JPH11200385A - Improved foundation-ground structure of existing cylindrical storage tank - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide the improved foundation-ground structure of an existing cylindrical storage tank, in which a required safety factor to a partial slip is ensured regarding the foundation ground of the existing cylindrical storage tanks regardless of the liquefaction of a peripheral sandy ground at the time of an earthquake even when the existing cylindrical storage tanks are installed onto the sandy ground in close order, differential settlement, etc., are not also generated and the hoop tensile breakdown of a doughnut-shaped improved region by excess pore water pressure generated is prevented and which can be constructed by reducing an improving material, etc. SOLUTION: The existing cylindrical storage tank 1c is built onto a part of a sandy ground 2c while using a part of the sandy ground 2c as a foundation ground. A doughnut-shaped region in the vicinity of the outer circumference of the existing cylindrical storage tank 1c is composed of an improved foundation ground 11 having the shear strength of high strength and a semi-improved foundation ground 12, in which shear strength is smaller than the improved foundation ground 11 in the doughnut-shaped region and which has shear strength, by which no liquefaction is generated, in the foundation ground at that time.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improved foundation structure of an existing cylindrical storage tank installed on a sandy ground which may be liquefied during an earthquake.

[0002]

2. Description of the Related Art With respect to the foundation ground structure of an existing cylindrical storage tank (for example, an oil tank, etc.), various measures for liquefaction during an earthquake have been studied for a long time. These are the foundation ground structures in which the foundation ground has been improved by a method of lowering the groundwater level, a drainage method, a steel ring method, or the like.

[0003] However, the foundation ground structure improved by the groundwater level lowering method is liable to cause the entire ground subsidence due to the lowering of the groundwater level, and requires long-term operation and maintenance of pumps and the like. High cost.

[0004] Further, in the case of improving the foundation ground by a drain type construction method, the construction machine is relatively large, so it is not suitable for construction in a place where the space between tanks is narrow, and the existing piping attached to the cylindrical storage tank is required. There is a problem with the dense foundation structure of the existing cylindrical storage tank because it needs to be removed.

[0005] Further, when the foundation ground is improved by a steel ring, there is a problem in the foundation ground structure of a large tank because the joint strength of the sheet pile is insufficient and the countermeasure effect has not been verified.

On the other hand, in recent years, in particular, as a method of improving a dense foundation ground of an existing cylindrical storage tank, in addition to the above-mentioned construction method, a foundation improvement method by injection solidification and a foundation ground structure by a partial ground improvement method have been studied.

As shown in FIG. 3, the foundation ground structure by the ground improvement method by the injection solidification is provided with a disc-shaped foundation ground 3 improved by the injection and solidification method on a part of the sandy ground 2a. An existing cylindrical storage tank 1a (tank or the like) is constructed. Reference numeral 4 indicates a groundwater level.

[0008] The foundation ground structure by the partial ground improvement method is disclosed in JP-A-8-128054, and as shown in FIG. 4, a grounded member of an existing cylindrical storage tank 1b (existing structure in the publication). That is, the ground is improved by the solidified columnar body 6 which is solidified only to the portion just below the outer peripheral portion of the bottom plate including the annular plate to the non-liquefied layer 5.
The upper end of the solidified columnar body 6 supports the outer peripheral portion of the bottom plate including the annular plate from below through the base portion 7. The inside of the solidified columnar body 6 arranged in an annular shape is a sandy ground 2b that may be liquefied during an earthquake.

[0009]

However, the foundation ground structure by the ground improvement method by injection solidification described above is
There are the following problems.

In a foundation ground structure by a ground improvement method by injection solidification, an improved ground diameter calculated by the following equation (1) is generally required. Diameter = tank diameter + 2 x (2/3) x liquefaction layer depth ... (1) The improved ground diameter is at least tank diameter + 1 from equation (1).
0 m or more, the volume of the improved ground is large, and the cost of the improvement is high.

On the other hand, in the foundation ground structure by the partial ground improvement method, an unimproved portion which may be liquefied is left inside the solidified columnar body 6 arranged in an annular shape, and the portion is settled during liquefaction. However, uneven settlement occurs between the improved portion that does not settle or the small settlement, and the possibility of adversely affecting the existing cylindrical storage tank 1b is large. In addition, existing cylindrical storage tank 1
Due to the excess pore water pressure generated in the unimproved portion immediately below b, hoop tension is likely to be generated in the solidified columnar body 6 arranged in an annular shape. In general, since the tensile strength of the improved ground by the solidified columnar body 7 is extremely small, there is a great possibility that the improved ground will cause a tensile failure with respect to the hoop tension and adversely affect the existing cylindrical storage tank 1b.

