Considering the deficiency of traditional anchors, we propose a new type of inflatable controlled anchor system in this paper. The working mechanism and its structural composition of newly designed inflatable controlled device are discussed in detail. To investigate the performance and pull-out capacity of this new anchor system, a series of field tests were carried out under different inflation pressure conditions. By comparing these test results with those of traditional grouting anchors, a full-process constitutive model of anchor-soil interface is proposed to depict the pull-out characteristics of the inflatable controlled anchor. The results show that the ultimate bearing capacity of the inflatable controlled anchor is greater than that of the traditional grouting anchor when the inflation pressure is greater than 0.2 MPa and the ultimate bearing capacity of this new anchor improves obviously with the increase of inflation pressure. When the inflation pressure reaches 0.4 MPa, the ultimate bearing capacity of the inflatable controlled anchor is 2.08 times that of the traditional grouting anchor. Through comparison with the experimental curves, the results of model calculation indicate that the proposed anchor-soil interface constitutive equation can describe the pull-out characteristics of the inflatable controlled anchor. The designed controlled anchor has the advantages of no grouting, recyclability, rapid formation of anchoring force, and adjustable anchoring force.
The geotechnical anchoring technology, which is used to bury the tensile rod into soil, can improve its own strength and stability of the stratum, and prevent the disaster damage of the soil such as collapse, landslide and land subsidence during the construction process [
Geotechnical anchoring technology has been fully applied in deep foundation engineering, antifloating structure engineering, high-steep slope engineering, dam reinforcement engineering, tunnel engineering, and highway engineering because of its advantages of safety, economy, and effectiveness. In recent years, the application scope of the geotechnical anchoring technique increases gradually. For meeting requirements of different geological conditions, working environment, and bearing capacity, a variety of new type anchors were developed successively, such as the split-set anchor, the water swelling anchor, the flexible pressurized grouting anchor, the inflatable anchor, and the inflatable anchor with the end baffle [
To overcome the disadvantages of aforementioned anchors, an inflatable controlled anchor system, the methodology, and the testing device are described in this paper. By conducting the pull-out test of the proposed anchor under different inflation pressures, the performance and uplift capacity are discussed, and then a full-process constitutive model is developed to describe the mechanical properties of the anchor-soil interface.
The construction of the current inflatable anchor includes a rubber membrane fixed on the outside the hollow steel tube, which is shown in Figure
Current inflatable anchor. (a) The anchor placed in the drilled hole. (b) The expansion of anchor.
In consideration of the defects of the current inflatable anchor, the following improvements have been made in this study: (1) Two baffles are provided at the front and rear ends of anchor to restrict the inflation deformation of rubber membrane in the axial direction. (2) The rubber membrane is separated from the steel tube and has a self-inflatable pipeline with a middle hole. (3) The steel tube is no longer used as an inflation channel but as the tensile rod to transmit the anchoring tension.
The inflatable controlled anchor is provided with an expansion device in the anchored zone, and the anchored effect is achieved by frictional resistance between the outer surface of expansion device and soil wall of drilled hole. The proposed inflatable controlled anchor system is shown in Figure
Proposed inflatable controlled anchor.
In the anchored zone, the inflatable controlled anchor is provided with a squeezing device, in which a special expansion device can control and set the squeezing force dynamically. When the expansion device is controlled to expand radially by inflation pressure, the squeezing device is pushed and pressed against the soil wall of drilled hole. After the anchor system is pulled out, the friction between the soil wall and squeezing device is generated, and thus the anchored force system is formed by the force transmission device. When the anchoring target is achieved, the expansion device shrinks by decreasing inflation pressure in the control device, and the squeezing device is detached from the anchor hole wall. Thus, the anchor system in the hole can be recovered, which has great environmental protection value and economic benefit for temporary anchorage of foundation pits and other temporary projects.
To facilitate the construction and recovery of the anchor, the inflatable controlled anchor system is made of four separate parts: squeezing device, expansion device, force transmission device, and control device, so as to realize a distributed manufacture and an easy-to-assemble construction process.
The squeezing device is located in the outermost layer of the inflatable controlled anchor, and its main function is to provide an anchoring force according to the generated resistance contacted with the soil. It is the key work to choose a suitable squeezing device with certain rigidity and strength as well as good springback function. The steel has high stiffness and strength, which is the first choice material for squeezing device as it can withstand a large tensile force in anchoring section, so as to provide the high load-bearing capacity.
