JPH0366836A - Design process for piping at structure - Google Patents

Abstract

PURPOSE:To prevent breakage due to liquefying phenomenon by setting the floatup preventive construction method for a structure in case the structure is afloat resulting from the liquefying phenomenon, presuming the amount of ground settling, and judging whether the obtained amount is below a certain tolerable value or not. CONSTITUTION:In advance the buoyancy applied to a manhole and a pipeline connecting thereto is calculated together with the amount of ground settling, and if it is judged that a structure concerned is afloat due to the liquefying phenomenon, a floatup preventive construction method for the structure is set. Further the amount of ground settling due to liquefying phenomenon is presumed, and it shall be judged whether the amount of ground settling remains below a certain allowable value for a liquefication-countermeasured pipe. In this procedure the design is pushed forward, and the ground liquefying phenomenon generated by an earthquake and the external force originating therefrom are presumed accurately, and a well-countermeasured method for execution of works is adopted for the structure and associate pipelines.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention is applicable to pipes installed near structures such as manholes when liquefaction phenomenon is predicted to occur in the ground due to the occurrence of an earthquake. The present invention relates to a method of designing a conduit section adjacent to a structure, which can prevent damage to the conduit section at the structure.

[Prior art] When ground liquefaction occurs due to an earthquake, structures and pipes buried underground may be damaged by buoyancy and ground deformation caused by the ground liquefaction. Are known. Therefore, in areas where there is a risk of ground liquefaction occurring during an earthquake, the following construction methods may be applied.

■The gravel drain method will be used when constructing structures such as manholes, with the aim of eliminating water pressure that forms in excess pores in the soil due to ground liquefaction.

■Pile foundation construction methods will be adopted to suppress deformation of pipelines due to buoyancy and ground deformation.

■In order to reduce the stress generated in the pipeline, a flexible pipe joint is inserted in the pipeline.

[Problems to be Solved by the Invention] When implementing the above-mentioned conventional construction method, the occurrence of liquefaction of the ground during an earthquake has been predicted. However, in the actual design of pipes between structures, the following problems remain.

■Since predictions of external forces associated with the occurrence of liquefaction have not been sufficiently incorporated into the design, it is not possible to ascertain the extent to which the above-mentioned countermeasure basic construction methods are actually effective.

■Flexible pipe joints are sometimes used as a means of dealing with pipe displacement, but it is not expected that changing the installation position of the pipe joint will make a difference in the risk of pipe breakage. However, in actual design, a design method for determining where the pipe joint should be installed has not yet been established.

- Design pipes in a shape that is durable against uplift and subsidence; specifically, what kind of liquefaction countermeasure pipes are suitable for preventing damage to the sound itself when the ground liquefies? There is no consideration as to whether to do so.

■As shown in ■ above, the use of durable pipes has not been considered, so of course there has been no consideration at all about combining this with the basic construction method mentioned above to formulate a comprehensive countermeasure. Ta.

■Particularly in pipes near structures such as manholes, it is predicted that the structures and pipes will be subjected to large external forces when the seven-night phenomenon occurs, but sufficient measures are taken to prevent damage to these pipes. Satisfactory technology had not been established.

Therefore, the inventors of the present invention have a basic idea of responding to the prediction of the occurrence of ground liquefaction phenomenon in buried piping planning areas,
Accurately estimate external forces acting on structures and pipelines, evaluate safety in pipelines adjacent to structures, consider adopting appropriate basic construction methods and liquefaction prevention pipes, and improve safety in pipelines adjacent to structures. We believed that it was necessary to design a new design, and completed the present invention after conducting various types of research.

[Means for Solving the Problems] The present invention provides a method for designing a structure-side pipe section to prevent the structure-side pipe section from being damaged due to the liquefaction phenomenon of the ground that occurs during an earthquake. At least the following (a) to (
The basic gist is to include and implement the steps shown in d) in order.

(a) determining whether the structure floats due to the liquefaction phenomenon; (b) setting a construction method to prevent the structure from floating when it is determined in step (a) that the structure floats; (c) ) Following step (a) or (b) above, a step of estimating the amount of ground subsidence due to the liquefaction phenomenon; (d) Is the amount of ground subsidence resulting from step (c) above less than the allowable value of the liquefaction countermeasure pipe? [Operations and Examples] FIG. 1 is a flowchart showing a typical example of the method of the present invention. In this example, when the occurrence of ground liquefaction phenomenon is predicted and each step in the above flowchart is performed, the buoyant force acting on the manhole and the pipe section fixedly connected to the manhole and the ground subsidence are determined in advance. (hereinafter both may be collectively referred to as liquefaction external force) and the ground spring constant shall be calculated in advance. However, since each of the three procedures detailed below can be calculated independently, they are not limited to the order of implementation related to the following proof, and may be arbitrarily replaced.

