An experimental device and method for simulating evolution of original fissures of over-consolidated expansive soil under unloading conditions

Abstract

This invention discloses an experimental apparatus and method for simulating the evolution of primary cracks in overconsolidated expansive soil under unloading conditions. The apparatus includes a main truss support system, a gravity loading / unloading and force transmission system, a specimen mold and visualization observation system, and a sensing and data feedback system. The specimen mold and visualization observation system includes a specimen mold containing a specimen. The gravity loading / unloading and force transmission system includes a Y-axis loading unit and an X-axis loading unit. The Y-axis loading unit applies axial pressure to the specimen inside the specimen mold; the X-axis loading unit applies lateral pressure to the specimen. Both the Y-axis and X-axis loading units are equipped with weight sets, and loading and unloading of the Y-axis and X-axis loading units are achieved by adding or removing weights. The sensing and data feedback system synchronously collects mechanical parameters and crack morphology data during the test. This invention solves the problems of poor long-term load stability, inability to observe crack evolution in real time, and difficulty in achieving continuous transformation of complex stress paths in existing triaxial testing systems.

Description

Technical Field

[0001] This invention relates to geotechnical engineering testing equipment, specifically to a testing device and method for simulating the evolution of primary cracks in overconsolidated expansive soil under unloading conditions. Background Technology

[0002] Expansive soils, rich in clay minerals such as montmorillonite and illite, exhibit significant swelling and shrinkage, fissures, and overconsolidation, resulting in extremely poor engineering properties. Slope instability caused by expansive soils is a global problem, having already caused numerous engineering disasters in the construction of infrastructure such as highways, railways, and water conservancy projects.

[0003] For excavated slopes of expansive soil, the formation process is essentially a process of unloading and stress release. The excavation method and speed directly correspond to the stress release path and rate in the soil. Inconsistent construction speeds and unloading rates inevitably lead to significant differences in the structure and mechanical response of expansive soil. Studies show that within the unloading rate range of 0.02 kPa / min to 2 kPa / min, the smaller the initial unloading rate, the greater the damage to the undisturbed expansive soil sample, and the more pronounced the initiation and propagation of microcracks. Furthermore, the unique overconsolidation of expansive soil results in greater lateral pressure than ordinary cohesive soil; the static earth pressure coefficient K0 is typically greater than 1.0, reaching 0.94 to 1.63 at a depth of 4 m. This leads to greater rebound expansion during unloading, resulting in more severe structural damage and strength reduction. Primary fissures in expansive soil are mostly located deep within expansive soil slopes. In their natural state, they are closed and exhibit a certain degree of continuity and dominance. These fissures are primarily generated during soil formation due to geological stresses caused by temperature, humidity, and uneven expansion effects. They only expand and develop during excavation or when significant slope deformation occurs. Landslides caused by primary fissures mainly manifest as the continuous expansion and gradual connection of these fissures within the slope body under the unloading action of excavation. Therefore, these landslides are larger in scale, have a distinct sliding surface, and possess greater destructive power.

[0004] Currently, experimental methods for studying the unloaded mechanical properties of expansive soil both domestically and internationally mainly rely on commercial stress path triaxial testing systems such as GDS. However, existing equipment has the following shortcomings:

[0005] (1) Conventional triaxial test systems use motor servo loading, which has poor load stability in long-term tests, and there are jitter and fluctuation problems, which cannot guarantee the long-term maintenance of constant stress state.

[0006] (2) In traditional triaxial tests, the specimen is wrapped in a rubber membrane in a sealed pressure chamber, making it impossible to observe the evolution of internal cracks in the soil, especially the original cracks in expansive soil, in real time during the unloading process. The acquisition of mechanical parameters and the changes in crack morphology cannot be synchronously correlated on the time axis, which restricts the in-depth understanding of the unloading damage mechanism of expansive soil.

[0007] (3) Existing equipment is difficult to flexibly realize the continuous and complex stress path from K0 consolidation and overconsolidation to undrained unloading shear on the same device, which limits the systematic study of the mechanical behavior of expansive soil under different overconsolidation ratios and stress path conditions.

[0008] Therefore, there is an urgent need to develop a special unloading test instrument for expansive soil that can simultaneously meet the requirements of constant gravity loading and unloading and has the function of visual monitoring of crack evolution. Summary of the Invention

[0009] Objectives of the Invention: The first objective of this invention is to provide a test apparatus for simulating the evolution of primary cracks in overconsolidated expansive soil under unloading conditions. This apparatus can solve the technical problem that existing equipment has poor load stability, vibration and fluctuation issues in long-term tests, and cannot guarantee the long-term maintenance of a constant stress state. The second objective of this invention is to provide a test method for simulating the evolution of primary cracks in overconsolidated expansive soil under unloading conditions. This method can solve the technical problem that existing equipment is difficult to flexibly realize the continuous and complex stress path from K0 consolidation and overconsolidation to undrained unloading shear on the same device.

[0010] Technical Solution: This invention provides an experimental device for simulating the evolution of primary cracks in overconsolidated expansive soil under unloading conditions, comprising a main truss support system, a gravity loading / unloading and force transmission system, a sample mold and visualization observation system, and a sensing and data feedback system; wherein, the sample mold and visualization observation system includes a sample mold containing an expansive soil sample; the sample mold is placed at the top center of the main truss support system; the gravity loading / unloading and force transmission system includes a Y-axis loading unit and an X-axis loading unit; the Y-axis loading unit is used to load the sample mold... Axial pressure is applied to the expansive soil sample inside the test chamber. The X-axis loading unit is divided into two groups, which are symmetrically arranged on the left and right sides of the test chamber. The X-axis loading unit is set on the main truss support system and is used to apply lateral pressure to the expansive soil sample inside the test chamber. Both the Y-axis loading unit and the X-axis loading unit are equipped with weight sets. The loading and unloading of the Y-axis loading unit and the X-axis loading unit are realized by adding or removing weights. The sensing and data feedback system is used to collect mechanical parameters and crack morphology data synchronously during the test.

[0011] The advantage of the above design using gravity loading with weights is that it can provide an extremely stable constant stress source. Loading and unloading are both achieved by adding or removing weights, which fundamentally eliminates the jitter and fluctuation problems of the motor servo system in long-term testing.

[0012] Furthermore, the Y-axis loading unit includes a Y-axis weight set, a Y-axis weight tray, a Y-axis gravity wheel, a Y-axis wire rope guide wheel, a Y-axis force-adding rod, a Y-axis guide wheel bearing seat, a first linear bearing, a Y-axis linear bearing seat, a bearing bracket, a first bearing seat, a Y-axis force-adding long shaft, a worm gear reducer, and a Y-axis thrust plate. The Y-axis force-adding rod is vertically positioned above the sample mold box, and its lower end is connected to the Y-axis thrust plate. The Y-axis guide wheel bearing seat is installed above the Y-axis linear bearing seat, and the Y-axis wire rope guide wheel is installed inside the Y-axis guide wheel bearing seat. The first linear bearing is coaxially positioned inside the Y-axis linear bearing seat, and the bearing bracket is connected to the side of the Y-axis linear bearing seat. The Y-axis force-adding long shaft is laterally arranged on one side of the worm gear reducer, and the Y-axis weight set is supported by the Y-axis weights. On the tray, the weight of the weights is transmitted through a steel wire rope to the Y-axis gravity wheel, driving the Y-axis to rotate. The Y-axis pulls the steel wire rope through a steel wire rope drum at its end. After the steel wire rope changes direction through the Y-axis guide wheel, it acts vertically downward on the Y-axis force rod, and then applies axial pressure to the upper surface of the expansive soil sample in the sample mold through the Y-axis thrust plate. The worm gear reducer is connected to the Y-axis thrust plate through a steel wire rope. In the initial state, the steel wire rope is taut, and the upward tension provided by the steel wire rope balances the downward gravity generated by the Y-axis weight set. During loading, the worm gear reducer starts and slowly releases the steel wire rope, gradually reducing the upward tension, thereby smoothly transmitting the downward driving force of the Y-axis weight set to the Y-axis thrust plate, achieving stable and controllable axial loading and preventing instantaneous impact.

[0013] Furthermore, the X-axis loading unit includes an X-axis weight set, an X-axis weight tray, an X-axis gravity wheel, an X-axis wire rope guide wheel, an X-axis force-applying rod, and an X-axis displacement reducer. The X-axis weight set is placed on the X-axis weight tray. The weight of the weights is guided by the wire rope through the X-axis wire rope guide wheel and the X-axis gravity wheel before acting on the X-axis force-applying rod. After being decelerated and buffered by the X-axis displacement reducer connected in series in the force transmission path of the X-axis force-applying rod, lateral pressure is applied to the expansive soil sample in the sample mold box. The speed ratio of the X-axis displacement reducer is not less than 40:1, which is used to achieve smooth force transmission and impact buffering during loading and unloading. The displacement changes caused by the increase or decrease of weights in the X-axis weight set are decelerated and transmitted to the end of the X-axis force-applying rod, ensuring the gradual nature of the load application and release process and avoiding disturbance of the overconsolidated expansive soil structure by sudden load changes.

