- Email: [email protected]
Evaluation of railway trackbed and formation: a case study
NDT&E International 36 (2003) 145–156 www.elsevier.com/locate/ndteint
Evaluation of railway trackbed and formation: a case study M. Brougha,*, A. Stirlingb, G. Ghataorab, K. Madelinb a
Scott Wilson Pavement Engineering, 9/10 Faraday Building, Nottingham Science and Technology Park, University of Boulevard, Nottingham NG7 2QP, UK b School of Civil Engineering, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK
Abstract To determine the causes of poor track geometry, a trackbed investigation should comprise a full assessment of ballast, sub-ballast and formation condition. If information about any of these layers is omitted, the true cause of poor formation sites cannot be ascertained satisfactorily and consequently appropriate ground improvement schemes may not be implemented. This has been observed in the UK rail network with examples of a lack of detailed site investigation and subsequent expensive remedial work being carried out without eliminating the initial cause of the problem. This paper summarises the trackbed investigation performed at a site requiring high maintenance, suspected as being caused by poor formation. The paper concludes with an analysis of ground conditions and possible choice of remedial schemes. q 2003 Elsevier Science Ltd. All rights reserved. Keywords: Ballast; Formation; Ground penetrating radar; Falling weight deflectometer; Railway trackbed
1. Introduction Trackbed investigation is considered to be crucial for assessing potential maintenance requirement and subsequent design of any remedial measures. In reality, track components are often maintained or replaced with little regard for properties of the formation and their subsequent effects on continued total asset life. Examples of expensive remedial work being carried out without eliminating the initial cause of the problem have been reported by Cope [1]. Ideally, site investigation data should be used not only for design of remedial ground improvement schemes, but to predict perceived track performance and more importantly schedule necessary work proposals. Sharpe [2] has proposed the ‘total route evaluation’ approach, a comprehensive methodology yielding sufficient information to enable optimisation of renewals and maintenance strategy for a ten-year period. This methodology was first suggested by Sharpe and Collop [3], comprising staged levels of investigation, identifying formation anomalies and enabling requirements for formation treatment to be identified many years in advance. The Association of American Railroads is also developing an expert system approach to diagnose and recommend the most cost-effective remedy for various track * Corresponding author. Tel.: þ44-115-922-9098; fax: þ 44-115-9431302. E-mail address: [email protected] (M. Brough).
substructure problems based on investigation methods and economic evaluations [4]. Hunt [5] has confirmed track quality,1 continued track component performance and subsequent maintenance are highly dependent upon the magnitude and variation of formation stiffness. This is exacerbated by consolidation and settlement of formation and the subsequent differential track geometry. The achievable track geometry can in turn influence the total asset life of elements of the track infrastructure, such as rails or sleepers, as suggested by Cope [1]. This situation has been compounded by an increase in rail traffic, pressure for higher line speeds and heavier axle loads. Consequently, there is an increasing importance in obtaining a consistent relative measure of trackbed stiffness in the physical and time restrictions applicable on live track. In addition to stiffness assessment, any trackbed investigation should include an assessment of ballast, subballast and formation. Cope [1] suggests that if this layer information is omitted, the cause of poor formation sites cannot be ascertained and consequently applicable remedial schemes cannot be implemented. Shahu et al. [6] suggested sub-ballast depth, after subgrade modulus, was the most influential track parameter on overall track response. Sharpe [2] recognised that the condition of the ballast would have 1 A track condition index based upon standard deviations of track geometry.
0963-8695/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 3 - 8 6 9 5 ( 0 2 ) 0 0 0 5 3 - 1
146
M. Brough et al. / NDT&E International 36 (2003) 145–156
a strong influence on track quality. Groundwater profiles and behaviour must also be assessed due to the deleterious effects on formation condition and track quality. Li and Selig [7] suggest that if the water table is within approximately 6 m of the surface, this is the major factor affecting the moisture content of the formation and subsequent problems. The nature of any site investigation will depend upon the specific nature of the problem and potential methods of remediation. In addition, physical and time restrictions in the UK rail network will limit investigation techniques that can be employed. An assessment of suitable testing techniques has been described previously by Brough [8]. This paper details a site investigation comprising trial pit excavation, dynamic cone penetrometer (DCP), ground penetrating radar (GPR), automatic ballast sampling (ABS), cone penetration testing (CPT) and falling weight deflectometer (FWD) testing. These were used in two distinct levels of trackbed investigation. Results of field trials performed at a suspected poor formation site and conclusions made are presented. The work forms part of a government funded EPSRC LINK research project (‘Improving the Stiffness of Existing Rail Track’—ISERT) in association with Railtrack, GTRM, Scott Wilson Pavement Engineering, WS Atkins Rail, Keller Ground Engineering, Fugro and Serco Railtest.
2. Site investigation 2.1. Trial site The site is located near Leominster station in Herefordshire in the UK. The initial site choice was based upon the need for repeated track maintenance, and occurrence of contaminated ballast in trial pits excavated as part of previous track maintenance contracts. 2.2. Test schedule The investigation, performed during a T22 and T33 possession, had several objectives. The preliminary assessment, carried out under a T2 possession, was to identify soft formation beneath track components and to highlight possible causes of differential track settlement. Consequently, omissions in data were highlighted along with recommendations for future site work that had to be done in a night-time possession. The subsequent more-detailed investigation confirmed the nature and cause of soft formation and provided more detailed geotechnical information that could be used for design and assessment of 2
Typically day-time work with intermittent access to track that is primarily governed by railway traffic and subsequent safety considerations. 3 Typically night-time work when there is no scheduled rail traffic with continuous access to track.
remediation techniques. The extent and methods of site investigation used at Leominster are summarised in Fig. 1. 2.3. Desk study Specific poor formation hot spots were identified during a visual survey, evidenced by associated wavy top and clean polished exposed sleeper surfaces. This was supported by track geometry measurements indicating excessive vertical track elevation standard deviations and associated poor track quality. This section of track had previously been identified by maintenance history as showing high potential for localised formation problems. Historical records of previously opened trial pits showed ballast on the up line was completely choked with grey clay and the water table was at or near the formation surface level. On the down line, trial pit records showed choked ballast was present but not across the entire trackbed section. This suggested possible ballast pockets, with choked ballast typically concentrated beneath rails. Water levels were also generally much deeper on the down line. Track maintenance history generally suggested the potential for and occurrence of formation problems was higher on the up line, possibly due to an increased traffic loading, observed higher water table and associated in situ moisture. Subsequently, it was decided to locate a significant proportion of the proposed trial pits on the up line, with some trial pit locations matched on the down line. This desk study highlighted the importance of using historical data to identify poor formation sites along a route. In addition to trial pit logs, information regarding trackbed condition can be ascertained from maintenance history and track surveys. For example, performance data from automatic ballast cleaning operations could provide information on the fines content of ballast, and roughly indicate formation condition. Unfortunately, potential access to relevant data is not always feasible due to regional ownership of track maintenance information by contract operators in a nonstandard format. Consequently, a consistent data collection procedure is required and subsequent data made accessible to supplement trackbed investigations. In addition to track maintenance history, track component type and location should be logged due to their obvious effects on track behaviour. Subsequent correlation of track component logs with identified poor formation sites may highlight possible relationships.
3. Ground investigation: stage 1 Trial pits, DCP and GPR were implemented to assess the overall conditions of the entire length of track, and identify sites with existing and potential problems. The site investigation performed has been summarised in Fig. 1.
M. Brough et al. / NDT&E International 36 (2003) 145–156
147
Fig. 1. Schematic showing site investigation performed at Leominster.
