Industrial Hazards Associated With Disposal of Municipal Waste in Underground Caverns

7/26/2019 Industrial Hazards Associated With Disposal of Municipal Waste in Underground Caverns

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A:IA YNE.IOY I CONERENCE ROCEEDING

Industrial hazards associated wih the isosal of mniciawaste in udergrond caverns

E. K S Passaris

Deparment of Civil Engieerng, Uiversity ofNewcaste upon Tyne, U

Introductio

The simplest method of disposing municipal waste has aways been the use of properymanaged and controled landl sites However, growing scarcity of suitable surface

landll sites and the requirements on the ong-term isolation of the polutants from theiosphere resulted in the development of underground disposa options.

he underground disposa of muicipal waste is goveed by severa standardrqurements which are uiversal to waste management, and are normaly appied as

asi good practice. To reduce irresponsile or ilega disposal of waste, the 1 UEnvironmental Protection Act intrduced responsibilities that relate to waste during itssorage, transport and nal disposa by imposing a duty of care on persons conceed

with controlld waste

Durng the design construction and operation of underground disposa faciities there is

a cear need to identi the industral hazards and estabish a risk assessmentmetodology that wll assess te safety f the waste disposa caves uch undergroundcaves are typically deveoped in sutae rocksalt formatons by employing soutioning techques that alow the constction of he required shape and dimensionsExapes of potential U storag caves in rocsalt are chacterized by an abiity toaccept mucipal waste at a rate ranging etween 1, to , tones per annumand a life that could extend typcaly to tee years.

For solution mined waste storae caverns the hazards are typicall liked with theeacing phase comprising the eaching of the caverns (incuding the eaching plant usedduring the cavern constrcton and the waste disposa phase comprising the waste

proessing plant operating during the lifetime of he storage cavern ince the hazardsand the safety aspects assocated with waste processing pants are general weldocumented this work prmar concentrates in examnng the detais petinent to theeachng phase and the rsk lked wth possble cave faures.

To assess the ikelihood of accidents the nown expeience for each possibe hazard isassessed the design precautons required to prevent accents re presented and ,in caseof a accident, the necessar measures to mze the consequences are indicat.

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Lechig of wste isposal cves

he U undegound salt cavns ae leaed in thickbedded salt fomations byinjecting wate that dissolves the ocksalt most cases the wate is taken om neaby

ives, pumped into the caven unde constuction tough a veical leacing stng andthe esulting bine is the utilized by cheical industies as a aw mateial fopocesses that equie sodium cloide supplies

A uncontolled leaching pocess will dissolve the salt on the entie intenal suface ofthe caven thus esulting in cave shapes tat could be unacceptable in tems of ocmechancs stuctual stability o pevent the uncontollable upwad development of acave use is made of a blanket medium hich is lighte than bine and does not dissolve

the salt. ypical blanket medium used in the development of suc undegound cavens iscompessed ai o liqueed petoleum gas (LPGas is a mixue of popane and butane).

he liquid blaket is stoed in low pessue vessels at suface stoage stations andpumped at high pessue into the uppe pa of the caven togh the annulus of thecemented casing and oute leacing sting. A inimum distance of 6 m om signicantpotential gas elease souces at the stoage station to the site bounday, safeguadsagainst seious impact of any sevee accidents that may happen at the stoage station.lthough tis 6 m distance satises the geneal pactice cuently in opeation inGemany, the possible cmbination of a accidents may cause damages beyond the site

bounday.

uing tesolution mning pocess the voume of the undegound caven i checked byemploying indiect ontinuou monitoing techniques Fuhemoe, a numbe of moe

accuate diect measuements employing sona tecques ae nomally pefomed duing the caven development phase hese measuements equie the inteution of theleacing opeations and the dismantling of the well head he withdawal of the blanket

takes place befoe the dismantle of the wel head and of couse it is e-injected pio tothe continuation of the leaching pocess

ing the leacing pocess, the geneal hazads that may inadvetently aect people andpopety outside the bondaies of the plant could be:

the elease and possible ignition ( oly ifLP-Gas is used) of the blaket medium; and the possible collapse of the caven cauing unacceptable subsidence of the gound

suaceStctual collapse of the caven is moe likely to take place duing the waste disposaopeations when te intenal cave could be as low as atmospheic, athe than duingthe leaching phase wen the intenal pessue is slightly ighe than the applied'hydostatic bine head.


