JP5576066B2 - Design method for composite ground - Google Patents

Description

  The present invention relates to a method for designing a composite ground comprising an original ground supporting a relatively small detached house and a small-diameter pile installed in the original ground.

  Relatively small detached houses, such as detached unit buildings, have a significantly lower ground bearing capacity than large-scale high-rise, super-high-rise condominiums and buildings. In general, there is no need for ground improvement or foundation structure. “Ground strength” here means both the weight of the unit type building, that is, the vertical supporting force of the foundation ground that supports the vertical load, and the amount of settlement that the ground sinks due to this vertical load. The foundation ground can support the building weight of the construction target when the building weight is within the allowable bearing capacity of the foundation ground and the amount of settlement (including non-uniform settlement) is within the tolerance settlement. It can be said that it has earth resistance.

  As mentioned above, even in small-scale unit buildings, where the soil bearing capacity and foundation structure of the foundation ground were not as important as those of large-scale structures so far, recently there are defective houses such as settlement of the foundation ground. Of the industry that seeks to reduce the cost of housing by rationally evaluating the strength of the foundation ground, and to increase demand for detached houses such as unit-type buildings. With the background of trends and the like, proposals for new design methods and improvements to the design methods for the foundation ground that supports detached houses have become popular among housing manufacturers.

  For example, with regard to the foundation ground structure that supports a conventional detached house, a strong foundation has been adopted in which a fabric foundation or a solid foundation is built directly on the ground, and a relatively soft ground (a foundation with a self-settled layer). In the case of viscous ground where there is concern about settlement such as consolidation, loose sandy ground where liquefaction is concerned, etc.), pile foundations, surface improvement ground where only a few meters of the surface layer is cemented, and pile foundations The foundation ground form combined with surface improvement is adopted.

  And the design method when adopting the pile foundation is to support all of the building weight with, for example, a pile whose tip is embedded in the support layer. The compression strength of the main body is checked, and the support force (tip support force) of the pile tip ground is checked (in some cases, the peripheral friction force between the pile peripheral surface and the ground is added to the pile tip support force) The allowable bearing capacity of the piles).

  That is, in the case of a pile foundation, no consideration is given to the vertical bearing capacity of the original ground around the pile directly below the solid foundation or the fabric foundation.

  Actually, for example, if the ground is subsidized due to a decrease in the groundwater level, etc., the reality is that the building will be supported only by the pile, and even if the ground is considered in addition to the pile, Considering the reality in structural mechanics that the pile installed under the pillar bears most of the building weight, there is no doubt that the design method that supports the building weight with only the pile is the design on the safe side There is no room.

  However, as described above, when the strength of the foundation ground is rationally evaluated, and when it is intended to further reduce housing costs while ensuring safety, the foundation considering both the pile and the original ground. Application of ground design techniques is desirable.

  And even in detached houses, the dimensions and shape are already set with several standard units, and the overall dimensions and shape of the building can be easily set by assembling these units in a predetermined number of planes and longitudinally. Especially in the case of unit type buildings that can be used, the weight of each planar area in the unit is easily determined, so even if the building weight is borne by both the pile and the ground, other structural types Compared with detached houses, it is relatively easy to calculate the share of load on piles and raw ground.

  In addition, the following two techniques can be mentioned as a public technique regarding the basic structure of a conventional detached house. One of them is that when building a building on soft ground, a foundation pile that joins the upper end to the concrete foundation is driven into the ground directly below the center of gravity of the entire building, including the concrete foundation laid under the building body. The foundation structure is such that the center of gravity of the building is supported by the foundation pile (see Patent Document 1). The other is that after confirming that the pile tip of the steel pipe pile has reached the support layer, the ground near the pile tip is loosened, and then the load of the superstructure is applied, so that only the loosened part The steel pipe pile sinks and the supporting force at the tip of the pile is exerted, and the ground reaction force acts on the bottom surface of the pressure plate that sinks along with the steel pipe pile to obtain the supporting force (patent) Reference 2).

  However, the technique disclosed in Patent Document 1 does not exceed the range of the conventional design method in which the building weight is supported only by the pile, and the technique disclosed in Patent Document 2 is used when the building weight is loaded. Because it is a technology that tries to demonstrate the bearing capacity of the pile tip, if considering the fact that the ground properties are different for each construction point of the building, it is extremely poor in versatility, and also has poor structural safety and reliability I must say.

JP 2008-223267 A JP 2007-113270 A

  The present invention has been made in view of the above-described problems, and is a foundation ground that supports a single unit building, and relates to a design method of a foundation ground that supports the weight of a building by both a pile and an original ground. An object of the present invention is to provide a rational foundation ground design method that is versatile, has high design reliability, and high reliability of the foundation structure formed by the design.

In order to achieve the above object, a composite ground design method according to the present invention is a composite ground design method comprising a base ground for supporting a detached house and a small-diameter pile installed in the base ground. The allowable ground bearing capacity after the small-diameter pile is installed, the allowable bearing capacity of the small-diameter pile, and the allowable settlement of the original ground after the small-diameter pile is installed are set. The first step, the ground spring of the original ground, and the pile spring of the small-diameter pile are set, and the load sharing rate of both the original ground and the small-diameter pile is determined according to the size of both the ground spring and the pile spring. The second step to be set, the weight of the detached house is apportioned for each of the plurality of divided areas, and the weight of the original ground and the weight of the small-diameter pile are calculated from the apportioned weight and the load sharing rate. And a third step of calculating a subsidence amount of the original ground, The load weight of each of the ground and the small-diameter pile is compared with each of the allowable support forces, the load weight is equal to or less than the allowable support force, and the subsidence amount and the allowable subsidence amount of the original ground In comparison, the fourth step of confirming both that the subsidence amount of the original ground is less than or equal to the allowable subsidence amount, in the second step, the size of the detached house by a basic analysis model, The load sharing rate is obtained from the result of analyzing the ground conditions in advance for each application location .

The “composite ground” as used in the present invention refers to a foundation ground that supports the weight of the building, consisting of both the original ground and small-diameter piles that are placed, pressed, or buried in the original ground. is there. Small-diameter piles generally refer to steel pipe piles, including piles with expanded tips, piles with wings at the tips, and piles for detached houses. Therefore, it means a pile having a diameter of 100 to 200 mm, particularly about 100 to 150 mm. In addition, this small-diameter pile generally supports a solid foundation etc. directly at its upper end, rather than being embedded in a solid foundation or cloth foundation, as in conventional pile foundations. It does not exclude the conventional pile foundation structure. Further, herein, the term "acceptable subsidence in the original ground" is ground is reinforced by the small-diameter piles is the allowable subsidence in the original ground constituting the composite ground after the modification in which designated finger.

