JP3862023B2 - Surface shear strength measuring device and method for measuring viscous ground - Google Patents

Description

The present invention relates to a surface shear strength measuring apparatus and a measuring method for measuring shear strength of a viscous soil having a depth of 5 mm or less from a surface in a room or field.

  The flowing water generated on the surface of the cohesive soil erodes the soil, and the degree of the erosion is affected by the shear strength (also referred to as adhesive force) of the cohesive soil surface in contact with the flowing water. Conventionally, there has been no device for measuring the surface shear strength of shallow ground with a depth of about 5 mm or less from the surface.

  Common methods for measuring the shear strength of soil include, for example, one-side shear test (JGS-0560, 0561), triaxial shear test (JGS-0521-0524), uniaxial compression test (JIS-A1216), vane shear test (JGS-1411), cone penetration test (JGS-1431) and so on. However, these standardized standard test methods could not measure the surface shear strength. In the conventional test method described above, even in the one-side shear test in which the thinnest specimen can be tested, the depth is limited to 10 mm. In addition, it was very difficult to collect viscous soil that was less than 10 mm deep and saturated with water and became soft and installed in the test apparatus.

  By the way, in order to estimate the soil erosion limit flow velocity and the erosion rate, an apparatus for measuring the tensile fracture stress of the soil, which is an influential factor of these, has been proposed (for example, see Patent Document 1).

  Further, although not intended for viscous soil, as a single-surface shear tester for sandy soil and gravel, a shear box comprising a bottom plate and side plates standing around the bottom plate, There is known a shear test apparatus in which at least a part of a sample that abuts on the lower surface of the loading plate is fixed to the loading plate described above (for example, patent document). 2).

Japanese Patent Laid-Open No. 7-259067 (0008, 0009, FIG. 1) JP-A-8-226886 (Claim 1, FIG. 1)

  However, since the device described in Patent Document 1 is a method of creating a square soil block of 100 mm on each side and measuring the tensile fracture stress of soil approximately 100 mm deep from the surface layer, It was impossible to measure the shear strength at very shallow depths that were directly subjected to the surface laminar flow forces.

  The apparatus described in Patent Document 2 has a structure in which a specimen (sample) is accommodated in a shear box. When the test object is sandy soil and gravel, this sample can be stored in a shear box. However, when a viscous soil is used as a test object, it is extremely difficult to house the softened viscous soil in a shear box without disturbing it, and it is impossible to perform a test on site. The device described in Patent Document 2 cannot be used for measuring the surface shear strength of the viscous ground.

  The present invention has been made to solve the above-mentioned drawbacks of the prior art, and a surface layer shear strength measuring device for simply measuring the shear strength of a viscous soil having a depth of 5 mm or less from the surface in a room or field, and It aims at providing the measuring method.

The present invention
(1) A pulling mechanism that pulls the loading plate in the horizontal direction, a shear strength measurement load cell, a shear displacement measurement displacement meter, a device main body, a coupling frame coupled to the shaft body of the device main body, The loading plate is connected to the connecting frame, and a plurality of plate-like shear blades are arranged at intervals of 10 to 20 mm on the lower surface of the loading plate, and the longitudinal direction of the shear blade intersects with the tensile direction of the loading plate. In addition, the surface shear strength measurement of the viscous ground is characterized in that the blade tip is provided so as to protrude downward from the loading plate, and the length protruding from the lower surface of the loading plate of the shear blade is 1 to 3 mm. apparatus,
(2) The surface layer shear strength measuring device of the viscous land according to (1), wherein the loading plate is formed of aluminum,
(3) In the above (1) or (2) , the loading plate pulling mechanism has a structure that can eliminate the moment generated when there is a distance between the resistance force generated at the cutting edge of the shearing blade and the point of action of the pulling force. Surface shear strength measuring device of the viscous land described,
(4) Using the measuring device according to (1) to (3) above, measuring the shear strength of the surface layer of the viscous soil, measuring the surface shear strength of the viscous ground,
Is the gist.

