1. Scope, Ths gude has been prepared to assst n the nvestgaton, desgn and rehabltaton of the exstng track formaton and ballast. It s ntended to be used n all constructon or mantenance actvtes amed at modfyng the exstng formaton condtons. It s recommended that a geotechncal engneer experenced n track geotechnology authorse the formaton desgn. 2. Reason and Nature of Change Document ressued as ARTC Engneerng Practce Manual. 3. Purpose of Track Recondtonng The purpose of track recondtonng s to reconstruct the track formaton and ballast to restore the track structure to a standard approprate to current track loadng. Recondtonng usually nvolves the removal of rals, sleepers and ballast and a predetermned depth of the exstng formaton and the reconstructon of those components to current earthworks standard. Modfcatons to the track dranage are necessary at the same tme. Recondtonng s usually carred out because of bearng capacty falure of the formaton or a falure of the dranage, but s at tmes carred out to replace foul ballast where local condtons prevent the use of ballast cleaners. 4. Determnng the Need to Carry Out Recondtonng Track mprovement requres an adequate nvestgaton. clear dea of what the problem s, and ts extent. It s mportant to have a Consderaton of the need to carry out track recondtonng should be gven when Page 1 of 22
any one of the followng features exst: a) Contnue poor track performance, e. Rapd deteroraton of top, or top and lne n spte of repeated attempts at resurfacng, ballastng, or ballast cleanng. b) Vsble sgns of formaton falure,e. heavng beyond the ends of sleepers or between sleepers. c) Bog Holes Track and ballast fouled and/or wth mud actvely pumpng through the ballast. When the track s removed and formaton lowered to ncrease heght clearances, the correct ballast depth must be provded and the reconstructon of the lowered formaton should follow the nvestgaton and desgn prncples set out n ths gude. Fgures 1, 2, 3, 4, 5, 6 and 7 llustrate examples of track formaton problems. Fgure 1 Page 2 of 22
Fgure 2 Fgure 3 Page 3 of 22
Fgure 4 Fgure 5 Page 4 of 22
Fgure 6 Fgure 7 Some of the above problems can be solved wthout removal of the track. Recondtonng, however, IS requred when the foundaton has faled. Alternatves that can be consdered nclude: Page 5 of 22
Methods that do not requre track removal: 1) Shoulder and crb ballast replacement (Manual and Machne) 2) Cross Drans (see secton 4.1) 3) Sleddng 4) Ballast Cleanng 5) Lme Slurry Injecton Methods that requre track removal: 1) Sk m Recond t on ng( s eesect on4. 2) 2) Formaton reconstructon 4.1. Formaton Dranage Opton Track condtons may mprove suffcently wthout recondtonng f dranage of the formaton s mproved. Cut off drans across the whole track to a level below the ballast formaton nterface wll dran depressons n the formaton. These cut off drans or fnger drans need to be at ntervals of 5m. Ths soluton may avod costly track possesson. Removal of the shoulder ballast and replacement wth clean ballast wll release water trapped n the track profle, and replacement of the crb ballast wth clean ballast wll mprove track performance. Cess drans must be lowered to below the formaton level to permt dranage. 4.2. Skm Recondtonng Where the formaton has not faled, there s no orgnal cappng, and t s possble to rase the track, skm recondtonng s a vable soluton. The track s removed and the clean top ballast excavated. The foul bottom ballast s graded and compacted to form a structural cappng layer. The track s then relad and ballasted. Ths method s used when replacng tmber sleepers wth concrete sleepers. 5. Investgatons Pror to Track Recondtonng The causes of the track formaton falure and detals of the subsurface condtons should not be assumed. All recondtonng projects should have a geotechncal nvestgaton carred out by an experenced geotechncal engneer at early plannng stages. The typcal geotechncal nvestgaton wll consder the followng: Subsurface condtons depth and condton of ballast, cappng and subgrade. Determnaton of the cause of falure foundaton falure, neffectve Page 6 of 22
