JP2009108491A - Method for checking end bearing capacity of cast-in-place pile - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for checking the end bearing capacity of a cast-in-place pile, which enables a load-settlement relationship of a leading end of the cast-in-place pile to be checked without the great underestimation of the end bearing capacity, on the basis of a relatively-small applied load. <P>SOLUTION: A load transfer member 10 is installed inside an excavated hole 1, which is formed by excavating ground G, in such a manner that its lower end is brought into contact with a hole bottom 1a of the excavated hole 1. The excavated hole 1 is at least partially backfilled with a back-filling material 12 (12b) with a specific gravity almost equal to that of the actual cast-in-place pile in the state of applying friction cutting 11 between it and the load transfer member 10. The load N is applied to the hole bottom 1a through the load transfer member 10 so that the relationship between the applied load and settlement can be determined. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a method for confirming the tip support force of a cast-in-place pile.

  Conventionally, cast-in-place concrete piles (cast-in-place piles) are constructed by inserting reinforcing bars into the excavation hole formed by excavating the ground and placing concrete into the excavation hole. Since large-diameter piles can be constructed relatively easily by widening (bottoming) the bottom side (tip side), it is widely used as a foundation for high-rise buildings that require high bearing capacity.

  When constructing such a cast-in-place concrete pile, as shown in FIG. 6, since the stress of the ground G is released together with the formation of the excavation hole 1, The ground near the hole bottom 1a of the excavation hole 1 (tip ground G1) is not less loose. And, as shown in FIG. 7, the tip ground G1 in which such looseness is produced can be expected to recover looseness to some extent (uploading effect) by placing concrete 2 in the excavation hole 1 and compacting again. However, according to the standards and guidelines, the long-term allowable tip bearing strength of the cast-in-place concrete pile A is suppressed smaller than other pile methods.

  On the other hand, when the ground is good, it is possible to adopt a supporting force that is greater than that specified by the standards and guidelines if the supporting force is confirmed by a loading test. Such a loading test is generally performed using a pile having the same specifications as the actual pile, a push test for loading the pile head, and a jack of the same type as the pile at the tip of the pile, The tip loading test which loads a pile tip is used.

  The indentation test is to place a load directly on the pile head and measure the amount of settlement of the pile on a cast-in-place concrete pile actually used or a test pile constructed in the same manner as the actually used pile. And the allowable bearing capacity of a pile is calculated | required and evaluated from the relationship between a loaded load and settlement. Since such an indentation test loads an actual pile under conditions close to actual conditions, it is possible to obtain the most accurate load load-settlement relationship and is the most desirable load test method. On the other hand, in order to evaluate the bearing capacity of a large-diameter cast-in-place pile with a tip diameter of several meters or more, a load of several thousand to 10,000 tons or more is required, and there are problems and costs of the loading device and the reaction force device. There is a problem that testing is practically impossible due to the enormous volume.

  In the tip loading test, as shown in FIG. 8, when the excavation hole 1 is formed in the ground G, the loading jack 6 is installed in the hole bottom 1 a, the reinforcing bar is inserted into the excavation hole 1, and the concrete 2 is placed. The cast-in-place concrete pile A is constructed, and the load jack 6 secures the reaction force by the weight of the cast-in-place concrete pile A and the frictional force of the peripheral surface, and loads the load N directly on the hole bottom 1a and performs the test. It is. However, even in this tip loading test, since the reaction force is the weight of the pile and the frictional force of the peripheral surface, it is difficult to load exceeding this value, and it is not possible to sufficiently handle a large load. In addition, since the peripheral frictional force acts in the opposite direction, the restraint pressure of the ground near the bottom of the pile is reduced, and there is a problem that the support force is evaluated to be small and that it can not be applied to the expanded pile. .