[0014] Furthermore, the fixed end of the X-direction force-adding rod is provided with an X-direction force-adding rod retraction device, and the moving side of the X-direction force-adding rod is provided with a moving force-adding rod retraction device. The X-direction force-adding rod retraction device and the moving force-adding rod retraction device cooperate with each other to ensure the smooth application and release of X-direction side pressure during loading and unloading, and realize the smooth retraction and impact-free reset of the force-adding rod during loading and unloading.

[0015] Furthermore, the sample mold and visualization observation system includes a PIV image acquisition module. The sample mold has a transparent back panel window composed of a movable perspective plate. The PIV image acquisition module is positioned facing the transparent back panel window and captures the crack propagation and displacement field evolution on the sample surface in real time.

[0016] Furthermore, by adjusting the weight ratio of the weight groups in the Y-axis loading unit and the X-axis loading unit, the proportional loading and unloading of the static earth pressure coefficient K0 within the range of 1.0 to 1.5 can be achieved to simulate the K0 consolidation stress state of expansive soil under different overconsolidation ratios.

[0017] Furthermore, the sample mold is equipped with a precast crack template assembly and a crack surface filling material; the precast crack template assembly includes multiple thin plates inserted into the template at different angles, ranging from 0° to 90°; the crack surface filling material is a mixture of montmorillonite and test expansive soil, with a montmorillonite mass content of 25% to 75%, a moisture content of 30% to 38%, and a coating thickness of 2 to 5 mm; the precast crack template assembly and the crack surface filling material are used together to precast primary cracks with specific angles and spatial distributions containing montmorillonite filling material during the layered compaction process, in order to simulate the physical and mechanical properties of the grayish-white clay filling material on the primary crack surface in natural expansive soil.

[0018] Furthermore, the bottom of the sample mold box is provided with a movable main board, which is mounted on the main truss support system via a linear guide pair. The movable main board is driven by a motor, which is fixed to the main truss support system by a displacement reducer fixing frame. The output end of the motor is connected to the movable main board for transmission, driving the movable main board to move horizontally along the linear guide pair to ensure the degree of freedom of the sample during shear deformation.

[0019] Furthermore, the sample mold box is equipped with a pore water pressure sensor and a volume change measuring device. The pore water pressure sensor is installed at the center of the bottom plate of the mold box, and the volume change measuring device includes a precision measuring tube, which is connected to the filter stone water channel plate through a drainage pipe.

[0020] Based on the same inventive concept, the present invention provides an experimental method for simulating the evolution of primary cracks in overconsolidated expansive soil under unloading conditions, applied to the aforementioned experimental apparatus for simulating the evolution of primary cracks in overconsolidated expansive soil under unloading conditions. The method includes the following steps:

[0021] S1. Sample preparation: After drying, crushing and sieving, the expansive soil is soaked in water according to the target moisture content and compacted in layers in the sample mold box. After the specified layer is compacted, the mixture of montmorillonite and the test expansive soil is evenly applied to the compacted surface of the specified layer as the crack surface filling material. Then, the thin template of the crack prefabricated template assembly is inserted into the coating layer to form a crack surface with a set inclination angle. After the specified layer is compacted, the template is removed to form a prefabricated original crack containing montmorillonite filling material.

[0022] S2, K0 consolidation: By adjusting the load ratio of the weight group in the Y-direction loading unit to the weight group in the X-direction loading unit, the ratio of the axial pressure applied by the expansive soil in the Y-direction loading unit to the lateral pressure applied by the X-direction loading unit to the sample in the sample mold reaches the set K0 value, and the consolidation pressure is maintained until the pore water pressure is completely dissipated.

[0023] S3. Formation of overconsolidation state: For test schemes with an overconsolidation ratio greater than 1, the sample is first loaded to a preset high consolidation stress level. After consolidation is stable, the sample is unloaded to the target stress state according to the K0 ratio to form an expansive soil sample with a set overconsolidation ratio.

[0024] S4. Unloading damage stage: Keep the confining pressure constant, gradually reduce the axial pressure by decreasing the weights in the Y-axis loading unit until the axial pressure is unloaded to zero. At the same time, the sample mold and the PIV image acquisition module in the visualization observation system record the sample surface displacement field, strain field and crack evolution process in real time. The sensing and data feedback system collects mechanical parameter data synchronously.

[0025] S5. Direct Shear Stage: After unloading damage is completed, the moving main board at the bottom of the sample mold box moves horizontally, causing the lower box of the sample to generate horizontal displacement to achieve direct shear. The shearing speed is controllable until the sample fails. Throughout the process, the PIV image acquisition module and the sensing and data feedback system synchronously acquire mechanical parameters and crack morphology data, fully capturing the mechanical characteristics and crack evolution law of the entire process from unloading damage to shear failure of expansive soil.

[0026] Beneficial effects: Compared with the prior art, the significant technical effects of the present invention are as follows:

[0027] (1) Constant load and smooth loading and unloading: Both the Y-axis loading unit and the X-axis loading unit use the weight gravity loading method to provide an extremely stable constant load source. Loading and unloading are achieved by adding or removing weights. The stress state can be maintained for a long time without complicated servo feedback control, which is particularly suitable for the characteristics of long consolidation time and long test cycle of expansive soil. In the Y-axis loading unit, the worm gear reducer is connected to the Y-axis thrust plate through the wire rope. By slowly releasing the wire rope, the upward tension is gradually reduced, and the downward driving force of the Y-axis weight group is smoothly transmitted to the Y-axis thrust plate. Finally, a smooth axial pressure is applied to the upper surface of the expansive soil sample in the sample mold box. In the X-axis loading unit, the X-axis displacement reducer is connected in series in the X-axis force transmission path for smooth force transmission and impact buffering during the X-axis loading and unloading process. The two work together to ensure the gradualness of the load application and release process, avoid the disturbance of the over-consolidated expansive soil structure by sudden load, and fundamentally solve the problem of poor long-term load stability of conventional servo motor triaxial testing machine.

[0028] (2) Visual monitoring of the entire process of original cracks: By using a transparent back plate window made of a moving perspective plate combined with a PIV image acquisition module, the soil mechanical parameters (stress, strain, pore water pressure) and the geometric evolution of micro cracks (inclination angle, spacing, opening degree, spatial distribution) are synchronized on the time axis for the first time, breaking through the technical bottleneck that cracks are not visible in traditional triaxial tests.

[0029] (3) Accurate simulation of complex stress paths: The test method of the present invention can flexibly realize the continuous stress path from K0 consolidation and overconsolidation to undrained unloading shear and subsequent direct shear on the same test device. It is particularly suitable for simulating the mechanical behavior of expansive soil under different overconsolidation ratios (OCR=1, 2, 3, 4), and provides a reliable test platform for systematically studying the unloading damage effect of expansive soil under different overconsolidation ratios. Attached Figure Description

[0030] Figure 1 This is a schematic diagram of the overall structure of the present invention;

[0031] Figure 2 for Figure 1 Side view;

[0032] Figure 3 for Figure 1 The front view;

[0033] Figure 4 This is a three-dimensional perspective view of the main truss support system in this invention;

[0034] Figure 5 This is a three-dimensional view of the gravity loading / unloading and force transmission system in this invention;

[0035] Figure 6This is a frontal perspective view of the gravity loading / unloading and force transmission system in this invention;

[0036] Figure 7 This is a three-dimensional side view of the sample mold and the visualization observation system in this invention.

[0037] Figure 8 This is a top-view perspective view of the sample mold and the visualization observation system in this invention.

[0038] Figure 9 This is a schematic diagram of the force transmission path of the upper half of the Y-axis loading unit in this invention;

[0039] Figure 10 This is a schematic diagram of the force transmission path of the lower half of the Y-axis loading unit in this invention;

[0040] Figure 11 The evolution diagram of the shear strain field on the sample surface during the unloading process obtained from PIV monitoring;

[0041] Figure 12 This is a diagram showing the concentrated strain evolution in a localized region of the crack obtained from PIV monitoring.

[0042] Figure 13 The image shows the evolution of the horizontal (X-direction) displacement field on the sample surface during the unloading process, obtained from PIV monitoring.

[0043] Figure 14 This is a diagram showing the evolution of the vertical (Y-direction) displacement field on the sample surface during the unloading process, obtained from PIV monitoring. Detailed Implementation

[0044] The technical solution of the present invention will now be described in detail with reference to specific embodiments and accompanying drawings.