dranage, rock pumpng. Adjonng structures effect on platform footngs, f any. Recondtonng recommendatons ballast clean, skm (remove ballast only) or full recondtonng. Recondtonng desgn depth of layers etc. Subgrade treatment geosynthetcs, rock fll, geocell etc. Dranage needs. 5.1. Investgaton Procedures The basc components of an adequate nvestgaton of track constructed on clay or fll consst of the followng procedures and ams: a) Excavaton of test pts at regular ntervals (10 to 20m, alternate Up & Down Sde, n the four foot, and at obvously faled locatons) to determne: - The depth and condton of the ballast. - The exstence of other ballast materals. - The exstence and condton of cappng. - The condton and type of subgrade. - The degree of saturaton of all materals. - Informaton about the nterface profle, f vsble, to ndcate the degree of ruttng or consoldaton. Investgatons should be located between two sleepers and the outsde ral or n the 4ft and extend to below the formaton level or about 1.2m below the ral level. See Fgure 8. Excavatons across the whole track between sleepers provde the most nformaton, especally about the formaton profle, but are expensve and ntrusve and should be done less frequently as a control. b) Instu strength tests usng Dynamc Cone Penetrometer (DCP) should be conducted close to the ral. Tests should start from below the ballast and contnue to a depth of 2.5m below ral level to provde an estmate of the nstu CBR value. c) Samples of the subgrade should be selected for classfcaton testng and to assst n the desgn of graded flters (sub ballast) or geotextles or to determne f stablzng materals wll assst n mprovng subgrade performance. Samples of the top and bottom ballast should be taken to determne ballast recovery, should the materal be recycled. Samples should be a mnmum of 30kg. d) A descrpton of the degree of saturaton of the track should be recorded, as should detals of the cess dranage and any system exstng and an Page 7 of 22
evaluaton of ts condton. e) Survey detals of the track, cess dran nverts, culvert nverts and adjacent areas, (and beyond boundary f necessary) are necessary for desgn of future dranage. Ballast Foul Ballast? (Wet/most/dry, foulng materal, ballast type) Excavaton n four foot or outsde ral Cappng? Formaton type of materal, mosture condton, hard/soft? Fgure 8 6. wet/most? condton? Subgrade Condton? DCP Test from top of cappng to 2.0m below ral level Ballast depth and formaton nvestgaton for recondtonng Track Formaton Desgn 6.1. Basc Desgn Model The basc desgn model conssts of the standard 300mm of ballast below the sleeper supported by 150mm of cappng materal. Ths s satsfactory provded the underlyng materal wll provde suffcent support. (Relate to TDS 11). Where the formaton conssts of hard rock or concrete the ballast thckness must not be less than 250mm otherwse ballast degradaton and rock abrason wll accelerate. (Relate to TDS 11). For track on sol or fll, the materal mmedately beneath the cappng layer (structural fll) s requred to have a soaked Calfornan Bearng Rato (CBR) 8. (See Fgure 9). The requred thckness of ths materal vares dependng on the qual t y oft he f ound ng mat er al. The t h c kness H ofs t r uc t ur alf l l / mat er al s recommended for the followng foundng materal: Foundng Materal CBR Th ckness H ( mm)st r uct ur almat er al 3 500 1-3 1000 Dynamc Cone Penetrometer (DCP) testng s therefore requred to 2.5m below ral. A correlaton between DCP tests and CBR s provded n Appendx 1. Page 8 of 22
Ballast depth see TDS 11 7. Cappng Materals 7.1. General Cappng as specfed n ARTC standard TDS 12 s used for some or all of the followng reasons: To mprove track support. (Generally a layer 150mm thck s suffcent for normal condtons wth ncreased thckness for soft subgrade). To fll n rregular over-break stuatons n rock excavatons to assst n dranage. To provde a dranage barrer to shed storm water from subgrade. The performance of the cappng depends on the qualty of the materal and the qualty of ts compacton on placement. The materal needs to comply strctly wth the requrements of TDS 12 and to be compacted n accordance wth RCP 01 to pr ov det he Bar r er / Suppor t f eat ur esex pect ed. 7.2. Flexble Impermeable Cappng The propertes specfed for cappng of ths type are desgned to provde a flexble mpermeable granular of hgh strength. For ths materal to perform correctly the propertes of the materal suppled must strctly comply wth the requrements of TDS 12 for flexble mpermeable cappng. Ths layer s desgned to seal the formaton and shed any surface water. Materal supped as DGB20, or smlar, n accordance wth specfcatons prepared for the RTA of NSW do not necessarly comply and wll not provde a seal to the formaton. The closest road specfcaton to ral r equ r ement s s Roadbasef orunseal edr oads.mat er alt obeused as cappng of ths type should be tested for complance pror to use. It s also very mportant that the correct compacton levels are obtaned. Poorly Page 9 of 22
