Search

本研究では、非排水条件における飽和土塊の液状化崩壊に必要な間隙水圧の変化予測モデルを評価した。モール・クーロン破壊基準おける土のせん断抵抗が、これと等しくなる時に発揮されるしきい間隙水圧が液状化崩壊した試料で示された。崩壊性地すべり地の滑落崖の試料を用いて様々な条件下で試験し、崩壊後に示される数秒間のせん断抵抗が土の密度を発達させた。しきい間隙水圧によりせん断開始直後に液状化した試料は、崩壊以前にもダイレタンシー傾向の強度が得られた。, A theoretical model predicting the pore pressure change necessary for liquefaction failure of saturated soil masses in undrained condition is assessed. It is shown that a threshold pore pressure, uc, derived from the Mohr Coulomb failure criterion when pore pressure at failure is equal to the corresponding shear resistance is enough to initiate liquefaction type of failure in sandy masses. Loading tests to failure on source-area sandy soils from a catastrophic landslide location, undertaken to verify the model, show that under definite conditions of loading, a threshold state, characterized by the equality and subsequent constancy of pore pressure and shear resistance from a few seconds after the commencement of shearing until failure, develops in the sands at a given density. Samples in which the threshold pore pressure was exceeded readily liquefied while those in which the pore pressure built-up was below the limit gained strength by tendencies to dilate. This paper demonstrates that while the stability of a slope founded on sandy soils may be breached when the pore pressure exceeds a certain limit, it is possible to make estimates of the limit. It is shown that where such estimates are accompanied with adequate field measurements of pore pressure, the efficiency of landslide prevention projects may be enhanced because only slopes whose stability is proven to constitute a real public threat are reinforced and reinforced adequately.

本稿は，豪雨時を対象とした表層斜面崩壊のモデリングについて示したものである。斜面の安定性は土層内の水分の消長に大きく依存するため、これを精度よく表現し得るリチャーズ式を導入している。降雨実験により、モデルの適用性を検討した後、無限長斜面を仮定した3層からなる斜面の安定性を、リチャーズ式によって評価される土壌水分の消長を考慮して検討している。この斜面安定性の解析法を木津川上流域のタコラ谷に適用し、実際の豪雨時斜面崩壊箇所との比較検討により、本モデルの適用性が確認された。さらに、透水係数、土層厚、安息角、土粒子密度など、モデル中のパラメータが崩壊発生個数に与える影響について感度分析を行い、斜面安定解析において重要なパラメータを特定している。, A landslide simulation model based on the Richards equation for the ground water flow and with assumptions of soil matrixes comprising three horizontal layers was prepared to simulate rainfall induced shallow landslides. The model is capable to explicitly calculate the change in pore pressure in response to transient rainfall. Infinite slope failure was assumed for the slope stability module. The numerical solution of the unsteady pore-water pressure was experimentally verified by the physical experiment. The model was used to reproduce the landslides occurred in 1973/10/26 at the Takora basin located upstream of Kizu river basin. Comparison of the result of transient rainfall model and steady state model was done by applying both models to simulate the landslide produced by the same rainfall. Performance of the transient response model is better compared to steady state model as the latter highly over-predicted the instability. Sensitivity analyses of some of the model parameter were also done which indicates that depth of the layers, soil angle of repose and soil density have great influence on the stability of the soil domain. Correct representation of their values and their proper spatial distribution are vital to enhance the performance of the model output.

The equations for describing the particle sustaining pressure and the shearing stress in a viscous debris flow are proposed under the assumption that the micro-flow structure of the interstitial fluid induced by approaching and receding of the lined up particles in the adjacent upper and lower layers plays major role. The two such fundamental equations give the vertical particle concentration distribution and the equilibrium particle concentration with which the flow continues to run down neither deposition nor erosion. The viscous debris flows on a rigid bed are classified into five categories by the relationship among the supplied particle concentration, the equilibrium particle concentration and the critical particle concentration to guarantee the particle sustainment by the action of micro-flow structure of the interstitial fluid. The vertical distribution of the first approximation of the mean velocity is obtained by assuming uniform particle concentration in the particle mixture layer which appears in the lower part or in the entire depth depending on the flow category it belongs.The theoretical considerations are proved valid by the laboratory experiments and the parameters included in the discussion are assessed. In general, the apparent viscosity of the debris flow is far larger than that of the interstitial fluid by both the effects of decrease in the shearing area and micro flow structure in the interstitial fluid.The experiments revealed that the ratio of the two effects is different depending on the viscosity of the interstitial fluid.The equilibrium concentration of the particle in the experiments is not much different from that of the stony type debris flow. Therefore, the mechanism of the viscous debris flow freighting much more particles than the experimental debris flow and still flows on gentle slope with fantastic speed that mainly appears in China needs further discussions and experiments using nonuniform materials.

