Yoichi Watabe

Burrowing Criteria and Burrowing Mode Adjustment in Bivalves to Varying Geoenvironmental Conditions in Intertidal Flats and Beaches

The response of bivalves to their abiotic environment has been widely studied in relation to hydroenvironmental conditions, sediment types and sediment grain sizes. However, the possible role of varying geoenvironmental conditions in their habitats remains poorly understood. Here, we show that the hardness of the surficial intertidal sediments varies by a factor of 20–50 due to suction development and suction-induced void state changes in the essentially saturated states of intertidal flats and beaches. We investigated the response of two species of bivalves, Ruditapes philippinarum and Donax semigranosus, in the laboratory by simulating such prevailing geoenvironmental conditions in the field. The experimental results demonstrate that the bivalve responses depended strongly on the varying geoenvironmental conditions. Notably, both bivalves consistently shifted their burrowing modes, reducing the burrowing angle and burial depth, in response to increasing hardness, to compensate for the excessive energy required for burrowing, as explained by a proposed conceptual model. This burrowing mode adjustment was accompanied by two burrowing criteria below or above which the bivalves accomplished vertical burrowing or failed to burrow, respectively. The suitable and fatal conditions differed markedly with species and shell lengths. The acute sensitivities of the observed bivalve responses to geoenvironmental changes revealed two distinctive mechanisms accounting for the adult–juvenile spatial distributions of Ruditapes philippinarum and the behavioral adaptation to a rapidly changing geoenvironment of Donax semigranosus. The present results may provide a rational basis by which to understand the ensuing, and to predict future, bivalve responses to geoenvironmental changes in intertidal zones.

Modeling and Implementation of the Isotache Concept for Long-Term Consolidation Behavior

AbstractA practical use of the isotache concept in creep settlement prediction is newly introduced in this study. The isotache concept introduces a unique relationship between the strain and the consolidation pressure corresponding to the strain rate in association with viscosity. The authors have proposed a method to simply introduce the isotache concept, which uses a compression curve normalized by preconsolidation pressure [e–log(σ′/σp′)] and a strain rate dependency relationship between the preconsolidation pressure (σp′) and strain rate (e˙=de/dt). The strain rate dependency relationship is modeled as an equation based on power law. According to the authors’ method, the slope of the strain rate dependency relationship (log⁡σp′–loge˙), which coincides with Cαe/Cc, can be calculated as a function of strain rate, showing that Cαe/Cc is not constant but decreases with decreasing strain rate. In engineering practice, consolidation settlement is generally estimated based on the compression curve obtained f...

Accounting for Carbon Stocks in Soils and Measuring GHGs Emission Fluxes from Soils: Do We Have the Necessary Standards?

Soil is a key compartment for climate regulation as a source of greenhouse gases (GHGs) emissions and as a sink of carbon. Thus, soil carbon sequestration strategies should be considered alongside reduction strategies for other greenhouse gas emissions. Taking this into account, several international and European policies on climate change are now acknowledging the importance of soils, which means that proper, comparable and reliable information is needed to report on carbon stocks and GHGs emissions from soil. It also implies a need for consensus on the adoption and verification of mitigation options that soil can provide. Where consensus is a key aspect, formal standards and guidelines come into play. This paper describes the existing ISO soil quality standards that can be used in this context, and calls for new ones to be developed through (international) collaboration. Available standards cover the relevant basic soil parameters including carbon and nitrogen content but do not yet consider the dynamics of those elements. Such methods have to be developed together with guidelines consistent with the scale to be investigated and the specific use of the collected data. We argue that this standardization strategy will improve the reliability of the reporting procedures and results of the different climate models that rely on soil quality data.

Use of Cement-Treated Lightweight Soils Made from Dredged Clay

Recently, a new runway designated “D-runway” was constructed at Tokyo Haneda Airport. In this project, a large amount of cement treated lightweight soils made from very soft and mildly contaminated dredged clay was used. In this paper, a seawall structure using a cement treated lightweight soil made from dredged clay is introduced and the quality control of these soils in the construction work is discussed. Two types of lightweight soils, “pneumatic mixing cement treated soil” and “air-foam treated lightweight soil,” were used as backfill in the seawall structures. Design and quality control of the lightweight soils were carried out, and the seawall structures were successfully constructed. The shear strength and bulk density of the lightweight soils placed in the sites met the required specifications in design.

Effects of Tide and River Discharge on Mud Transport on Intertidal Flat

Fine-grained sediment budgets based on the monthly bed level and net sediment flux monitoring from October 2006 to October 2007 are estimated to examine the relative contributions of tides and river discharge to mud transport on an intertidal flat adjacent to a river mouth in a semiclosed sound. The estimated sediment budgets are interpreted using the water mass balance equation and the horizontal tidal current pattern measured using high frequency (HF) radar. The intertidal flat accreted during normal discharge conditions are primarily attributable to the alongshore sediment flux toward the river mouth. However, the flat was eroded when the large offshore suspended sediment transport occurred on the flat during the large river discharge. The net alongshore tidal current causes alongshore sediment fluxes toward the river mouth on this intertidal flat adjacent to the river mouth. These findings may be specific to this site, but an adopted comprehensive approach may be applicable to other sites.

