JP2007270542A - Road bridge foundation design method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a road bridge foundation design method that uses measured values that are concrete to an extent that they are practicable from an original position shear friction test (SBIFT). <P>SOLUTION: First, the shear stress-vertical stress curve and the water supply volume-vertical stress curve are calculated for each layer of the ground to which this road bridge foundation design applies. Then the circumferential frictional force of each ground layer is calculated using an adhesive force and an inner friction angle obtained from the shear stress-vertical stress curve, and the side pressure value obtained from the water supply volume-vertical stress curve. Further a deformation coefficient is obtained from the water supply volume-vertical stress curve. An ordinary correction coefficient is set up at 2 and an earthquake correction coefficient is at 4 for use in the estimation of a subgrade reaction coefficient. The circumferential friction force of each ground layer is calculated using the adhesive force and the inner friction angle obtained from the shear stress-vertical stress straight line and the side pressure value obtained from the water supply volume-vertical stress curve if the layer is a clay layer, and may be estimated from the value N if the layer is a sand and gravel layer. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a bridge design method for roads, and more particularly to a road bridge foundation design method for determining the arrangement and number of foundation piles.

  Bridges on roads are designed and constructed based on “Technical Standards for Bridges, Elevated Roads, etc.” (Director of City / Regional Development Bureau, Director General of Road Bureaus). The described road bridge specifications are widely used.

  According to the conventional design method based on the above road bridge specifications, the peripheral frictional force and deformation coefficient of the pile used when setting the bearing capacity of the foundation pile are indirect from the N value obtained in the standard penetration test. The value evaluated for was adopted. However, this N value tends to cause a mechanical error or an artificial error at the time of measurement, and depending on the soil, it may not be an index that accurately represents the property. Therefore, when performing foundation design based on the ground constant estimated from the N value, it is necessary to expect a safety factor, and as a result, the number of foundation piles is originally necessary to ensure sufficient strength. There was a tendency to become more than the number. In the actual construction, the cost is increased and the construction period is prolonged.

  In some cases, the deformation coefficient is measured by a method in which a measuring machine for measuring the deformation coefficient is inserted into a hole that has been drilled in advance, and the measured value is corrected for design using a correction coefficient. However, in this method, since the looseness due to stress release occurs in the ground in the already drilled hole, there is a high possibility that the exact strength of the actual ground cannot be measured. Therefore, as in the case of the peripheral frictional force, it is necessary to allow for a safety factor.

In recent years, therefore, research has been conducted on a design method that can measure the properties of the ground more accurately and determine necessary and sufficient piles without considering an excessive safety factor. An in-situ shear friction test (SBIFT) has been proposed as a method for accurately measuring the properties of the ground, which is the premise of such a design technique. This SBIFT is a test method for excavating by self-boring without relaxing the stress on the natural ground and utilizing the borehole, and integrally measuring the strength and deformation characteristics of the ground by the expansion and shearing of the probe. According to this test method, the shear strength constant (c, φ, or δ) and the deformation coefficient E0 can be directly obtained in any soil. In addition, SBIFT has been proposed as a literature, for example, the introduction of new technology “In-situ shear and friction strength measurement of soil” published on page 50 of soil and foundation Vol 45 No12, JSCE Proceedings No. 617 / III- 46, pages 191-200, "Estimation of ground strength and deformation constants by in-situ friction test and application to practical use", or pages 620-621 of the summary of lectures at the National Conference of the Japan Society of Civil Engineers There is “About estimation of frictional force on the surface of piles by SBIFT”. Further, for example, there is a utility model registration No. 3065030 as a document relating to improvement of an apparatus for performing SBIFT.
Utility Model Registration No. 3065030 Road bridge indication (I common edition, IV substructure edition) same comment issued by Japan Road Association New technology introduction “In-situ shear / friction strength measurement of soil”, soil and foundation, Vol45, No12, pp50 Estimation of soil strength and deformation constant by in-situ friction test and application to practical work, Proceedings of Japan Society of Civil Engineers, 617 / III-46, pp191-200 About the estimation of the frictional force of the peripheral surface of the pile by SBIFT, Japan Society of Civil Engineers National Conference Research Presentation Lecture Collection, pp 620-621

  Since the SBIFT can directly determine the shear strength constant of any soil, the deformation coefficient that accurately reflects the strength of the ground can be obtained by designing the structure foundation using these measured values. Therefore, it is theoretically possible to obtain a more accurate result than the conventional design method by using the deformation coefficient. However, a concrete design method using the measurement value obtained by SBIFT has not been proposed yet.

