JPS6272813A - Formation of composite foundation pile and composite foundation pile thereby - Google Patents

Abstract

PURPOSE:To increased the baring capacity of a ground for a composite foundation pile by a method in which the lower part of a SC pile is inserted into the upper inside of a reinforcing bar cage set to an in-situ pile, and the lower part of the SC pile is connected with the upper part of the in-situ pile. CONSTITUTION:A reinforcing bar cage 3 is inserted into the lower inside of a casing 5 penetrated into a ground, and a given amount of concrete 6 is placed into the cage 3. A SC pile (a) whose outside is fixed with a closing plate 8 is hung down by a hanger C and slowly lowered into the casing 5. The pile (a) is then lowered at the center of the casing 5 by the inducing action of the connecting plate 7 of the hanger C and the closing plate 8 and its lower end is positioned at the center of the cage 3. When the concrete 6 hardens, the SC pile (a) is integrated with the in-situ pile and a composite foundation pile having great ground bearing capacity can thus be obtained.

Description

Detailed Description of the Invention The present invention provides a cast-in-place reinforced concrete pile (abbreviated as cast-in-place pile) above a concrete pile with an outer shell steel pipe (SC).
It is abbreviated as a pile. ), and the structure of this composite foundation pile. and this S
Insert a C pile of the required length into the cast-in-place pile's reinforcing bar cage, and transmit the axial force that increases during an earthquake to the cast-in-place concrete from the tip of the SC pile and the outer peripheral surface of the SC pile in the reinforcing bar cage, respectively. In addition, the upper part of the SC pile is covered with a cylindrical artificial layer with a deformation coefficient of the required size to further improve the horizontal seismic performance due to the small diameter effect of the SC pile, and the large diameter effect of the cast-in-place pile The purpose is to increase the bearing capacity of the ground for foundation piles.

Next, an example of a construction method for constructing this composite foundation pile will be shown, and the following description will be made assuming that a reverse construction method is used. 1st
Several outer guide bars 4 are protruded from the upper end of the reinforcing bar cage 3 of the cast-in-place pile b shown in the figure, and the reinforcing bar cage 3 is located at a predetermined position with respect to the casing 5. Water is filled above the ready-mixed concrete 6 that has been poured in the required amount, so insert the suction pipe that communicates with the suction pump into the casing 5, and remove the water, some remaining slime, and a small amount of ready-mixed concrete. 6 and check the top position of the fresh concrete 6.

When the SC pile a connected to the suspension device C is gradually lowered into the casing 5 in the above condition, the SC pile a is connected to the connecting plate 7 of the suspension device C and the outer surface of the SC pile a. It descends through the center of the casing 5 by the guiding action of the annular closing plate 8, is guided by several inner guide bars 9 fixed to the upper end of the reinforcing bar cage 3, and descends through the center of the reinforcing bar cage 3. On the other hand, the fresh concrete b rises inside the casing 5 as the SC pile a whose lower end is closed by an obtuse cone 10 descends, and the upper surface of the fresh concrete 6 reaches the position of the closing plate 8.

The connecting position of the closing plate 8 on the SC pile a is a position that is at least 50 cm above the upper end of the reinforcing bar cage 3 when the SC pile a is inserted into the reinforcing bar cage 3 for a predetermined length.

In other words, the pouring amount of ready-mixed concrete 6 is the amount by which the upper surface of ready-mixed concrete 6 rises to a position 50 cm from the upper end of reinforcing bar cage 3 when the lower end of SC pile a is inserted into reinforcing bar cage 3 for the required length. shall be.

By timely and accurately measuring the top surface of the fresh concrete 6, the final amount of fresh concrete to be carried in can be directed, and the amount of fresh concrete 6 placed will be approximately as planned. The diameter of the closing plate 8 is 15 m from the inner diameter of the casing 5.
m or 20 mm smaller, and several rubber plates 11 are fixed to the outer peripheral surface of the closing plate 8, so that the closing plate 8 is smaller than the casing 5.
Never lean towards one side. The water remaining on the fresh concrete 6 overflows through the gap, but the highly viscous fresh concrete 6 cannot pass through the narrow gap between the casing 5 and the closing plate 8, and is sealed by the closing plate 8 and SC.
Pile a stops at the position shown in FIG.

