한국지반공학회논문집제 35 권 1 호 2019 년 1 월 pp. 5 ~ 15 JOURNAL OF THE KOREAN GEOTECHNICAL SOCIETY Vol.35, No.1, January 2019 pp. 5 ~ 15 ISSN 1229-2427 (Print) ISSN 2288-646X (Online) https://doi.org/10.7843/kgs.2019.35.1.5 제주도북동부및북서부현무암반의변형계수추정 Estimation of Deformation Modulus of Basaltic Rock Masses in Northeastern and Northwestern Jeju Island 양순보 1 Yang, Soon-Bo 부상필 2 Boo, Sang-Pil Abstract In this study, the in situ deformation moduli, which were measured by borehole loading tests at basaltic rock masses located in the northeastern onshore and offshore and the northwestern onshore of Jeju Island, were examined in relation to RQD and RMR. The measured deformation moduli were also compared with the estimated deformation moduli from conventional empirical formulas using RQD and RMR. In addition, the measured deformation moduli were analyzed with respect to both the velocity ratio (V P/V S) and dynamic Poisson s ratio, which were obtained from the elastic wave velocities measured by velocity logging tests. As results, with only RQD, it was inappropriate to evaluate the quality of the Jeju island basaltic rock masses, which are characterized by vesicular structures, to select a measurement method of in situ deformation moduli, and to estimate the deformation moduli. On the other hand, it was desirable to evaluate the quality of the Jeju Island basaltic rock masses, and to estimate the deformation moduli by using RMR. The conventional empirical formulas using RMR overestimated the deformation moduli of the Jeju Island basaltic rock masses. There was qualitative consistency in the relation between velocity ratio and deformation moduli. To estimate appropriately the deformation moduli of the Jeju Island basaltic rock masses, empirical formulas were proposed as the function of RMR and velocity ratio, respectively. 요 지 본연구에서는제주도북동부육 해상및북서부육상에위치한현무암반에대한공내재하시험으로부터계측된변형계수와 RQD 및 RMR의관계를각각살펴보았으며, 기존의경험식들로부터추정된변형계수값과비교 분석하였다. 뿐만아니라, 속도검층시험을통하여산정된탄성파속도비및동적포아송비와변형계수의관계에대해서도각각살펴보았다. 그결과, 다공성구조및층상구조가특징인제주도현무암반의경우, RQD 값만을이용한암반등급의결정, 변형계수계측방법의선정및변형계수의추정은부적절하며, 최소한 RMR을통하여암반등급을결정하고, 변형계수를추정하는것이바람직하다는것을알수있었다. RMR을이용한기존의변형계수추정식은제주도현무암반의변형계수에비해큰값을예측하는경향을보였으며, 탄성파속도비및동적포아송비와변형계수는서로정성적인특성이일치하는관계에있었다. 그리고, 적절한제주도현무암반의변형계수를추정하기위한 RMR과탄성파속도비를이용한변형계수추정식을각각제시하였다. 1 정회원, 일본국립연구개발법인해상 항만 항공기술연구소항만공항기술연구소연구관 (Member, Researcher, Port and Airport Research Institute, National Institute of Maritime, Port and Aviation Technology, Japan, Tel: +81-46-844-5054, Fax: +81-46-844-4577, yan-s2@pari.go.jp, beeo17@hotmail.com, Corresponding author, 교신저자 ) 2 정회원, 연웅 E&C 전무 (Member, Executive Director, Yonung Engineering & Construction) * 본논문에대한토의를원하는회원은 2019년 7월 31일까지그내용을학회로보내주시기바랍니다. 저자의검토내용과함께논문집에게재하여드립니다. Copyright 2019 by the Korean Geotechnical Society This is an Open-Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited. 제주도북동부및북서부현무암반의변형계수추정 5
