Gonghui Wang

Pore-pressure generation and movement of rainfall-induced landslides: effects of grain size and fine-particle content

Abstract Using a small flume, a series of tests was conducted to trigger rainfall-induced landslides. By performing tests on silica sand no. 7 (D50=0.13 mm), no. 8 (D50=0.05 mm) at different initial dry densities, the effects of grain size on pore-pressure generation and failure behavior of a landslide mass were analyzed. Results from tests of different initial densities showed that the optimal density index, at which the pore-pressure build-up after failure reached its maximum value, differed for samples of different grain size. Moreover, observed failure phenomena showed that the failure mode also depended greatly on the grain size. In fact, flowslides were initiated in the tests on finer silica sand (no. 8), whereas retrogressive sliding occurred in the tests on silica sand no. 7. Results of tests on mixtures of silica sand no. 8 with different contents of loess by weight showed that the existence of fine-particle soil (loess) could significantly change the flow behavior of a landslide mass during motion. The flow behavior of soils with 20% and 30% loess was different from these two silica sands and the mixture with 10% loess, showing greater velocity without deceleration. This suggests the existence of a mechanism that maintains high pore pressures during motion for these soils. In addition, by rotating saturated samples in a double-cylinder apparatus, a mechanism was examined in which pore pressure in saturated soils during motion was maintained. The results showed that the pore pressure of the saturated mixture increased with velocity because of the “floating” of sand grains that accompanied the movement for each test. In addition, the sample with finer grain sizes or greater fine-particle (loess) contents floated more easily, and high pore pressure could be maintained during motion. The floating ratios of grains reached a high value (>0.85) at a very slow velocity for samples with 20% and 30% loess. Based on these test results, it is concluded that grain size and fine-particle contents can have a significant impact on the mobility of rainfall-induced landslides.

PERFORMING UNDRAINED SHEAR TESTS ON SATURATED SANDS IN A NEW INTELLIGENT TYPE OF RING SHEAR APPARATUS

The design and construction of an undrained ring shear apparatus, which was newly developed at Disaster Prevention Research Institute, Kyoto University, are presented in detail. This apparatus is suited for undrained shear tests under all types of loading, and enables observing the undrained shear behavior of soils in high-speed motion (maximum rotating speed 2.24 m/s) to an endless displacement level. Using rubber edges and a gap control system, leakage of pore water was effectively prevented, and the friction of rubber edges was controlled with high credibility during shearing. For undrained shearing tests on sand, the related experimental procedures are introduced and the undrained shear behavior of sand in ring shear test is presented. Test results showed that high pore water pressures could be built up in both loose and dense sands, given the shear displacement is great enough.

Mechanism of a long-runout landslide triggered by the August 1998 Heavy rainfall in Fukushima Prefecture, Japan

Abstract Heavy rainfall from 26 to 31 August 1998 triggered many landslides in Nishigo Village of southern Fukushima Prefecture, Japan. The Hiegaesi landslide, a long-runout landslide with travel angle of 11°, which occurred in loamy volcanic-ash/pumice layer and was deposited in a nearby rice paddy, was investigated. In an observation pit dug in the middle part of the landslide deposit, the sliding zone just above the deflected rice plants was observed, and it was confirmed that grain crushing occurred in the sliding zone. The triggering and sliding mechanisms of this landslide then were investigated by ring-shear tests in laboratory. For the triggering mechanism, one saturated naturally drained test (test A: torque-controlled test) and one saturated undrained test (test B: speed-controlled test) were conducted on the samples taken from the source area of the landslide. Even in the naturally drained test opening the upper drain valve of the shear box, a temporary liquefaction occurred. In the undrained test, excess pore-pressure was generated along with shearing, and “sliding-surface liquefaction” phenomenon was observed. The effective stress and shear resistance finally decreased to near zero. These results can explain the observed phenomenon of small friction resistance like a flow of liquid when the sliding mass slid out of the source area. For the sliding mechanism of the landslide in the rice paddy, saturated undrained test (test C: speed-controlled test) was performed on soil sample above the deflected rice plants. The apparent friction angle obtained in this test was 8°. In addition, the residual friction angle measured after test B and test C was the same value of 41°. Combining with the observation on the shear zone in the ring-shear box after test C, it is concluded that, during the sliding in rice paddy, the undrained shear strength of the soil layer itself mainly influenced the high mobility of the landslide, probably because the friction between rice plants and soils is greater than the undrained shear strength inside the soil mass.