Next, a ring 13 protruding from the cylindrical body 12 of the hanging device C
When the fixed hexagon nut 15 at the upper end of the plurality of tie bolt bolts 14 attached to the Tie rod nut 17 comes off and the SC
The connection between the stake a and the suspension device C is released. As described above, the hanging device C is released from the SC pile a and the hanging device C is recovered. The hollow portion at the upper end of the SC pile a is closed in advance. Reference numeral 18 of the hanging device C is a tightening nut, which serves to tighten the tie rod bolt 14 onto the ring 13 when connecting the SC pile a and the hanging device C. In addition, as another structure, when attaching a pair of hanging rings to the upper outer surface of the SC pile a, a wire rope is removably hooked to the hanging rings, and the SC pile a is suspended inside the casing 5. The installation work is simplified,
Work time is reduced.

In the next step, sand or sand mixed with gravel in the circulation tank is poured into the space formed between the SC pile a and the casing 5, filling the space considerably above the upper end of the SC pile a. After this, a vibrator (not shown in the drawing) is attached to the casing 5 and activated, and when the casing 5 is gradually pulled out, the ready-mixed concrete 6 protrudes toward the original ground on soft ground, and the SC
The pile a may descend to some extent, and the distance between the closing plate 8 and the upper end of the reinforcing bar cage 3 may become approximately 25 cm.

On the other hand, if sand or sand mixed with gravel is also used on soft ground, casing 5
According to the low speed of pulling out, it protrudes towards the original ground,
It compresses the original ground and also undergoes vibration compaction to form a dense structure, and excess water rises and becomes floating water. In other words, a dense cylindrical artificial layer is formed around SC pile a, the surrounding ground is further compressed, the deformation coefficient of the cylindrical artificial layer is improved, and the bending moment acting on the SC pile during an earthquake is considerably reduced. It is expected that this will have the effect of

Figure 3 shows the structure of the upper part of the composite foundation pile created by the above construction method, and when creating the footing, the head of SC pile a is dug out as shown. S.C.
The connection between pile a and footing is appropriately designed and constructed. Reference numeral 20 is a cylindrical artificial layer made of tightly compacted sand mixed with gravel. The layer thickness of this cylindrical artificial layer 20 is
When the outer diameter of the SC pile a is 60 cm to 100 cm, the distance from the bottom of the footing is preferably 4 meters to 6 meters.

The method for constructing a synthetic foundation pile of the present invention is not limited to the above method. For example, if the illustrated closing plate 8 is not attached to the SC pile a, the buoyancy of the fresh concrete 6 and the upper layer of water will act on the SC pile suspended within the casing;
The acting weight, minus the buoyant force, remains and cannot maintain and secure the SC pile in position. Therefore, in this case, the cylindrical body 12 of the hanging device C shown in the figure is further lengthened, a pair of I-beams are protruded from its upper part, and the I-beams are received by a hydraulic jack installed on the upper outer surface of the casing 5. The SC pile a is maintained at a predetermined position by the supporting force of the casing 5. If left in this state for 30 minutes, the ready-mixed concrete 6 will change its properties to be difficult to flow, and even if the hydraulic jack is activated to tighten the lot and the I-beam is not supported, the I-beam will not descend. After confirming this condition, the suspension device C is uncoupled from the SC pile a and the suspension device C is recovered. Next, sand or sand mixed with gravel is allowed to flow down into the casing 5, and this layer is removed from the SC pile a. Fill it to a level higher than the top surface. During this process, most of the water on the fresh concrete flows out of the casing. Next, after pulling out and collecting the casing 5, a plurality of vibroflots are placed at equal intervals on the outside of the SC pile a, and while replenishing supplies, the vibroflots are operated to remove sand or gravel around the SC pile. Compact the mixed sand and surrounding ground. In this case as well, a dense cylindrical artificial layer is created around the upper part of the SC pile. Furthermore, instead of the gravel-mixed sand, carboxymethyl cellulose or bentonite is added as a concrete admixture to increase the yield value of the concrete.The concrete is made of air-foamed concrete, in which countless air bubbles are dispersed in the concrete, and the casing 5 and the SC pile a. Another effective method is to create a cylindrical artificial layer made of concrete containing numerous air bubbles. The deformation coefficient of this cylindrical artificial layer made of special foamed concrete can be equal to or greater than the deformation coefficient of hard ground.
This is an effective construction method when the lower layer of the footing is on hard ground. Although various other construction methods may be possible,
Whichever method is used, it is desirable that the construction method be able to create a cylindrical artificial layer around the upper part of the SC pile, which has a deformation coefficient E0 larger than that of the original ground and also has plasticity.

