JPH0424330A - Earthquake-proof reinforcing structure of construction - Google Patents

Abstract

PURPOSE:To prevent a liquefying of the ground by providing penetration holes in the axial direction to plugs installed to numerous holes engraved on the pipe wall, and using a liquefying preventive tube in which filters are arranged in the penetration holes as a part of holding pillars of the foundation of a construction. CONSTITUTION:To each small hole of the holes 2 of a liquefying preventive pipe 1, a plug 4 is installed. And the plug 4 is composed of a shaft 5, plug removal-preventive members 9 and 10, and a filter protective member 8, and a penetration hole 6 is formed in the axial direction. And the pipe 1 is used as a basic pile set on a ground liable to be liquefied, and the void water in the ground is exhausted into the hollow pipe 1 through the plugs 4 so as to escape the excessive void water generated in the liquefied ground in an earthquake. Consequently, a generation of liquefying can be prevented.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention is applicable to bridges, buildings,
This relates to seismic reinforcement structures for pile foundations of structures such as storage tanks or direct foundations of houses.

[Conventional technology]

Conventionally, as a countermeasure against liquefaction, there is a construction method in which a large number of perforated pipes are installed discretely in the ground, and excess pore water pressure generated during an earthquake is dissipated into the perforated pipes to prevent liquefaction (Japanese Patent Laid-Open No. 61 -146910 Publication, JP-A-62-211416
(Refer to the publication number, etc.)

That is, as shown in FIGS. 28(a) to 28(C), a perforated pipe 61 is provided with a large number of small holes 62 in the longitudinal direction and the circumferential direction, and a water-permeable filter 63 is provided in each hole. By erecting the pipe at a predetermined depth in sandy ground that is at risk of liquefaction, excess pore water pressure generated in the sandy ground during an earthquake can be dissipated into the perforated pipe 61 through the hole 62. can. The water-permeable filter 63 is provided to prevent sand from entering the perforated tube 61 together with interstitial water.

Furthermore, an earthquake-reinforced structure using such a perforated pipe having a liquefaction inhibiting function as a foundation pile of a structure has been proposed.

[Problem to be solved by the invention]

The above-mentioned water-permeable filters are usually made of synthetic fibers such as Tetron and nylon, synthetic resins, or thin metal materials, and are easily damaged by contact with earth and sand, construction machinery, and the like. If the hole is damaged, it will not be able to sufficiently prevent sand from entering through the hole. Therefore, there is a problem in that the perforated pipe must be handled with great care when transporting it, installing it on the ground, etc.

In addition, in order to reduce the possibility of damage due to contact with earth and sand, construction machinery, etc., it is usually preferable to install the above filter from the inside of the perforated pipe. Due to the narrow space, installation is very complicated.

Furthermore, as mentioned above, filters are easily damaged, so conventionally, for example, when installing a perforated pipe with a filter on the entire outer circumferential surface of the pipe in the ground, the steel casing pipe was rotated with an auger to the desired depth. After the construction is complete, a perforated pipe is passed through the casing pipe and erected to a specified depth, then the tip of the casing pipe is opened, the casing pipe is pulled out of the ground, and the perforated pipe is left in the liquefaction layer. A method is being taken. However, this method not only requires large-scale construction, but also creates a gap between the outer diameter of the casing pipe and the outer diameter of the perforated pipe. will inevitably become loose, and if left as is, it may increase the risk of liquefaction.

Therefore, it is practically difficult to use the above-mentioned perforated pipes as foundation piles for structures, and not only is construction difficult, but also in terms of liquefaction prevention function and function as foundation piles. It was insufficient.

The present invention is an attempt to solve the above-mentioned problems, and uses a liquefaction prevention pipe that is easy to handle during transportation and installation on the ground, and has a reliable liquefaction prevention function. The aim is to provide a large and safe seismic reinforced structure.

[Means to solve the problem]

The liquefaction countermeasure pipe used in the seismic reinforced structure of the present invention has many holes in the pipe wall, and plugs equipped with water-permeable filters are attached to the holes, making it easy to transport the perforated pipe. In addition to preventing damage to the filter during installation in the ground, the installation of the filter greatly facilitates the work. That is, the shaft portion of the stopper has a through hole in the axial direction, and a filter is provided within the through hole.

In the through hole of the stopper, a water-permeable filter protection member can be provided together with the filter to prevent damage to the filter due to earth and sand. Further, in order to prevent the plug from separating from the pipe when the pipe is transported or installed on the ground, it is possible to provide a detachment prevention portion having a diameter larger than the diameter of the hole in the pipe at one or both ends of the plug.

As a method for manufacturing a liquefaction countermeasure pipe, a perforated pipe with a filter can be easily manufactured by fitting the above-mentioned plug into a hole in a perforated pipe.

In addition, the liquefaction countermeasure pipe used in the seismic reinforcement structure of the present invention is
The filter is installed inside the through-hole of the stopper, and a filter protection member is installed if necessary, so it can be easily removed using a diesel hammer, hydraulic hammer, vibro hammer, press-fit machine, etc. without using a casing vibrator to protect the filter. , can be installed directly into the ground.

The seismic reinforcement structure of the structure of the present invention uses the above-mentioned liquefaction prevention pipe for at least part of the support piles of the foundation of a structure, such as a building, a bridge pier, an abutment, or a storage tank, or Furthermore, by installing liquefaction prevention pipes around the support piles, it is possible to prevent liquefaction of the ground and form a safe seismic reinforcement structure with high resistance.

In addition, in residential buildings with direct foundations, the above-mentioned liquefaction countermeasure pipes are installed below the direct foundations of houses installed in strata where there is a risk of ground liquefaction during an earthquake, or between the direct foundations of houses or By installing liquefaction countermeasure pipes around the area, it is possible to create a safe seismic reinforced structure with high resistance to residential construction.

[For production]

The principle of liquefaction prevention is that liquefaction prevention pipes are used for foundation piles installed in ground where liquefaction is a concern, and pore water in the ground can be drained into the hollow pipes through plugs attached to the pipes serving as piles. By doing so, the excess pore water pressure generated in the liquefied ground during an earthquake is dissipated and the occurrence of liquefaction is prevented.

