AN EXTENDED HEAD PILE WITH INSIDE AND OUTSIDE REINFORCEMENT

Technical Field

[1] The present invention relates to a head-extended pile, and more particularly to a head-extended pile having an improved bearing capacity against load of a structure. Also, the present invention relates to a head-extended pile with inside and outside reinforcements for providing a pile construction method to ensure stability, construction workability, and economic efficiency in constructing the pile.

[2]

Background Art

[3] When a building or a structure is built, construction of a foundation for reinforcing the ground is generally carried out according to conditions of the ground or the load of the building or the structure so that the building or the structure can be stable on the ground. Construction of a shallow foundation or a deep foundation is carried out on various conditions, such as load of a structure. The shallow foundation is a foundation having a penetration width ratio below 1 while the deep foundation is a foundation having a penetration width ratio above 1. In the case of the shallow foundation, a structure is directly supported on the ground without using piles. In the case that a structure is not sufficiently supported on the ground, on the other hand, piles are used to reinforce a bearing capacity of the ground.

[4] A pile foundation system is a foundation system characterized in that heads of the constructed piles are connected to a structure. Piles may be classified into steel piles, concrete piles, and composite piles based on their material. Installation of the pile may be classified into pile driving, auger drilled piling, and in-place casting.

[5] The pile driving is a method characterized in that a pile is erected, and is then forcibly driven into the ground by hammering until the pile is fully penetrated into the ground. When the pile is forcibly penetrated into the ground by means of hammering energy, it is penetrated while the pile pushes through the soil around the pile. Consequently, a bearing capacity of the pile is excellent, and the pile is simply constructed.

[6] However, the pile driving method has drawbacks in that it is difficult to penetrate the pile vertically when the pile needs to be penetrated deeply into the ground and also excessive vibration and noise are generated when the pile driving method is carried out. Consequently, the pile driving method is limited to use in urban areas due to certain restrictions.

[7] On the other hand, the auger drilled piling is a method characterized in that a hole is

previously bored in the ground, a cement paste is poured halfway into the hole, and then a pile is fixedly inserted into the hole. The auger drilled piling method can solve the drawbacks of the pile driving method. Currently, the pile burying method is mainly used to construct pile foundations in urban areas.

[8] The construction of the foundation is an extremely important in building construction. Various pile installation methods are used without consistency on the basis of the personal experiences of constructors because conditions of the ground vary with the sites, or operation of a pile-driving machine is not fully understood. As a result, the pile constructing work is not easily carried out.

[9] FIG. 1 is a conceptual drawing showing the relation between an inherent proof stress of a pile and a constructional proof stress of the constructed pile. As shown in FIG. 1, load (PF) of a structure 11 is supported by means of constructional proof stress of a plurality of piles 12 penetrated into the ground below the structure 11. The constructional proof stress of the constructed pile 12 is the sum of a front end bearing force (TF) at the front end of the pile and a surrounding frictional force (SF) around the outer circumference of the pile. Generally, the inherent proof stress of the pile is larger than constructional proof stress. However, the constructional proof stress of the pile is decreased due to its bad construction workability.

[10] For example, a Φ400 pretensioned spun high-strength concrete pile (hereinafter, referred to as "PHC pile") has an inherent proof stress of 112 tf and a constructional proof stress of 60 to 80 tf. As a result, 32 to 52tf of the inherent proof stress of the pile is wasted. Especially when an auger drilled-piling method is used, a drilled hole having a diameter larger than that of a pile, the pile is penetrated into the hole, and cement paste is poured between the pile and the ground to increase a surrounding frictional force. However, a proof stress test of the pile after the construction is completed shows that the surrounding frictional force is insignificant and most of the constructional proof stress is the front end bearing force. Consequently, it is necessary to increase the constructional proof stress near to the inherent proof stress of the pile in the auger drilled piling method to improve the efficiency in use of the pile.

[11] To improve the efficiency of the pile, there are various proposals as follows:

[12] Japanese utility model laid-open publication No. S55-50142 discloses a front end supporting steel pipe pile attaching a front-end steel plate having a through hole to the front end of a steel pipe, and adding a reinforcement rib between the steel pipe and the front end steel plate.