[0045] Example 1

[0046] like Figure 1-3 As shown, this invention provides an experimental device for simulating the evolution of primary cracks in overconsolidated expansive soil under unloading conditions. It is suitable for simulating the stress path evolution and stress-strain relationship during the excavation of expansive soil slopes, and for achieving visualized synchronous monitoring of the propagation and penetration behavior of primary cracks during unloading damage. The experimental device includes a main truss support system 100, a gravity loading / unloading and force transmission system 200, a sample mold and visualization observation system 300, and a sensing and data feedback system 400.

[0047] like Figure 1 and Figure 4As shown, the main truss support system 100 includes a main truss 101 and a wheel assembly 102 located at the bottom of the main truss 101. The main truss 101 is welded from Q235B structural steel with a rectangular tube cross-section to ensure the overall rigidity and stability of the equipment under heavy loads. The wheel assembly 102 includes four swivel casters (purchased parts, quantity 4), two of which have locking functions to facilitate the movement and positioning of the equipment in the laboratory and the fixing of samples during loading and unloading. The upper part of the main truss 101 is provided with a bearing plate 103 to support components such as bearing seats and reducer base plates on the Y-axis loading force transmission path; the side of the main truss 101 is provided with a Y-axis force-adding left frame 104, which serves as the reaction support frame for the entire Y-axis loading unit.

[0048] like Figure 5 and Figure 6 As shown, the gravity loading / unloading and force transmission system 200 includes a Y-axis loading unit and an X-axis loading unit. Both the Y-axis loading unit and the X-axis loading unit are equipped with weight sets. The loading and unloading of the Y-axis loading unit and the X-axis loading unit are realized by adding or removing weights.

[0049] The Y-axis loading unit is used to apply axial pressure to the expansive soil sample inside the sample mold 301. The Y-axis loading unit includes a Y-axis weight group 201, a Y-axis weight tray 202, a Y-axis gravity wheel 203, a Y-axis wire rope guide wheel 204, a Y-axis force-applying rod 205, a Y-axis guide wheel bearing seat 206, a first linear bearing 207, a Y-axis linear bearing seat 208, a bearing bracket 209, a first bearing seat 210, a Y-axis force-applying long shaft 211, a worm gear reducer 212, a reducer base plate 213, a second bearing seat 214, a bearing 215, a fixed central shaft 216, and a Y-axis thrust plate 217.

[0050] To further illustrate the specific connection methods and cooperation relationships of the various components of the Y-axis loading unit, the following is a summary: Figure 2 , Figure 9 and Figure 10 Let me introduce it. Figure 9 The process of force transmission is demonstrated by the weight of the Y-axis weight group 201, which drives the Y-axis force-adding shaft 211 to rotate after the steel wire rope passes over the Y-axis gravity wheel 203, and the steel wire rope is pulled by the steel wire rope drum at the shaft end of the Y-axis force-adding shaft 211; weight—steel wire rope—Y-axis gravity wheel—Y-axis force-adding shaft and steel wire rope drum at the shaft end. Figure 10 The demonstration shows the force transmission process where the wire rope pulled by the drum changes direction after being guided by the Y-axis wire rope guide wheel 204, and then acts vertically downward on the Y-axis force-applying rod 205, applying axial pressure to the upper surface of the sample inside the sample mold box 301 through the Y-axis thrust plate 217. (Example) Figure 10As shown, the force transmission path of the lower half of the Y-direction loading unit is: wire rope drum—wire rope—Y-direction wire rope guide wheel—Y-direction force-adding rod—Y-direction thrust plate—upper surface of the sample. The main truss 101 is welded from Q235B steel; the bearing plate 103 and the Y-direction force-adding left frame 104 are connected to the top and side of the main truss 101 by bolts, forming the supporting foundation of the Y-direction loading unit. On the support plate 103, the Y-axis guide wheel bearing housing 206 is installed above the Y-axis linear bearing housing 208, the Y-axis wire rope guide wheel 204 is installed inside the Y-axis guide wheel bearing housing 206, the first linear bearing 207 is coaxially arranged inside the Y-axis linear bearing housing 208, and the bearing bracket 209 is connected to the side of the Y-axis linear bearing housing 208; the bottom flange of each support component is fastened to the support plate 103 by bolts; the first linear bearing 207 is press-fitted into the inner hole of the Y-axis linear bearing housing 208, and the other support bearings are similarly installed into their corresponding bearing housings. The Y-axis force bar 205 passes through the first linear bearing 207 in the vertical direction and forms a clearance fit (sliding fit) with the first linear bearing 207, thereby achieving low-friction axial sliding; the lower end of the Y-axis force bar 205 is fixed to the Y-axis thrust plate 217 through a threaded connection (with a shoulder and a lock nut), and the Y-axis thrust plate 217 directly contacts the upper surface of the expansive soil sample in the sample mold box 301 to transmit axial pressure. The Y-axis force-adding long shaft 211 is arranged laterally on one side of the worm gear reducer 212. The Y-axis weight group 201 consists of several standard weights placed sequentially from bottom to top on the Y-axis weight tray 202. The weight tray 202 is connected to the end of the wire rope through a lifting ring. During operation, the gravity of the Y-axis weight group 201 is transmitted to the Y-axis force-adding long shaft 211 through the wire rope and the Y-axis gravity wheel 203, driving the Y-axis force-adding long shaft 211 and its shaft end wire rope drum to rotate. The wire rope pulled by the drum changes direction after being changed by the Y-axis wire rope guide wheel 204, and then acts vertically downward on the Y-axis force-adding rod 205, and finally applies axial pressure to the upper surface of the sample in the sample mold box 301 through the Y-axis thrust plate 217. The housing of the worm gear reducer 212 is connected to the reducer base plate 213 by bolts. The reducer base plate 213 is then fixed to the Y-axis force-adding left frame 104 (located on the upper part of the main truss 101) by bolts. The fixed central shaft 216 passes through the worm gear reducer 212 and is connected to the worm gear inside the reducer by a key and a lock nut. Bearings 215 (model F6006, specification 30×55×13) are inserted into both ends of the fixed central shaft 216 in sequence. The outer ring of the bearing 215 is interference-fitted with the inner hole of the second bearing seat 214, and the inner ring is clearance-fitted (rotational fit) with the fixed central shaft 216. The second bearing seat 214 is fixed to the Y-axis force-adding left frame 104 by bolts, thereby ensuring the stable support and axial positioning of the reducer itself.The worm gear reducer 212 is connected to the Y-direction thrust plate 217 via a steel wire rope. In the initial state, the steel wire rope is taut, and the upward tension it provides balances the downward gravity generated by the Y-direction weight set 201. During loading, the worm gear reducer 212 starts and slowly releases the steel wire rope, and the upward tension gradually decreases, thereby smoothly transmitting the downward driving force of the Y-direction weight set 201 to the Y-direction thrust plate 217, which is converted into axial pressure acting on the expansive soil sample in the sample mold box 301, achieving stable and controllable loading and preventing instantaneous impact.

[0051] In this embodiment, the first linear bearing 207 is model KCMH30, and the bearing 215 is model F6006 with specifications of 30×55×13. The Y-axis weight group 201 consists of cast iron standard weights, including multiple mass grades such as 50 g, 100 g, 200 g, 500 g, 1 kg, 2 kg, 5 kg, 10 kg, and 20 kg, with a minimum mass grade difference of 10 g. The total weight of the weights meets the loading requirement of a maximum axial pressure of 320 kPa. The Y-axis force-adding rod 205 is vertically arranged above the sample mold box 301, and its lower end is connected to the Y-axis thrust plate 217. The Y-axis force-adding long shaft 211 is arranged laterally on one side of the worm gear reducer 212 and is pulled by a wire rope drum at the shaft end. The worm gear reducer 212, model NMRV050 (purchased component), has a speed ratio (i.e., transmission ratio, referring to the ratio of input speed to output speed) of 40:1. It is used to adjust the upward pulling force acting on the Y-direction thrust plate 217 by slowly releasing the wire rope, so that the downward driving force generated by the Y-direction weight group 201 is transmitted to the Y-direction thrust plate 217 at an extremely low rate and in a gradual manner. This ensures the smoothness of the axial load application and release process and avoids disturbance to the over-consolidated expansive soil structure caused by sudden load changes. The housing of the worm gear reducer 212 is fixed to the Y-direction force-adding left frame 104 on the upper part of the main truss 101 via the reducer base plate 213. The fixed central shaft 216 passes through the worm gear reducer 212 and, together with the bearing 215, provides stable axial positioning for the reducer. The Y-axis force bar 205 is provided with high-precision linear guidance and support by the first linear bearing 207 (installed in the Y-axis linear bearing housing 208), the Y-axis guide wheel bearing housing 206, the bearing bracket 209, and the first bearing housing 210. The bearing bracket 209 and the first bearing housing 210 provide additional lateral constraints and support for the upper end of the Y-axis force bar 205. The Y-axis tension sensor 401 is installed at the end of the Y-axis force bar 205 via the Y-axis pressure sensor connector 402 to monitor the axial load acting on the sample in real time.