[I] First, in estimating the amount of ground subsidence during ground liquefaction, the type of ground in the buried piping planning area is determined based on Table 1.

Table 1 (Note) A): The thickness of the strata here is the thickness from the ground surface.

(mouth): Includes alluvial compact sand layers, gravel layers, and cobblestone layers.

h): Includes new sedimentary layers due to fM collapse, etc.

Next, we determine the occurrence of an earthquake based on the past epicenter distribution and magnitude shown in Figure 2, calculate the magnitude and epicenter distance at the time of earthquake occurrence in the design area, and then calculate the magnitude and epicenter distance in Table 1 using the calculation formula from Table 2. Maximum horizontal acceleration for each ground type shown [A,,,,-(gal)]
seek. The maximum horizontal acceleration is based on the estimation formula shown in the specifications for highway bridges.

Table 2 However, in Table 2, M is the predicted magnitude in the design area, and Δ is the epicenter distance, indicating the distance from the epicenter to the design area.

The amount of ground subsidence δ is calculated using the following equation (1-a) or (1-b
) is estimated by the formula.

(1-a) When there is embankment; Height of embankment (m) × Amount of settlement [δ (cm)] = o, oaa Table 3 (I-b) When there is no embankment; ... (1-a ) +2.52 ... (1-b) However, the above estimation formula is 1964: Niigata Earthquake, 1968:
It was derived using a regression analysis method based on subsidence damage data from 404 points in five earthquakes: the Tokachi-Oki Earthquake, 1973: Nemuro Peninsula-Oki Earthquake, 1978: Miyagi Prefecture-Oki Earthquake, and 1983: Japan Sea Chubu Earthquake. The applicable range of this estimation formula is shown in Table 3.

In addition, the average N value [N] of the sand layer is the total number of layers n, i
When the N value of one layer is Ni, it is calculated by N=-ΣNi.

[+1]Next, the buoyant force acting on manholes and pipes when the ground liquefies is calculated by the unit volume weight of saturated sand [r s (kgf/
cm')] is 1.8 x 10-3 kgf/cm3, and estimated using the following equations (2-a) and (2-b).

(II-8) Manhole buoyancy [F, (kgf)];
F, = γ5 ·V, -γ.・■1 ...(2-
a) (II-b) Buoyant force acting per unit length of the pipe [f p (kgf/cm)]; fp = γs ' π・D2/4 Wp ・=(
2-b) However, γ is the unit volume weight of the manhole (kg
f/cm3), (2) is the volume of the manhole (cm3), W is the dead weight per unit length of the pipe (kgf/cll13), and D is the outer diameter of the pipe (cm).

[1rr] Also, the ground spring constant against subsidence during liquefaction of the ground is determined by the following equation (3-a) based on the results of an upper pipe resistance test on sandy ground.

In other words, the spring constant for sinking is 10g+ok = A-103+oδ+B ・”(
Set according to 3-a). However, δ is the estimated amount of ground subsidence (c+++), A and B are coefficients corresponding to the ground around the pipeline and the estimated amount of ground subsidence, and the coefficients A and B are the location of the buried pipe when calculating the maximum strain described below. The value differs depending on the burial depth depending on whether it is deeper than the groundwater level (saturated sand ground) (see Table 4) or shallower than the groundwater level (drained sand ground) (see Table 5).

Table 4 Table 5 On the other hand, the ground spring constant when a pipeline is subjected to buoyant force during ground liquefaction is based on the results of a pipeline-manhole flotation experiment using a large shearing sand tank, and is shown in the gas pipeline seismic design guidelines. 1/1000 of the ground spring constant (0.6 kgf/cm3)
It has been confirmed that it is 1/3000, and in the pipe response calculation described later, O,OO06kgf/cm3...(
3-b).

Using the amount of ground subsidence, ground spring constant, and buoyancy of the manhole and pipe as basic data, the design of the pipe next to the manhole is carried out in the order shown in the flow chart shown in FIG.

(a) Step: First, the following judgment is made regarding the floating of the manhole.