Keywords : Jeju Island, Basaltic rock mass, Borehole loading test, Deformation modulus, RQD, RMR, Velocity logging, Velocity ratio, Dynamic Poisson s ratio 1. 서론제주도는약 200만년전부터역사시대까지발생한여러차례의화산활동에의해형성된섬으로 (Yoon and Ko, 2011), 제주도의화산암은다공성구조인현무암과같은특수한지질학적조건을갖고있으며, 지역마다다양한지질특성을보이고있다. 그리고제주도화산암석의물리 역학적특성과관련하여제주도내의다양한지역에서채취된암석에대하여연구가이루어져왔으며 (Kim and Choi, 1991; Eum, 2002; Kim, 2006; Nam et al., 2008a, 2008b; Nam et al., 2009; Cho et al., 2009; Moon et al., 2014), 기존연구의종합적인비교분석을통하여, 제주도화산암석의물리 역학적특성또한지역마다차이를나타내고있으며, 다공성구조의특성을나타내는파라미터인흡수율및유효공극률에대한겉보기비중의관계가화산암석의일축압축강도는물론점착력과내부마찰각과같은강도정수를지배하고있음이밝혀졌다 (Yang, 2014, 2015a, 2015b, 2016; Yang and Sassa, 2016, 2017). 뿐만아니라, 제주도기반암은육지부와달리화산암층사이에연약층인화산쇄설물및공동이불규칙하게발달된층상구조로이루어져있으며, 이로인해이전부터대규모토목공사및지하수개발에있어서수많은시행착오를경험하고있다 (Kim, 2006). 그리고, 최근에건설되고있는육상및해상풍력발전시스템의기초공사와향후건설예정인대규모의토목공사에대비하여제주도화산암반에대한공학적관점에서의물리적및역학적특성에대한심도깊은연구가요구되고있는실정이다. 암반층에대하여말뚝기초의수평하중에대한안정성을검토할시사용되는 p-y곡선법의수평지반반력의산정에있어서, 공내재하시험 (Borehole loading test) 등에의해계측된변형계수를사용할것을권장하고있으며, 그렇지않은경우기존의경험식을통하여변형계수를추정하도록되어있다. 그리고변형계수를추정하기위한경험식으로서, RQD(Rock Quality Designation), RMR(Rock Mass Rating), GSI(Geological Strength Index), Q 값등암반의등급을분류하는데사용되는파라미터를이용한경험식들이국내 외에서많이제안되어있으 나 (Deere et al., 1967; Bieniawski, 1978; Serafim and Pereira, 1983; Kim, 1993; Aydan et al., 1997; Gokceoglu et al., 2003; Hoek and Diederichs, 2006; Barton, 2007; Chun et al., 2009; Ahn, 2011), 현재까지제주도화산암반에대한변형계수를추정하기위한노력은전무하였다. 변형계수를추정하기위한경험식의파마리터로서사용되는 RQD는시추장비의선정및시추방법상의문제때문에실제의현장조건을고려한값의결정이어렵고 (Koo et al., 1994), Bieniawski(1978) 에의하면, RQD 는불연속면의방향성및연속성등의암반의조건을나타내는데한계를갖고있으며, 암반의변형계수를결정하는데있어서적절하지않다고보고하고있다. 그럼에도불구하고 RQD는현재까지여러현장에서변형계수를추정하기위해여전히사용되고있으며, 특히, 다공성및층상구조가특징인제주도화산암반의경우 RQD만으로암반의등급을결정하는것은부적절하다고판단된다. 한편, RMR 암반분류법은 6가지의변수 ( 암석의일축압축강도, RQD, 불연속면의간격, 불연속면의상태, 지하수상태, 불연속면의방향성 ) 에각각할당된점수의합으로산정되는 RMR 값을근거로암반의상태를 5등급으로분류하는방법 (Bieniawski, 1989) 으로, 국내에서는암반사면과암반내터널구조물설계및안정해석에주로적용되고있다. 그러나 RMR 암반분류법은기술자및전문가의개괄적인추정과경험적인지식및주관적인판단에따라각각의변수에할당된점수를결정하는사례가많고, 그에따른 RMR 값의편차가발생하기쉽기때문에 RMR 분류법은불확정성에서자유로울수없다 (Koo et al., 1994). 물리검층및물리탐사기법중에서속도검층 (Velocity logging) 또는탄성파탐사 (Seismic survey) 를통해서측정되는탄성파속도는암반을포함한지반상태의평가및동적특성등을파악하기위해지반조사에서활발히활용되고있으며 (Sunwoo et al., 2001; Kwon, 2006; Leem et al., 2009), 특히, 본연구에서다루어질탄성파속도비 (V P /V S ; V P : P파속도, V S : S파속도 ) 는암석의성분및암반구조의상태파악, 그리고간극안에존재하는유체의예측등다양한목적으로사용되고있다. 또한, 6 한국지반공학회논문집제 35 권제 1 호
탄성파속도비는간극률, 압밀정도, 불연속면에충진된세립자의양, 간극구조, 간극안의유체등의영향을받으며, 동적포아송비 (Dynamic Poisson s ratio: ν d ) 와관련이있다 (Barton, 2007). 본연구에서는제주도북동부육 해상및북서부육상에위치한현무암반에대하여공내재하시험을통하여계측된변형계수와 RQD 및 RMR의각각의관계는물론, 기존의경험식들로부터추정된변형계수값과비교 분석하였다. 뿐만아니라, 속도검층시험으로부터측정된탄성파속도로부터산정된탄성파속도비및동적포아송비와변형계수의관계를각각살펴보았으며, 적절한제주도현무암반의변형계수를추정하기위하여위의각각의파라미터에대한변형계수추정식을제안하였다. 2. 현장시험 2.1 공내재하시험 (Borehole loading test) 암반의변형특성은암반을구성하는암석의성질, 암반의불연속면, 응력상태및지하수에의한간극수압등에의해좌우되며, 공내수평재하시험은시추공벽면의원주전체또는일부에하중을가하여가압하중에따른공벽면의변형량을측정함으로서강도및변형특성을측정하는방법으로, 재하방식에따라등분포하중방식과등분포변위방식으로크게나뉜다. 등분포하중방식은시추공내에팽창성고무튜브를삽입하고액체또는기체에의하여시추공의공벽면에하중을가하는방법으로, 장비로는 Elastmeter, Pressuremeter 가있으며, 풍화암또는연암정도의암반에주로사용된다. 한편, 등분포변위방식은강재재하판을장착한프루브 (Prove) 를시추 Fig. 1. Location of the boring areas 공안에삽입하고, 유압을이용하여재하판을공벽면에작용시켜하중을가하는방법으로, 장비로는 Goodman jack 또는 KKT가있으며, 등분포하중방식보다큰하중을작용시킬수있기때문에경암정도의암반에주로사용되고있다. 