compacted cappng wll become saturated quckly, foul the track and become the cause of further track and ballast problems. 7.3. Sub-Ballast Where the foundaton conssts of free dranng materals such as rock fll or sand a sub-ballast may be more approprate than an mpermeable cappng. Sub-ballast s not mpermeable and s desgned as a flter materal to lmt foulng by subgrade materal and to prevent tself from foulng the ballast. Its purpose s to: Spread vertcal load from the ballast. Prevent mgraton of fnes from the subgrade. Permt mgraton of mosture. The gradng of the sub-ballast s mportant and must be specfcally desgned for a partcular subgrade and ballast. Laboratory testng of both the subgrade and the ballast s requred to be able to desgn a gradng for sub-ballast. The performance of the sub-ballast s not destroyed by saturaton as s that of the cappng and t s much more easly compacted. It may provde a better soluton than cappng n areas where the subgrade s constantly wet due to, say, a sprng or n soft rock formaton stuatons. 7.4. Sem Rgd Cappng In stuatons where t s dffcult to obtan materal complyng wth the specfcatons for mpermeable cappng and the formaton condtons are relatvely stff a sem rgd cappng may be approprate. It s also approprate as a cappng over soft frable rock foundatons. Ths materal may consst of a stablzed materal and propertes are specfed n TDS 12. The use of ths materal over soft formaton s not recommended as t wll crack on flexure and rapdly cause track pumpng problems. It s advsed that thorough geotechncal nvestgaton and analyss be performed before choosng ths type of cappng. 8. Dranage The success of any track or reconstructed track formaton s governed by the effectveness of the dranage of water from the ballast above the cappng and the subsol water beneath the cappng. An effectve surface dranage system must be constructed as part of all recondtonng. For surface dranage to be effectve, grades must be unform, wthout local hollows. Pondng of water n cess drans or surface drans produces gradual deteroraton of the formaton strength. Recondtonng must not create a condton where water ponds beneath the track. If the nvestgaton dscovers saturated condtons a subsol dranage system must be nstalled. Subsol dranage s requred to: Dran any porous subsol materal. Lower water table where t s near the ground surface. Page 10 of 22
Reduce pore water pressure due to tran loadngs n rock foundatons or artesan pressure eg.-sprngs. Subsol dranage systems requre headwalls at the outlets and flushng ponts for mantenance. These features need to be clearly vsble and sgnposted. More detaled nformaton on surface and subsol drans s avalable n ARTC Engneerng Practce RTS 3433. 9. Geosynthetcs CAUTION Geotextles are not Magc They cannot be used to short cut the project or make up for poor constructon materals Geotextles have been used extensvely n track recondtonng for some years because t was beleved that beneft could be ganed n subgrade support and separaton of the ballast from the cappng. More recent experence and nvestgatons reveal that the geotextle provdes lttle beneft unless used approprately and may also create problems. The most approprate uses of geosynthetcs n track formaton are: Fltraton and separaton. Subgrade support. The geotextles for these purposes have sgnfcantly dfferent propertes and the better performance n one means a reduced performance n the other. 