FASHE DUTSE KUSA DA KOGO: GAJERUN TSINKAYEN NE SUKA FI FUSKANTAR HAƊARI

A tsaunukan Vercors na kudu maso gabashin Faransa, kogon Choranche yana karɓar baƙi kusan 100,000 a kowace shekara. Taskarsa wani daji ne na soda straws (“tsinkayen shan lemo”): stalactites (duwatsun da ke rataye daga rufin kogo) masu rami a ciki, masu haske-haske, waɗanda ke girma zobe bayan zobe na calcite a kewayen ɗigon ruwan da ke rataye a ƙarshensu. Suna kai tsawon har mita 3 da faɗin kusan milimita 5, da bango mai kaurin rabin milimita. Da yake suna girma tsakanin milimita 0.09 da 0.92 a kowace shekara, mafi tsayinsu na iya fara samuwa shekaru 3,200 zuwa 33,000 da suka wuce.

Sai wani aikin gini ya zo. Ƙaramar hanyar ƙafa da ke rataye a gindin bangon dutsen tsauni, wadda ke kai baƙi zuwa kogon, dole ne a matsar da ita kusa da bangon dutsen a tsawon kusan mita 100. Manyan injuna ba za su iya kaiwa wurin ba, don haka dole ne a yanke dutsen da abubuwan fashewa (explosives) — a nisan ’yan mita goma-goma daga tsinkayen. Lalacewar irin waɗannan halittun dutse ba za a iya gyara ta ba.

Auna sautin stalactite

P. Bottelin da abokan aiki a ADRGT, Université Grenoble Alpes da wani kamfanin injiniyan ƙasa (geotechnical) na yankin, tare da kamfanin da ke gudanar da kogon wanda kuma ya ɗauki nauyin binciken, sun fara da ɗaukar kowane tsinke a matsayin abin da yake a zahiri ta fuskar makanikanci: siririn bututu da aka kafe a ƙarshe ɗaya. Irin wannan bututu yana girgiza a jerin mitocin halitta (natural frequencies) — “sautukansa” na kansa — waɗanda suka dogara da tsawonsa: duk yadda bututun ya fi gajarta, sautinsa yana ƙara sama.

Ƙungiyar ta tattara tsinkaye 15 da suka karye, yawancinsu daga ƙasan tafkin kogon, kuma ta auna, da na’urar laser mai auna girgiza (laser vibrometer), girgizar huɗu daga cikinsu a ɗakin gwaji, sannan ta tsinke ɗaya da har yanzu ke rataye a cikin kogon, inda ɗigon ruwan da ke ƙarshensa ya mayar da hasken laser. Ma’aunan sun yi daidai da tsarin lissafin littattafai na katako mai girgiza (vibrating beam), da tauri (Young’s modulus) na gigapascal 60. Tsinken da ke cikin kogon, wanda babu wanda zai iya kaiwa ya auna, ya yi ƙara a hertz 1.2 da kuma a wasu sautuka 13 masu ƙara sama har zuwa hertz 650: sautukan sun ba da tsawonsa, mita 1.78. Tsinkaye kusan ba sa rage girgizarsu: da zarar an sa su ƙara, suna ci gaba.

Gwaje-gwajen lanƙwasawa a kan samfurori 25 sun ba da matsin (stress) da tsinke ke karyewa a kansa: tsakanin megapascal 2.5 da 16.2. Ƙungiyar ta ɗauki iyaka ta taka-tsantsan ta megapascal 1.25, rabin mafi raunin ƙimar.

Gwajin fashe-fashe

Daga nan aka yi ƙananan fashe-fashen gwaji, har zuwa kilogram 0.2, waɗanda na’urorin ji (sensors) da aka kafa a bangon kogon suka naɗa. A kusan mita 55 daga na’urar jin da ke cikin kogon, fashewar kilogram 0.2 ta girgiza dutsen a kusan milimita 1 a cikin daƙiƙa ɗaya na tsawon kashi ɗaya cikin goma na daƙiƙa. Girgizarta ta baje daga hertz 10 zuwa 500, da ƙoli kusan hertz 175.