본연구에서는 Fig. 1에나타낸제주도북동부육 해상및북서부월령에위치한현무암반의시추공내에서 Elastmeter-2(Oyo Corp., Japan) 와 Goodman jack을이용하여각각계측된변형계수를대상으로분석하고자한다. 2.2 속도검층 (Velocity logging) 속도검층이란지반내에서탄성파가전달되는속도, 즉, 탄성파속도의심도방향분포를측정하는기법을말한다. 탄성파속도는토목및환경분야의지반조사에서많이측정되고있으며, 연약지반의평가, 암반의분류, 연안지역매립정도의평가, 지반의굴착난이도평가및지반의동적특성의파악등에활용되고있으며, 다양한시험법이제안되어있다 (Jo et al., 2006). 현장에서흔히사용되고있는시험법으로는다운홀시험 (Down-hole seismic test) 과부유형 PS 속도검층 (Suspension-PS velocity logging) 이있으며, 이밖의업홀 (Up-hole) 시험, 크로스홀 (Cross-hole) 시험에비하여시험수행이비교적간단하고수월하며, 장소의제약을받지않는다. 다운홀시험은 1개의시추공이필요하며, 시추공부근의지표면에서탄성파를송신하고시추공안에설치된수신기의심도를변화시키면서탄성파를수신하여구간속도를계산하는방법으로, 측정심도가 30m 이하일경우와지하수위가없을경우측정값의정밀도가높다. 한편, 부유형 PS 속도검층은시추공안에설치된송 수신기가부착된프루브 (Prove) 를이용하여탄성파속도를측정하는방법으로, 심도에대한제약은없지만, 지하수위가존재하는구간에대해서만측정이가능하다. 그리고송수신기사이의거리가짧고, 고주파의음원을발생하기때문에, 이밖의속도검층시험에비해수직분해능이높고, 신뢰도가높다 (Jo et al., 2006). 본연구에서는다운홀시험 (Data logger: McSeis-Sx (Oyo Corp., Japan); Geophone: New borehole pick 3040 (Oyo Corp., Japan)) 과부유형 PS 속도검층시험 (Data logger: Geologger-3(Oyo Corp., Japan); Prove: Model- 3302A(Oyo Corp., Japan)) 을통하여각각측정된탄성파속도를이용하였다. 제주도북동부및북서부현무암반의변형계수추정 7
3. 제주도현무암층에대한탄성파속도분석본장에서는제주도북동부육 해상의현무암반에대한물리 역학적특성을파악하기위해실시된시추조사로부터얻어진결과를중심으로논하고자한다. 시추조사의구체적인내용은다음과같다. 각시추공에서채취한암편을통하여 RQD를산정하였으며, 채취된일부암편에대하여물성시험및일축압축강도시험등을실시하였다. 그리고 RQD, 일축압축강도및불연속면의특성등을통하여 RMR을산정하였다. 뿐만아니라, 각시추공에대하여공내재하시험및속도검층시험을수행하였으며, 육상에위치한시추공에서는다운홀시험, 해상의시추공에서는부유형 PS 속도검층시험을실시하였다. 한편, 제주도북동부육 해상및북서부월령에서채취한암석은박편현미경분석과 X-선회절분석을통하여현무암에해당하였다 (Yang, 2014, 2015a). 제주도북동부육상의시추공 JB-01에서는다운홀시험과부유형 PS 속도검층시험으로부터각각측정된탄성파속도의특성을비교 분석하기위해, 두속도검층시험을각각실시하였으며, Fig. 2에시추공 JB-01에서각각의속도검층시험으로부터측정된탄성파속도와그속도비 (V P /V S ) 는물론, RQD 및지질조사로부터얻어진암층에대한정보를심도별로각각나타내었다. 그리고다운홀시험으로부터측정된탄성파속도의경 Down-hole 우, 지층별탄성파속도또한함께나타내었다. 각각의속도검층시험방법에의한탄성파속도의심도분포를통하여, 심도가약 14m까지의시추공주변의암반은약 4개의암층 (1~3m, 3~7m, 8~10m, 11~14m) 으로이루어져있으며, 이는지질주상도를통해서도확인할수있다. 한편, 각각의속도검층시험으로부터얻어진탄성파속도비 (V P/V S) 의경우, 탄성파속도의크기및암층에따라크게달라지고있으며, 탄성파속도가큰경암층에서는탄성파속도가작은연암층에비해탄성파속도비 (V P/V S) 가작게나타나고있음을알수있다. 이러한결과는, 불연속면이적은경암반에서는불연속면이많은연암반에비해평균적으로작은탄성파속도비 (V P/V S) 를나타낸다고보고하고있는 Barton (2007) 의결과와정성적으로일치하고있다. Fig. 3은시추공 JB-01 에서다운홀시험과부유형 PS 속도검층시험으로부터각각측정된 P파속도 (V P) 와 S 파속도 (V S ) 의관계를나타내고있다. 각각의속도검층시험에의해측정된 P파속도와 S파속도는서로선형관계에있으며, 부유형 PS 속도검층시험으로부터측정된탄성파속도는다운홀시험에의한것보다큰값을나타내고있음을알수있다. 이러한속도검층시험방법에따른탄성파속도의차이는널리알려져있으며, 그원인으로는 1) 사용하는송신주파수와송수신기배열간격차이로인해측정되는매질규모의차이, 2) 각탐사 Suspension PS Fig. 2. Depth distributions of elastic wave velocities, velocity ratios and RQD obtained at the borehole JB-01 8 한국지반공학회논문집제 35 권제 1 호
에쓰이는측정장치의오차와자료분석시에발생할수있는부정확성, 3) 두탐사방법에서이용되는파의서로다른전파특성등크게 3가지이유를들수있다 (Stewart et al., 1984; Jo et al., 2006). 그리고각각의속도검층시험에의해얻어진 S파속도에대한 P파의속도 (Fig. 3 그래프상의기울기 ), 즉, 탄성파속도비 (V P /V S ) 는대체적으로일직선상에놓여있으며, 비슷한값을갖고있음을알수있다. 위의두속도검층시험에의해각각얻어진탄성파속도비 (V P/V S) 에있어서유의한차이가있는지를확인하기위해 t 테스트를수행하였으며, 이에대한결과를 Fig. 4에나타내었다. Fig. 4에의하면, 각각의탄성파속도비 (V P/V S) 의평균및표준편차는거의비슷한값을보이고있으며, 유의한차이가없음을알수있다 (p>0.05). 이러한결과로부터, 탄성파속도비는속도검층시험방법이다르더라도공통적인파라미터로서사용할수있다는것을알수있다. 시추공 JB-01에서다운홀시험과부유형 PS 속도검층시험으로부터각각얻어진탄성파속도비 (V p/v s) 와 RQD의관계및각각의선형근사관계를 Fig. 5에, 그리고각각의선형근사의절편과기울기에대한 t테스트결과 를 Table 1에나타내었다. 각각의탄성파속도비 (V P/V S) 와 RQD는음 (-) 의선형관계에있으며, RQD값이증가함에따라각각의탄성파속도비 (V P /V S ) 는유의하게감소하고있다 (p<0.001). 