Method and measuring device for managing displacement of the excavation bottom.

Abstract

To classify vertical displacement behavior generated in an excavation bottom surface into heave behavior and rebound behavior, and manage the behaviors.SOLUTION: A management method of an excavation bottom surface for managing vertical displacement behavior generated in an excavation bottom surface measures an amount of settlement and a pore pressure, at least on an upper part of a confined aquifer, and a lower part and an upper part of an aquiclude positioned above the confined aquifer, below the excavation bottom surface.SELECTED DRAWING: Figure 8

Description

Technical Field

[0001] The present invention relates to an excavation bottom surface displacement management method and a measuring device for managing vertical displacement behavior occurring on an excavation bottom surface.

Background Art

[0002] In excavation work, vertical displacement may occur at the excavation bottom due to rebound or heaving. Rebound is a deformation behavior in which the load is removed when the upper ground is removed by excavation, elastic strain is released, and the ground deforms to uplift upward. On the other hand, heaving is a behavior in which the upper cohesive soil layer (low-permeability layer) is pushed up by the groundwater pressure in the sandy soil layer, causing the ground to heave up.

[0003] In order to manage such vertical displacement behavior of the excavation bottom, for example, Patent Document 1 discloses a method of measuring ground displacement at an open shaft excavation site using an optical ground displacement measuring device. In addition, Patent Document 2 discloses a method of estimating the rebound amount by measuring pore water pressure and vertical displacement.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Summary of the Invention

Problem to be Solved by the Invention

[0005] As described above, various methods and devices have been studied for measurement and management of ground displaced along with excavation work. However, a method for effectively measuring heaving has not been established. For this reason, there has been a problem that it cannot be determined whether the excavation bottom surface is uplifted by rebound due to stress release during excavation, or is pushed up from below and swollen by heaving caused by groundwater.

[0006] The present invention has been made in view of the above problems, and its main objective is to distinguish and manage the vertical displacement behavior occurring at the bottom of the excavation from ground swelling behavior and rebound behavior. [Means for solving the problem]

[0007] To achieve this objective, the present invention provides a method for managing the displacement of an excavation bottom, which distinguishes and manages the vertical displacement behavior occurring at the excavation bottom from ground swelling behavior and rebound behavior, and uses a single borehole to measure the settlement amount and pore water pressure at least above the confined aquifer, and below and above the impermeable layer located above the confined aquifer. Same position in the depth direction in A settlement gauge including a target guide tube oriented vertically, and a ring-shaped measuring target through which the target guide tube is inserted and which moves vertically with the ground, and a pore water pressure gauge having a built-in pressure sensor and being provided so as to protrude from the center outward toward the outer circumference of the measuring target, and inserted into the hole wall at the same position as the measuring target in the depth direction, The method is characterized by measuring and estimating ground swelling behavior when both the vertical displacement and the change in pore water pressure are similar in the lower and upper parts of the low-permeability layer, and estimating rebound behavior when the change is greater in the upper part of the low-permeability layer than in the lower part, and then managing the vertical displacement behavior occurring at the bottom of the excavation by distinguishing between the estimated behaviors.

[0009] The measuring device of the present invention is designed to manage the vertical displacement behavior of the excavation bottom by distinguishing between ground swelling behavior and rebound behavior. Installed in a borehole The measuring device comprises a settlement meter for measuring the amount of settlement below the bottom of the excavation and a pore water pressure meter for measuring pore water pressure, wherein the settlement meter includes a target guide pipe that runs vertically through the ground and a ring-shaped measuring target through which the target guide pipe is inserted and which moves vertically with the ground, and the pore water pressure meter has a pressure sensor built in and protrudes from the center side toward the outer circumference of the measuring target. Furthermore, it can be inserted into the hole wall at the same position in the depth direction as the measurement target. It is characterized by being provided on the measurement target.

[0010] The measuring device of the present invention is The settlement gauge includes a target anchor connected to the lower part of the measurement target. It is characterized by the following:

[0012] According to the displacement management method and measuring device for the excavation bottom of the present invention described above, settlement and pore water pressure can be simultaneously obtained below the excavation bottom, at least in the upper part of the confined aquifer, and in the lower and upper parts of the impermeable layer located above the confined aquifer.

