Four-Year Monitoring Study of Shallow Landslide Hazards Based on Hydrological Measurements in a Weathered Granite Soil Slope in South Korea

To build a comprehensive understanding of long-term hydro-mechanical processes that lead to shallow landslide hazards, this study explicitly monitored the volumetric water content (VWC) and rainfall amount for a weathered granite soil slope over a four year period. From the 12 operational landslide monitoring stations installed across South Korea, the Songnisan station was selected as the study site. VWC sensors were placed in the subsurface with a grid-like arrangement at depths of 0.5 and 1.0 m. Shallow landslide hazards were evaluated by applying an infinite slope stability model that adopted a previously proposed unified effective stress concept. By analyzing the variations in the monitored VWC values, the derived matric suctions and suction stresses, and the calculated factor of safety values, we were able to obtain numerous valuable insights. In particular, the seasonal effects of drainage and evapotranspiration on the slope moisture conditions and slope stability were addressed. Preliminary test results indicated that continuous rainfall successfully represented the derived matric suction conditions at a depth of 1.0 m in the lower slope, although this was not the case for the upper and middle slopes. The significance of a future study on cumulative field monitoring data from various sites in different geological conditions is highlighted.

Variation in strength owing to solidification treatment of weathered granite soil

Recently, soil-cement is being increasingly used in various applications such as road pavements, slope protection, backfilling of earth walls, and improving soft ground, in order to increase the strength of the raw material soil. Therefore, in this study, the characteristics of changes in the unconfined compression strength of soil-cement were analyzed by adding a solidifying agent targeting granite soil, which is representatively distributed in Korea. Laboratory tests were conducted to analyze the changes in strength according to 1) curing conditions and fine contents, 2) inorganic solidifying agent, and 3) repeated effects of dry and wet conditions. The unconfined compression strength of soil-cement increased as the curing period and mixing ratio increased, and it was constant after 14 days of curing. In addition, weathered granite soil with relatively low fine contents showed a large increase in the unconfined compression strength. In addition, the strength increased with an increase in the solidifying agent added. X-ray-diffraction analysis showed that vermiculite was generated by adding cement and admixtures, and the strength was increased by filling the gap between the particles. Considering the results of this study, a relationship between the unconfined compression strength and the elastic modulus of soil-cement by treatment with a solidifying agent was proposed.

Application Ranges of EPB Shield TBM in Weathered Granite Soil: A Laboratory Scale Study

Soil conditioning is a key factor in increasing tunnel face stability and extraction efficiency of excavated soil when excavating tunnels using an earth pressure balance (EPB) shield tunnel boring machine (TBM). Weathered granite soil, which is abundant in the Korean Peninsula (also in Japan, Hong Kong, and Singapore), has different characteristics than sand and clay; it also has particle-crushing characteristics. Conditioning agents were mixed with weathered granite soils of different individual particle-size gradations, and three characteristics (workability, permeability, and compressibility) were evaluated to find an optimal conditioning method. The lower and upper bounds of the water content that are needed for a well-functioning EPB shield TBM were also proposed. Through a trial-and-error experimental analysis, it was confirmed that soil conditioning using foam only was possible when the water content was controlled within the allowable range, that is, between the upper and lower bounds; when water content exceeded the upper bound, soil conditioning with solidification agents was needed along with foam. By taking advantage of the particle-crushing characteristics of the weathered granite soil, it was feasible to adopt the EPB shield TBM even when the soil was extremely coarse and cohesionless by conditioning with polymer slurries along with foam. Finally, the application ranges of EPB shield TBM in weathered granite soil were proposed; the newly proposed ranges are wider and expanded to coarser zones compared with those proposed so far.

Effect of Loading Path on the Mechanical Properties of Completely Decomposed Granite Soil Based on the Multiscale Method

Mechanical properties of intact completely decomposed granite (CDG) soil, widely distributed in South China, decrease drastically as encountering external load-related disturbance or soaked by water. In this study, staged triaxial consolidated drained (CD) test and microscopic scanning were conducted using intact CDG specimens extracted from a construction site to investigate the mechanical behavior and microscopic pore distribution. The results show that the stress-strain relationship reveals a shrinking behavior in the first-stage loading and a brittle behavior in the second-stage loading. The development of cracks is affected by the principal stress, which causes the pores and cracks to shrink or partially close. In addition, Esec-1 increases linearly with the increase of confining pressure, but Esec-2 decreases exponentially.

Experimental assessment of the relationship between rainfall intensity and sinkholes caused by damaged sewer pipes

In several countries, the rising occurrence of sinkholes has led to severe social and economic damage. Based on the mechanism of sinkhole development, researchers have investigated the correlation between rainfall intensity and sinkholes caused by damaged sewer pipes. In this study, the effect of rainfall intensity on the formation of eroded zones, as well as the occurrence of sinkholes caused by soil erosion due to groundwater infiltration through pipe defects, has been analyzed through model tests. The ground materials in Seoul were represented by weathered granite soil, which is generally used for backfill sewer pipes, and groundwater levels corresponding to three different rainfall intensity conditions were considered. The ground level changes and ground displacements were measured continuously, and the particle image velocimetry (PIV) algorithm was applied to measure the displacement at each position of the model ground. The results indicate that impeding the excessive rise in groundwater levels by securing sufficient sewage treatment facilities can effectively prevent the development of sinkholes caused by pipe defects.

Experimental assessment of the relationship between rainfall intensity and sinkholes caused by damaged sewer pipes

In several countries, the rising occurrence of sinkholes has led to severe social and economic damage. Based on the mechanism of sinkhole development, researchers have investigated the correlation between rainfall intensity and sinkholes caused by damaged sewer pipes. In this study, the effect of rainfall intensity on the formation of eroded zones, as well as the occurrence of sinkholes caused by soil erosion due to groundwater infiltration through pipe defects, has been analyzed through model tests. The ground in Seoul was adopted using weathered granite soil, which is generally used for backfill sewer pipes, and groundwater levels corresponding to three different rainfall intensity conditions were considered. The ground level changes and ground displacements were measured continuously, and the particle image velocimetry (PIV) algorithm was applied to measure the displacement at each position of the model ground. The results indicate that impeding the excessive rise of groundwater levels by securing sufficient sewage treatment facilities can effectively prevent the development of sinkholes caused by pipe defects.