JP2003082648A - Bearing capacity calculation method of soil cement composite pile - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a new formula capable of accurately calculating bearing capacity of a soil cement composite pile by simple calculation. SOLUTION: When bearing capacity of the soil cement composite pile obtained by twistedly penetrating the existing pile having spiral wings into a soil cement columnar body is calculated, the calculation of friction stress τacting on the circumferential surface of the composite pile is made with one formula τ=α+βN (α, β represents a fixed number obtained by correlation of a diameter of the existing pile body, a diameter of the spiral wing and a diameter of the soil cement columnar body) for using N value through a standard penetration test without depending on sorts of the ground.

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for calculating a bearing capacity of a soil cement composite pile in which a ready-made pile having a spiral blade is inserted into a soil cement column. About. 2. Description of the Related Art The vertical supporting force P of a pile installed in the ground can be obtained as the sum of the tip supporting force Po of the pile and the peripheral frictional force Pf of the pile P = Po + Pf. Among them, the peripheral frictional force Pf of the pile acting on the boundary surface between the pile and the ground is obtained as Pf = τ · Af.
τ is the friction stress at the interface between the pile and the ground, and Af is the area of the interface. In the conventional pile method, the value of τ differs depending on the type of the ground. For sandy ground, τ = 10N / 3, for viscous ground, τ = Qu / 2 (where N is the standard penetration test of sandy ground. The value and Qu were determined using empirical formulas such as the uniaxial compressive strength of cohesive soil. [0003] For this reason, piles are generally installed through a plurality of strata, so that it is necessary to divide each stratum into sandy soil and cohesive soil, determine the peripheral frictional force for each stratum, and perform a complicated calculation for summing them. there were. When the bearing capacity of the soil cement composite pile by this method was evaluated using these conventional formulas, it became 1/5 to 1/10 of the actual test result, and it was found that the bearing capacity calculation formula was not applicable at all. SUMMARY OF THE INVENTION An object of the present invention is to provide a new formula capable of accurately calculating the bearing capacity of a soil cement composite pile by a simple calculation. SUMMARY OF THE INVENTION The present inventor has focused on the fact that a pile supporting mechanism of a ready-made pile having a soil cement column and a spiral blade is different from that of a conventional pile construction method. Reached. That is, the present invention is as follows. In calculating the bearing capacity of a soil cement composite pile obtained by screwing a ready-made pile with spiral blades into the soil cement column, the calculation of friction stress τ acting on the peripheral surface of the composite pile is independent of the type of ground, It is characterized by a formula τ = α + βN (α and β are constants obtained by the correlation between the diameter of the ready-made pile body, the diameter of the spiral blade, and the diameter of the soil cement column) using the N value obtained by the standard penetration test. Calculation method for bearing capacity of soil cement composite pile. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The supporting force calculation method according to the present invention will be described below using examples. FIG. 2 is a diagram showing a general outline of the soil cement composite pile method. In FIG. 2, this composite pile is installed through four types of ground. Type of soil are alternating layers of sandy soil and cohesive soil from the top, respectively N ₁ from the mean is the surface portion of each layer of the N-value by SPT indicating the strength of the ground, N _2, N _3, N ₄ Further, the layer thickness is L ₄ from L _1. According to the conventional bearing capacity calculation formula, L ₂ layer and L ₄ layer is not used N values in the calculation of skin friction Pf of piles for a cohesive soil layer, cohesive soil in place of N value It was necessary to obtain and calculate the uniaxial compressive strength Qu by some method. The peripheral friction force Pc of the sandy soil part obtained from the N value and the peripheral friction force Ps of the sandy soil part obtained from the N value are added to obtain the peripheral friction force Pf of the pile.
Has to be obtained by Pf = Pc + Ps. Specifically, in the sandy soil portion of the ground, the peripheral frictional force Ps of the sandy soil is represented by Ps = τ · Af
_{= (10 · N 1/3} · L 1 +10 · N 3/3 · L 3) ψ, (Af wherein
Is the peripheral surface area of the pile, and 杭 is the peripheral length of the pile.