To ensure the expansion deformation capacity and springback performance of steel tube, the circular steel tube is equally divided into longitudinal strips as squeezing steel. When the steel tube is not subjected to the inflation pressure, the steel strips can be folded into a circular tube, which is shown in Figure
The section sketch of circular steel tube divided into two, four, and six parts.
Structural form comparison of steel strips.
Structural form | Disadvantage | Advantage | Testing result |
---|---|---|---|
The steel tube is divided into two equal parts longitudinally | Uneven expansion of squeezing device | The structure is simple, the processing is convenient, and the expansion is easy | Poor performance |
The steel tube is divided into four equal parts longitudinally | Slightly complicated structure | The processing is convenient, the expansion of steel strips is easy. The steel strips have large contact area with the soil wall | Good anchoring performance |
The steel tube is divided into six equal parts longitudinally | Difficult processing and construction | The expansion of steel strips is easy. The steel strips have large contact area with the soil wall | Complicated structure |
Considering the expansion property of steel strips, structure form, and manufacture processing, the four-section cutting structure, that is, the steel tube divided into four equal parts longitudinally, is applied as the squeezing device in the proposed anchor system (as shown in Figure
Squeezing device. (a) Steel strips. (b) Closure of steel strips.
Schematic diagram of two ends of steel strips. (a) Upper end. (b) Lower end.
The expansion device is designed in the innermost layer of the anchor system (as shown in Figure
Expansion device. (a) Rubber membrane. (b) Rubber membrane and steel strips.
To ensure the sealing, pressure resistance, large deformation, and other characteristics of the rubber membrane, the expansion device adopts butyl rubber as the material for expansion membrane. The rubber membrane used in this study is all made by the rubber factory.
The function of transmission device is to transfer load, and it should have higher tensile strength. As an excellent tensile material, the steel bar is the preferred material for the force transmission device. According to the ultimate bearing capacity tests of tensile force, the HRB335 (Hot-rolled Ribbed Bar) with a diameter of 8 mm is selected as the tension bar for force transmission device, which is shown in Figure
The tension bar.
The main function of control device is inflation and pressure relief of rubber membrane. The control pipeline is mainly composed of gas filled pipeline, valves, and quick connectors, which should have good pressure-bearing capacity and tightness. According to experimental research, PU tube (polyurethane) meets the requirements of the gas filled pipeline.
To ensure the air tightness of the connection between the rubber membrane and the gas filled pipe, a quick connector is used to connect the above two parts, which not only enables the anchor section and the control device to be processed independently but also can be quickly disassembled and assembled. The PU tube and the quick connector are shown in Figure
Control device. (a) PU tube. (b) The quick connector.
The main equipment used in field test include the following: (a) the anchor pull gauge and pressure gauge used to measure pulling force, (b) the dial gauge used to measure the displacement, (c) the counterforce frame used for anchor resistance, (d) the air compressor and its control device used to provide inflation pressure, and (e) the auxiliary equipment such as counterforce transferring plates and sleeve. The structure diagram of the field test model is shown in Figure
Schematic diagram of field test.
The field tests were conducted in the open ground of a proposed project located at Jingzhou City in Hubei Province. The soil hole is formed by the mechanical drilling method in advance, and then the anchor system is placed into the hole. To eliminate the test error, it is necessary to ensure that the hollow jack and the tension bar are on the same axis when the anchor bar are pulled out, which is also the requirement of Chinese regulations for supporting foundation pit of buildings.
The field test system is shown in Figure
Field test set-up.
According to the geotechnical survey result, the field site where the foundation pit is located belongs to the first-order terrace geomorphic unit of the Yangtze River. The surface of the site is a miscellaneous fill soil with a thickness of 1.2 m, the second soil layer is silty clay with a thickness of 5.63 m, and the third soil layer is silty sand with a thickness of 6.04 m. According to the geotechnical test method standard [
Physical and mechanical properties of soil layers.