  In the present invention, a plurality of plate-like shear blades are arranged on the lower surface of the loading plate with a predetermined interval, the longitudinal direction of the shearing blade intersects the tensile direction of the loading plate, and the cutting edge is below the loading plate. By adopting a configuration provided so as to be convex, a surface layer shear strength measuring device for easily measuring the shear strength of viscous soil having a depth of 5 mm or less from the surface in a room or field can be obtained. Since there is almost no restraining pressure on the ground surface, it has been impossible to directly measure the adhesive strength of soil on the surface layer below 10 mm. Can be measured. As a result, it has become possible to accurately evaluate the erosion phenomenon of viscous soil caused by surface flow. Further, the device of the present invention has a simple structure as compared with the conventional triaxial testing machine, and can easily measure the shear strength on site.

  Moreover, the surface layer shear strength measuring method of the present invention employs a method for measuring the shear strength of the surface layer of the clay soil on site using the above-described shear strength measuring device, so that the measuring method is simple, Therefore, it is possible to easily obtain a lot of data for soil existing in various states, and it is possible to improve the credibility of the data.

  The present invention will be described in detail below. FIG. 1 shows an example of the device of the present invention, where (a) is a plan view and (b) is a side view of (a). As shown in FIG. 1, the surface shear strength measuring device 1 of the viscous ground of the present invention comprises a screw jack 3 as a tension mechanism for pulling the loading plate 2 in the horizontal direction, a load cell 4 for measuring shear strength, and a shear displacement. The apparatus main body 6 includes a measurement displacement meter 5, a connection frame 13 connected to the shaft body 7 of the apparatus main body 6, and a loading plate 2 connected to the connection frame 13.

  As shown in FIGS. 1 (a) and 1 (b), the apparatus main body 6 is mounted on a pedestal 8 assembled with a steel frame or the like, on the screw jack 3, the load cell 4 for measuring shear strength, and the displacement meter for measuring shear displacement. Each part such as 5 is attached. Further, as shown in FIG. 1 (b), the pedestal 8 is fixed on a pedestal stabilizing plate 12 [not shown in FIG. 1 (a)] made of a plywood plate and installed on the ground B.

  Further, the drive of the screw jack 3 used as the pulling mechanism may be either a manual type or a motor type. In the apparatus shown in FIG. 1, a handle 9 for driving the screw jack 3 by hand is attached.

  The screw jack 3 is connected to the shaft body 7 via the load cell 4 so that the shear strength is transmitted from the shaft body 7 to the loading plate 2. The shaft body 7 is inserted through a guide 11 provided with bearings 10 having bearings at both ends of the guide, and the guide 11 is fixed to the base 8. The shaft body 7 is formed by a bearing 10 so as to be movable in the front-rear direction of the loading plate 2. The front-rear direction means the pulling direction of the loading plate 2.

  2 shows the loading plate of FIG. 1, in which (a) is a side view, (b) is a bottom view, and (c) is a front view. As shown in FIGS. 2 (a) to 2 (c), the loading plate 2 has a large number of shearing blades 22 provided at predetermined intervals on the bottom surface of a substrate 21 made of a plate-like body. The longitudinal direction of the shearing blade 22 is a direction orthogonal to the front-rear direction of the loading plate, and the respective shearing blades 22, 22... Are arranged in parallel. Further, the shearing blade 22 is disposed so that the cutting edge is convex below the loading plate 2.

  The size of the substrate 21 is preferably formed in a square shape having a side length of 100 to 150 mm. As shown in FIG. 2 (a), the shearing blade 22 is preferably arranged such that the distance D between the shearing blades 22 (the distance between the cutting edge of adjacent shearing blades and the cutting edge) is 10 to 20 mm. As shown in FIG. 2 (c), the shearing blade 22 is formed so that the length G of the blade protruding from the bottom surface of the substrate 21 of the loading plate (sometimes referred to as the protruding amount) is 1 to 3 mm. Is preferred. The thickness E of the shearing blade 22 shown in FIG. 2 (b) is preferably 0.3 to 0.5 mm. If the thickness of the shearing blade is less than 0.3 mm, it is difficult to stably attach the shearing blade to the loading plate.If the thickness exceeds 0.5 mm, depending on the interval between the shearing blades, In some cases, the amount of distortion applied to the surface increases, making accurate measurement difficult. The shearing blade preferably has a sharp tip as shown in FIG. 2 (a) in order to suppress the disturbing action when the tip is inserted from the ground surface.