  In addition, unlike a conventionally known group pile foundation, it is not intended to compact the original ground between the piles by placing the piles densely. For example, one small-diameter pile under the pillar of a detached house Therefore, the strength inherent to the original ground is evaluated as it is.

  Furthermore, the design method of the present invention is suitable for the design of a foundation ground that supports a unit type building among detached houses. This is because, in the case of a unit type building consisting of a rectangular parallelepiped or a cubic hexahedron, the dimensions and shapes have already been set with several standard units as described above, and these units are arranged in a predetermined number of planes and longitudinally. As a result, the overall dimensions and shape of the building can be easily set.Therefore, it is relatively easy to calculate the building weight borne by each of the piles constituting the composite ground and the original ground. This is because it is suitable for clear and rapid design. That is, since the apportioned weight for each area of the unit type building is already known, the load sharing ratio of both the original ground and the small-diameter pile is calculated by calculating the load sharing ratio of both the original ground and the small-diameter pile described below. Comparing the burden load of both sides with the allowable bearing capacity (or design standard value) of both the original ground and the small diameter pile, it is confirmed whether it is within the allowable bearing capacity. For example, it can be determined that the originally set composite ground specifications (strength specific to the original ground, small diameter pile specifications and length, etc.) are acceptable, and if the allowable bearing capacity is exceeded, the composite ground specifications can be changed and redesigned. I will do it.

  First, it is necessary to set the allowable bearing capacity of the original ground that constitutes the composite ground, the allowable bearing capacity of the small-diameter pile, and the allowable settlement of the original ground that constitutes the composite ground. It is preferable to refer to various calculation formulas and various allowable values described in the small-scale building basic design guidelines (Architectural Institute of Japan, 2008) as appropriate. In addition, the allowable subsidence amount of the original ground is the allowable subsidence amount of one or both of the allowable subsidence amount of the original ground directly below the floor of the building and the original subsidence depth deeper than the pile tip. In the case where uneven settlement of buildings is a problem, it means that the amount of relative settlement (relative deformation angle that can affect the building structure) is included. The “composite ground design method” according to the present invention is not described in the small-scale building foundation design guidelines, and the technical concept does not exist in the conventional ground foundation design method for detached houses. Needless to say, it is a design philosophy.

  In addition, the present inventors independently obtained the standard penetration test result of the original ground throughout Japan and the Swedish sounding test result (hereinafter referred to as “SWS test”) which is a simple ground investigation method. In general, the SWS test is carried out in the basic design of a detached house, and various characteristics of the ground are evaluated based on the test results. In forming the design method of the present invention, these results are used as appropriate. The reliability and validity of the design method are guaranteed.

  First, in the first step, the allowable bearing capacity of the original ground after the small-diameter pile is installed, the allowable bearing capacity of the small-diameter pile, and the allowable subsidence amount of the original ground after the small-diameter pile is installed Then, as a second step, both the ground spring when the original ground is evaluated as a ground spring and the pile spring when the small-diameter pile is evaluated as a pile spring are set.

  The ground spring and the pile spring are used for both setting the load sharing ratio of both the original ground and the small-diameter pile and calculating the settlement amount of the original ground.

  Here, the setting method of the ground spring is preferably set in accordance with the setting method indicated by the small-scale building foundation design guideline described above.

  Moreover, as one embodiment of the setting method of the pile spring, the peripheral friction spring between the pile and the surrounding ground, the pile tip spring of the pile tip ground, and the pile main body when the compressive force is applied There is a method of setting from a pile body spring based on elastic deformation.

  This peripheral friction spring is different from the peripheral friction spring in a long-term design, that is, a design that normally supports only the building weight in general time, and a short-term design, that is, a design such as during an earthquake. The present inventors have found that this is preferable from the viewpoints of economy, rationality, and safety. Needless to say, the same peripheral friction spring may be used for the long-term design and the short-term design.

  Therefore, in the long-term design, the peripheral friction spring is calculated by multiplying the value specified by the maximum peripheral friction force when a small-diameter pile sinks by a predetermined amount by a predetermined coefficient less than 1. The setting method of setting the circumferential friction spring at the time to be twice that of the circumferential friction spring at the time of long-term design can be applied.

  For example, the friction force at the time of settlement of 1/10 of the pile diameter of a small-diameter pile is set as the maximum peripheral friction force (extreme friction force), and the peripheral friction spring is 1/3 of the ultimate friction force. It can be set from the amount of settlement when the load is 2/3.

  Moreover, you may apply the method of changing a surrounding surface friction spring by the case where the above-mentioned load sharing rate is calculated | required, and the case where the amount of subsidence is calculated | required.

  That is, among the peripheral friction springs that constitute the pile spring, the peripheral friction spring at the long-term design for calculating the settlement amount of the original ground is the maximum peripheral friction force when the small-diameter pile sinks by a predetermined amount. It is calculated by multiplying the specified value by a predetermined coefficient of 1 or more, and the circumferential friction spring at the short-term design for calculating the subsidence amount of the original ground is 1/2 times the circumferential friction spring at the long-term design. Can be set to In addition, according to the present inventors, although the result that the short-term spring is lowered due to the creep is obtained in the experiment, in the actual ground, the short-term allowable bearing force becomes larger than the long-term. Therefore, the short-term circumferential friction spring may be larger than that of the long-term.

  According to the knowledge of the present inventors, in order to perform a safe design in the evaluation of ground subsidence, the peripheral friction spring at the time of long-term design has a predetermined peripheral amount of friction (for example, a small-diameter pile) It is specified that it can be matched with the experimental value by making it 10 times the value divided by the settlement amount of 1/10 of the pile diameter) and making the peripheral friction spring at the time of short-term design 5 times. However, since the result that the short-term spring is lowered due to the creep is obtained, the design can be doubled for both the long-term and the short-term. As described above, both the peripheral friction springs may be set to the same value during the long-term design and during the short-term design.