9.1. Ballast/Cappng Separaton Geotextle was orgnally placed beneath the ballast n the belef that t would reduce foulng of the ballast by fne materal from the formaton. In other cases t was placed because the cappng had not been suffcently compacted. However, research of ballast performance has shown that where the formaton and cappng are constructed properly the major contrbuton to ballast foulng s from ballast breakdown and contamnaton from the surface. The presence of geotextle therefore fals to protect the ballast from these contamnatons. Local experence has shown n many cases that the materal above the geotextle has been more fouled than below. Furthermore the geotextle suffers severely from abrason by the ballast and very quckly loses ts separatng capabltes. Where the geotextle has been used n wet and soft condtons or when cappng has not been compacted adequately, the hgh dynamc loads from ral traffc nduces extreme pressures at the ballast formaton nterface and the flterng acton of the geotextle s neglgble. Ballast cleanng operatons have also been severely hampered by the presence of the geotextle. It s therefore not recommended that geotextle be placed beneath ballast at the ballast/cappng nterface. Page 11 of 22
A geotextle s necessary, however, between two materals of sgnfcantly dfferent gradng. When placng cappng or sub-ballast materal over coarse rock fll a geotextle s necessary to prevent the fne materal of the cappng from fallng nto the vods of the coarse fll. Fgure 10 Excavat edgeot ext l eshow ngcompl et e cl ogg ng nser v ce 9.2. Fltraton Geotextles are used to great advantage n subsol dranage systems and the costly process of graded aggregate flters have been avoded. A geotextle wrapped around coarse aggregate or a slotted ppe provdes substantal cost savngs. It s, however, necessary to sample and test the sols n whch the subsol dran wll be placed to determne the approprate geotextle to use. Some sols are not fltered by all geotextles. Sols wth hgh content of fne slts and clays requre close nvestgaton and n some cases cannot be draned by subsol drans wrapped n geotextle. The mportant propertes of the geotextle are ts flterng propertes and the strength propertes so that the geotextle wll mantan ts ntegrty durng the constructon process. The propertes set out n the followng tables are for average condtons. Where there s doubt about the sol/geotextle compatblty, a geotechncal nvestgaton s requred. 9.3. Renforcement A geotextle s placed below the structural fll to reduce subgrade deformaton due to the load and to reduce the amount of structural fll requred. A geotextle for such a purpose s requred to have a hgh tensle strength and low elongaton and to develop good bond wth the contact materal. The best materal for ths purpose s a geogrd and t s recommended n very soft Page 12 of 22
areas. Other geotextles provdng good renforcement are the woven type fabrcs or bonded fabrcs. The propertes recommended as mnmum for track work are gven n Secton 9.4 below. 9.4. Propertes of Geotextles n Track Projects 9.5. Subsol Dranage Property Test Method Value Tearng Strength, Trapezodal * AS 3706.3 200N mn Burst Strength, CBR Method * AS 3706.4 1700N mn Puncture Resstance, Drop Cone * AS 3706.5 d500 H50 = 33.5mm Max = 900mm Mn Pore Sze * AS 3706.7 240 max Permttvty AS 3706.9 3.4 sec* mn * - Important when used as a separaton for rock fll 9.6. Subgrade Renforcement (Below Cappng) Property Test Method Value Tensle Strength, Wde Strp AS 3706.2 25KN/m mn peak Elongaton at Peak Strength AS 3706.2 25% max Burst Strength CBR Method AS 3706.4 2800N mn Permttvty AS 3706.9 0.12 sec mn -1 9.7. Propertes of Geogrds A geogrd s specfcally desgned to provde tensle renforcement to the track substructure. Geogrds are superor to geotextles n ths functon because of ther hgher modulus and tensle strength and the performance s not destroyed by coarse aggregate. The geogrd selecton should be on the bass of the gradng of the materal n contact wth t ant the grd openng sze s related to the maxmum sze of the aggregate wth whch t nterlocks. The manufacturers provde a selecton crtera for ther products for the varous uses. The recommended mnmum strength s 20KN/m transverse and 20KN/m longtudnal to the track. The nterlockng wth aggregates s mportant. When selectng a geogrd, careful attenton should be pad to the profle and rgdty of the grd webbng. A geogrd wth a square web profle and hgh rgdty wll perform well n nterlockng wth aggregate. 10. Track Reconstructon Solutons Recondtonng s usually requred because the subgrade has not met the above crtera. Methods of mprovng the performance of the exstng subgrade materal need to be consdered n the plannng of the recondtonng. The followng optons are suggestons only and each stuaton needs to be assessed by a geotechncal engneer for condtons pecular to the ste. Page 13 of 22