[13] The Japanese publication discloses the technical scope inserting the pile on exposed ground or into excavated hole, and situating and burying the pile in the ground. However, it has some problems that proof stress of the pile is easily wasted because a surrounding frictional force is not present by method of filling soil without cement in

the pile, and the work of reinforcement (especially, the reinforcement of the central) is very difficult. As a result, it is necessary to pay excessive cost in execution of construction.

[14] Also, Japanese patent laid-open publication H2003-138561 discloses a pile with multi-stage improving a continuous underground wall. This publication discloses the method of a concrete placement using a continuous excavator in the longitudinal direction in place. However, it also has some problems in construction cost and workability. Furthermore, it is not used in actual because it is necessary to simultaneously use a truck mixer for curing the concreted surface and a iron rod.

[15]

Disclosure of Invention Technical Problem

[16] As mentioned above, the present invention has been made to solve the above problems, and it is an object of the present invention to provide a head-extended pile with inside and outside reinforcements for supporting load of a structure so that an efficiency in use and an economic efficiency of the pile are improved by increasing constructional proof stress of a pile without affecting the weight and the volume of the pile used in an auger drilled piling method, the head-extended pile is adapted to a pile construction method to ensure stability, construction workability, and economic efficiency in constructing pile.

[17] It is an anther object of the present invention to provide a head-extended pile with inside and outside enforcement part that can be applied to the auger drilled piling method and increase a constructional proof stress by hammering after a drilled piling.

[18] The present invention provides a head-extended pile with inside and outside reinforcement parts that make a head having a diameter larger than that of the pile. The head extends right and left with respect to a central axis at the front end of the pile to increase the front end supporting stress of the pile.

[19] Especially, the present invention enables the design and the production of a pile to achieve easily by a head-extended pile extending to the same length with respect to a central axis, for a general pile with a diameter of Φ300 to Φ500, and enables to obtain a more accurate value of the proof stress of the pile without an error by having the area in which the sum of a supporting force with respect to a central axis is same, for a large pile having a diameter over Φ500.

[20]

Technical Solution

[21] To accomplish the objects of the present invention, there is provided a head- extended pile for supporting load of a structure, comprising: a first head portion having

the same length extending right and left with respect to a central axis of a first supporting wall; and a second head portion having the same length extending right and left with respect to a central axis of a second supporting wall, wherein a circular structure is formed symmetrically by the first and the second supporting walls, also and the first and the second head portions; a hammering surface being provided on the top surface of the circular structure; and a central borehole being formed between the first head part and the second head part.

[22] In the head-extended pile, the first and the second head portions have a uniform thickness respectively, and each of the first and second head portions may include inclined surfaces formed at both sides thereof.

[23] In the head-extended pile, the uniform thickness set at the first and the second head portions is achieved by sequentially laminating a plurality of head portions.

[24] The desired effect is also achieved by having the inner and outer area so that the sum of the inside and the outside supporting force of the first and the said second head portions is same.

[25]

Advantageous Effects

[26] According to the present invention as mentioned above, the constructional proof stress of a pile can be increased to near the inherent proof stress of the pile without increasing the weight or the volume of the pile used in an auger drilled piling method. Consequently, the quantity of the piles to be used in the work of burying the pile is reduced, therefore efficiency in use of the pile and economic efficiency of the pile are improved. Also, the reinforcement part or the reinforcement disk can be attached to a conventional pile in market. Consequently, applicability of the conventional pile is also improved.

[27] A designed proof stress of the pile is generally decided by pile manufacturers in such a manner that the designed proof stress corresponds to the proof stress of the pile decided on the basis of the ground conditions or the construction workability, which is lower than the inherent proof stress of the pile. As a result, the designed proof stress of the pile is suffered a loss of approximately 30 to 40 % compared to inherent proof stress. However, 100 % of the proof stress can be utilized in this construction method when the head-extended pile according to the present invention method is used.

[28] In view of constructing the pile, it is possible to improve the proof stress of the pile in early stage thereby progressing construction in safe and prompt. In view of designing the pile, it is possible to decrease the quantity of the piles thereby leading to saving the cost and time of construction.

[29] It is expected that the length of the pile can be decreased when the pile is designed

by little proof stress.