3.1. Trial pits Trial pits would not normally be excavated in the formation layers beneath the trackbed. However, due to their perceived relevance to track geometry deterioration they were included and logged as in a traditional site investigation. Open trial pits suggested that the poor formation sections were more prevalent on the up line, due to contaminated ballast encountered at shallower depths. In all trial pits on the up and down line, ballast below depths of 0.3 – 0.6 m was completely choked and contaminated with a soft grey clay/silt. In some cases this layer was saturated or groundwater seepage was observed below the ballast. This was particularly evident on the up line, with all trial pits opened showing a typically saturated soft mid layer, consisting of ballast and fines. Although it would appear that excavated material from the up and down line was of a similar composition, the contaminated fines layers on the down line did not exhibit the same high in situ moisture content. Excavation of bottom layers (. 0.7 m) showed the formation ranged from a firm to stiff intermixed mid grey and red brown slightly sandy clay/silt with occasional gravel, to a firm to stiff brown slightly sandy clay/silt. As with other layers, these bottom layers exhibited higher moisture content on the up line. Trial pit logs confirmed initial observations regarding the identification of poor formation at Leominster station, particularly on the up line. However, although trial pit logs identified poor formation at discrete points along the track, the extent, depth and nature of the poor formation section could not be assessed. This was due to current limitations in the trial pit method currently being employed on the UK railway network. First, depth of trial pit excavation was limited, not including formation layers where problems are often sourced. Second, trial pit excavation was often infrequent resulting in an incomplete
assessment of poor formation sites. Third, as a rule geotechnical engineers did not log trial pits, with a good possibility that important information may not be recorded or recorded incorrectly. Sharpe [2] has also highlighted these limitations in his assessment of current state of the art methods of trackbed investigation. As shown in this study, there is the potential for a more consistent method, with logging and sample excavation performed using traditional geotechnical principles and improved quality control. Samples can be analysed off-site for classification testing and results used for design of any relevant remedial schemes. Although depth of trial pit excavation is limited, geotechnical test devices such as the shear vane and DCP could be used in trial pits and improve the current investigation practices in the UK railways industry. Needless to say, trial pit excavation, with improved quality control, still represents a useful accepted tool for site investigation of trackbed. Trial pit logs were used to calibrate the performance of other test devices used in this investigation. 3.2. Dynamic cone penetrometer Due to the improved access to the formation in excavated trial pits, there are substantial benefits of using simple hand held devices. Due to the relatively easy operating principles, semi-nondestructive nature, low cost and simple results interpretation of the DCP, tests were performed at the maximum depth of open trial pits. The DCP also offered the potential for formation assessment without the removal of ballast. The DCP relies upon a penetration cone attached to a rod, being driven into the soil by means of a manual drop hammer, and the resulting penetration and number of blows recorded. Following testing, results can be used to obtain relative measures of bearing capacity in terms of depth of
148
M. Brough et al. / NDT&E International 36 (2003) 145–156
cone penetration per blow. Livneh et al. [9] suggests that a universal correlation exists between the DCP and California bearing ratio (CBR) for a wide range of pavement and subgrade materials, testing conditions, and technologies. In the current study, the DCP was used following trial pit excavation. Ideally, the DCP would be better implemented without prior excavation of the ballast. Initial trials would suggest that penetration of DCP rods through compacted ballast is feasible. The effect of ballast on DCP measurements is under investigation at present and will be reported in the future. DCP tests on the up line suggested that formation (approximately 1 m deep) underlying the open trial pits was very soft, typically comprising three layers with increasing strength with depth. Correlated CBR values confirmed the nature of this soft formation layer with values ranging from 0.8 to 2.4% and an average value of 1.5%. Similar layers were observed in trial pits opened on the down line. However, results suggested the formation was slightly stronger than on the up line. Although DCP tests identified the soft formation layer of concern, the technique still gave no indication of stiffness variation with depth. CBR is used extensively in the highways industry and has been considered a reliable relative measure of combined stiffness and shear strength; however, its application in a dynamic load environment is increasingly subject to scepticism when compared with other more relevant parameters such as elastic stiffness. Although the reliability of the DCP as a tool for providing geotechnical parameters relevant to the railway engineer is questionable, it could provide a useful relative indicator of formation condition. It is quick, easy to use and maintain and could be applied in a T2 possession to the measurement of relative formation condition along a track length. This should provide an insight into the heterogeneity of the formation in the direction of traffic, now known to be a major influence on track quality deterioration. Application rates of six tests per hour in trial pits excavated in the trackbed were observed on site. 3.3. Ground penetrating radar The inspection of ballast using the ground probing radar has already been described in detail by Sharpe and Collop [3], Hugenschmidt [10] and Jack and Jackson [11]. GPR was used to give an indication of ballast depth, as well as condition of ballast and the formation. GPR is a nondestructive technique that uses electromagnetic radiation to identify the presence of layer interfaces between the different materials comprising the trackbed construction. Short pulses of electromagnetic radiation are directed into the ground where they are reflected back at interfaces between different layers. The equipment records the strength of reflected radiation as well as travel time for the waves. Depths of layers can be estimated from the travel time assuming the velocity of wave propagation.
There is normally a strong reflection from the base of the ballast layer in the trackbed. If the ballast has deteriorated or become contaminated, the radar waves cannot penetrate as easily, so the strength of reflections from the base of the ballast will be low. However, if there is a distinct layer of contamination within the ballast (e.g. clay slurry) there may be a strong reflection from its upper surface. Typically a radar reading is taken between each pair of sleepers, and all the results are combined to give an effectively continuous profile. The speed of the survey is typically about 1 km/h. The GPR trace is plotted using a grey scale to indicate the strength of the reflected signal. A constant gain has been applied throughout, which gives an indication of the relative strength of the reflected signal along the track, while filtering out much of the noise which is characteristic of most GPR data. The particular gain function used has been specially developed to bring out the main features of the ballast layer. Lower layers may also be apparent, but the deeper information is less reliable with respect to both depth and amplitude of reflection. For the radar used in this study a 900 MHz antenna was used, with a typical frequency range between 75 MHz and 1 GHz. A 900 MHz antenna was used to give a shorter pulse. This method of assessment can be used for qualitative first indication of ballast condition and potential fines migration at poor formation sites, and also facilitates bettertargeted intrusive site investigations. Sharpe [3] also suggests the GPR can be used as an excellent tool for the quality control of trackbed renewal items, providing continuous ballast depth data. At the Leominster site, GPR traces were obtained for the up and down line, to identify poor formation hot spots and areas with potential for track geometry deterioration (Fig. 2). In agreement with results of trial pits previously opened (marked as £ in Fig. 2), there was a section on the up line that exhibited a significant upward migration of fines, and insufficient variable nonslurried ballast depth. This was confirmed in automatic ballast samples, with nonslurried ballast depth (below ballast surface) ranging from 220 to 680 mm. As can be seen in Fig. 2, the interface highlighted by the GPR on the up and down line correlated very well with the noncohesive or nonslurried ballast depth observed in ABS (this interface has been taken from ABS sections and is discussed later). GPR analysis also suggested that the slurried ballast layer extended over further track length than that investigated with trial pits, demonstrating the potential benefits of using GPR for highlighting the limits of any suspected trackbed anomalies. The slurried layer was confirmed following ABS (discussed later), and highlighted the previously discussed limitations of trial pit excavation. Other sections on the up line were also identified that could exhibit an upward migration of fines. In summary, on the up line the GPR identified three distinct sections of trackbed which required further investigation to assess existing and
M. Brough et al. / NDT&E International 36 (2003) 145–156
149
Fig. 2. GPR trace at Leominster trial site.
potential future formation problems (Fig. 2). As a rule, the extent and nature of trackbed anomalies highlighted in the GPR trace correlates well with trackbed layers observed in this additional site investigation (discussed later). On the GPR trace for the down line, three sections are again evident. However, even though similar soils are present on the down line, the current formation problems observed on the up line are not apparent. This is in agreement with conclusions from trial pit excavation, sampling and subsequent laboratory testing. As similar soils are present on the down line, with increased loading
and higher line speeds, there may be the potential for future problems as observed on the up line. The current study confirmed the suitability of GPR for initial identification and characterisation of existing and potential poor formation sites. Limitations with the current methods of trackbed investigation, such as trial pit excavation, were also highlighted. Although GPR can be a useful tool for assessment of trackbed condition, reducing the frequency of sampling or trial pit excavation required, a minimal supplementary disturbed sampling should be carried out. Suitable application to quality control of ballast
150
M. Brough et al. / NDT&E International 36 (2003) 145–156
depth during renewal works was also confirmed in the field tests. 3.4. Summary The trackbed from 38 ml 1108 yd4 to 38 ml 1197 yd, to an approximate depth of 1.8 m, comprised a four layer system typically consisting of clean ballast (0 –0.3 m), choked ballast (0.3 –0.6 m), a low bearing capacity, high in situ moisture content slightly sandy clay silt (0.6 –1 m) and an underlying soft clay (1 – 1.8 m) exhibiting increasing strength with depth. This underlying soft clay exhibited low bearing capacity on the up and down line, however, this was slightly lower on the up line due to an observed higher in situ moisture content. Although bearing capacity was low, analysis suggested sufficient existing or potential strength to resist the applied dynamic loading, with many of the problems as a result of the slurried ballast and high in situ moisture content. Trial pit excavation suggested a water table extending up into the ballast layer with the trackbed typically saturated below 0.5 m. The observed increasing strength with depth (to a maximum depth of 1.8 m) in the identified clay layer, and shallow water table, could be due to surface ponding in subgrade depressions. In combination with repeated loading, this could cause the observed slurried soft surface clay layer, with gradual in situ moisture content reduction with depth and associated increased strength. This layer arrangement has been presented in Fig. 3. GPR suggested the formation problem was more extensive on the up line than was targeted with trial pits, with further more detailed evaluation required.
4. Ground investigation: stage 2 Following the stage 1 evaluation, omissions in data were highlighted along with recommendations for future site work that had to be done in a night-time possession. This detailed site investigation focussed on the trial pitted section of track and further along the track to investigate other potential localised formation problems. The ABS, CPT and FWD were used to confirm the major causes of the problem and provide geotechnical parameters for design and assessment of any proposed remedial measures. The site investigation performed has been summarised in Fig. 1. 4.1. Automatic ballast sampler Sample tubes are driven dynamically into the trackbed, through ballast cribs, to obtain a disturbed continuous core sample of the ballast and underlying formation. Samples are sealed in a plastic tube, allowing examination off-site under 4 In the UK railway industry track locations are referenced in miles (ml) and yards (yd). Metric conversion 1 ml ¼ 1760 yd ¼ 1609 m.
controlled conditions by a trained geologist or engineer, and a subsequent accurate record of the formation and trackbed layers [2]. The technique gives an idea of the condition and residual life of ballast directly beneath track components, and overcomes many of the limitations of the trial pit method previously discussed. Not only is a full assessment of ballast, sub-ballast and formation condition feasible, the technique is easy to use and does not require ballast excavation. Sharpe [2] suggests depth of application is also much greater than is feasible with trial pits opened beneath the trackbed, with depths of up to 4 m achieved in 1 h. Concern has been raised about the ABS breaching the integrity of filtering or blanket layers, or even geomembranes installed at cess heave sites. Ayres [12] has suggested a fine to medium sand backfill should be used to maintain filter properties. Alternatively, a cationic bituminous emulsion could be used in the case of geomembrane removal. Following analysis of data from the stage 1 evaluation, targeted ABS samples were taken at a total of 16 locations, to a typical maximum depth of 2 m below the underside of rails. Following retrieval, samples were returned for logging and moisture content determination. A typical photograph and sample log for the up line can be found in Fig. 4. The trackbed typically consisted of a clean ballast layer, underlain with contaminated ballast and soft to firm clay. On the up line, the contaminated ballast layer consisted of high moisture content slurried ballast. On the down line, the clean ballast layer was shallower and the contaminated ballast layer consisted of stiffer clayey gravel. This was also observed in the GPR trace, with an obvious section on the up line demonstrating upward formation migration as suggested by a shallow variable depth interface. On the adjacent down line, there was a reasonably obvious more uniform depth interface, possible evidence of the stiffer contaminated ballast layer observed. Both GPR and subsequent targeted ABS highlighted the difference in moisture content and subsequent nature of fines migration observed in the up and down line trackbed. Observations in ABS agreed with trial pit logs previously discussed, demonstrating suitable application of the ABS instead of hand dug trial pits. The slurried ballast on the up line was also evidenced by moisture content values recorded in ABS samples ranging from 25 to 30%, in good agreement with moisture contents of disturbed samples taken from trial pits. A relatively shallow water table was also evident in the slurried ballast layer in the up line ABS samples that was not evident in the down line ABS samples. ABS samples further supported conclusions from GPR analysis that the poor formation area extended beyond the trial pitted section of track, covering a maximum track length of 230 m. Trial pits had previously identified only a 100 m section of track with poor formation. If trial pit logs had been relied upon alone, the poor formation section of trackbed would not have been completely identified, possibly resulting in insufficient remedial treatments. This
M. Brough et al. / NDT&E International 36 (2003) 145–156
151
Fig. 3. Typical trackbed profile in poor subgrade section on the up line (38 ml 1108 yd –38 ml 1197 yd).