  If the ground spring of the original ground and the pile spring of the small-diameter pile are set, the load sharing ratio of both the original ground and the small-diameter pile is automatically set according to the size of both the ground spring and the pile spring.

  In the second step, it is preferable that a composite spring that combines a ground spring and a pile spring is further set.

  This composite spring may be calculated from the sum of the ground spring and the pile spring, or a corrected ground spring obtained by multiplying the ground spring already set by a predetermined coefficient less than 1 And the sum of the pile springs.

  In particular, the calculation method for the latter composite spring is based on the actual load flow in structural mechanics, that is, the actual phenomenon that the load is easily transmitted to the pile with respect to the original ground. The springs are reduced as desired, thereby relatively increasing the value of the pile springs.

  The predetermined coefficient less than 1 multiplied by the ground spring may be an arbitrary value such as 0.3, 0.4, or 0.8, or may be set uniquely. Both the superstructure of the unit type building and the composite ground (original ground and small-diameter pile) may be modeled in a computer, and the coefficient multiplied by the ground spring or the ground spring itself may be set. .

  As described above, when the detached house is a unit type building, the weight is already set (in advance), and the weight for each of the plurality of divided areas in the plan view shape, that is, the apportioned weight is already set. Therefore, the burden weight of the original ground and the burden weight of the small-diameter pile in each divided area are calculated from the apportioned weight and the load sharing rate, and the subsidence amount of the original ground is also calculated (third) Step). In addition, the superstructure of a detached house (for example, a unit type building) is appropriately modeled in a computer (of course, the weight of the superstructure is also set in the computer). A method may be used in which a spring model of a model and a pile spring is attached, and an apportioned weight borne by each spring is automatically calculated in the computer.

  In the third step, the burden weight of the original ground and the burden weight of the small-diameter pile are calculated, and the subsidence amount of the original ground is calculated. Finally, in the fourth step, the burden weight of each of the original ground and the small-diameter pile is calculated. Comparing each allowable bearing capacity, it is confirmed that the burden weight is not more than the allowable bearing capacity and that the subsidence amount of the original ground is not more than the allowable settlement amount.

  If each burden weight is within the allowable bearing capacity and the subsidence amount of the original ground is within the allowable subsidence amount, the specifications of the initially set composite ground (specific strength of the original ground, The design is finished at this point.

  In addition, the specifications of the pile set initially, that is, the pile diameter and the pile length of the small-diameter pile, etc., depend on the ground characteristics (hardness of soil layer, level of hard ground as support layer, etc.) Since it may be changed as desired and is intended for small-sized unit-type buildings, it is reasonable to design by changing the specifications of the piles for each place in this way. And by changing the pile specifications and pile lengths for each location, it becomes possible to make the amount of settlement of all piles uniform, and by carrying out such a design, the above-mentioned uneven settlement of buildings is suppressed. It becomes possible to do.

  In addition, even in the composite ground whose safety has been confirmed in the fourth step, if there is a margin in the safety factor, the diameter of the pile can be made smaller or the pile length can be made shorter. It is possible to design an optimal composite ground that is as cheap as possible and excellent in safety by conducting a review and conducting a safety check again.

  In the fourth step, if it is determined that the allowable bearing capacity is exceeded, it is needless to say that the specifications of the composite ground are changed and the design is performed again to obtain the specifications that guarantee the safety. Yes.

  According to the composite ground design method of the present invention described above, pile springs and ground springs are set as desired based on simple SWS test results, their load sharing ratio, and composite ground composed of both springs. The spring can be set as desired, and it is possible to realize an optimum design of the composite ground supporting the unit type building which is rational, economical and highly safe.

  As can be understood from the above description, according to the composite ground design method of the present invention, the foundation ground supporting a small united unit building, more specifically, the composite ground composed of a pile and an original ground. With regard to the design of a small-diameter pile, an appropriate pile spring is used to evaluate the original ground using an appropriate ground spring, the load sharing ratio is appropriately evaluated using both springs, and a composite spring consisting of both springs is used appropriately. By evaluating the above, it is possible to realize the optimum design of the composite ground, and thus it is possible to design a composite ground that is as cheap as possible and sufficiently safe.

It is the top view which showed typically one embodiment of the unit type building. It is the II-II arrow line view of FIG. 1, and is the longitudinal cross-sectional view which showed the composite ground typically. It is a schematic diagram of the pile spring model and ground spring model of a composite ground. It is the flowchart which showed the design method of the composite ground of this invention. It is the graph which compared the conversion N value by a SWS test, and the N value by a standard penetration test. Is a graph showing the relation between the pile tip average conversion N value and the ultimate tip bearing capacity of q _p. It is a graph showing the relation between the adhesive force c and ultimate tip bearing capacity of q _p pile tip bottom. It is the graph which compared the calculated value and the experimental value of ultimate tip bearing force. It is the graph which showed the relationship between the peripheral ground average conversion N value and the limit peripheral surface frictional force degree (tau) _d . It is the graph which showed the relationship between circumferential surface average conversion adhesive force c and ultimate circumferential surface frictional force degree (tau) _d . It is the graph which compared the calculated value and experimental value of the ultimate frictional force. It is a graph regarding the calculated value and experimental value (at the time of long-term load) of a ground spring. It is the graph which showed the relationship between the load degree of ground and the amount of settlement. It is the graph which showed the relationship between the load degree of ground and the amount of settlement. It is the graph which showed the relationship between the load degree of ground and the amount of settlement. It is a figure explaining the view of a circumferential friction spring. It is a figure explaining the setting of the surrounding surface friction spring at the time of long-term design and short-term design. It is the graph which showed the relationship between frictional force and the amount of settlement. It is the graph which showed the pile tip spring at the time of long-term design and short-term design. It is the graph which showed the relation between pile tip load and pile tip settlement. (A) is the figure explaining the compressive force concerning a steel pipe pile, (b) is the figure explaining the method of evaluating (a) on design. It is the graph which compared the calculated value and experimental value of a pile head spring. It is a graph regarding the calculated value and experimental value of a compound spring. It is a graph regarding the calculated value and experimental value of the load share of a pile. It is the schematic diagram which showed the composite ground model at the time of calculating the load sharing rate of a pile by analysis. It is the graph which showed one Example of the setting chart of the load sharing rate of a pile. It is a graph regarding the calculated value and experimental value of the load sharing rate of the ground.