[30] When the head-extended pile is prepared and driven in the same manner as the conventional pile constructing method and the pile construction is completed at an N value of approximately 50/20 ~ 50/10, a required supporting force is obtained as compared to a pile construction designed in the field. [31] When the designed proof stress of the pile is low, the length of the pile can be reduced by approximately 20 to 40 %. Consequently, cost of construction is reduced. [32] An friendly environmental construction can be achieved because of decreasing the amount of the head-arrangement for connecting a pile with a pile foundation. [33] Consequently, the pile constructing method according to the present invention can guarantee the economic efficiency and safety, and improve construction workability as compared to the conventional pile constructing method. [34] In the case that the designed proof stress of the pile is increased by the use of the head-extended pile according to the present invention, the total number of the piles may be reduced approximately 30 to 40 %. [35] Also, because the size of pile cap is decreased, and thus the amount of concrete cement and required reinforcing steel is reduced approximately 10 to 20 %.

Furthermore, construction time is considerably reduced as the total number of the piles is decreased. Also construction material and labor cost, indirect cost, personnel expenses, and financial expenses related to construction duration are considerably reduced. Therefore, the present invention is very useful method for building and civil construction. [36]

Brief Description of the Drawings [37] The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: [38] FIG. 1 is a conceptional view of a pile's proof stress showing the relation between an inherent proof stress of a pile and a constructional proof stress of the constructed pile; [39] FIG. 2 is a sectional view showing a change of moment varying as the position of a force applied on the axis when a structure is loaded; [40] FIG. 3 is a plane view and partial sectional view showing a head-extended pile with inside and outside reinforcement parts according to the present invention; [41] FIG. 4 is a partial sectional view showing a difference between when hammering a head-extended pile with reinforcement parts as an embodiment of the present invention and a case of a conventional extended pile;

[42] FIG. 5 is a partial sectional view showing a case of hammering a head-extended pile with inside and outside reinforcement parts related to another embodiment of the present invention; and

[43] FIG. 6 is a partial sectional view of a head-extended pile with inside and outside reinforcement part in laminated forms as another embodiment of the present invention.

[44]

Best Mode for Carrying Out the Invention

[45] Now, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

[46] FIG. 2 is a sectional view showing a change of moment when a structure is loaded.

[47] As shown in FIG. 2(a), a structure is applied a load from a structure itself to the center of the structure in the direction as indicated in an arrow.

[48] However, a moment of the force to turn is separately generated in addition to a load of a structure when a separate force 9 is applied at the side of the structure as shown in

[49] Therefore, when a separate force 9 is applied in a state of applying of a structure s load as shown in FIG. 2(c), the first moment of the force to turn is generated and a displacement is generated so that the additional second moment is applied to the point until the displacement stops. As a result, it has a bad influence on a structure.

[50] The above statements should be considered when reinforcements are formed because the above problems also are generated in the case of a head-extended pile.

[51] FIG. 3 is a plane view and partial sectional view showing a head-extended pile with the inside and the outside reinforcement parts according to the present invention;

[52] As shown FIG. 3, a pile 5 is a cylindrical structure with the first supporting wall 1 and the second supporting wall 2 formed in the center. The pile 5 includes an inside reinforcement part Rl formed inside a pile 5 as a cylindrical structure, an outside reinforcement part R2 formed outside a pile 5, and a central borehole 7 formed in the center.

[53] That is to say, as shown at the lower end in a partial sectional view of FIG. 3, a first head portion 3 has extended parts extending right and left with respect to a central axis of a first supporting wall 1, and a second head portion 4 has extended parts extending right and left with respect to a central axis of a second supporting wall 2. The length of each extended part is the same.

[54] The first and second supporting walls 1 and 2, and the first and second head portions 3 and 4 are a pile of circular structure 5 formed symmetrically, a hammering surface 6 is provided on the top surface of the circular structure 5.

[55] A central borehole 7 is formed between the first and the second head parts 3 and 4.

The central borehole 7 prevents the buoyant force from being generated by the slime or

water flowed by hammering at supporting surface in low end of a pile, and guides smoothly the stream of force in hammering as a medium of a hard ground and a weak pile.

[56] FIG. 4 is a partial sectional view showing a difference generated when hammering a head-extended pile illustrated as an embodiment of the present invention and a conventional extended pile.

[57] In FIG. 4, a left drawing represents in case of hammering a conventional extended pile, and a right drawing represents in case of hammering the pile of FIG. 3 according to an embodiment of the present invention.