[0013] Therefore, based on the measured settlement and pore water pressure, it becomes possible to estimate at an early stage whether the vertical displacement behavior occurring at the bottom of the excavation is ground swelling or rebound behavior. This makes it possible to manage the vertical displacement behavior occurring at the bottom of the excavation by distinguishing between the estimated behaviors, and to provide support information for implementing countermeasures corresponding to the estimated behavior.

[0014] Furthermore, the settlement gauge and pore water pressure gauge can be placed in the ground at horizontally separated locations, such as near the upper surface of the confined aquifer and near the lower and upper surfaces of the low-permeability layer located above the confined aquifer. Therefore, it becomes possible to use commercially available measuring instruments, for example, without having to construct any special structures to combine the settlement gauge and pore water pressure gauge.

[0015] Furthermore, since the settlement meter and pore water pressure gauge are installed in a single borehole, only one borehole needs to be drilled for their installation, simplifying the work and reducing working time. Also, because they are installed in the same borehole, it becomes possible to more reliably measure the settlement and pore water pressure of the same ground, making it possible to more accurately estimate either ground swelling behavior or rebound behavior.

[0016] Furthermore, by equipping the settlement element of the settlement meter, which moves along with the settlement of the ground, with a pore water pressure gauge, it becomes possible to install both the settlement meter and the pore water pressure gauge in a single borehole, making it easier to install the measuring devices. [Effects of the Invention]

[0017] According to the present invention, the vertical displacement behavior of an excavated bottom surface can estimate either heaving behavior or rebound behavior, and it becomes possible to efficiently manage the excavated bottom surface by distinguishing it for each estimated behavior. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] [Figure 1] It is a diagram showing a state where a measurement device used in a displacement management method for an excavated bottom surface according to an embodiment of the present invention is installed. [Figure 2] It is a diagram showing a measurement device according to an embodiment of the present invention. [Figure 3] It is a schematic diagram for explaining a target unit. [Figure 4] It is a diagram for explaining a method of fixing a measurement target. [Figure 5] It is a diagram for explaining the procedure until a first target unit is fixed. [Figure 6] It is a diagram for explaining the procedure until a third target unit is fixed. [Figure 7] It is a diagram for explaining the procedure until installation of the measurement device is completed. [Figure 8] It is a diagram for explaining a displacement management method for an excavated bottom surface, FIG. 8(a) is a diagram for explaining heaving behavior, and FIG. 8(b) is a diagram for explaining rebound behavior. MODE FOR CARRYING OUT THE INVENTION

[0019] The present invention relates to a method for managing an excavated bottom surface that exhibits vertical displacement behavior accompanying excavation and a measurement device used therefor, and is applicable to excavated bottom surfaces formed by any excavation work, such as the excavation bottom of foundation excavation work and the invert portion of tunnel work. In the present embodiment, as shown in FIG. 1, in foundation excavation work where a pair of retaining walls 1a are oppositely arranged in the ground 1 and excavation is performed between them, the excavation bottom 1b generated when excavation is progressed is taken as an example, and the details thereof will be described.

[0020] First, we will describe the measuring device 2 (hereinafter referred to as the measuring device) used in the displacement management method of the excavation bottom surface in this embodiment.

[0021] <Measuring device> As shown in Figure 2, the measuring device 2 includes a layered settlement meter 3 and a pore water pressure gauge 4. In this embodiment, a layered settlement meter 3 using a measurement target is given as an example, but any settlement meter that measures the amount of settlement at multiple different depths in the ground can be used. Furthermore, the settlement element of the layered settlement meter 3 can be any member that is buried in the ground and moves with the settlement of the ground, such as a settlement plate or an anchor, but in this embodiment, the target unit 12, which will be described later, is given as an example.

[0022] The layered settlement meter 3 includes multiple ring-shaped measurement targets 5 placed underground and displaced together with the ground 1, a target guide pipe 6 embedded vertically in the ground through which the measurement targets 5 are inserted, multiple sensors 7 placed vertically at intervals within the target guide pipe 6, and a data logger 8 that records data from the sensors 7.

[0023] As shown in Figure 3, the layered settlement gauge 3 has a ring-shaped measurement target 5 formed by a magnet, and multiple sensors (magnetostrictive displacement sensors) 7 are arranged vertically in a target guide pipe 6 installed vertically in the ground.