The present invention has been made in order to solve the above-mentioned problems. Even when existing cylindrical storage tanks are densely installed on a sandy ground, the surrounding sandy ground due to an earthquake can be obtained. Regardless of the liquefaction of the soil, the required safety factor against local sliding of the foundation ground of the existing cylindrical storage tank is secured, without uneven settlement, etc., and the hoop tensile fracture of the improved donut-shaped area due to excess pore water pressure generated there It is an object of the present invention to provide an improved basic ground structure of an existing cylindrical storage tank which can be constructed while preventing the occurrence of the improvement material and reducing the amount of the improving material.

[0013]

SUMMARY OF THE INVENTION The present invention relates to a foundation structure of an existing cylindrical storage tank installed on a sandy ground, wherein the existing cylindrical storage tank has a doughnut-shaped region near the outer periphery having a high strength shearing force. An improved foundation ground having strength, and the inside of the donut-shaped region is constructed on a foundation ground comprising a semi-improved foundation ground having a shear strength smaller than that of the improved foundation ground and having a shear strength not causing liquefaction. This is an improved foundation ground structure for an existing cylindrical storage tank.

In the first aspect of the present invention, it is preferable that the semi-improved foundation ground has a shear strength of not less than 0.5 kgf / cm ^{2 and} less than 2.0 kgf / cm ² as uniaxial compressive strength by using the improving material.

According to the present invention, with the above configuration, the improved foundation ground structure of the existing cylindrical storage tank has a donut-shaped region formed of a high-strength improved foundation ground and an inner region formed of a semi-improved foundation ground. The required safety factor against local slip of the improved foundation ground can be secured, and unequal settlement of the donut-shaped internal region and hoop tensile fracture of the donut-shaped improved region due to excessive pore water pressure generated therein can be prevented.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a sectional view showing one embodiment of the present invention. The existing cylindrical storage tank 1c is constructed on a part of the sandy ground 2c as a base ground. The improved foundation ground structure according to the present invention includes an improved foundation ground 11 in which a donut-shaped area near the outer periphery of the existing cylindrical storage tank 1c has a high shear strength, and an interior of the donut-shaped area in the improved foundation ground 1
And a semi-improved foundation ground 12 having a shear strength smaller than 1 and a shear strength that does not cause liquefaction. Reference numeral 4 indicates a groundwater level.

The improved foundation ground 11 has a shear strength of 2.0 kgf / cm ² or more and 30.0 kgf as uniaxial compressive strength.
/ Cm ² or less, but practically 2.0
kgf / cm ² or more 6.0 kgf / cm ² or less in the range are employed.

If the improved foundation ground 11 has a shear strength within the above range, the existing cylindrical storage tank 1c constructed on the improved foundation ground 11 as a doughnut-shaped area near the outer periphery is a local part of the improved foundation ground 11 due to an earthquake or the like. Slip is prevented, and the structural safety of the existing cylindrical storage tank 1c can be ensured.

Since the improved foundation ground 11 is intended for the vicinity of the outer periphery of the existing cylindrical storage tank 1c, it is easy to improve the foundation ground. In general, the improved ground having the above-mentioned shear strength is obtained by using a cement-based improving material. The foundation ground 11 can be formed.

The semi-improved foundation ground 12 has a shear strength of 0.5 kgf / cm ² or more as a uniaxial compressive strength.
It is preferable to set the range to less than 0 kgf / cm ² .

The semi-improved foundation ground 12 is the improved foundation ground 11
In this case, it is only necessary to prevent unequal settlement due to an earthquake or the like and hoop tensile fracture of the donut-shaped improved region due to excessive pore water pressure generated therefrom. It has a lower shear strength than that of the first embodiment and has a shear strength that does not cause liquefaction.

The semi-improved foundation ground 12 can be provided with the above-mentioned shear strength by using an improvement material such as a water glass type or a cement type. Since the semi-improved foundation ground 12 has a smaller shear strength than the improved foundation ground 11, it is easy to select an improved material, and the semi-improved foundation ground 12 can be easily formed by an injection solidification method or the like. Can be reduced, and the construction time can be shortened.