The unconfined compressive strength Qu value of the ground is obtained by any method without using the value, and the peripheral friction force Pc of the cohesive soil portion is calculated as Pc = τ · Af = (Qu _{2
/ 2 · L 2 + Qu 4} /2 · L 4) determining the [psi. Thereafter, the peripheral frictional force of the pile, Pf = Pc + Ps, was calculated as the sum of the two. Therefore, conventionally, it has been necessary to use two formulas, and it has been very complicated to change the N value to the Qu value in the cohesive soil. The bearing capacity calculation method of the present invention does not need to consider the type of the ground such as sandy soil and cohesive soil, and can be calculated only by the N value and the layer thickness L of the ground. That is, the formula for calculating the frictional force on the peripheral surface of a pile according to the present construction method can be easily obtained by one calculation formula represented by Pf = τ · Af = (α + βN) · Af. Here, N is the average N value of all layers, so that N = (N ₁ · L ₁ +
N ₂ · L ₂ + N ₃ · L ₃ + N ₄ · L ₄ ) / L (L is the sum of L ₁ to L ₄ )
You can ask for it. α and β are constants that are experimentally determined if the correlation between the outer diameters of the pile (the diameter of the ready-made pile main body, the diameter of the spiral blade, and the diameter of the soil cement column) is determined. Conventionally, the empirical value of the ground friction stress τ obtained from the N value and the Qu value has been obtained by performing a vertical loading test. In the present invention, a load test may be performed in the same manner as in the prior art. For example, when the spiral blade diameter D is set to 1.5 times the outer diameter d of the pile body and the diameter of the soil cement column is set to a constant value such as 3 times, a vertical loading test is carried out at several sites with different N values. The stress τ can be obtained as an empirical value using α and β as constants. A specific method for obtaining α and β will be described in the following examples. [0010] Table 1 shows an example of the results of a vertical loading test at six sites. [Table 1] The pile uses a steel pipe pile, and the pile outer diameter d is 190.
7mm to 216.3mm, the length L of the pile is 4.3m to 22m, and the spiral blades having the outer diameter D of about twice the outer diameter d of the pile body are fixed at equal pitches of about 2m. . The outer diameter Dc of the soil cement column is 200 mm larger than the outer diameter D of the blade.
600 mm and 650 mm. Therefore, the correlation between the pile outer diameters in the present embodiment is that D / d is about 2, and Dc /
D is about 1.5. As shown in Table 1, the average N value of each area where the pile was installed was in the range of 2.3 to 8.4. The perimeter area of the pile was calculated in two types. One is the area surrounded by the outer diameter D of the spiral blade as shown by the broken line in FIG. 2, and Aw = π · DL. The other is the area surrounded by the outer diameter Dc of the soil cement column. Ac = π ・
Dc · L. Aw and Ac of each pile are as shown in Table 1. Friction stress τ acting on the pile is obtained by dividing the peripheral frictional force Pf of the pile obtained from the load test result by the surface area of the pile. Friction stress τw acting on the part surrounded by the spiral blade
Is τw = Pf / Aw, and the frictional stress τc acting on the outer peripheral portion of the soil cement is obtained by τc = Pf / Ac. FIGS. 3 and 4 show the relationship between τw and τc and the average N value.
It is. As is clear from all the figures, τ is a straight line proportional to the N value when approximated by the least squares method, and it can be found that τ can be obtained by almost one straight line regardless of the type of ground or the pile length. As is apparent from this embodiment, when the frictional stress acting on the outer diameter portion of the spiral blade shown in FIG. 3 is τw, it can be obtained by a linear equation of τc = α + βN = 50 + 10N. On the other hand, when the friction stress τc acting on the outer peripheral portion of the soil cement shown in FIG. 4 is obtained, τc = 30 + 8N is obtained. Although the values of α and β differ depending on where the friction stress τ is calculated on the soil cement composite pile,
In any case, it can be obtained by a simple equation of τ = α + βN. A comparison between the test results shown in Table 1 and the frictional force obtained by the conventional formula for calculating the frictional force revealed that they were quite different. In the conventional formula for calculating sandy soil using the N value, τ = 10/3 · N.