이는 Fig. 2에서언급한탄성파속도비 (V P /V S ) 의특성과정성적으로일치하고있으며, Barton(2007) 에의하면, 평균적으로탄성파속도비 (V P /V S ) 가작은암반에서 Q 분류법에의해산정된 Q 값이크다 Fig. 4. Comparison between the velocity ratios (V P/V S) obtained from Down-hole seismic test and Suspension PS logging test at the borehole JB-01 Fig. 3. Relation between P-wave velocity (V P) and S-wave velocity (V S) obtained from Down-hole seismic test and Suspension PS logging test at the borehole JB-01 Fig. 5. Relation between velocity ratio (V P/V S) and RQD at the borehole JB-01 Table 1. Results of t tests for Y-intercepts and slopes of the linear approximations between velocity ratio (V P/V S) and RQD, which were indicated in Fig. 5 Y-intercept Slope N Mean SD T value DF p Mean SD T value DF p Down-hole 14 2.129 0.139-0.00281 0.00213-0.871 19.599 0.394 1.225 19.921 0.235 Suspension PS 12 2.188 0.194-0.00405 0.0029 제주도북동부및북서부현무암반의변형계수추정 9
4.1 변형계수와 RQD 의관계 Fig. 6. Relation between velocity ratio (V P/V S) and RQD 제주도북동부육 해상, 북서부월령및제주도도내의항만공사에서실시된시추조사에서심도에따라시추된일부무결암에대한물성및일축압축시험결과, 공내재하시험 (Goodman Jack, Elastmeter 등 ) 으로부터계측된변형계수및 RQD값을 Table 2에각각정리하였다. 공내재하시험으로부터계측된변형계수와 RQD의관계를 Fig. 7에나타내었다. 전체적으로 Goodman Jack (GJ) 에의해계측된변형계수가 Elastmeter(EM) 에의한것보다큰값을나타내고있으며, 각각의시험방법에의해서계측된변형계수는 RQD값이증가함에따라지수적으로유의하게증가하고있다 (p<0.001). 또한, Fig. 7 고보고하고있다. 그리고 Table 1의결과를통하여, 두선형근사의절편과기울기사이에는유의한차이가없음을알수있다 (p>0.05). Fig. 6은제주도북동부육 해상에위치한시추공에대하여다운홀시험 ( 육상의시추공 ) 과부유형 PS 속도검층시험 ( 해상의시추공 ) 으로부터각각얻어진탄성파속도비 (V P/V S) 와 RQD의관계를나타내고있다. 탄성파속도비 (V P /V S ) 와 RQD는 Fig. 5와같이음 (-) 의선형관계에있으며, RQD값이증가함에따라탄성파속도비 (V P/V S) 는유의하게감소하고있다 (p<0.0001). 그리고부유형 PS 속도검층에의한탄성파속도비 (V P /V S ) 가부분적으로다운홀시험에의한탄성파속도비 (V P /V S ) 보다 95% 신뢰구간내에서작은값을나타내고있다. 이는서론에서언급한바와같이탄성파속도비는암반의절리상태및간격, 그리고암석의다공성구조에영향을받으며, 이러한사실로부터 RQD값이같더라도, 해상에위치한현무암반의상태가육상의암반에비해절리의상태및간격이양호하다고판단되며, 이에대한추가적인연구가필요하다고사료된다. Fig. 7. Relation between deformation modulus E m and RQD; EM: Elastmeter, GJ: Goodman Jack, PM: Pressuremeter 4. 제주도현무암반의변형계수 본장에서는제주도도내의육 해상에위치한현무암반의시추공에대하여 Elastmeter와 Goodman jack을이용하여계측된변형계수와 RQD, RMR, 탄성파속도비 (V P /V S ) 및동적포아송비 (ν d ) 의관계에대하여각각살펴보고자한다. Fig. 8. Relation between E m /E i and RQD; E m : Deformation modulus of rock mass, E i : Elastic modulus of intact rock, EM: Elastmeter, GJ: Goodman Jack 10 한국지반공학회논문집제 35 권제 1 호
의 C port의경우, Pressuremeter(PM) 에의해계측된변형계수를나타내고있으며, Elastmeter(EM) 에의한변형계수값과거의차이가없음을확인할수있다. Fig. 8은제주도북동부육상및북서부월령의시추공에서 채취한무결암의탄성계수에대한변형계수의비 (E m/e i) 와 RQD의관계및 Bieniawski(1978) 가제시한선형상관관계를나타내고있다. Fig. 7의결과와같이, Goodman Jack(GJ) 에의해계측된변형계수를통하여산정된비 Table 2. Results of physical and mechanical properties of the basaltic intact rocks, and in situ deformation modulus and RQD of the basaltic rock masses of Jeju Island Jeju northeast onshore Jeju northeast offshore Wollyeong A port B port B on Borehole No. Depth (m) G w (%) UCS (MPa) E i (Gpa) ν RQD (%) E m (Gpa) Borehole loading test JB-02 10.3-10.5 2.596 1.830 67.86 45.002 0.23 94 1.113 Elastmeter JB-03 JB-04 BH-11 BH-12 HJB-01 HJB-02 HJB-03 5.0-5.5 2.188 3.300 24.60 15.287 0.24 12 0.038 Elastmeter 12.7-12.9 