Figure 16 shows the results of a free damping wave excitation experiment using a model of a liquefaction countermeasure pipe installed in liquefaction ground. In the ground where liquefaction countermeasure pipes are installed, the excess pore water pressure ratio is well below 0.5, which has a liquefaction prevention effect, whereas in the case of only the ground where no liquefaction pipes are installed, the excess pore water pressure ratio is 0. ．．
When it becomes 6 or more, it becomes almost liquefied.

Figure 17 shows the displacement of the pipe during vibration compared to the normal displacement without vibration when the same load is applied to the pipe in the ground where liquefaction countermeasures pipes and ordinary pipes without liquefaction countermeasures are installed. This shows the tube. The displacement of the pipe during vibration is nearly three times as much in the case of a pipe without countermeasures as compared to that under normal conditions, while in the case of a pipe with liquefaction countermeasures it is 1.5 times as much.
It fits about twice as much. This indicates that the ground where liquefaction prevention pipes have been installed can be expected to have considerable horizontal resistance even in the event of an earthquake.

Figure 18 shows the horizontal load P when a horizontal load is applied by a loading plate to the ground where a liquefaction countermeasure pipe or no countermeasure pipe is installed, and the ground where no pipe is installed, and the entire ground is vibrated in the middle. This figure shows the relationship between δ and loading plate displacement δ. If pipes with liquefaction countermeasures are installed, horizontal resistance can be expected even during excitation, whereas if pipes without countermeasures are installed, no horizontal resistance can be expected during excitation. In other words, in the ground where liquefaction countermeasures pipes are installed, the area around the pipe does not liquefy and the pipe exerts horizontal resistance, whereas in the ground where no countermeasures are installed, the area around the pipe also liquefies and the pipe exerts horizontal resistance. Almost no power left.

As described above, by installing liquefaction countermeasure pipes, it is possible to prevent liquefaction of the ground during an earthquake, and at the same time, it is possible to expect horizontal resistance.

Therefore, when such liquefaction prevention pipes are used for foundation piles of structures, it is possible to prevent liquefaction near the foundation piles where liquefaction is most likely to occur, thereby providing highly safe foundation piles. be able to.

By using a method of attaching a stopper with a filter inside to the hole of the perforated pipe, damage to the filter can be prevented when the perforated pipe is transported or erected on the ground. Further, by providing the filter protection member, it is possible to prevent damage to the filter even if dirt enters in the axial direction of the plug, especially when installing the liquefaction countermeasure pipe in the ground.

In addition, by providing a detachment prevention part at the end of the plug, it is possible to reliably prevent the plug from detaching even when the pipe is installed on the ground.

Regarding the construction method, there is no need to adopt the conventional method of press-fitting a casing pipe into the ground, or rotating a casing auger to make an installation hole in the ground, and then installing a drainage pipe. It can be press-fitted into the ground with a press-fitting machine, and directly with a vibrohammer or diesel hammernato equipment. ! ! It can be placed on the board.

Therefore, construction is much easier than before, and there is a wide range of construction methods available.

Furthermore, regarding the manufacturing method of the liquefaction countermeasure pipe, simply fitting a plug with a filter inside into the hole of a perforated pipe is much easier than installing a filter on a conventional perforated pipe. It becomes easy to use.

〔Example〕

Next, embodiments of the present invention will be described based on the attached drawings.

FIG. 1 shows an outline of a liquefaction countermeasure pipe 1 according to the present invention. A plug 4 is attached to each small hole in a perforated portion 2 of the pipe 1, and a filter is installed inside the plug 4. It is installed internally.

The length of the perforated portion 2 of the pipe may be greater than or equal to the layer thickness of the target liquefied ground.

FIG. 2 shows an example of a cross section of the plug 4 and an example of a method of attaching the plug 4 to the perforated pipe 1. Also, Figure 3 (a
) is a side view of the stopper 4 in FIG. 2, and FIG.
Figure (C) is a front view and a rear view.

In this embodiment, the plug 4 includes a shaft portion 5, a plug detachment prevention portion 9
, 10, and a filter protection member 8, and has a through hole 6 in its axial direction.

Then, the tube 1 of the filter protection member 8 in the through hole 6
A water-permeable filter 7 is provided on the inner surface.

The outer diameter of the plug shaft portion 5 is approximately the same as the diameter of the hole 3 of the tube 1. The diameter of the stopper separation preventing portion 9°10 is larger than the diameter of the hole 3 of the tube 1. The filter protection member 8 is preferably provided integrally with the stopper shaft 5, and this protection member 8 is provided with many small holes to ensure water permeability.

In addition, the liquefaction prevention pipe according to the present invention is usually transported with a plug attached to the hole of the pipe, and is further erected in the ground and used to prevent liquefaction of the ground. The stopper needs to be strong enough so that it is not damaged in the process.

Furthermore, in order to insert the stopper into the hole of the tube, at least the stopper separation prevention part needs to be made of a material with some degree of flexibility. In addition, the filter protection member is used to transport relatively thin and weak filters such as nylon filters in liquefaction prevention pipes.
Since it is intended to protect it from damage during erection and use, it is necessary to ensure the strength to satisfy this purpose.

Also, from this point of view, it is desirable to manufacture the filter protection member integrally with the stopper, but it is of course possible to attach it later as a separate body. The material for the plug is not particularly limited as long as it satisfies the above conditions, but based on the results of experiments on the fit of the plug into the hole and experiments on driving liquefaction prevention pipes into the ground, for example, Synthetic resins such as vinyl chloride, polyethylene, nylon, and polypropylene, rubber, etc. are considered suitable.

Further, although the filter protection member is provided with small holes for water permeability, the shape of the small holes is not particularly limited, such as circular or square. It is desirable that the diameters and meshes of each small hole in the filter protection member are approximately the same size. Also,
If the filter does not have a protective member, there is a risk that it will be damaged by friction with relatively large sand (rough sand) when the liquefaction countermeasure pipe is erected. It should be smaller than the size of the rough sand. The material of the filter protection member may be, for example, the aforementioned vinyl chloride, synthetic resin such as polyethylene, nylon, or polypropylene, or rubber, and the required strength may be ensured by adjusting the pore diameter or mesh and pitch of the protection member.

In the embodiments shown in FIGS. 2 and 3, the filter protection member 8 also serves as a primary filter for preventing coarse sand from flowing into the pipe 1, and the filter 7 located inside the pipe of this protection member 8 serves to prevent relatively fine soil particles that have passed through the filter protection member 8 from flowing into the pipe.