[58] On the top surface of the circular head-extended pile 5 a hammering surface 6 hammered after drilled-piling of the pile is provided. The cylindrical pile 5 having a diameter of 350, 400, 450, 500, or 600 mm may be a concrete pile, a PHC pile or a steel tube.

[59] The inside and the outside reinforcement parts Rl and R2 are disk-shaped structures having a predetermined thickness fixed to the lower end of the cylindrical pile 5 by means of a fixing unit. Herein, the fixing unit can bind the pile with the reinforcement disk using a conventional welding. On the other hand, the reinforcement disk is integrally formed, or further comprises a steel reinforcing band wrapping the lower end of the cylindrical pile if the cylindrical pile 5 is a concrete pile or a PHC pile. In this case, the pile is fixed when the steel reinforcing band welds the inside and the outside reinforcement parts.

[60] Table 1 shows dimensions and dynamism of the pile and the reinforcements.

[61] Table 1

[62] [63] Where σc = 3 front end bearing force / cross-sectional area of reinforcement disk [64] It can be seen from Table 1 that the front end bearing force of 58 tf is necessary to obtain an inherent proof stress of 89 tf in the case of the Φ350 PHC pile. However, a front-end bearing force of 58 tf cannot be obtained by the conventional pile having a diameter of 350 mm and the cross-sectional area of 961.625 cm . Consequently, it is required that a cross-sectional area of the front end of the pile be increased to obtain a front end bearing force of 58 tf. In Table 1, the cross-sectional area of the rein- forcement disk is 1759 cm , the diameter of the reinforcement disk is 473 mm, and the thickness of the reinforcement disk is 10.8 mm.

[65] It can be also seen from Table 1 that in the case of the Φ400, Φ450 and Φ350 PHC piles, the cross-sectional areas of the reinforcement disks are increased to 2216 cm , 2707 cm , and 3419 cm respectively, to fully utilize the inherent proof stress corresponding to the diameter of the respective pile. Also, the diameters of the reinforcement disks are increased to 531 mm, 587 mm, and 660 mm, respectively. Table 1 shows that the thickness of the reinforcement disk is increased as the protruded length of the reinforcement disk is increased.

[66] In the case of above embodiments, the magnitude of the increase varies with the stress of the front end by the change of the frictional force, and it means that the dimension of the pile is decided according to the condition of the ground.

[67] The central borehole 7 formed between the first and the second head parts 3 and 4 that makes the reinforcement part prevents a head-extended pile 5 from being floated because of an underground leachate. This leachate is flowed into and filled in the circular structure.

[68] Then, the central borehole 7 guides unnecessary slime into the pile in inter-

penetrating of the pile by hammering, also serves to guides soil and cement pastes flowed by hammering into the pile, and makes the slime and cement pastes mix and harden.

[69] FIG. 5 representing another embodiment of the present invention is a cross- sectional view showing the structure of the head-extended pile with the inside and the outside reinforcement parts. As shown in FIG. 5, the head extended pile 5 comprises the cylindrical pile part, and the inside and the outside reinforcement parts Rl and R2.

[70] On the top of the pile a hammering surface 6 hammered after drilled-piling of the pile is provided. The cylindrical pile 5 having a diameter of 350, 400, 450, 500, or 600 mm may be a concrete pile, a PHC pile, a steel tube, an H-shaped steel, or any other complex tube or wooden tube.

[71] The reinforcement parts formed at lower end has a disk-shaped structure with a predetermined thickness integrally formed or fixed to the lower end of the cylindrical pile part using a fixing unit.

[72] Furthermore, FIG. 5 similar to as FIG. 3, represents a pile 5 having a circular structure formed symmetrically by the first and the second head portions 3 and 4, and the first and the second supporting walls 1 and 2.

[73] Each of the first and second head portions 3 and 4 has a uniform thickness, and includes inclined surfacs 10 formed at both sides thereof.

[74] Table 2 shows dimensions and dynamism of the pile and the reinforcements.

[75] Table 2

[76] [77] It can be seen from Table 2 that the front end bearing force of 58 tf is necessary to obtain an inherent proof stress of 89 tf in the case of the 350 PHC pile. However, a front end bearing force of 58 tf cannot be obtained by the conventional pile with the diameter of 350 mm and the cross-sectional area of 961.625 cm . It is required that a cross-sectional area of the front end of the pile be 1759 cm to obtain a front end bearing force of 58 tf. In other words, it is required that the diameter of the reinforcement part be increased to 473 mm.

