To properly construct impermeable embankment, typically the core zone of earth-fill dam, it is required to ensure sufficiently low values of the coefficient of hydraulic conductivity, k. To this end, the grading characteristic of backfill and the moisture content and dry density after compaction in the field are measured and controlled. This conventional method is, however, time-consuming therefore restricting the number of measurements. To develop an indirect but much faster method that can control the field k values to be sufficiently low by upper-bound control of field soil stiffness index, S, we performed a comprehensive series of laboratory and field tests changing the compaction energy level, CEL, and particle size distribution characteristics related to the construction of a 139 m-high fill dam. It was confirmed that, when the CEL and the soil type are controlled to be identical to those used in field tests by which the allowable upper bound of S is determined, the k value is kept lower than the allowable upper bound if all measured S values are kept lower than the specified upper bound.
Recently, due to the heavy rain or large scale earthquake, collapses of cut and embankment slope on the expressway have seriously influenced to economic activities and daily lives. About 40% of the expressway network has already passed more than 30 years since the construction period, and the deterioration risk becomes higher than ever. In this study, the authors scored many factors, such as topography, geology, and embankment material, based on the actual 213 collapsed cases on the expressway in Tohoku Region. And evaluated the stability and sustainability of the cut and embankment slope from the viewpoint of preventative maintenance. In addition, reviewed the evaluation result by adopting to the actual collapsed case of typhoon-19 in 2019. As a result, the possibility of the collapse can be prioritized by this scored method, and it’s useful to preventative maintenance countermeasures.
With regard to the seismic behavior of small scale earth dams, many studies focusing on the foundation ground have been conducted. However, there are few researches on the embankment itself, considering the influences of the water level, the degree of compaction, and countermeasure techniques. In this study, a series of centrifuge tests are carried out considering the seepage flow to clarify the effect of countermeasure techniques on the seepage-seismic coupled behavior of an earth embankment. Primary conclusions are summarized as follows. Under a higher water table elevation, a large settlement is induced due to the deformation in both the upstream and downstream directions. When the water level of reservoir is low, the settlement at the crest is less compared with the case explained above. A drainage method, which lowers phreatic line at the downstream side, is effective for reducing deformation. Counterweight fill installed at the upstream side may restrain the slide of the upstream side.
It is well known that the pore pressure fluctuates due to the earth tide and ocean tide. In the Tomakomai CCS conducted in Tomakomai City, Hokkaido, pore pressure fluctuation due to ocean tides were detected from the pore pressure data measured at a CO2 injection well. It has been reported that after CO2 injection, the amplitude decreased and the time lag to ocean tide occurred. This phenomenon is due to the increase in the compressibility of pore fluid induced by inflow of CO2. The phenomena can be explained by mechanical equilibrium under ocean tide loadings and flow of pore fluids in the reservoir. This indicates the possibility of a monitoring method to estimate the CO2 behavior in the reservoir by analyzing the variation of pore fluid pressure due to ocean tide. For this purpose, in order to deal with three-dimensional mechanical equilibrium and CO2 movement, a numerical analysis for fluid-geomechanical problem is suitable. However, there has been no example in which the relationship between the change in the pressure fluctuation and the movement of the CO2 plume has been investigated by numerical analysis. Therefore, in this study, we have developed a fluid-geomechanical coupled analysis program (SWN3D) based on the theory of poroelasticity in the presence of two-phase flow. We have conducted a simulation for the reduction of amplitude and time delay of the water pressure response during CO2 injection. As a result, it was confirmed that the decrease in amplitude and time delay of the pore pressure fluctuation appeared in the numerical analysis results. In this analysis, the amplitude and time delay of the pore pressure fluctuation change in conjunction with the CO2 behavior in the reservoir. That indicates possibility that the CO2 behavior in the reservoir can be monitored by analyzing the pore pressure fluctuation due to ocean tide.