2.840 1.080 110.89 38.329 0.21 9814.2 8Goodman Jack 1.5-1.9 2.103 2.510 28.14 34.399 0.24 85 1.239 Elastmeter 4.0-4.1 2.131 3.290 17.67 9.952 0.22 41 0.155 Elastmeter 5 74 6.413 Goodman Jack 8 80 2.111 Elastmeter 13 92 1.776 Elastmeter 14 95 6.859 Goodman Jack 27 80 1.444 Elastmeter 4 80 1.033 Elastmeter 7 90 1.039 Elastmeter 12 90 6.314 Goodman Jack 16 75 1.399 Elastmeter 18 95 1.445 Elastmeter 5.1-5.2 2.098 9.370 26.70 9.682 0.27 37 0.296 Elastmeter 6.1-6.2 2.476 5.780 70 18.59 0.267 76 6.440 Goodman Jack 7.3-7.4 2.263 8.730 27.1 7.532 0.397 62 0.103 Elastmeter 9.0-9.1 2.391 6.780 36 9.275 0.297 82 5.094 Goodman Jack 9.0-9.1 2.391 6.780 36 9.275 0.297 82 0.477 Elastmeter 5.4-5.5 2.446 5.658 73.8 18.774 0.256 40 0.684 Elastmeter 6.8-7.0 2.272 9.130 45.3 13.933 0.284 34 0.561 Elastmeter 7.8-8.0 2.186 11.216 28.3 24.045 0.166 34 0.069 Elastmeter 18 61 3.884 Goodman Jack 3.3-3.4 2.354 6.063 45.9 12.687 0.309 55 6.930 Goodman Jack 7.1-7.3 2.636 2.922 97.3 16.774 0.235 784.622 Goodman Jack 7.1-7.3 2.636 2.922 97.3 16.774 0.235 781.203 Elastmeter 11 46 0.080 Elastmeter DH-2 13 76 2.616 Elastmeter DH-5 24 67 1.769 Elastmeter DH-5 27 8 9 4.28 2 Elastmeter DH-13 13 73 2.205 Elastmeter NB-7 14 0 3.690 Goodman Jack NB-11 12 30 5.260 Goodman Jack BB-1 21.3 77 1.139 Elastmeter BB-2 5.7 70 1.593 Elastmeter BB-8 10 70 0.956 Elastmeter C port NBH-22 28.0-28.8 0 0.120 Pressuremeter G: Bulk specific gravity, w: Water absorption ratio, UCS: Unconfined compressive strength, E i: Elastic modulus, ν: Poisson s ratio, E m: Deformation modulus 제주도북동부및북서부현무암반의변형계수추정 11
(E m/e i) 가 Elastmeter(EM) 에의한변형계수를통하여산정된비 (E m /E i ) 보다훨씬큰값을나타내고있음을알수있으며, Elastmeter에의한변형계수비 (E m /E i ) 는 RQD 가증가함에따라유의하게증가하고있다 (p<0.01). 위결과를통하여, 제주도현무암반의변형계수를 RQD에대하여나타낼시, 변형계수의계측방법에따른값의차이가나타나고있음을알수있으며, 제주도의현무암반과같이다공성및층상구조가특징인암반에대하여 RQD값만으로암반의등급을결정하고, 변형계수의계측방법을결정하는것이타당한지의문이생긴다. 계수와 RQD의관계와달리, 변형계수의계측방법에따른값의차이가나타나고있지않으며, 이러한결과로부터 RQD만으로다공성구조가특징인제주도현무암반의암질을결정하고, 변형계수의계측방법을선정하는것은부적절하며, 최소한 RMR을통하여암반등급을결 4.2 변형계수와 RMR 의관계 제주도북동부육상의현무암반에대한 RMR 산정과정 (Bieniawski, 1989), 그리고산정된 RMR과 Elastmeter (EM) 와 Goodman Jack(GJ) 에의해계측된변형계수의관계를 Table 3과 Fig. 9에각각나타내었다. 뿐만아니라, 변형계수를추정하기위해제안된기존의경험식들과비교하기위해, Table 4에나타낸기존의경험식들을통해서산정된변형계수추정값을 Fig. 9에함께나타내었다. RMR값이증가함에따라변형계수는지수적으로유의하게증가하고있으며 (p<0.001), 기존에제안된경험식들은제주도현무암의변형계수에비해서전체적으로 2~10배에달하는값을추정하고있다. 한편, Ahn (2011) 이제안한육지부의화산암에대한변형계수경험식이제주도현무암반의변형계수에가장가까운값을추정하고있지만, 이또한크게추정하고있음을알수있다. 그리고변형계수와 RMR의관계의경우, 변형 Fig. 9. Relation between RMR and deformation modulus E m ; EM: Elastmeter, GJ: Goodman Jack Table 4. List of empirical correlations for estimating the rock deformation modulus with RMR Correlations (E m in GPa) References E m=2 RMR-100 (RMR>50) Bieniawski (1978) E m=10 ((RMR-10)/40) (RMR 50) Serafim and Pereira (1983) E m=0.0097 RMR 3.54 10-3 Aydan et al. (1993) E m=0.0736 Exp (0.0755RMR) Gokceoglu et al. (2003) E m=1.3326 Exp (0.0364RMR) Chun et al. (2009) E m=685.58 Exp (0.0401RMR) 10-3 (Volcanic rocks) Ahn (2011) Table 3. Rock mass ratings using the RMR system (Bieniawski, 1989) for the basaltic