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th cas fLPGas blant on should considr th possibility of its rlas om:

th low prssur tans, pump and pips at th storag station; th high prssur pump and piping; and th wll had of th cavrn.

In addition, as a cosqunc of possibl laag or rptur of th laching strings,blant may b rlasd into th sh watr or brin pips rsulting in thir possiblruptur

Rasons of possibl larg rlass could b quipmnt rptur associatd with:

t ovrprssur;xtrnal impact;

structurally wa pips or prssur vssls; or0 ronous opning of th systm (such as dismantling of th wll had during th

sonar survy of th cavrn).

Small rlass ar tyically associatd with

faild angs; ump sals; pacing tc

ny ras to th atmosphr of thLPGas blant will immdiatly vaporatrsulting in a ammabl gas cloud wch is havir than th ambint air. Th vaporatdgas miur will b disprsd by th jt ct of th rlas and by th subsqunt actionof gravity and th prvailing winds If igntd, th cloud will ithr burn or xplod if itcontains mor than a fw hundrd ilograms of blat

As mntiond arlir U practic is typid by th us of comprssd air blant (whichlimnats all th aformntiond riss) but th adoption ofLPGas blat in Grmanyhas so far producd vry ncouraging rsults, whr oly on accidntal rlas wasrcordd in a population of mor than twnty carns Ts paricular incidnt was thoutcom of an rronous opnng of a valv and in fact th rsulting gas cloud did notignit. On rhr ncouraging factor, is th vidnc that comlt loss of th lacingstrings has nvr tan plac in storag cavs in Dmar, Grmany, Franc and U(;indicating that laag through th lacing strings constituts a vry improbablpossbility.

To niz th ris of th accidtal rlas of th blant mdium th followingprcautions ar tan

Th w hads and th blat pips ar dsignd to1oprat at prssurs abo thmaximum oprating blant prssur.

Doubl protcton of th blant pips and tns against ovrprssur is normallyprovidd in th following sns. If th prssr ontrol valv fails to contrl th

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pssible pessue educin f blae an addiinal nline valve will be acivaedand if is fails as well a safey elief valve will elieve he vepessue heeducin f he envimenal isks all safey valves ae designed dischage in

he anks All pipe secins lcaed abve gund and can be shu ae euipped wih

hydsaic elief valves he vlumeic changes f he blanke edium ae cninuusly mnied duing

injecin and wihdawal peains and he assciaed anks ae peced againsvelling by high level alams and gh level ips designed shu h pipe

he pssibiliy f failue f he leaching sings, esuling in he leakage f highpessue blanke bine wae pipes, is pevened by w pessue ips clsing asping laded valve and check valves especively

All safey valves used in he safey aangemens ae secically designed as "failsafe

As pa f he adped sandad pcedues pi he dismanling f he cavewell head, leaching cnacs ae bliged duble check ha all blanke mediumhas been wihdawn and ha he cespnding pessue is elieved.

All suface insallains assciaed wih blanke peains ae peced againsexenal impac ypically linked mving vehcles impac

iniize he cnseuences f any accidens ha may happen he fllwing seps aeimplemened

If he blake piping pues he uaniy f he blanke eleased will be esiced small amuns y means f lw check valves a he anks In addiin, impan secinvalves will be shu when he sysem is n peaingl elecical insallains ae explsin pf educe any igniin pbabiliyAund he blake insallains a he sage sain e deecin nd e ghingsysems ae insalled faciliae emegency peains.