[78] Referring to Table 2, it is required that the cross-sectional areas of the front ends of Φ400, Φ450, and Φ500 PHC piles be increased to 2216 cm ² , 2707 cm ² , and 3419 cm ² , respectively, to fully utilize the inherent proof stress corresponding to the diameter of the respective pile. To this end, it is required that the diameters of the reinforcement parts of the pile be increased to 531 mm, 587 mm, and 660 mm, respectively.

[79] In the case of the embodiments, the increased magnitude varies with the stress of the front end by the change of the frictional force, and it means that the dimension of the pile is decided according to the condition of the ground.

[80] As shown in FIG. 4, the difference in case of hammering a head-extended pile with reinforcement parts according to the present invention and a conventional extended pile is as follows;

[81] It should be taken note that a conventional extended pile is a pile used in the drilled piling method, but not used in the hammering after drilled piling. In fact, the hammering after drilled piling has not been utilized up to now.

[82] When the pile 5 is hammered after drilled piling, it enters the ground to X depths. Then, a moment of force F2 bending a reinforcement disk is generated because slime is forced to enter into the central borehole 7, and an additional moment is continuously generated by the time of stopping the displacement as described in FIG. 1 (c), thereby having a bad influence on a structure installed on the pile.

[83] On the other hand, according to the present invention, it is possible to minimize the force bending the inside and the outside reinforcements Rl and R2, because the reinforcements with the same length extended in right and left are provided to evenly distribute the force generated when soil is pushed into the central borehole 7 by hammering after drilled piling. As a result, it is unnecessary to additionally reinforce the pile because the vertical force is applied to the central of the first and second supporting walls 1 and 2.

[84] FIG. 5 is a partial sectional view showing a case of hammering a head-extended pile with inside and outside reinforcement parts according to another embodiment of the present invention. When hammering after drilled piling, the pile 5 is penetrated into the ground by the depth of X thereby increasing the proof stress of the pile.

[85] In this case, it is possible to ensure the stable proof stress because the reinforcements with the same length extended in right and left are provided to evenly distribute the force generated when soil is pushed into the central borehole 7 by hammering after drilled piling. That is, it is possible to ensure the stable proof stress because the force is applied to the pile in vertical, and the eccentric force by the external factor is not generated. Also, it is possible to prevent the pile from being weakened by soil because the inside and the outside reinforcements prevent the soil elevated by the weight of upper part of the pile from being flowed into the pile through the borehole.

[86] The extension of the inside and the outside reinforcement parts is decided by two following methods.

[87] First, it is decided by the method for equally distributing the force. The method is achieved by minimizing the eccentric force of the reinforcement parts and transferring the weight of the pile to the ground by forming equally the inside and the outside reinforcement parts (FIG.3: Rl and R2).

[88] The length of the inside and the outside reinforcement part (FIG.3: Rl and R2) is set with the condition in which the square of a separate distance to the unit stress is same (the moment is same) and a separate distance to the power of the fourth to the unit stress (the settlement is same) is same. The moment is satisfied with sinking limit condition by a designed standard and a designed stress that the materials of the reinforcement part has.

[89] Such a way is useful for a typical pile with a diameter from Φ300 and Φ500, and characterized in an extended head part extends to the same length with respect to a central axis, so that the design and the production of the pile is simple.

[90] Second, the protruded length of the inside and the outside reinforcement part

(FIG.3: Rl and R2) is based on the conditions in which the total stress applied an inner reinforcement area and the total stress applied an outer reinforcement area are same so that the same unit stress applies at an inner reinforcement area and an outer reinforcement area. Therefore, the moment is satisfied with settlement limit condition by a designed standard and a designed stress that the materials of the reinforcement part.

[91] Such a way is useful for a large pile having a diameter above Φ500. It enables to obtain more accurate stress value of the pile without an error in the case that the pile is large-sized or the design and construction work is complicated, because the head- extended part allows to have the area so that the sum of supporting force with respect

to a central axis is same.

[92] The above pile-construction method is carried out at a distance of 2.5 times of a pile's diameter, a trial is done with one per 250 under the dynamic pile loading testing or the static pile loading testing. The dynamic pile loading testing used in the field especially includes as E.O.I.D(End of Initial Driving), which is carried out before the surrounding cement paste harden and carried out 1 -3 weeks after the cement paste harden.