[0024] Each sensor 7 has a housing 9 containing an electronic circuit (not shown), and a magnetostrictive wire 10 connected to the electronic circuit and suspended downward from the housing 9, having a predetermined length. The magnetostrictive wire 10 is arranged vertically. The electronic circuit is connected to a data logger 8 located on the ground.

[0025] The layered settlement gauge 3 has a magnetostrictive wire 10 of an associated sensor (magnetostrictive displacement sensor) 7 inserted through each measurement target (magnet) 5, and each measurement target 5 is positioned to be displaceable while the magnetostrictive wire 10 is inserted through it. When an electric current pulse is applied to the magnetostrictive wire 10 from the electronic circuit, a circumferential magnetic field is generated along the entire axis of the magnetostrictive wire. When this magnetostrictive wire 10 is inserted through the measurement target (magnet) 5, an axial magnetic field is generated only in that part. When these two magnetic fields collide, the magnetostrictive wire 10 vibrates mechanically (torsional strain occurs). This vibration is detected by a pulse detector in the electronic circuit, and the absolute position of the measurement target 5 is measured by measuring the time difference between pulse generation and detection of the propagating ultrasonic wave. Therefore, the amount of displacement of the measurement target 5 can be measured by the change in its absolute position.

[0026] As shown in Figure 3, the measurement target 5 is unitized with a target anchor 11 for installing the measurement target 5 at a predetermined position in the ground 1. Hereinafter, the unit having the measurement target 5 and the target anchor 11 will be referred to as the target unit 12.

[0027] The target anchor 11 has an anchor base 13 through which the target guide tube 6 is inserted, and a plurality of leaf springs 14 provided on the anchor base 13. The anchor base 13 is an annular shape, almost identical in shape to the measurement target 5, and the leaf springs 14 are strip-shaped, with one end in the longitudinal direction fixed to the anchor base 13. With the target guide tube 6 inserted into the anchor base 13, each leaf spring 14 is positioned so that its longitudinal direction aligns with the target guide tube 6, and the plurality of leaf springs 14 surround the target guide tube 6.

[0028] The anchor base 13 is positioned below the measurement target 5 at a distance, and the anchor base 13 and the measurement target 5 are connected by connecting members 11a at multiple points in the circumferential direction. The leaf spring 14 is provided so as to rise from the upper surface of the anchor base 13 upward, i.e., towards the measurement target 5. The leaf spring 14 is formed to be longer than the distance between the anchor base 13 and the measurement target 5, and when the measurement target 5 is held and the anchor base 13 is suspended, the upper end of the leaf spring 14 is positioned outside the outer circumference of the measurement target 5. Note that in Figure 3, some of the multiple leaf springs 14 are omitted.

[0029] Therefore, as shown in Figure 4, when the anchor base 13 remains stationary and the measurement target 5 is moved downwards to approach the anchor base 13, the measurement target 5 pushes down the leaf spring 14 so that its upper end moves away from the target guide tube 6, and the multiple leaf springs 14 are configured to spread out radially around the target guide tube 6. At this time, the connecting member 11a undergoes plastic deformation, maintaining the state in which the measurement target 5 is close to the anchor base 13.

[0030] The pore water pressure gauge 4 is buried underground and measures the water pressure in the ground beneath it. The pore water pressure gauge 4, for example, has a cone-shaped tip and measures pore water pressure using a built-in pressure sensor (not shown).

[0031] The pore water pressure gauge 4 is paired with, for example, a measurement target 5 located at the same depth in the layered settlement gauge 3, and the measurement target 5 and the pore water pressure gauge 4 are associated with each other, as they are located at the same depth. As shown in Figure 3, the pore water pressure gauge 4 is installed together with the measurement target 5, and is configured to have a conical tip that protrudes horizontally from the center of the measurement target 5 toward the outer circumference, powered by an externally installed hydraulic pump or the like.

[0032] <Installation of measuring devices> The measuring device 2 used for managing the displacement of the excavation bottom is preferably installed approximately in the center of the area where the excavation work is to be carried out, as shown in Figure 1. When installing the measuring device 2, a ground survey is conducted in advance by boring to understand the conditions of the ground 1 and the depth of the geological boundary at the location where the measuring device 2 is to be installed. In this embodiment, the depth at which the boundary between the poorly permeable layer 15, such as a clay layer, and the confined aquifer 16, such as a sand layer, is determined in advance. Hereinafter, the location at which the boundary between the poorly permeable layer 15 and the confined aquifer 16 is defined as the layer boundary location S.