  As shown in FIGS. 2 (b) and 2 (c), the horizontal width H of the shearing blade 22 is formed to the same length as the horizontal width of the substrate 21. In addition, the loading plate 2 has a mounting surface 23 on which the upper surface of the substrate 21 is placed for loading weights (weights), and a fixing pin 24 is provided on the mounting surface 23. A load weight having a through hole is inserted through the fixing pin 24. By changing the weight of this weight in various ways, the restraining pressure of the loading plate can be arbitrarily changed.

In order to measure the adhesive strength of soft clay soil, it is necessary to apply a very weak restraining pressure equivalent to a weak shear resistance force to the ground via a loading plate. For that purpose, it is preferable to reduce the weight of the loading plate. The material of the specific loading plate 2 (the substrate 21 and the shearing blade 22) is preferably made of aluminum because it has a certain degree of rigidity, is light and inexpensive. For example, the restraining pressure of only the loading plate of the shape shown in FIG. 2 is 0.005 kgf / cm when the material is made of aluminum, the size of the substrate 21 is 100 mm square, the thickness is 1 mm, and 10 shearing blades 22 are provided. ²

  FIG. 3 is an enlarged view of the vicinity of the connecting frame in FIG. 1 (b). The connection frame 13 includes a shaft body side frame 13A and a loading plate side frame 13B. As shown in FIG. 3, the shaft body 7 side end portion of the shaft body side frame 13A includes a shaft body receiving portion 16 provided with a recess into which the end portion of the shaft body 7 can be fitted. Further, the shaft body receiving portion 16 and the shaft body 7 end portion are provided with a through hole and a groove through which the connecting pin 14 can be inserted, and after fitting the shaft body 7 end portion to the shaft body receiving portion 16, The shaft pin 16 is connected through the connecting pin 14 from above.

  Further, as shown in FIG. 3, the connecting frame 13 includes a horizontal member in which the end of the shaft body side frame 13A on the loading plate 2 side is positioned below the tip position (shear surface) of the shear blade, and the end of the horizontal member. The horizontal member is formed in the shape of a cross section made up of a vertical member 31 continuously connected upward, and the end of the loading plate side frame 13B on the shaft body 8 side is located above the shear plane and the horizontal member It was formed in a cross-sectional saddle shape consisting of a vertical member connected downward to the end. Then, the vertical member 31 of the shaft body side frame 13A and the vertical member 32 of the loading plate side frame 13B are arranged to engage with each other, and the shear strength transmission pin 15 is arranged between the vertical members 31 and 32. The position of the central axis of the shear strength transmission pin 15 was made to be the same position as the shear plane.

  Thus, the structure of the connecting frame 13 shown in FIG. 3 is such that the shear surface of the tip of the shearing blade 22 and the action line F that pulls the loading plate 2 in the direction of arrow P coincide with each other, and the shaft body side frame 13A and the loading plate side frame 13B Rotation that occurs when there is a distance between the resistance force generated at the cutting edge of the shearing blade and the point of action of the tensile force because it is connected to the contact point via the shear strength transmission pin 15 consisting of a round steel rod for load transmission There is no moment.

  Hereinafter, a method for measuring the shear strength of the surface layer of the viscous ground according to the present invention will be described. The method of the present invention uses the above-described measuring device to measure the shear strength of the viscous soil several times while changing the restraint pressure on the site, to obtain the relationship between the shear strength and the restraint pressure, and from the relationship, the adhesion of the viscous soil surface layer The force (the adhesive strength in this case) is to obtain the shear strength when the restraint pressure is zero. Specifically, the loading plate 2 of the surface shear strength measuring device is directly installed on the ground surface to be measured. At this time, the loading plate 2 is installed on the ground B so as to be horizontal using a level. The screw jack 3 is driven to pull the loading plate 2 in the horizontal direction, and the resistance force at that time is measured by the load cell 5 for shear strength measurement to determine the shear strength (τ).