A big landslide occurred at Jizukiyama, Nagano Pref., 1985. This landslide was remarkedthat the sliding mass overcame a gentle slope of 8 and went 100-200m during only someten minutes. The long distance of motion is seemed to be supported by the pore-pressureoccurance and while the sliding mass loaded on fully or nearly saturated ground, and itresulted in the decrease of the frictional resistance of soil. So, soil tests were carried out onthe Jizukiyama soil, and the pore-pressure coefficients and other constants were measured.Further, these coefficients were applied to Sassa's "The Improved Sled Model", and the apparentfriction angle of Jizukiyama Landslide was calculated. The obtained apparent friction angleis reasonable to explain its long travel distance of the Jizukiyama landslide.

A review of field performance of sandy soil deposits during past earthquakes is conductedwith emphasis being placed on Static Cone Penetration Test qc-values and fines content.The field relationship between dynamic shear stress ratio and qc1-values (qc normalized foreffective overburden pressure of 1.0 kgf/cm2) indicate that (1) sands containing more than15% fines, or having smaller mean grain size than 0.25 mm, show much greater resistanceto liquefaction than clean sands which have the same qc-values, (2) extensive damage wouldnot occur for clean sands with qc1-values greater than about 150 kgf/cm2. On the basis ofthese findings, an empirical relationship separating liquefiable and non-liquefiable conditionsis presented in terms of dynamic shear stress ratio, qc1-values, and fines content.

A large scale landslide occurred at Ichinomiya, Hyogo Prefecture, 13th September, 1976.Water and clay mineral samples of both the sliding and unsliding part of the landslide area werecollected to estimate whether rock forming silicates are weathered or not. The authors examined therelationships between the chemical species contents of the waters and the clay minerals. Theresults were as follows.(1) The concentration of sodium, calcium and bicarbonate ions and soluble silica in thenatural waters of the sliding part show higher than those of the unsliding part.(2) The calculated dissolved carbon dioxide gas of sliding part is l0-4.1～2.4 atmosphere.(3) Most of chloride and sulfate ions in the waters of both parts are supplied through pre-cipitation and dry fallout.(4) The waters are in equilibrium with kaolinite and montmorillonite.(5) Most of the interlayer clays in the welded tuff consist of montmorillonite and the domi-nant species of clay minerals in the weathered granodiorite are kaolinite and illite.

Soil Characteristics of the Jizukiyama Landslide and the Ontake Debris Avalanche wereexamined in use of a newly developed High-Speed High-Stress Ring Shear Apparatus.Glass beads were also tested for comparison with them.This paper mainly presents following results. That is:1) The friction angle tends to begreater when shearing speed becomes greater, for both of the Jizukiyama and the Ontakesample. For glass beads, however, the friction angle comes to be smaller. 2) No effect onthe friction angle by normal stress was recognized in the whole range of 0-about 3.5kgf/cm2. 3) When increasing normal stress on a saturated sample of the Jizukiyama soil, excess pore-pressure was generated, and it took so long time to dissipate. It is caused byvery small permeability of the Jizukiyama soil. While the Ontake soil did not generate suchexcess porepressure during the usual procedure to increase normal stress, because of theirhigh permeability.

Water samples have been collected from the landslide areas of Tertiary and fractured zones toinvestigate the water quality of geologically different landslide areas. The authors have examinedthe relationships among the chemical species contents of the waters and the following have beenobtained: In general, the concentrations of calcium and bicarbonate ions show high value amongdissolved ions in the natural waters of the two landslide areas, the concentration of soluble silica inthe natural waters of landslide areas of fractured zone is low value, calcium to sodium ratio and tomagnesium of dissolved elements can draw a fairly clear between two landslide areas, montmorillonit-es and kaolinite are in equilibrium with the waters of the Kuraisbi, Maruyama, Kozuehiyama andMinamikaminaka landslide areas, and equilibrium between kaolinite and montmorillonite isattained at the concentrations of bicarbonate ions over about 80 mg/I in the natural waters oftheir investigated fields.