LBTJ-50 Digital Display Rock Mass Plane Push‑Test In‑situ Direct Shear Testing Apparatus

Product Introduction

This test is used to determine the in-situ shear‑fracture strength of the concrete–rock interface in dam structures, the interlayer bonding surfaces of roller‑compacted concrete, and the body of the roller‑compacted concrete itself. It complies with standards such as SL 264‑2020 and GB/T 50266‑2013 and is suitable for field direct shear tests.

Technical specifications:

1. Maximum load range: 0–100 t; Maximum displacement: 0–50 mm.
2. Hydraulic cylinders: 2 units
3. Displacement sensors: 4 units
4. Load-transfer bar: 1 piece
5. Meter bracket: 4 units
6. Shim plate and ball bearing array: 1 set
7. Pre‑test preparation and specimen fabrication shall comply with the following requirements:

1. On the structure, select representative locations and test levels; it is recommended to conduct these tests on a construction trial section or on the dam body itself. The area of the selected test zone shall be no less than 2 m × 8 m, and the test specimens shall be arranged at the same elevation, with a total of 4 to 5 specimens.

2. The shear area of each test specimen shall not be less than 500 mm × 500 mm, and the clear spacing between specimens shall not be less than 1.5 times the minimum dimension of the specimen.

3. The height of the specimen above the shear plane should be slightly greater than two-thirds of the specimen’s side length.

4. During the test setup, the direction of the horizontal thrust applied to the specimen’s surface shall be consistent with the direction of the structural load.

4. The excavation, preparation, and curing of test specimens shall be carried out in accordance with the following provisions:

1. At the time of core extraction, the specimen shall be at least 21 days old. The concrete surrounding the specimen within the test area shall be removed manually to prevent any disturbance to the specimen.

2. The excavation depth of the test specimen shall extend to the test plane; however, for test planes subjected to horizontal thrust, excavation shall proceed below the plane, leaving sufficient space for the installation of the jack. The jack must be precisely positioned, with its centerline parallel to the intended shear plane and at a distance from the shear plane not exceeding 5% of the specimen’s side length in the shear direction. The dimensional tolerances after excavation of the specimen should be ±20 mm, and the surface shall be smoothed with cement mortar having a strength comparable to that of the test block. When conducting shear tests on bedding planes, a shear joint approximately 10 mm wide shall be left around the perimeter of the shear plane.

3. After completing the excavation of the test specimen and the test area, the test pit shall be filled with water or backfilled with moist sand, and the specimen shall be properly cured and protected until, prior to the specified age, installation of the instrumentation can commence, at which point the backfill material may be removed. At the same time, measures must be in place to ensure that the specimen and its shear surfaces remain fully saturated with water.

5. The installation of the normal load system shall comply with the following requirements:

1. A layer of cement mortar shall be spread on the top of the specimen, and a bearing plate shall be placed thereon. The bearing plate should be gently tapped to ensure it is parallel to the designated shear plane. Alternatively, rubber or fine sand may be laid on the specimen’s top surface before positioning the bearing plate.
2. Place the roller stringer, bearing plate, jack, load‑transfer column, top bearing plate, and reaction device in sequence on the base plate.
3. When working in an open‑air site or when the tunnel roof cannot be used as a reaction support, the pile‑cap method or ground anchors may be employed as reaction devices; for relatively small normal loads, the ballast‑weight method can also be adopted.
4. After installation is complete, the jack may be activated to apply slight pressure, ensuring that the entire system is securely assembled.
5. All components of the normal‑load system shall be aligned on the same axis as the loading direction and shall be perpendicular to the designated shear plane. The resultant of the normal load shall pass through the centroid of the designated shear plane.
6. The normal-load system shall possess adequate strength and stiffness. When the shear plane is inclined or when the load system exceeds a specified height, the normal-load system shall be provided with support.
7. Before installation, the piston of a hydraulic jack should be operated through a partial stroke.

6. The installation of the shear load system shall comply with the following requirements:

1. When conducting a direct shear test using the push‑off method, a bearing plate coated with cement mortar shall be bonded to the loaded face of the specimen, and the bearing plate must be perpendicular to the intended shear plane. Behind the bearing plate, the load‑transfer block, the hydraulic jack, and another bearing plate should be arranged in sequence. Concrete (or mortar) shall be placed between the bearing plate and the reaction frame.