research highlights the benefits of combined GPR analysis, supported with ABS testing, for the initial identification of poor formation sites. Penetrometer readings were taken throughout the clay layer in ABS samples. Initial evaluation of the technique suggested the narrow internal diameter of sampling tubes could cause sample disturbance and inaccurate shear strength profiles. However, test results on the whole agreed with DCP and CPT (discussed later) results. DCP tests suggested that formation (roughly 1 m depth) underlying the open trial pits was very soft, typically comprising three layers with increasing strength with depth. Shear strength measurements of the clay layer in ABS samples also suggested a soft to firm clay layer with a shear strength ranging from 50 to 140 kPa. Average shear strength of 56 and 63 kPa were recorded in the up and down line, respectively. This lower shear strength on the up line was as expected, due to the higher moisture content observed. ABS shear strength measurements suggested that the clay layer comprised alternating layers of stiff and soft clay or consisted of an upper stiff crust underlain with soft clay. CPT results tended to agree with this alternating stiff–soft layer formation.
In summary, testing highlighted the benefits of combined GPR analysis, supplemented with ABS, for identification and assessment of poor formation sites. Extent of poor formation could be identified with GPR and targeted for more detailed investigation with ABS. ABS gave relatively undisturbed continuous profiles, to a typical depth of 2 m, of ballast, sub-ballast, formation, moisture content, shear strength and extent of fines migration. If further depths need targeting, for sample extraction or soil profiling and characterisation, the CPT can be implemented. Sharpe [2] suggests ABS is a relatively simple technique, allowing up to ten 1 m samples to be taken in 1 h with a four-man team. 4.2. Static cone penetration test (SCPT) Horsnell and Adam [13] suggest the SCPT provides detailed information on soil type and stratigraphy, allowing on-site assessment of various engineering parameters and can be adapted to allow detection of hydrocarbon contamination in ballast and formation. A more detailed assessment of soil stratigraphy can be obtained using a piezocone penetration test in which pore pressures established in the soil during cone penetration are measured. The CPT is
Fig. 4. ABS log and picture.
152
M. Brough et al. / NDT&E International 36 (2003) 145–156
a useful tool for trackbed investigation and provides information on the quality, quantity and in situ characteristics of the ballast and the underlying subgrade material. This data can provide a complete understanding of the geotechnical properties which will govern drainage characteristics, ground stability and ballast/subgrade deformation behaviour. A test can be performed in the same time, or less, than it takes to excavate and investigate traditional trial pits. Chrismer and Li [4] using CPT test results, have suggested a failure mode similar in nature to the one suggested as occurring at the Leominster site. They suggest due to a distorted subgrade surface profile, water becomes trapped within the granular layer, further weakening the subgrade and driving its displacement. Simply increasing the granular layer thickness has proven ineffective. This situation appears to be similar in nature to problems experienced at the Leominster site, i.e. a very shallow water table in the trackbed possibly due to subgrade depressions with underlying slurried ballast and high moisture content soft clay at the layer interface. Continued ballast replacement, effectively increasing ballast depth, has not remedied the problem with continued onset of poor track geometry. Automatic ballast cleaning, may have exacerbated the problem, with remaining fouled ballast shoulders creating a channel for water ponding. For the failure mode discussed above, literature would suggest the CPT could be applied to assessment for subsequent design of a remedial scheme. However, due to a relatively high ballast shoulder and nearby adjacent
buildings, the CPT could not be used to obtain profiles across the track and confirm the existence of subgrade depressions. CPT was performed eight and seven times along the track using hydraulic penetrometer equipment on the up and down line, respectively. A 7.5 tonne capacity electric cone was used for each of the tests. Following data processing, friction ratios were calculated and soil type, shear strength and coefficients of volume compressibility (Mv) determined. CPT plots suggested a ballast layer from 0.9 to 1.3 m thick (Fig. 5). Although this was classified as ballast, with no evidence of contaminated ballast reported, a more detailed analysis of data could be performed to highlight these distinct layers. This was proposed for any future CPT analysis to provide a more detailed assessment of the trackbed and underlying shallow surface layers. Evidence of ponding in shallow subgrade depressions could not be confirmed from CPT results without a more detailed evaluation of friction ratio data at shallow depths. Immediately beneath the ballast was a clay layer (Fig. 5). On the down line, this layer typically comprised a firm to very soft clay and in some instances contained thin layers of sand. Of particular interest was the progression of strength through this clay layer. Typically, the layer became increasingly soft with depth, up to the underlying sand and gravel layer. On the up line, similar conditions were observed. The clay layer typically comprised a firm to very soft clay with no evidence of sand layers. This clay layer
Fig. 5. Typical CPT results from the up line (soil type profiles).
M. Brough et al. / NDT&E International 36 (2003) 145–156
had a fairly consistent depth on the up and down line, and was still evident at maximum depths of 2.9 –3.8 and 2.9 – 4.4 m, respectively. The stiffer clay at the top of this layer could be evidence of ballast penetration due to repeated dynamic loading or of consolidation, with the underlying clay demonstrating less consolidation and subsequently being much softer in nature. Calculated coefficients of volume compressibility support this conclusion, with clay from 2.5 to 3.5 m depth exhibiting high compressibility and potential for consolidation. The incidence of very soft clay seemed greater on the up line, presumably due to the increased loading, reduced ballast thickness and higher in situ moisture contents. On the up and down line beneath this clay layer, was a medium dense to dense sand and gravel layer. Although other defined thin layers were often sandwiched between this clay and sand and gravel layer, these intermediate layers seemed to be transitional, consisting of a combination of overlying clay and underlying sand and gravel layers. In summary, although these minor transitional layers existed (roughly 0.6 m deep), stratigraphy consisted of a firm becoming soft clay underlain with a medium dense to dense sand and gravel. Although the sand and gravel layer would not be targeted with any ground improvement technique, the layer could be used as a drainage channel, providing no artesian pressures exist. A more or less continuous sand and gravel layer can be found at a depth of 3.8– 5.3 and 3.9 – 5.4 m on the up and down line, respectively. Any drainage system used would have to be installed to these depths if the sand and gravel layer were to be used effectively. Vibrating wire piezometers have been installed to assess the pore water pressure in the formation, and the potential for using the gravel layer as a drainage channel. In summary, CPT result suggested that the stratigraphy seemed to consist of a firm becoming soft clay underlain with a medium dense to dense sand and gravel. This clay layer had a fairly consistent depth on the up and down line, and was still evident at maximum depths of 2.9 –3.8 and 2.9 –4.4 m, respectively. Although soils were the same on the up and down line, there appeared to be a localised moisture problem and shallower clean ballast depth on the up line. Beneath the clay layer, a more or less continuous sand and gravel layer could be found at a depth of 3.8 –5.3 and 3.9 –5.4 m on the up and down line, respectively. If the clay is analysed further, it would appear that shear strength gets lower and compressibility gets higher with depth. This could be evidence of ballast penetration at the top of the clay layer or possible consolidation causing the observed shallow stiffer clay conditions. There seems to be a layer from 2.5 to 3.5 m of particular concern exhibiting very low shear strength and high compressibility. CPT results could not be correlated with GPR results at excessive depth due to the shallow depth of application of the radar survey. As has been discussed previously, in
153
the upper 1.5 m of the trackbed, there is good correlation between the results of the GPR, trial pits, ABS and CPT. 4.3. Falling weight deflectometer In the highways industry, the FWD was developed to provide detailed information on the structural response of pavements to simulated traffic loading. An impulsive load is applied to the pavement surface and the velocity response is measured using geophones [2]. These velocity response time histories are integrated, and the peak displacements are used to give a ‘static’ deflection bowl that characterises the quasi-static deflection response of the pavement [14]. With minor modifications, the FWD has been applied to the measurement of railway track. A pre-determined impulse load is applied close to the rail seat area of a sleeper, and the transient deflections of the sleeper and ballast accurately measured using geophones [3]. The deflections of the loaded sleeper, the ballast in the adjacent crib and the ballast in the next crib, are interpreted as corresponding to the sleeper deflection, the deflection of the loaded ballast and the deflection of the formation, respectively [1]. Currently, the FWD is suggested as the only satisfactory way of obtaining sufficient data to assess trackbed stiffness magnitude and variation for an individual site [2]. The track loading vehicle (TLV) is not applicable, as scheduling for and implementation at a targeted section of track in the rail network would not be cost effective. The TLV is currently being developed to measure parameters such as dynamic track stiffness and track receptance whilst moving, and should potentially be more applicable for whole route assessments [5]. Hunt suggests that the most effective monitoring should be vehicle mounted and aim to give a continuous track stiffness profile, identifying sites for consideration for remedial treatment. These sites could be investigated further by targeted stiffness measurement and site investigation techniques. Research would suggest, including Hunt [5], Sharpe [2] and Sharpe and Collop [3], that the FWD is the most applicable method for determining quasi-static sleeper stiffness, an important parameter when assessing sites exhibiting track geometry deterioration. In this trial, FWD was performed on the up line, over a 130 m section. Six months later, further testing was carried out across the whole site on the up and down line (Fig. 1). Following testing, deflection data retrieved was analysed off-site to provide track stiffness profiles. Deflection and stiffness profiles can be found in Fig. 6. Deflection and stiffness profiles agree with observations from previous testing performed. Test results from the initial 130 m section on the up line suggest subgrade deflections (d1000) are not ideal for track support, with relatively large deflections observed (Fig. 6(a)). However, problems have not yet manifested in the section tested. This may be due to the relatively uniform consistency of stiffness across this section (from 38 ml 969 yd to the platform end). If FWD profiles are compared with previous test results,
154
M. Brough et al. / NDT&E International 36 (2003) 145–156
Fig. 6. Deflection and trackbed stiffness profiles for (a) up line (b) down line.