On the other hand, as a method for evaluating the supporting force of the cast-in-place pile with a small load, for example, as shown in FIG. 9, a load rod 4 having a small diameter is used for the excavation hole 1 and the lower end of the load rod 4 is used. A method of loading at the hole bottom 1a with a small loading plate 5 provided at the (tip) has been proposed (for example, see Patent Document 1). This is because a predetermined load N is loaded on the hole bottom 1a via the loading rod 4 and the loading plate 5, and the amount of settlement of the loading rod 4 is measured with a dial gauge or the like to obtain the relationship between the loading load and the settlement. This is a method of expressing the relationship between the degree of loading load (loading load / loading plate area) and the settlement ratio (sinking amount / loading plate diameter) to give the tip load-sinking relationship of a large-diameter pile.
JP 2001-64954 A

  By the way, as shown in FIG. 10 (a), the actual pile tip is somewhat recovered from loosening during excavation by receiving the weight of the pile from above, and is mounted on the ground G around the pile tip according to the pile length. Pressure is acting. On the other hand, in the method shown in FIG. 8, there is no ground G around the loading plate 5 as shown in FIG. I can't expect. For this reason, it becomes a state of “shallow foundation” with the foundation near the ground surface, which is different from the failure mode of the pile tip which is “deep foundation”, and it is highly possible that the bearing capacity will be greatly underestimated. There was a problem.

  In view of the above circumstances, the present invention provides a tip bearing capacity of a cast-in-place pile capable of confirming the load-settlement relationship at the tip of the cast-in-place pile without significantly underestimating the support capacity with a relatively small load. The purpose is to provide a confirmation method.

  In order to achieve the above object, the present invention provides the following means.

  The method for confirming the tip bearing capacity of the cast-in-place pile according to the present invention is a method for confirming the tip bearing capacity of the cast-in-place pile, and the bottom end of the drilled hole is formed at the bottom of the borehole formed by excavating the ground. The load transmission member is placed in contact with the load transmission member, and at least one of the excavation holes is filled with a backfill material having a specific gravity substantially equal to that of an actual cast-in-place pile with a friction cut between the load transmission member and the load transmission member. The portion is backfilled, and a load is loaded on the bottom of the hole through the load transmission member to obtain a relationship between the loaded load and the settlement.

  In this invention, even when the load is loaded with a load transmission member having a lower diameter smaller than the bottom of the excavation hole, the tip ground on the outside of the load transmission member is actually cast into place by the backfill material. As in the case of an actual pile, the test can be performed by recovering to some extent the looseness of the tip ground on the outside of the load transmission member together with the tip ground on the center side as in the actual pile. In particular, even when the cast-in-place pile is an expanded pile, it is possible to perform the test by recovering the looseness of the tip ground outside the load transmission member to some extent by the backfill material. As a result, it is possible to obtain a relationship between the loaded load and the settlement (the relationship between the loaded load and the settlement amount, and the relationship between the loading load degree (load / loading plate area) and the settlement ratio (sinking amount / loading diameter)). The allowable bearing force can be obtained from the relationship between the load level and the settlement ratio. That is, it becomes possible to eliminate the fact that a support force that is significantly smaller than the support force when an actual pile is constructed like a small-diameter loading performed at a normal hole bottom is obtained.

  Further, since the loosened area by excavation is the largest at the center of the pile, loading the load directly on the center of the hole bottom with the load transmitting member is to evaluate and evaluate the support force on the safe side.

  Further, in the method for confirming the tip bearing capacity of a cast-in-place pile according to the present invention, a depth range greater than the diameter of the bottom of the excavation hole to the lower end of the load transmitting member is filled with a strength equal to or less than that of the surrounding ground. It is desirable to backfill with return material.

  In this invention, the depth range from the bottom of the excavation hole to the diameter of the lower end of the load transmitting member is filled with a backfill material having a strength equal to or less than that of the surrounding ground, and the test is performed more accurately. It becomes possible to ask for support force. That is, in general, the bearing capacity of a cast-in-place pile is the depth of the tip diameter (hole bottom diameter) on the top and the tip diameter (hole bottom diameter) on the bottom, with the tip of the pile (hole bottom of the excavation hole) in between. It is expressed by the average N value of the ground in the area, but the bearing capacity is overestimated by performing the test with backfilling this depth range with backfill material having the same or lower strength as the surrounding ground. It is possible to prevent this, and it becomes possible to determine the supporting force more accurately.

  Furthermore, in the method for confirming the tip bearing force of the cast-in-place pile according to the present invention, the load transmitting member is configured to include a loading rod and a loading plate integrally provided at the lower end of the loading rod. Is more desirable.