  Embodiments of the present invention will be described below with reference to the drawings. In the following, among the detached houses, the unit type building is taken up and the design method of the present invention is explained. However, it goes without saying that the design method of the present invention can be used for designing a detached house other than the unit type building. is there.

  FIG. 1 is a plan view schematically showing an embodiment of a unit type building, and FIG. 2 is a view taken in the direction of arrows II-II in FIG. 1 and schematically showing an outline of the composite ground. FIG. 3 is a longitudinal sectional view, and FIG. 3 is a diagram schematically showing a pile spring model and a ground spring model of a composite ground.

  A unit type building 20 shown in FIG. 1 is a unit type building having four units 10,. It should be noted that the shape of the unit in plan view may be a square, the radix of the unit may be 5 or more, and it may be a unit type building with 2 or 3 floors. That is.

  The unit 10 is a hexahedron having a frame composed of a column 1 made of square steel pipe, a ceiling beam 2 and a solid foundation 3 (floor slab), and is configured by installing a wall panel and a floor panel (not shown) on the frame. Is done. Further, below each column 1, steel pipe piles 4 </ b> A, 4 </ b> B, 4 </ b> C having a small diameter of about 100 to 150 mm exist. The solid foundation 3 is directly supported by the original ground G (surface first layer ground G1, second layer ground G2, third layer relatively hard ground G3). In the illustrated example, for example, the first layer ground G1 and the second layer ground G2 are both relatively loose viscous ground, and the third layer relatively hard ground G3 supports the tip of the small-diameter pile. However, the ground G3 is not necessarily hard.

  Thus, the unit type building 20 is supported by the small-diameter piles 4A, 4B, 4C and the original ground G, and the composite ground 30 from the small-diameter piles 4A, 4B, 4C and the original ground G. This is a supporting ground structure targeted by the design method of the present invention.

  Then, by setting a plurality of types of units 10 whose shapes and dimensions are changed in advance, when the unit type building 20 is arranged as illustrated, the pillar 1 surrounded by the four units 10,. , A divided area A in the longitudinal direction adjacent to divided area A (two divided areas B, one pillar 1 exists in the center of B), and divided area C in the short direction adjacent to divided area A (One pillar 1 exists in the center of the two divided areas C and C), and a divided area D (one pillar 1 exists) at a position diagonal to the divided area A is automatically set.

That is, in each divided area, the plane area borne by the corresponding pillar 1 is automatically set, and the small-diameter piles 4A, 4B below the respective pillars 1,... , vertical load _W a of unitary building 20 flowing through _4C, W B, so that _{the W C} is easily set.

Further, the total weight W of the unit type building 20, small-diameter piles 4A, 4B, 4C are vertical load _W A _bear, W B, _{W C} and, distributed load w of original ground G burden through the mat foundation 3 It will be divided and divided. Here, the superstructure of the unit building 20 is appropriately modeled in the computer (of course, the weight of the superstructure is also set in the computer), and the ground spring as shown in FIG. A spring model of a pile, a spring model of a pile spring (a pile spring k _t that models a small-diameter pile, and a spring according to the size of a ground spring k _s that models an original ground) are automatically installed in a computer The weight borne by each spring may be calculated. In FIG. 3, the linear beam model such as mat foundation, consisting attached this pile spring k _t and ground spring k _s, is shown a model of the complex ground is. Here, the pile spring k _t is a pile based on a circumferential friction spring k _pf between the pile and the surrounding ground, a pile tip spring k _{ps on} the pile tip ground, and an elastic deformation of the pile body when a compressive force is applied. and it is set from the main spring k _p.

  Next, the composite ground design method of the present invention will be outlined with reference to the design flow diagram of FIG. Here, each step is outlined, and details of each step will be described separately below.

  First, based on the SWS test results and the various characteristics of the small-diameter steel pipe piles (cross-sectional rigidity, pile length, etc.), the allowable bearing capacity of the original ground after the small-diameter steel pipe piles are installed, the allowable support of the small-diameter piles Each of the allowable subsidence amount of the ground after the force and the small-diameter steel pipe pile are installed is set in accordance with, for example, a small-scale building foundation design guideline (Architectural Institute of Japan, 2008) (step S1). At this stage, the pile lengths of the small-diameter piles 4A, 4B, and 4C are different according to the layer distribution of the hard ground G3, which is the support layer that supports the pile tip, and thus the support ground By changing the pile length according to the position, it is possible to equalize the amount of settlement of each pile when all the piles support the building weight, thereby suppressing the uneven settlement.

  Once each allowable value (design standard value) is set, the ground spring of the original ground and the pile spring of the small-diameter pile are similarly set according to the small-scale building foundation design guidelines, and then both springs Based on this, one sets the load sharing ratio of both the small-diameter pile and the original ground, and the other sets a composite spring based on both springs (step S2). Here, the pile spring is composed of three friction springs: a circumferential friction spring between the pile and the surrounding ground, a pile tip spring at the pile tip ground, and a pile body spring based on elastic deformation of the pile body when a compressive force is applied. A spring element is synthesized.

  Next, the weight of the original ground and the small-diameter pile is calculated from the apportioned weight and the load sharing rate of the unit type building, and the amount of settlement of the original ground is calculated (step S3). This apportioned weight is a weight that is known in advance for each divided area shown in FIG. 1, and the load borne by both springs is determined based on the load sharing ratio according to the size of both the ground spring and the pile spring. It is uniquely required. As described above, the upper structure of the unit type building is appropriately modeled in the computer, and the spring model of the ground spring and the spring model of the pile spring are attached to the upper structure model, and each spring is automatically set in the computer. May be a method of calculating the weight borne by.

  Compare the calculated load weight of the original ground and small-diameter piles with their allowable bearing capacity, and compare the amount of settlement of the original ground with the amount of allowable settlement. As a result of the comparison, if each burden weight is within the allowable bearing capacity, and the amount of settlement of the original ground is within the allowable amount of settlement, the specification of the small-diameter pile set as intended and the composite ground consisting of the original ground will be accepted. To do. If there is a margin in safety factor, it is better to rank down the specifications of small-diameter piles and check the optimal composite ground while redesigning.

  On the other hand, if, as a result of comparison, the burden weight of one of them does not fall within the allowable bearing capacity, or if the amount of subsidence of the original ground does not fall within the allowable amount of subsidence, the pile specifications for small diameter piles, for example, are upgraded. The design is repeated again until all the allowable values are satisfied.

