SLAB TRACK Recent developments in slab track - …

1 With the growth of traffic intensity itbecomes more and more difficult to carryout maintenance and renewal work. InThe Netherlands, night time possessions often lastno longer than 5 hours, and on the future high speedline in Korea (435 km from Seoul to Pusan) the max-imum effective possession is estimated at no morethan 11/2hours per night. In this respect, the currentincrease in popularity of low-maintenance trackdesigns is not the past, new projects were mainly assessed onthe basis of investment costs, whereas today the prin-ciple of life cycle costing is strongly emerging.

2 As aresult of this new attitude, ballastedtrack concepts will loose attractive-ness in favour of slab TRACK considerationsPresently all over the world non-bal-lasted TRACK concepts are beingapplied, although still at a moderatevolume. The great advantages of suchstructures can be summarised as fol-lows: Reduction of structure height; Lower maintenance requirementsand hence higher availability; Increased service life; High lateral TRACK resistancewhich allows future speedincreases in combination with tilt-ing technology.

3 No problems with churning ofballast particles at the low-maintenance characteristics of slab trackon open line are to be retained, great care must betaken to ensure that the subgrade layers are homoge-nous and capable of bearing the loads imposed. Theslabs may be prefabricated or poured on site. Thehigh level of investment required has preventedwidespread use of slab TRACK on open line so , on the basis of life cycle costs a differentpicture is obtained [1]. The greatest savings will beachieved in tunnels and on bridges.

4 The use of moreefficient construction methods, of the type used inthe road construction industry, could reduce con-struction costs further principlesGenerally speaking there are two ways of designingslab TRACK . According to the German school, basedon highway design, the supporting layer should havea substantial bearing stiffness, represented by themodulus of elasticity Ev2 *120 N/mm2. Thisrequirement refers in particular to slabs on a sub-grade and is originated by the fact that incoherentblock structures, having no bending resistance, couldbe used.

5 This requirement also covers the conditionthat no differential settlements may occur. Slabsdesigned in this way only contain reinforcement ineuropeanrailway review81 slab TRACKR ecent developments inslab TRACK Coenraad Esveld,Professor of Railway Engineering, Delft University of Technology DelftWith the design of railway lines factors like life cyclecost, construction time, availability and durability playan increasingly important role. In this respect, non-ballasted TRACK concepts offer good opportunities. Fig. 1: Shinkansen slab 23/5/03 10:08 am Page 1europeanrailway review82the neutral axis to control crack width in the con-crete.

6 This is standard practice in Germany. On engi-neering structures like bridges and tunnels, providingrigid supporting conditions, this is a logical , when slabs are built on subgrades oftenmassive soil improvements are required, which makeslab tracks financially second way of constructing slabs, especiallyon soils where some settlements may be expected, iswith reinforcement at the top and at the bottom ofthe slab to take bending stresses and axial studies at TU Delft have shown that rela-tively high reinforcement percentages of about %for a B35 concrete [2, 3, 4, 5, 6].

7 On the other handonly very limited soil improvements are no bending resistance is required, prefabri-cated slab sections can be used. On bridges this hasthe advantage that the influence of bending of thebridge has no influence on the bending stresses in thetrack (semi) continuous slabs are applied the con-struction method can consist of either pouring theconcrete into a casing, or constructing continuouslyusing a slipform paver. The price is, to a large extent,determined by the building for slab tracks it is important to con-trol the TRACK geometry carefully during construc-tion, as corrections afterwards are difficult.

8 Perhapseven more important is to make the welds at verytight tolerances to avoid impact loads which have astrong negative influence on the service life of theentire systems in useJapan was effectively the birthplace of high speedrail. development work on the Shinkansen networkstarted at the end of the 1950s, and the first line(between Tokyo and Osaka) opened in autumn lines are currently in service and a sixth is underconstruction. Government plans dating back to 1970specify a national Japanese high speed network of 3500 km of double TRACK .

9 By 1993, a good 1 400 kmof this had been built (double TRACK ), of which morethan 1 000 km consists of ballastless double TRACK . InJapan, ballastless TRACK always consists of prefabri-cated slabs just under 5 m long. The percentage ofballastless TRACK varies considerably from line to newer lines include a higher percentage (up to96 %). The slab TRACK design has remained virtuallyunchanged since the first sections were laid in Shinkansen slab TRACK (Figure 1, pg. 81) con-sists of a sublayer stabilised using cement, cylindrical stoppers to prevent lateral and longitudinal move-ment, reinforced pre-stressed concrete slabs measur-ing m x m x m ( m x m x in tunnels) and asphalt cement mortar injectedunder and between the slabs.

10 The slabs weighapproximately 5 TRACK was undergoing rapid develop-ment in Germany. Since 1996, DB has been operat-ing a test TRACK in Karlsruhe consisting of seven newtypes of ballastless TRACK . The best-known Germandesign is the Rheda system. There are numerousvariants which indicate that the initial designs werenot completely successful. The most developedRheda system is Rheda 2000 [7]. By eliminating theconventional concrete trough, a significant simplifi-cation of the overall system con-figuration wasachieved.