  In this invention, it is more accurate to form a loading plate with a diameter as large as possible within the range of the loading capacity by loading a load on the bottom of the excavation hole with a loading plate provided at the lower end of the loading rod. It is desirable to obtain a proper load-sink relationship. With a small loading board, only the pile center part with a large looseness is loaded, and the settlement tends to be overestimated.

  Moreover, in the tip supporting force confirmation method of the cast-in-place pile according to the present invention, the load rod described above may be a double tube rod, and the load plate described above may be integrally provided at the lower end of the inner tube.

  In this invention, the loading rod is a double tube rod, so that the friction cut between the inner tube and the backfill material for transmitting the loading load to the loading plate can be reliably performed by the outer tube, and the predetermined loading The load can be reliably loaded on the tip ground (hole bottom).

  According to the method for confirming the tip bearing capacity of a cast-in-place pile according to the present invention, even when the load is loaded with a load transmission member having a lower diameter smaller than the bottom of the excavation hole, the tip on the outside of the load transmission member It is possible to perform the test by recovering the looseness of the ground to some extent, and it is possible to determine the bearing capacity of the pile on the safe side and without significantly underestimating from the relationship between the loaded load and the settlement.

  Hereinafter, with reference to FIG.1 and FIG.2, the tip supporting force confirmation method of the cast-in-place pile which concerns on one Embodiment of this invention is demonstrated. The present embodiment relates to a method for confirming the tip bearing capacity of a cast-in-place concrete pile (cast-in-place pile) that is frequently used as a foundation of a building such as a high-rise building, and in particular, the tip load of a large-diameter pile having a tip diameter of several meters. The tip bearing capacity of cast-in-place piles, where the relationship between (loading load) and settlement can be obtained with a small loading load, and the tip bearing capacity can be obtained from the relationship between this loading load and settlement, on the safe side without significant underestimation. It relates to the confirmation method.

  In the method for confirming the tip bearing capacity of a cast-in-place concrete pile according to the present embodiment, first, as shown in FIG. 1 (see FIG. 6), the ground G is excavated by the same construction method as the actual cast-in-place pile. An excavation hole 1 having the same shape and size is formed. In this embodiment, it is assumed that the actual cast-in-place pile is a widened pile (large diameter pile) with the bottom side (tip side) widened, and accordingly, the excavation hole 1 has a pile axis O1 on the upper side. The pile shaft portion 8 having the same diameter is provided over the entire length in the direction, and the bottom portion is formed with the bottom expanded portion 8 that gradually increases in diameter from the upper side toward the lower side in the direction of the pile shaft O1.

  In this embodiment, for example, a load transmission member 10 in which a circular and flat metal loading plate 10b is integrally provided at the lower end of a loading rod 10a such as a steel pipe is provided on the bottom 1a of the loading plate 10b. It is inserted and installed in the inside of the excavation hole 1 so that the lower surface (loading surface) contacts. At this time, the load transmitting member 10 is installed such that the loading plate 10b has a larger diameter than the loading rod 10a, and the axis O2 is coaxially arranged with the pile axis O1. Thereby, the loading board 10b is installed in the center part of the hole bottom 1a. In the present embodiment, a coating material for reducing friction with the backfill material 12 (12a, 12b) described later is attached to the outer surface of the load transmitting member 10, that is, the outer surface of the loading rod 10a or the loading plate 10b. Thus, the load transmitting member 10 is formed by applying a friction cut 11.

Then, at the stage when the load transmitting member 10 as described above was set at a predetermined position, depth or diameter D _PL of loading plate 10b from bottom hole 1a of the borehole 1 (lower than the diameter D _PL of the load transmitting member 10) The range L is filled with a backfill material 12a (12) such as gravel or mixed soil having a strength equal to or less than that of the surrounding ground G, and the bottom side of the excavation hole 1 is backfilled.

  Furthermore, when the bottom side of the excavation hole 1 is backfilled with the backfilling material 12a, the backfilling material 12b (12 such as concrete having a specific gravity substantially equal to that of the actual cast-in-place pile is provided above the backfilling material 12a. ) To fill the remaining space (upper side) of the excavation hole 1.

