What is Liquefaction?
- Mechanisms and Liquefaction Damages -
RiverLevee Damaged by Liquefaction
Manhole Damaged by Liquefaction
(Urayasu city, Chiba)
The Great East Japan Earthquake in March 2011 caused liquefaction damages over a wide area including Tokyo Bay, Kasumigaura and its surroundings and the upper reaches of the Tone River, which are far from the epicenter. The mechanism of liquefaction has already been clarified, but since this earthquake focused attention on liquefaction again, liquefaction and how it causes disasters are outlined below.
The liquefaction mechanism will be described with reference to diagrams. When loosedeposit sand is slowly disturbed, the engagement among particles is changed, reducing the volume. If this sand layer is located below the groundwater level and water (pore water) permeates between sand grains, then pore water is discharged to the outside as the volume is reduced. However, when an earthquake shakes many times in a short period, the sand grains try to quickly reduce their volume but the discharge of pore water cannot keep pace. The sand particles thus become suspended in pore water, the strength of the sand is suddenly reduced, and the sand may become like a liquid. After the earthquake has finished, the pore water is progressively discharged, and the ground sinks by an amount equal to the lost water, and then stabilizes almost in the previous state. This phenomenon is called liquefaction, and is known to occur easily in a loose sand layer below the groundwater level.
Photographs of damages to river levees and sewerage facilities are shown as examples of liquefaction damages to civil engineering structures caused by the Great East Japan Earthquake. Liquefaction prevents the ground from supporting the weight of the river levee, causing the levees to subside and deform. If the subsidence causes the height of the levee to fall below the water level of the river, the river water will flood into protected lowland and cause a large secondary disaster. Since buried facilities that contain air, such as manholes, have a small apparent weight, buoyancy causes the facilities to float during liquefaction. Floating sewerage pipes impede the discharge of sewerage and obstruct road traffic.
Liquefaction has caused much damage in previous earthquakes, such as the 1964 Niigata Earthquake and the 1995 Kobe Earthquake. Accordingly, the PWRI has been continuously studying methods of predicting the occurrence of liquefaction and measures to protect civil engineering structures susceptible to liquefaction.
We are currently assessing the liquefaction damages caused by the recent earthquake, and reviewing a method for predicting liquefaction. We will continue with our research to improve the technique for predicting liquefaction and protection measures developed so far.
(Contact: Soil Mechanics and Dynamics Research Team)
Creation of Spawning Grounds for Sandfish
- Use of artificial seaweed to restore the functions of lost spawning grounds -
Fig. 1 Changes in sandfish catch from
the Sea of Japan off the Hokkaido coast
Artificial seaweed and sandfish egg masses
Sandfish spawning on artificial seaweed
The sandfish (Arctoscopus japanicus) is a cold-water benthic species inhabiting muddy or sandy seabed areas at depths of 150 ~ 300 m. It matures with well-developed gonads and spawns on seaweed beds along coasts in November when the water temperature drops to 10℃ or below. However, the area of Japans seaweed beds, which generally provide spawning grounds and nursery functions, has decreased by 40% in the past 30 years (2010 White Paper on Fisheries), and this has had a variety of effects in many areas throughout the nation. The annual catch of sandfish from the Sea of Japan off the Hokkaido coast was approximately 1,000 tons in the 1970s, but has rapidly decreased since 1983 (Fig. 1). This is considered due in part to spawning ground deterioration caused by a decrease in the amount of large seaweed (e.g., Sargassum fulvellum (Turner) C. Agardh), which provides spawning grounds for sandfish, as a result of barren ground along the Sea of Japan coast.
Possible measures to restore sandfish spawning environments are: 1) promoting the growth and development of communities of large seaweed species to act as spawning grounds; and 2) installing artificial seaweed in place of Sargassum fulvellum (Turner) C. Agardh. As implementing measure 1) would take a significant amount of time, the Fisheries Engineering Research Team has been working on research relating to artificial seaweed toward the implementation of measure 2). Artificial seaweed must satisfy the following requirements: 1) it must stand on its own to allow sandfish to spawn; and 2) it must be durable enough to withstand the forces of waves. In this context, artificial seaweed was developed using the Civil Engineering Research Institute for Cold Regions irregular oscillatory water tunnel, which reproduces the back-and-forth currents seen on the seabed. Artificial seaweed was installed at Ofuyu Fishing Port in the town of Mashike on Hokkaidos Sea of Japan coast, and related surveys were carried out. The results showed that a large number of sandfish eggs were laid there (Photo 1). Sandfish spawning is closely related to the water depth/temperature and waves of spawning migration routes. The Fisheries Engineering Research Team succeeded in filming sandfish spawning (Photo 2), and also obtained data on waves, flow rate, water temperature and other variables from a survey conducted in December 2010. These activities represent the teams efforts to clarify suitable physical conditions for spawning grounds. Further field surveys and studies on sandfish spawning environments are expected to enable the proposal of suitable locations for the installation of artificial seaweed in other regions.
(Contact: Fisheries Research Team,CERI)