Downslope volume enlargement of a debris slide–debris flow in the 1999 Hiroshima, Japan, rainstorm

Abstract Following a heavy rainstorm on 29 June 1999, hundreds of slope failures occurred in granitic mountains in Hiroshima Prefecture, Japan. Among these events, a highly mobile landslide (termed the Kameyama landslide in this paper), which occurred in Kameyama area of Hiroshima city, was the most catastrophic, and was investigated in detail. The displaced soil mass from the source area of this landslide traveled about 300 m and deposited a volume more than 10 times as great as that in the source area. The landslide originated in and traversed a valley-shaped concave slope covered by pre-existing colluvial debris deposits. In addition, a spring was visible in the source area and very shallow ground water was observed in an observation pit dug in the source area. Thus, it is inferred that the ground-water table rose quickly during the rainfall, and that this rise triggered the slope failure in the source area. Based on a field survey along the landslide cross section, a possible explanation for the mechanism of the landslide was obtained: the displaced soil mass from the source area impacted the debris deposit in the path of the landslide, thus triggering liquefaction failure of the saturated part of debris. The original landslide and the liquefied debris then moved downslope as a single mass. To examine this assumption, ring-shear tests were performed on samples taken from the source area. Undrained ring-shear tests on the saturated samples showed that the sample is highly liquefiable, and liquefaction failure could have been triggered in the debris deposits by a very small impact from the displaced soil mass of the initial failure. In addition, laboratory tests simulating the impacts on the debris deposits at natural water content, i.e., unsaturated (at the survey time, 2 days after the failure) showed that although shear failure could be caused by the assumed impact force, the displaced soils stopped after a few centimeters displacement, indicating that existence of a saturated zone in debris deposits is prerequisite for this kind of failure.

Displacement Monitoring and Physical Exploration on the Shuping Landslide Reactivated by Impoundment of the Three Gorges Reservoir, China

The Three Gorges Dam construction on the Yangtze River in China is the largest hydro-electricity project in the world. After the first impoundment in June 2003, many landslides occurred or reactivated. Shuping landslide is one of the most active landslides among them. In this paper, the deformation of the Shuping landslide monitored by GPS, extensometers, and crack measurements are summarized. Also, for the investigation of the groundwater situation, 1 m depth ground temperature measurement was conducted, and the groundwater veins were estimated. Based on the monitoring data and exploration results, a deformation model of the landslide caused by impoundment of reservoir was proposed.

Effect of pore-water chemistry on undrained shear behaviour of saturated loess

Pore-water chemistry can have a fundamental influence on the shear behaviour of soil. To study the effects of salt concentration and desalinization on undrained shear behaviour, a series of ring shear tests was conducted on Chinese loess saturated with various NaCl solutions of differing concentration, under the same initial void ratio and consolidating stress conditions. Results showed that the peak and steady-state shear strengths increase as the NaCl concentration increases only up to a certain level. When this level is exceeded, any further increase of NaCl concentration results in lower shear strengths. After desalinization the peak and steady-state shear strengths recover to those of the original sample, showing that the salinization of loess is reversible. A pore pressure ratio of 0.6 can be used as a criterion to assess whether liquefaction can be induced in saturated loess. These findings may provide a useful explanation for progressive or seasonal activity of some irrigation-induced loess landslides in the Chinese Loess Plateau.

Residual shear strength variability as a primary control on movement of landslides reactivated by earthquake‐induced ground motion: Implications for coastal Oregon, U.S.