  Further, a loading device (not shown) for installing the load transmitting member 10 and backfilling the excavation hole 1 with the backfilling material 12 (12a, 12b) and loading a predetermined loading load N on the load transmitting member 10 is performed. Is installed on the ground and preparation for the tip bearing capacity confirmation test is completed. The loading device is the same as the conventional loading device used when confirming the tip supporting force of the cast-in-place pile. For example, a reaction force pile, a reaction force girder, and a predetermined loading load N are loaded. A loading jack and measuring means such as a displacement meter for measuring the amount of settlement of the loading rod 10a are provided.

Then, when the preparation for the tip supporting force confirmation test is completed as described above, the load N is statically loaded on the hole bottom 1a through the load transmission member 10 by the loading device, and the amount of settlement of the load transmission member 10 is measured. As shown in FIG. 2, the relationship between the loading load and the settlement (the relationship between the loading load (load / loading plate area) and the settlement ratio (sinking amount / loading plate diameter)) is obtained. From the relationship between the loading load and settlement set forth in this way, for example, the loading load degree when the settlement ratio is 0.1 (the loading load when sinking with a sinking amount of 10% with respect to the loading plate diameter (pile diameter)) determined, the applied load degree was evaluated as a reference bearing capacity of q _d, the evaluation by calculating a long-term tolerance bearing capacity of q _a a further reference bearing capacity of q _d. Note that the load N applied to the load transmitting member 10 is not limited to static loading, and may be appropriately and selectively loaded according to the situation such as rapid loading.

Then, in the tip bearing capacity check method place pile of the present embodiment, so as to loading the load N in loading plate 10b of the smaller diameter D _PL than the diameter of the conventional as well as borehole 1 hole bottom 1a Even in this case, the backfilling material 12b (12) having a specific gravity substantially equal to that of the actual cast-in-place pile is filled to backfill the excavation hole 1, and the backfilling material 12 and the load transmission member 10 are friction cuts 11. Since the edge is cut, the tip ground G1 outside the loading plate 10b (load transmitting member 10) is pressed by the weight of the backfill material 12b (12) in the same manner as when an actual cast-in-place pile is constructed, and loaded to some extent. It is possible to recover the looseness of the tip ground G1 outside the plate 10b. In particular, even in the case where the cast-in-place pile is an expanded bottom pile as in the present embodiment, it is possible to fill the backfilling material 12 on the outside (the expanded bottom portion 7 and the pile shaft portion 8) of the loading plate 10b. The backfill material 12 can surely recover the looseness of the tip ground G1.

By providing the backfill material 12 in this way, and by excavating the ground G and forming the excavation hole 1 by the same construction method as the actual cast-in-place pile as in this embodiment, the looseness and slime of the tip ground G1 It is possible to reproduce the situation of the piles close to the actual cast-in-place pile. For this reason, by carrying out a test by loading a predetermined load N on the load transmitting member 10 in such a state, the relationship between the loaded load and the settlement is obtained more accurately than in the conventional test method. From the relationship between the load and the settlement, the supporting force (the standard supporting force degree q _d and the long-term allowable supporting force degree q _a ) can be obtained.

  At this time, the load transmitting member 10 is installed with its axis O2 coaxially arranged with the pile axis O1, and the loading plate 10b is installed at the center of the hole bottom 1a. The weight of the load transmission member 10 and the load N from the load transmission member 10 are loaded on the part. For this reason, the self-weight and load of the direct load transmitting member 10 with respect to the tip ground G1 at the center of the pile that is largely loosened (decrease in rigidity is large) due to the decrease in the confining pressure during the formation of the excavation hole 1 and the occurrence of shear strain associated therewith. The load N is loaded from the transmission member 10, and the test is performed with the subsidence rigidity given to the tip ground G1 at the center where the rigidity is greatly reduced. As a result, it is possible to evaluate and evaluate the bearing force on the safe side while eliminating the fact that a significantly smaller bearing force is obtained as a result of the conventional cast-in-place pile. Become. In addition, when the load degree loaded in the center part of the hole bottom 1a by the own weight of the load transmission member 10 is smaller than the load degree loaded on the outer side of the loading board 10b by the own weight of the backfilling material 12, it is before a test. It is desirable to preload the load transmitting member 10 to make the load levels of both equal.