By the way, an SC pile a with an outer diameter of 100 cm has a steel pipe 1 as an outer shell.
By designing the cylindrical thickness of the high-strength concrete 2 to the required thickness, the outer diameter is 160 cm.
It is clear from the nature of the material that constitutes the SC pile that the allowable axial force, allowable bending moment, and allowable shear force can be approximately equivalent to the allowable axial force, allowable bending moment, and allowable shear force of a cast-in-place pile of m. When an SC pile is rigidly connected to a footing, the maximum bending moment and shear force that will act during an earthquake will be applied at the pile cap, and if the SC pile can absorb the applied force, the maximum bending moment and shear force will be in the underground area. Since the underground shear force is smaller than the pile cap restraint moment and the pile cap shear force, it is easy to design the cast-in-place pile to resist this with the resistive bending moment and shear resistance force. The problem lies in the mechanism for transmitting the axial force between the different types of piles, SC pile a and cast-in-place pile b. In particular, the most important issue for this composite foundation pile is how to transmit the dramatic increase in axial force that acts impulsively on the SC pile a during an earthquake to the cast-in-place pile b. Standard compressive strength F of hardened cast-in-place concrete 6'
If C is 210 kg/cm2, this concrete 6
The permissible compressive stress degree δ'C during an earthquake for ' is δ'C=FC/4.
5 X 2 = 210 It is determined by the total area of the tip of pile a. This vertical transmission force P1 is P1=(50X50X3.14)X93.3=732.
It is desirable that the maximum allowable axial force of a 4-ton cast-in-place pile with an outer diameter of 160 cm during an earthquake is 1000 tons, and the above P
1 is insufficient. Due to hardening and shrinkage of the cast-in-place concrete 6', a strong adhesion force is generated between the thick concrete 6' and the steel pipe 1 of the SC pile.

There is enough concrete covering the steel pipe 1, and the entire surface of the concrete 6' is pressed against the entire circumferential surface of the steel pipe 1 due to hardening shrinkage of the concrete 6' in the radial direction. Taking into consideration the fact that no cavities will occur, the allowable adhesion stress of cast-in-place concrete 6' to steel pipe 1 during an earthquake is determined by
' is assumed to be 1/3 of the allowable adhesion stress during an earthquake for deformed reinforcing bars. According to this standard compressive strength 210kg/cm
A trial calculation of the allowable adhesion stress for the concrete steel pipe in No. 2 during an earthquake is 7 kg/cm2, which is considered to be a reasonable value. SC pile a housed in reinforcing bar cage 3
The length of the parallel part (excluding the length of the cone 10) is 35
0 cm, and calculate the transmitted axial force P2 due to adhesion.

P2=(100X3.14)X7X350=769.3
Therefore, the total transmitted axial force P is P=P1+P2=732.4+769.3=1501.
7≒1500 tons This total transmitted axial force P is the allowable axial force of cast-in-place pile b during an earthquake of 10
00 tons, and the safety factor is 1.5.
becomes. The most important thing to keep in mind is the concern that slime may remain in the concrete 6' in the reinforcing bar cage 3, and when carefully treating the slime on fresh concrete as in this example, the above safety factor should be set to 1.4 or above. It is considered that even if it is lowered to 1.3, there will be no shortage in transmitting the axial force. The fact that the area of action of the axial force transmitted from the tip of the SC pile to the cast-in-place concrete 6' is limited to the total area of the tip including the hollow part 19 of the SC pile a means that the axial force is not transmitted by the adhesive force. S
Since it acts on the concrete 6' outside of the C pile, the idea is to avoid duplication of the transmitted axial force for safety reasons. A more specific analysis of the above axial force transmission mechanism shows that if the length of the SC pile inserted into the reinforcing bar cage 3 is extremely shortened, the axial force that increases dramatically during an earthquake will be applied from the tip of the SC pile to the concrete. However, in the axial force transmission mechanism of the present invention, the SC pile a is inserted into the reinforcing bar cage for the required length, and the spot driving of the SC pile is performed. The structure makes full use of the adhesion of concrete 6', so SC
The axial force transmitted from the tip surface of pile a decreases accordingly, and SC
There is no risk of localized destruction or damage to the cast-in-place concrete 6' below the pile.