As the filter, a stainless steel filter, a synthetic fiber filter, or a synthetic resin filter may be used. When there is a concern about rust, the latter filter is preferable, and for example, warp knitted fabrics such as nylon, polyethylene monofilament, or polyester, or nonwoven fabrics such as polypropylene or polyester fibers can be used. In addition, combining synthetic fibers, synthetic resins, or synthetic fibers and synthetic resins,
They can also be layered to form a filter.

For example, on one or both sides of a synthetic fiber filter made of thin warp knitted fabric or non-woven fabric, a synthetic resin filter that is thicker and has a larger mesh than the synthetic fiber filter,
Alternatively, it is also possible to use a filter reinforced by overlapping synthetic resin nets that are thicker than synthetic fiber filters and have many small holes larger than the mesh. In this case, a synthetic resin filter or net with a large mesh will play the role of the above-mentioned filter protection member, and in that case, there is no need to use a separate filter protection member.

The mesh or gap of warp knitted fabric, non-woven fabric, or stainless steel filter as a filter is determined by considering the particle size distribution of the ground (generally sandy ground) that is targeted for liquefaction prevention. It is best to set it to prevent clogging. This mesh or gap can be set as large as 1 to 3 mm in ground where there is a possibility of liquefaction and where the soil particle size is particularly large and relatively uniformly distributed. There is little ground, and in general ground with the possibility of liquefaction, 1 to 1
It is best to set it considerably smaller than about 3 mm. Also,
The thinner the filter, the better to prevent clogging. If the mesh or gap of the filter can be set large to about 1 to 3 holes, the filter should be made of the same material and strength as the above-mentioned filter protection member, and the pore size or mesh should be set to about 1 to 3M. It is also possible for the protective member alone to function as a filter.

The permeability coefficient of a filter for liquefaction countermeasures depends on the ground where the liquefaction countermeasure pipe is installed, but it is generally 10-2 cm/s.
ec to about 10-'Cm/sec or more is preferable. In addition, the pore diameter or mesh of the filter protection member is generally larger than the mesh or gap of the filter, so if the filter has a specified water permeability coefficient, the water permeability coefficient when the filter protection member and filter are stacked will also ensure the specified specification. can do.

In order to fit the plug 4 with a filter into the pipe 1 in the embodiment shown in FIG. 2, the side of the stopper separation prevention part 10 is applied to the hole 3 of the pipe 1, and the side of the stopper separation prevention part 9 is struck with a hammer or the like. Therefore, it can be easily attached to the hole 3 in the tube wall.

Since the diameter of the stopper removal prevention part 10 is larger than the hole diameter of the tube wall, it is reduced to the hole diameter of the tube wall when it is installed, and when the stopper removal prevention part 10 passes through this hole, it expands and the installation is completed. Therefore, the material of the stopper needs to have elasticity or flexibility. In order to facilitate attachment of the stopper 4, a slit 11 may be provided extending from the shaft portion 5 of the stopper 4 to the stopper detachment prevention portion 10, as shown in FIG. In the example shown in Fig. 3, slits 11 are provided at four locations, but the width, length, and number of slits 11 are as follows:
The material may be appropriately changed depending on the material of the plug 4 and the length of the shaft portion 5 so that the plug 4 can be easily fitted and the plug 4 does not come off after fitting.

If the stopper 4 is attached as described above, the stopper separation prevention part 9,
10, the stopper 4 is mechanically fixed to the hole 3 of the pipe 1, and will not come off during transportation or construction of the liquefaction countermeasure pipe.

The main body of the pipe to which the stopper is attached may be any steel pipe, cast iron pipe, concrete pipe, steel/concrete composite pipe, PVC pipe, synthetic resin pipe such as polyethylene, etc., and is suitable for liquefied ground that is subject to liquefaction measures. The holes are perforated over a pipe length that is equal to or greater than the layer thickness.

In order to dissipate excess pore water pressure in the ground and prevent liquefaction during an earthquake, it is necessary to increase the ratio of the total area of the hole (sum of the areas of individual holes) to the total surface area of the hole (open area ratio). call) 1
% or more is desirable. The area of this opening depends on the nature of the ground (grain size, density, etc. of the soil),
It may be determined based on the nature of the earthquake, the properties of the pipe (pipe diameter, size of the hollow part inside the pipe, roughness of the inner surface, etc.). For example, experimental results using pipes with a porosity of approximately 1 to 6% and a diameter of 20 to 1,400 mm show that liquefaction can be reliably prevented if these liquefaction prevention pipes are placed at an appropriate pitch on the ground. .

As another embodiment of the plug with a filter, a filter protection member 8b is further added to the inner surface of the tube 1 in addition to the above-mentioned plug embodiment, as shown in FIGS. 4 and 5. This example is the second
The members of the plug 4 are the same as the embodiment shown in FIGS.
A filter protection member 8b is added to prevent damage to the filter 4 in the event that dirt enters the inner surface of the pipe 1 during construction of the liquefaction countermeasure pipe 1. The filter protection member 8b can also be integrally molded with the shaft portion 5 of the stopper 4 and the like.

Further, after integrally molding the filter protection member 8a with the shaft portion 5 of the plug 4, etc., the filter protection member 8b is attached to the shaft portion 5 of the plug 4.
The stopper 4 may be manufactured by adhesively bonding the material to the material.

This stopper 4, which has a filter 7 protected by protective members 8a and 8b, can be used, for example, in a pipe 1 as shown in FIG.
Since the tip of the filter 7 is open, damage to the filter 7 can be prevented even if dirt or sand enters the pipe during construction.

The inside of the liquefaction prevention pipe located in the liquefaction layer of the ground needs to be hollow in order to drain pore water in the ground during an earthquake, so if earth and sand enters this part during construction, the liquefaction pipe should be hollow. After erecting the liquefaction countermeasure pipe into the ground, use a grab hammer or a sand suction/discharge machine 31 (a vibrator that applies suction force to the pipe 32 and placed inside the liquefaction countermeasure pipe 1) as shown in Fig. 6 to remove the sediment that has entered the pipe. 32 Suction of earth and sand 33 inside the pipe from the tip,
The water can be discharged out of the liquefaction prevention pipe using a machine that discharges it onto the ground, such as a submersible pump.

FIGS. 7, 8, 9, and 10 show other embodiments of the plug 4 with a filter. In these examples, the stopper shaft portion 5 has a truncated conical outer shape.