  Accordingly, an object of the present invention is to provide a road bridge foundation design method based on SBIFT measurement values, which is practical enough to be practically used.

  In the road bridge foundation design method according to the present invention, first, by the in-situ shear friction test (SBIFT), the shear stress-vertical stress curve and the water supply amount-vertical are obtained for each layer of the ground which is the target site of the road bridge foundation design. Obtain the stress curve. Then, using the adhesive force and internal friction angle obtained from the shear stress-normal stress curve and the lateral pressure value obtained from the water supply amount-normal stress curve, the peripheral friction force degree for each layer is calculated, A deformation coefficient is obtained from the water supply amount-normal stress curve. In addition, the constant correction coefficient used for estimation of the ground reaction force coefficient is 2 and the earthquake correction coefficient is 4.

  When the ground that is the target site of the road bridge foundation design is composed of a clayey layer, a sandy layer, and a gravel layer, the circumferential frictional force for each layer is Calculated using the adhesive force and internal friction angle obtained from the shear stress-vertical stress line and the lateral pressure value obtained from the water supply amount-normal stress curve, and if the layer is a sandy layer and a gravel layer, the N value It may be estimated by

  According to the design method according to the present invention, the specific calculation of the peripheral frictional force and the ground reaction force coefficient using the shear stress-vertical stress line and the water supply amount-vertical stress curve for each ground layer obtained by SBIFT. Since the method has been decided, the necessary minimum number of road bridge foundation piles can be actually decided.

  In addition, if the ground subject to the road bridge foundation design is composed of a clayey layer, a sandy layer and a gravel layer, and the sandy layer and gravel layer are particularly fragile and easily broken, A more accurate design can be performed by estimating the surface frictional force from the N value.

  A specific example of the road bridge foundation design method according to the present invention will be described with reference to FIGS. FIG. 1 is a flowchart of the design method, FIG. 2 is a cross-sectional view of the ground showing the appearance of a test apparatus used for SBIFT, FIG. 3 is a graph showing an example of a stress-shear displacement curve obtained by SBIFT, and FIG. FIG. 5 is a graph showing an example of a water supply amount-vertical stress curve obtained by SBIFT, FIG. 5 is a graph showing a shear stress-normal stress line obtained based on the graph of FIG. 3, and FIG. FIG. 7 is a graph showing the conceptual shape of the curve obtained when the continuous part is made continuous. FIG. 7 compares the horizontal ground reaction force coefficient calculated from the deformation coefficient and the horizontal ground reaction force coefficient obtained by the loading test. FIG. 8 shows the pier structure dimensions of the road bridge foundation to be designed, (a) is a side view, and (b) is a front view seen from the bridge axis direction.

  The basic flow of this design method follows the road bridge specifications as in the past. Specifically, as shown in FIG. 1, load conditions and ground conditions were arranged in step S1, pile types and pile diameters were set in step S2, and pile design conditions were arranged in step S3. Then, pile arrangement | positioning and the number of piles are set by step S4. However, in this design method, the numerical setting method in organizing the pile design conditions in step S3, specifically the peripheral friction force degree setting method and the ground reaction force coefficient setting method are in accordance with the conventional road bridge specifications. There is a difference in these numerical value setting methods. Therefore, these numerical value setting methods will be described in detail below.

  In this design method, in order to organize the pile design conditions, first of all, by in-situ shear friction test (SBIFT), the shear stress-vertical stress curve and water supply amount- Obtain the normal stress curve.

  As described above, SBIFT is a known technique, and as shown in FIG. 2, is a test performed using a probe 1 having a drilling means 2 at its tip and a peripheral surface freely expanding. The probe 1 includes a peripheral wall made of an elastic material such as rubber surrounding an internal space, and has a structure that expands by injecting high-pressure water into the space by a hydraulic pump 3.