[0033] In this embodiment, we will describe an example in which measurement targets 5 are placed at four locations with intervals in the vertical direction. The four measurement targets 5 are placed at point A in the confined aquifer 16, slightly below the layer boundary position S; point B in the low-permeability layer 15, slightly above the layer boundary position S; point C, approximately in the vertical center of the low-permeability layer 15; and point D in the low-permeability layer 15, slightly below the excavation bottom 1b. As shown in Figure 5(a), the installation of the measuring device 2 begins by forming a borehole 1c vertically to a predetermined depth using a boring machine.

[0034] Next, a target guide pipe 6, equipped with a tip shoe 6a at its lower end, is inserted into the borehole 1c, and the tip shoe 6a is driven into the ground at the bottom of the borehole 1c. With the target guide pipe 6 standing upright, grout 17 is injected up to the height of point A in the borehole 1c. At this time, the grout 17 is injected to a depth near the layer boundary position S in the confined aquifer 16. The grout 17 is used to stop the infiltration of groundwater into the borehole 1c, and if it can prevent water caused by ground swelling from entering the borehole 1c, no further hardness or rigidity is required.

[0035] Next, as shown in Figure 5(b), the first target unit 12a, which is equipped with a first measurement target 5a and a first pore water pressure gauge 4a to be placed at point A, is inserted into the borehole 1c by passing it through the target guide pipe 6. At this time, as shown in Figure 5(c), a push pipe (not shown) for pushing in the measurement target 5 is also inserted into the borehole 1c together with the first target unit 12a, and the first measurement target 5a of the first target unit 12a, which is placed on the upper end of the grout 17 which is point A, is pushed downward with the push pipe. As a result, the first measurement target 5a approaches the first anchor base 13a, and the leaf spring 14a opens radially and is inserted into the borehole wall 1d, fixing the first measurement target 5a to the borehole wall 1d in close proximity to the first anchor base 13a.

[0036] After fixing the first measurement target 5a to the borehole wall 1d, as shown in Figure 5(d), the first pore water pressure gauge 4a is extended using a hydraulic pump and inserted into the borehole wall 1d at the same position (point A) in the depth direction as the first measurement target 5a and fixed therein. For this reason, the upper end of the grout 17 to be injected is aligned so that the position where the first measurement target 5a and the first pore water pressure gauge 4a are fixed is the same position (point A) in the depth direction in the confined aquifer 16. Here, the same position in the depth direction includes, for example, the range in which the first measurement target 5a and the first pore water pressure gauge 4a are installed on the same first target unit 12a, or the range in which there is a difference in the depth direction between the positions of the first measurement target 5a and the first pore water pressure gauge 4a, including installation errors during construction.

[0037] After fixing the first measurement target 5a and the first pore water pressure gauge 4a, as shown in Figure 6(a), more grout 17 is injected on top of the already injected grout 17 to a depth near point B. The injected grout 17 is near the layer boundary position S in the low-permeability layer 15, and is injected to a position higher than the layer boundary position S.

[0038] Next, as shown in Figure 6(b), the second target unit 12b, which is equipped with a second measurement target 5b and a second pore water pressure gauge 4b ​​positioned at the depth of point B, is inserted into the borehole 1c by passing it through the target guide pipe 6. In this case as well, by pushing the second measurement target 5b of the second target unit 12b, which is placed on the upper end of the injected grout 17, downward with the push pipe, the leaf spring 14b of the second anchor base 13b is opened radially by the second measurement target 5b and inserted into the borehole wall 1d, and the second measurement target 5b is fixed to the borehole wall 1d in close proximity to the second anchor base 13b.

[0039] After fixing the second measurement target 5b to the borehole wall 1d, the second pore water pressure gauge 4b ​​is extended using a hydraulic pump and inserted into the borehole wall 1d at the same position (point B) in the depth direction as the second measurement target 5b, and then fixed in place. For this reason, the upper end of the grout 17 to be injected is aligned so that the position where the second measurement target 5b and the second pore water pressure gauge 4b ​​are fixed is at the same position (point B) in the depth direction in the low-permeability layer.

