Non-Invasive Geophysical Investigation of Failure Along a Section of Ago-Iwoye Market Road, Ago-Iwoye, Southwestern Nigeria

GeoScience Enginee ing Vol. 69 (2023), No. 1 geoscience.cz pp. 11–20, ISSN 1802-5420 DOI 10.35180/gse-2023-0085 NON-INVASIVE GEOPHYSICAL INVESTIGATION OF FAILURE ALONG A SECTION OF AGO-IWOYE MARKET ROAD, AGO-IWOYE, SOUTHWESTERN NIGERIA Ganiyu Omo ola MOSURO1 , Oluwaseyi John DADA1, Ola eju Ola unji BAYEWU1 , Mo oo Olasunbo OLORUNTOLA2 , Oluwadamila e Oluwa obiloba ODUGBESAN1 1 Olabisi Onabanjo Uni e si y, Depa men o Ea h Sciences, Ogun, Nige ia 2 Uni e si y o Lagos, Depa men o Geosciences, Lagos, Nige ia E-mail: odugbesanolu[email p o ec ed] ABSTRACT A de ailed geophysical in es iga ion was ca ied ou o de e mine i he subsu ace geology is esponsible o he ailu es along his oad sec ion. This s udy aims o image and iden i y compe en , mode a ely compe en , and incompe en zones o he subsu ace soil and hus o ind ou i geology is esponsible o he ailu e along his sec ion o he oad. Elec ical Resis i i y Imaging (ERI) using he Wenne a ay 2D Imaging, and Ve ical Elec ical Sounding (VES) using he Schlumbe ge a ay we e adop ed o his su ey. The geoelec ic sec ion om he VES e ealed he p esence o 3 li hological laye s; opsoil, wea he ed laye (clay, sandy clay, clayey sand), and basemen , which is also ue o he elec ical esis i i y images e ealed by he 2D elec ical esis i i y imaging. The esul s showed ha he shallow subsu ace benea h he oad sec ion is mos ly unde lain wi h clay ha is geo echnically incompe en and no sui able o cons uc ion. The causes o oad ailu e a e belie ed o be he esul o he incompe en clayey opsoil and he wea he ed laye seen in he p o iles, which expand when abso bing wa e and sh ink when d ying, hus causing ins abili y benea h he pa emen due o he low shea s eng h and high comp essibili y, and also due o ac u ed basemen ocks a shallow dep hs. Keywo ds: Basemen ; Clay; Resis i i y; Road ailu e, Subsu ace; Topsoil. 1 INTRODUCTION Roads a e buil o connec di e en ci ies, owns, and e en communi ies. Road anspo a ion is one o he mos impo an and mos common modes o anspo a ion in Nige ia oday due o he poo s a e o al e na i e modes o anspo a ion in he coun y [1]. Road a ic depends on he pa e n o human se lemen s, accoun ing o mo e han 90 % o he con ibu ion o he sub sec o o he G oss Domes ic P oduc (GDP) [2]. I a oad does no p o ide i s equi ed use by no ha ing a smoo h su ace o ehicles o mo e on, ha oad ails. Pa emen ailu e has been a ibu ed o ac o s such as excessi e a ic load o unde es ima ion o a ic olume, design p oblems such as poo s uc u al design, he use o subs anda d cons uc ion ma e ials o une hical cons uc ion p ac ices [3]. O he ac o s include inadequa e in o ma ion on he unde lying soil laye s and he local subsu ace geology [4] exis ence o geological s uc u es such as ac u es and aul s, s eam channels, and shea zones [5]. This ailu e is no es ic ed o only one geological e ain, as i can occu in bo h he basemen complex and he sedimen a y e ain. The geophysics me hod o explo a ion has p o ed qui e ele an in oad and si e in es iga ions and se e al o hese enginee ing and geological p oblems ha e been success ully sol ed by geophysical me hods [6,7]. Acco ding o Kamil e al. [8], he e alua ion o exis