Next, using the test results by the present inventors, the setting method of the allowable bearing force of the small-diameter pile, the setting method of the allowable bearing force of the original ground, the setting method of the ground spring and the pile spring will be described in detail in this order.
[How to set the allowable bearing capacity of small-diameter piles]
The small-diameter steel pipe piles that make up the composite ground are not connected to floor beams and the like, and therefore, support performance is evaluated as bearing only a vertical load. Therefore, it is not necessary to consider the force acting from the pile in the design of floor beams. The following formulas are based on the small-scale building foundation design guidelines (Architectural Institute of Japan, 2008).

(Long-term allowable bearing capacity)
Long allowable bearing capacity R _a steel pipe pile is represented by the following equation. The short-term allowable bearing capacity is twice the long-term allowable vertical bearing capacity.
R _a = 1/3 · R _u (kN)
R _a : Long-term allowable vertical bearing capacity (kN) of steel pipe pile
R _u : Ultimate vertical bearing capacity of steel pipe pile (kN)

Long allowable bearing capacity R _a of the steel pipe pile is a long-term permissible vertical bearing force R _a1 by ground piles circumferential surface and the tip, of the allowable compressive force R _a2 of the pile body, and a small value.

(Long-term allowable vertical bearing capacity)
R _a1 = 1/3 · (R _p + R _f ) (kN)
R _a1 : Long-term allowable vertical bearing capacity (kN) of steel pipe pile
R _p : Ultimate tip bearing capacity (kN) at the tip of steel pipe pile
R _f : Ultimate friction force (kN) by the ground around the steel pipe pile
The short-term allowable bearing capacity is twice the long-term allowable bearing capacity.
A method for evaluating the tip support force and the ultimate support force will be described later.

(Long-term allowable compression force)
R _a2 = 1/3 · F _c · A _p (1-α ₁ ) (kN)
R _a2 : Long-term allowable compressive force of steel pipe pile (kN)
F _c : Design standard strength
In the case of 0.01 <t _e /r<0.08 F _c = F (0.8 + 2.5 × t _e / r)
When t _e /r≧0.08 F _c = F
F: Reference strength 235 (N / mm ² ) for STK400, 325 (N / mm ² ) for STK490
t _e : Pile wall thickness (m) excluding corrosion allowance (1mm outer surface)
r: Effective radius of pile (m)
A _p : Effective cross-sectional area of steel pipe pile (m ² )
α ₁ : Reduction ratio by slenderness ratio When L / D> 100, α ₁ = (L / D−100) / 100
L: Pile length (m) D: Effective diameter of pile (m)
The short-term allowable compression force is 1.5 times the long-term allowable compression force.

(About the evaluation range of the tip support force)
Regarding the target range of the tip, in the case of a pile of general construction method, the target range of N value in the evaluation of sandy soil is defined as “1D below, up 1D range ”(D: pile diameter). Other construction methods and literature have even larger ranges.

  Here, when the penetration resistance value obtained in the SWS test near the tip of the pile is changing abruptly, an extremely large value may be adopted in a narrow range.

  FIG. 5 compares the average N value of the tip portion converted from the SWS test adopted for actual evaluation for each test site and the N value by the standard penetration test for each tip range. In this way, the SWS test may be evaluated more greatly if the range is reduced in this way. On the other hand, considering the roughness of 1 m unit of sampling in the standard penetration test, it is appropriate to evaluate at about 5D.

(Extreme tip support force)
In the target steel pipe pile, since the support performance by friction is also expected, as long as the required support performance is obtained, the tip of the steel pipe pile does not necessarily reach the hard support layer. Therefore, the evaluation is basically based on the N value, and it is appropriate to evaluate the relatively soft clay soil at a level that does not reach the support layer by the adhesive force. FIG. 6 shows a comparison of the tip average converted N value in the experiment when the lower ground at the tip of the steel pipe pile is sandy soil or hard viscous soil, and the ultimate tip bearing strength q _p in the experiment.

  The gradient in FIG. 6, the so-called tip support force coefficient, can be appropriately evaluated by setting α = 200. However, the range where the converted N value exceeds 20 is not fully supported by this experiment, so the upper limit is N = 20. Note that the hard cohesive soil is N> 15, which can be determined to be sufficient as a support layer.

Then in the experiment when the lower ground of the steel pipe pile tip of Clay, 7 adhesion and c of the tip bottom, a comparison of ultimate tip bearing capacity of q _p in the experiment.

  From the above, the coefficient relating to the adhesive strength c used for the calculation of the tip support force degree can be set with the coefficient 6 shown in the small-scale building foundation design guidelines (Japan Architectural Institute).

  Moreover, the ultimate tip bearing force can be appropriately evaluated by the following formula according to the soil quality of the lower ground of the steel pipe pile tip.

(For sandy soil)
R _p = q _p × A _p = α × N ₀ × A _p (kN)
q _p : Ultimate tip bearing strength (kN / m ² )
A _p : Cross-sectional area of the tip of the steel pipe pile (m ² )
α: Tip bearing force coefficient. In this method, evaluation is performed with α = 170 (where N = 18 is the upper limit).
N ₀ : Average value of converted N values in the range of 1D down from the tip of the steel pipe pile and 5D up (D: steel pipe pile diameter). The converted N value of each layer is W _SW , N _SW obtained by the SWS test From the following formula.
N = 2.0W _SW + 0.067N _SW (Sandy soil)
N = 3.0W _SW + 0.050N _SW (cohesive soil, when intervening in calculation range)
W _SW : Load of SWS test (kN)
N _SW : SWS test half rotation speed (times / m)

(For clay soil)
Based on the knowledge of the present inventors, in the case of cohesive soil, it is preferable to change the calculation formula as follows according to the hardness. Note that the design method for changing the calculation formula in accordance with the hardness of the clay is not in the conventional design method.