Further, in this embodiment, filled with backfill material 12a having a peripheral ground G equal to or less strength a diameter D _PL more depth range L of loading plate 10b from bottom hole 1a of the borehole 1 (12) It is returning. In general, the supporting force of the cast-in-place pile is equal to the tip diameter (hole bottom diameter) of the pile on the top of the pile (hole bottom 1a of the excavation hole 1), and the tip diameter (hole bottom diameter) below. The depth range L is expressed by the average N value of the ground G in the depth range, and the depth range L is backfilled with the backfilling material 12a having the strength equal to or lower than that of the surrounding ground G as in this embodiment, and the test is performed. Thus, it becomes possible to accurately obtain the relationship between the loaded load and the settlement, and it is possible to accurately obtain the supporting force without overestimating the supporting force.

  Therefore, according to the method for confirming the tip bearing capacity of the cast-in-place pile according to this embodiment, a relatively small load N is loaded by the load transmitting member 10 (loading plate 10b) having a smaller diameter than the bottom 1a of the excavation hole 1. Even in this case, it is possible to recover the looseness of the tip ground G1 outside the load transmission member 10 to some extent and to reproduce the case where an actual cast-in-place pile is constructed, From the relationship, it is possible to determine the bearing capacity of the pile on the safe side and without significantly underestimating.

  In addition, since the relationship between the loading load and settlement is relatively accurately obtained in this way, the calculation accuracy of settlement during design load action is improved.For example, buildings with different settlement amounts, such as high-rise buildings and low-rise buildings, are joined. It is also possible to improve the prediction accuracy of the joining timing when it is necessary to do so.

  The method for confirming the tip bearing capacity of a cast-in-place pile according to the present invention is applied to a case where a ground layer G deeper than the tip of the pile (hole bottom 1a) has a support layer continuous three times or more of the pile tip diameter (hole bottom 1a diameter). it can.

  As mentioned above, although embodiment of the tip supporting force confirmation method of cast-in-place pile concerning the present invention was described, the present invention is not limited to the above-mentioned one embodiment, and can be suitably changed in the range which does not deviate from the meaning. is there. For example, in the present embodiment, the actual cast-in-place pile is assumed to be an expanded bottom pile (large-diameter pile), but the present invention is applied when confirming the tip support force of the cast-in-place pile not provided with the expanded portion 8. May be. Further, in this embodiment, the ground G is excavated by the same construction method as that of an actual cast-in-place pile to form the excavation hole 1 having the same shape and size as the actual cast-in-place pile. For example, a small-diameter excavation hole 1 may be formed on the cast-in-place pile and the tip support force may be confirmed using the excavation hole 1.

Furthermore, in the present embodiment, the lower end of the diameter D _PL more depth range L of the load transmission member 10 from the hole bottom 1a of the borehole 1, backfill material 12a (12 have equivalent or less intensity and surrounding ground G ) And backfilling the bottom side of the excavation hole 1 so that the support force is not overestimated, but the pile tip support force is less than the pile tip (hole bottom 1a) ground ( Since there is also a research result that it is almost dominantly determined by the properties of the tip ground (G1), when it is determined by prior examination that the influence of the shallower than the pile tip (upper than the hole bottom 1a) is small, for example, as shown in FIG. Thus, at the stage where the load transmitting member 10 is installed inside the excavation hole 1, the actual cast-in-place pile and Backfill material 12 such as concrete having substantially the same specific gravity You may make it backfill the excavation hole 1 by filling b (12).

  In the present embodiment, the load transmission member 10 is configured by integrally providing a circular and flat metal loading plate 10b at the lower end of the loading rod 10a such as a steel pipe. The rod 10a and the loading plate 10b are not particularly limited to metal materials, and may be formed using, for example, concrete as long as the material is subjected to a friction cut process for cutting off the adhesion to the backfill material 12. Good.

Furthermore, as shown in FIG. 4, for example, the load transmission member 10 may include only the loading rod 10 a without the loading plate 10 b, and in this case, from the hole bottom 1 a of the excavation hole 1, by backfill diameter D _PL more depth range L of the lower end of the loading rod 10a in contact with the bottom of the hole 1a in the surrounding ground G equal to or backfill material 12a having a less strength (12), the present embodiment In the same manner as described above, it is possible to accurately obtain the relationship between the loaded load and the settlement, and it is possible to accurately obtain the supporting force without overestimating the supporting force.