M. Brough et al. / NDT&E International 36 (2003) 145–156
then the consistent stiffness across the 130 m section initially tested correlates with the well-defined interface suggested by the GPR trace. Initial test results suggested a well-defined interface, constant adequate ballast depth and generally consistent track support stiffness. Subsequently, no apparent track geometry deterioration problems are evident on this section of track. This consistent stiffness was still apparent across the 130 m section following FWD testing six months later. Even though stone blowing had been performed in the interim, there were no significant changes in the deflection characteristics of the sleeper, ballast and formation (Fig. 6(a)). FWD testing across the whole site on the up and down line further defined the formation problem area of concern previously highlighted (Fig. 6(a) and (b)). There appeared to be a length of track on the up and down line that was exhibiting a less consistent deflection and trackbed stiffness profile than was observed across the whole site. From 38 ml 1276 yd to 38 ml 1014 yd, the sleeper, ballast and formation deflections appear more variable than further along the track. This agrees with previous testing that suggested the poor formation section stretched from the highways over bridge (38 ml 1276 yd) to 38 ml 1024 yd. If FWD data are analysed further the source of the track geometry problem can be further understood. Trackbed stiffness magnitude does not appear to be the main cause for concern, with the range of measured stiffness similar along the site. However, increased trackbed stiffness variation does appear to be correlated with the poor formation section (38 ml 1276 yd – 38 ml 1014 yd). Previous results suggested ponding of water in subgrade depressions on the up line, further evidenced by nonuniform subgrade deflections across this section of track. As can be seen in Fig. 6, deflections of the formation are less consistent and relatively high in this section on the up and down line. This can be seen when compared with other FWD data from other sites as presented by Sharpe [1]. Even though track geometry problems manifest on the up line, no significant differences can be observed in trends of formation deflection data on the up and down line. If deflection data are examined further, it would appear that ballast deflections are slightly higher on the up line, possibly suggesting the problem is sourced in this layer. Alternatively, this could suggest a problem at the ballast/subgrade interface, such as fines migration and ponding. 4.4. Summary The poor subgrade extended beyond the trial pitted section of track, stretching from the cross drain (38 ml 1276 yd) to roughly 38 ml 1024 yd on the up line, a maximum length of 230 m. The trackbed typically consisted of a clean ballast layer, underlain with contaminated ballast and soft to firm clay. The incidence of high moisture content slurried ballast and potential
155
ponding in subgrade depressions seemed greater on the up line with a relatively shallow water table evident. This was also highlighted by penetrometer readings, with shear strength measurements in up line ABS samples suggesting a soft becoming stiff clay layer with values ranging from 50 to 140 kPa. The softening of clay at the top of this layer could be evidence of the ponding discussed previously. The incidence of very soft clay seemed greater on the up line, presumably due to the increased loading, reduced ballast thickness and higher in situ moisture contents. On the down line, the clean ballast layer was shallower and the contaminated ballast layer consisted of stiffer clayey gravel. Ponding in shallow subgrade depressions could not be confirmed from CPT results without a more detailed evaluation of friction ratio data at shallow depths. CPT result suggested that the stratigraphy beneath the ballast seemed to consist of a firm becoming soft clay underlain with a medium dense to dense sand and gravel. If the clay was analysed further, it appeared that shear strength became lower and compressibility became higher with depth. This could be evidence of ballast penetration or consolidation at the top of the clay layer. A layer from 2.5 to 3.5 m of particular concern exhibiting very low shear strength and high compressibility could be evidence of the clay prior to any consolidation. With increased loading or line speeds, this layer could cause more severe long-term settlement and associated track geometry deterioration. In agreement with ABS and CPT results, FWD subgrade deflections are indicative of an underlying soft clay. However, trackbed stiffness magnitude does not appear to be the main cause for concern, with the range of measured stiffness similar along the up line. However, increased trackbed stiffness variation does appear to be correlated with the poor subgrade (38 ml 1276 yd– 38 ml 1014 yd). Deflections of the formation are less consistent and relatively high in this section on the up and down line. If data are examined further, it would appear that ballast deflections are typically higher on the up line, possibly suggesting the problem is sourced in this layer. This would suggest that the problem is shallow seated and could be remedied by treatment of the contaminated ballast and upper formation layers. In summary, there appeared to be several suggested causes for the observed track geometry deterioration on the up line. Ponding of water in shallow subgrade depressions causing localised softening, fines migration into the ballast, heterogeneous stiffness and consequently nonuniform track settlement. This wavy top in turn has exacerbated the rate of track geometry deterioration. Although there is underlying soft clay exhibiting a high potential for consolidation, this layer should not need targeting in any remedial scheme, providing load transfer issues can be addressed in shallow subgrade layers.
156
M. Brough et al. / NDT&E International 36 (2003) 145–156
5. Remedial ground improvement
References
Results would suggest that track geometry deterioration on the up line at Leominster is due to a combination of insufficient clean ballast thickness, high in situ moisture content and increased loading from freight traffic. Continued maintenance in the form of repeated addition of ballast and tamping has resulted in excessive ballast thickness, much of which extends into the underlying clay layer, forming ballast pockets along the trackbed. Ponding of water in resultant formation depressions has caused a continual softening of the upper clay formation layers, extensive fines migration into the clean ballast layer and a nonconsistent trackbed stiffness. This differential stiffness and localised ponding has exacerbated the problem, minimising potential benefits of repeated ballast addition. As stiffness or shear strength magnitude appears to be sufficient across the site, this should not be the main focus of any remedial programme. To overcome the problems at Leominster, either consistency of the formation condition should be improved and subsequently any formation depressions disrupted, or the ponding water and slurried ballast layer should be removed. At present, shallow soil mixing treatments or novel drainage technologies are being considered. A full assessment of all available ground improvement technologies for remediation of trackbed formation is reported in Ref. [15].
[1] Cope D. Track technology for the future. Second course on vehicle/ track interaction, programme and notes, Cambridge: Emmanuel College; 2000. [2] Sharpe PS. Trackbed investigation. PWI J Rep Proc 2000;118(3): 238 –55. [3] Sharpe P, Collop AC. Trackbed investigation—a modern approach. Proceedings of the First International Conference on Maintenance and Renewal of Permanent Way and Structures, London, UK: Engineering Technics Press, Brunel University; 1998. [4] Chrismer SM, Li D. Cone penetrometer testing for track substructure design and assessment. Sixth International Heavy Haul Railway Conference—Strategies Beyond 2000, Cape Town, South Africa; 1997. [5] Hunt GA. EUROBALT optimises ballasted track. Railway Gazette International; December 2000. [6] Shahu JT, Kameswara Rao NSV, Yudhbir. Parametric study of resilient response of tracks with a sub-ballast layer. Can Geotech J 1999;1137– 50. [7] Li D, Selig BT. Evaluation of railway subgrade problems. Transpn Res Rec 1995;1489:17 –25. [8] Brough MJ, Ghataora GS, Stirling AB, Madelin KB, Rogeus CDF, Chapman DN. Investigation of railway track subgrade. I. In situ assessment. Institute of Curl Engineering, Transport Journal. (2003). [9] Livneh MI, Ishai I, Livneh AL. Effect of vertical confinement on dynamic cone penetrometer strength values in pavement and subgrade evaluations. Transpn Res Rec 1995;1473:1–8. [10] Hugenschmidt J. Ballast inspection using ground penetrating radar. Proceedings of the Second International Conference on Railway Engineering, London, UK; 1999. [11] Jack R, Jackson P. Imaging attributes of railway track formation using ground probing radar (GPR). Railway Engineering—98, First International Conference on Maintenance and Renewal of Permanent Way and Structures, London, UK: Engineering Technics Press, Brunel University; 1998. [12] Ayres DJ. Trackbed investigation. J Rep Proc Permanent Way Institution 2000;118(4):356–60. Comments on paper of same title by Phil Sharpe. [13] Horsnell MR, Adam CH. Geotechnical investigations for railway maintenance and renewal. Proceedings of the Third International Conference on Railway Engineering, London, UK; 2000. [14] De Almedia JR, Brown SF, Thom NH. A pavement evaluation procedure incorporating material non-linearity. ASTM STP 1198 1994;. [15] Brough MJ, Stirling A, Ghataora G, Madelin K. Improving railway subgrade stiffness—assessment of traditional in situ ground improvement techniques. Proceedings of the Third International Conference on Railway Engineering, London, UK; 2000.
Acknowledgements The authors gratefully acknowledge the combined financial and technical support of the EPSRC and the industrial partners Railtrack, GTRM, Scott Wilson Pavement Engineering, WS Atkins Rail, Keller Ground Engineering, Fugro and Serco Railtest. The authors also gratefully acknowledge the specific technical advice and discussion provided by Scott Wilson Pavement Engineering regarding the GPR, ABS and FWD. Similar thanks go to Fugro for advice and discussion regarding the CPT and rig applications, and to WS Atkins Rail for supervising and logging the trial pits.
