

Segregation remains one of the major problems for traditional and self-compacting concrete. The consequences of this pathology are numerous and may affect the long-term properties of the structures. In order to ensure the expected characteristics of the concrete, it is essential to be able to check its homogeneity.

Some tests allow the checking of the fresh concrete properties at the concrete mixing plant, but there is at the present time no method to assess concrete segregation on site.

The development of a quick and low disturbance method allowing quantification of the segregation phenomenon automatically within structures constitutes an advance in the pathology detection area. The method presented here relies on the use of geoendoscopy and automatic image processing techniques. After a short presentation of the tools and the auscultation methodology, the image processing techniques developed in order to measure the concrete homogeneity and to control the concrete particle size distribution are exposed.

Results obtained with this methodology in laboratory experiments are then compared with those obtained with the traditional video counting technology. Finally, the last part is devoted to the application of this method to a real self-compacting concrete structure.

New diagnosis methodology for old tunnels in service – P Bruel, P Goirand & Y Haddani – TUNNELS ET ESPACE SOUTERRAIN – No.245 – Oct 2014

Author: Younés HADDANI, Pierre BREUL, Patrick GOIRAND

Given the issues surrounding the long­term future of ageing underground structures, agencies responsible for these struc­tures need to optimize their maintenance policy. This involves a better evaluation of deterioration in structures and their ability to perform their function over time.

Based on geophysical testing and non­aggressive probes, this methodo­logy offers quantitative evaluation of the condition of masonry, the state of contact between the structure and the surrounding terrain, and a description of the surrounding terrain and its variability. Based on properties obtained using this new ins­pection method, a scoring system for structures was defined in order to prioritize maintenance works.

At the same time, mo­delling work incorporating this data was performed in order to study the behaviour of the structures once the deterioration and properties measured on site had been integrated. Development and validation of this research was performed using actual structures on the RATP metro network in Paris. This paper pre­sents the whole of the fully­ developed diagnosis methodology for these structures, and its application to an actual structure.

Performance Evaluations of Pavement Working Platforms Constructed with Large-Sized Unconventional Aggregates – H Kazmee & E Tutumluer, University of Illinois & D Mishra, Boise State University – 2015

Author: Hasan Kazmee, Erol Tutumluer, Debakanta Mishra

Date: 2015

Use of unconventional aggregate materials, such as primary crusher run and concrete demolition waste, have become viable for the construction of pavement working platforms over very weak and often wet subgrade soils. To this end, a research study was undertaken at the Illinois Center for Transportation to evaluate the adequacy and field performances of such large-sized aggregate materials and validate new material specifications.

A state-of-the-art image analysis technique was utilized to characterize the size and morphological properties, e.g. shape, texture and angularity of two large-sized aggregates, referred to herein as primary crusher run and crushed concrete. For the field evaluation, full-scale test sections were constructed with these large-sized aggregate materials over a very weak engineered subgrade and subjected to accelerated pavement testing. Construction quality control was achieved through in-place density and modulus measurements on conventional aggregate capping surface layers using nuclear gauge, lightweight deflectometer and soil stiffness gauge type devices. Periodic rut measurements were carried out on the pavement surface throughout the accelerated loading process using an Accelerated Transportation Loading Assembly (ATLAS). Contributions of the underlying pavement layers to the total rut accumulation was evaluated through innovative applications of ground penetrating radar (GPR), a light weight penetrometer device, known as the French Panda, as well as a geo- endoscopy probe. Layer intermixing and material migration at the aggregate subgrade and subgrade interface was found to improve the layer stiffness and pavement performance results significantly.

Methods of Track Stiffness Measurements – INNOTRACK GUIDELINE – Project No. TIP5-CT-2006-031415 (2006)

Date: 2006

Vertical track stiffness is an important parameter in railway track engineering, both from a design and maintenance point of view. This guideline presents important aspects of track stiffness as well as different measurement methods to gather stiffness information of the track.

A method called Panda, for determining local track stiffness has been used and developed. Panda is a lightweight penetrometer which determines the cone-resistance of the layers of the track substructure rapidly.

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