  For example, as shown in FIG. 5, the load transmitting member 10 may be configured by making the loading rod 10 a a double tube rod and integrally providing a loading plate 10 b at the lower end of the inner tube 10 c of the loading rod 10 a. . In this case, by making the loading rod 10a a double tube rod, the outer tube 10d reliably performs the friction cut between the inner tube 10c and the backfilling material 12 for transmitting the loading load N to the loading plate 10b. Without applying a coating material (or applying a coating agent) for reducing the friction with the backfill material 12 on the outer surface of the loading rod 10a as in the present embodiment, the predetermined loading load N can be obtained. It is possible to reliably load the tip ground G1 (hole bottom 1a). Thereby, it becomes possible to obtain the relationship between the loaded load and the settlement more accurately, and it is possible to accurately obtain the support force from the relationship between the loaded load and the settlement.

It is a figure which shows the state filled with the backfilling material while installing the load transmission member in the inside of an excavation hole in the tip supporting force confirmation method of the cast-in-place pile which concerns on one Embodiment of this invention. It is a figure which shows an example of the relationship between the loading load and subsidence obtained by the tip supporting force confirmation method of the cast-in-place pile which concerns on one Embodiment of this invention. In the method for confirming the tip bearing capacity of a cast-in-place pile according to an embodiment of the present invention, it is a diagram illustrating a state in which a backfill material having a specific gravity equivalent to that of an actual pile is filled on the entire bottom side and top side of the excavation hole. is there. It is a figure which shows an example using the load transmission member comprised without providing a loading board in the tip supporting force confirmation method of the cast-in-place pile which concerns on one Embodiment of this invention. It is a figure which shows an example using the load transmission member comprised by applying the double pipe rod to the loading rod in the tip supporting force confirmation method of the cast-in-place pile which concerns on one Embodiment of this invention. It is a figure which shows the state which formed the excavation hole in the ground. It is a figure which shows the bottom expansion pile formed by filling concrete inside the excavation hole. It is a figure which shows the conventional pile tip loading test method performed by installing a loading jack in the hole bottom. It is a figure which shows the conventional pile tip loading test method performed by installing the rod for loading in the inside of an excavation hole. It is a figure which shows the difference of the top pressure effect around the actual pile front-end | tip and the loading board of the conventional test method.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Excavation hole 1a Hole bottom 2 Concrete 4 Loading rod 5 Loading plate 6 Loading jack 7 Expanded part 8 Pile shaft part 10 Load transmission member 10a Loading rod 10b Loading plate 10c Inner tube 10d Outer tube 11 Friction cut 12 Backfill material 12a Backfill material 12b Backfill material A Cast-in-place pile D Diameter of lower end of _PL load transmission member G Ground G1 Tip ground L Depth range N Load load O1 Pile shaft O2 Axis of load transmission member

Claims (4)

A method for confirming the tip bearing capacity of a cast-in-place pile,
In the state of the excavation hole formed by excavating the ground, the load transmission member is installed with the lower end in contact with the bottom of the excavation hole, and the friction cut is performed between the load transmission member and the actual A backfill material having substantially the same specific gravity as the cast-in-place pile is used to backfill at least a part of the excavation hole, and a load is loaded on the bottom of the hole through the load transmission member to obtain a relationship between the load and the settlement. A method for confirming the tip bearing capacity of cast-in-place piles, characterized by In the method for confirming the tip bearing capacity of the cast-in-place pile according to claim 1,
The cast-in-place pile is characterized in that the depth range from the bottom of the excavation hole to the diameter of the lower end of the load transmitting member is backfilled with a backfill material having a strength equal to or less than that of the surrounding ground. Tip support force confirmation method. In the method for confirming the tip bearing capacity of the cast-in-place pile according to claim 1 or claim 2,
A method for confirming the tip bearing force of a cast-in-place pile, wherein the load transmitting member includes a loading rod and a loading plate provided integrally with a lower end of the loading rod. In the method for confirming the tip bearing capacity of the cast-in-place pile according to claim 3,
A method for confirming the tip bearing capacity of a cast-in-place pile, wherein the load rod is a double tube rod, and the load plate is integrally provided at a lower end of the inner tube.

Images