rock mass in the northeastern onshore of Jeju Island Spacing of Condition of discontinuties UCS RQD Groundwater Orientation of Borehole Depth discontinuities (MPa) (%) Discontinuity Separation conditions discontinuities No. (m) (m) Roughness Infiliing Weathering RMR length (m) (mm) Rating Rating Rating Rating Rating Rating Rating Rating Rating Rating JB-02 10.3 67.86 94 0.28 3~10 1~5 Smooth HF<5mm MW wet VF -10.5 7 20 10 2 1 1 4 3 7 0 55 JB-03 5.0 24.63 12 0.05 10~20 >5 Slickensided SF>5mm Decomposed flowing VU -5.5 2 3 5 1 0 0 0 0 0-25 -14 12.7 110.9 98 0.75 1~3 0.1~1 Rough None Unweathered damp VF -12.9 12 20 15 4 6 5 6 6 10 0 84 JB-04 1.5 28.14 85 0.15 3~10 1~5 Smooth SF<5mm MW wet VF -1.9 4 17 8 2 1 1 2 3 7 0 45 4.0 17.67 41 0.18 3~10 >5 Slickensided SF>5mm HW dripping U -4.1 2 8 8 2 0 0 0 1 4-15 10 HF: Hard filling, SF: Soft filling, HW: High weathered, MW: Moderately weathered, VF: Very favourable, VU: Very unfavourable, U: Unfavourable 12 한국지반공학회논문집제 35 권제 1 호
정하고변형계수를추정하는것이바람직하다고사료된다. 4.3 변형계수와탄성파속도비 (V P /V S ) 의관계제주도북동부육 해상에위치한시추공에서계측된변형계수와다운홀시험과부유형 PS 속도검층시험으로부터각각얻어진탄성파속도로부터산정된탄성파속도비 (V P/V S) 의관계를 Fig. 10에나타내었다. 탄성파속도비 (V P /V S ) 가감소함에따라변형계수는유의하게증가하고있으며 (p<0.0001), 탄성파속도비 (V P /V S ) 가약 1.8에서변형계수는거의연직으로급격하게증가하고있음을알수있다. 서론에서언급을했듯이, 탄성파속도비 (V P /V S ) 는 Eq. (1) 과같이동적포아송비 (Dynamic Poisson s ratio) 와관련이있으며, 변형계수와동적포아송비 (ν d ) 의관계를 Fig. 11에나타내었다. 포아송비 (ν) 는암반을포함한지반의수치해석및설계분야에서지반의측면변형을반영하는중요한파라미터로서, 지반의강도및변형특성에영향을미친다. 한편, Barton (2007) 에의하면, 공극이거의존재하지않는암반의경우동적포아송비 (ν d) 는약 0.25의값을나타내고, 비압밀퇴적층에있어서는 0.45이상의값을보인다고보고하고있다. Fig. 11을통하여알수있듯이, 동적포아송비 (ν d) 가감소함에따라, 즉암반의강성이증가함에따라변형계수는지수적으로유의하게증가하고있으며 (p<0.0001), 동적포아송비가약 0.28에서변형계수는연직으로급격하게증가하고있음을알수있다. 이러한 결과로부터, 탄성파속도비와변형계수는정성적인특성이일치하는관계에있음을알수있으며, 탄성파속도비를통하여변형계수를추정할수있을것이라사료된다. 뿐만아니라, 탄성파속도비는, RMR 산정시경험적지식또는주관적인판단이필요한암반의불연속면의평가에있어서, 객관적으로판단할수있는파라미터로서사용할수있을것이라사료된다. V P V S d VP V S (1) Fig. 11. Relation between deformation modulus E m and dynamic Poisson s ratio ν d; EM: Elastmeter, GJ: Goodman Jack 5. 결론 본연구에서는제주도북동부의육 해상및북서부육상에위치한현무암반에대하여공내재하시험 (Elastmeter, Goodman jack) 을통하여계측된변형계수와 RQD, RMR, 탄성파속도비 (V P /V S ) 및동적포아송비 (ν d ) 의관계를각각비교 분석하였다. 그리고변형계수와 RMR의관계의경우, 변형계수를추정하기위해제안된기존의경험식들과비교 분석하였으며, 얻어진결과를종합하면다음과같다. Fig. 10. Relation between deformation modulus E m and velocity ratio (V P/V S); EM: Elastmeter, GJ: Goodman Jack (1) 변형계수와 RQD의관계의경우, 변형계수의계측방법에따른값의차이가나타나고있으며, 다공성및층상구조가특징인제주도현무암반의경우, RQD만으로암질을결정하고, 변형계수의계측방법을선정하는것은부적절하며, RQD를이용한변형계수의추정또한부적절하다고사료된다. 제주도북동부및북서부현무암반의변형계수추정 13
(2) 변형계수와 RMR의관계의경우, 변형계수와 RQD 의관계와달리, 변형계수의계측방법에따른값의차이가없으며, 변형계수와 RMR사이에는지수적인상관관계가있다. 변형계수를추정하기위한기존의경험식들은제주도현무암반에대하여계측된변형계수의 2~10배에달하는값을추정하고있으며, 본연구에서제안한추정식을통하여양호한값을추정할수있을것이라사료된다. 그리고제주도현무암반의경우최소한 RMR을통하여암반등급을결정하고, 변형계수를추정하는것이바람직하다고사료된다. (3) 탄성파속도비 (V P /V S ) 및동적포아송비 (ν d ) 와변형계수의관계의경우, 각각유의한상관관계에있으며, 탄성파속도비와변형계수사이에는정성적으로특성이일치하는관계에있음을알수있었다. 탄성파속도비는변형계수를추정하기위한파라미터는물론, RMR암반분류의불연속면평가에있어서, 보다객관적인암반등급의추정을위한하나의파라미터로서이용이가능하다고사료된다. 참고문헌 (References) 1. Ahn, T. (2011), The Correlation Between RMR and Deformation Modulus by Rock Masses Using Pressuremeter, Journal of the Korean Geo-Environmental Society, Vol.12, No.1, pp.5-12 (in Korean). 