The material of the plug 4 and the characteristics of the filter 7 are the same as in the above example, and if there is a risk of damage to the filter 7, it is desirable to provide a filter protection member 8.

The embodiment shown in FIGS. 7 and 8 differs in the shape of the shaft portion 5 from the embodiment shown in FIGS. 2 to 5, and has a structure in which the plug detachment prevention portion 10 at the tip is omitted. The embodiment shown in FIGS. 9 and 10 is an example in which the detachment prevention part 9 is further removed from the embodiment shown in FIGS. 7 and 8.

The plug 4 in these embodiments is inserted into the hole 3 of the tube 1 by pushing its shaft 5 with the required pushing force. At this time, the shaft portion 5 having the external shape of a truncated cone is subjected to a compressive force in its diametrical direction, so that the diameter of the base portion is approximately the same as that of the tip portion.

Therefore, a frictional force is generated between the circumferential surface of the shaft portion 5 and the inner circumferential surface of the hole 3, and the plug 4 is fixed to the hole 3 by this frictional force. Therefore, when the liquefaction prevention pipe 1 according to the present invention is installed in the ground, the plug 4 remains fixed in the hole 3 without falling off even if it receives resistance from the surrounding ground.

Furthermore, in order to securely fix the plug 4 to the hole 3 of the tube 1, after applying adhesive to the circumferential surface of the shaft portion 5 of the plug 4, the plug 4 is attached to the tube 1.
It may be fitted into the hole 3 of.

In the case of the plug 4 shown in FIGS. 7 and 8, there is a plug detachment prevention part 9 on the surface side of the pipe 1, and when the outer surface of the pipe 1 comes into contact with earth and sand during construction of the liquefaction countermeasure pipe 1. However, the dirt is in the through hole 6.
This prevents the filter 7 from being damaged.

The example shown in Figs. 9 and 10 does not have the stopper detachment prevention parts 9 and 10, so if the intrusion pressure of earth and sand is applied in the radial direction of the countermeasure pipe 1 during construction of the liquefaction countermeasure pipe 1, or if the liquefaction countermeasure pipe 1 is Measures When the pipe 1 is in service, when the pipe vibrates horizontally and the pressure of earth and sand is applied in the pipe radial direction, the friction force between the outer circumferential surface of the shaft portion 5 of the plug 4 and the inner circumferential surface of the hole 2 of the pipe 1, You should consider whether you can resist the above intrusion pressure. Experiments have shown that appropriate adjustment of the outer diameter of the stopper part relative to the bore of the tube provides the required resistance. Additionally, an adhesive may be used as described above to securely fix the stopper to the tube hole. In addition,
In the example shown in FIGS. 9 and 10, the stopper 4 has a stopper separation prevention part 9,
Since there is no 10, there is an advantage that less plugging material is required.

In the plugs having the structure shown in FIGS. 2, 3, 4, and 5, the plug separation prevention portion 10 is
is formed integrally with the shaft portion 5 of the stopper 4 in advance.

On the other hand, in the plug described below, the plug detachment prevention part 10 is connected to the hole 3 of the pipe 1.
It is expanded after being inserted into. The purpose of this plug is to make it easier to fit the pipe into the hole, and to prevent the plug from coming off once it has been fitted.

FIG. 11 shows a state in which the plug 4 is attached to the hole 3 in this embodiment, FIGS. 12 and 12) are side views of the plug 4 in this state, and FIG. 12(C) is a sectional view of the same. In this example, it is attached to the hole 3 by the following method. That is, as shown in FIG. 12(a), before the stopper 4 is attached to the hole 3, the stopper separation preventing portion 10 is not enlarged. However, an injection pipe 12 is provided inside the shaft part 5 in advance so that the plug detachment prevention section 10 can be expanded by injecting the hardening material after insertion into the hole 3, and the plug detachment prevention section 10 accommodates the hardening material. Keep it in a bag that can be used. After inserting the plug 4 into the hole 3,
By injecting the hardening material into the injection tube 12, the hardening material is filled into the bag of the stopper detachment prevention part 10 and expanded at the tip of the shaft part 5, as shown in FIGS. 11 and 12(b). C
), the stopper detachment prevention portion 10 expands. As a result, the plug 4 is locked and fixed in the hole 3. The positions where the filter 7 and the filter protection member 8 are installed are the same as in the previous embodiment.

Note that the hardening material is a material that hardens over time, such as cement mortar, polymer mortar, synthetic resin, and the like.

The five types of plugs described above may be selected depending on the transportation, construction, and service conditions of the liquefaction prevention pipe.

In general, the stopper detachment prevention part 9 shown in FIGS. 2 to 5 is easy to fit into a perforated pipe and securely locks and fixes in the hole.
．． By using a stopper having a diameter of 10, it is possible to obtain a liquefaction prevention pipe that satisfies the specified performance and conditions.

The part of the liquefaction prevention pipe where the stopper is fitted (the perforated part) is used as a drainage route for pore water in the ground after the pipe is installed in the liquefaction ground and is used for preventing liquefaction of the ground. Therefore, it must be hollow.

In the case of the liquefaction countermeasure pipe shown in FIG. 1 whose tip is open, there is a risk that earth and sand may enter the pipe in the perforated section 2 when the pipe is erected on the ground. If earth and sand enters the perforated pipe, the earth and sand can be ejected out of the pipe using a grab hammer, a sand suction ejector, etc., as described above, to make the inside of the perforated pipe hollow. A means for preventing the intrusion of earth and sand may be provided between the lower end of the perforated portion 2 of the liquefaction prevention pipe 1 shown in FIG. 1 and the tip of the liquefaction prevention pipe.

FIGS. 13(a) and 13(b) show, as an example, a cap 13 provided at the lower end of the liquefaction countermeasure pipe 1 to prevent soil intrusion (FIG. 13 et al.)) and a perforation of the liquefaction countermeasure pipe 1. This shows an example in which a dirt intrusion prevention lid 14 is provided at the lower end of the part 2 (Fig. 13B). In this example, the lid 14 is
5, and to facilitate the installation of the tube 1 on the lid 14,
A small air vent hole 16 is provided. This lid 14 can be easily attached by welding, for example, to a steel pipe having a diameter of about 500 to 600 mm or more. Also, the pipe diameter is 40
For steel pipes of up to 200 plates, a lid or cap without ribs or air vents may be attached to the tip of the pipe by welding. Furthermore, in the case of a resin pipe having a diameter of about 40 to 200 times, the rib or air vent hole may be eliminated and the resin pipe may be attached to the tip of the tube by resin welding, or a cap may be similarly attached to the tip of the tube.