Strctral failre of the wate dipol cavern

In evaluaing he hazads assciaed wih he scual failue f a wase dispsal cavei is impan n nly assess he pbabiliies f he pssible failues bu als ideni he cnseuences f suchailues

Becuse he available infmain m failue f sal caves is ahe liied andpimaily f descipive naue heeical cnsideains and numeical calculainscupled wih specal ck mechanics ess us e adped ideni he assciaedfailue pbabiliies

1 ssible failue mdes

f he sss disibuin in he cksal mass suunding he wase dispsal caveexceeds cean liiing cndiins i culd inadveenly cause pa f he side

walls bm f he caven cllapse. Such scual failues may ake place duing


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the leacing phase, although more likel the are typicall associated with the igh stressconcentrations developing during the pumping out of the bine and the resuting depressuriation down to atmospheric pressure

Rocksalt is a material characteried by creep propeties and as sch is tpied b timedependent deformatons when subjected to stress dierences that exceed a cerain limitSince the waste disposal cave will be sbjected to atmospheric pressre, the internalpressre of the cave is signicantl lower than the stresses existing in the surounding

undisturbed geologic formations forcing the rocksat to creep into the cave Becauserocksalt has an ability to accept ve igh strains without failing, tis time-dependentcavern convergence is not considered as a stctural failre owever, if the cavernclosre reaches a level wich may cause unacc


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the ceep become exceve, the cloue ma n pncple develop nto an ntablteultng n he poble collape of par of the cave

If the cemetg between cang and alt bad o damaged, ga ma mgate upwad

fom the cave though the cementng untl an aea of good cementg eached wchwll top he penetaton of te ga. O f the ga encounte a permeable ol laebefoe eachng the ecent cementng, the thee a poblt of a ga lea ntogoundwate zone. On the othe hand, f cac hae developed n the alt mauoudng the caven, the ga wll peetate nto thee cac Howeve, than to theceep epoe of ocalt, uch cac ae not expected to each fa out fom the cave

wall and a a eult the conequence ae gncant

n condeng the poblt of leaage though the cang, leaage though cementedad potecton cag wll caue eleae of bne n the annulu to the uoundng ocfomaton whch ma eult n lght polluton of goundwate

Peventonthrough d gn

adop ato

The tablt degn of the caven baed on oc mechanc pcple and the ue ofnumecal tecque, uch a nte element of nte deence method to calculate thetee and dplacement n the oc ma uoundng the wate dpoal caven a eult of uch numecal calculaton, the dtace (cente t cente dtance) between

the caven detemned o that an pobe nteacto betwee caven wthn theacceptable lmt dctated b the adopted oc falue ctea

The mateal paamete ued n the numecal calculaton ae noall deved fom a

ee of pecall degned lboato tet and ae rthe veed b n tu tet(tpcall pumpng out tet dung the leachng phae) that ae conducted n the cavenunde contructon. The laboatoy tet c0pe the unaxal compeve tet thatdetemne the elatc contant, taxal compeon and exteon tet that detemne

the tength paamete and taxal ceep tet that detemne the ceep paamete ofthe vco-elatc model ued n the umecal anale.

Dung leachng the eult of the oluton Jnng poce ae _contnuoul aeed bmontong the bne ow and bne content and b egula caven hape meauement

ung ona tecnque n th wa, t poble t detect elatvel fat thedevelopment of an unntended caven hape th allowng the adopton of appopate

countemeaueA mentoned pevoul, n tu tet ae nomall pefomed dung leachng, wheebthe caven teal peue loweed, b pumpng out a pedetemned quantt ofbne. The eultng ceep cloue of the cav montoed and ued to ve theextent of the valdt of the model emploed n the degn calculaton.

The degn calculaton ae eentall povdng the ba fo etablng the opeatonof the cave ude atmophec ntenal eue. Unde uch condton, the tmedependent ceep cloue ate hould be kept below a pedeteed lmt, tpcall n theange of % pe annum If the cloue of th caven exceve, condeaton hould


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b givn to th possibilit of oprating th cav lld with brin rathr than mp.

Howvr such a solution is bound to hav nvironmntal iplications lid to thcration of pollutd brin and normall oprators tnd to avoid tis option.