2. When affixing the shim to the loaded face of the test specimen, a 1 cm gap shall be maintained between the bottom of the shim and the shear plane.

3. When installing the shear‑load jack, the shear direction shall be aligned with the intended thrust direction, and the projection of its axis onto the shear plane shall pass through the center of the designated shear plane. For the flat‑push method, the axis of the shear load shall be parallel to the designated shear plane, and the distance between the axis and the shear plane shall not exceed 5% of the side length of the specimen in the shear direction.

7. The installation of the measurement system shall comply with the following requirements:

1. The dial gauge bracket used for measuring absolute displacement during installation shall be securely mounted on a support point, and the support point of the bracket shall be located outside the zone affected by deformation.

2. The measuring instruments shall be mounted on the support using magnetic dial stands. Shear and normal displacement transducers shall be installed at symmetrical locations on the test specimen, with no fewer than two transducers of each type.

3. As required, dial gauges may be installed between the test specimen and the base to measure the relative displacement of the specimen.

4. All measuring instruments and markers shall be oriented so as to be either perpendicular or parallel to the designated shear plane, respectively.

5. The cement mortar and concrete poured during installation shall be cured.

8. The test preparation shall include the following items:

1. The applied loads at each stage and the corresponding pressure gauge readings shall be calculated based on the hydraulic jack’s calibration curve and the shear area of the test specimen.

2. The operating status of each measuring instrument shall be checked, and the initial reading shall be recorded.

9. The method for applying normal loads shall comply with the following requirements:

1. A different normal load shall be applied to each specimen, with each load being an equal fraction of the maximum normal load. The maximum normal stress on the shear plane shall not be less than the specified normal stress.

2. For each specimen, the normal load should be applied in 1 to 3 increments, with the number of increments determined by the magnitude of the normal stress and the lithology.

3. Loading shall be controlled by time: a load increment shall be applied every 5 minutes, and the normal displacement at each load level shall be measured immediately after loading; a second measurement shall be taken after 5 minutes before proceeding to the next load increment. Once the predetermined load has been reached, measurements shall be taken every 5 minutes. When the difference between two consecutive measurements of normal displacement does not exceed 0.01 mm, shear loading may be initiated.

4. During the shearing process, the normal stress should be kept constant at all times.

10. The method for applying shear loads shall comply with the following requirements:

1. Prior to applying the shear load, the shear‑load system and the measuring instruments shall be inspected, and adjusted if necessary.

2. The load shall be applied in 8 to 12 increments, corresponding to the estimated maximum shear load. When the shear displacement increases significantly under the applied shear load, the number of load increments may be appropriately increased.

3. The application of shear loading shall be governed by time control. A new load level shall be applied every 5 minutes, and each displacement transducer shall be read both before and after the application of each load level. As the specimen approaches failure, the load variations and corresponding displacements must be closely monitored and recorded; load and displacement measurements shall be taken simultaneously.

4. After the specimen is sheared, the shear load shall continue to be applied until a stable shear load value is obtained.

1. The shear load is slowly unloading to zero, and the specimen’s elastic recovery is observed; at this point, the direct shear test is concluded. During the unloading of the shear load to zero, the normal stress must remain constant.
2. As required, after the shear‑failure test is completed, the normal stress may be kept constant while the testing apparatus and measuring instruments are adjusted. Shear (friction) tests shall then be conducted along the shear plane in accordance with paragraphs 2 through 6 of this article. The shear load may be applied in increments based on the final stable value obtained during the shear‑failure test.
3. After the shear test is completed, repeated friction tests—specifically, single-point friction tests—may be conducted at various normal loads as required.

11. During the testing process, provide detailed descriptions and records of the operating conditions of the loading equipment and measuring instruments, any unusual noises observed during the test, and any signs of loosening or spalling in the test specimens and surrounding rock mass, as well as the development of cracks.

12. Upon completion of the test, the equipment shall be promptly dismantled. After clearing the test site, the specimens shall be turned over, and the shear surface shall be described. The description of the shear surface shall include the following:

1. Measure the shear surface area.

2. The failure characteristics of the shear plane, as well as the distribution, orientation, and length of the striations.

3. The location and area of localized shear failure within the rock mass or concrete specimen.

4. The nature and distribution of clastic material on the shear plane.