On the other hand, the first example shows an example in which the structure of the tube tip is different from the one described above.
It is shown in Fig. 4 and Fig. 15.

FIG. 14(a) is an example of a liquefaction prevention pipe 1 in which a pipe with a circular cross section is bored and a filter sealing plug 4 is fitted therein.

An earth and sand infiltration prevention lid 24 made of a flat plate is provided at the tip of the pipe. Even if the pipe has a large diameter, such a lid 24 is sufficiently practical as long as the strength of the ground on which the pipe is installed is small. Further, even if the tube has a small diameter, such a lid 24 is sufficiently practical. The upper end of the tube 1 is closed with a lid 19. In this case, the pore water in the ground is discharged from the upper part of the liquefaction countermeasure pipe 1 to the outside of the pipe, and the excess pore water pressure in the liquefaction ground is dissipated. If the pipe diameter is about 40 to 200 mm, the liquefaction countermeasure pipe 1 having the structure shown in FIG. 14(a) may generally be used.

Further, if necessary, the number of holes in the upper part of the tube 1 may be increased compared to other parts.

FIGS. 14(a) and 14(b) show ten examples of liquefaction countermeasure pipes having a circular cross section and a structure different from that shown in FIG. 14(a).

The tip of the pipe 1 has a bellows tip 17 that facilitates the installation of the pipe on the ground and prevents dirt from entering the pipe 1. A filter 18 is provided at the upper end of the pipe 1, as shown in the figure, to prevent earth and sand from flowing into the pipe from the upper part of the pipe when the pipe 1 for liquefaction prevention is installed in the ground and used to prevent liquefaction. ing. This filter 18 may be made of the same material as the filter 7 attached to the stopper 4, but since the filter is easily damaged, it is desirable to protect it with a water-permeable resin net or welded wire mesh.

Alternatively, the upper end of the liquefaction countermeasure tube 1 may be completely closed with a lid 19 as shown in FIG. 14(C), for example. In this case, the pore water in the ground will be discharged from the top of the liquefaction countermeasure pipe 1 to the outside of the pipe, so it is necessary to secure the necessary hole area at the top of the liquefaction countermeasure pipe 1 to ensure smooth discharge of the pore water. Bye. Generally, even if the upper end of the tube 1 is closed with the lid 19, the pore diameter and number of holes in the upper portion of the tube 1 may be approximately the same as in the lower portion of the tube 1. As another example of the structure of the upper part of the countermeasure tube, as shown in the figure, the hole in the upper part of the tube 1 may be made larger than other parts, and the filter sealing plug 20 having the above-described structure may be fitted therein.

In this case, the interstitial water is discharged to the outside through the plug 20 or the plug 4 below the plug 20. In addition, in Fig. 14 (company), reference numeral 21 indicates an example of a chuck gripping jig used when driving the liquefaction countermeasure pipe 1 with a vibrohammer, and it is fixed to the lid 19 part. .

The liquefaction countermeasure pipe 1 shown in FIG. 14(d) is
This is an example in which a spiral fin (auger) 23 is attached to the lower part of the tube 1 similar to that shown in a). In this case, the pipe 1 can be easily constructed by rotating and press-fitting the pipe 1 into the liquefied ground. In addition, when installing the pipe 1 by rotary press-fitting, in order to reduce friction between the pipe 1 and the ground and make installation easier, the lower part of the pipe 1 should have the structure shown in Figure 13 (a), or the pipe 1 A structure in which a flat plate 22 is attached in a band shape to the outer peripheral surface of the lower part (Fig. 14 (C)
) may also be used.

Further, the cross section of the liquefaction countermeasure pipe 1 does not necessarily have to be circular, but may be square as shown in FIGS. 15(a), 15(a) and 15(c). Furthermore, the pipe may be polygonal or have any cross section, but since the inside of the pipe serves as a drainage route for pore water in the ground, it is sufficient that the cross-sectional area of the hollow part is ensured for smooth drainage. .

In addition, if the liquefaction countermeasure pipe is installed upright to the non-liquefaction layer below the liquefaction layer, the part where the pipe enters the non-liquefaction layer is shown in Figure 14 (a).
), (C), and there is no need to drill holes as shown in FIGS. 15(a) and (C).

FIG. 19(a) shows a situation in which a liquefaction prevention pipe is installed in liquefaction ground 41 in order to prevent liquefaction of the ground.

et al.).

Liquefaction prevention pipes will be installed at necessary intervals to prevent ground liquefaction. When relatively small diameter tubes IB are used, the inner diameter of the tubes may generally be on the order of 40 to 200 mm or more, and the spacing between the tubes may generally be on the order of 200 to 3000 mm or more. As the main body of the liquefaction countermeasure pipe IB, steel pipes,
A liquefaction countermeasure pipe IB using something with high rigidity and strength such as a concrete composite pipe (for example, a so-called SC pile) or a concrete pile is installed in the compacted ground 42 below the liquefied ground 41.
When penetrated to this point, the countermeasure pipe IB not only prevents the liquefied ground 41 from liquefying, but also has the effect of restraining and stabilizing the liquefied ground 41. In particular, it has been confirmed that when the ground surface or the lower surface of the liquefaction layer is sloped, the ground may slip due to liquefaction, leading to a major disaster. It will be effective.

For example, as shown in FIGS. 20(a) and 20(b), when an embankment 43 such as a railway road or embankment is constructed on liquefied ground 41, or when a new embankment 43 is constructed, the embankment 43
3. If the liquefaction countermeasure pipe IB is installed in the liquefied ground 41 below or in the liquefied ground near the toe of the embankment, the liquefied ground 4
This not only prevents the liquefaction of the liquefied ground 41 and the embankment 43 that may liquefy, but also increases the strength of the liquefied ground 41 and the embankment 43, and prevents the embankment from sliding and collapsing. In the figure, 44
is a crushed stone layer that drains the pore water discharged from the upper end of the liquefaction countermeasure pipe IB.