= 16.7, and N = 10, τ = 33.3. On the other hand, in the proposed formula, the frictional stress at the portion of the spiral blade diameter is τ = 50 + 10N as shown in FIG. 3, so when the N value is 5, τ = 100,
When the N value is 10, τ = 150, which is about five times different from the conventional type. The conventional formula is τ calculated from the outer diameter of the ready-made pile main body, and the proposed formula is τ calculated at the outer diameter portion of the spiral blade. The ratio of the outer diameters of the spiral blades is about 2, and the value of τ in the proposed formula is twice the above, and τ = 200 and 300, respectively. This value is about 10 times the value obtained from the conventional formula, and it is understood that the conventional formula cannot be applied to the soil cement composite pile at all. Conventionally, the frictional force acting on the peripheral surface of the pile differs depending on the type of the ground, and it has been necessary to calculate the ground separately for two types of clay soil and sandy soil. The inventor of the present invention has found a calculation formula that does not need to be divided into two types and can be obtained by a simple calculation formula and that can easily calculate an actual friction stress that is about 10 times the value obtained by the conventional formula. . The reason that the conventional method cannot be applied is that, in many cases, the pile and the ground are in contact with each other in the conventional pile method, whereas the soil cement composite pile has a relatively large soil cement column around the ready-made pile body. Covering. As a result, ready-made piles and ground are not in direct contact. Further, the ground is agitated at the time of forming the soil cement, and it is expected that a hard ground having a large N value and a viscous ground having a small N value are mixed to form a soil cement column having an average strength. As a result, it is expected that the frictional force acting on the peripheral surface of the ready-made pile does not reflect the strength of the ground sensitively and that the entire ground functions in an averaged state. Next, a preferred embodiment of the soil cement composite pile construction method of the present invention will be described with reference to the drawings. FIG. 1 is a sectional view of the ground showing an outline of a construction process of a construction method according to the present embodiment. In this embodiment, an example is shown in which a steel pipe pile is used as a ready-made pile, but a concrete pile can be used instead of the steel pipe pile. In the case of a concrete pile, it is preferable to attach a spiral blade via a metal band or the like. First, as shown in FIG. 1A, a soil cement column forming apparatus 5 is installed at a target position on the ground 1, and a soil cement column 3 is formed by a mechanical deep mixing method. Is preferred. Here, the mechanical deep mixing method is a stirring and mixing device equipped with an excavating blade and a stirring blade while pouring a slurry prepared by kneading cement or a solidifying material containing cement as a main component and water into the ground. Means a soil improvement method for mechanically stirring and mixing the ground and slurry to form a soil cement column. The method for forming the soil cement column may be not only the mechanical deep mixing method, but also a conventional pre-boring method using a ready-made pile. The pre-boring method is to excavate a pile pit using an auger before laying a ready-made pile, and inject a solidifying material containing water or cement as a main component to form a soil cement column of a pit. In penetrating the column forming apparatus 5 into the ground, it is preferable to provide a non-forming layer 6 of a predetermined length which does not form the soil cement column 3 on the surface of the ground. In this case, neither cement milk nor water is used. When the column forming device 5 reaches the depth of the non-formed layer 6 or more, the formation of the soil cement column 2 is started.