[0052] The X-axis loading unit is divided into two groups, which are symmetrically arranged on the left and right sides of the sample mold 301. The X-axis loading unit is set on the main truss support system 100 and is used to apply lateral pressure to the expansive soil sample inside the sample mold 301. The X-axis loading unit includes an X-axis weight group 218, an X-axis weight tray 219, an X-axis gravity wheel 220, an X-axis wire rope guide wheel 221, an X-axis force-applying rod 222, an X-axis force-applying rod retraction device 223, a moving force-applying rod retraction device 224, a motor 225, a displacement reducer fixing frame 226, and an X-axis displacement reducer 227. In this embodiment, the motor 225 is model C20807345. The X-axis weight set 218 is placed on the X-axis weight tray 219. The weight's gravity is guided by a steel wire rope through the X-axis steel wire rope guide wheel 221 and the X-axis gravity wheel 220, and then acts on the X-axis force-applying rod 222. After being decelerated and buffered by the X-axis displacement reducer 227 connected in series in the force transmission path of the X-axis force-applying rod 222, lateral pressure is applied to the expansive soil sample in the sample mold 301. The speed ratio of the X-axis displacement reducer 227 is also 40:1, which is used to achieve smooth force transmission and impact buffering during loading and unloading. The displacement changes caused by the addition or removal of weights in the X-axis weight set 218 are decelerated and transmitted to the end of the X-axis force-applying rod 222, ensuring the gradual nature of the load application and release process and avoiding disturbance of the overconsolidated expansive soil structure by sudden load changes. The X-direction displacement reducer 227 works in conjunction with the Y-direction worm gear reducer 212 to ensure the smooth synchronization of X-direction loading and unloading with the Y-direction. The X-direction force bar retraction device 223 is located at the fixed end of the X-direction force bar 222, while the movable force bar retraction device 224 corresponds to the movable side of the X-direction force bar 222. The X-direction force bar retraction device 223 and the movable force bar retraction device 224 work together to ensure the smooth application and release of X-direction lateral pressure during loading and unloading. The X-direction force bar 222 ultimately applies lateral pressure to the sample through the X-fixed mold box push plate 313 and the X-moving mold box push plate 314 (guided by the second linear bearing 315). It should be noted that the X-direction displacement reducer 227 is integrated within the force transmission path of the X-direction force bar 222. Figure 3 It is not labeled as an independent component, but its function corresponds to the Y-axis worm gear reducer 212.

[0053] The advantage of this invention, which uses gravity loading with weights, is that it can provide an extremely stable constant stress source. Loading and unloading are both achieved by adding or removing weights, which fundamentally eliminates the jitter and fluctuation problems of the motor servo system in long-term testing. At the same time, the slow release of the steel wire rope connected to the Y-axis thrust plate 217 by the Y-axis worm gear reducer and the buffering effect of the X-axis displacement reducer avoid the impact load of the weights when they are placed or removed from disturbing the soil sample structure, ensuring the gradual nature of the load application and release process.

[0054] It should be noted that the Y-axis weight group 201, Y-axis weight tray 202, X-axis weight group 218, and X-axis weight tray 219 are external accessories of the test apparatus and are placed on the ground outside the test bench during actual use. To clearly demonstrate the mechanical structure of the apparatus, Figure 2 and Figure 3 The aforementioned weight sets and weight trays are not shown in the figures. The Y-axis weight set 201 is connected to the Y-axis force-applying rod 205 via a wire rope, a Y-axis gravity wheel 203, a Y-axis force-applying long shaft 211 and a wire rope drum at the shaft end, and a Y-axis wire rope guide wheel 204. The X-axis weight set 218 is connected to the X-axis force-applying rod 222 via a wire rope, an X-axis wire rope guide wheel 221 and an X-axis gravity wheel 220, thus forming a complete gravity loading system.

[0055] The motor 225 is installed at the lower part of the main truss 101 in the main truss support system 100 via the displacement reducer fixing bracket 226. The output end of the motor 225 is connected to the moving main board 311 at the bottom of the sample mold box 301 for transmission, and is used to drive the moving main board 311 to move horizontally along the linear guide pair 312 to realize the controllable shear displacement loading in the direct shear stage and ensure the degree of freedom of the sample during the shear deformation process.

[0056] like Figure 3 As shown, from the front of the test apparatus, the sample mold 301 is clearly visible in the middle of the main truss 101. The main truss 101 has an X-direction wire rope force transmission system symmetrically arranged on its left and right sides, consisting of X-direction gravity wheels 220 and X-direction wire rope guide wheels 221. (The weight set and weight tray are located on the ground outside the test platform, not on the test bench.) Figure 3 (As shown in the figure), the transparent back panel window, which is composed of a movable perspective plate 310, faces the operator's side, facilitating the installation and focusing of the PIV image acquisition module.

[0057] Both the Y-axis weight set 201 and the X-axis weight set 218 include multiple standard weights of different mass grades, with a minimum mass difference of no more than 10 g, used to achieve fine adjustment of axial and lateral pressures within the range of 0 to 320 kPa. By adjusting the weight ratio of the Y-axis weight set 201 and the X-axis weight set 218, proportional loading and unloading can be achieved within the range of the static earth pressure coefficient K0 from 1.0 to 1.5, to simulate the K0 consolidation stress state of expansive soil under different overconsolidation ratios. For example, when simulating the initial consolidation state of K0=1.5, the confining pressure σr=120 kPa and the axial pressure σa=80 kPa can be set by placing weights of corresponding weights on the X-axis and Y-axis weight trays, respectively.

[0058] In this embodiment, the coupling working principle of the worm gear reducer 212 and the X-direction displacement reducer 227 with the gravity loading system is as follows: During loading, the weights are placed on the Y-direction weight tray 202 (or the X-direction weight tray 219). The gravity of the weights is guided by the wire rope through the wire rope guide wheel and the gravity wheel in sequence, and then acts on the corresponding loading rod. After being decelerated and buffered by the corresponding reducer, the sample is finally pressed downward by the Y-direction thrust plate 217 (Y-direction loading) or by the X-direction fixed mold push plate 313 and the X-direction moving mold push plate 314 (X-direction loading). During unloading, the weights on the weight tray are removed step by step. The speed ratio of both reducers is 40:1, which converts the larger input displacement into a smaller output displacement and amplifies the torque, making the pressurization and unpressurization process of the loading rod end on the soil sample more stable. After the weights are placed on the tray, the force is not immediately applied to the soil sample, but rather buffered and transmitted through the reducer. When the weights are removed during unloading, the stress release is also gradual, without abrupt changes. This is particularly important for expansive soils, which are sensitive to disturbance. The X-axis loading unit is also equipped with an X-axis force bar retraction device 223 and a moving force bar retraction device 224, which work in conjunction with the X-axis displacement reducer 227 to ensure smooth movement and impact-free resetting of the force bar during X-axis loading and unloading.

[0059] like Figure 7 and Figure 8 As shown, the sample mold and visualization observation system 300 includes a sample mold 301 and a PIV image acquisition module. The sample mold 301 contains an expansive soil sample.

[0060] The sample mold 301 is placed in the middle of the main truss support system 100. The sample mold 301 is a rectangular box structure with internal dimensions of length × width × height = 100 mm × 100 mm × 200 mm, suitable for plane strain testing conditions. The sample mold 301 is assembled from components such as a first mold back plate 302, a second mold back plate 303, a mold bottom plate 304, a fixed mold plate 305, a fixed mold guide plate 306, a movable mold guide plate 307, a movable frame fixing plate 308, a movable short shaft 309, a movable viewing plate 310, a movable main plate 311, a linear guide pair 312, an X-fixed mold push plate 313, an X-moving mold push plate 314, a second linear bearing 315, and a filter stone waterway plate 316. In this embodiment, the linear guide pair 312 is model GGB65AB, and the second linear bearing 315 is model FCMH25. The first mold box back plate 302 and the second mold box back plate 303 form a double-layer structure of the rear side wall of the mold box, providing high-rigidity constraint; the filter stone water channel plate 316 is located in the center of the mold box bottom plate 304 for sample drainage. The mold box bottom plate 304 and the filter stone water channel plate 316 form the bottom structure of the mold box, and the filter stone water channel plate 316 has the dual functions of supporting the sample and drainage. The fixed mold box plate 305, together with the fixed mold box guide plate 306, forms the upper box (fixed box); the movable mold box guide plate 307, the movable frame fixed plate 308, and the movable short shaft 309 form the movable frame of the lower box, and the lower box can achieve X-direction lateral pressure transmission under the guidance of the X fixed mold box push plate 313 and the X movable mold box push plate 314 via the second linear bearing 315. The movable viewing plate 310 is located on the opposite side of the back plate 302 of the first template (i.e., the operator's side). It is made of a 20 mm thick high-strength transparent acrylic sheet with a light transmittance of 93%. The inner surface is coated with a nano-level anti-fog coating to effectively prevent fogging of the viewing window in high humidity environments over a long period of time, thus avoiding the impact on the observation effect. The remaining side plates and bottom plate of the sample template 301 are made of high-strength stainless steel, and the inner wall is precision machined and treated with anti-corrosion measures.