In order to ensure the ground restraint effect of the liquefaction countermeasure pipe IB, the countermeasure pipe IB should be built up to the tight non-liquefaction ground 42 as shown in Figures 21 (a) and (b). The head of the countermeasure pipe 18 may be tied with a binding material 45 (for example, steel bar, shaped steel, etc.). Although the figure shows an example where the tube is tied in two directions, the tube head may be tied in only one direction. In this case, countermeasure pipe I
The displacement is restricted at the upper and lower ends of B, and therefore the liquefied ground 41
Since the soil is also restrained, the liquefaction prevention effect is even greater, and the stabilization of the ground can be ensured. This method of installing countermeasure pipes IB can be expected to have the greatest effect if applied when the ground surface or the lower surface of the liquefaction layer is sloped.

The liquefaction prevention pipe according to the present invention can be installed by directly driving it into the ground using a vibro/'% pump, a hydraulic hammer, a diesel hammer, a press-fitting machine, etc., and is extremely easy to install.

As mentioned above, conventionally, in order to install a resin pipe with many small holes and a thin filter wrapped around the pipe that is easy to break, the steel casing pipe is rotated and enlarged with an auger until it reaches a specified depth. After construction, the resin pipe is passed through the casing pipe and erected to a predetermined depth, then the tip of the casing pipe is opened, the casing pipe is pulled out of the ground, and the resin pipe is left with a liquefied layer. (This method is similar to the method used in the conventional crushed stone drain method, in which crushed stone is left in the liquefaction layer and crushed stone piles are installed in the ground.) However, in the conventional method, the outer diameter of the casing pipe (usually 100 to 125
During construction, there is a difference between the outside diameter of the resin pipe (usually about 50 to 100 mm), and the ground around the resin pipe inevitably becomes loose after it is installed in the ground. Become.

Loosening the liquefaction layer before installing drainage pipes (perforated resin pipes, etc.) means further loosening the ground, which is already loose and at risk of liquefaction. Excess pore water pressure tends to increase in a short period of time. In other words, the risk of liquefaction becomes greater.

Therefore, the porosity of the installed drainage pipe is larger,
In addition, unless the arrangement intervals are made closer, excess pore water pressure in the ground cannot be dissipated in a short time.

On the other hand, if the porosity of the drain pipe is increased, the rigidity and strength of the pipe will decrease, and the number of holes will increase, making it more expensive. Furthermore, if the spacing between drain pipes is made closer, the cost of countermeasures against liquefaction will increase, and the rate of loosening of the ground due to the installation of drain pipes will further increase.

As mentioned above, when installing drainage pipes, the liquefied ground can be loosened,
The conventional construction method, which causes softening, has a major drawback and can be said to be less effective as a countermeasure against liquefaction. In addition, we actually conducted a shaking table experiment by installing drainage pipes with low rigidity on loose ground, and found that even if the drainage pipes were arranged very closely together, they had little effect on preventing ground liquefaction.

Conventional drainage pipes have a filter attached around the pipe to prevent dirt from entering the pipe, and this thin filter breaks due to friction with the ground, making it impossible to drive the pipe directly into the ground.

However, the drain pipe with a stopper (liquefaction countermeasure pipe) according to the present invention has a structure in which the filter does not break even if it is directly driven into the ground, and has a great feature.

In fact, the particle size distribution range is 0.05 to 2.0 degrees, average particle size I)
s.

= 0.48mm, after driving a liquefaction countermeasure pipe with multiple plugs into the ground with a standard penetration test N value of 35 or less using a hammer, pull out the liquefaction countermeasure pipe and check for damage to the filter in the frame. When we conducted an experiment to confirm, the filter was not damaged at all. The ground to which liquefaction countermeasures are applied usually has an N value of 15 or less, and the liquefaction countermeasure pipe according to the present invention can withstand direct driving and has sufficient practicality.

Therefore, the liquefaction countermeasure pipe according to the present invention can be installed directly on liquefied ground using a vibro hammer, hydraulic hammer, diesel hammer, press-fit machine, etc. without using a conventional casing pipe, and loosens the ground. Never.

Rather, the erection of the pipe will compact the ground.

In other words, when installing the pipe into the ground, ■ The pipe moves the ground laterally and compacts it.

■ Local liquefaction occurs in the ground around the pipe, and pore water flows into the pipe, causing a reorganization of the sand skeleton.In other words, the gaps between sand become smaller, and the ground around the pipe becomes compacted. Become. This phenomenon is particularly noticeable when using a vibrohammer.

Another advantage is that driving into the ground is easier than driving a regular pipe of the same size due to the effect of dissipating excess pore water pressure.

An example of construction of liquefaction prevention pipes is shown in Figure 22. In this example,
A liquefaction countermeasure pipe 1 having a structure as shown in FIG. 14 is used, and is erected on the liquefaction ground 41 by a vibrohammer 23 attached to a hydraulic excavator 22. In this case, the liquefaction ground was quite loose with an N value of 10 or less before the liquefaction countermeasure pipe 1 was erected, but after the liquefaction countermeasure pipe 1 was erected, the N value increased and It was confirmed that the installation work went smoothly and the filter did not break.

Therefore, even if the strength of the ground is increased and excessive pore water pressure in the ground increases, the pore water can be smoothly discharged upward through the hollow countermeasure pipe.
By avoiding an increase in excess pore water pressure, liquefaction of the ground during an earthquake can be effectively prevented.

FIG. 23 shows an example in which the seismic reinforcement structure of the present invention using the above-mentioned liquefaction prevention pipe is applied to a pile foundation. In this case, it is sufficient to provide the liquefaction countermeasure section only in the portion of the liquefaction countermeasure pipe IA that is located in the liquefaction ground 42. As the building 490 foundation pile, it is desirable to use steel pipes that have a large bearing capacity, and the diameter of steel pipes is generally about 400 to 800 squares.

The pile foundation according to the present invention shown in Fig. 23 can prevent liquefaction around the liquefaction countermeasure pipe IA and can take horizontal ground reaction force even during an earthquake, so ordinary piles without liquefaction countermeasures can be used. It is a much safer foundation than the case.

Furthermore, if you want to make it safer, liquefaction countermeasure pipes IB can also be installed in the ground around the building 49, as shown in FIG. In this case as well, it is preferable to use a steel pipe pile or the like that is provided with a plug to prevent liquefaction as the liquefaction prevention pipe IA as the foundation pile of the building 49. In addition, the main body material of the liquefaction countermeasure pipe IB is preferably a material with rigidity and strength such as steel pipe from the viewpoint of being able to restrain the ground, but if the purpose is simply to prevent liquefaction, this material should be a synthetic resin pipe. It may be. However, liquefaction countermeasure pipe IB is also liquefaction countermeasure pipe IA.
Similarly, since it is installed on liquefied ground using a diesel/'% hammer, hydraulic hammer, vibrohammer, press-in machine, etc., even if the material of the countermeasure pipe IB body is a synthetic resin pipe,
It is necessary to have sufficient strength and rigidity to withstand installation.