  A bit is provided as the excavating means 2 at the tip of the probe 1 (the end disposed on the lower side when inserted into the soil), and it is possible to reach the measurement point by self-digging. Further, a boring rod 4 is connected to the base end (the end disposed on the upper side when inserted into the soil), and the probe 1 in the target ground can be pulled through the boring rod 4. It has become. At the measurement point, the probe 1 is expanded and a vertical stress is loaded on the wall surface of the target ground, and a tensile force is loaded on the probe 1 and a shear stress is loaded on the wall surface of the target ground. By measuring the shearing stress generated by the measuring machine 5, a stress-shear displacement curve as shown in FIG. 3 and a water supply amount-normal stress curve as shown in FIG. 4 can be obtained. However, since the stress-shear displacement curve obtained by SBIFT does not directly indicate various values necessary for the design, a shear stress-normal stress straight line as shown in FIG. 5 is obtained from this curve. On the other hand, the water flow-normal stress curve obtained by SBIFT has a discontinuity due to the nature of the test, but the essence is the same as the continuous curve as shown in FIG. It is meant to be shown. The vertical axis in FIGS. 4 and 6 represents the water supply amount, but the water supply amount is proportional to the expansion of the peripheral wall of the probe, that is, the deformation of the ground, and thus substantially indicates the strain of the ground.

  If the shear stress-vertical stress line and the water supply-vertical stress curve can be obtained, the adhesive force C is determined from the value of the shear stress axis intercept of the shear stress-normal stress line, and the internal friction angle φ is determined from the inclination with respect to the vertical stress axis. Can be sought. Moreover, the side pressure value P0 can be calculated | required from the normal stress in the 0 point side inflection point P of a water supply amount-normal stress curve. Then, the peripheral surface frictional force f for each layer is calculated by an equation of f = C + P0tanφ.

On the other hand, for the water supply amount-vertical stress curve, the deformation coefficient E0 is obtained from the inclination with respect to the vertical stress axis in the region exceeding the zero point inflection point P. For example, in FIG. 4, when the inflection point P exists in the area of the water supply amount V2, the deformation coefficient E0 is an average value of the slopes of the straight lines L3, L4, and L5 of the areas V3, V4, and V5. Then, the ground reaction force coefficient (vertical direction and horizontal direction) is obtained from the following mathematical formulas 1 and 2. In addition, Formula 1 and Formula 2 are also shown in the road bridge bridge manual.

In these equations 1 and 2, the correction factor (using coefficients to estimate the subgrade reaction coefficient) alpha, where there must be set in accordance with the method of estimating deformation coefficient E _0, if the deformation coefficient E ₀ calculated in SBIFT The correction coefficient α has not been clarified yet. Therefore, the correction coefficient α when the deformation coefficient E ₀ is calculated by SBIFT is set as follows.

As described above, SBIFT uses a bored hole drilled by probe self-boring without drilling the borehole in advance, so that it can obtain a deformation coefficient with relatively high accuracy without relaxing the stress of natural ground. Can do. However, the horizontal ground reaction force coefficient calculated from the deformation coefficient was found to be slightly different from the horizontal ground reaction force coefficient obtained by the loading test. It was also found that when the deformation coefficient calculated by SBIFT was doubled, the horizontal ground reaction force coefficient calculated therefrom was in good agreement with the horizontal ground reaction force coefficient obtained by the loading test. FIG. 7 shows the horizontal ground resistance calculated when the deformation coefficient calculated by SBIFT is doubled (correction coefficient α = 2) and when the deformation coefficient calculated by SBIFT is adopted as it is (correction coefficient α = 1). A comparison of the force coefficient and the horizontal ground reaction force coefficient obtained by the loading test is shown. The horizontal axis is the ratio of the ground surface displacement amount S for loading the width D of the basic, the specifications for highway bridges, when estimating the deformation coefficient E ₀ from N values, the ratio is approximately 1% point A matching relational expression E ₀ = 2800N is adopted. For reference, the horizontal ground reaction force coefficient calculated when the deformation coefficient E ₀ is estimated to be 2800 N is also shown in FIG.

  Considering these facts, the correction coefficient for obtaining the ground reaction force coefficient is always 2 (always correction coefficient). In addition, as in the conventional design method, the earthquake (correction coefficient at the time of earthquake) is set to twice the normal time. That is, it is 4 in this design method.

  In addition, if the ground that is the target site for the road bridge foundation design is composed of clayey layer, sandy layer, and gravel layer, and the sandy layer and gravel layer are particularly brittle and easily collapse, The peripheral surface friction force is estimated from the N value.