If the liquefaction resistance index pL (described later) exceeds 5, there is a very high possibility that liquefaction will occur, and consideration must be given to the possibility of manhole floating. If the manhole is buried in the ground, excess pore water pressure will not act directly on the manhole, so there is no need to consider floating. The above liquid resistivity FL is ('tJJ shear strength: R)
/ (Earthquake shear stress ratio; L) and is used to determine the liquefaction of the soil layer (based on the road bridge specifications/V seismic design line). Also, the liquefaction resistance index PL is i2'F
This is one index of the degree of influence of the liquefaction phenomenon on a structure, which is represented by W(z)dz.

However, F=1-FL (FL≦1.0) F=O (FL >1.0) W (z) = 10-0.52 Z: Depth from the ground surface, that is, safety against manhole floating The study of the liquefaction resistance is performed on manholes installed in the ground where the value of the liquefaction resistance index PL exceeds 5 and the liquefaction resistance coefficient FL is 1 or less, and is calculated using the safety factor Fu of floating shown in equation (4). .

However, W: Load on top (t) [including weight of water]
, W,: Self-weight of the manhole (1), Qs: Shear resistance on top (1), Q,: Frictional resistance on the side of the manhole (1), U,: Lifting force due to hydrostatic pressure acting on the bottom of the manhole (1) , Ud: uplift pressure due to excess pore water pressure acting on the bottom of the manhole (1), and the above uplift pressure [Ud] is as follows: Ud=Δu-A=Lu-av'-A...(5)
Find it by

However, A: Manhole bottom area (m2), σv°: Effective overburden pressure at the manhole bottom position under hydrostatic pressure (t/
m2).

Lu: Excess pore water pressure ratio, Lu=ΔU/ σ■, Δu: A surplus water pressure (t/m3) shall be.

(b) Step; When the above-mentioned floating safety factor Fu becomes 1 or less and it is determined that the manhole will float,
Considering the reliability of the floating prevention effect, workability, and economic efficiency, we recommend using crushed stone replacement method (gravel, drain method), crushed stone drain pile method, or gravel net method to prevent the manhole from floating. Adopt floating method.

(c) Step: (1-a), (1
-b) The amount of ground subsidence σ obtained by equation corresponds to the estimation of the amount of ground subsidence in this step.

(d) Step: When the value of 0 ground subsidence estimated in step (c) above is adopted, the liquefaction countermeasure pipe has a structure as shown in Fig. 5, which will be described later. Determine whether it is within the range. In other words, when it is determined that the amount of ground subsidence is such that the maximum bending strain and joint rotation angle described below can satisfy the allowable values by adopting the liquefaction countermeasure pipe, the process moves to step (f) described later, and the ground subsidence that cannot be handled is When it is determined that the amount of subsidence is the amount of subsidence, the following (e)
Move to step.

(e) Step: As a result of the judgment in step (d) above, it can be expected that a judgment conclusion will be made that the amount of ground subsidence exceeds the limit that cannot be countered even by the application of liquefaction countermeasure pipes. A structure that can prevent damage to the pipeline due to ground subsidence will be created by using a combination of methods such as supporting and fixing with piles, fixing with sheet piles, or backfilling with crushed stone for the outer pipeline section and the pipeline.

(f) Step: Using the ground subsidence σ estimated in step (c) and the ground spring constant k obtained in the previous section [III], while performing successive response calculations using differential equations based on beam theory on elastic floors. , - Design an optimal structure-to-structure conduit section using membrane tubes.

For the above design, we used the general pipe model shown in Figure 3 (explanatory cross-sectional view), and the distance between the joint installation position and the manhole wall was 1! ! Change tL sequentially and perform the following (6-a) and (6-b)
) formula to calculate the optimal joint position in the pipe section near the manhole where the generated stress and joint rotation angle are below the allowable values. That is, the maximum bending strain [ε, 18] is as follows (6-a)
Calculate the joint rotation angle [θ,. t] is (6-b
) is calculated using the formula.

(fl) Maximum bending strain calculation formula: % formula % (6) (f2) Joint rotation angle calculation formula; θ,. t = However, β' = / (4EI), = πkD, where is the cross-sectional rigidity of the Erz pipe, D: pipe outer diameter, k: ground spring constant, δ: ground subsidence, L: from the fixed end. Distance to the joint, KR: Joint rotation characteristics.

(g) Step: Determine whether or not the maximum bending strain and joint rotation angle exceed allowable values by changing the distance ML.