[0024]

[Embodiment] The calculation results of the safety factor against local slippage of the improved foundation ground of a cylindrical storage tank assuming that a design seismic load is applied will be described below. FIG. 2 is a sectional view of the improved foundation ground near the side plate of the virtual cylindrical storage tank according to the present invention.

As shown in FIG. 2, a virtual cylindrical storage tank 13 has a dimension of 16 m in diameter, a liquid depth of 13.7 m, a capacity of 3000 kl, and a load p (tf / m ² ) in the direction of the arrow. . The virtual cylindrical storage tank 13 includes a part of the sandy ground 2c, an improved foundation ground 11a, and a semi-improved foundation ground 12a.
It is built on the foundation ground. 2d is a sandy ground at a position higher than the groundwater level. Table 1 shows the material strength of these grounds.

In Table 1, for convenience, the uniaxial compressive strength (qu) is indicated by the adhesive force C (tf / m ² ) converted according to the following (2) (Coastal Development Research Center: Handbook for Liquefaction Countermeasures in Landfills, p. 203, August 1997). C = (qu / 2) · (1-sinφ) / (cosφ) (2) where φ (°): internal friction angle

[0027]

[Table 1]

Next, under a load condition of a horizontal seismic coefficient of 0.48 and a vertical seismic coefficient of 0.24, the adhesion (C) and the improved width (L) of the improved foundation ground 11a are used as parameters, and Table 2 shows the results of calculating the safety factor against local slip on the foundation ground.

[0029]

[Table 2]

In Table 2, the range without shading is a range where the safety factor against local slip is 1.1 or more. For example, the adhesive strength (C) of the improved foundation ground 11a is 15 tf /
In the case of m ² (5.8 kgf / cm ² in uniaxial compressive strength conversion), the improvement width may be 2 m. As a method of studying local slip on the foundation ground, a method based on arc slip calculation was applied ([Fire and Disaster Management Agency: Report on Ensuring the Structural Safety of Old Tanks and Opening Cycle of Specific Outdoor Tank Storage, p.1)
9-20, March 1994]).

As is clear from Table 2, the improved foundation ground 1
By properly setting the adhesive strength (C) of 1a, that is, the shear strength, and by setting the semi-improved foundation ground 12a corresponding to the improved foundation ground 11a, the required safety against local slip is improved by the improved foundation ground 11a. It was estimated that the semi-improved foundation ground 12a could prevent the uneven settlement and the hoop tensile fracture of the donut-like improved area due to the excessive pore water pressure generated there.

[0032]

As described above, according to the improved foundation ground structure of the present invention, even if the existing cylindrical storage tanks are densely installed on the sandy ground, liquefaction of the surrounding sandy ground due to an earthquake occurs. Regardless of the basic soil of the existing cylindrical storage tank, the required safety factor against local slip is ensured, without causing uneven settlement, etc., and preventing hoop tensile fracture of the donut-shaped improvement area due to excess pore water pressure generated there. In addition, it is possible to reduce the amount of improving materials and the like.

[Brief description of the drawings]

FIG. 1 is a sectional view showing an embodiment of the present invention.

FIG. 2 is a cross-sectional view of an improved foundation structure near a side plate of a virtual cylindrical storage tank according to another embodiment of the present invention.

FIG. 3 is a cross-sectional view showing an example of a foundation ground structure by a conventional ground improvement method by injection solidification.

FIG. 4 is a sectional view showing an example of a foundation ground structure by a conventional partial ground improvement method.

[Explanation of symbols]

 1a to 1c Existing cylindrical storage tanks 2a to 2c Sandy ground 2d Sandy ground (sandy ground at a position higher than groundwater level) 4 Groundwater level 11, 11a Improved foundation ground 12, 12a Semi-improved foundation ground 13 Virtual cylindrical storage tank

Claims (2)

[Claims] 1. A foundation structure of an existing cylindrical storage tank installed on a sandy ground, wherein the existing cylindrical storage tank has an improved foundation ground in which a donut-shaped region near the outer periphery has high strength shear strength. The interior of the donut-shaped region has a lower shear strength than the improved foundation ground, and is constructed on a foundation ground comprising a semi-improved foundation ground having a shear strength not causing liquefaction. Improved foundation structure of existing cylindrical storage tank. 2. The semi-improved foundation ground is provided by an improving material.
Shear strength is 0.5 kgf / cm ² as uniaxial compressive strength
^2. The improved foundation ground structure of an existing cylindrical storage tank according to claim 1, wherein the range is less than 2.0 kgf / cm ² .