This paper presents consolidation property of three cement-treated marine clays through constant strain rate consolidation test. In addition, chemical properties such as exchangeable cations (Ca2+, Mg2+, K+. Na+) and cation exchange capacities of them were investigated to evaluate permeabilities by Komine and Ogata’s method (2008). Following conclusions were obtained from this study. 1) Permeability of cementtreated clays on were 2.98 ~ 4.26 times higher than untreated clays by their solidification effect and exchange of exchangeable cations between Ca2+ and Mg2+. 2) Permeability of untreated clay were well corresponded to estimated values of Komine and Ogata’s method. They showed 0.65 ~ 1.96 times as much of experimental values on average. However, estimated permeabilities of cement-treated clays were 3.50 ~ 8.51 times higher estimated than obtained values when consolidation pressure approached to 5000 kPa. It can be assumed that estimated component of exchange capacity of Ca2+ were higher than the actual values without considering solidification of cement.
It is necessary to evaluate the deformability and initial stresses of rock masses in order to study the stability of cavities and tunnels excavated in deep underground. For initial stress measurement rock masses, in Japan, hydraulic fracturing method and stress release method have been used. However, these measurement methods are costly and time consuming.
On the other hand, borehole jack test is an effective method to measure rock deformabilities. The stiffness of rock masses is calculated from the slope of the linear part of the pressure-displacement curve obtained by the test. In this study, it is theoretically revealed that the initial stresses in rock masses are analized from pressure-displacement curves obtained from a set of tests where three or more loading orientations are assigned.
In this paper, the theory to find initial stress from borehole jack test is derived using exact elastic solution, and the result that its validity is verified by laboratory experiments using large concrete specimens is shown.
In the high-level radioactive waste geological disposal, bentonite buffer material will be keeping exposed to complex environmental of high pressure and high temperature. Therefore, bentonite buffer material will be consolidated and its physical properties such as swelling and permeability will be changed. In this study, the authors thought that bentonite raw ore, consolidated in the natural ground, will have similar properties of consolidated buffer material. Three sodium type bentonite ore were choosen whose mined places and age were different. For experimental evaluation of influence of cementation on swelling properties, the authors made undisturbed and reconstituted specimens, and conducted swelling pressure and one-dimensional swelling deformation tests. As a result, for the undisturbed specimens, it was tended to have lower swelling properties than the reconstituted specimens by the effect of cementation. Also, the swelling properties tended to decline as the geological age of the ores was older.
Product extension of the precast L-type retaining wall is short and the products are not rigidly connected. So it receives lateral pressures directly from wheel loads each time vehicle passes. The lateral pressures increase as traveling position of vehicle approaches the wall. In addition, as the retaining wall height decreases, the ratio of lateral pressures due to wheel loads to the total earth pressures increase. In this study, loading was repeatedly performed on the backfill material packed in a box made of lightweight section steel, and the lateral pressures acting on the wall of the box were measured. The measured lateral pressures were represented by an approximate equation obtained by multiplying the Boussinesq equation by a coeﬃcient. The shear force and bending moment acting on the stem of the precast L-type retaining wall were calculated by using the approximate equation. As a result, it was found that the bending moment acting on the stem under repeated loading due to the vehicle is bigger than a uniform load of 10 kN/m2 in some case.
In the Green Tuff region at Tohoku district, many embankments have been constructed fragile rocks from taken the near site. The embankment has been constructed to tightly compact so that the air porosity is less than 15%. However, in recent years, the fragile rock embankment has been deformed and collapsed due to aged deterioration, which is a problem at the site of maintaining and managing expressways. In this paper, we investigated the physical property change of the valley filling embankment in which cracks intermittently occurred on the pavement surface after several years of service start, and considered the causal relationship between changes in physical property of the embankments and their deformation. As a result, it was found that the slaking rock slag was considered to have been refined from the initial stage of the service. In addition, fine particles moved downward during the infiltration and discharge of rainwater, and loosening and hollowing occurred at the upper shoulder. In addition, it was found that the groundwater level into the embankment was raised by making the finely divided soil into a clay below the embankment, which resulted in instability of the embankment slope. In addition, the change in water permeability due to the actually adjusted particle size distribution was also evaluated.