ing oad s uc u es has a majo po en ial o he applica ion o he elec ical esis i i y me hod, as he e is an inc easing in e es in main aining and imp o ing he s anda d o se ice o he 12 GeoScience Enginee ing Vol. 69 (2023), No. 1 geoscience.cz pp. 11–20, ISSN 1802-5420 DOI 10.35180/gse-2023-0085 cu en oad ne wo k. Al hough he impo ance o he geophysical me hod in enginee ing geophysics explo a ion wo k has no been p o ed, i has also been conside ed ha ano he me hod o geoscience explo a ion app oach can se e as a supplemen o ga he in o ma ion on he condi ion o pa emen dep h and laye hickness, especially o a long con inuous pa emen assessmen , hus educing he numbe o co ing es s ha can cause damage o he pa emen s uc u e. A ypical example is he combina ion o he geophysical and geo echnical app oach o enginee ing p oblems [9,10,11]. Ebhohimen and Mamah [4], who wo ked on he geophysical in es iga ion o oad ailu e in he case o Opoji Nige ia using elec ical esis i i y, ound ou om he esis i i y da a ha he subsu ace, jus a shallow dep h, was unde lain wi h clays/shale and sands one, and he pseudo-sec ion was able o show ha he clays domina ed he uppe pa o he subsu ace and his was esponsible o he c acks and po holes on he oad. Ade emi and Adeola [12] ca ied ou a geophysical in es iga ion o he causes o oad ailu e along he Abadina communi y oad Uni e si y o Ibadan, Nige ia, using VES and he ailed segmen o he oad con ained opsoil, clay, a wea he ed basemen / esh basemen , and ha he ailu e o he oad a ose om he di e en ial se lemen o he subg ade clay due o high po osi y and e y low pe meabili y ha causes i o swell a e i abso bs wa e . The pu pose o his s udy is o app ecia e he impo ance o geophysical su eys in iden i ying compe en , mode a ely compe en , and incompe en zones, and hence o de e mine whe he he subsu ace geology is esponsible o he ailu e along his oad sec ion. The objec i es a e o de e mine he esis i i y and hickness o he subsu ace li hology, o deduce he subsu ace li hology, o classi y he subsu ace li hology in o compe en , mode a ely compe en , and incompe en zones, o sugges solu ions o mi iga e oad ailu e along his oad and o p e en u u e occu ence. 2 LOCATION OF STUDY AREA AND GEOLOGICAL SETTING The in es iga ed oad is wi hin Ago-Iwoye in sou hwes e n Nige ia (Figu e 1) and lies be ween he longi udes 3°53’60” E o 3°56’6.00” E and he la i udes 6°55’30” N o 6°57’00” N. The oad se es as a link be ween Olabisi Onabanjo Uni e si y and he main Ago-Iwoye ma ke . A he ime o he s udy, some majo c acks and po holes can be seen on he oad sec ions, which a e signs o oad ailu e. The s udy a ea is loca ed wi hin he opical ain o es egion o Wes A ica wi h a clima e cha ac e ized by al e na ing d y and we seasons wi h hea y ain all, high empe a u e, and humidi y. The mean annual ain all is be ween 1200 and 1500 mm, eaching peaks in he mon hs o June and July [13]. The we season occu s be ween Ma ch and Oc obe o ea ly No embe . The d y season is a he sho wi h e y ho days, which begins in la e No embe and con inues un il he end o Feb ua y. The elie is gen ly ugged, anging om low o high wi h an ele a ion g adien o abou 30 m, and has a dend i ic d ainage pa e n; su ace wa e low is in he no heas -sou hwes di ec ion. The P ecamb ian basemen ocks o Sou