Takenori Hino

Anisotropic Consolidation Behavior of Ariake Clay from Three Different CRS Tests

Three types of constant rate of strain (CRS) consolidation tests were conducted on samples of undisturbed Ariake clay, using a newly developed consolidometer, to investigate the anisotropic consolidation behavior of the clay. CRS tests conducted using vertically cut specimens (with respect to the in situ condition) tested with vertical (or end) drainage (with respect to test condition) were designated as CRS-V-V tests. Specimens cut vertically but with radial drainage were designated CRS-V-R, whereas those cut horizontally and tested with vertical drainage were designated CRS-H-V. The test results show that the ratio of the consolidation yield stress of a horizontally cut specimen (pch) to that of a vertically cut specimen (pcv) is in a range from 0.5 to 1.0. Both pcv and pch increased about 15 % with a tenfold increase in strain rate, but there was no clear difference in the degree of strain-rate dependency for pcv and pch. Values of the coefficient of consolidation obtained from CRS-H-V (chh) and CRS-V-R (chv) test are larger than those measured in CRS-V-V (cv) tests, and it has been identified that these differences arise mainly from the anisotropy of hydraulic conductivity (k). The ratio of k in the horizontal direction (kh) measured in a CRS-V-R test to that in the vertical direction (kv) from a CRS-V-V test is about 1.65, and the ratio of chv/cv is about 1.54. The value of kh from a CRS-H-V test is generally smaller than that from a CRS-V-R test.

Density and undrained shear strength of bed sediment from ND-CPT

The density and the undrained shear strength (su) of bed sediments at either side of the Isahaya Bay dike in the Ariake Sea, Japan, were investigated using nuclear density cone penetration tests (ND-CPTs). The nuclear density cone penetrometer (ND-CP) was operated from a boat, conducting 71 ND-CPTs. Furthermore, 26 undisturbed samples were obtained for soil density and su measurements to calibrate the ND-CPT data. The results show that the density and su obtained from in situ with the ND-CPTs agree well with those from the laboratory tests on undisturbed samples, and the obtained density profiles show good repeatability. The vertical variation in density and su of the bed sediment can be obtained from the ND-CPT results. The relations between the density and su show that su increases with an increase in density, but that this relation is site specific. The values of su show an exponentially growing trend and the values of log su show a linearly increasing trend with density. The vertical distribution of the bed sediments can be described using the density values obtained with the ND-CPT.

Environmental changes in Ariake Sea of Japan and their relationships with Isahaya Bay reclamation

This paper reviews the recent environmental deterioration in Ariake Sea, Japan, including an increased frequency of red tides and hypoxic waters and decreased fishery production. Analysis of the mechanisms of environmental deterioration suggests that it is possibly induced by the decrease in tidal flat area, decreases in the tide and tidal current and changes in the sediment environment. The Isahaya Bay reclamation project resulted in the loss of 1550 ha of tidal flats, and is one of the possible reasons for the decreases in the tide and tidal current. Therefore, some fishermen and researchers believe that opening the reclamation project dikes floodgates is an effective environmental restoration countermeasure for Ariake Sea. However, the central government decided not to open the floodgates at present due to strong opposition from local farmers, and some researchers believe that the influences of the Isahaya Bay reclamation project on the environmental changes outside of Isahaya Bay are minor. Several lawsuits regarding these relationships and the opening of the dikes floodgates are currently under dispute. To revive Ariake Sea as a sustainable ecosystem, other countermeasures for environmental restoration are suggested and discussed in this paper.

Undrained Shear Strength Prediction and Stability Analysis of Geocomposite Reinforced Embankment with Clayey Backfill

A method of predicting undrained shear strength (S u ) within a dual function geocomposite reinforced embankment with clayey backfill is described, in which the effects of discharge capacity (Q w ) of the geocompoiste, spacing (2B) between geocomposite layers, construction speed (V), and the coefficient of consolidation (C v ) of the backfill are considered. Then Q w values of three geocomposites were measured by laboratory tests under the confinement of clayey soils. The test results indicate that to maintain a higher long-term (more than 1 month) confined in clayey soil Q w value, a geocomposite must have a drainage core or tube. Then by referring the measured Q w values, the effects of Q w , 2B, as well as V on S u distribution within an assumed 5 m high embankment were investigated by the proposed method. With the predicted S u values, the factor of safety (FS) of the assumed embankment has been investigated by Bishop’s slip circle method, and the results demonstrate that a 5 m high embankment with clayey backfill can be successfully constructed using dual function geocomposite.