Soil Parameters in the New Design Code for Port Facilities in Japan

In the new design code for port facilities in Japan, the depth profile of the derived values is modeled as the profile of the estimated values so as to be either the mean value or the regression line, then the correction factors are multiplied to the estimated value according to the coefficient of variation (if COV > 0.1) and the number of the data entries (if n < 10). The new port-design code is applied to both the unconfined compression test results and recompression triaxial test results in order to evaluate the undrained shear strengths to emphasize that the latter test has many advantages in design over the former test.

Simplified Modelling of Isotache Concept for Consolidation

The authors have proposed a simplified method based on the isotache concept by using a compression curve and a relationship between the preconsolidation pressure and the strain rate. The former and the latter can be obtained from one constant rate of strain consolidation test (CRS test) and one long-term consolidation test (LT test), respectively. The latter is expressed by an equation with three isotache parameters initially determined for Osaka Bay clays but generalized to worldwide inorganic clays with various characteristics.

Long-Term Properties of Airfoam-Treated Lightweight Soil Made from Dredged Clay

Airfoam-treated soil is lightweight and provides for the early age development of shear strength and effective utilization of dredged soils. This paper summarizes a 10-year follow-up study of the lightweight soils that were placed as backfill at the seawall in Kobe Port Island and Tokyo International Airport. These two sites are the early case examples of construction undertaken in 1996 including the recovery project after the Kobe earthquake disaster and the offshore expansion project of the Tokyo International Airport. Sampling and laboratory testing were carried out in order to investigate the physical and mechanical properties to compare the initial material condition. The depth profiles of the soil parameters such as bulk density, water content, pH, Ca content, shear strength, and needle penetration resistance were examined. Consequently, it was confirmed that the physical and mechanical properties of the airfoam-treated lightweight soil satisfied their required performances, indicating that the lightweight soil has sufficient durability for use as geomaterial for construction in coastal areas.

Discussion of “Settlement of the Kansai International Airport Islands” by G. Mesri and J. R. Funk

Technical information about the Kansai International Airport has been published by some of those involved in the project. Although the authors have not been involved in the project, they have gathered the published information and unpublished data, and then integrated them into a well-documented technical report from a viewpoint focusing on the consolidation settlement of the artificial islands. First, the discusser would like to express his appreciation to the authors for their excellent work. In the latter part of the article, the authors calculated the consolidation settlement using the ILLICON software program, which is based on the uniqueness of the end-of-primary (EOP) void ratio–effective vertical stress relationship and constant Cα=Cc concept. From the discusser’s experiences, as well as many criticisms on the first author’s model (e.g., Leroueil 2006; Degago et al. 2011, 2013), the assumptions adopted in ILLICON are highly questionable. Thus, the discusser doubts the authors’ prediction about the consolidation settlement of the artificial islands at Kansai International Airport.

Sedimentation History of Sandbars in Flood-Tidal Delta Evaluated by Seismic Method in Lake Tofutsu, Japan

ABSTRACT Watabe, Y. and Sassa, S., 2016. Sedimentation history of sandbars in flood-tidal delta evaluated by seismic method in Lake Tofutsu, Japan. An extensive pattern of sandbars within a flood-tidal delta complex can be seen at the tidal inlet of Lake Tofutsu, one of the lagoons located along the Okhotsk Sea in Hokkaido, Japan. The primary aim of this study was to investigate the sequential sedimentation history of these sandbars. The shear-wave velocity structure, which is equivalent to the stiffness variation, of these sandbars was examined using a surface-wave method called the multichannel analysis of surface waves (MASW) to identify and describe the stratigraphy of the flood-tidal delta at the tidal inlet of Lake Tofutsu. The MASW results demonstrated that even though the sandbars appeared to comprise the same sandy material, the sediment under the sandbars had a very varied stratigraphy, comprising the river-borne muddy sediment derived from the upstream side of the lagoon and wave-induced marine sandy sediment derived from the coastal sand of the Okhotsk Sea. The second aim of this study was to verify the sedimentation history described on the basis of the MASW results through laboratory testing of sediment samples collected at characteristic points along the MASW array. The sampling method was modified to be applicable to low-cost sediment investigation at tidal flats and was conducted by human power without the aid of an engine. The collected samples were examined in the laboratory to obtain the depth profiles of grain-size distribution and carbon 14 dating. The results obtained from the sampling are consistent with the sequential sedimentation history evaluated by MASW.