By the road bridge foundation design method according to the present invention, a road bridge foundation under the following conditions was designed.
(Basic conditions)
1) Upper construction weight: Continuous narrow box girder bridge between 5 steel diameters 2) Bridge length: 389.000m
3) Girder length: 388.000m
4) Branch length: 69.800m + 78.000m + 86.000m + 87.000m + 66.800m
(Loading condition)
1) Live load: B live load 2) Dead load reaction force: Rd = 13100kN
3) Live load reaction force: RL = 3600 kN
4) L1 design seismic intensity: kh = 0.25 (Lg), kh = 0.26 (Tr)
5) Earthquake horizontal force: H = 3450 kN (Lg), H = 3450 kN (Tr)
6) L2 sharing weight: Wu = 13100 kN (Lg), Wu = 13100 kN (Tr)
(Classification for seismic design)
1) Classification by importance: Class B (National Highway Expressway)
2) Regional distinction: Region B (Niigata Prefecture)
3) Correction factor by region: Cz = 0.85
4) Ground type: Type III ground (structure shape and dimensions)
1) Foundation type: Steel pipe soil cement pile (φ1.2m)
The structural dimensions are as shown in FIG.

The SBIFT and the standard penetration test were performed on the ground to be designed, and the peripheral frictional force and the corrected deformation coefficient shown in Table 1 were obtained. In Table 1, the peripheral frictional force of the layers of As1, Ag2, Dg1-1, Dp-2, Ds1, Dc1-2, and Dg1-2 was determined from the N value. And the result shown in Table 2 was able to be obtained.

For comparison, Tables 3 and 4 show the results obtained when the peripheral frictional force levels and the correction deformation coefficients of all layers are obtained from the N value, that is, the results obtained by the conventional design method.

  As described above, according to this design method, the specifics of the peripheral frictional force and the ground reaction force coefficient using the shear stress-vertical stress line and the water supply amount-vertical stress curve for each ground layer obtained by SBIFT are used. Since the calculation method is determined, the minimum necessary number of road bridge foundation piles can be actually determined. Moreover, since the number of the piles is smaller than that determined by using the N value, it was confirmed that the necessary and sufficient piles can be determined by this design method without expecting an excessive safety factor.

It is a flowchart figure of the specific example of the road bridge foundation design method concerning this invention. It is sectional drawing of the ground which shows the external appearance of the test apparatus used for SBIFT. It is a graph which shows an example of the stress-shear displacement curve calculated | required by SBIFT. It is a graph which shows an example of the water supply amount-perpendicular | vertical stress curve calculated | required by SBIFT. It is a graph which shows the shear stress-normal stress straight line obtained based on the graph of FIG. It is a graph which shows the conceptual shape of the curve obtained when the discontinuous part of the graph of FIG. 4 is made continuous. It is a graph which compares and shows the horizontal direction ground reaction force coefficient calculated from a deformation coefficient, and the horizontal direction ground reaction force coefficient obtained by the loading test. The pier structure dimension of the road bridge foundation used as design object is shown, (a) is a side view, (b) is the front view seen from the bridge axis direction.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Probe 2 Drilling means 3 Hydraulic pump 4 Boring rod 5 Measuring machine S1 Load condition and ground condition arrangement process S2 Pile type and pile diameter setting process S3 Pile design condition arrangement process S4 Pile arrangement and number of piles setting process

Claims (2)

By the in-situ shear friction test (SBIFT), the shear stress-vertical stress line and the water supply-vertical stress curve are obtained for each layer of the ground that is the target site of the road bridge foundation design,
Using the adhesive force and internal friction angle determined from the shear stress-vertical stress line and the lateral pressure value determined from the water supply amount-vertical stress curve, the peripheral frictional force for each layer was calculated,
The deformation coefficient is obtained from the water supply amount-normal stress curve,
A road bridge foundation design method characterized in that a constant correction coefficient is 2 and an earthquake correction coefficient is 4, which is used for estimating a ground reaction force coefficient. If the layer is a clayey layer, the peripheral frictional force for each layer is determined by the adhesive force and internal friction angle obtained from the shear stress-normal stress line, and the lateral pressure value obtained from the water supply amount-normal stress curve. The road bridge foundation design method according to claim 1, wherein if the layer is a sandy layer and a gravel layer, it is estimated by N value.

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