The Use of Falling-Weight Deflectometers in Determining Critical Velocity Problems Craig Govan, URS, Trackbed Technology Railway Track Science & Engineering Workshop, UIC, Paris December 5th, 2013
Content of Presentation URS & Trackbed Technology Overview FWD / HWD Overview How the FWD Works Calculating Critical / Permissible Velocity Case Study 1: Midland Mainline Linespeed Improvements, UK Case Study 2: Lac La Biche Sub, Canada COMPANY CONFIDENTIAL: Use this space for disclaimer if needed. 2
About URS & Trackbed Technology URS Corporation has more than 47,000 employees in a network of offices in more than 40 countries URS (formerly Scott Wilson) started work on trackbed around 15 years ago, with Dr Phil Sharpe (ex-br research) Team expanded to incorporate trackbed technology (consultancy), geophysics, laboratory and operations teams Team now part of Transportation Asset Management Team Approx 30 full time staff, comprising both Consultancy and Operations Undertaking works around the globe including Australia, North America, Africa and Europe
FWD / HWD Overview FWD / HWD used extensively in pavement testing URS have developed the technique for use in the rail environment The technique works by dropping a weight to create a pulse load This creates a wave within the ground The displacement of the ground created by this wave is measured using geophones The equipment also records the time taken for the wave to reach each geophone this is plotted as a time history
Falling Weight Deflectometer (FWD) Overview Loading Beam 0.3m Geophones Geophone Peak Load on Sleeper 12 Tonnes, equivalent to 24 Tonne axle Automated Test Cycle takes 1 minute, typically 30 tests/hr 3 drops per test cycle, both peak load + displacement, time histories Results displayed immediately on monitor 5
Heavy Weight Deflectometer (HWD) Overview Geophone Loading Beam 0.3m Geophones Loading Beam Peak Load on Sleeper 16 Tonnes, equivalent to 40 Tonne axle Automated Test Cycle takes 3 minutes, typically 10 tests/hr 3 drops per test cycle, both peak load + displacement, time histories Results displayed immediately on monitor 6
How the FWD / HWD Works Geophones placed at set offsets from load-beam based on BR research: In load-beam 300mm offset 1000mm offset 2000mm offset Weight dropped from pre-determined height to generate pulse load can either select height or contact pressure Cycle comprises three drops 1 st drop displacement only 2 nd drop displacement only 3 rd drop displacement and time histories COMPANY CONFIDENTIAL: Use this space for disclaimer if needed. 7
How the FWD / HWD Works Deflection mm 2.0 1.5 1.0 0.5 Load 0 0 20 40 60 80 100 120 Time - Milliseconds
Dynamic displacement/static Displacement Determination of Permissible Train Speed 3.5 3 Permissible Velocity 2.5 2 Limit Strain in Ballast 1.5 1 Limit Accelerations in Ballast 0.5 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Normalised Train Speed (Velocity/Critical Velocity) 9
Case Study 1: Midland Mainline Linespeed Improvement 10
Site Description The site is located on the Midland Mainline The line runs from London St Pancras Chesterfield The line primarily used for passenger traffic From Bedford to London St Pancras (approx. 50 miles) the line is electrified The majority of the route is proposed for linespeed improvement in order to meet timetable demands Network Rail identified sites which it considered to be a potential problem when linespeed increased URS were asked to investigate sites with both FWD and ABS to determine site conditions and critical velocity for sections 11
MML Sub Site 13 FWD Testing 74m 220y to 74m 880y Up Main Current linespeed = 100mph Proposed linespeed = 110mph UIC60 with G44 sleepers Mudstone and Ironstone Geology Located in cutting Overbridge in centre of sub-site 6ft drainage run at high mileage end of site Historic problems in area suspected spring at around location of signal gantry 12
MML Sub Site 13 FWD Testing Results 13
MML Sub Site 13 FWD Testing Conclusions ABS data supported the indication of a possible spring at the location of the signal gantry Extensive slurrying of the trackbed layers High peak deflections in the HWD data throughout the sub-site Permissible velocity is around the proposed linespeed (110mph) Data indicates that the low permissible velocity is in part due to extensive slurrying and a high water table A recommendation was carried forward by Network Rail for ground improvement works in order to remediate poor ground conditions Maintenance liability was reduced post hand-back of the track through identification of the problems 14
Case Study 2: Lac La Biche Sub, Alberta, Canada 15
Site Description Site is located in Northern Alberta 170 miles of track The site takes freight from Fort McMurray to Lac La Biche and onwards The section is strategically important for development of oil sands area in Northern Alberta The plan is to design for an axle load of 36 tons in next 12 months There are also plans to increase frequency of traffic currently only two trains per day Implications for maintenance access and resourcing 16
Site Specifics Concerns that the site is located on highly variable glacial geology Areas of biggest concern are where there is Muskeg subgrade deep peats Also problems associated with plastic clays, especially where peat lenses create highly variable stiffness conditions Track failures experienced range from typical dirty ballast failure and subgrade erosion to cess heave in plastic clays and sink holes Some sections of track require tamping every two weeks to maintain adequate geometry Initial indications prior to investigation were that a high proportion of the route required deep-seated treatment 17
Site Investigation Site investigated using a range of techniques: ABS HWD GPR Rolling Stiffness Measurement Results of various techniques were compared to determine what comprised the significant problems HWD was used due to the need to replicate high axle loads on site (outside range possible with FWD) 18
Results Critical Velocity Prior to investigation there were concerns that critical velocity was responsible for maintenance problems on site However, there is an extremely low line speed on site only 25mph The critical velocity results are amongst the lowest ever measured using the FWD / HWD (approx. 62mph maximum permissible velocity of 54mph) Results indicate that although the site has very low critical velocity, critical velocity on the site is significantly higher than the current or proposed linespeeds From data collected the problems are associated with peak deflections / displacement, rather than critical velocity Significant impact can be expected from increase in axle load 19
Results Peak Deflections Using the data from the Lac La Biche Sub site, URS modelled the effect of increased tonnage on ballast strain and formation displacement URS were able to determine the limits for FWD deflections based on sleeper spacings and rail sections All track on Lac La Biche Sub has a sleeper spacing of 500mm Design requirements are for a 36 ton axle load From the data, URS determined that for 36 ton axle loads, required formation deflections will be approximately: 136lb/yd = 1.10 100lb/yd = 1.35 136lb/yd Rail 100lb/yd Rail 20
Outcomes On the Lac La Biche Sub, some of the track does not achieve a formation deflection of lower than the just maintainable threshold However, the majority of track not achieving the threshold is on 100lb/yd rail This track could predominately be rectified using heavier rail weight (136lb/yd) On the track which could not be rectified using heavier rail, ABS sampling has indicated that many of these sections contain shallow trackbed depths, with Muskeg at shallow depth these sections can be remediated with track lifts Only a small proportion of the sites will require some form of deep-seated treatment to achieve the maintainable threshold of formation deflection HWD testing and analysis has delivered efficiency in the initial remediation plans and reduced the amount of deep-seated treatment that will be required in order to support 36 ton axle loads 21
Conclusions 22
Conclusions FWD / HWD provides an accurate method of assessing the critical velocity and permissible velocity FWD / HWD can also provide information on deflection of the trackbed layers Works undertaken on Lac La Biche Sub have demonstrated that much of the proposed deep-seated remediation can be omitted and replaced with changes to track componentry (significant cost reduction) Works undertaken on Midland Mainline proved the need for deepseated remediation which was not previously considered (although higher initial cost, reduction in whole-life cost and maintenance liability) 23