In the above explanation, the conical body 10 and the SC pile a were closed, but
Fresh concrete 6 is introduced into the hollow part 19 of the SC pile a,
The hollow portion 19 may be closed with concrete in a plug-like structure. In this case, the lower end of the cylinder of the SC pile is formed into a V-shaped slope, so that the slime rises along the slope and no slime remains at the lower end of the SC pile.

As described above, the total amount of axial force transmitted from the SC pile a to the cast-in-place pile b is the sum of P1 and P2. Among them, the vertical transmitted axial force P1 is a fixed value because the total area of the tip of the SC pile a is constant, and the transmitted axial force P2 due to the adhesion force is the length of the parallel part of the SC pile a housed in the reinforcing bar cage 3, that is, it is a variable value. The adhesion area that can be applied determines the magnitude of the total transmitted axial force. Therefore, by designing P2 that satisfies P with a safety factor of 1.3 to 1.5, a composite foundation pile that can withstand a dramatic increase in axial force during a huge earthquake can be obtained.

The above explanation is based on the adhesion force when no processing is applied to the outer surface of the steel pipe 1 of the SC pile a, but there are methods for welding multistage reinforcing bar rings to the lower part of the outer surface of the steel pipe 1 of the SC pile, Or, when using a special steel pipe with spiral protrusions on its outer surface, the steel pipe 1 and cast-in-place concrete 6'
The adhesion force with the S
It is expected that the length of the parallel part of the C pile will be significantly shortened.

Next, the features of the synthetic foundation pile of the present invention described above will be listed.

(1) Small diameter effect of an SC pile with an outer diameter of 1 meter (effect that the moment of inertia of an SC pile is 15% to 18% of the moment of inertia of a cast-in-place pile with an outer diameter of 1.6 meters)
Therefore, the bending moment acting on an SC pile due to horizontal force during an earthquake will not exceed 80% of the bending moment acting on a cast-in-place pile with an outer diameter of 1.6 meters due to the same horizontal force. Therefore, when the resistance bending moment required for a cast-in-place pile is 200 ton-m, the SC pile can be designed to have a maximum resistance bending moment of 160 ton-m, and the SC pile can be obtained economically.

(2) Due to the small diameter effect mentioned in the previous section (1), if the bending moment resistance of an SC pile with an outer diameter of 1 meter is made equal to the bending moment resistance of a cast-in-place pile with an outer diameter of 1.6 meters, then The resistance bending moment is essentially the outer diameter of 1.6
This is more than 1.25 times the bending moment resistance of a meter-meter cast-in-place pile. By utilizing this feature, special moment foundation piles in the civil engineering field, which require a large resistance bending moment compared to the required axial strength, can be designed to reduce the diameter of the cast-in-place pile itself within the range of allowable axial force. , can provide an economical moment composite foundation pile.

(3) Due to the small diameter effect of the SC pile, the footing becomes small and lightweight, the cost of constructing the footing becomes small, and the horizontal force that acts on it is reduced, albeit slightly.

(4) For composite piles in which the upper outer surface of cast-in-place piles is wrapped with steel pipes,
The short-term allowable compressive stress of cast-in-place concrete is usually 100.
kg/cm2 or less, the force of the steel pipe cannot be fully utilized. On the other hand, with SC piles, the short-term allowable compressive stress of high-strength concrete 2 is 400 kg/cm2, making it possible to fully utilize the force of steel pipe 1, and due to the small diameter effect of SC piles, the short-term allowable compressive stress of high-strength concrete 2 is relatively the same as that of cast-in-place piles. It is possible to provide a synthetic foundation pile that can be designed to have an allowable bending moment, has high utilization efficiency of the steel pipe 1, and is highly economical.

(5) In normal ground, excluding areas of ground subsidence, the large diameter effect of cast-in-place piles increases the circumferential frictional force with the ground, which can increase the frictional bearing capacity of synthetic foundation piles.

(6) Due to the large-diameter effect of cast-in-place piles, when creating an expanded root at the tip of the pile, the contact area with the ground is expanded, and the supporting capacity of the tip of the composite foundation pile can be increased.

(7) Due to the adhesion between the outer surface of the SC pile inserted into the reinforced cage of the cast-in-place pile and the cast-in-place concrete, the
Even if a pulling force is applied to the C pile, the SC pile will not separate from the cast-in-place pile and will not come up.