[0061] The sample mold 301 has a movable main plate 311 at its bottom, which is mounted on the base of the main truss 101 in the main truss support system 100 via two linear guide rail pairs 312. The linear guide rail pairs 312 have an extremely low coefficient of friction, ensuring that the sample mold 301 can slide freely in the horizontal direction during the shear deformation process, effectively reducing the interference of mechanical friction on the test results. The X-fixed mold push plate 313 and the X-moving mold push plate 314 correspond to the opposite sides of the sample mold, respectively, and accurately transmit the lateral force output by the X-direction force bar to the sample via the second linear bearing 315. The direct shearing stage is driven by a motor 225, which is fixedly mounted on the main truss 101 via a displacement reducer mounting bracket 226. Its output end is connected to the movable main plate 311 and drives its horizontal movement.

[0062] The sample mold 301 has a transparent backplate window composed of a movable viewing plate. The PIV image acquisition module is positioned directly opposite the transparent backplate window composed of a movable viewing plate 310. The PIV image acquisition module captures the crack propagation and displacement field evolution of the sample surface in real time. The PIV image acquisition module includes a high-resolution industrial camera, a ring LED supplementary light source, and an image processing workstation. The high-resolution industrial camera has a resolution of 2448×2048 pixels and a frame rate of 50 fps. It is equipped with a fixed-focus macro lens and is fixed to the main truss 101 via a focusing bracket. The ring LED supplementary light source is installed around the camera lens, providing a uniform surface light source and eliminating the influence of shadows and reflections on image quality. The image processing workstation has built-in PIV particle image velocimetry software based on a cross-correlation algorithm, which can calculate the two-dimensional displacement and strain fields of the sample surface in real time, with a temporal resolution of 20 ms and a spatial resolution better than 0.1 mm.

[0063] The sample mold 301 is also equipped with a pore water pressure sensor and a volume change measuring device. The pore water pressure sensor is installed at the center of the mold base plate 304 (near the filter stone channel plate 316), with a range of -100 kPa to 500 kPa and an accuracy of 0.1 kPa. The volume change measuring device includes a precision measuring tube, which is connected to the filter stone channel plate 316 through a drain pipe, and the measuring tube has an accuracy of 0.01 mL.

[0064] The sample mold 301 is equipped with a precast crack template assembly, which includes multiple thin-sheet insert templates with different inclination angles ranging from 0° to 90°. The precast crack template assembly includes a set of stainless steel thin-sheet insert templates with different inclination angles, each 0.3 mm thick, with inclination angles of 0°, 15°, 30°, 45°, 60°, 75°, and 90°. The sample mold 301 is also equipped with crack surface filling material, which is a mixture of montmorillonite and test expansive soil, prepared by mixing montmorillonite and test expansive soil in a certain proportion. Natural expansive soil often contains grayish-white clay filling material in its original cracks, with higher water content and montmorillonite content than the soil on both sides, resulting in a crack surface strength much lower than the strength of the intact soil. This invention uses a mixture of montmorillonite and expansive soil to simulate the infill material. The montmorillonite content is controlled at 25% to 75% (approximately 30% recommended), the moisture content is controlled at 30% to 38% (approximately 32% recommended), and the coating thickness is 2 to 5 mm. A precast crack template assembly is used in conjunction with the crack surface filling material to precast primary cracks with a specific inclination angle and spatial distribution containing montmorillonite infill material during layered compaction. This simulates the physical and mechanical properties of the grayish-white clay infill material on the primary crack surface in natural expansive soil. Specifically, after compaction of a designated layer, the montmorillonite mixture prepared according to the specified ratio is evenly applied to the compacted surface of that layer using a scraper, ensuring consistent application height. Then, the precast crack template is inserted into the designated position of the applied layer, and the upper layer of soil is compacted. After compaction of this layer, the template is slowly and uniformly withdrawn along the insertion direction, thus forming a primary crack surface with a set inclination angle containing montmorillonite infill material in the soil sample, realistically simulating the physical and mechanical properties of the primary crack surface in natural expansive soil.

[0065] The sensing and data feedback system 400 is used to simultaneously acquire mechanical parameters and crack morphology data during the test. The sensing and data feedback system 400 includes a Y-axis tensile force sensor 401, a Y-axis compressive force sensor connector 402, an X-axis tensile / compressive force sensor 403, and a multi-channel data acquisition instrument. The Y-axis tensile force sensor 401 is installed at the end of the Y-axis force-applying rod 205 via the Y-axis compressive force sensor connector 402, and the X-axis tensile / compressive force sensor 403 is installed at the end of the X-axis force-applying rod 222. Both sensors have a range of 0 to 5 kN and an accuracy of 0.05% of full scale. The Y-axis tensile force sensor 401, Y-axis compressive force sensor connector 402, and X-axis tensile / compressive force sensor 403 are used to monitor the actual load acting on the soil in real time, ensuring that stress loss after the weight is applied through the lever / pulley system is compensated and calibrated. The multi-channel data acquisition instrument synchronously acquires signals from the tension / compression sensor, pore water pressure sensor, and volume change measurement device, with a sampling frequency of no less than 10 Hz. The data is then transmitted in real time to the image processing workstation via a USB interface, achieving time synchronization of mechanical parameters and fracture morphology data.

[0066] In this invention, the gravity loading system applies axial and lateral pressure to the sample via a weight set, a wire rope guide wheel, a gravity wheel, and a force-applying rod. Loading and unloading are achieved by adding or removing weights. A worm gear reducer is connected in series in the Y-axis loading path, and an X-axis displacement reducer is connected in series in the X-axis loading path. The speed ratio of both is not less than 40:1, which is used for smooth force transmission and impact buffering during loading and unloading, ensuring the gradual nature of load application and release, and avoiding disturbance to the overconsolidated expansive soil structure by sudden load changes. The sample mold is equipped with a transparent back panel window composed of a movable perspective plate. The PIV image acquisition module is positioned directly opposite the window to capture the crack propagation and displacement field evolution on the sample surface in real time. The sample mold is also equipped with a crack prefabrication template assembly and crack surface filling material. The filling material is a mixture of montmorillonite and the test expansive soil. In conjunction with the thin sheet template, the original cracks containing the filling material are prefabricated during the layered compaction process to simulate the physical and mechanical properties of the original crack surface of natural expansive soil. The bottom of the mold box is equipped with a movable main board and a linear guide rail pair, driven by a motor. The motor is fixed to the main truss via a displacement reducer bracket, and its output end is connected to the movable main board for transmission, driving the movable main board to move horizontally and thus moving the lower box to achieve direct shear testing. This invention solves the technical defects of existing triaxial testing systems, such as poor long-term load stability, inability to observe crack evolution in real time, and difficulty in achieving continuous transformation of complex stress paths. It is particularly suitable for simulating the unloading mechanical behavior of expansive soil slope excavation under different overconsolidation ratios.

[0067] It should be noted that: (1) in order to clearly show the mechanical structure of the device, Figure 2 and Figure 3 The Y-axis weight group 201, Y-axis weight tray 202, X-axis weight group 21, and X-axis weight tray 219 are not shown in the figure. The above four items are external accessories of the test device. In actual use, they are placed on the ground outside the test bench and connected to the Y-axis wire rope guide wheel 204, X-axis wire rope guide wheel 221, Y-axis gravity wheel 203, and X-axis gravity wheel 220 inside the device through wire ropes to transmit the weight of the weights to the force-adding rod; (2) The X-axis displacement reducer 227 is integrated inside the force transmission path of the X-axis force-adding rod 222. Figure 2 It is not labeled as an independent component. Its function corresponds to the Y-axis worm gear reducer 212, which is used to achieve smooth transmission of loads in the X-axis loading and unloading process; (3) Due to the limitations of view direction and scale, Figure 2 , Figure 3Components not separately indicated by lead lines include: X-axis tensile and compressive sensor 403, multi-channel data acquisition instrument, pore water pressure sensor, volume change measurement device, fracture prefabrication template assembly, PIV image acquisition module, high-resolution industrial camera, ring LED supplementary light source, and image processing workstation. Among the above components, the X-axis tensile and compressive sensor 403 is installed at the end of the X-axis force bar 222, the PIV image acquisition module is arranged facing the moving perspective plate 310, and the pore water pressure sensor, volume change measurement device, and fracture prefabrication template assembly are internal functional components and supporting tooling of the sample mold 301.