Evaluation of railway trackbed and formation: a case study M. Brougha,*, A. Stirlingb, G. Ghataorab, K. Madelinb a
Scott Wilson Pavement Engineering, 9/10 Faraday Building, Nottingham Science and Technology Park, University of Boulevard, Nottingham NG7 2QP, UK b School of Civil Engineering, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK
Abstract To determine the causes of poor track geometry, a trackbed investigation should comprise a full assessment of ballast, sub-ballast and formation condition. If information about any of these layers is omitted, the true cause of poor formation sites cannot be ascertained satisfactorily and consequently appropriate ground improvement schemes may not be implemented. This has been observed in the UK rail network with examples of a lack of detailed site investigation and subsequent expensive remedial work being carried out without eliminating the initial cause of the problem. This paper summarises the trackbed investigation performed at a site requiring high maintenance, suspected as being caused by poor formation. The paper concludes with an analysis of ground conditions and possible choice of remedial schemes. q 2003 Elsevier Science Ltd. All rights reserved. Keywords: Ballast; Formation; Ground penetrating radar; Falling weight deflectometer; Railway trackbed
1. Introduction Trackbed investigation is considered to be crucial for assessing potential maintenance requirement and subsequent design of any remedial measures. In reality, track components are often maintained or replaced with little regard for properties of the formation and their subsequent effects on continued total asset life. Examples of expensive remedial work being carried out without eliminating the initial cause of the problem have been reported by Cope [1]. Ideally, site investigation data should be used not only for design of remedial ground improvement schemes, but to predict perceived track performance and more importantly schedule necessary work proposals. Sharpe [2] has proposed the ‘total route evaluation’ approach, a comprehensive methodology yielding sufficient information to enable optimisation of renewals and maintenance strategy for a ten-year period. This methodology was first suggested by Sharpe and Collop [3], comprising staged levels of investigation, identifying formation anomalies and enabling requirements for formation treatment to be identified many years in advance. The Association of American Railroads is also developing an expert system approach to diagnose and recommend the most cost-effective remedy for various track * Corresponding author. Tel.: þ44-115-922-9098; fax: þ 44-115-9431302. E-mail address: [email protected] (M. Brough).
substructure problems based on investigation methods and economic evaluations [4]. Hunt [5] has confirmed track quality,1 continued track component performance and subsequent maintenance are highly dependent upon the magnitude and variation of formation stiffness. This is exacerbated by consolidation and settlement of formation and the subsequent differential track geometry. The achievable track geometry can in turn influence the total asset life of elements of the track infrastructure, such as rails or sleepers, as suggested by Cope [1]. This situation has been compounded by an increase in rail traffic, pressure for higher line speeds and heavier axle loads. Consequently, there is an increasing importance in obtaining a consistent relative measure of trackbed stiffness in the physical and time restrictions applicable on live track. In addition to stiffness assessment, any trackbed investigation should include an assessment of ballast, subballast and formation. Cope [1] suggests that if this layer information is omitted, the cause of poor formation sites cannot be ascertained and consequently applicable remedial schemes cannot be implemented. Shahu et al. [6] suggested sub-ballast depth, after subgrade modulus, was the most influential track parameter on overall track response. Sharpe [2] recognised that the condition of the ballast would have 1 A track condition index based upon standard deviations of track geometry.
0963-8695/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 3 - 8 6 9 5 ( 0 2 ) 0 0 0 5 3 - 1
146
M. Brough et al. / NDT&E International 36 (2003) 145–156
a strong influence on track quality. Groundwater profiles and behaviour must also be assessed due to the deleterious effects on formation condition and track quality. Li and Selig [7] suggest that if the water table is within approximately 6 m of the surface, this is the major factor affecting the moisture content of the formation and subsequent problems. The nature of any site investigation will depend upon the specific nature of the problem and potential methods of remediation. In addition, physical and time restrictions in the UK rail network will limit investigation techniques that can be employed. An assessment of suitable testing techniques has been described previously by Brough [8]. This paper details a site investigation comprising trial pit excavation, dynamic cone penetrometer (DCP), ground penetrating radar (GPR), automatic ballast sampling (ABS), cone penetration testing (CPT) and falling weight deflectometer (FWD) testing. These were used in two distinct levels of trackbed investigation. Results of field trials performed at a suspected poor formation site and conclusions made are presented. The work forms part of a government funded EPSRC LINK research project (‘Improving the Stiffness of Existing Rail Track’—ISERT) in association with Railtrack, GTRM, Scott Wilson Pavement Engineering, WS Atkins Rail, Keller Ground Engineering, Fugro and Serco Railtest.
2. Site investigation 2.1. Trial site The site is located near Leominster station in Herefordshire in the UK. The initial site choice was based upon the need for repeated track maintenance, and occurrence of contaminated ballast in trial pits excavated as part of previous track maintenance contracts. 2.2. Test schedule The investigation, performed during a T22 and T33 possession, had several objectives. The preliminary assessment, carried out under a T2 possession, was to identify soft formation beneath track components and to highlight possible causes of differential track settlement. Consequently, omissions in data were highlighted along with recommendations for future site work that had to be done in a night-time possession. The subsequent more-detailed investigation confirmed the nature and cause of soft formation and provided more detailed geotechnical information that could be used for design and assessment of 2
Typically day-time work with intermittent access to track that is primarily governed by railway traffic and subsequent safety considerations. 3 Typically night-time work when there is no scheduled rail traffic with continuous access to track.
remediation techniques. The extent and methods of site investigation used at Leominster are summarised in Fig. 1. 2.3. Desk study Specific poor formation hot spots were identified during a visual survey, evidenced by associated wavy top and clean polished exposed sleeper surfaces. This was supported by track geometry measurements indicating excessive vertical track elevation standard deviations and associated poor track quality. This section of track had previously been identified by maintenance history as showing high potential for localised formation problems. Historical records of previously opened trial pits showed ballast on the up line was completely choked with grey clay and the water table was at or near the formation surface level. On the down line, trial pit records showed choked ballast was present but not across the entire trackbed section. This suggested possible ballast pockets, with choked ballast typically concentrated beneath rails. Water levels were also generally much deeper on the down line. Track maintenance history generally suggested the potential for and occurrence of formation problems was higher on the up line, possibly due to an increased traffic loading, observed higher water table and associated in situ moisture. Subsequently, it was decided to locate a significant proportion of the proposed trial pits on the up line, with some trial pit locations matched on the down line. This desk study highlighted the importance of using historical data to identify poor formation sites along a route. In addition to trial pit logs, information regarding trackbed condition can be ascertained from maintenance history and track surveys. For example, performance data from automatic ballast cleaning operations could provide information on the fines content of ballast, and roughly indicate formation condition. Unfortunately, potential access to relevant data is not always feasible due to regional ownership of track maintenance information by contract operators in a nonstandard format. Consequently, a consistent data collection procedure is required and subsequent data made accessible to supplement trackbed investigations. In addition to track maintenance history, track component type and location should be logged due to their obvious effects on track behaviour. Subsequent correlation of track component logs with identified poor formation sites may highlight possible relationships.
3. Ground investigation: stage 1 Trial pits, DCP and GPR were implemented to assess the overall conditions of the entire length of track, and identify sites with existing and potential problems. The site investigation performed has been summarised in Fig. 1.
M. Brough et al. / NDT&E International 36 (2003) 145–156
147
Fig. 1. Schematic showing site investigation performed at Leominster.