The above allowable value will vary depending on the diameter of the pipe material, the structure of the joint, etc., but for example, in the case of PVC pipe for power cables, the yield stress of the material is 100 OJ/cm2, so the safety factor is set to 3 and the maximum allowable bending strain 330 kgt/cm
' Regarding the joint rotation angle, since the angle at which conduction is lost when the cable is pulled in is 4.5 degrees, the allowable value is set at 4 degrees with some margin in mind.

(h) Step: In the response calculation in step (g) above, if the maximum bending strain and joint rotation angle exceed the allowable values, apply a liquefaction countermeasure pipe as shown in Figure 5 to the pipe section near the manhole. Do this.

(i) Step: In step (g) above, when the maximum bending strain and joint rotation angle are below the allowable values, it is necessary to consider the buoyancy of the pipe section next to the manhole when the liquefaction phenomenon occurs. That is, in this step, it is determined whether the buried depth of the pipe section is deeper or shallower than the groundwater level.

Step (j): If the buried depth of the pipe section is shallower than the groundwater level, there is no need to consider the buoyancy that follows, so the design of the pipe section next to the manhole ends here.

(k) Step: On the other hand, for pipes that are buried deeper than the groundwater level, consideration must be given to floating the pipe section as shown below. In other words, the buoyancy fP of the pipe section determined by equation (2-b) and the ground spring constant k shown in (3-b)
Using this method, we examine the safety of pipes adjacent to structures against the buoyancy of pipes during liquefaction. First of all, the company is shown in Figure 4 (A
) (cross-sectional explanatory diagram), the distance 11iiL is estimated as the liquefaction range where the maximum response occurs using the following equation.

coshfi L-L si°f3 L-cos/3L
) ++ (7-,.

Based on the distance 11iL estimated by equation (7-a) above, the response calculation of the maximum bending strain [ε(3] and joint rotation angle θ) was performed under the condition of L=Ll +L2 for the model shown in FIG. 4(B). The following is shown (7-b).

This is performed sequentially using equation (7-c).

(kl) Maximum bending strain calculation formula for pipe section near manhole; % formula % ( ( ) (cosβL1cosβLe-sinβ(Ll +L2
)-coshβL1-coshβLe-sinhβ(
Ll”L2) (sinβL, ・coshβ12-c
osβLIHsinhβL2) cosβLl-cosh
βLe+ (sin βLe −coshβL, −cos
βLe-sinhβL, ) cos73 Le -cos
hβL+)] -(]7-b(k2) Joint rotation angle calculation formula: % formula %) ) ) (7) However, β' = ni/(4EI), ni = πkD, where EI: cross section of the pipe Rigidity, D2 pipe outer diameter, k: ground spring constant, fP: liquefaction countermeasure pipe, L: distance from fixed end to joint, KR: iJi hand rotation characteristics.

(1) Step: Based on the above response calculation, consider whether the maximum bending strain and joint rotation angle exceed the allowable values, and if the maximum bending strain and joint rotation angle exceed the allowable values, take the following measures (m
) Use special measures pipes in the pipes near the manholes as shown in the step. Note that the allowable value in this step is as described above (
g) Shall comply with step tolerances.

(n) Step: In step (1) above, if the maximum bending strain and joint rotation angle are below the allowable values, it is considered that the countermeasure against floating during liquefaction is sufficient, and the design is finished for the time being.

(m) Step: Fig. 5 is an explanatory diagram showing an example of the liquefaction countermeasure pipe in the pipe section near the manhole. A similar laminated tube is used for the receiving tube 2 connected to this, and the length of the laminated portion of the laminated tube is appropriately selected to maintain appropriate safety against external forces generated during liquefaction. Design it as you like.

The order of the design method for the pipe near the manhole above can be changed arbitrarily to the extent possible, and the above (1-
The complicated calculations shown in equations a) to (7-c) are preferably performed using a computer.

It is also known that when the ground liquefies during an earthquake, the flow of the liquefied layer causes permanent ground deformation in the horizontal direction. Prediction of whether or not permanent ground deformation will occur can be made by (a) liquefaction occurrence prediction, (b) topographical prediction, and (c) geological prediction, and ground deformation measurement is based on actual measured values. Statistical methods [for example, the following (8)
) method] has been devised.

D=0.75 H・3 θ...(
8) However, H is the liquefaction layer thickness (m), and θ is the maximum value of the earth surface gradient and the lower surface gradient of the liquefaction layer.