In coction with possibl gas lakag thrugh cmnting or trough salt th wakstpar of th salt cavn is th ara around th casing sho o safgard against gaslakag through th salt at ts location oprators ar normall mplo a maximumalowabl casing sho prssur. his limit is normal st to approximatl 7% of thxtrnal gostatic prssur calculatd b using tpicall a gostatic gradint of25 kPm. Sinc th prssur of th gas associatd with th mthanognsis issigcantl lowr than th maxmum allowabl casing sho prssur thr appars to bno problm with rspct to gas lakag Futhrmor during th lacng phasprssur in css of th allowabl casing sho prssur is prvntd b high prssurswitchs shutting o watr and brin suppl. owvr during th itial blanktinction whn this is not sucint strict oprating procdurs ar followd to rduc

th haardsWith rspct to possibl lakag throgh th casings it is imporant to clari that thcmnting of th cmntd casing will to a crtain xtnt protct th casing againstxtrnal corrosion Futhrmor a cathodic protction sstm prvnts corrosion of thxal surfac of th casing whr th cmn coating is not covring compltl thpi h cathodic protction sstm will b cint down to th lvl of th rocksaltformation low that point protction is not cssa du to th high rsistivit of throcksal A corrosion inbitor additiv is introducd in th brin i th aulus btwn

th ir casing and cmntd casing thus prvnting intal corrosion of th cmntdcasing and xtrnal corrosion of th innr casing

3 tction ofossible

failurs

Most o th invstigatd risks capabl o causing signicant haards to th nviromntman or uipmnt ar possibl dtctabl ithr whil corroborativ conditions ar stilldvloping in wch cas prvntativ actions r normall activatd; or whn thacuall occur whrb immdiat action is tan

Cavrn instabilit cannot b dtctd immdial unlss signicant collaps of thcavrn rsults in masurabl surfac subsidnc. tction in tis cas will dpnd onth iunc that subsidnc will hav on th casings wllhads foundations of surfac

installations tc. gular lvlling of th surfa corrsponding to th ntir wastdisposal plant will provid imporant subsidnc information although cr should btan to dirntiat from ground movmnts associat b moistur contnt changs.

Possibl gas lakag through cmnting or tough salt can b dtctd indirctl via thrsulting incrasd gas contnt in ground watr

3 Minimiation of th consequences

T fasibl actions that a wast disposal cav oprator ma tak ar basd ontchicall possibl solutions but th spcic ations cannot b dtrmnd in advanc

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uess a reevant ata have been coeced so that the approprate contngenc pn canbe actvate. The na ecison s base not o on techca factors but on economccriteria as e an the opton of abanong a cave shoud not be dscared as it ma

turn out to be the on avaiabe souton. Detecton of cavern nstabt depenng on

the degree of ts severit ma actua require the ng of the cav ith a chemicainert materia prior to ts na abanonment.

Faiure of the nner casing above pacer eve norma requires the instation of pugsat the bottom remova of the inner casing repair or renea and subsequentreinstaation On the other han repair or renea of the cemente casing is verdicut an occasona technca mpossibe

4. Colusios

The potentia faures of aste sposa caverns and the associate unergroundinstaations are

comete coapse of a aste disposa cavern resuing from rocsat instabiities thadeveope into a progressve faiure;

excessive cosure of a cavern triggered b high eviatoric stresses that resuted increep eformation of sat;

gas eaage om the aste isposa cavern through cementing aroun the casing orthrough rocsat; and

gas eaage through the casings

Numerica methos are use to etermine the cavern shape and safe cavern distances thati revent instabtes an excessve cosure he the caverns are operated uneratmospherc nterna pressure. Practica exerience om storage caverns and theoreticaanases inicate that even f nstabiit occurs the consequences oud probab beimte

Adequate esgn and corrosion rotection revent probems associated ith thestrctura performance of the casings

Caverns an the associated instaations are monitored agaist possbe riss to assureear etection of possibe hazars. Furthermore aequate countermeasures are

incorporate n the contingenc pans to mimize the consequences of these hazars

Industrial Hazards Associated With Disposal of Municipal Waste in Underground Caverns

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