[0040] After fixing the second measurement target 5b and the second pore water pressure gauge 4b, as shown in Figure 6(c), more grout 17 is injected on top of the already injected grout 17 to a position near point C. The grout 17 to be injected is injected to a depth near the center in the vertical direction of the low-permeability layer 15.

[0041] Next, as shown in Figure 6(d), the third target unit 12c, which is equipped with a third measurement target 5c and a third pore water pressure gauge 4c positioned at the depth of point C, is inserted into the borehole 1c by passing it through the target guide pipe 6. The third measurement target 5c of the third target unit 12c is then pushed downward with a push pipe over the grout 17, causing the leaf spring 14c of the third anchor base 13c to open radially and be inserted into the borehole wall 1d, thereby fixing the third measurement target 5c to the borehole wall 1d in close proximity to the third anchor base 13c.

[0042] After fixing the third measurement target 5c to the borehole wall 1d, the third pore water pressure gauge 4c is extended using a hydraulic pump and inserted into the borehole wall 1d at the same position (point C) in the depth direction as the third measurement target 5c, and then fixed in place. For this reason, the upper end of the grout 17 to be injected is aligned so that the position where the third measurement target 5c and the third pore water pressure gauge 4c are fixed is the same position (point C) near the center of the low-permeability layer 15.

[0043] After fixing the third measurement target 5c and the third pore water pressure gauge 4c, as shown in Figure 7(a), more grout 17 is injected on top of the already injected grout 17 to a depth near point D. The grout 17 to be injected is injected to a depth near the excavation bottom 1b in the low-permeability layer 15, that is, near the upper end of the low-permeability layer 15.

[0044] Next, as shown in Figure 7(b), the fourth target unit 12d, which is equipped with a fourth measurement target 5d and a fourth pore water pressure gauge 4d positioned at the depth of point D, is inserted into the borehole 1c by passing it through the target guide pipe 6. The fourth measurement target 5d of the fourth target unit 12d is then pushed downward with a push pipe over the injected grout 17, causing the leaf spring 14d of the fourth anchor base 13d to open radially and be inserted into the borehole wall 1d, thereby fixing the fourth measurement target 5d to the borehole wall 1d in close proximity to the fourth anchor base 13d.

[0045] After fixing the fourth measurement target 5d to the borehole wall 1d, the fourth pore water pressure gauge 4d is extended using a hydraulic pump and inserted into the borehole wall 1d at the same position (point D) in the depth direction as the fourth target 4d, and then fixed in place. For this reason, the upper end of the grout 17 to be injected is aligned so that the position where the fourth measurement target 5d and the fourth pore water pressure gauge 4d are fixed is at the same position (point D) in the depth direction near the excavation bottom 1b in the low-permeability layer 15. After fixing the fourth measurement target 5d and the fourth pore water pressure gauge 4d, grout 17 is injected on top of the already injected grout 17 up to the position of the excavation bottom 1b, as shown in Figure 7(c).

[0046] After installing the four target units 12a, 12b, 12c, and 12d, four sensors 7a, 7b, 7c, and 7d are placed inside the target guide tube 6, corresponding to each of the measurement targets 5a, 5b, 5c, and 5d, as shown in Figure 7(d). Each of the four sensors 7a, 7b, 7c, and 7d is suspended inside the target guide tube 6 so that the magnetostrictive wire 10 is located on the lower side of the housing 9 and aligns vertically. At this time, one magnetostrictive wire 10 of each sensor 7a, 7b, 7c, and 7d is inserted into each of the four installed measurement targets 5a, 5b, 5c, and 5d, and each sensor 7a, 7b, 7c, and 7d is positioned so that it does not detach from the magnetostrictive wire 10 when each of the measurement targets 5a, 5b, 5c, and 5d is displaced together with the ground 1.

[0047] The cables for each of the installed sensors 7a, 7b, 7c, and 7d are routed out from the target guide tube 6 and connected to the data logger 8, and together with the cables for the pore water pressure gauges 4a, 4b, 4c, and 4d, they are connected to a processing unit equipped with a monitor. The measured values ​​from the installed measuring device 2 can be confirmed on the monitor or the like.