3.1. Trial pits Trial pits would not normally be excavated in the formation layers beneath the trackbed. However, due to their perceived relevance to track geometry deterioration they were included and logged as in a traditional site investigation. Open trial pits suggested that the poor formation sections were more prevalent on the up line, due to contaminated ballast encountered at shallower depths. In all trial pits on the up and down line, ballast below depths of 0.3 – 0.6 m was completely choked and contaminated with a soft grey clay/silt. In some cases this layer was saturated or groundwater seepage was observed below the ballast. This was particularly evident on the up line, with all trial pits opened showing a typically saturated soft mid layer, consisting of ballast and fines. Although it would appear that excavated material from the up and down line was of a similar composition, the contaminated fines layers on the down line did not exhibit the same high in situ moisture content. Excavation of bottom layers (. 0.7 m) showed the formation ranged from a firm to stiff intermixed mid grey and red brown slightly sandy clay/silt with occasional gravel, to a firm to stiff brown slightly sandy clay/silt. As with other layers, these bottom layers exhibited higher moisture content on the up line. Trial pit logs confirmed initial observations regarding the identification of poor formation at Leominster station, particularly on the up line. However, although trial pit logs identified poor formation at discrete points along the track, the extent, depth and nature of the poor formation section could not be assessed. This was due to current limitations in the trial pit method currently being employed on the UK railway network. First, depth of trial pit excavation was limited, not including formation layers where problems are often sourced. Second, trial pit excavation was often infrequent resulting in an incomplete
assessment of poor formation sites. Third, as a rule geotechnical engineers did not log trial pits, with a good possibility that important information may not be recorded or recorded incorrectly. Sharpe [2] has also highlighted these limitations in his assessment of current state of the art methods of trackbed investigation. As shown in this study, there is the potential for a more consistent method, with logging and sample excavation performed using traditional geotechnical principles and improved quality control. Samples can be analysed off-site for classification testing and results used for design of any relevant remedial schemes. Although depth of trial pit excavation is limited, geotechnical test devices such as the shear vane and DCP could be used in trial pits and improve the current investigation practices in the UK railways industry. Needless to say, trial pit excavation, with improved quality control, still represents a useful accepted tool for site investigation of trackbed. Trial pit logs were used to calibrate the performance of other test devices used in this investigation. 3.2. Dynamic cone penetrometer Due to the improved access to the formation in excavated trial pits, there are substantial benefits of using simple hand held devices. Due to the relatively easy operating principles, semi-nondestructive nature, low cost and simple results interpretation of the DCP, tests were performed at the maximum depth of open trial pits. The DCP also offered the potential for formation assessment without the removal of ballast. The DCP relies upon a penetration cone attached to a rod, being driven into the soil by means of a manual drop hammer, and the resulting penetration and number of blows recorded. Following testing, results can be used to obtain relative measures of bearing capacity in terms of depth of
148
M. Brough et al. / NDT&E International 36 (2003) 145–156
cone penetration per blow. Livneh et al. [9] suggests that a universal correlation exists between the DCP and California bearing ratio (CBR) for a wide range of pavement and subgrade materials, testing conditions, and technologies. In the current study, the DCP was used following trial pit excavation. Ideally, the DCP would be better implemented without prior excavation of the ballast. Initial trials would suggest that penetration of DCP rods through compacted ballast is feasible. The effect of ballast on DCP measurements is under investigation at present and will be reported in the future. DCP tests on the up line suggested that formation (approximately 1 m deep) underlying the open trial pits was very soft, typically comprising three layers with increasing strength with depth. Correlated CBR values confirmed the nature of this soft formation layer with values ranging from 0.8 to 2.4% and an average value of 1.5%. Similar layers were observed in trial pits opened on the down line. However, results suggested the formation was slightly stronger than on the up line. Although DCP tests identified the soft formation layer of concern, the technique still gave no indication of stiffness variation with depth. CBR is used extensively in the highways industry and has been considered a reliable relative measure of combined stiffness and shear strength; however, its application in a dynamic load environment is increasingly subject to scepticism when compared with other more relevant parameters such as elastic stiffness. Although the reliability of the DCP as a tool for providing geotechnical parameters relevant to the railway engineer is questionable, it could provide a useful relative indicator of formation condition. It is quick, easy to use and maintain and could be applied in a T2 possession to the measurement of relative formation condition along a track length. This should provide an insight into the heterogeneity of the formation in the direction of traffic, now known to be a major influence on track quality deterioration. Application rates of six tests per hour in trial pits excavated in the trackbed were observed on site. 3.3. Ground penetrating radar The inspection of ballast using the ground probing radar has already been described in detail by Sharpe and Collop [3], Hugenschmidt [10] and Jack and Jackson [11]. GPR was used to give an indication of ballast depth, as well as condition of ballast and the formation. GPR is a nondestructive technique that uses electromagnetic radiation to identify the presence of layer interfaces between the different materials comprising the trackbed construction. Short pulses of electromagnetic radiation are directed into the ground where they are reflected back at interfaces between different layers. The equipment records the strength of reflected radiation as well as travel time for the waves. Depths of layers can be estimated from the travel time assuming the velocity of wave propagation.
There is normally a strong reflection from the base of the ballast layer in the trackbed. If the ballast has deteriorated or become contaminated, the radar waves cannot penetrate as easily, so the strength of reflections from the base of the ballast will be low. However, if there is a distinct layer of contamination within the ballast (e.g. clay slurry) there may be a strong reflection from its upper surface. Typically a radar reading is taken between each pair of sleepers, and all the results are combined to give an effectively continuous profile. The speed of the survey is typically about 1 km/h. The GPR trace is plotted using a grey scale to indicate the strength of the reflected signal. A constant gain has been applied throughout, which gives an indication of the relative strength of the reflected signal along the track, while filtering out much of the noise which is characteristic of most GPR data. The particular gain function used has been specially developed to bring out the main features of the ballast layer. Lower layers may also be apparent, but the deeper information is less reliable with respect to both depth and amplitude of reflection. For the radar used in this study a 900 MHz antenna was used, with a typical frequency range between 75 MHz and 1 GHz. A 900 MHz antenna was used to give a shorter pulse. This method of assessment can be used for qualitative first indication of ballast condition and potential fines migration at poor formation sites, and also facilitates bettertargeted intrusive site investigations. Sharpe [3] also suggests the GPR can be used as an excellent tool for the quality control of trackbed renewal items, providing continuous ballast depth data. At the Leominster site, GPR traces were obtained for the up and down line, to identify poor formation hot spots and areas with potential for track geometry deterioration (Fig. 2). In agreement with results of trial pits previously opened (marked as £ in Fig. 2), there was a section on the up line that exhibited a significant upward migration of fines, and insufficient variable nonslurried ballast depth. This was confirmed in automatic ballast samples, with nonslurried ballast depth (below ballast surface) ranging from 220 to 680 mm. As can be seen in Fig. 2, the interface highlighted by the GPR on the up and down line correlated very well with the noncohesive or nonslurried ballast depth observed in ABS (this interface has been taken from ABS sections and is discussed later). GPR analysis also suggested that the slurried ballast layer extended over further track length than that investigated with trial pits, demonstrating the potential benefits of using GPR for highlighting the limits of any suspected trackbed anomalies. The slurried layer was confirmed following ABS (discussed later), and highlighted the previously discussed limitations of trial pit excavation. Other sections on the up line were also identified that could exhibit an upward migration of fines. In summary, on the up line the GPR identified three distinct sections of trackbed which required further investigation to assess existing and
M. Brough et al. / NDT&E International 36 (2003) 145–156
149
Fig. 2. GPR trace at Leominster trial site.
potential future formation problems (Fig. 2). As a rule, the extent and nature of trackbed anomalies highlighted in the GPR trace correlates well with trackbed layers observed in this additional site investigation (discussed later). On the GPR trace for the down line, three sections are again evident. However, even though similar soils are present on the down line, the current formation problems observed on the up line are not apparent. This is in agreement with conclusions from trial pit excavation, sampling and subsequent laboratory testing. As similar soils are present on the down line, with increased loading
and higher line speeds, there may be the potential for future problems as observed on the up line. The current study confirmed the suitability of GPR for initial identification and characterisation of existing and potential poor formation sites. Limitations with the current methods of trackbed investigation, such as trial pit excavation, were also highlighted. Although GPR can be a useful tool for assessment of trackbed condition, reducing the frequency of sampling or trial pit excavation required, a minimal supplementary disturbed sampling should be carried out. Suitable application to quality control of ballast
150
M. Brough et al. / NDT&E International 36 (2003) 145–156
depth during renewal works was also confirmed in the field tests. 3.4. Summary The trackbed from 38 ml 1108 yd4 to 38 ml 1197 yd, to an approximate depth of 1.8 m, comprised a four layer system typically consisting of clean ballast (0 –0.3 m), choked ballast (0.3 –0.6 m), a low bearing capacity, high in situ moisture content slightly sandy clay silt (0.6 –1 m) and an underlying soft clay (1 – 1.8 m) exhibiting increasing strength with depth. This underlying soft clay exhibited low bearing capacity on the up and down line, however, this was slightly lower on the up line due to an observed higher in situ moisture content. Although bearing capacity was low, analysis suggested sufficient existing or potential strength to resist the applied dynamic loading, with many of the problems as a result of the slurried ballast and high in situ moisture content. Trial pit excavation suggested a water table extending up into the ballast layer with the trackbed typically saturated below 0.5 m. The observed increasing strength with depth (to a maximum depth of 1.8 m) in the identified clay layer, and shallow water table, could be due to surface ponding in subgrade depressions. In combination with repeated loading, this could cause the observed slurried soft surface clay layer, with gradual in situ moisture content reduction with depth and associated increased strength. This layer arrangement has been presented in Fig. 3. GPR suggested the formation problem was more extensive on the up line than was targeted with trial pits, with further more detailed evaluation required.
4. Ground investigation: stage 2 Following the stage 1 evaluation, omissions in data were highlighted along with recommendations for future site work that had to be done in a night-time possession. This detailed site investigation focussed on the trial pitted section of track and further along the track to investigate other potential localised formation problems. The ABS, CPT and FWD were used to confirm the major causes of the problem and provide geotechnical parameters for design and assessment of any proposed remedial measures. The site investigation performed has been summarised in Fig. 1. 4.1. Automatic ballast sampler Sample tubes are driven dynamically into the trackbed, through ballast cribs, to obtain a disturbed continuous core sample of the ballast and underlying formation. Samples are sealed in a plastic tube, allowing examination off-site under 4 In the UK railway industry track locations are referenced in miles (ml) and yards (yd). Metric conversion 1 ml ¼ 1760 yd ¼ 1609 m.
controlled conditions by a trained geologist or engineer, and a subsequent accurate record of the formation and trackbed layers [2]. The technique gives an idea of the condition and residual life of ballast directly beneath track components, and overcomes many of the limitations of the trial pit method previously discussed. Not only is a full assessment of ballast, sub-ballast and formation condition feasible, the technique is easy to use and does not require ballast excavation. Sharpe [2] suggests depth of application is also much greater than is feasible with trial pits opened beneath the trackbed, with depths of up to 4 m achieved in 1 h. Concern has been raised about the ABS breaching the integrity of filtering or blanket layers, or even geomembranes installed at cess heave sites. Ayres [12] has suggested a fine to medium sand backfill should be used to maintain filter properties. Alternatively, a cationic bituminous emulsion could be used in the case of geomembrane removal. Following analysis of data from the stage 1 evaluation, targeted ABS samples were taken at a total of 16 locations, to a typical maximum depth of 2 m below the underside of rails. Following retrieval, samples were returned for logging and moisture content determination. A typical photograph and sample log for the up line can be found in Fig. 4. The trackbed typically consisted of a clean ballast layer, underlain with contaminated ballast and soft to firm clay. On the up line, the contaminated ballast layer consisted of high moisture content slurried ballast. On the down line, the clean ballast layer was shallower and the contaminated ballast layer consisted of stiffer clayey gravel. This was also observed in the GPR trace, with an obvious section on the up line demonstrating upward formation migration as suggested by a shallow variable depth interface. On the adjacent down line, there was a reasonably obvious more uniform depth interface, possible evidence of the stiffer contaminated ballast layer observed. Both GPR and subsequent targeted ABS highlighted the difference in moisture content and subsequent nature of fines migration observed in the up and down line trackbed. Observations in ABS agreed with trial pit logs previously discussed, demonstrating suitable application of the ABS instead of hand dug trial pits. The slurried ballast on the up line was also evidenced by moisture content values recorded in ABS samples ranging from 25 to 30%, in good agreement with moisture contents of disturbed samples taken from trial pits. A relatively shallow water table was also evident in the slurried ballast layer in the up line ABS samples that was not evident in the down line ABS samples. ABS samples further supported conclusions from GPR analysis that the poor formation area extended beyond the trial pitted section of track, covering a maximum track length of 230 m. Trial pits had previously identified only a 100 m section of track with poor formation. If trial pit logs had been relied upon alone, the poor formation section of trackbed would not have been completely identified, possibly resulting in insufficient remedial treatments. This
M. Brough et al. / NDT&E International 36 (2003) 145–156
151
Fig. 3. Typical trackbed profile in poor subgrade section on the up line (38 ml 1108 yd –38 ml 1197 yd).
