Determination of strength parameters for landslide slope stability analysis
by laboratory test and inverse calculation engagement
Toshiyuki MITACHI, Takayuki KUDA, Masafumi OKAWARA and Masahiro ISHIBASHI |
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Abstract
A practical method of determining strength parameters for stability calculation of landslide slope is proposed in which the strength parameters are given by combining the conventional inverse calculation method, which has been frequently used in engineering practice, with the laboratory shear test results. In addition, the authors have newly developed a fully automated reversal direct box shear （RDBS） test apparatus using a high‐precision digital servomotor. Test results on ten kinds of clay samples using the new apparatus are presented. Case studies for three sites of landslide which prove the suitability of design strength parameters determined by using the proposed method based on the test data obtained by newly developed RDBS test apparatus are also presented.
Key words:landslide, design strength parameter, reversal direct shear test, residual strength, inverse calculation method |
水質からみた福島県滝坂地すべり地における排水機構
Hydrochemical evaluation of subsurface drainage in the Takisaka Landslide Area,
Fukushima Prefecture, Japan
鈴木将之・佐藤 修
Masayuki SUZUKI and Osamu SATO |
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Abstract
Hydrochemical observations of discharge waters have been carried out from subsurface drainages and spring in the Takisaka landslide area, Fukushima Prefecture, Japan. The samples have been collected from the Sodenosawa drainage tunnel, collecting wells, and the Matsuzaka spring in the landslide area. The main objectives of this study are to identify the change in dissolved components after rainfalls and to access the relation in term of landslide stability. On the basis of the major solutes in groundwater and geology of parent rocks, groundwaters in this area are chemically classified into two types. One is the enriched Na‐SO４ type derived from the mudstone unit and the other is Na‐HCO３ type derived from the arkose unit that is close to the slip surface. The corrected HCO３ − concentrations with that of the Matsuzaka spring water are almost constant among the groundwaters. The Sodenosawa drainage tunnel discharges large volumes of Na‐HCO３ type groundwater during a few days after the ＞３０mm/day rainfalls. These indicate that discharge of Na‐HCO３ type groundwater is closely related to the stability of the landslide mass.
Key words: drainage tunnel, collecting well, spring water, hydrogencarbonate concentration, rainfall
和文要旨
福島県西会津町滝坂地すべり地において，地すべり地全域の排水に対する地球化学的評価手法の確立を目的に，袖の沢排水トンネル出口における電気伝導度の自動観測と，１９９８年から２００２年にかけて採取した湧水，トンネル排水および集水井排水試料の水質分析を行った。排水は，浅部の泥岩に由来する高濃度のNa‐SO４型地下水とやや深部のアルコースに由来する比較的濃度の低いNa‐HCO３型地下水に大別される。トンネルおよび集水井排水のHCO３ −濃度から松坂湧水のHCO３ −濃度の変動を差し引くと，それぞれのHCO３ −濃度はほぼ一定となる。数１０mm以上の降雨後１日程度の短時間に，トンネルはアルコース由来の水をとらえる。排水のHCO３ −濃度の増減から，これらの施設は長期的にアルコース由来の水を排水し続けていると推定される。
キーワード：排水トンネル，集水井，湧水，炭酸水素塩濃度，降雨 |
Estimation of residual shear strength for bentonite‐kaolin‐Toyoura sand
mixture
Binod TIWARI and Hideaki MARUI |
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Abstract
Soil samples collected from a few landslides, debris flow, slope failure and volcanic areas in Japan have shown smectite, kaolinite, halloysite, feldspar and quartz as main dominating minerals. Commercial bentonite having quartz, crystballite and smectite and commercial kaolin, consisting of kaolinite, halloysite, feldspar and quartz has been mixed up with Toyoura sand to make the samples of various proportions of smectite, feldspar, halloysite and kaolinite in quartz. Residual internal friction angle（phi r）of the bentonite‐sand mixture was not varied considerably for the bentonite proportion upto １５％. Likewise, phi r was more or less constant with the minimum value i.e. ３．７° for more than ５０％ bentonite. Value of phi r was gradually decreased for the mixture from １５％ to ５０％ bentonite. Value of phi r for kaolin‐sand mixture had decreased continuously from １５％ to １００％ kaolin proportion. In case of kaolin‐bentonite mixture, the minimum phi r was obtained for more than ３０％ bentonite. The shear strength of the mixture was controlled mainly by bentonite proportion. The effect of kaolin was high for the proportion of kaolin less than １０％. For proportion of kaolin more than ２０％, sand less than ５０％ and bentonite more than ３０％, the phi r was close to minimum. Strong relationship was obtained with liquid limit, plasticity index and weighted smectite proportion. Value of phi r for actual disaster area samples were fitted well inside the envelope for kaolin and bentonite in the phi r‐liquid limit and phi r‐plasticity index charts. The research showed that the residual shear strength could be estimated roughly with the proportion of clay, liquid limit and plasticity index, for certain clay mineralogy.
Key words:residual shear strength, bentonite, kaolin, Toyoura sand, consistency |
振動台実験による斜面の地中変位量の計測
Shaking table tests to measure the underground displacement of the slope
浅野志穂・松浦純生・岡本 隆・松山康治
Shiho ASANO, Sumio MATSUURA, Takashi OKAMOTO and Koji MATSUYAMA |
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Abstract
To understand the mechanisms of slope failure during earthquakes, shaking table tests were conducted using a large‐scale slope model, and the seismic response acceleration of the displacement of the slope was measured. Using the data of the slopes that collapsed during the Hyogo‐ken Nanbu Earthquake, a slope model was prepared, which measured approximately４m long and３m high, and was inclined at ４５ degrees and had a soil layer １．２ m thick. Forces that imitated seismic waves were repeatedly applied until the slope fell by changing the seismic acceleration. The test showed１）sudden increases in response acceleration at the upper part of the slope, especially along the knick line where the head of the fallen soil mass was formed, ２）accumulation of displacement even in deep ground sections during the initial stages of failure, and３）sudden formation of the sliding surface in the shallow section of the slope, which caused the overlying soil mass to slide.
Key words：shaking table test, slope failure, underground displacement, response acceleration
和文要旨
地震発生時の斜面崩壊機構を明らかにするため大型実験斜面による振動台実験を行い，崩壊発生時の斜面の地震応答加速度と地中変位量の計測を行った。兵庫県南部地震で発生した崩壊地を参考に斜面長約４m，高さ３m，斜面勾配４５度，層厚１．２mの実験斜面を作成し，仮想の地震波を入力して実験を行った。実験は崩壊が発生するまで，地震加速度を変えて繰り返し実施した。その結果から，崩壊発生直前には斜面上部の特に崩壊地頭部に当たる遷急線付近で応答加速度が急激に増大したこと，崩壊時には初期段階で深部まで変位の累積があったこと，その後浅層部分にすべり面を形成して土塊が急速に滑落したこと等が明らかとなった。
キーワード：振動実験，斜面崩壊，地中変位，応答加速度 |