[0068] Example 2

[0069] This invention also includes a test method for simulating the evolution of primary cracks in overconsolidated expansive soil under unloading conditions. This method is applied to the test apparatus described in Example 1 for simulating the evolution of primary cracks in overconsolidated expansive soil under unloading conditions. The method includes the following steps:

[0070] S1. Sample preparation: After drying, crushing and sieving the expansive soil, water is added according to the target moisture content and the mixture is compacted in layers in the sample mold 301. After the specified layer is compacted, the mixture of montmorillonite and the test expansive soil is evenly applied to the compacted surface of the specified layer as a crack surface filler. Then, the thin template of the crack prefabricated template assembly is inserted into the coating layer to form a crack surface with a set inclination angle. After the specified layer is compacted, the template is removed to form a prefabricated original crack containing montmorillonite filler.

[0071] S2, K0 Consolidation: By adjusting the load ratio of the weight group in the Y-direction loading unit to the weight group in the X-direction loading unit, the ratio of the axial pressure applied by the expansive soil in the Y-direction loading unit to the lateral pressure applied by the X-direction loading unit to the sample in the sample mold 301 reaches the set K0 value, and the consolidation pressure is maintained until the pore water pressure is completely dissipated.

[0072] In step S2, by adjusting the load ratio between the Y-axis weight group 201 and the X-axis weight group 218, the ratio of the axial pressure transmitted by the Y-axis thrust plate 217 to the lateral pressure transmitted by the X-axis fixed mold box push plate 313 and the X-axis moving mold box push plate 314 reaches the set value K0, and the consolidation pressure is maintained until the pore water pressure is completely dissipated.

[0073] S3. Formation of overconsolidation state: For test schemes with an overconsolidation ratio greater than 1, the sample is first loaded to a preset high consolidation stress level. After consolidation is stable, the sample is unloaded to the target stress state according to the K0 ratio to form an expansive soil sample with a set overconsolidation ratio.

[0074] S4. Unloading Damage Stage: Maintain constant confining pressure, gradually reduce axial pressure by decreasing the weights in the Y-axis loading unit until the axial pressure is unloaded to zero. At the same time, the sample mold and the PIV image acquisition module in the visualization observation system 300 record the displacement field, strain field and crack evolution process of the sample surface in real time. The sensing and data feedback system 400 synchronously acquires mechanical parameter data.

[0075] In step S4, the axial pressure is gradually reduced by decreasing the weights in the Y-axis weight group 201 until the axial pressure is unloaded to zero. At the same time, the PIV image acquisition module records the displacement field, strain field and crack evolution process of the sample surface in real time through the transparent back plate window formed by the moving perspective plate 310. The multi-channel data acquisition instrument simultaneously acquires mechanical parameter data.

[0076] S5. Direct Shear Stage: After the unloading damage is completed, the moving main board 311 at the bottom of the driving sample mold 301 moves horizontally, causing the lower box of the sample mold 301 to generate horizontal displacement to achieve direct shear. The shearing speed is controllable until the sample fails in shear (the shear strain reaches 15%). Throughout the process, the PIV image acquisition module and the sensing and data feedback system 400 synchronously acquire mechanical parameters and crack morphology data, fully capturing the mechanical characteristics and crack evolution law of the entire process from unloading damage to shear failure of expansive soil.

[0077] In step S4, the motor 225 is started, and the output end of the motor drives the moving main board 311 to move horizontally along the linear guide pair 312, so that the sample lower box generates horizontal displacement to achieve direct shearing.

[0078] The effectiveness of the experimental method described in this invention will be verified through a specific embodiment below.

[0079] The following uses expansive soil from Nanyang, Henan Province as an example to illustrate the specific test methods and operating procedures of the test device of this invention.

[0080] The expansive soil samples used in the experiment were taken from the slope of a highway in Nanyang, Henan Province, at a depth of 4 to 5 m. The undisturbed soil samples were brownish-yellow. Its basic physical and mechanical parameters were: natural water content 26.1%, initial saturation 97.2%, natural density 1.98 g / cm³, preconsolidation pressure 160 kPa, and free swelling rate 58.5%, classifying it as a moderately expansive soil.

[0081] S1. Sample Preparation: The undisturbed expansive soil was dried, crushed, and sieved, then water was added at its natural moisture content and allowed to stand for 24 hours. The soil was compacted in five layers within sample mold 301, each layer being 40 mm thick. Before compacting the second and fourth layers, a mixture of montmorillonite and expansive soil (approximately 30% montmorillonite content and 32% moisture content) was evenly applied to the compacted surface using a scraper, to a thickness of approximately 5 mm, ensuring consistent application height. Then, a 0.3 mm thick, 45° inclined thin-sheet template was selected from the precast crack template assembly and inserted into the designated position of the applied layer before compaction. After compaction, the template was slowly and uniformly withdrawn along the insertion direction. Because the template is only 0.3 mm thick and its surface is polished, the disturbance to the soil on both sides of the crack surface is minimal when it is removed. After the template is removed, the montmorillonite filling material fills the crack space, forming a primary crack surface containing grayish-white filling material, realistically simulating the physical and mechanical properties of primary cracks in natural expansive soil. The compaction load of subsequent layers is applied uniformly through a rigid platen, with the compaction direction vertically downward. The normal pressure component on the already formed inclined crack surface is small and will not cause the crack to close again. The size of the sample after compaction is 100 mm × 100 mm × 200 mm.

[0082] S2. Sample Consolidation: After installing the sample mold 301, adjust the weight ratio in the Y and X directions. Taking K0=1.5 as an example, place weights corresponding to an axial pressure of 80 kPa on the Y-direction weight tray 202 in stages (applying pressure to the sample via the Y-direction force transmission system through the Y-direction thrust plate 217), and place weights corresponding to a confining pressure of 120 kPa on the X-direction weight tray 219 (applying pressure to the sample via the X-direction force transmission system through the X-direction fixed mold push plate 313 and the X-direction movable mold push plate 314). Keep the consolidation pressure constant and monitor the pore water pressure dissipation using a pore water pressure sensor. Consolidation is considered complete when the pore water pressure change rate is less than 0.5 kPa / h.

[0083] S3. Formation of Overconsolidation: The overconsolidation ratio (OCR) is defined as: OCR = pc / p0, where pc is the maximum consolidation pressure the soil sample has historically experienced (i.e., the axial effective stress during the initial high-pressure consolidation stage), and p0 is the current axial effective consolidation stress. Taking OCR=4 under consolidation conditions of K0=1.5 as an example, high-pressure consolidation is first carried out by increasing the axial pressure to 320 kPa and the confining pressure to 480 kPa (keeping σr / σa=480 / 320=1.5). After consolidation stabilizes, the load is simultaneously unloaded to axial pressure of 80 kPa and confining pressure of 120 kPa according to the ratio of K0=1.5. At this time, OCR=320 / 80=4. Similarly, by changing the initial high-pressure consolidation stress level and unloading to the same target stress (σa=80 kPa, σr=120 kPa), standard specimens with different overconsolidation ratios can be formed. The specific stress parameters corresponding to each overconsolidation ratio are as follows: When OCR=1, the initial consolidation axial pressure σa=80 kPa and confining pressure σr=120 kPa remain unchanged after unloading; when OCR=2, the initial consolidation axial pressure σa=160 kPa and confining pressure σr=240 kPa are unloaded to σa=80 kPa and σr=120 kPa; when OCR=3, the initial consolidation axial pressure σa=240 kPa and confining pressure σr=360 kPa are unloaded to σa=80 kPa and σr=120 kPa; when OCR=4, the initial consolidation axial pressure σa=320 kPa and confining pressure σr=480 kPa are unloaded to σa=80 kPa and σr=120 kPa. For isobaric consolidation (K0=1.0) conditions, the initial consolidation stresses corresponding to each OCR are σa=σr=80, 160, 240, and 320 kPa, respectively, and are all unloaded to σa=σr=80 kPa.

[0084] S4. Unloading Damage Stage: After consolidation, maintaining a constant confining pressure, the axial pressure is gradually reduced by progressively decreasing the weights in the Y-axis weight group 201 until the axial pressure is unloaded to 0. Throughout the unloading process, the PIV image acquisition module automatically captures images of the sample surface at set time intervals through the transparent backplate window formed by the moving perspective plate 310. The image processing workstation calculates the displacement and strain fields in real time. Simultaneously, the Y-axis tensile sensor 401, the X-axis tensile and compressive sensor 403, and the pore water pressure sensor continuously collect mechanical parameter data.