hwes e n Nige ia a e ound in he Dahomey basin, which consis s o banded gneisses, migma i e, and g ani e gneisses wi h low-g ade me asedimen a y and me a olcanic schis s uled by g ani es and cha nocki es o Pan A ican age [14]. Ago-Iwoye is in sou hwes e n Nige ia and con ains some o hese ocks (Figu e 2). G ani e gneiss and bio i e gneiss a e o en in uded by pegma i e eins and dykes. Olde g ani e, g anodio i es, and syeni es wi h dole i e dykes also o m pa o he P ecamb ian basemen o Nige ia. The c ys alline basemen ocks include bio i e-ho nblende gneiss, kyani e gneiss, migma i e gneiss, and g ani es a e well ac u ed [15]. 13 GeoScience Enginee ing Vol. 69 (2023), No. 1 geoscience.cz pp. 11–20, ISSN 1802-5420 DOI 10.35180/gse-2023-0085 Figu e 1. Base map o he s udy a ea showing he oad along which he s udy was ca ied ou Figu e 2. Geological map o Ago-Iwoye (adap ed om [16]) 3 METHODOLOGY In his s udy, Ve ical Elec ical Sounding and Elec ical Resis i i y Imaging (ERI) we e used o he su ey. The la e was used o ge he 2D esolu ion o he subsu ace because i p obes e ically downwa d and hen la e ally along he p o ile while he o me p obes only e ically downwa d, and can show he app oxima e hickness o he o e bu den seen in he 2D elec ical image go en om he ERI. The ins umen s used o his su ey a e he Ohmega esis i i y me e , 4 eels o cable and connec ing wi e, 4 elec odes, 4 hamme s, a handy GPS ecei e , ield no es and measu ing apes. The e ical elec ical sounding me hod en ails he es ablishmen o 10 VES poin s o a cu en sp ead (AB/2 = 100 m), while he ield p ocedu e o he 2D imaging su ey en ails he use o he Wenne a ay con igu a ion o es ablish a o al o 6 p o iles (Figu e 3), 3 o which we e 120 m long and he o he s 100 m in leng h wi h a leng h o 120 m and 100 m, espec i ely, o a 5-le el measu emen . 14 GeoScience Enginee ing Vol. 69 (2023), No. 1 geoscience.cz pp. 11–20, ISSN 1802-5420 DOI 10.35180/gse-2023-0085 The da a p ocessing o he e ical elec ical sounding me hod was achie ed by plo ing he ob ained esis i i y alues agains he hal -cu en elec ode spacing (AB/2) on a bi-loga i hm g aph. The ob ained cu e is hen manually ma ched wi h a Schlumbe ge mas e cu e o delinea e he numbe o laye s and es ima e he co esponding esis i i y and hickness o he laye s, a e which he geoelec ic pa ame e s we e modeled using compu e i e a ion on a WinResis p og am o ob ain he geoelec ic pa ame e s o he delinea ed laye s. Simila ly, he 2D appa en esis i i y da a we e p ocessed and in e ed concu en ly using he RES2DINV in e sion code [17]. The RES2DINV p og am uses a non-linea op imiza ion echnique ha au oma ically de e mines he in e se model o he 2D esis i i y dis ibu ion o he subsu ace o he measu ed appa en esis i i y da a [18]. 4 RESULTS AND DISCUSSION 4.1 Ve ical Elec ical Sounding (VES) Resul s The esul s o he VES in e sion p oduced he geoelec ic pa ame e s in he o m o laye esis i i y, dep h, hickness, and li hology, and a e p esen ed in Table 1 below. The geology o he s udy loca ion, which is a basemen e ain, aided in he in e ence o he li hology o he s udy a ea o be opsoil, wea he ed ma e ials (clay, sandy clay, clayey sand, sand), and basemen ock, which helped o classi y he subsu ace in o compe en , mode a ely compe en , and incompe en zones. The