Though physical properties of shear layer are much related to water in soil, the relation have not been so much studied. The author studied on soil structures and permeabilities of shear layer with different stages of direct shear test.
(1) The relations between soil structures and water content
The structures of share layers in soil samples with different water contents were examined with X-lay photograph.
Many small shear planes generally gathered to make a shear layer.
In case of optium water content W_opt% or W_opt-10%, the shear layer was formed in thin range along neutral line of soil sample. In case of W_opt+10%, the layer was formed in thick range around the line.
The author consider that the former case suggests the shear layer formed in solid body, the latter case suggests that formed in the body like fluid.
(2) The permeability of shear layer
Permeabilities of the shear layers were measured with different directions.
The permeability parallel to the shear layer was of the order of 10^-6cm/sec, and the permeability across that was of the order of 10^-7cm/sec.
The author consider that for the above results, ground water is able to permeate very slowly only through landslide layer in clyey landslide area, therefor landslide occurs repeatedly along the same landslide layer by ground water.

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The authors have thought that one of the most effective factors on the movement of a large crystalline schist landslide “Zentoku” in TokushimaPrefecture, Japan is the underground erosion, and have monitored the sediment discharge due to the erosion and subsequent transportation by the ground water at spring point in there. This paper aims to reveal the relationship between underground erosion and landslide movement on the basis of the monitoring results by the apparatus to catch the sediment, the extensometers and the rain gauge.
Due to the results of the preceding monitorings, the sediment discharge is roughly proportional to the precipitation. However, sediment data scatter, which seems due to the factor other than the precipitation. In order to examine the condition of the sediment yield, the authors have induced two parameters, one is the displacement ratio (D/D_AVn) to evaluate the degree of landslide movement and the other is the sediment discharge ratio (W/W_P) to evaluate the degree of the sediment discharge affected by the factor other than the precipitation. The relationship between the discharge sediment and the landslide movement, which based on the result of the time dependent analysis of these two parameters, leads to the following conclusions;
(1) Undoubtedly the precipitation is the most effective factor on the sediment discharge. The landslide movement affects the sediment discharge as the subsequent factor, which causes the scattering of data on the sediment discharge versus precipitation diagram.
(2) According to the relationship between the landslide displacement under active period (D/D_AVn>2) and the sediment discharge ratio (W/W_P), the sediment discharge at the spring point has two factor groups, first group is under the effect of precipitation and the second is affected by the precipitation and the landslide movement itself. The sediment discharge, which produced one month after the active period of the slide, shows good proportionality to the landslide displacement.

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Observation of slope deformation using thirteen extensometers being set continuously and investigation by a portable penetration test apparatus were done on a slope which consists of weathered granodiorite in Todoroki area, where the Nojima Fault appeared at the toe at the time of the 1995 Hyogoken-Nanbu Earthquake. Analysis of the result of extensometer observation suggests that slope deformation is composed of two different types of ground movement; one is creeping movement of surface layer, and the other is movement of soil block which displaces as one body. Positive correlation between rate of surface layer creeping and rainfall intensity can be seen during precipitation but the creeping lasts for a long period (more than ten days) after rain stopped. Two active soil blocks are found; one is across the Nojima Fault and the other is uphill of it. Their movements begin almost one day after a precipitation and show stick-slip type displacement. Result of the penetration test shows structure of resistance which agrees with the result of analysis on slope deformation stated above.

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[in Japanese]

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Some of the methods commonly used for analyzing slopes utilizing the principles of limit equilibrium are the Ordinary method, the Janbu method, the Bishop method, the Morgenstern-Price method, and the Spencer methods.
Spencer proposed two general limit equilibrium methods of slices, the method (1967) of the resultant inter-slice force Q in a slice and the other method (1973) of divided two inter-slice forces Z in a slice. The Spencer method (1967) is simply introduced in an assumption of constant angle θ of inter-slice force Q in all slices. The property of moment equilibrium equation in conversing calculation is simple and this method is sometimes used in tree-dimensional analysis. On the other hand, the Spencer method (1973) is introduced in an assumption of different angles θ; of inter-slice forces Z in each slice and is more general and advanced than the Spencer method (1967). However, the property of moment equilibrium equation of the Spencer method (1973) is complicated like that of the Morgenstern-Price method and this has a disadvantage of computer programming.
It is considered mainly on the moment equilibrium equation of the Spencer method (1973) that is ∑ [J] =0. It is shown that the equation is sum of the moment of inter-slice forces Z in all slices about the start point of a slip surface, although this is introduced from the principle that moment equilibrium condition is satisfied if sum of the moments of all forces in each slice about a middle point of base is zero and these conditions are realized in all slices of a slip surface. A general moment equilibrium equation is proposed, and the statical signification is discussed. The properties of these equations and relative merits are discussed.

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The characteristics of the landslides along the Median Tectonic Line occurred by the road construction are considered. Most of the landslides are the sliding of the colluvial deposit by the cutting. The scale of the landslides is relatively small, up to 200m in the width and length, up to 30m in the depth of the slip surface. The angle of shear resistance of the clay in the fractured zone at the Median Tectonic Line is relatively small. This is one reason that the counter measure of the landslides at the Median Tectonic Line is difficult.

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Shear strength characteristics (fully-softened and residual) and the recovery of the shear strength by re-consolidation following the large displacement shear were investigated for the soil sample collected from the moving mass of a reactivated and large-scale landslide which occurred at the middle part of Taiwan in 1994 triggered off by the heavy rainfall after road construction. Geology of the landslide area is the Miocene formation composed mainly of shale. The soil sample was characterized by the low clay content and the low plasticity and dominated by quartz accompanied by mica, chlorite, and feldspars.
Slurried subsamples passed through a 420-μm sieve were normally consolidated at the respective normal stresses ranging from 50 to 300kPa. The subsamples were sheared using a ring shear apparatus under drained condition with a slow shearing rate until about 700mm of displacement. They were then re-consolidated at the corresponding normal stresses for 2 days and re-sheared.
The fully-softened strength was C_sf=4.6 kPa and φ_sf=35.3°. The relationship between residual strength and effective normal stress tended to be curve, showing the dependence of the residual strength on the effective normal stress. The residual strength was calculated to be φ_γ=31.2° under σ_n'of 50-150 kPa and φ_γ=25.4°underσ_n'of 200-300 kPa with a assumption of cγ=0 kPa.The recovery of the shear strength ofthe soil subjected to large displacement shear by re-consolidation was not detected under σ_n'of 200-300 kPa but clearly observed under σ_n' of 50-150kPa. The recovery rate in strength increased with the decreasing normal stress but the strength did not recover to the level of the fully-softened strength. The above results are important to clarify the mechanism of the recurrence of the reactivated landslide.

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Since particular soil structures are formed by the shear failure and the following shear creep caused by landslide, the study on the soil structures is important to make the mechanism of landslide clear.
The authors made a direct shear test apparatus as a trial in order to study the progress of the soil structures of shear layer in process of shear test.
The structural features of the apparatus are that shear boxes have 600mm length, the shear deformation of the boxes can be theoretically established from 0mm to 600mm in order to form the soil structures by long shear creep, and the boxes can be disjointed with simple procedures in order to sample the undisturbed soil structures.
We were able to make it clear, with the apparatus, that almost deformations in the length direction of the sample are about uniform, and the particular soil structures are formed in the creep layer. Therefor we conclude that the apparatus is available for the same kind of study.

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