Most large seismogenic landslides are reactivations of preexisting landslides with basal shear zones in the residual strength condition. Residual shear strength often varies during rapid displacement, but the response of residual shear zones to seismic loading is largely unknown. We used a ring shear apparatus to perform simulated seismic loading tests, constant displacement rate tests, and tests during which shear stress was gradually varied on specimens from two landslides to improve understanding of coseismic landslide reactivation and to identify shear strength models valid for slow gravitational failure through rapid coseismic failure. The landslides we studied represent many along the Oregon, U.S., coast. Seismic loading tests resulted in (1) catastrophic failure involving unbounded displacement when stresses represented those for the existing landslides and (2) limited to unbounded displacement when stresses represented those for hypothetical dormant landslides, suggesting that coseismic landslide reactivation may be significant during future great earthquakes occurring near the Oregon Coast. Constant displacement rate tests indicated that shear strength decreased exponentially during the first few decimeters of displacement but increased logarithmically with increasing displacement rate when sheared at 0.001 cm s−1 or greater. Dynamic shear resistance estimated from shear strength models correlated well with stresses observed during seismic loading tests, indicating that displacement rate and amount primarily controlled failure characteristics. We developed a stress-based approach to estimate coseismic landslide displacement that utilizes the variable shear strength model. The approach produced results that compared favorably to observations made during seismic loading tests, indicating its utility for application to landslides.

Landslides Induced by a Combined Effect of Earthquake and Rainfall

Many landslides are triggered by rainfall and many landslides are triggered by earthquakes. The probability that rainstorms (typhoons or hurricanes) and earthquakes attack same area is not high or rare. Combined effects of rainstorms and earthquakes have not been examined. The 2004 Mid-Niigata Prefecture earthquake (M6.8) caused twelve landslides more than one million cubic meters, and many landslide dams were formed by large-scale displaced landslide masses. While, the 1995 Hyogo-ken Nambu earthquake (M7.2) did neither cause any large-scale landslide, nor landslide dam although it has a greater magnitude and a similar depth of earthquake. One major difference is: a typhoon attacked Niigata Prefecture in three days before the earthquake, and the 1995 Hyogo-ken Nambu area was very dry before the earthquake. Combined effects of two triggering factors were examined for two cases which the authors investigated: the 2006 Southern Leyte landslide possibly triggered by a nearby small earthquake (M2.6), and the Higashi- Takezawa landslide triggered by the 2004 Mid-Niigata Prefecture landslide.

Influence of initial dry density and water content on the soil-water characteristic curve and suction stress of a reconstituted loess soil

In the northwest of China, many loess landslides have occurred without clear triggering factors (i.e., rainfall, earthquake, human activities, etc.). To better understand and analyze the hydro-mechanical properties of these slopes and then provide evidence for their stability analysis subjected to matric suction, it is essential to clarify the soil–water characteristic curve (SWCC). In this study, we conducted a set of experimental trials to examine the influences of initial dry density, water content upon the SWCCs of a loess soil taken from a loess landslide area, by using a conventional volumetric pressure plate extractor. Two common SWCC models have been investigated to evaluate which one is better for loess soil. The suction stress characteristic curves (SSCCs) were also estimated and analyzed. We found that behaviors of SWCCs would be different when the matric suction was greater than a certain value. The two SWCC equations have approximately the same performance in describing the SWCC. The rates of desorption decrease and residual water content increases with increasing the initial dry density, while the initial dry density has little, if any, influence on the air-entry value (AEV). The specimen compacted under higher initial water content would exhibit a higher AEV value and residual water content but lower rate of desorption as compared with the lower initial water content. The magnitude of suction stress had an approximately linear relationship with matric suction before the AEV value, the SSCC shapes will be markedly varied with the initial dry density and water content.

Shear Resistance Variations in Experimentally Sheared Mudstone Granules: A Possible Shear‐Thinning and Thixotropic Mechanism

We present results of ring-shear frictional resistance for mudstone granules of different size obtained from a landslide shear zone. Little rate dependency of shear resistance was observed in sand-sized granules in any wet or dry test, while saturated gravel-sized granules exhibited significant and abrupt reversible rate-weakening (from μ = 0.6 to 0.05) at about 2 mm/s. Repeating resistance variations occurred also under constant shear displacement rate. Mudstone granules generate mud as they are crushed and softened. Shear-thinning and thixotropic behavior of the mud can explain the observed behavior: with the viscosity decreasing, the mud can flow through the coarser soil pores and migrate out from the shear zone. This brings new granules into contact which produces new mud. Thus, the process can start over. Similarities between experimental shear zones and those of some landslides in mudstone suggest that the observed behaviour may play a role in some landslide kinematics.