As shown in FIG. 3B, when the column building apparatus 5 is pulled up to complete the soil cement column 3, it is preferable to stop the rotation and the cement milk injection and pull out the non-formation layer 6 near the ground surface. As shown in FIG. 1C, a plurality of spiral blades 4 are provided at predetermined positions, and a steel pipe pile with a spiral blade (hereinafter, simply referred to as a "steel pipe pile") 2 is rotated while the uncured soil is rotated. The cement column 3 is screwed and penetrated. Steel pipe pile 2
Is penetrated into the soil cement column 3 to integrate the two into a soil cement composite steel pipe pile as shown in FIG. The spiral blade 4 provided around the steel pipe pile 2 is preferably formed such that the root portion welded to the steel pipe pile is thicker and thinner toward the outer periphery. The spiral blade is obtained, for example, by bending an in-plane steel plate with equal thickness in a plane by rolling. The inner peripheral side is formed thicker than the strip steel sheet thickness, and the outer peripheral side is formed thinner. It is also possible to use an unequal thickness strip from the beginning and roll it and bend it to the inner surface to further increase the inner diameter, and further reduce the outer circumference to increase the difference in unequal thickness. In terms of strength design, when the thickness on the outer peripheral side is 1, the thickness on the inner peripheral side is preferably 1.2 to 3.0, and more preferably 1.2 to 1.5. Since the design strength is determined by the root thickness of the blades, the amount of steel material can be relatively reduced as compared with the case of using blades of equal thickness. Further, it is possible to prevent the occurrence of a cutting loss that occurs when the spiral blade is cut from a flat steel plate. The steel pipe pile 2 preferably has spiral blades near the pile head and near the lower end, and at least one spiral blade between them. Therefore, it is preferable that the steel pipe pile 2 is provided with at least three spiral blades 4. The diameters of the spiral blades may be the same or different. Here, the vicinity of the pile head refers to a footing to which the steel pipe pile 2 is connected, or an area within a range equal to the diameter Dc of the soil cement column 3 from the bottom surface of the cloth foundation. In addition, the vicinity of the lower end of the pile refers to a range equal to the diameter Dc of the soil cement column 3 from the lower end of the pile. Generally, the largest vertical load acts on the foundation pile of civil engineering and building structures at the pile head. Therefore, when the steel pipe pile 2 has the spiral blades 4 near the pile head, the soil cement column 3 below the spiral blades 4 is compressed by the pressing effect of the spiral blades 4, and the vertical load is increased. It is possible to resist not only the steel pipe pile 2 but also the soil cement column 3. Further, even with respect to a horizontal load, the horizontal rigidity can be increased by the pressing effect of the spiral blade 4 and the restraining effect of the soil cement column 3. In the soil cement composite steel pipe pile according to the present embodiment, spiral blades 4 are provided near the pile head and the lower end of the steel pipe pile 2 and at least one spiral is provided between the two. Since the blades 4 are provided, the load is dispersed from the spiral blades 4, and the entire vertical supporting load can be increased. In addition, the installation interval of the spiral blade 4 between the spiral blades 4 provided near the pile head and the lower end of the steel pipe pile 2 is not particularly limited.