cu e ypes ob ained in he a ea a e H, HA, and KH, which means he subsu ace is h ee o ou laye s, he opsoil and wea he ed laye a e composed o clay, sandy clay, and clayey sand, wi h clay being he p edominan ma e ial while he basemen ock is esh, wea he ed, o ac u ed. The esis i i y o he opsoil a ies om 17.8 o 197 Ωm and has an a e age hickness o 0.85 m, he wea he ed laye has a esis i i y ange o 9.3 o 50.1 Ωm, and he basemen ock has a esis i i y ange o 603.3 o 7710.6 Ωm. A summa y o he VES esul s can be seen in Table 1, and a c oss sec ion o he six VES poin s on he p o iles can also be seen in Figu e 3. I can be seen ha he esis i i y o he opsoil and he wea he ed laye o all he VES poin s a e <200 Ωm and he oads a e in poo o bad condi ions. VES 1, 3, 4, and 5 ha e opsoil esis i i y alues be ween 17.8 and 109 Ωm, which co esponds o clay and sandy clay soils which a e incompe en soils ha should no be used o cons uc ion. While VES 2 and 6 ha e esis i i y alues o 165 and 197 Ωm, which co esponds o mo e compe en clayey sand, he laye is no hick enough (0.5–1.4 m), and also he wea he ed laye benea h all he VES poin s is illed wi h clay (9.3–38.9 Ωm), he e y low esis i i y means he conduc i i y is highe , showing ha he clay con ains a high mois u e con en . This low esis i i y is p oo ha soil is no ideal o enginee ing cons uc ion due o he absence o d ainage, low shea s eng h, and high comp essibili y. F om li hology (Table 1), i can be in e ed ha clay <100 Ωm is incompe en , sandy clay is mode a ely compe en , clayey sand is compe en , and he basemen is compe en when no ac u ed and p esen a a shallow dep h. Figu e 3. Geoelec ic sec ion o VES poin s on p o ile showing dep h, esis i i y and he co ela ion o li hology along he a e se 15 GeoScience Enginee ing Vol. 69 (2023), No. 1 geoscience.cz pp. 11–20, ISSN 1802-5420 DOI 10.35180/gse-2023-0085 Table 1. Geoelec ic pa ame e s o he Ve ical Elec ical Sounding poin s VES No Resis i i y (Ωm) Thickness (m) Dep h (m) Cu e ype Li hology 1 109.7 0.8 0.8 HA Topsoil 50.1 5 5.7 Clay 649 13 18.7 Wea he ed basemen 6380 F esh Basemen 2 165 0.6 0.6 H Topsoil 16 3.7 4.2 Clay 476.5 F esh Basemen 3 27.2 0.8 0.8 H Topsoil 9.3 6.5 7.3 Clay 7710.6 F esh Basemen 4 90 0.5 0.5 H Topsoil 22.7 9.5 10 Clay 725.8 Wea he ed basemen 5 17.8 0.8 0.8 H Topsoil 12.2 9.7 10.4 Clay 603.3 Wea he ed basemen 6 197 1.4 1.4 H Topsoil 38.7 5.6 7 Clay 6724.3 F esh Basemen 7 97.1 1.1 1.1 HA Topsoil 31.6 17.8 19 Clay 149.9 27.3 46.2 Sandy Clay 2246.6 F ac u ed Basemen 8 68.4 1.1 1.1 HA Topsoil 37.4 0.9 2 Clay 101.5 7.5 9.6 Sandy Clay 1782.9 F ac u ed Basemen 9 87.3 0.7 0.7 KH Topsoil 149.9 1 1.8 Sandy Clay 32.1 6.9 8.7 Clay 7709.4 F esh Basemen 10 91.7 0.7 0.7 KH Topsoil 184.7 1.8 2.5 Clayey Sand 49.3 6.8 9.2 Clay 1523.1 F esh Basemen 4.2 Elec ical Resis i i y Imaging (ERI) Resul s The da a we e collec ed in o de o au oma ically gene a e a wo-dimensional 2D esis i i y image o he subsu ace ha can be e e ed o as he ‘Elec ical Image’. These alues ob ained we e used o he quali a i e in e p e a ion o he p o iles. The essence o he da a collec ed om hese p o iles was o unde s and he subsu ace geology in he s udy a ea and o deduce how i a ec s he pa emen abo e i . In e p e a ion o p o ile 1: bad oad po ion; he elec ical image shows ha he