(In case of hard clay (N ₀ > 15))
R _p = q _p × A _p = α × N ₀ × A _p (kN)
q _p: ultimate tip bearing capacity of (kN / m ²⁾
A _p : Cross-sectional area of the tip of the steel pipe pile (m ² )
α: Tip bearing coefficient, evaluated by α = 200 (however, N = 20 is the upper limit) in this construction method.
N ₀ : Average value of converted N values in the range of 1D down from the tip of the steel pipe pile and 5D up (D: steel pipe pile diameter). The converted N value of each layer is W _SW , N _SW obtained by the SWS test From the following formula.
N = 3.0W _SW + 0.050N _SW (Cohesive soil)
N = 2.0W _SW + 0.067N _SW (sandy soil, when intervening in calculation range)
In addition, it is also possible to calculate by the following formula from the result shown in FIG. 10 by a conventional method.
R _p = 6 × c × A _p (kN)
c: The average value of the converted adhesive force in the range of 1D down from the tip of the steel pipe pile and 5D up (D: steel pipe pile diameter). The converted N value of each layer is from W _SW and N _SW obtained by the SWS test. Calculated using the following formula.
c = 1/2 (45 W _SW +0.75 N _SW ) (kN / m ² )

(For other clay soils)
R _p = 6 × c × A _p (kN)
c: 1D from steel pipe pile tip down, top to 5D: the average value of the adhesive strength in the range (D steel pipe pile ^{diameter) (kN / m 2),} W adhesion of each layer was obtained by SWS test _SW , _NSW is calculated by the following formula.
c = 1/2 (45 W _SW +0.75 N _SW ) (kN / m ² )
FIG. 8 shows a summary of the calculated values and experimental values of the ultimate tip support force.
From FIG. 8, it is determined that the value of the ultimate tip supporting force can be evaluated on the safe side.

(Extreme frictional force)
FIG. 9 shows a comparison between the average converted N value of the steel pipe pile peripheral ground and the experimental limit peripheral surface frictional force τ _d from the experimental results.

From Fig. 9, the evaluation of the ultimate peripheral frictional force from the converted N value is based on the guideline for small-scale building foundation design (The Architectural Institute of Japan). Τ _d = 10/3 · N (≒ 3.3) for sandy soil N), and other data such as data showing large values of viscous soil are also mentioned. However, in this construction method, τ _d = 3.0N for viscous soil and sandy soil Is considered to be appropriate to evaluate at τ _d = 2.0N.

The ultimate friction force can be calculated as follows.
R _f = D × Σ (τ _d × L _i ) × π (kN)
R _f : Ultimate frictional force (kN) due to the ground of the pile-shaped ground reinforcement
D: Pile-shaped ground reinforcement diameter (m)
τ _d : Ultimate circumferential frictional force of each layer (kN / m ² )
For cohesive soil by conversion N value of each τ _{d =} 3.0N, in the case of sandy soils tau _{d =} 2.0 N
The converted N value is W _SW , N _SW obtained by the SWS test.
From the following formula.
N = 3.0W _SW + 0.050N _SW (Cohesive soil)
N = 2.0W _SW + 0.067N _SW (Sandy soil)
W _SW : Load of SWS test (kN)
N _SW : SWS test half rotation speed (times / m)
L _i : Layer thickness of each layer (m)

FIG. 11 shows a summary of calculated values and experimental values of the ultimate friction force.
From FIG. 11, it is determined that the calculated value of the ultimate friction force is appropriate in light of the experimental value.

[How to set the allowable bearing capacity of the ground]
The long-term allowable bearing capacity of the ground is set by the following formula from the small-scale building foundation design guidelines (Architectural Institute of Japan). In normal design, the average value up to 2 m from the bottom of the foundation and the lowest value of each measuring point are adopted. In addition, the short-term allowable bearing capacity is set to be twice the long-term allowable vertical bearing capacity.
q _a = 30 W _SW +0.64 N _SW (kN / m ² )
q _a : Long-term allowable bearing capacity of the ground (kN / m ² ) The short-term is double the long-term.
W _SW : Load of SWS test (kN)
N _SW : SWS test half rotation speed (times / m)

[Setting method of ground spring and pile spring]
(Ground spring)
The ground spring when the ground is assumed to be a uniform semi-infinite elastic body is set by the following equation.
k _s = p / S _E (kN / m)
k _s : Ground spring (kN / m)
p: Load acting on the ground surface (kN)
S _E : Ground subsidence (m)
Here subsidence S _E of the ground surface can be calculated by the following equation.
S _E = I _s {(1-ν _s ² ) / E _s } q × B (m)
I _s : Subsidence coefficient (This is a coefficient determined by the shape of the foundation, according to the Building Foundation Structural Design Guidelines (Architectural Institute of Japan).)
ν _s : Poisson's ratio of the ground Set appropriately according to the soil, but when the viscosity is weak or when the soil cannot be clearly determined, ν _s = 0 in subsidence calculation, when calculating the load sharing of the ground Is appropriately set such that ν _s = 0.50.
q: Degree of load acting on the foundation (kN / m ² )
B: Width of base bottom (m)
E _s : Elastic modulus of ground (kN / m ² )

Here, the average value E _s0 within 2 m from the base bottom surface of the elastic modulus E _si of the ground of each layer calculated by the above formula is used.
E _si = 5000 W _SW +85 N _SW (kN / m ² )
W _SW : Load of SWS test (kN)
N _SW : SWS test half rotation speed (times / m)

The load degree q acting on the foundation is a B × L foundation where a part of the foundation corresponding to the housing unit is taken out,
q = p / (B × L) (kN / m ² ) (where the long side of the foundation is L and the short side is B)
Substituting this into the above formula,
S _E = I _s {(1−ν _s ² ) / (E _s × L)} p (m)
k _s = (E _s × L) / {I _s (1−ν _s ² )} (kN / m)
Compared with the experimental results, the validity of the evaluation by the above equation is shown in FIG.

As can be seen from FIG. 12, the ground spring k _s value in the experiment exceeds the calculated value, and the calculation of the subsidence amount (examination of the non-uniform subsidence) can be safely evaluated by the set calculation formula. Moreover, regarding the calculated value and the experimental value, FIG. 13 shows the relationship between the degree of load and the amount of settlement. Even for a short-term load degree in design, this evaluation method can be evaluated sufficiently safely.

(Pile spring)
(Surface friction spring)
The ground friction spring is generally considered to have a constant characteristic after reaching the maximum peripheral friction force _Rf as shown in FIG.
k _pf = R _f / (D / 10) (kN / m)
By multiplying by a constant factor, long-term and short-term springs can be set. Note that D: pile diameter settlement amount D / 10 is regarded as the limit.
FIG. 15 shows a peripheral friction spring corresponding to a long-term and a short-term relative to the peripheral friction spring at the extreme time in the experiment.