2. Aydan, Ő., Ulusay, R., and Kawamoto, T. (1997), Assessment of Rock Mass Strength for Underground Excavations, International Journal of Rock Mechanics, Mining Science, Vol.34, No.3-4, pp. 1-17. 3. Barton, N. (2007), Rock quality, seismic velocity, attenuation and anisotropy, Taylor & Francis, p.729. 4. Bieniawski, Z.T. (1978), Determining Rock Mass Deformability: Experience from Case Histories, International Jour. of Rock Mechanics, Mining Science and Geomechanics, Vol.15, pp.237-247. 5. Bieniawski, Z.T. (1989), Engineering rock mass classifications, John Wiley & Sons, Inc. p.250. 6. Cho, T.C., Lee, S.B., Hwang, T.J., and Won, K.S. (2009), Variations of Mechanical Properties of Hallasan Trachyte with Respect to the Degree of Weathering, Tunnel and Underground Space, Vol.19, No.4, pp.287-303. 7. Chun, B., Ryu, W., Sagong, M., and Do, J. (2009), Indirect Estimation of the Rock Deformation Modulus based on Polynomial and Multiple Regression Analyses of the RMR System, International Journal of Rock Mechanics & Mining Sciences, Vol.46, pp.649-658. 8. Deere, D.U., Hendron, A.J., Patton, F.D., and Cording, E.J. (1967), Design of Surface and Near-surface Construction in Rock Proceedings of the 8 th U.S. Symposium on Rock Mechanics, pp.237-302. 9. Eum, K.Y. (2002), Study on the Mechanical Properties of Volcanic Rocks in Chejudo, Master thesis, Yonsei University, p.63. 10. Gokceoglu, C., Sonmex, H., and Kayabasi, A. (2003), Predicting the Deformation Moduli of Rock Masses, International Journal of Rock Mechanics & Mining Sciences, Vol.40, pp.701-710. 11. Hoek, E. and Diederichs, M.S. (2006), Empirical Estimation of Rock Mass Modulus, International Journal of Rock Mechanics & Mining Sciences, Vol.43, pp.203-215. 12. Jo, C., Byun, J., and Hwang, S. (2006), Considerations on the Difficulties in Velocity Logging in the Near Surface Environments, Geophysics and Geophysical Exploration, Vol.9, No.3, pp.185-192 (in Korean). 13. Kim G.W. (1993), Revaluation of Geomechanics Classifications of Rock Masses, Proceedings of the Korean Geotechnical Society Conferences, pp.33-40 (in Korean). 14. Kim, J.H. (2006), A Study on the Mechanical Characteristics and the Strength in Pyoseonri Basalt, Master thesis, Cheju National University, p.93 (in Korean). 15. Kim, Y.K. and Choi, O.G. (1991), Engineering Geological Characteristics of Volcanic Rocks of the Northwestern Cheju Island, Korea, The Journal of Engineering Geology, Vol.1, No.1, pp. 19-37. 16. Koo, H.B., Bae, G.J., Moon, H.D., and Hong, S.W. (1994), An Evaluation of Rock Mass Rating System as Design Aids in Korea, Proceedings of the Korean Geotechnical Society Conferences, pp. 209-216 (in Korean). 17. Kwon, B.D. (2006), On-Land Seismic Survey of Korea, Economic and Environmental Geology, Vol.39, No.4, pp.441-446 (in Korean). 18. Leem, K.M., Park, J.O., Yu, Y.C., and Song, M.Y. (2009), Geophysical Well-log Analysis of Andesite and Sedimentary Rocks in Myodo-dong, Yeosu, Southern Korea, Journal of Korean Earth Science Society, Vol.30, No.4, pp.401-408 (in Korean). 