10.1. Reconstructon by Structural Fll Methods Reconstructon of the formaton s conducted by excavaton of unsutable materal and replacement wth sutable materal to sut the basc desgn model. See Fgure 9. For soft subgrade ths opton would requre suffcent tme and space to permt the removal and replacement of what could consst of a substantal amount of materal to provde the support defned n the model. If, however, nvestgatons ndcate that the subgrade has a CBR of 8 or greater and that track falure s due to loss of the frctonal propertes of the ballast due to poor dranage only, mnmal excavaton s necessary to remove the thn layer of soft materal mmedately beneath the ballast. In conjuncton wth the necessary reconstructon of effectve dranage, a reduced thckness of structural fll can be used n these stuatons. The new materal must comply wth the requrements of ARTC Standards C1100 and TS 3422 and the work requred ncludes preparaton and compacton as normally apples to earthworks constructon. A cappng layer of 150mm (mn) s recommended unless the structural fll also meets the requrements of TS 3422, n whch case the fll would contnue to below the ballast. 10.2. Reconstructon Usng Renforcements of Rock Fll and Geosynthetcs The use of geotextles and/or geogrds, geocells and rock fll can assst to reduce the amount of excavaton and fll materal especally n soft subgrade. 10.3. For Subgrade wth CBR = 3-8 Where the subgrade has been found to have an nstu CBR of between 3 and 8 a geotextle below a hgher grade mported fll materal wll reduce the excavaton necessary. The fll materal s recommended to have CBR of at least 20 at 100% standard compacton and a mnmum thckness of 350mm. The geotextle renforcement must be a hgh strength, low elongaton type materal or the benefts wll not be realzed. See Fgure 11. 1m (mn) 1m (mn) 1.5 1 Ballast depth see TDS 11 1:30 Cappng 150mm (mn) Subgrade CBR = 3 8 use Geotextle Subgrade CBR = 1 3 use Geogrd Geotextle separaton H H=350mm, Subgrade CBR = 3 8 H=500mm, Subgrade CBR = 1-3 Fgure 11 Geogrd Renforcement Page 14 of 22
10.4. For Subgrade wth CBR = 1-3 Where the subgrade has been found to have a CBR of between 1 and 3 a geogrd s recommended n place of a geotextle at the subgrade level. The same recommendatons for the type of fll as n Secton 11.3.1 apply wth a thckness of 500mm. See Fgure 11. 10.5. For Subgrade wth CBR = 1 or Less (Rock Fll) Where the subgrade had been found to have a CBR of 1 or less t wll be necessary to use coarse rock fll to reduce the amount of excavaton. A mnmum excavaton of 500mm s recommended. Obtan geotechncal advce for resolvng these condtons. Coarse rock fll, 50-150mm n sze (no fne materal) s tpped or rolled nto the soft materal. More s added untl the rock fll no longer penetrates the mud and the surface s just free of mud. A geogrd s then lad and fllng contnued wth rock fll up to the undersde of cappng, wth approprate statc rollng. No wheeled or tracked machnery should be permtted on the subgrade untl suffcent rock fll has been placed to support t. An addtonal layer of geogrd should be added where the fllng exceeds 500mm. The surface of the rock fll must be covered wth a geotextle for separaton between rock fll and cappng. See Fgure 12. 1m (mn) 1m (mn) 1.5 1 Ballast depth see TDS 11 Ballast depth see TDS 11 1:30 Cappng 150mm (mn) Rockfll 50 to 150mm statc rolled H=500mm (mn) Subgrade CBR<1 Rock rolled nto soft formaton Geotextle separaton Addtonal layer of Geogrd where H>500mm Geogrd at base Fgure 12 Rockfll Renforcement 10.6. For Subgrade wth CBR = 1 or Less (Geocells) An alternatve to the solutons outlned n 10.3.3 s to nstall Geocells to mprove the load support capacty of soft subgrade. It s often recommended for swampy condtons where the ground water s close to the surface. Geocells consst of an array of plastc hexagonal cells nflled wth granular materal. These are nstalled mmedately below the ballast and flled wth ballast. Geocells are avalable n panels 2.44m x 6.1m wth thckness varyng between 102 and 203mm. Page 15 of 22