Soil layer | Thickness (m) | Density (kN/m³) | Cohesion (kPa) | Internal friction angle (°) | Compression modulus (MPa) |
---|---|---|---|---|---|
Miscellaneous fill | 1.20 | 18.0 | 12.0 | 9.0 | — |
Silty clay | 5.63 | 19.1 | 18.4 | 7.9 | 4.5 |
Silty sand | 6.04 | 19.3 | 11.1 | 9.7 | 8.5 |
To test the effectiveness of the proposed inflatable controlled anchor, four cases of pull-out tests were performed with different inflation pressures. The depth and diameter of the soil hole are 2 m and 150 mm, respectively. The length, thickness and diameter of the squeezing device are 120 cm, 5 mm, and 127 mm, respectively. The length, thickness, and diameter of the rubber membrane corresponding to the expansion device are 100 cm, 5 mm, and 100 mm, respectively.
The field test procedure for the inflatable controlled anchor is set as follows: (1) Assemble the inflatable controlled anchor. (2) Position the soil hole and put the anchor in the design depth. (3) Install displacement gauges and jacks. (4) Connect the gas filled pipeline with the rubber membrane. (5) Set and record the initial value of each gauge. (6) Connect the gas source, and pressurize the gas to the designed pressure value. (7) Conduct the pull-out test. (8) Recycle the inflatable controlled anchor.
The assembly sequence of inflatable controlled anchor is carried out as follows: (1) Surround the cylindrical rubber membrane by four steel strips and fix the closed steel tube by rubber bands. (2) Connect the tension bars to the upper end of squeezing device (as shown in Figure
The field tests are carried out by multistage loading method. The load is applied every 3 minutes and each level of load is about 1/15 of the ultimate load. The data are recorded every minute after loading. When the reading displayed on the gauges is close to the preset value, the loading speed should be slowed down and the load should be stopped when the displacement continues to increase. The schematic diagram of the field test program setting of the inflatable controlled anchor is shown in Figure
Schematic diagram of field test program.
The pull-out tests of inflatable controlled anchor are carried out under different inflation pressure. The load-displacement curves are shown in Figure
The load-displacement curves under different inflation pressure.
According to ultimate bearing capacity and corresponding displacement of inflatable controlled anchor under different inflation pressure (shown in Table
Ultimate bearing capacity and corresponding displacement of inflatable controlled anchor.
Test number | Inflation pressure (MPa) | Ultimate bearing capacity (kN) | Residual bearing capacity (kN) | Ultimate displacement (mm) |
---|---|---|---|---|
1 | 0.1 | 10.82 | 9.32 | 18.61 |
2 | 0.2 | 19.83 | 17.45 | 29.87 |
3 | 0.3 | 29.46 | 27.50 | 32.68 |
4 | 0.4 | 40.98 | 36.02 | 41.43 |
From the perspective of the ultimate value of the pull-out capacity, the increment of the ultimate bearing capacity is 83.27%, 48.5%, and 39.10% corresponding to the previous level of inflation pressure, respectively. In addition, with the increase of the inflation pressure, the residual bearing capacity and ultimate displacement also increased obviously. For the ultimate displacement of pull-out tests, the increment of ultimate displacement is 60.51%, 9.41%, and 26.77% corresponding to the previous level of inflation pressure, respectively.
To verify the structure reasonableness of the inflatable controlled anchor, the test results of inflatable controlled anchor are compared with those of the traditional grouting anchor.
3 groups of traditional grouting anchor have been designed for pull-out tests at the same test site condition. The length of traditional grouting anchor is 2.0 m, in which the anchoring length is 1.2 m and free length is 0.8 m. The anchor body adopts the hollow steel pipe with diameter of 48 mm and thickness of 3 mm. The cement mortar with mixture ratio of 1 (cement) to 1 (sand) is selected as the grouting liquid for anchoring. When the cement mortar is cured to a coagulation period of 28 days, the traditional grouting anchors are carried out pull-out tests. The test device is shown in Figure
The measured load-displacement curves of grouting anchors are shown in Figure
The load-displacement curves of traditional grouting anchor.
In general, the curve characteristics of 3 grouting anchors are basically the same during the pullout stage: (1) When the pull-out force is small, the load-displacement curves are approximately linear. (2) After the pull-out force reaches the ultimate value, the load suddenly drops to a certain value and remains basically unchanged, and then the anchor quickly generates a large displacement, which is named as plastic stage.