It is preferable to appropriately set the length to about 1 m to 3 m. In this embodiment, two spiral blades 4 are provided near the lower end of the steel pipe pile 2 and near the pile head, and a total of four spiral blades 4 are provided. I have. In the present embodiment, the presence of the non-formation layer 6 serves as a lid, so that the excavated soil is hardly discharged to the ground during excavation. In addition, since piles are also buried by rotation, they penetrate at the pitch of the blades without disturbing the ground, so that the non-formation layer functions sufficiently as a lid, and the piles hardly discharge earth and sand to the ground. The thickness of the non-formation layer 6 is preferably at least half the outer diameter of the soil cement column, more preferably at least the outer diameter. In some cases, the steel pipe pile 2 may be used alone, but is not necessarily limited to the use of a single piece, and a plurality of steel pipe piles connected by means of welding or screwing in the longitudinal direction are used. Is also good. In the steel pipe pile 2 configured as described above, if the lower end is closed by the bottom plate, as the steel pipe pile 2 penetrates into the soil cement column 3, the soil cement of the volume penetrated is pressurized, and It is possible to increase the strength. It is also possible to fill the hollow steel pipe pile 2 with high quality cement milk, mortar and concrete. When the filler is filled inside the steel pipe pile 2, the effective cutting of the steel pipe pile 2 can be achieved. It is possible to increase the area and prevent deformation of the cross-sectional shape. As a result, the vertical load and the horizontal load that the soil cement composite pile can bear can be further increased. In order to effectively integrate the soil cement column 3 with the steel pipe pile 2 and to disperse the vertical load, as shown in FIG. The diameter of the spiral blade 4 having a diameter is D
And when the diameter of the soil cement column 3 is Dc,
The diameter D of the spiral blade 4 having the largest diameter is preferably at least 1.5 times the diameter d of the steel pipe pile 2 from the viewpoint of load distribution and the effect of integrating the steel pipe pile and soil cement, and the bending stress generated in the spiral blade 4 The range of 3.0 times or less is preferable from the viewpoint of suppressing the above. In order to effectively integrate the soil cement column 3 with the steel pipe pile 2 and to disperse the vertical load, the diameter Dc of the soil cement column 3 is set to the diameter D of the spiral blade 4 having the maximum diameter. It is preferable to be in the range of 1.0 to 2.0 times,
It is more preferable to set the range of 1.2 times to 1.5 times. The penetration speed of the steel pipe pile 2 into the soil cement column 3 is Vp (m /
), The rotation speed at the time of screwing the steel pipe pile 2 is Rp (times /
Minute), when the spiral pitch of the spiral blades 4 is tp (m), a method of penetrating the steel pipe pile 2 into the soil cement column 3 is as follows.
Method to penetrate only the pitch or less than the pitch,
That is, the penetration speed V of the steel pipe pile 2 into the soil cement column 2
It is desirable that p be Vp ≦ Rp · tp. By setting the penetration speed of the steel pipe pile 2 as described above, the integrity of the steel pipe pile 2 and the soil cement column 3 can be effectively secured. Further, in the present invention, it is not necessary that the head of the soil cement column and the head of the steel pipe pile coincide with each other, and the head of the steel pipe pile may jump out of the soil cement column and protrude to near the ground surface. Although the embodiment has been described above using the steel pipe pile, the same can be said for a concrete pile. According to the present invention, it is possible to simply and accurately calculate the bearing capacity of a soil cement composite pile to which the conventional calculation method cannot be applied. In the calculation method of the present invention, it is not necessary to consider the types of the ground such as sandy soil and clayey soil, and the calculation can be performed only by the N value and the layer thickness of the ground.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a sectional view of a ground showing an outline of a construction process of a soil cement steel pipe pile according to an embodiment. FIG. 2 is a cross-sectional view of a case where the present invention is applied to an alternate layer of clayey soil and sandy soil in the embodiment. FIG. 3 shows the frictional force τ acting on the pile peripheral surface obtained from a plurality of test results, which is shown in relation to the average N value of the entire ground.
FIG. 14 is a -N correlation diagram. In FIG. 3, the value of τ is calculated as the frictional force τw acting on the surface surrounded by the spiral blade outer diameter Dw shown in FIG. FIG. 4 is a τ-N correlation diagram similar to FIG. 3; In FIG. 4, calculation is made as a frictional force τc acting on a surface surrounded by the outer diameter Dc of the soil cement column shown in FIG. [Description of Signs] D Diameter of spiral blade 4 having maximum diameter d Diameter of steel pipe pile body Dc Diameter of soil cement column 1 Ground 2 Steel pipe pile 3 Soil cement column 4 Spiral blade 5 Soil cement column forming device 6 Non-stratified layer

Claims (1)

Claims: 1. Friction stress acting on the peripheral surface of a composite pile in calculating the bearing force of a composite pile made of soil cement obtained by screwing a prefabricated pile with spiral blades into a soil cement column. τ is calculated by using a standard penetration test N value regardless of the type of ground. A method for calculating the bearing capacity of a soil-cement composite pile, characterized in that it is calculated using a constant.