The thckness and number of geocell layers must be ndvdually desgned for each partcular stuaton. An excavaton s made to the desgned level of the geocell base. A geotextle s lad over the excavaton to mnmze the transmsson of fnes through the geocell nto the ballast. The geocell s lad out, flled and the nfll lghtly compacted. No cappng s requred above the geocell, however n stuatons where the subgrade needs to be protected from surface water, an mpermeable geomembrane may be nstalled at the geocell base nstead of a geotextle. Impermeable membranes are not requred when Geocells are nstalled n swampy ground condtons. See Fgure 13. 1.5 1 1:30 Ballast depth see TDS 11 Geocell 100 to 200 deep Geotextle or Impermeable geomembrane Fgure 13 Geocell Renforcement, Formaton CBR<1 It should be noted that you stll need to nstall the depth of ballast specfed n TDS 11 above the geocell. The geocell nstallaton s a soluton to provde support, not to reduce ballast. 11. Chemcal Stablsaton Stablsaton of subgrade s the mprovement of the subgrade by mxng n lme or njecton of lme or a lme/fly ash mxture. Lme stablsaton s very relevant for hgh plastcty clays because of the reacton of lme wth the clay mnerals and can be used to reduce the shrnk/swell behavour of actve sols. For the best results the subgrade must be scarfed and lme spread and mxed unformly. Ths requres adequate track possesson and work space. Lme njecton methods requre numerous nspecton ponts to guarantee that the lme s unformly dstrbuted n the subgrade. For less plastc clays the effect of lme s more a cementaton acton that s not strong. Procedures are avalable for lme and fly ash njecton that provdes an mprovement n strength of the subgrade. These procedures provde hgh-pressure njecton to sgnfcant depths at close spacng. In general, lme njecton or stablsaton s not recommended wthout suffcent nvestgaton to support ther use. Geotechncal advce and laboratory testng s necessary to determne the approprate percentage of lme for the sol n queston. Stablsaton by the njecton of cement slurry s not recommended. Other chemcals are offered to assst n stablsaton of the subgrade and need to be evaluated as to practcablty or effectveness. Obtan geotechncal advce to assst n the evaluaton of products that are beng marketed as possble stablsaton Page 16 of 22
measures. 12. Constructon Issues 1) Mnmse or elmnate constructon traffc over exposed subgrade materal. After excavaton of the ballast the formaton s usually not frm and the passage of wheeled or tracked machnery wll cause sgnfcant addtonal damage. Where possble, the constructon method must be desgned to avod traffckng over the exposed formaton. Excavatng spol by placng excavatng machnery on adjacent undsturbed track or ground and pushng new fll n front of constructon equpment s necessary when dealng wth soft formaton. The structure of some clay s destroyed by the remouldng acton of constructon equpment. Ths damage to the sol structure s permanent and may reduce the strength and affect the future performance of the formaton. 2) Compact new fll n layers no deeper than 200mm loose. Thnner for lght compacton equpment. 3) Use proper compacton equpment and compact to specfed densty. 4) Construct the correct cross fall. 5) Construct adequate dranage. 13. Rock Foundaton The nvestgaton of track on a rock base follows the basc steps set out earler. Where there s evdence of track foulng as a result of foundaton problems and the nvestgaton reveals that the track s constructed on hard shale or sandstone, the potental of foundaton rock pumpng must also be nvestgated by geotechncal personnel. 13.1. Foundaton Rock Pumpng Foundaton rock pumpng occurs n horzontally bedded rock under track work where the cyclc loadng condtons cause deflectons of the rock strata and abrason along rock beddng planes. In the presence of water, fne materal, resultng from the abrasve acton, s forced through fractures n the rock and contamnates the ballast. Dependng on dranage condtons at the track level, solated cones of rock fnes wll exst n the ballast or the ballast wll eventually be completely fouled wth mud. See Fgure 14. Ths pumpng can exst up to 2.0m below the track formaton level. In cases where foundaton rock pumpng s beleved to be occurrng more detaled nvestgaton methods are recommended wth a geologcal evaluaton of whatever rock s exposed n adjacent cuttngs. The cores would be examned for sgns of abrason at beddng planes to determne at whch depth ths s takng place. Ths type of nvestgaton must be carred out by geotechncal engneers or geologsts experenced wth ths problem. When the foulng of ballast due to pumpng s severe and the track geometry s Page 17 of 22