research highlights the benefits of combined GPR analysis, supported with ABS testing, for the initial identification of poor formation sites. Penetrometer readings were taken throughout the clay layer in ABS samples. Initial evaluation of the technique suggested the narrow internal diameter of sampling tubes could cause sample disturbance and inaccurate shear strength profiles. However, test results on the whole agreed with DCP and CPT (discussed later) results. DCP tests suggested that formation (roughly 1 m depth) underlying the open trial pits was very soft, typically comprising three layers with increasing strength with depth. Shear strength measurements of the clay layer in ABS samples also suggested a soft to firm clay layer with a shear strength ranging from 50 to 140 kPa. Average shear strength of 56 and 63 kPa were recorded in the up and down line, respectively. This lower shear strength on the up line was as expected, due to the higher moisture content observed. ABS shear strength measurements suggested that the clay layer comprised alternating layers of stiff and soft clay or consisted of an upper stiff crust underlain with soft clay. CPT results tended to agree with this alternating stiff–soft layer formation.
In summary, testing highlighted the benefits of combined GPR analysis, supplemented with ABS, for identification and assessment of poor formation sites. Extent of poor formation could be identified with GPR and targeted for more detailed investigation with ABS. ABS gave relatively undisturbed continuous profiles, to a typical depth of 2 m, of ballast, sub-ballast, formation, moisture content, shear strength and extent of fines migration. If further depths need targeting, for sample extraction or soil profiling and characterisation, the CPT can be implemented. Sharpe [2] suggests ABS is a relatively simple technique, allowing up to ten 1 m samples to be taken in 1 h with a four-man team. 4.2. Static cone penetration test (SCPT) Horsnell and Adam [13] suggest the SCPT provides detailed information on soil type and stratigraphy, allowing on-site assessment of various engineering parameters and can be adapted to allow detection of hydrocarbon contamination in ballast and formation. A more detailed assessment of soil stratigraphy can be obtained using a piezocone penetration test in which pore pressures established in the soil during cone penetration are measured. The CPT is
Fig. 4. ABS log and picture.
152
M. Brough et al. / NDT&E International 36 (2003) 145–156
a useful tool for trackbed investigation and provides information on the quality, quantity and in situ characteristics of the ballast and the underlying subgrade material. This data can provide a complete understanding of the geotechnical properties which will govern drainage characteristics, ground stability and ballast/subgrade deformation behaviour. A test can be performed in the same time, or less, than it takes to excavate and investigate traditional trial pits. Chrismer and Li [4] using CPT test results, have suggested a failure mode similar in nature to the one suggested as occurring at the Leominster site. They suggest due to a distorted subgrade surface profile, water becomes trapped within the granular layer, further weakening the subgrade and driving its displacement. Simply increasing the granular layer thickness has proven ineffective. This situation appears to be similar in nature to problems experienced at the Leominster site, i.e. a very shallow water table in the trackbed possibly due to subgrade depressions with underlying slurried ballast and high moisture content soft clay at the layer interface. Continued ballast replacement, effectively increasing ballast depth, has not remedied the problem with continued onset of poor track geometry. Automatic ballast cleaning, may have exacerbated the problem, with remaining fouled ballast shoulders creating a channel for water ponding. For the failure mode discussed above, literature would suggest the CPT could be applied to assessment for subsequent design of a remedial scheme. However, due to a relatively high ballast shoulder and nearby adjacent
buildings, the CPT could not be used to obtain profiles across the track and confirm the existence of subgrade depressions. CPT was performed eight and seven times along the track using hydraulic penetrometer equipment on the up and down line, respectively. A 7.5 tonne capacity electric cone was used for each of the tests. Following data processing, friction ratios were calculated and soil type, shear strength and coefficients of volume compressibility (Mv) determined. CPT plots suggested a ballast layer from 0.9 to 1.3 m thick (Fig. 5). Although this was classified as ballast, with no evidence of contaminated ballast reported, a more detailed analysis of data could be performed to highlight these distinct layers. This was proposed for any future CPT analysis to provide a more detailed assessment of the trackbed and underlying shallow surface layers. Evidence of ponding in shallow subgrade depressions could not be confirmed from CPT results without a more detailed evaluation of friction ratio data at shallow depths. Immediately beneath the ballast was a clay layer (Fig. 5). On the down line, this layer typically comprised a firm to very soft clay and in some instances contained thin layers of sand. Of particular interest was the progression of strength through this clay layer. Typically, the layer became increasingly soft with depth, up to the underlying sand and gravel layer. On the up line, similar conditions were observed. The clay layer typically comprised a firm to very soft clay with no evidence of sand layers. This clay layer
Fig. 5. Typical CPT results from the up line (soil type profiles).
M. Brough et al. / NDT&E International 36 (2003) 145–156
had a fairly consistent depth on the up and down line, and was still evident at maximum depths of 2.9 –3.8 and 2.9 – 4.4 m, respectively. The stiffer clay at the top of this layer could be evidence of ballast penetration due to repeated dynamic loading or of consolidation, with the underlying clay demonstrating less consolidation and subsequently being much softer in nature. Calculated coefficients of volume compressibility support this conclusion, with clay from 2.5 to 3.5 m depth exhibiting high compressibility and potential for consolidation. The incidence of very soft clay seemed greater on the up line, presumably due to the increased loading, reduced ballast thickness and higher in situ moisture contents. On the up and down line beneath this clay layer, was a medium dense to dense sand and gravel layer. Although other defined thin layers were often sandwiched between this clay and sand and gravel layer, these intermediate layers seemed to be transitional, consisting of a combination of overlying clay and underlying sand and gravel layers. In summary, although these minor transitional layers existed (roughly 0.6 m deep), stratigraphy consisted of a firm becoming soft clay underlain with a medium dense to dense sand and gravel. Although the sand and gravel layer would not be targeted with any ground improvement technique, the layer could be used as a drainage channel, providing no artesian pressures exist. A more or less continuous sand and gravel layer can be found at a depth of 3.8– 5.3 and 3.9 – 5.4 m on the up and down line, respectively. Any drainage system used would have to be installed to these depths if the sand and gravel layer were to be used effectively. Vibrating wire piezometers have been installed to assess the pore water pressure in the formation, and the potential for using the gravel layer as a drainage channel. In summary, CPT result suggested that the stratigraphy seemed to consist of a firm becoming soft clay underlain with a medium dense to dense sand and gravel. This clay layer had a fairly consistent depth on the up and down line, and was still evident at maximum depths of 2.9 –3.8 and 2.9 –4.4 m, respectively. Although soils were the same on the up and down line, there appeared to be a localised moisture problem and shallower clean ballast depth on the up line. Beneath the clay layer, a more or less continuous sand and gravel layer could be found at a depth of 3.8 –5.3 and 3.9 –5.4 m on the up and down line, respectively. If the clay is analysed further, it would appear that shear strength gets lower and compressibility gets higher with depth. This could be evidence of ballast penetration at the top of the clay layer or possible consolidation causing the observed shallow stiffer clay conditions. There seems to be a layer from 2.5 to 3.5 m of particular concern exhibiting very low shear strength and high compressibility. CPT results could not be correlated with GPR results at excessive depth due to the shallow depth of application of the radar survey. As has been discussed previously, in
153
the upper 1.5 m of the trackbed, there is good correlation between the results of the GPR, trial pits, ABS and CPT. 4.3. Falling weight deflectometer In the highways industry, the FWD was developed to provide detailed information on the structural response of pavements to simulated traffic loading. An impulsive load is applied to the pavement surface and the velocity response is measured using geophones [2]. These velocity response time histories are integrated, and the peak displacements are used to give a ‘static’ deflection bowl that characterises the quasi-static deflection response of the pavement [14]. With minor modifications, the FWD has been applied to the measurement of railway track. A pre-determined impulse load is applied close to the rail seat area of a sleeper, and the transient deflections of the sleeper and ballast accurately measured using geophones [3]. The deflections of the loaded sleeper, the ballast in the adjacent crib and the ballast in the next crib, are interpreted as corresponding to the sleeper deflection, the deflection of the loaded ballast and the deflection of the formation, respectively [1]. Currently, the FWD is suggested as the only satisfactory way of obtaining sufficient data to assess trackbed stiffness magnitude and variation for an individual site [2]. The track loading vehicle (TLV) is not applicable, as scheduling for and implementation at a targeted section of track in the rail network would not be cost effective. The TLV is currently being developed to measure parameters such as dynamic track stiffness and track receptance whilst moving, and should potentially be more applicable for whole route assessments [5]. Hunt suggests that the most effective monitoring should be vehicle mounted and aim to give a continuous track stiffness profile, identifying sites for consideration for remedial treatment. These sites could be investigated further by targeted stiffness measurement and site investigation techniques. Research would suggest, including Hunt [5], Sharpe [2] and Sharpe and Collop [3], that the FWD is the most applicable method for determining quasi-static sleeper stiffness, an important parameter when assessing sites exhibiting track geometry deterioration. In this trial, FWD was performed on the up line, over a 130 m section. Six months later, further testing was carried out across the whole site on the up and down line (Fig. 1). Following testing, deflection data retrieved was analysed off-site to provide track stiffness profiles. Deflection and stiffness profiles can be found in Fig. 6. Deflection and stiffness profiles agree with observations from previous testing performed. Test results from the initial 130 m section on the up line suggest subgrade deflections (d1000) are not ideal for track support, with relatively large deflections observed (Fig. 6(a)). However, problems have not yet manifested in the section tested. This may be due to the relatively uniform consistency of stiffness across this section (from 38 ml 969 yd to the platform end). If FWD profiles are compared with previous test results,
154
M. Brough et al. / NDT&E International 36 (2003) 145–156
Fig. 6. Deflection and trackbed stiffness profiles for (a) up line (b) down line.