[0048] <Method for estimating either bulging behavior or rebound behavior> From the measuring device 2 installed in the ground 1, the displacement amounts of the measurement targets 5a, 5b, 5c, and 5d at each measurement position from point A to D, measured by the sensors 7a, 7b, 7c, and 7d of the layer settlement gauge 3, and the change in pore water pressure at each measurement position from point A to D, measured by the pore water pressure gauges 4a, 4b, 4c, and 4d, are displayed on a monitor via the processing device.

[0049] Ground swelling is a phenomenon in which the bottom of an excavation bulges when a confined aquifer 16 is located below the excavation bottom 1b of a low-permeability layer 15, and the high-pressure groundwater in the confined aquifer 16 uniformly pushes up the low-permeability layer. Therefore, as shown in Figure 8(a), the overall volume change of the low-permeability layer 15 is small, and the vertical displacement ΔX for each measurement point B to D is large from the top to the bottom of the low-permeability layer 15, but is almost the same. Also, since the entire low-permeability layer 15 is pushed up, the change in pore water pressure is small.

[0050] On the other hand, rebound is a phenomenon in which the soil above is removed by excavation, etc., and the stress is released, causing loosening in the gaps and lifting the excavation bottom 1b. For this reason, in the case of rebound, as shown in Figure 8(b), the stress release has a greater impact at the upper part of the low-permeability layer 15, whose upper surface is the excavation bottom 1b, so the vertical displacement amounts ΔX1 to ΔX3 for each measurement point B to D are larger at the upper part than at the lower part. Also, the loosening of the gaps in the low-permeability layer 15 that makes up the excavation bottom 1b is greater, and the change in volume is larger at the upper part than at the lower part, so the pore water pressure decreases more significantly at the upper part than at the lower part of the low-permeability layer 15. In other words, when rebound occurs, both the vertical displacement amount and the change in pore water pressure are smaller at the lower part of the low-permeability layer 15 and larger at the upper part.

[0051] Thus, when swelling occurs, the vertical displacement is similarly large from the top to the bottom of the low-permeability layer 15, and the change in pore water pressure is similarly small from the top to the bottom of the low-permeability layer 15. When rebound occurs, both the vertical displacement and the change in pore water pressure are smaller in the lower part of the low-permeability layer 15 and larger towards the top. Therefore, it is possible to more accurately estimate either the swelling behavior or the rebound behavior based on the measurements of the layer-specific settlement gauge 3 and pore water pressure gauge 4 of the measuring device 2.

[0052] Furthermore, by comparing the vertical displacement at points A and B, and by comparing the pore water pressure at points A and D, it becomes possible to more accurately estimate either the swelling behavior or the rebound behavior.

[0053] In this embodiment, we have described an example where four measurement points, A to D, were used. However, for example, it is possible to estimate the bulging behavior and rebound behavior even without measuring point C. Furthermore, there may be more measurement points than just four.

[0054] As described above, according to the displacement management method and measuring device 2 of this embodiment, settlement and pore water pressure can be simultaneously obtained below the excavation bottom 1b, at least at the upper part (point A) of the confined aquifer 16, and at the lower part (point B) and upper part (point D) of the impermeable layer 15 located above the confined aquifer 16.

[0055] Therefore, based on the measured settlement and pore water pressure, it becomes possible to estimate at an early stage whether the vertical displacement behavior occurring at the excavation bottom 1b is ground swelling or rebound behavior. This makes it possible to manage the vertical displacement behavior occurring at the excavation bottom 1b by distinguishing between the estimated behaviors, and to provide support information for implementing countermeasures corresponding to the estimated behavior.

[0056] Furthermore, the settlement meter and pore water pressure gauge can be placed in the ground at horizontally separated locations, such as near the upper surface (point A) of the confined aquifer 16 and near the lower surface (point B) and upper surface (point D) of the low-permeability layer 15 located above the confined aquifer 16. Therefore, commercially available measuring instruments can be used without the need for any special construction to combine the settlement meter and pore water pressure gauge.

[0057] Furthermore, as in the embodiment described above, if a pore water pressure gauge 4 is provided on the target unit 12 that moves with the settlement of the ground and is attached to the settlement gauge 3, it becomes possible to install both the settlement gauge 3 and the pore water pressure gauge 4 in a single borehole 1c, making it easier to install the measuring device 2. Even if the settlement gauge and pore water pressure gauge of the measuring device are configured as separate components, they may still be installed in a single borehole.