In the evaluation of ground subsidence, the multiplier for the extreme time is set to 1 for both long-term and short-term. That is,
k _pfL = 1 × R _f / (D / 10) (kN / m)
k _pfS = 1 × R _f / (D / 10) (kN / m)
FIG. 16 shows the relationship between the friction force and the amount of settlement regarding the calculated value and the experimental value under these conditions.

(Pile tip spring)
As with the ground spring described above, the pile tip spring is based on the following formula, assuming that the shape is the cross-section of the pile.
k _ps = E _sp0 × D × π / {4I _s (1−ν _s ² )} (kN / m)
The settlement coefficient in the above equation is I _s = 1.00.

Poisson's ratio ν _s is set appropriately according to the soil quality, but ν _s = 0 in subsidence calculation and ν _s in ground subsidence calculation when the viscosity is soft or the soil quality cannot be determined clearly. Set appropriately, such as = 0.50 .
Long-term and short-term pile tip springs can also be set with an appropriate coefficient.

Unlike ground friction springs, the behavior of steel pipe pile tip springs is not necessarily bilinear and stable. However, the long-term and short-term equivalent pile tip springs for the steel pipe pile tip springs at the extreme time in the experiment are as shown in FIG.
About the calculated value and experimental value on this condition, FIG. 18 shows the relationship between the axial force of the pile tip and the amount of settlement.

(Pile spring)
The pile spring is basically due to distortion caused by the compressive force applied to the steel pipe. However, it is considered that the compressive force applied to the steel pipe is reduced by the frictional force that becomes a reaction force from the pile head to the deep part, so that the strain is reduced (see FIG. 19a).

However, it is impractical to calculate the strain of the steel pipe for each layer in consideration of the frictional force at each location, so in the end the design may take into account variations in the value of the spring at the head of the pile, Here, it evaluates simply as distortion with respect to the load concerning the whole pile (refer FIG. 19b).
From ε = N / (EA), the pile body spring k _p (kN / m) in the case of the pile length L (m) is calculated by the following equation.
k _p = EA / L (kN / m)
E: Young's modulus of steel pipe material (kN / m ² )
For example, E = 205940 (N / mm ² ) = 20594 × 10 ⁴ (kN / m ² )
A: Cross-sectional area of the pile (m ² )
L: Pile length (m)
In addition, the pile body is evaluated in the same formula as within the elastic range in the long and short term of the ground.

(About variation of pile springs )
Pile spring k _t of the pile head part can be considered pile tip spring k _ps, skin friction spring k _pf, consisting of a "steel pipe pile spring" Synthesis of Kuitai spring k _p and (series).
k _t = {(k _pf + k _ps ) × k _p } / {(k _pf + k _ps ) + k _p }

Here, FIG. 20 shows a summary of calculated values and experimental values of steel pipe pile head springs during long-term loading (1/3 of the ultimate vertical bearing force). In addition, a pile spring can also be set by carrying out a direct loading experiment of a pile body other than setting with the said calculation formula.

  Although the concept up to this point is based on the assumption that the amount of settlement will be evaluated, the safety side is set as a whole. However, when setting the design axial force of the pile from the pile head load degree, a large load is applied to the pile side ( The steel pipe pile spring is multiplied by a factor of 4.0 and applied.

(Composite spring)
From the ground spring k _s and the pile spring k _t calculated above, a spring k _c (kN / m) in the case of a composite ground is calculated. Basically, the composite spring (composite ground spring) is the sum of each spring,
k _c = k _s + k _t
Where k _s = {E _s (L + t)} / {I _s (1−ν _s ² )} (kN / m), k _t = {(k _pf + k _ps ) × k _p } / {(k _pf + k _ps ) + k _p } (kN / m), but if the influence of the ground spring is reduced due to the fact that the load point of the superstructure is close to the pile head, the rigidity of the foundation, and variations including the pile body Conceivable. Here, we consider a method for evaluating the amount of settlement uniquely from the experimental results on the safe side.

From FIG. 21, which compares the experimental value and the calculated value of the composite ground spring, when evaluating on the safe side uniquely, for example, the ground spring can be set at 80% as in the following equation.
k _c = 0.8 × k _s + k _t
In the design, the composite spring can be set by appropriately modeling the ground spring, the pile spring, and each part of the foundation.

(Load sharing ratio of small diameter piles)
The load sharing rate of the pile is set by the ratio of the ground spring and the steel pipe pile spring.

(Pile load sharing ratio in pile design)
The load sharing ratio of the pile can be set by experiment, for example, at a magnification of 25 in the pile spring .
Pile load sharing rate = (25 × k _t ) / (k _s + 25 × k _t )
In FIG. 22, the result of the calculated value regarding the load share of a pile and the experimental value is shown.

  In addition, since it is thought that there is no difference in the load sharing rate of a pile in a long term and a short term from an experimental result, it is thought that load sharing can be prescribed | regulated with the same type | formula.

In the design, the ground spring, the pile spring, and each part of the foundation can be appropriately modeled. Here, the load on the pile head basically depends on the ratio of the spring between the pile and the ground, but it is necessary to reduce the load on the foundation due to the influence of the foundation and the ground. FIG. 23 shows a basic analysis model.

  As shown in the figure, it is possible to obtain the load sharing with high accuracy by changing the ground conditions in advance for each unit size and application location.

Furthermore, as shown in Figure 24, depending on a degree to which the spring pile against the roots if ground under example basic, it is also possible to define a ratio to reduce the spring. In addition, Table 1 below shows an example of a comparison result of experimental values and analysis values of the spring constant of the single pile and the ground (only the foundation) and the pile head sharing ratio by the present inventors.

From the above, it is possible to appropriately evaluate the pile head load degree based on the experimental result or by increasing the pile spring by analysis.

(Load sharing ratio of ground in the examination of ground pressure)
Corresponding to the load sharing rate of the pile, the pile spring is set on the minimum side as the load sharing rate of the ground.
Load sharing rate of the ground _{= 1-k t / (k} s + k t)
In FIG. 25, the calculated value and the experimental value regarding the load sharing ratio of the ground are shown.

  From the above, it is possible to appropriately evaluate the degree of load on the ground based on the experimental results. In the design, it is possible to appropriately model each part of the ground spring, the pile spring, and the foundation to calculate the load degree of the ground.