s udy a ea e eals he p esence o 3 li hological laye s co esponding o he opsoil, he wea he ed laye , and he basemen in VES1 (Figu e 4). The RMS e o is 4.1 %, and he elec ical image shows shallow o nea -su ace laye s, he esis i i y o which is be ween 17 o 120 Ωm and a hickness o abou 4.5 m, which shows ha he opsoil down o he wea he ed laye is clayey o sandy clay. I is qui e simila o VES1 and no compe en o enginee ing cons uc ion because he e would be a di e en ial se lemen o he sub-base and sub-g ade, which will lead o ailu e. The e a e also low esis i i y a eas be ween high esis i i y, which is diagnos ic o a shallow ac u e/ aul p esen a he sou he n and middle pa o he p o ile, which could also ha e con ibu ed o he ailed sec ion. In e p e a ion o p o ile 2: bad oad condi ion; he elec ical image shows ha he s udy a ea e eals he p esence o 3 li hological laye s co esponding o he opsoil, he wea he ed laye , and he basemen in VES 2 (Figu e 5). The elec ical image shows shallow o nea -su ace laye s, he esis i i y o which is be ween 10 and 200 Ωm wi h 16 GeoScience Enginee ing Vol. 69 (2023), No. 1 geoscience.cz pp. 11–20, ISSN 1802-5420 DOI 10.35180/gse-2023-0085 a hickness o abou 7 m, below he e a e laye s wi h highe esis i i y, which means he en i e nea -su ace is mos ly clayey in na u e, which is no compe en ma e ial o oad pa emen cons uc ion. A po ion o he p o ile has a esis i i y be ween 10 and 60 Ωm, indica ing a highly sa u a ed zone wi h high mois u e con en , as clay swells upon abso bing wa e and sh inks on d ying, which is likely esponsible o he po holes seen in his oad sec ion. In e p e a ion o p o ile 3: bad oad condi ion; he elec ical image shows ha he s udy a ea e eals he p esence o 3 li hological laye s co esponding o he opsoil, he wea he ed laye , and he basemen in VES 3 (Figu e 6). The elec ical image shows shallow o nea -su ace laye s, he esis i i y o which is be ween 9 and 100 Ωm wi h a hickness o app oxima ely 4.5 m, which means he en i e p o ile is clayey. In he p esence o clay, he pe meabili y o a geo echnical ma e ial dec eases [19] as i can e ain wa e . The mid- egion o he p o ile has a esis i i y be ween 10 and 25 Ωm, indica ing a highly sa u a ed zone wi h high mois u e con en . The da kes pa o he middle po ion is wi hin he wea he ed laye and has a e y low esis i i y o <10 Ωm indica ing a wea he ed laye o VES 3. This low- esis i i y soil is likely esponsible o he bad s a e o he oad sec ion. In e p e a ion o p o ile 4: bad oad condi ion; he elec ical image shows ha he s udy a ea e eals he p esence o 3 li hological laye s co esponding o he opsoil, he wea he ed laye , and he basemen in VES4 (Figu e 7). The elec ical image shows shallow o nea -su ace laye s, he esis i i y o which is be ween 12 and 100 Ωm wi h a hickness o abou 5 m, which means he en i e nea -su ace is clayey. The da k blue po ions o he p o ile ha e esis i i y <30 Ωm, which is a highly sa u a ed zone ha occu s a di e en dep hs along a la e al dis ance o 0 o 100 m o he a e se. I may ha e helped o di e en ially se le he subg ade and hen he subsequen ailu e o he oad sec ion [20, 21]. The lowe middle po ion is wi hin he wea he ed laye and has a e y low esis i i y be ween 20 and 30 Ωm, which co esponds o he wea he ed laye o VES4. A la ge po hole