[0058] Furthermore, when installing a settlement gauge and a pore water pressure gauge in different boreholes located at horizontally separated positions, it is desirable to place the boreholes in which both are installed closer together, as this makes it easier for the geological and soil layers at the same depth to coincide. For example, it is desirable to place the two boreholes at the closest possible location where they can be drilled in close proximity.

[0059] It should be noted that the displacement management method for the excavation bottom of the present invention, and the measuring device 2 used in this displacement management method for the excavation bottom, are not limited to the above-described embodiments, and various modifications are possible without departing from the spirit of the present invention.

[0060] In the above embodiment, an example was given in which a so-called magnetostrictive type layered settlement meter 3 is used, which has a sensor 7 equipped with a magnetostrictive wire 10 and a measurement target 5 made of a magnet inserted through the magnetostrictive wire 10. However, it is not limited to this, and for example, a so-called differential transformer type settlement meter that detects the displacement of the sensor may be used. In this case, the settlement meter sensor and the pore water pressure gauge are fixed in the ground at the same depth, and the settlement meter and pore water pressure gauge are installed. That is, when a differential transformer type settlement meter is used, the sensor itself corresponds to a settlement element that moves with the settlement of the ground. [Explanation of Symbols]

[0061] 1 Ground, 1a Retaining wall, 1b Bottom of excavation, 1c Borehole, 1d Borehole wall, 2. Measuring device, 3. Layer-specific settlement gauge, 4. (4a, 4b, 4c, 4d) Pore water pressure gauge, 5 (5a, 5b, 5c, 5d) Measurement target, 6 Target guide tube, 6a Tip shoe, 7 (7a, 7b, 7c, 7d) Sensors, 8 Data loggers, 9 Housings, 10 Magnetostrictive wire, 11 Target anchor, 11a Connecting member, 12 (12a, 12b, 12c, 12d) Target Units, 13 (13a, 13b, 13c, 13d) Anchor base, 14 (14a, 14b, 14c, 14d) Leaf spring, 15. Impermeable layer, 16. Confined aquifer, 17. Grout, S layer boundary position, Δx(Δx1, Δx2, Δx3) Vertical displacement,

Claims

1. A method for managing the excavation bottom, which distinguishes and manages the vertical displacement behavior occurring at the excavation bottom from ground swelling behavior and rebound behavior, Below the bottom of the excavation, at least in the upper part of the confined aquifer, and in the lower and upper parts of the impermeable layer located above the confined aquifer, Settlement and pore water pressure are measured at the same location in the depth direction using a single borehole. A settlement meter including a target guide pipe oriented vertically, and a ring-shaped measuring target through which the target guide pipe is inserted and which moves vertically with the ground, The measurement is performed using a pore water pressure gauge, which has a built-in pressure sensor and is mounted on the measurement target so as to protrude from the center outward, and is inserted into the hole wall at the same position as the measurement target in the depth direction. When the vertical displacement and the change in pore water pressure are similar in the lower and upper parts of the low-permeability layer, it is estimated to be a bulging behavior, and when the displacement in the upper part of the low-permeability layer is greater than that in the lower part, it is estimated to be a rebound behavior. A method for managing the excavation bottom, characterized by managing the vertical displacement behavior occurring at the excavation bottom by distinguishing it from the estimated behavior.

2. A measuring device installed in a borehole to distinguish and manage the vertical displacement behavior of the excavation bottom between ground swelling behavior and rebound behavior, The system includes a settlement gauge for measuring the amount of settlement below the excavation bottom and a pore water pressure gauge for measuring pore water pressure, The settlement gauge includes a target guide pipe that runs vertically through the ground, and a ring-shaped measuring target through which the target guide pipe is inserted and which moves vertically with the ground. The pore water pressure gauge is a measuring device characterized in that it has a built-in pressure sensor and is provided on the measurement target so as to be able to be inserted into the hole wall at the same position as the measurement target in the depth direction, protruding from the center of the measurement target toward the outer circumference.

3. In the measuring device according to claim 2, The measuring device is characterized in that the settlement gauge includes a target anchor connected to the lower part of the measurement target.

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