  This shear strength measurement is performed a plurality of times by changing the restraining pressure (σ) of the loading plate 2. The shear strength is measured at least three times while changing the vertical restraint pressure (hereinafter simply referred to as restraint pressure). The restraining pressure can be changed by changing the load on the loading plate. For example, the restraint pressure (σ1) with only the weight of the loading plate without placing the weight on the loading plate, the restraining pressure (σ2) with the weight applied to the loading plate, and the weight further increased The maximum shear strength (τ1, τ2, τ3, τ4) is measured with the constraint pressure (σ3) and the constraint pressure (σ4) with the weight further increased.

  Next, as shown in FIG. 4, the relationship (shear failure line) between the restraining pressure (σ1 to σ4) and the shear strength (τ1 to τ4) is plotted in a straight line. From this graph, the shear strength (τ0) when the vertical restraint pressure is zero (σ0) can be used as the adhesive strength of the surface layer of the viscous land. The method for determining the adhesive strength is a method according to “JGS0561 soil consolidation and constant pressure single surface shear test method”.

[Experimental example]
From the adhesive strength obtained from the shear strength obtained by the triaxial compression test device, and the shear strength obtained using the device of the present invention, using the viscous soil of the water content ratio at the limit where the specimen is self-supporting. The obtained adhesive force was compared, and the validity of the measurement apparatus and measurement method of the present invention was tested. The results are shown below.

Using the apparatus of the configuration shown in FIG. 1, as the loading plate, the vertical (X) × horizontal (Y) is a 100 mm × 100 mm square shape and the thickness (Z) is 10 mm, the shape is as shown in FIG. Using aluminum loading plate, (1) No loading weight (σ1 = 0.005kgf / cm ² ), (2) Weight (σ2 = 0.01kgf / cm ² ), (3) Second heavy weight ( σ3 = 0.015 kgf / cm ² ), (4) In each state with the heaviest weight (σ4 = 0.02 kgf / cm ² ), pulling the loading plate, respectively, the maximum shear strength (τ1, τ2, τ3, τ4) Asked. Next, in the method described in the method for measuring adhesive strength, as shown in FIG. 5, the relationship between the restraining pressure (σ) and the shear strength (τ) (shear fracture line) is plotted in a straight line, The shear strength when the restraint pressure was zero) was taken as the adhesive strength of the viscous ground surface. Table 1 shows the composition and physical properties of the soil used in the test.

(table 1)

  Fig. 5 shows the results of the surface shear test and the standard triaxial test (consolidated undrained condition), which were carried out by preparing the specimens in the same state shown in Table 2 for the soil shown in Table 1. However, in the case of the triaxial test apparatus, there is a structural limit, and the same low restraint pressure as the apparatus of the present invention cannot be applied. Therefore, the adhesive force was obtained by extrapolating the shear fracture line to the low restraint pressure side.

(Table 2)

As shown in FIG. 5, the difference between the adhesive strength of 2.0 kN / m ² obtained using the surface shear strength measuring apparatus of the present invention and the adhesive strength of 2.45 kN / m ² obtained by the triaxial test is about 0.45 kN / a m ^2. This difference is extremely small, and the measurement accuracy is sufficiently practical. In other words, the surface shear strength measuring device of the present invention has a structure such as a triaxial testing device that can control with high accuracy the confining pressure before shearing and the drainage conditions that occur with volume changes occurring in the soil during shearing. Despite being a simple device that does not, it is possible to measure the shear strength that is generally consistent with the triaxial test.