M. Brough et al. / NDT&E International 36 (2003) 145–156
then the consistent stiffness across the 130 m section initially tested correlates with the well-defined interface suggested by the GPR trace. Initial test results suggested a well-defined interface, constant adequate ballast depth and generally consistent track support stiffness. Subsequently, no apparent track geometry deterioration problems are evident on this section of track. This consistent stiffness was still apparent across the 130 m section following FWD testing six months later. Even though stone blowing had been performed in the interim, there were no significant changes in the deflection characteristics of the sleeper, ballast and formation (Fig. 6(a)). FWD testing across the whole site on the up and down line further defined the formation problem area of concern previously highlighted (Fig. 6(a) and (b)). There appeared to be a length of track on the up and down line that was exhibiting a less consistent deflection and trackbed stiffness profile than was observed across the whole site. From 38 ml 1276 yd to 38 ml 1014 yd, the sleeper, ballast and formation deflections appear more variable than further along the track. This agrees with previous testing that suggested the poor formation section stretched from the highways over bridge (38 ml 1276 yd) to 38 ml 1024 yd. If FWD data are analysed further the source of the track geometry problem can be further understood. Trackbed stiffness magnitude does not appear to be the main cause for concern, with the range of measured stiffness similar along the site. However, increased trackbed stiffness variation does appear to be correlated with the poor formation section (38 ml 1276 yd – 38 ml 1014 yd). Previous results suggested ponding of water in subgrade depressions on the up line, further evidenced by nonuniform subgrade deflections across this section of track. As can be seen in Fig. 6, deflections of the formation are less consistent and relatively high in this section on the up and down line. This can be seen when compared with other FWD data from other sites as presented by Sharpe [1]. Even though track geometry problems manifest on the up line, no significant differences can be observed in trends of formation deflection data on the up and down line. If deflection data are examined further, it would appear that ballast deflections are slightly higher on the up line, possibly suggesting the problem is sourced in this layer. Alternatively, this could suggest a problem at the ballast/subgrade interface, such as fines migration and ponding. 4.4. Summary The poor subgrade extended beyond the trial pitted section of track, stretching from the cross drain (38 ml 1276 yd) to roughly 38 ml 1024 yd on the up line, a maximum length of 230 m. The trackbed typically consisted of a clean ballast layer, underlain with contaminated ballast and soft to firm clay. The incidence of high moisture content slurried ballast and potential
155
ponding in subgrade depressions seemed greater on the up line with a relatively shallow water table evident. This was also highlighted by penetrometer readings, with shear strength measurements in up line ABS samples suggesting a soft becoming stiff clay layer with values ranging from 50 to 140 kPa. The softening of clay at the top of this layer could be evidence of the ponding discussed previously. The incidence of very soft clay seemed greater on the up line, presumably due to the increased loading, reduced ballast thickness and higher in situ moisture contents. On the down line, the clean ballast layer was shallower and the contaminated ballast layer consisted of stiffer clayey gravel. Ponding in shallow subgrade depressions could not be confirmed from CPT results without a more detailed evaluation of friction ratio data at shallow depths. CPT result suggested that the stratigraphy beneath the ballast seemed to consist of a firm becoming soft clay underlain with a medium dense to dense sand and gravel. If the clay was analysed further, it appeared that shear strength became lower and compressibility became higher with depth. This could be evidence of ballast penetration or consolidation at the top of the clay layer. A layer from 2.5 to 3.5 m of particular concern exhibiting very low shear strength and high compressibility could be evidence of the clay prior to any consolidation. With increased loading or line speeds, this layer could cause more severe long-term settlement and associated track geometry deterioration. In agreement with ABS and CPT results, FWD subgrade deflections are indicative of an underlying soft clay. However, trackbed stiffness magnitude does not appear to be the main cause for concern, with the range of measured stiffness similar along the up line. However, increased trackbed stiffness variation does appear to be correlated with the poor subgrade (38 ml 1276 yd– 38 ml 1014 yd). Deflections of the formation are less consistent and relatively high in this section on the up and down line. If data are examined further, it would appear that ballast deflections are typically higher on the up line, possibly suggesting the problem is sourced in this layer. This would suggest that the problem is shallow seated and could be remedied by treatment of the contaminated ballast and upper formation layers. In summary, there appeared to be several suggested causes for the observed track geometry deterioration on the up line. Ponding of water in shallow subgrade depressions causing localised softening, fines migration into the ballast, heterogeneous stiffness and consequently nonuniform track settlement. This wavy top in turn has exacerbated the rate of track geometry deterioration. Although there is underlying soft clay exhibiting a high potential for consolidation, this layer should not need targeting in any remedial scheme, providing load transfer issues can be addressed in shallow subgrade layers.
156
M. Brough et al. / NDT&E International 36 (2003) 145–156
5. Remedial ground improvement
References
Results would suggest that track geometry deterioration on the up line at Leominster is due to a combination of insufficient clean ballast thickness, high in situ moisture content and increased loading from freight traffic. Continued maintenance in the form of repeated addition of ballast and tamping has resulted in excessive ballast thickness, much of which extends into the underlying clay layer, forming ballast pockets along the trackbed. Ponding of water in resultant formation depressions has caused a continual softening of the upper clay formation layers, extensive fines migration into the clean ballast layer and a nonconsistent trackbed stiffness. This differential stiffness and localised ponding has exacerbated the problem, minimising potential benefits of repeated ballast addition. As stiffness or shear strength magnitude appears to be sufficient across the site, this should not be the main focus of any remedial programme. To overcome the problems at Leominster, either consistency of the formation condition should be improved and subsequently any formation depressions disrupted, or the ponding water and slurried ballast layer should be removed. At present, shallow soil mixing treatments or novel drainage technologies are being considered. A full assessment of all available ground improvement technologies for remediation of trackbed formation is reported in Ref. [15].
[1] Cope D. Track technology for the future. Second course on vehicle/ track interaction, programme and notes, Cambridge: Emmanuel College; 2000. [2] Sharpe PS. Trackbed investigation. PWI J Rep Proc 2000;118(3): 238 –55. [3] Sharpe P, Collop AC. Trackbed investigation—a modern approach. Proceedings of the First International Conference on Maintenance and Renewal of Permanent Way and Structures, London, UK: Engineering Technics Press, Brunel University; 1998. [4] Chrismer SM, Li D. Cone penetrometer testing for track substructure design and assessment. Sixth International Heavy Haul Railway Conference—Strategies Beyond 2000, Cape Town, South Africa; 1997. [5] Hunt GA. EUROBALT optimises ballasted track. Railway Gazette International; December 2000. [6] Shahu JT, Kameswara Rao NSV, Yudhbir. Parametric study of resilient response of tracks with a sub-ballast layer. Can Geotech J 1999;1137– 50. [7] Li D, Selig BT. Evaluation of railway subgrade problems. Transpn Res Rec 1995;1489:17 –25. [8] Brough MJ, Ghataora GS, Stirling AB, Madelin KB, Rogeus CDF, Chapman DN. Investigation of railway track subgrade. I. In situ assessment. Institute of Curl Engineering, Transport Journal. (2003). [9] Livneh MI, Ishai I, Livneh AL. Effect of vertical confinement on dynamic cone penetrometer strength values in pavement and subgrade evaluations. Transpn Res Rec 1995;1473:1–8. [10] Hugenschmidt J. Ballast inspection using ground penetrating radar. Proceedings of the Second International Conference on Railway Engineering, London, UK; 1999. [11] Jack R, Jackson P. Imaging attributes of railway track formation using ground probing radar (GPR). Railway Engineering—98, First International Conference on Maintenance and Renewal of Permanent Way and Structures, London, UK: Engineering Technics Press, Brunel University; 1998. [12] Ayres DJ. Trackbed investigation. J Rep Proc Permanent Way Institution 2000;118(4):356–60. Comments on paper of same title by Phil Sharpe. [13] Horsnell MR, Adam CH. Geotechnical investigations for railway maintenance and renewal. Proceedings of the Third International Conference on Railway Engineering, London, UK; 2000. [14] De Almedia JR, Brown SF, Thom NH. A pavement evaluation procedure incorporating material non-linearity. ASTM STP 1198 1994;. [15] Brough MJ, Stirling A, Ghataora G, Madelin K. Improving railway subgrade stiffness—assessment of traditional in situ ground improvement techniques. Proceedings of the Third International Conference on Railway Engineering, London, UK; 2000.
Acknowledgements The authors gratefully acknowledge the combined financial and technical support of the EPSRC and the industrial partners Railtrack, GTRM, Scott Wilson Pavement Engineering, WS Atkins Rail, Keller Ground Engineering, Fugro and Serco Railtest. The authors also gratefully acknowledge the specific technical advice and discussion provided by Scott Wilson Pavement Engineering regarding the GPR, ABS and FWD. Similar thanks go to Fugro for advice and discussion regarding the CPT and rig applications, and to WS Atkins Rail for supervising and logging the trial pits.