It is Niigata earthquake that occurred in 1964 (Showa 39) that liquefaction was had close-up of. Many buildings and bridges sank in Niigata or collapsed till then as the thick sand layer trusted as the ground which supported building softened instantly.
Liquefaction was greatly reported by the Great East Japan Earthquake, the Great Hanshin-Awaji Earthquake, the earthquake in western Tottori Prefecture, Geiyo earthquake.
Particle of soil overlaps in the ground. This soil particle gets caught and becomes in a state with groundwater in the gap on the ground where is with water level under the ground or less of. However, coming together of soil particle gradually comes off when shaken from side to side by earthquake. Soil particle breaks up at last and is in a condition to have loosened in groundwater. This is liquefaction.
Damage by liquefaction is classified in two of next mainly.
- Subsidence and slants such as buildings which occur by support power of the ground decreasing
- Damage by *sa (phenomenon that water and sand blow up from the underground)
With sandy area board which the ground which may liquefy most is sandy area board and is loose, groundwater is the ground existing to shallow depth of the neighborhood of earth surface more. On the contrary, it is said that it is hard to have liquefaction when we give case and the ground improvement that are in condition not to be dipped into groundwater even if it is sandy area board.
Collapse of apartment by liquefaction
(1964 Niigata earthquake/Kishimachi, Ichikawa, Niigata)
odai*sakou by liquefaction
(1983 Sea of Japan central part earthquake/Sharikimura, Aomori)
※We extract from "liquefaction map and measures method" (Gyosei)
We may not give up immediately because the ground is bad. If we keep the ground under control with number properly and take proper appropriate measures, we can build building soundly.
In reference to liquefaction map, you carry out the ground investigation about place very likely to be liquefaction, and please grasp the situation such as stratum or water level under the ground.
In addition, we recommend that boring survey is carried out as much as possible as the ground investigation has difficult judgment of liquefaction by simple methods such as Swedish style sounding.
We just looked at the ground, and it is difficult to grasp the property. If, at first, we start with knowing the property and take measures strengthening earthquake proofing performance of the ground and building about the ground where we are going to build land, building which we are going to purchase newly, we can reduce damage.
※With Swedish style sounding
The simple investigation into ground method to check strength. We place screw-formed stick in the ground and judge strength of layer of the rotary ground which let you penetrate.
"Raft" is distinguished from "the cloth basics" that steel reinforced concrete is installed only in main part in a belt of lower part of building about board-formed full-scale foundations of steel reinforced concrete and is used.
In other words, the cloth basics support load in line whereas raft supports building with aspect. Therefore because raft contains reinforcing rod in mesh of a net to length and breadth, it may be said that it is the basics that are more firm than the cloth basics and is the basics that can expect action to equalize uneven subsidence (say nonequivalent subsidence) to occur from structural characteristic that bend is hard to appear.
As raft is very effective as preventive measures against nonequivalent subsidence, we are used many on the soft ground. Even so, we do not say all possible measures if it is raft.
- Self-respect of the basics itself becoming heavy, and stimulating the soft ground accordingly
- It may rather cause nonequivalent subsidence in place that balance of the ground has bad
There is nadono demerit, and judgment by expert is required when we adopt.
In addition, there are not ups and downs in flatland where the neighborhood is large, and stratum not changing and precondition that alien substance does not enter laying earth on the ground, or it is grasped beforehand by result of the ground investigation that it is the good ground of balance are required whether we may adopt raft.
In addition, by "the Great East Japan Earthquake" generated on March 11, 2011, house of raft was hit by subsidence by liquefaction, too. As raft has heavy weight of concrete, it may be said that liquefaction measures include problem.
When the ground which supported load of building becomes like fluid till then, and the basics sink into one of the ground, and building inclines when liquefaction happens once, two ways of phenomena when the basics are lifted by water and sand (*sa) blowing up adversely from the underground occur. Building inclines by sedimentation or upheaval if we lose support of the ground.
Liquefaction occurred on unprecedented scale, and, by the Great Hanshin-Awaji Earthquake, there was damage such as building inclining in Port Island and Rokko Island which were artificial island, house of reclaimed land of seaside part, but liquefaction damage of high-rise building was relatively small in apartments.
As this lets building arrive at the hard ground with support stake and stake and building were fixed closely, it is said that upheaval avoided subsidence.
But there is not danger of life, but it is necessary for liquefaction to take measures against such a facility when the outskirts ground sinks by liquefaction even if subsidence and upheaval are not caused by pile foundation structure as for high-rise building itself in apartments because facilities such as water supply, gas, sewage which is public utilities are buried underground when these facilities suffer because difficulties are forced to in life.
We recommend that it is talked about confirmation with construction supplier and expert what kind of design, structure building to live is built with.
Generally it is not performed that dozens of meters hit support stake with detached residence like the medium- and high-levels layer building and take liquefaction measures. The reason is simply that we say significant increase in construction cost, but it may be said that considerably wide construction space is necessary to cast solid support stake.
In addition, not only support stake, it is extremely only several kinds of construction methods that can just apply liquefaction measures such as large-scale engineering works and various ground improvement methods validated on the site of building in detached residence.
As liquefaction measures of detached residence, "the outer layer ground improvement methods" generally spread as possible measures. When there is stratum which does not liquefy from the ground to depth of around 2m from damage experience of past earthquake, there is findings that damage decreases. Thus, we tighten the ground to this depth and harden, and the ground improvement such as rearranging in soil which it is hard to liquefy is said to be effective method of liquefaction measures.
In addition, "drain pipe construction method" to bury draining off pipes (pipe which there are a large number of small holes in) of 5-10cm in diameter every 50-120cm in the ground is one of the damage reduction plans. This is Yokohama-shi and construction method by joint development with company and lets the water pressure in the ground rising at the time of major earthquake flow in this pipe and is construction method to restrain rise of the basement water pressure causing liquefaction.
In any case, in the case of new construction, we build new chief priest's house who performed construction or the ground improvement of pile foundation, and, in the case of existing, measures improving the ground are thought about by construction methods such as medicinal solution infusion.
Drain pipe method
We take measures that we accepted structure "not to produce" liquefaction in area very likely to be liquefaction of the ground. This "structure" greatly has three things, and the following measures method is adopted each.
In addition, in the reclaimed land of Yokohama, we use construction outbreak soil which liquefaction is hard to relatively happen and add to subsidence measures with it is Minato Mirai 21 district, and liquefaction further takes measures. In this way, with large-scale earthquake, *sagensho (phenomenon that water and sand blow up from the underground) is expected, but it is thought that early restoration remains for possible small damage.
Structure "and measures method not to produce" liquefaction
|Structure not to produce liquefaction||Main measures method|
|We make drain (pipe with high permeability) in the ground and reduce the water pressure in the ground which we produced by earthquake immediately.||Paper drain method|
Rubble drain method
Sponge gourd drain methods
|We push sandpile in the ground and by dropping weight on the ground, tighten the loose ground and harden.||Sand compaction pile method|
|We mix solidification materials such as cement in the ground and harden the loose ground.||The depths mixture processing construction method|