The pipe diameter of the liquefaction countermeasure pipe IB installed in the surrounding ground may generally be smaller than the pipe diameter of the countermeasure pipe IA determined from the study of the foundation of the building, and as mentioned above, it is about 40 to 200 mm,
Or more is fine.

FIGS. 25(a) and 25(b) show, as an earthquake-reinforced structure of the present invention, a liquefaction countermeasure pipe IB is installed at the bottom of the direct foundation 47 of a house 46 installed in a stratum where there is a risk of ground liquefaction during an earthquake. This is an example of a case in which

Direct foundation 4 of a private residence 46 built on liquefied ground 42
When a liquefaction countermeasure pipe is used to prevent subsidence and reinforce 7 (cloth foundation), a steel pipe with a diameter of about 40 to 200 mm may be used as the main body of the liquefaction countermeasure pipe IB. At this time, the tip of the liquefaction countermeasure pipe IB should be placed on the ground 4, which can provide as much support as necessary.
It is enough to penetrate up to 2. In this way, the liquefaction countermeasure pipe I
If B is used, it will become the foundation of a house that is safe against earthquakes.

In the figure, 48 is the crushed stone section, and the liquefaction countermeasure pipe IB in the event of an earthquake
The interstitial water discharged from the upper part of the pipe is led to the side gutter 48a and drained.

In addition, in order to further improve safety against ground liquefaction in seismic reinforcement structures for residential buildings, similar liquefaction prevention pipes should be installed directly between the foundations of the house and in the liquefied ground around the house. is possible. In this case, the main body of the liquefaction prevention pipe installed directly between the foundations or on the liquefied ground around the house is not limited to a steel pipe, but may be a synthetic resin pipe if the purpose is simply to prevent liquefaction.

Next, FIG. 26 shows an embodiment in which the seismic reinforcement structure of the present invention is applied to a bridge pier 50 or a foundation pile of an abutment portion of a bridge. In this case as well, the material of the main body of the liquefaction countermeasure pipe IA is preferably a steel pipe, and the liquefaction countermeasure part only needs to be a part of the foundation pile corresponding to the liquefaction ground 41. In this example, the plug 4 of the present invention is also attached to a part of the pile that enters the 42nd layer of non-liquid ground for safety reasons, taking into account the construction error of the pile.

During an earthquake, pore water in the ground flows through the plug 4 into the liquefaction countermeasure pipe IA, which also functions as a support pile, and is discharged from the upper part of the countermeasure pipe IA to the upper ground through the crushed stone section 51, causing liquefaction of the ground. prevent.

Therefore, the horizontal ground reaction force of the pile can be reliably taken when the pile is in the ground, and a safe foundation can be provided.

The safety of pile foundations will further increase if liquefaction of the ground is reliably prevented far from the foundation piles. Therefore, as shown in FIG. 27, the liquefaction prevention pipe IB can be driven around the liquefaction prevention pipe IA as a foundation pile.

The pipe diameter of the liquefaction countermeasure pipe IA is determined by considering the function of the foundation pile, and if the pipe body material of the countermeasure pipe IA is steel pipe, it is usually 50
A length of about 0 to 1400 mm is often used.

As with the embodiment shown in Figure 25, the liquefaction countermeasure pipe IB should have a diameter of about 40 to 200 plates, and the main material of the pipe should preferably be steel, but a synthetic resin pipe can be used as long as it has the specified rigidity and strength. You can.

The method for discharging pore water in the ground during an earthquake to the outside of the ground through the liquefaction countermeasure pipe IA may be the same as that shown in Fig. 26, but as shown in Fig. 27, A drain pipe 52 having a hollow cross-sectional area necessary for smooth discharge of pore water in the ground is installed, and this drain pipe 52 is installed inside the footing of the pier 5o to drain the pore water in the liquefied ground to the outside of the ground. You can.

As described above, the seismic reinforcement structure of the present invention can reliably prevent liquefaction of the ground and can also function as a safe pile foundation against earthquakes.

〔Effect of the invention〕

■ The seismic reinforcement structure of the pile foundation of the present invention prevents liquefaction of the ground around the foundation pile, so even in the event of an earthquake, the horizontal ground reaction force of the pile is removed, and the cross-section determined by the pile thickness, outer diameter, etc. Alternatively, the number of piles can be significantly reduced compared to when ordinary piles without liquefaction measures are used.

Therefore, it is possible to provide a safe and economical pile foundation that is resistant to earthquakes.

■ If liquefaction countermeasure pipes made of small-diameter steel pipes are used as supporting members for direct foundations (fabric foundations) of houses built on liquefied soil, conventional improvement methods such as compaction of liquefied soil can be avoided. It is possible to provide a safe and economical direct foundation (fabric foundation) that can prevent liquefaction and subsidence even in the event of an earthquake without using it, and the construction period can be shortened.

■ In addition, if liquefaction prevention pipes are erected at appropriate intervals so as to surround the group of foundation piles, the ground around the group can be It is also possible to prevent liquefaction of water and provide a safe pile foundation with greater resistance to earthquakes.

■ In the seismic reinforcement structure using the liquefaction prevention pipe of the present invention, liquefaction can be reliably prevented by erecting piles with a liquefaction prevention function at specified intervals, and the filters in the holes of the piles are Since it is not clogged with earth and sand, the liquefaction prevention effect is maintained even after repeated earthquakes.

■ The liquefaction prevention pipe according to the present invention not only prevents liquefaction by being installed in the ground, but also has the effect of stopping and restraining the movement of the ground, and improves the ground to have greater resistance to earthquakes. I can do it. In particular, a more effective ground restraint effect can be expected when the liquefaction layer is sloped, when reinforcing embankments, and when reinforcing the butt of a caisson front mound.

(2) The liquefaction prevention pipe according to the present invention does not damage the filter during transportation or installation on the ground, and is extremely easy to handle.