[0085] Figures 11 to 14 The results of the evolution of the displacement and strain fields on the sample surface monitored by the PIV system in a typical unloading test of the test device of the present invention intuitively show the whole process of crack initiation to penetration.

[0086] Figure 11 The sample surface shear strain field cloud map monitored by the PIV system during the unloading process is shown. Figure 11 Figure (a) shows the shear strain field contour map during the initial unloading stage. Figure 11 Figure (b) shows the shear strain field contour plot after unloading. Figure 11 As shown in Figure (a), in the initial stage of unloading, the shear strain distribution on the sample surface is relatively uniform, with only a small area of ​​strain concentration (color scale range approximately ±3×10⁻³) appearing near the pre-existing crack surface, indicating that microcracks are beginning to initiate; Figure 11 As shown in Figure (b), in the later stage of unloading, the color scale range is the same as in the early stage, but the spatial distribution range of the shear strain concentration zone is significantly expanded. The high strain region (red, positive strain) and the low strain region (blue, negative strain) have developed from local point-like concentration in the early stage to strip-like distribution throughout the sample. The strain gradient is significantly increased, corresponding to the stage of full expansion and mutual penetration of multiple cracks.

[0087] Figure 12 The results of high-resolution monitoring of localized fracture regions using the PIV system are presented. Figure 12 Figure (a) shows a schematic diagram of strain concentration in a local area of ​​a fracture during the fracture initiation stage. Figure 12 Figure (b) shows a schematic diagram of strain concentration in a localized area of ​​the crack during the crack propagation stage. Figure 12 Figure (c) shows a schematic diagram of strain concentration in a localized area of ​​the fracture during the fracture penetration stage. Figure 12 As shown in Figure (a), during the crack initiation stage, a weak strain concentration phenomenon occurs near the pre-fabricated crack surface; as... Figure 12 As shown in Figure (b), during the crack propagation stage, the strain concentration zone extends significantly along the crack surface, forming a clearly defined elliptical high-strain region; as Figure 12 As shown in Figure (c), during the fracture penetration stage, the strain concentration zone further develops and penetrates multiple pre-fabricated fracture surfaces, with the extreme strain value significantly increasing to the order of ±6×10⁻³. The above evolution sequence fully reveals the progressive failure process of expansive soil primary fractures under unloading damage, from initiation to expansion and penetration.

[0088] Figure 13 The evolution of the horizontal (X-direction) displacement field on the sample surface monitored by the PIV system during the unloading process is shown. Figure 13 Figure (a) shows a schematic diagram of the X-direction displacement field during the initial stage of unloading. Figure 13 Figure (b) shows a schematic diagram of the X-direction displacement field during the unloading process. Figure 13 Figure (c) shows a schematic diagram of the X-direction displacement field during the later stage of unloading. Figure 13 As shown in Figure (a), in the initial stage of unloading, the overall displacement in the X direction is small and relatively uniformly distributed; as Figure 13 As shown in Figure (b), during the mid-unloading stage, a significant positive displacement (red) appears on the left side of the specimen, while the right side shows a negative or weak displacement (blue), forming a differential displacement characteristic with the crack surface as the boundary; Figure 13As shown in Figure (c), in the later stage of unloading, the displacement difference in the X direction further increased, and the region with the maximum displacement gradient coincided highly with the direction of crack propagation. The displacement magnitude increased from 0.05 mm in the early stage to more than 0.3 mm.

[0089] Figure 14 This demonstrates the evolution of the vertical (Y-direction) displacement field on the sample surface monitored by the PIV system during the unloading process. Figure 14 Figure (a) shows the Y-direction displacement field during the initial unloading phase. Figure 14 Figure (b) shows the Y-direction displacement field before and during the unloading process. Figure 14 Figure (c) shows the Y-direction displacement field during the mid-term after unloading. Figure 14 Figure (d) shows the Y-direction displacement field at the end of unloading. Figure 14 As shown in Figure (a), in the initial stage of unloading, the displacement in the Y direction is uniform and of a small magnitude; as Figure 14 As shown in Figure (b), in the early to mid-stages of unloading, the top region of the specimen begins to show a sinking trend (warm red tone), while the bottom displacement is smaller; as Figure 14 As shown in Figure (c), in the mid-term after unloading, the difference in displacement in the Y direction increased significantly, and obvious uneven settlement formed between the upper and lower parts of the sample. The displacement gradient concentration zone corresponds to the crack development region; as shown in Figure (c), the displacement gradient concentration zone corresponds to the crack development region. Figure 14 As shown in Figure (d), at the end of the unloading period, the displacement field in the Y direction exhibits a clear stratification characteristic, with a maximum vertical displacement of 0.7 mm. Obvious expansion and rebound deformation can be seen on the sample surface, which fully reflects the mechanical nature of overconsolidation and rebound during the unloading process of expansive soil.

[0090] S5. Direct Shear Stage: Immediately after unloading damage is completed, a direct shear test is performed. The motor is started, and the motor output drives the moving main plate 311 at the bottom of the specimen mold 301 to move horizontally along the linear guide pair, causing horizontal displacement of the lower specimen box to achieve misaligned shearing. The shearing speed is controllable until the specimen fails (shear strain εa reaches 15%). PIV image acquisition and mechanical parameter data acquisition are performed continuously throughout the process, completely recording the data from unloading damage to shear failure.

[0091] Through the above steps, the testing instrument of this invention can simultaneously acquire the following data: stress-strain relationship curves under different overconsolidation ratios, pore water pressure-strain relationship curves, effective stress paths, and a time-synchronized sequence of images showing the entire process of fracture evolution (e.g., ...). Figures 11 to 14 (As shown). By comparing and analyzing the mechanical parameters with the geometric parameters of the cracks (inclination angle, spacing, opening, density) on a unified time axis, we can gain a deeper understanding of the initiation, expansion, and penetration mechanisms of primary cracks during the unloading damage stage, as well as the influence of different overconsolidation ratios on the mechanical properties of expansive soils.

[0092] The testing instrument of this invention can flexibly realize the following continuous stress path: K0 consolidation → overconsolidation state transition → passive compression unloading damage (constant pressure remains unchanged while axial pressure decreases) → direct shear test, which fully covers the stress path of expansive soil slopes from the original consolidation state to excavation unloading and even catastrophic failure.

[0093] The PIV visualization monitoring results of this invention ( Figures 11 to 14 The results show that the displacement and strain fields on the surface of the expansive soil sample exhibit significant spatiotemporal evolution characteristics during unloading: in the initial stage of unloading, both strain and displacement are small and relatively uniformly distributed; as unloading continues, strain concentration zones gradually form and develop along the direction of the pre-fabricated crack surface; in the later stage of unloading, the displacement field shows obvious zoning characteristics, with differential displacements forming on both sides of the crack surface, revealing the progressive failure mechanism of cracks from initiation to penetration. The above measured results are highly consistent with the theoretical analysis, verifying the reliability and effectiveness of the PIV visualization monitoring system of the present invention.

Claims

1. An experimental device for simulating the evolution of primary fractures in overconsolidated expansive soil under unloading conditions, characterized in that: It includes a main truss support system (100), a gravity loading / unloading and force transmission system (200), a sample mold and visualization observation system (300), and a sensing and data feedback system (400); wherein, the sample mold and visualization observation system (300) includes a sample mold (301), and the sample mold (301) contains an expansive soil sample. The sample mold (301) is placed in the middle of the main truss support system (100). The gravity loading / unloading and force transmission system (200) includes a Y-axis loading unit and an X-axis loading unit; the Y-axis loading unit is used to apply axial pressure to the expansive soil sample in the sample mold (301); The X-axis loading unit is divided into two groups, and the two groups of X-axis loading units are symmetrically arranged on the left and right sides of the sample mold (301). The X-axis loading unit is set on the main truss support system (100). The X-axis loading unit is used to apply lateral pressure to the expansive soil sample in the sample mold (301). Both the Y-axis loading unit and the X-axis loading unit are equipped with weight sets, and the loading and unloading of the Y-axis loading unit and the X-axis loading unit can be realized by adding or removing weights; The sensing and data feedback system (400) is used to collect mechanical parameters and crack morphology data synchronously during the test.