(8) When performing a construction method that creates a cylindrical artificial layer 20 with a deformation coefficient E01 larger than the deformation coefficient E02 of the original ground around the SC pile placed on the upper stage of the composite foundation pile, the SC pile According to the improvement effect of E01, the bending moment acting on the SC pile is lower than when the SC pile is in direct contact with the original ground. This deformation coefficient improvement effect and the small diameter effect described in the previous section 1 have a synergistic relationship, and the synergistic effect of the two further reduces the resistance bending moment of the SC pile required during an earthquake. The cost of SC piles used for foundation piles will naturally decrease.

The composite foundation pile of the present invention has the multifaceted effective features listed above, and can reliably increase the seismic capacity of the foundation pile by the SC pile placed in the upper stage, and can improve the foundation pile by the cast-in-place pile placed below. It has very desirable properties that can increase the earthquake bearing capacity of piles, and its effectiveness is high.

In addition, the suspension device C that connects the SC pile a shown in FIG.
The outer diameter of the cylindrical body 12 is made almost equal to the outer diameter of the SC pile a, and the beam protruding from the head of this cylindrical body is received by a hydraulic jack installed in the casing. 5, and the cylindrical body 12 and SC pile a, if sand mixed with gravel or foamed concrete is poured to create a cylindrical artificial layer, the cast-in-place concrete 6 can be poured during the elapsed time of this pouring process. Almost all fluidity is lost, and even if the suspension device C and the SC pile a are uncoupled, the SC
The pile a is prevented from settling into the ready-mixed concrete 6. In other words, there is almost no waiting time during construction,
The construction of the composite foundation pile is streamlined and the outer diameter of the annular closure plate 8 may be reduced or omitted in this construction method. In this way, various modifications can be made to the construction of this composite foundation pile.

In addition, the most important condition required for the construction method of this synthetic foundation pile is that the deformation coefficient E01 of the cylindrical artificial layer built around the upper part of the SC pile a should be at least equal to the deformation coefficient E02 of the surrounding original ground. Furthermore, the E02 of the surrounding ground should be increased. This is extremely important; if the E01 of the cylindrical artificial layer is smaller than the E02 of the surrounding original ground, the displacement resistance (passive earth pressure) of the artificial layer will be weaker, and the bending force acting on the SC pile during an earthquake will weaken. The moment increases, and the displacement of the head of the SC pile during an earthquake increases, and the seismic performance of the SC pile clearly deteriorates. That is,
In this construction method, increasing the displacement resistance of the artificial layer by making the deformation coefficient E01 of the cylindrical artificial layer larger than the E02 of the surrounding original ground is an important factor in improving the seismic performance of the synthetic foundation pile.

[Brief explanation of drawings]

Figure 1 is a diagram showing the state in which ready-mixed concrete cast in place is sealed off with a closure plate connected to an SC pile when constructing a composite foundation pile according to the present invention, and Figure 2 is a view taken along line A-A in Figure 1. cross section,
Figure 3 is a structural diagram of the upper part of the composite foundation pile after the cast-in-place concrete has hardened. In the drawings, code a...SC pile, b...cast-in-place reinforced concrete pile, C...hanging device, 1...
Steel pipe that becomes the outer shell of the SC pile, 2... High-strength concrete of the SC pile, 3... Rebar cage of the cast-in-place pile, 5...
・Casing, 6...Ready-mixed concrete with cast-in-place piles,
6'...Hardened concrete of cast-in-place piles, 8.
...Closing plate connected to the SC pile, 19...Hollow part of the SC pile, 20...Cylindrical artificial layer made of sand mixed with gravel around the SC pile.

Claims (2)

[Claims] (1) Insert the lower part of the SC pile to the required length inside the reinforcing bar cage placed on the cast-in-place pile, and create a cylindrical artificial layer that covers the SC pile above the ready-mixed concrete of the cast-in-place pile. , A method for constructing a synthetic foundation pile, characterized by engineering the cylindrical artificial layer so that its deformation coefficient is equal to or greater than the deformation coefficient of the surrounding original ground. (2) The lower part of the SC pile, which has the area required to bear part of the axial force that increases during an earthquake, through the adhesion force between the outer surface of the SC pile and the concrete of the cast-in-place pile, is placed inside the reinforced cage of the cast-in-place pile. A composite foundation pile is constructed by encasing the SC pile above the cast-in-place concrete with a cylindrical artificial layer having a deformation coefficient of the required size.