badly affected remedal measures to address rock pumpng are requred. The presence of water close to the formaton level s a major contrbutor to the problem and deep drans are necessary to control the pumpng. These drans may need to be n excess of 1m below the formaton. Alternatve solutons to recondtonng may need to be consdered where deep drans cannot be acheved. These solutons may nclude structural concrete slabs and ground anchors and specalst geotechncal and structural advce s requred. Volcano mound of slt and clay caused by ejecton of fnes under pressure Bedrock fnes (sand, slt and clay) are pumped nto ballast and cess Bedrock separaton Vertcal fracture due to flexng or natural dscontnuty Cavtes can exst at depths of 2m below formaton Bedrock flexes under load Cavty caused by abrason, opens and closes wth loadng ejectng water and fnes Sandstone/Sltstone or shale Pumpng and foulng of the track occurs prmarly n the presence of water Fgure 14 Bedrock Formaton pumpng mechansm Fgure 15 Example of Bedrock Formaton pumpng Page 18 of 22
13.2. Irregular Rock Excavaton Track recondtonng may be requred because of the foulng of the ballast n rock foundaton areas due to falure of the cappng or abrason of the rock. In rock cuttng s where the excavated rock surface s rregular water can be trapped n depressons under the track. Ths condton wll produce accelerated abrason of the surface of the rock formaton and foulng of the ballast. Ths stuaton may also arse durng the recondtonng exercse where rock has been removed by rppng or hydraulc rock breaker. It s mportant to ensure that there s a unform cross fall on the top of the rock formaton. In these stuatons any over-break or depresson should be flled wth materal wth characterstcs as close as possble to those of the rock. A cement stablzed road base materal, sem rgd cappng or roller compacted dry concrete are satsfactory. Layers of cement stablzed materal need to be greater than 50mm to prevent dsntegraton under traffc. Page 19 of 22
Appendx 1 Dynamc Cone Penetrometer (DCP) Gude to Use and Interpretaton The dynamc cone penetrometer (DCP) has become a valuable tool n geotechncal engneerng to dentfy soft materal and the boundary between sutable and unsutable materal. Because of past correlatons by Scala (1956) and the Australan Road Research Boar d( ARRB,1973)ameasur eofso lst r engt hcanbef ound.scal a swor kwascar r ed out predomnantly on clays and the report by ARRB was for flays n the Bankstown area. The results of the dynamc cone penetrometer are recorded on the attached record sheet as blows for 50mm of penetraton and usng the followng graph, an equvalent Calfornan Bearng Rato (CBR) can be obtaned. For clay materal a reasonable level of confdence s possble, however wth sands and ashes lttle correlaton exsts to obtan shearng or bearng capacty values. The results obtaned from layers of sand or ash wll ndcate much lower resstance to penetraton than for clay but do not ndcate correspondngly low bearng capactes. Experence n geotechncal engneerng s necessary to make relable nterpretaton of low DCP results n ashes and sand. The nformaton from the DCP soundng n blows per 50mm s plotted on a lnear scale wth depth and correlated f possble wth nformaton from boreholes or test pts to dentfy crtcal layers. Consstently hgh values n excess of 3 blows/50mm over 350-500mm can be nterpreted as a layer of materal that may provde good track support. Inconsstent values ndcate non homogeneous materal and the possblty of doubtful support. Rocky fll materal wll provde extreme varatons and needs to be nterpreted carefully. Page 20 of 22
CBR 100 10 1 0.1 1 10 Blows/50mm Page 21 of 22
Pr o j e c tno. :. DYNAMIC CONE PENETROMETER TEST.. Project:..... Date:... Feature:..... Tested by:... Locaton:..... Supervsor:. Hole No. Ground Level (m) Depth below ground level (m) Number of Blows per 50mm 0.000 +2.000 +4.000 0.000 +2.000 +4.000 0.000 +2.000 +4.000 0.000 +2.000 0.000-0.050 0.050-0.100 0.100-0.150 0.150-0.200 0.200-0.250 0.250-0.300 0.300-0.350 0.350-0.400 0.400-0.450 0.450-0.500 0.500-0.550 0.550-0.600 0.600-0.650 0.650-0.700 0.700-0.750 0.750-0.800 0.800-0.850 0.850-0.900 0.900-0.950 0.950-1.000 1.000-1.050 1.050-1.100 1.100-1.150 1.150-1.200 1.200-1.250 1.250-1.300 1.300-1.350 1.350-1.400 1.400-1.450 1.450-1.500 1.500-1.550 1.550-1.600 1.600-1.650 1.650-1.700 1.700-1.750 1.750-1.800 1.800-1.850 1.850-1.900 1.900-1.950 1.950-2.000 Page 22 of 22 +4.000