was p esen a he ime o he su ey; o he sec ions o his oad will soon ollow as ehicles con inue o ply he oad. In e p e a ion o p o ile 5: ai oad condi ion; he elec ical image o his p o ile e eals he p esence o 3 li hological laye s co esponding o he opsoil, he wea he ed laye , and he basemen , as in VES 5 (Figu e 8). The image shows shallow o nea -su ace laye s, he esis i i y o which is be ween 8 and 200 Ωm wi h a hickness o abou 10 m, which means he en i e nea -su ace is clayey. The deep blue po ions o he p o ile ha e esis i i y <50 Ωm, indica ing a highly sa u a ed zone wi h high mois u e con en ha may be esponsible o oad condi ions because he e is a di e en ial se lemen as ehicles mo e o e hem. Al hough his oad is no as bad as he o he s, he e a e signs all o e he pa emen ha i will ail soon. In e p e a ion o p o ile 6: bad oad po ion; he elec ical image shows ha he s udy a ea e eals he p esence o 3 li hological laye s co esponding o he opsoil, he wea he ed laye , and he basemen in VES 6 (Figu e 9). The elec ical image shows shallow o nea -su ace laye s, he esis i i y o which is be ween 23 and 250 Ωm and a hickness o abou 10 m, which shows ha he i s ew me e s a e mos ly clayey wi h a small amoun o clayey sand. I is no compe en o enginee ing cons uc ion because he e would be di e en ial se lemen o he sub- base and sub-g ade, which could lead o ailu e and may be esponsible o he bad s a e o he oad sec ion. Figu e 4. A 2D elec ical esis i i y image o p o ile 1 17 GeoScience Enginee ing Vol. 69 (2023), No. 1 geoscience.cz pp. 11–20, ISSN 1802-5420 DOI 10.35180/gse-2023-0085 Figu e 5. A 2D elec ical esis i i y image o p o ile 2 Figu e 6. A 2D elec ical esis i i y image o p o ile 3 Figu e 7. A 2D elec ical esis i i y image o p o ile 4 18 GeoScience Enginee ing Vol. 69 (2023), No. 1 geoscience.cz pp. 11–20, ISSN 1802-5420 DOI 10.35180/gse-2023-0085 Figu e 8. A 2D elec ical esis i i y image o p o ile 5 Figu e 9. A 2D elec ical esis i i y image o p o ile 6 5 CONCLUSION A geophysical in es iga ion was ca ied ou using he elec ical esis i i y me hod (2D ERI and VES) o de e mine whe he he geology benea h he subsu ace is esponsible o he incessan ailu e along a sec ion o he Ago- Iwoye ma ke oad. The elec ical image and he VES c oss sec ion indica ed ha he subsoil benea h he pa emen is cha ac e ised by low esis i i y soils as bo h sub-base and sub-g ade and a e hus geo echnically incompe en . The low- esis i i y soils a e clay, being highly sa u a ed a some loca ions along he a e se, sandy clay, and clayey sand, which is mo e compe en han he o he s bu was no hick enough whe e p esen . The esul ob ained a he end o his geophysical su ey has helped o de e mine he esis i i y and hicknesses o he unde lying laye s a he subsu ace. I aided in in e ing he subsu ace li hology, classi ying hem in o compe en , mode a ely compe en , o incompe en laye s, and has e ealed ha clay is he p edominan soil in he opsoil and he wea he ed laye . This clay could swell upon abso bing mois u e (wa e ) and sh ink upon d ying, hus causing ins abili y benea h he pa emen as ehicles mo e due o he low shea s eng h and high comp essibili y. Also, he p esence o shallow aul s/ ac u es could be esponsible o he oad ailu e. 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