  The embodiment of the present invention has been described in detail with reference to the drawings. However, the specific configuration is not limited to this embodiment, and there are design changes and the like without departing from the gist of the present invention. They are also included in the present invention.

  DESCRIPTION OF SYMBOLS 1 ... Column, 2 ... Ceiling beam, 3 ... Solid foundation, 4,4A, 4B, 4C ... Small-diameter pile, 10 ... Unit, 20 ... Unit type building, 30 ... Composite ground, G ... Original ground

Claims (12)

A method for designing a composite ground consisting of an original ground for supporting a detached house and a small-diameter pile installed in the original ground,
The first is to set the allowable bearing capacity of the original ground after the small-diameter pile is installed, the allowable bearing capacity of the small-diameter pile, and the allowable subsidence amount of the original ground after the small-diameter pile is installed. Step,
A ground spring of the original ground and a pile spring of a small-diameter pile are set, and a load sharing ratio of both the original ground and the small-diameter pile is set according to the size of both the ground spring and the pile spring. Step,
The weight of the detached house is apportioned for each of the plurality of divided areas. From the apportioned weight and the load sharing rate, the weight of the original ground and the weight of the small-diameter pile are calculated, and A third step of calculating the amount of settlement,
The burden weight of each of the original ground and the small-diameter pile is compared with each of the permissible supporting forces, the burden weight is equal to or less than the permissible supporting force, and the settlement amount of the original ground and the permissible settlement Comparing the amount, and confirming both that the amount of subsidence of the original ground is less than or equal to the allowable amount of subsidence,
In the second step, the composite ground design method for obtaining the load sharing ratio from the result of analyzing the ground conditions in advance for each size and application location of a detached house using a basic analysis model . The composite ground design method according to claim 1, wherein a reduction ratio of the ground spring under the foundation is obtained from a spring ratio of the ground spring under the foundation and the pile. A method for designing a composite ground consisting of an original ground for supporting a detached house and a small-diameter pile installed in the original ground,
The first is to set the allowable bearing capacity of the original ground after the small-diameter pile is installed, the allowable bearing capacity of the small-diameter pile, and the allowable subsidence amount of the original ground after the small-diameter pile is installed. Step,
A ground spring of the original ground and a pile spring of a small-diameter pile are set, and a load sharing ratio of both the original ground and the small-diameter pile is set according to the size of both the ground spring and the pile spring. Step,
The weight of the detached house is apportioned for each of the plurality of divided areas. From the apportioned weight and the load sharing rate, the weight of the original ground and the weight of the small-diameter pile are calculated, and A third step of calculating the amount of settlement,
The burden weight of each of the original ground and the small-diameter pile is compared with each of the permissible supporting forces, the burden weight is equal to or less than the permissible supporting force, and the settlement amount of the original ground and the permissible settlement Comparing the amount, and confirming both that the amount of subsidence of the original ground is less than or equal to the allowable amount of subsidence,
The subsidence amount of the original ground is calculated using an elastic coefficient obtained from the load of the SWS test and the half rotation number of the SWS test,
In the first step, when setting the allowable bearing capacity of the small-diameter pile and the allowable subsidence amount of the original ground after the small-diameter pile is installed, The ultimate tip bearing force R at the tip of the small diameter pile according to its hardness _p A composite ground design method that changes the calculation formula. When the original ground is hard viscous soil (No> 15), the extreme tip support force R _p The
R _p = Q _p × A _p = Α x No x A _p (KN)
q _p : Ultimate tip bearing strength (kN / m ² )
A _p : Cross-sectional area of steel pipe pile (m ² )
α: Tip bearing force coefficient, evaluated by α = 200 (however, N = 20 is the upper limit) in this method
No: The average value of the converted N value in the range of 1D down from the tip of the steel pipe pile and 5D up (D: diameter of the steel pipe pile). _SW , N _SW Than,
N = 3.0W _SW + 0.050N _SW (Cohesive soil)
N = 2.0W _SW + 0.067N _SW (Sandy soil, when intervening in the calculation range)
The composite ground design method according to claim 3, which is calculated by: When the original ground is other viscous soil, the ultimate tip bearing force R _p The
R _p = 6 × c × A _p (KN)
c: The average value of the converted adhesive force in the range of 1D down from the tip of the steel pipe pile and 5D up (D: steel pipe pile diameter). The converted N value of each layer is the W obtained by the SWS test. _SW , N _SW Than,
c = 1/2 (45 W _SW + 0.75N _SW ) (KN / m ² )
The composite ground design method according to claim 3, which is calculated by: The composite ground design method according to any one of claims 1 to 5 , wherein in the second step, a composite spring calculated from the sum of the ground spring and the pile spring is further set. The said 2nd step WHEREIN: The composite spring calculated from the sum of the ground spring after correction | amendment and the pile spring after multiplying the predetermined coefficient less than 1 to the already set ground spring is further set. The design method of the composite ground in any one of 1-5 . The pile spring includes a circumferential friction spring between the pile and the ground surface of the pile, a pile tip spring of the pile tip ground, and a pile body spring based on elastic deformation of the pile body when a compressive force is applied. The composite ground design method according to any one of claims 1 to 7 , which is set. When calculating the load distribution rate of the small-diameter piles, using the pile spring after correction multiplied by a large predetermined coefficient than 1 already on the pile spring being set, claim 1-8 The composite ground design method described in 1. Of the circumferential friction springs of the pile spring, the circumferential friction spring is a predetermined value less than 1 at a value defined by the maximum circumferential frictional force when the small diameter pile sinks by a predetermined amount at the time of long-term design. The composite ground design according to claim 8 or 9 , wherein the peripheral friction spring at the short-term design is set to be twice the peripheral friction spring at the long-term design. Method. Among the peripheral friction springs of the pile spring, the peripheral friction spring at the long-term design for calculating the subsidence amount of the original ground is defined by the maximum peripheral friction force when the small-diameter pile sinks by a predetermined amount. The circumferential friction spring at the short-term design for calculating the subsidence amount of the original ground is 1 / of the circumferential friction spring at the long-term design. The composite ground design method according to claim 8 or 9 , wherein the composite ground is set to double. The composite ground design method according to any one of claims 1 to 11 , wherein the detached house is a unit type building, and the apportioned weight for each divided area and each divided area is set in advance.

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