Sai wani samfurin kwamfuta na tsinkaye masu tsawon santimita 2.5 zuwa mita 3 ya sake kunna girgizar da aka naɗa. Sakamakon ya saɓa wa hankali. Mafi munin matsi ya auka wa tsinkaye masu tsawon kusan santimita 15, waɗanda babban sautinsu (fundamental note) ke daidai a hertz 175: suna yin amsa-kuwwa (resonance) da fashewar. Tsinkaye tsakanin santimita 10 da mita 1 su ne suka fi fuskantar barazana; masu tsayi fiye da haka, duk da sun fi nauyi, wahalarsu ta yi ƙasa. Amsa-kuwwa na iya ninka matsin har sau biyar.

Fasa dutse na aikin gini kan kunna caji (charges) da dama da tazarar ’yan millisecond goma-goma tsakaninsu, don hana raƙuman girgiza su haɗu su ƙaru. A cikin samfurin, caji goma masu tazarar millisecond 20 zuwa 30 kusan ba su ƙara ƙolin girgizar ba, daga kusan milimita 1 zuwa 1.3 a cikin daƙiƙa ɗaya, amma sun ninka matsin sau 3.5: jerin raƙuman yana da yawan mitoci masu sama (high frequencies), daidai waɗanda gajerun tsinkaye ke kamawa.

Iyakar gudu ga dutse

Ta hanyar ƙara girman girgizar har sai matsin ya kai iyakar taka-tsantsan, an sami wani ma’auni: milimita 2.4 a cikin daƙiƙa ɗaya a bangon kogon ga fashe-fashe masu jinkiri (delayed blasts), 8.6 ga fashewa ɗaya tak. Sai wata doka da aka daidaita da yadda girgiza ke dusashewa da nisa ta mayar da wannan zuwa iyakar cajin da aka yarda da shi a kowane nisa. An gudanar da fashe-fashen haƙar dutsen cikin wannan iyaka, kuma babu tsinken da ya lalace.

Batun girgizar ƙasa

Ƙungiyar ta kuma kwaikwayi wata girgizar ƙasa ta yanki, ta amfani da girgizar ƙasar Barcelonnette ta 2014, mai ƙarfin 5.2, wadda aka naɗa a kusan kilomita 10 daga cibiyarta (epicentre), a matsayin madadin girgizar ƙasar da ake ɗauka a matsayin ma’auni a yankin, ta 1962, wadda ba a taɓa naɗa ta ba. A can zurfin ƙarƙashin ƙasa, ana sa ran girgizar za ta ragu zuwa kusan kashi ɗaya cikin goma na ƙimarta a saman ƙasa. Girgizar ƙasa tana girgiza a hankali, ƙasa da kusan hertz 10, don haka a wannan karon tsinkaye masu tsawon mita 1 zuwa 2 ne ke yin amsa-kuwwa. Matsin ya kasance ƙasa da iyakar: irin wannan lamari mai yiwuwa ba zai karya su ba.

Darasin ya wuce Choranche. Hanyoyin da ke auna haɗarin stalactite daga ƙolin hanzari (peak acceleration) kaɗai, ba tare da la’akari da amsa-kuwwa ba, na iya raina haɗarin sosai — kuskure ga binciken da ke karanta tsoffin girgizar ƙasa daga halittun dutsen kogo da suka karye ko suke nan daram. Binciken, wanda aka wallafa a 2020 a cikin Journal of Seismology kuma yanzu aka sanya shi a arXiv, ya shafi tsinkaye masu cikakkiyar siffar bututu kawai; stalactites masu kauri, ginshiƙai (columns) da labulen dutse (draperies) kowanne zai buƙaci nasa samfurin.