[93] The two methods can continue a succeeding construction step after measuring a pile-driving immediate proof stress with E.O.I.D method to continue to carry out a pile construction. On the contrast, the method carried out after the filling materials harden is difficult to continue a construction step, because it needs the time to harden the filling materials. To continue a pile construction, it is required that the pile-driving equipments run over the pile, therefore more cost and time are required to protect the pile.

[94] For this reason, the pile construction can not be carried safely until after the proof stress is enough in E.O.I.D method. For example, in the case of a D400 PHC A-type pile, it has an outer diameter of 400mm and an inner diameter of 270mm. This is a conventional pile with a hollow that the central part is empty in a cross section to use the cross section efficiently, and allows to have a proof stress efficiently using a structural mechanics. If the area that meets a supporting surface in which a pile touches slime is 683.74 cm , and a reinforcement disk is reinforced 25mm at inside and outside due to the nature of the ground, it is reinforced with a structure having an outer diameter of 450mm and an inner diameter of 220mm. As a result, the area that meets a supporting surface in which a pile touches the slime is increased to 1209.69 cm , and it means that the supporting surface is increased to 177%. This allows a pile construction to proceed because an increase of a construction and immediate proof stress is made certainly by increasing of 111 % of the area that meets a supporting surface in which a pile touches the slime, compared with the conventional supporting area.

[95] The effect of 127% increase is achieved in comparison with the conventional area

(area considering blocking effect) when the cement paste is hardened and an outer area only without considering the area of a borehole is 1256.00 cm , or the outer area is to increase to 1589.63 cm . That is to say, a final pile becomes an efficient pile with an outer diameter of 450mm and an inner diameter of 270mm from a conventional pile having an outer diameter of 400mm and an inner diameter of 220mm.

[96] It can be seen that the side area of the front end (E.O.I.D) is achieved to the increase of 177% by drilled piling as described above, and the pile proof stress is increased to 127% when the filling materials harden.

[97] FIG. 6 represents a partial sectional view of a head-extended pile with an inside and

an outside reinforcement parts in laminated form as another embodiment of the present invention.

[98] As shown in FIG. 6(A), a pile 5 is a cylindrical structure with the first supporting wall 1 and the second supporting wall 2 formed in the center. It includes laminated inside reinforcement parts Rl and Rl formed inside a pile 5 as a cylindrical structure, and laminated outside reinforcement parts R2 and R2 formed outside a pile 5, and a central borehole 7 formed in the center.

[99] That is to say, in FIG. 3, the first head portion 3 having the same length extending right and left with respect to a central axis of the first supporting wall 1 is composed of Rl + R2; and the second head portion 4 has the same length extending right and left with respect to a central axis of the second supporting wall 2.

[100] In FIG. 6, the first head portion 3 forming the first layer is composed of Rl'+R2' and the first head portion 3 forming the second layer has the length of Rl + R2. The second head portion 4 is also composed of the first and the second layer having the same length in the same manner, and it is sequentially laminated to compose the first head portion and the second head portion.

[101] At this time, the lamination is carried out by means of welding, etc. when a metal pile is used as shown in FIG. 6(B), and a pile is used after multiple forming and hardening when a concrete pile is used as in FIG. 6(D).

[102] When a metal extending part is connected to the concrete pile as shown in FIG.

6(C), a band-shaped metal is wound around the concrete pile as an iron plate-reinforcing band, and then metal extending parts (inside and outside reinforcement parts) are fixed by means of welding, etc.

[103] The first and second supporting walls 1 and 2, and the first and second head portions 3 and 4 are the pile of circular structure 5 formed symmetrically. A hammering surface 6 is provided on the top surface of the circular structure 5.

[104] A central borehole 7 formed between the first and the above second head parts 3 and 4 enables to prevent the buoyant force caused by the slime or water flowed by hammering at supporting surface in low end of a pile, and to guide smoothly the stream of force in hammering as a medium of a hard ground and a weak pile. Furthermore, in hammering, it is possible that a laminating part delivers the force stream from the upper laminating part to the lower laminating part, and continues to guide uniformly the force to the ground, thereby delivering ideally the force stream to the ground.

[105] When the laminated head part is used, comparing with the case of using a head part of the same thickness (t), the proof stress of a pile per the unit area is almost same, but the area of the head part is decreased to be able to save the materials considerably.