報告書番号

U94007

タイトル（和文）

１９９３年北海道南西沖地震における礫地盤液状化の原因解明（その１）－地盤調査・試験と液状化判定－

タイトル（英文）

INVESTIGATION OF CAUSES LIQUEFACTION OF GRAVELLY-DEBRIS AVALANCHE DEPOSIT DURING HOKKAIDO-NANSEI-OKI EARTHQUAKE OF 12 JULY 1993(PART 1) -GEOTECHNICAL INVESTIGATION AND LIQUEFACTION ASSESMENT-

概要 （図表や脚注は「報告書全文」に掲載しております）

１９９３年北海道南西沖地震においては，駒ヶ岳の南側に位置している森町赤井地区や七飯町で火山性堆積物から成る礫地盤（岩屑なだれ堆積物）が液状化し，多くの家屋が全半壊するとともに，道路に亀裂が生じた。今回液状化した礫地盤は重要電力施設の基礎地盤に成り得る河川堆積性の洪積砂礫地盤とは異なるが，大きな被害を伴った礫地盤の液状化事例は記録にないため，その重要性に鑑み，液状化の原因を究明するため，地質調査，弾性波探査，電気探査，貫入試験，凍結サンプリング試料による室内試験などの詳細な地盤調査・試験を実施した。さらに室内試験から得られた液状化強度と当サイトの近くで得られた地表面加速度を用いて，当サイトにおける液状化判定を実施し，液状化安全率が１．０を下回ることを確認した。

概要 （英文）

A GRAVELLY-DEBRIS AVALANCHE DEPOSIT LIQUEFIED EXTENSIVELY DURING THE HOKKAIDO-NANSEI-OKI EARTHQUAKE OF 12 JULY 1993,CAUSING SEVERE DAMAGE TO WOODEN HOUSES AND ROADS. GRAVELLY GROUND IS OFTEN USED FOR FOUNDATION GROUND OF IMPORTANT ELECTIC POWER FACILITIES BECAUSE OF ITS HIGH RESISTANCE TO LIQUEFACTION. THUS,IT IS NECESSARY TO INVESTIGATE THE CAUSE OF THIS LIQUEFACTION. SITE INVESTIGATIONS,INCLUDING GEOLOGICAL SURVEYS,ELECTRICAL PROSPECTING BY ELECTRICAL RESISTIVITY METHOD,RS-LOGGING,LARGE PENETRATION TESTS,ELASTIC WAVE EXPLORATION BY THE REFRACTION METHOD,IN-SITU FREEZE SAMPLING AND LABORATORY TESTS WERE CONDUCTED. THE TEST RESULTS SHOW: (1)THE AGE OF THE ORGANIC SOIL LAYERSSANDWICHING THE GRAVELLY-SOIL DEPOSIT WAS ABOUT 2000 YEARS ESTIMATED BY RADIOCARBON DATING. THUS,THE GRAVELLY-SOIL DEPOSIT IS RELATIVELY YOUNG AND OF THE ALLUVIAL AGE. (2)PS-LOGGING AT HOLE B1 LOCATED NORTH OF A TWO-STORY RESORT HOTEL GAVE AN AVERAGE SHEAR WAVE VELOCITY FOR GROUND SHALLOWER THAN 7 M OF ABOUT 100 M/S,WHICH IS VERY LOW FOR GRAVELLY SOIL.AN LPT (LARGE PENETRATION TEST) AT HOLE B1,GAVE AN AVERAGE LPT BLOW COUNT FOR THE SAME PART OF THE GROUND OF LESS THAN 10,WHICH IS ALSO VERY LOW FOR GRAVELLY SOIL. (3)THE SHEAR WAVE VELOCITY OF THE GROUND MEASURED BY THE REFRACTION METHOD APPROXIMATES THAT BY PS-LOGGING. (4)LIQUEFACTION TESTS GAVE UNDRAINED CYCLIC STRENGTHS (DEFINED AS STRESS RATIO REQUIRED TO REACH DOUBLE AMPLITUDE OF AXIAL STRAIN 2% IN 20 LOADING CYCLES) FOR SAMPLES PROCURED BY IN-SITU FREEZING OF ABOUT 0.2. (5)THE RELATIONSHIPS BETWEEN UNDRAINED CYCLIC STRENGTHS AND LPT BLOW COUNTS,AND BETWEEN UNDRAINED CYCLIC STRENGTHS AND INITIAL SHEAR MODULI AGREE WITH THOSEOF OTHER SITES ALREADY INVESTIGATED BY CRIEPI. LIQUEFACTION SUSCEPTIBILITY WAS EXAMINED BY PRACTICAL LABORATORY TESTS AND THE MAXIMUM SURFACEACCELERATION WAS ESTIMATED FROM DATA RECORDED NEAR THE INVESTIGATION SITE. IT WAS CONCLUDED THAT LIQUEFACTION AT THE SITE WAS HIGHLY PROBABLEBECAUSE THE INFERRED SHEAR STRESS

報告者

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