  Next, experimental examples in the case where the interval between the shearing blades and the length of the shearing blade are changed are shown below. A shear blade suitable for measuring shear strength for soft clay soil was evaluated. The results are shown below. The evaluation method was to determine the adhesive strength by measuring the shear strength of the same soil surface while changing the interval (D) and the amount (G) of the blades. The results are shown in FIG. 6 and FIG.

  The relationship between the shearing blade spacing (D) and the adhesive strength (C) shows that when the spacing between the shearing blades exceeds 20 mm, the plane area sheared by the tensile force spreads outside the loading plate. As shown in FIG. 6, the shear strength may increase. If the distance between the shearing blades is less than 10 mm, the cutting edge may easily hit the gravel or wood chips scattered on the ground surface, and the test may not be possible. If the distance between the shearing blades is 10 mm or more, the test is possible even if there are some gravel and wood chips on the ground surface. From the above results, it is preferable that the interval between the shearing blades 22 of the loading plate 2 is 10 to 20 mm.

  The relationship between the amount (G) of the shearing blade and the adhesive force (C) is as shown in FIG. If the length of the shearing blade protruding from the lower surface of the loading plate (sometimes referred to as the protruding amount) exceeds 3 mm, the cutting edge will be in a region where the depth to be sheared is too deep and softening is insufficient (the strength is relatively high). Therefore, not the shear strength of the surface layer of the weakened clay soil, but the exfoliation action of the ground with a depth exceeding 3 mm is outstanding. When the peeling action is excellent, a phenomenon that the loading plate floats is observed. As a result, there is a possibility that a large shear strength is expected. In addition, when the protruding amount of the shearing blade is less than 1 mm, the cutting edge becomes less than the unevenness of the ground surface, and the tip of the shearing blade does not pierce the ground uniformly, and there is a possibility that accurate measurement cannot be performed. From the above points, it is preferable that the protruding amount of the shearing blade is 1 to 3 mm.

An example of the shear strength measuring device of the present invention is shown, (a) is a plan view and (b) is a side view of (a). FIG. 1 shows the loading plate of FIG. 1, (a) is a side view, (b) is a bottom view, and (c) is a front view. FIG. 2 is an enlarged view of the vicinity of a connecting frame in FIG. 1 (b). It is a graph for demonstrating how to obtain | require adhesive force. It is a graph which shows the shear test result by this invention apparatus and a triaxial test apparatus. It is a graph which shows the relationship between the space | interval of the shear blade of an experiment example, and adhesive force. It is a graph which shows the relationship between the amount of cutting edges of the shear blade of an experiment example, and adhesive force.

Explanation of symbols

1 Surface shear strength measuring device for viscous ground
2 Loading plate
3 Screw jack
4 Load cell for shear strength measurement
5 Displacement meter for shear displacement measurement
6 Main unit
7 shaft
13 Connecting frame
13A Shaft side frame
13B Loading plate side frame
21 Board
22 Shear blade

Claims (4)

An apparatus main body provided with a pulling mechanism for pulling the loading plate in the horizontal direction, a load cell for measuring shear force, and a displacement meter for measuring shear displacement, a connection frame connected to a shaft body of the apparatus main body, and a connection frame A plurality of plate-like shearing blades are arranged at intervals of 10 to 20 mm on the lower surface of the loading plate, the longitudinal direction of the shearing blade intersects the tensile direction of the loading plate, and the cutting edge A surface shear strength measuring device for a viscous ground , wherein the length of the shear blade protrudes from the lower surface of the loading plate is 1 to 3 mm .   2. The apparatus for measuring the surface shear strength of a viscous ground according to claim 1, wherein the loading plate is made of aluminum. 3. The structure of the viscous ground according to claim 1 or 2 , wherein the loading plate tensioning mechanism has a structure capable of eliminating the moment generated when there is a distance between the resistance force generated at the cutting edge of the shearing blade and the point of action of the tensile force. Surface shear strength measuring device. Using a measuring device according to claim 1 to 3, the surface layer shear strength measuring method of the viscous land board, characterized by measuring the shear strength of the surface layer of cohesive soil.

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