19. Moon, K., Park, S., Kim, Y., and Yang, S. (2014), Mechanical Properties of basalt in Jeju Island with Respect to Porosity, Journal of the Korean Society of Civil Engineering, Vol.34, No.4, pp.1215-1225 (in Korean). 20. Nam, J.M., Yun, J.M., Song, Y.S., and Kim, J.H. (2008a), Analysis of Engineering Properties to Basalt in Cheju Island, Journal of Korean Geosynthetics Society, Vol.7, No.1, pp.12-21. 21. Nam, J.M., Yun, J.M., Song, Y.S., and Kim, J.H. (2008b), Analysis of Influence Factors to Compressive and Tensile Strength of Basalt in Cheju Island, The Journal of Engineering Geology, Vol.18, No.2, pp.215-225. 22. Nam, J.M., Yun, J.M., and Song, Y.S. (2009), Estimation to the Strength of Basalt in Jeju Island according to Rock Failure Criterions, The Journal of Engineering Geology, Vol.19, No.2, pp.153-163. 23. Serafim, J.L. and Pereira, J. P. (1983), Consideration of the Geomechanical Classification of Bieniawski, Proceedings of International Symposium on Engineering Geology in Underground Construction, Lisbon, Portugal, Vol.1, No.2, pp.33-44. 24. Stewart, R.R., Huddleston, P.D., and Kan, T.K. (1984), Seismic Versus Sonic Velocities: A Vertical Seismic Profiling Study, Geophysics, Vol.49, No.8, pp.1153-1168. 25. Sunwoo, C., Chung, S.K., Han, K.C., Hwang, S.H., and Lee, S.K. (2001), Correlation Between the Rock Mass Classification Methods, Journal of the Korean Geotechnical Society, Vol.17, No.4, pp. 127-134 (in Korean). 26. Yang, S.B. (2014), Comparative Study on Physical and Mechanical 14 한국지반공학회논문집제 35 권제 1 호
Characteristics of Volcanic Rocks in Jeju Island, Journal of the Korean Geotechnical Society, Vol.30, No.11, pp.39-49 (in Korean). 27. Yang, S.B. (2015a), Physical and Mechanical Characteristics of Basalts in Northwestern and Southeastern Jeju Island, Journal of the Korean Geotechnical Society, Vol.31, No.7, pp.41-52 (in Korean). 28. Yang, S.B. (2015b), Cohesion and Internal Friction Angle of Basalts in Jeju Island, Journal of the Korean Geotechnical Society, Vol.31, No.11, pp.33-40 (in Korean). 29. Yang, S.B. (2016), Strength Parameters of Basalts in Jeju Island according to Rock Failure Criterions, Journal of the Korean Geotechnical Society, Vol.32, No.3, pp.15-27 (in Korean). 30. Yang, S. and Sassa, S. (2016), Mechanical Characteristics and Design Strength Parameters due to Vesicular Property of Basalts, Journal of Japan Society Civil Engineers, Ser. B3, Vol.72, No.2, pp.437-442 (in Japanese). 31. Yang, S. and Sassa, S. (2017), Effects of Porosity on Mechanical Characteristics of Vesicular Basalts, Proceedings of the 27 th International Ocean and Polar Engineering Conference, pp.705-711. 32. Yoon, S.H. and Ko, K.W. (2011), Topography, Geology and Underground Water in Jeju Island, Nae Ha Publishing Company, pp.5-57 (in Korean). Received : October 1 st, 2018 Revised : January 9 th, 2019 Accepted : January 11 th, 2019 제주도북동부및북서부현무암반의변형계수추정 15