2. The experimental apparatus for simulating the evolution of primary fractures in overconsolidated expansive soil under unloading conditions according to claim 1, characterized in that: The Y-axis loading unit includes a Y-axis weight group (201), a Y-axis weight tray (202), a Y-axis gravity wheel (203), a Y-axis wire rope guide wheel (204), a Y-axis force-adding rod (205), a Y-axis guide wheel bearing seat (206), a first linear bearing (207), a Y-axis linear bearing seat (208), a bearing bracket (209), a first bearing seat (210), a Y-axis force-adding long shaft (211), a worm gear reducer (212), and a Y-axis thrust plate (217). The Y-axis force bar (205) is vertically positioned above the sample mold box (301), and the lower end of the Y-axis force bar (205) is connected to the Y-axis thrust plate (217); the Y-axis guide wheel bearing seat (206) is installed above the Y-axis linear bearing seat (208), and the Y-axis wire rope guide wheel (204) is installed inside the Y-axis guide wheel bearing seat (206); the first linear bearing (207) is coaxially positioned inside the Y-axis linear bearing seat (208), and the bearing bracket (209) is connected to the side of the Y-axis linear bearing seat (208); The Y-axis force-adding long shaft (211) is arranged laterally on one side of the worm gear reducer (212). The Y-axis weight group (201) is placed on the Y-axis weight tray (202). The weight gravity is transmitted to the Y-axis force-adding long shaft (211) through the steel wire rope via the Y-axis gravity wheel (203), driving the Y-axis force-adding long shaft (211) to rotate. The Y-axis force-adding long shaft (211) pulls the steel wire rope through the steel wire rope drum at the shaft end. After the steel wire rope changes direction through the Y-axis steel wire rope guide wheel (204), it acts vertically downward on the Y-axis force-adding rod (205). Then, axial pressure is applied to the upper surface of the expansive soil sample in the sample mold (301) through the Y-direction thrust plate (217); The worm gear reducer (212) is connected to the Y-direction thrust plate (217) via a wire rope. In the initial state, the wire rope is taut, and the upward tension provided by the wire rope is balanced with the downward gravity generated by the Y-direction weight group (201). During loading, the worm gear reducer (212) starts and slowly releases the wire rope, so that the upward tension gradually decreases, thereby smoothly transmitting the downward driving force of the Y-direction weight group (201) to the Y-direction thrust plate (217), achieving smooth and controllable axial loading.

3. The experimental apparatus for simulating the evolution of primary fractures in overconsolidated expansive soil under unloading conditions according to claim 1, characterized in that: The X-axis loading unit includes an X-axis weight group (218), an X-axis weight tray (219), an X-axis gravity wheel (220), an X-axis wire rope guide wheel (221), an X-axis force rod (222), and an X-axis displacement reducer (227). The X-axis weight set (218) is placed on the X-axis weight tray (219). The weight gravity is guided by the steel wire rope through the X-axis steel wire rope guide wheel (221) and the X-axis gravity wheel (220) and then acts on the X-axis force rod (222). After being decelerated and buffered by the X-axis displacement reducer (227) connected in series in the force transmission path of the X-axis force rod (222), it applies lateral pressure to the expansive soil sample in the sample mold box (301). The speed ratio of the X-direction displacement reducer (227) is not less than 40:

1. It is used to achieve smooth force transmission and impact buffering during the loading and unloading process. The displacement change caused by the increase or decrease of weights in the X-direction weight group (218) is transmitted to the end of the X-direction force rod (222) after deceleration, ensuring the gradual nature of the load application and release process.

4. The experimental apparatus for simulating the evolution of primary fractures in overconsolidated expansive soil under unloading conditions according to claim 3, characterized in that: The fixed end of the X-direction force bar (222) is provided with an X-direction force bar retraction device (223), and the movable side of the X-direction force bar (222) is provided with a movable force bar retraction device (224). The X-direction force bar retraction device (223) and the movable force bar retraction device (224) cooperate with each other to ensure the smooth application and release of X-direction lateral pressure during loading and unloading.

5. The experimental apparatus for simulating the evolution of primary fractures in overconsolidated expansive soil under unloading conditions according to claim 1, characterized in that: The sample mold and visualization observation system (300) includes a PIV image acquisition module. The sample mold (301) has a transparent back panel window composed of a movable perspective plate (310). The PIV image acquisition module is set facing the transparent back panel window. The PIV image acquisition module captures the crack propagation and displacement field evolution of the sample surface in real time.

6. The experimental apparatus for simulating the evolution of primary fractures in overconsolidated expansive soil under unloading conditions according to claim 1, characterized in that: By adjusting the weight ratio of the weight groups in the Y-axis loading unit and the X-axis loading unit, the proportional loading and unloading of the static earth pressure coefficient K0 is achieved within the range of 1.0 to 1.5, so as to simulate the K0 consolidation stress state of expansive soil under different overconsolidation ratios.

7. The experimental apparatus for simulating the evolution of primary fractures in overconsolidated expansive soil under unloading conditions according to claim 1, characterized in that: The sample mold (301) is equipped with a crack prefabrication template assembly and crack surface filling material; The prefabricated template assembly for the crack includes multiple thin-sheet insert templates with different inclination angles, ranging from 0° to 90°; The crack filling material is a mixture of montmorillonite and test expansive soil, with a montmorillonite mass content of 25% to 75%, a moisture content of 30% to 38%, and a coating thickness of 2 to 5 mm. The precast template assembly for fissures is used in conjunction with the fissure filling material to precast primary fissures with specific inclination angles and spatial distributions containing montmorillonite filling material during the layered compaction process, in order to simulate the physical and mechanical properties of the gray-white clay filling material on the primary fissure surface in natural expansive soil.

8. The experimental apparatus for simulating the evolution of primary fractures in overconsolidated expansive soil under unloading conditions according to claim 1, characterized in that: The sample mold box (301) is provided with a movable main board (311) at the bottom, and the movable main board (311) is installed on the main truss support system (100) through a linear guide pair (312); The movable motherboard (311) is driven by a motor (225). The motor (225) is fixed on the main truss support system (100) by means of a displacement reducer fixing frame (226). The output end of the motor (225) is connected to the movable motherboard (311) for transmission, driving the movable motherboard (311) to move horizontally along the linear guide pair (312) to ensure the degree of freedom of the sample during the shear deformation process.

9. The experimental apparatus for simulating the evolution of primary fractures in overconsolidated expansive soil under unloading conditions according to claim 1, characterized in that: The sample mold (301) is equipped with a pore water pressure sensor and a volume change measuring device. The pore water pressure sensor is installed at the center of the bottom plate (304) of the mold. The volume change measuring device includes a precision measuring tube, which is connected to the filter stone water channel plate (316) through a drainage pipe.

10. A test method for simulating the evolution of primary fractures in overconsolidated expansive soil under unloading conditions, characterized in that, This method is applied to the experimental apparatus described in claim 1 for simulating the evolution of primary fractures in overconsolidated expansive soil under unloading conditions. The method includes the following steps: S1. Sample preparation: After drying, crushing and sieving the expansive soil, water is added according to the target moisture content and the material is compacted in layers in the sample mold (301). After the specified layer is compacted, the mixture of montmorillonite and the test expansive soil is evenly applied to the compacted surface of the specified layer as the crack surface filling material. Then, the thin template of the crack prefabricated template assembly is inserted into the coating layer to form a crack surface with a set inclination angle. After the specified layer is compacted, the template is removed to form a prefabricated original crack containing montmorillonite filling material. S2, K0 consolidation: By adjusting the load ratio of the weight group in the Y-direction loading unit to the weight group in the X-direction loading unit, the ratio of the axial pressure applied by the expansive soil in the Y-direction loading unit to the sample in the sample mold (301) to the lateral pressure applied by the X-direction loading unit to the sample in the sample mold (301) reaches the set K0 value, and the consolidation pressure is maintained until the pore water pressure is completely dissipated. S3. Formation of overconsolidation state: For test schemes with an overconsolidation ratio greater than 1, the sample is first loaded to a preset high consolidation stress level. After consolidation is stable, the sample is unloaded to the target stress state according to the K0 ratio to form an expansive soil sample with a set overconsolidation ratio. S4, unloading damage stage: Keep the confining pressure constant, gradually reduce the axial pressure by gradually reducing the weights in the Y-axis loading unit until the axial pressure is unloaded to zero. At the same time, the sample mold and the PIV image acquisition module in the visualization observation system (300) record the sample surface displacement field, strain field and crack evolution process in real time. The sensing and data feedback system (400) collects mechanical parameter data synchronously. S5. Direct shear stage: After the unloading damage is completed, the moving main board (311) at the bottom of the driving sample mold (301) moves horizontally, so that the lower box of the sample mold (301) generates horizontal displacement to achieve direct shear. The shearing speed is controllable until the sample is sheared and fails. Throughout the process, the PIV image acquisition module and the sensing and data feedback system (400) synchronously acquire mechanical parameters and crack morphology data, and fully capture the mechanical characteristics and crack evolution law of the entire process from unloading damage to shear failure of expansive soil.

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