Therefore, when the occurrence of permanent ground deformation is predicted, as a countermeasure for the permanent ground deformation in the conduit section near the manhole, the joint part should have excellent elasticity and flexibility and be equipped with a separation prevention mechanism. In addition, a joint structure will be adopted that takes measures to prevent excessive bending at the joint.

In addition, in order to prevent horizontal flow in structures and pipelines, and to improve the stability of the connection to the fixed ground, the method of securing structures and pipelines using piles and sheet piles will be accepted.

[Effects of the Invention] Since the present invention follows the steps described above, it becomes possible to accurately estimate the liquefaction phenomenon of the ground that occurs during an earthquake and the external force caused by this, and prevent damage etc. from occurring in the pipe section adjacent to the structure. As a result, appropriate liquefaction countermeasure pipes can be adopted, and appropriate countermeasure construction methods can be adopted for structures and pipelines. Therefore, it has become possible to quickly design a pipe section between structures that will not cause damage due to the occurrence of a liquefaction phenomenon.

[Brief explanation of drawings]

Fig. 1 is a flowchart showing a typical implementation procedure example of the present invention, Fig. 2 is an explanatory diagram showing the epicenter distribution and magnitude since recorded history, and Fig. 3 is response calculation in the pipe near the manhole due to ground pouring. Figure 4 (A) is a cross-sectional explanatory diagram showing a pipe model for calculating the response to buoyancy during liquefaction, and Figure 4 (B) is a cross-sectional diagram showing a pipe equipped with a joint. FIG. 5 is an explanatory diagram showing an example in which a liquefaction countermeasure pipe is provided in a pipe section near a manhole. 1... Short pipe near the manhole 2... Intubation Figure 2

Claims (7)

[Claims] (1) In the method of designing pipes adjacent to structures to prevent them from being damaged by the ground liquefaction phenomenon that occurs during earthquakes, (a) the pipes are fixed due to the liquefaction phenomenon; (b) When it is determined that the structure will float as a result of the determination in step (a), setting a construction method to prevent the structure from floating; (c) Step (a) above. When it is determined that the structure will not float, or following step (b), a step of estimating the amount of ground subsidence due to the liquefaction phenomenon; (d) If the amount of ground subsidence resulting from step (c) above is determined as a result of the liquefaction countermeasures. A method for designing a conduit section adjacent to a structure, comprising: determining whether or not the permissible value of the pipe is exceeded; and further proceeding with the design based on the determination result of step (d) above. (2) In claim (1), (e) when it is determined in step (d) above that the allowable value is exceeded, liquefaction countermeasure pipes are applied to the pipes adjacent to the structure, and the ground and pipes are A method for designing a pipeline section adjacent to a structure, which includes the following steps: applying a liquefaction countermeasure method to the pipeline and completing the design. (3) In claim (1), (f) when it is determined in step (d) that the allowable value is not exceeded, the optimum joint in the pipe section adjacent to the structure is (g) determining whether or not the maximum bending strain and joint rotation angle exceed allowable values in the response calculation of step (f) above; , (h) When it is determined in step (g) above that the allowable value is exceeded, liquefaction prevention pipes are applied to the pipe section adjacent to the structure and the design is completed. Design method for pipes between structures. (4) In claim (3): (i) When it is determined in step (g) above that the permissible value is not exceeded, whether the buried depth of the structure-side conduit is deeper than the groundwater level. (j) When it is determined that the burial depth is not deeper than the groundwater level by the determination in step (i) above, the step of terminating the design. . (5) In claim (4), (k) when it is determined in step (i) that the burial depth is deeper than the groundwater level, the buoyancy acting on the pipe section and the ground spring constant (l) setting a liquefaction range that generates a maximum response for the boundary pipe section model, and further calculating the response of the maximum bending strain and joint rotation angle in the structure boundary pipe section based on the liquefaction range; a step of determining whether the maximum bending strain and joint rotation angle in step (k) above exceed the allowable values; (n) when it is determined that the maximum bending strain and joint rotation angle in step (l) above do not exceed the allowable values, the design is A method for designing a conduit section between structures by adding the following steps. (6) In claim (5), (m) a step of performing step (h) above when it is determined that the allowable value is exceeded by the determination in step (l); Department design method. (7) Following any of steps (e), (h), (j), (n), and (m) above, prediction of the occurrence of horizontal deformation in the permanent ground and estimation of the amount of ground deformation are performed. make an estimate,
7. The method for designing a pipe line adjacent to a structure according to any one of claims (2) to (6), comprising the step of applying a permanent ground deformation countermeasure method to the pipe line adjacent to the structure.