The average ultimate bearing capacity of traditional grouting anchors is 19.7 kN. Compared with the inflatable controlled anchor, the ultimate bearing capacity of grouting anchors is less than that of inflatable controlled anchor if the inflation pressure is increased to 0.2 MPa. The ultimate bearing capacity of inflatable controlled anchor is 1.50 times and 2.08 times that of grouting anchor for the inflation pressure 0.3 MPa and 0.4 MPa, respectively.
Comparing Figure
According to the field test layout of inflatable controlled anchor shown in Figure
Before the pull-out test for inflatable controlled anchor, the inflation pressure is gradually increased, the rubber membrane expands and gradually contacts with the steel strips. When the inflation pressure reaches a certain value, the steel strips begin to contact with the soil wall of hole closely, and the diameter of the soil hole gradually enlarges. The essence of inflatable controlled anchor is a type of small enlarged end anchor [
Field tests show that the steel strips contacts closely with the wall of the soil hole when the inflation pressure is increased to 0.1 MPa, and the small enlarged diameter of the hole can be neglected. When the inflation pressure increases to 0.4 MPa, the enlarged diameter of soil hole is about 200 mm, increasing by 50 mm. Assuming
From the load-displacement curves of the inflatable controlled anchor shown in Figure
Division of deformation stage for inflatable controlled anchor.
The friction resistance of steel strips plays a decisive role in the ultimate pull-out capacity of the inflatable controlled anchor. The friction resistance is mainly related to the inflation pressure, the stress state of the soil, and the friction coefficient between the steel and the soil. To study the influence of inflation pressure and other factors on the ultimate bearing capacity of inflatable controlled anchor, a whole process constitutive relation of the anchor-soil interface is proposed in this study [
The load
The normalized curves
Comparison between calculated curves and measured curves under different inflation pressure. (a) 0.1 MPa, (b) 0.2 MPa, (c) 0.3 MPa, and (d) 0.4 MPa.
The relationship between the ultimate bearing capacity of anchor and the inflation pressure basically meets the linear relationship, which is shown in Figure
Relationship between bearing capacity and inflation pressure.
The residual shear strength
After the
Through the fitting analysis of the test results (shown in Table
Fitting results under different inflation pressure.
Inflation pressure (MPa) |
|
|
---|---|---|
0.1 | 0.8315 | 2.1209 |
0.2 | 0.7998 | 2.2306 |
0.3 | 1.0293 | 1.3912 |
0.4 | 1.6201 | 0.4418 |
Based on the field tests of an inflatable controlled anchor system, the following conclusions can be drawn: A new type of inflatable anchor system is designed and developed, which mainly includes four separate parts: squeezing device, expansion device, force transmission device, and control device. The expansion device is enclosed with the squeezing device, which can be dynamically controlled and set. The radial enlargement of expansion device pushes the squeezing device to contact with the hole wall tightly, and the friction resistance generated by the expansion device is transmitted directly to the anchor plate outside the hole, thus forming the anchorage system. The material selection, performance requirements, processing, and assembly for anchor system were determined, and then a complete set of inflatable controlled anchor was manufactured for field tests. The field test results show that the ultimate bearing capacity of the inflatable controlled anchor increases with inflation pressure and is greater than that of the traditional grouting anchor when the inflation pressure is greater than 0.2 MPa. When the inflation pressure is 0.3 MPa and 0.4 MPa, the ultimate bearing capacity of inflatable controlled anchor are 1.50 and 2.08 times that of grouting anchor, respectively. The bearing capacity of inflatable controlled anchor is composed of end resistance and friction resistance, and the friction resistance plays a decisive role. A full-process constitutive model for the interface between anchor and soil is proposed. The calculation results show that the calculated curve of the proposed model is in good agreement with the experimental curve and can reflect the actual deformation of the anchor-soil interface. After the anchorage target is achieved, the expansion device can shrink by unloading the gas pressure and then the squeezing device is separated from the wall of the anchor hole, so the steel strips and rubber membrane in the anchor hole can be recovered completely. A series of field tests prove that the inflatable controlled anchor has good engineering application potential. It not only has the advantages of simple construction and low price, but also has the advantages of recovery, quick formation of anchoring force, and adjustable anchoring force.
The data used to support the findings of this study are included within the article.
There are no conflicts of interest.
The authors gratefully acknowledge the support of the Natural Science Foundation of China (grant no. 51678066).