■ Piles using the liquefaction prevention pipe according to the present invention can be constructed directly using ordinary construction machinery such as a vibro hammer, hydraulic hammer, diesel hammer, or press-in machine, without using a casing pipe to protect the filter. It is extremely easy to install because it can be driven into the ground, and pore water in the ground can be extracted from the pile using liquefaction prevention pipes during driving. Also, the ground will not be loosened during construction.

■ The liquefaction countermeasure pipe according to the present invention allows a stopper with a filter provided in the through hole to be fitted and fixed into the hole of the pipe with a hole in it very easily without using expensive machinery. Easy to manufacture.

In addition, if it is fixed at a - degree, it will be affected by shocks during transportation, construction, and use.
Since it does not come off from the hole in the pipe due to friction, quality is also guaranteed.

[Brief explanation of drawings]

Fig. 1 is a perspective view showing an outline of the liquefaction countermeasure pipe according to the present invention, Fig. 2 is a vertical sectional view showing an example of how a plug is attached to a perforated pipe, and Figs. 3 (a) to (C). 2 are a side view, a front view, and a rear view of the stopper shown in FIG. 2, FIG. 4 is a vertical cross-sectional view showing the state in which the stopper is installed in another embodiment, and FIGS.
C) is a side view, front view, and rear view of the plug in FIG. 4, FIG. 6 is a vertical cross-sectional view showing an overview of the intrapipe sediment suction and discharge system, and FIG. 7 is a state in which the plug is installed in yet another embodiment. 8(a) to (C) are respectively a front view, a side view, and a sectional view of the stopper in FIG. 10(a) and 10(a) are respectively a side view and a sectional view of the stopper in FIG. 9, FIG. 11 is a vertical sectional view showing the state of attachment of the stopper in yet another embodiment, and FIG. 12 (a)～(
C) is a side view before enlargement, a side view after enlargement, and a sectional view after enlargement of the detachment prevention part of the stopper shown in Fig. 10, and Fig. 13 (
Figures 14 (a) to (d) and 15 (a) to (C) are perspective views showing examples of liquefaction prevention pipes each having a cap or lid for preventing soil intrusion at the tip; are perspective views of different embodiments of the liquefaction countermeasure pipe according to the present invention, FIG. 16 is a graph showing the change in excess pore water pressure ratio over time in a free damping wave excitation experiment, and FIG. 17 is an experiment of the liquefaction countermeasure pipe. Figure 18 is a graph showing the relationship between horizontal resistance and loading plate displacement in the liquefaction countermeasure pipe experiment, Figure 19 (a),
(b) is a plan view and a vertical cross-sectional view showing how the liquefaction countermeasure pipe is installed on the ground, and Figure 20 (a) and (a) are vertical cross-sectional views showing an example of application when there is embankment on liquefaction ground. Figure 21 is a plan view and vertical sectional view showing an example of the arrangement when the heads of liquefaction prevention pipes are combined, and Figure 22 is a vertical sectional view showing the state of driving the liquefaction prevention pipe into the ground. Fig. 23 is a vertical sectional view showing an example of application of the seismic reinforcement structure of the present invention to a building foundation, Fig. 24 is a vertical sectional view showing application to a building foundation and surrounding ground, and Fig. 25 (a), et al. 26 is a vertical cross-sectional view and a horizontal cross-sectional view of the foundation, respectively, showing an example of application of the seismic reinforcement structure of the present invention to a direct foundation of a house, and FIG. Figure 27 is a vertical sectional view showing the application of the seismic reinforcement structure of the present invention to the pier foundation and surrounding ground, and Figure 28 (a
) to (C) are perspective views showing examples of conventional liquefaction prevention pipes,
They are a front view (partial view) and a horizontal sectional view. 1... Liquefaction countermeasure pipe, 2... Perforated part, 3... Hole,
4... Plug, 5... Shaft, 6... Through hole, 7...
Filter, 8... Filter protection member, 9.10.
... Stopper separation prevention part, 11 ... Slit, 12 ... Injection pipe, 13 ... Cap, 14 ... Earth and sand intrusion prevention lid, 15 ... Rib, 16 ... Air vent hole, 17 ...・・・
Pointed end, 18... Filter 19... Lid, 20.
... Filter sealing plug, 21 ... Chuck gripping jig,
22... Band-shaped flat plate, 23... Spiral fin, 24...
・Earth and sand intrusion prevention lid, 25...Hydraulic excavator, 26...
・Vibrohammer, 31... Sediment suction discharge machine, 32
... Pipe, 33... Earth and sand, 41... Liquefied ground, 42... Non-liquefied ground, 43... Embankment, 44...
- Crushed stone layer, 45... Binding material, 46... Housing, 47.
... Direct foundation, 48... Crushed stone section, 48a... Side ditch,
49... Building, 50... Pier, 51... Stone crushing section,
52... Drain pipe] Figure (a) Figure (b) (C) Figure (a) Figure (b) Figure 13 (a) (b) (C) ] 4 Figure (a) Figure (b) Figure (C) Figure Figure Figure Figure Liquefaction Countermeasures Pipe Installation Temporary Direction δ (mm) Figure

Claims (5)

[Claims] (1) A plug is attached to each hole of a pipe formed by drilling a large number of holes in the pipe wall, the plug has a through hole in the axial direction, and a filter is provided in the through hole. 1. An earthquake-resistant reinforcement structure for a structure, characterized in that a liquefaction countermeasure pipe made of liquefaction prevention pipe is used for at least a part of a support pile of a structure foundation installed in a stratum where there is a concern about ground liquefaction during an earthquake. (2) An earthquake-resistant reinforcement structure for a structure, characterized in that the liquefaction countermeasure pipe is also installed in a stratum around the support pile of the structure foundation, where there is a concern that the ground may liquefy during an earthquake. (3) The seismic reinforcement structure for a structure according to claim 1 or 2, wherein the through hole of the plug is provided with a water-permeable filter protection member for preventing damage to the filter together with the filter. (4) A plug is attached to each hole of a steel pipe formed by drilling a large number of holes in the pipe wall, the plug has a through hole in the axial direction, and a filter is provided in the through hole. 1. A seismic reinforcement structure for a house, characterized in that a liquefaction countermeasure pipe is installed directly below the foundation of the house, which is installed in a stratum where there is a risk of ground liquefaction during an earthquake. (5) An earthquake-resistant reinforcement structure for a house, characterized in that the liquefaction prevention pipe is installed directly between the foundations of the house or in the strata around the house, where there is a concern that the ground will liquefy in the event of an earthquake.