Abstract
This paper describes an investigation into the performance and pullout capacity of a new inflatable anchor system embedded in deep excavation engineering. The proposed inflatable recyclable anchor system consists of a rubber pneumatic bag, movable steel plate, and force transmission bar, which can all be recycled after use. By conducting a number of field pullout tests, it is found that the pullout bearing capacity of the proposed anchor is related to the inflation pressure, the length of the rubber pneumatic bag, and the embedment depth. It is found that, with the exponential increase of inflation pressure or length of the rubber pneumatic bag, the pullout bearing capacity of the novel inflatable anchor increases exponentially. The proposed inflatable recyclable anchor can meet the bearing requirements of traditional grouted anchors. Moreover, the proposed anchor not only has better supporting effects than traditional grouted ones, but it also has the advantages of recyclability, repeated use, and rapid formation of anchorage force. It is a new green and low-carbon anchorage, so it will have good application prospects in temporary foundation pit slope engineering.
1. Introduction
With the development of society, human beings have increasingly developed and utilized the underground space. For a long time, deep foundation pit supporting technology has always been a hot issue in the engineering and academic community. Due to its advantages in safety, economy, and effectiveness, bolt anchoring technology has been widely used in deep excavation engineering [1–3], antifloating structure engineering [4], slope engineering [5], dam reinforcement engineering [6], tunnel engineering [7], and so on. Foundation pit support is a temporary project. The pullout capacity of the anchor is generated by the friction between the anchor body and the rock and soil mass. The anchor body is composed of reinforcement rod material and a grouting agent. Common grouting agents are cement slurry [8], resin syrup [9], and kaolin slurry [10]. Once the foundation pit construction is complete, the anchor bars left behind in the rock and soil will lose their use value, and these anchorages will cause many adverse effects on the subsequent development and utilization of the underground space, for example, negative effects such as increasing the cost of construction, affecting a project, or more seriously, causing a project to be abandoned and redesigned. Therefore, research and development of novel anchors that can replace traditional grouting anchors is very necessary; the recyclable anchor is one of these alternative replacements.
Since TF Herbst proposed the application of recyclable anchor rods in underground engineering [11], various types of recyclable anchors have been rapidly developed. One of them is the screw recyclable anchor [12, 13]. In their research, the rigid anchor body is made into a spiral type, and a specific tool or the rod body itself can be used to recycle the rigid anchor body. The second type is the rotary recyclable anchor proposed by Wang et al. [14], Kramadzhyan et al. [15], and others. In this type of anchor, the anchor end of the inflation anchor is made into a U-shaped lock. When a special rod body is installed and externally forced, the U-shaped lock is unlocked, and the inflation anchor body is recycled. The third type is an umbrella-shaped anchor proposed by Mei et al. [16], Zhu et al. [17], and others. In this kind of anchor, the anchor body is designed as an umbrella-like structure that can be extended and closed. When the auxiliary rod body is extended, it provides an anchoring force. When the anchoring force is released, the rod body is recycled. Although the above three representative types of anchors solve the problem of anchor recovery, there are still many problems in the application of practical deep foundation pit support engineering. For example, the screw recyclable anchor has a limited pullout bearing capacity and application range, even though it has good supporting capacity and recyclability. In the rotary anchor, although the metal bar can be recovered, grouting stones are still left in the soil, which can affect subsequent construction. The umbrella-shaped anchor is often difficult to recycle due to too many pricks in the soil. Based on the above considerations and the research done by Newson et al. [18, 19], Liang et al. [20, 21], and Cao et al. [22, 23]on inflatable anchors, this study proposes a novel inflatable recyclable anchor [24]. Compared with the inflatable anchors of the others, the main differences between the two anchors are as follows. (1) The structure of the anchors is different. The anchor studied by others consists of a cylindrical steel tube around which is fastened to a rubber membrane. The anchor has a simple structure and a small size. The new type of the inflatable anchor is composed of a special anchoring section and a free section, which is more in line with the characteristics of the engineering anchor. (2) The bearing performance of the anchors is different. The pullout bearing capacity of the former inflatable anchor is provided by the friction between the rubber bladder and the soil. And the pullout bearing capacity of the new type of the anchor is provided by the combination of the steel sheet and the soil.
By conducting a number of field pullout tests, the characteristics of the pullout bearing capacity of the proposed anchor are investigated, and the possibility of its application in practical engineering is explored.
2. Experimental Studies
2.1. Materials and Properties
The experiment was conducted in the open ground of a proposed project located at Yangtze University. The soil profile of the test site is shown in Figure 1: the surface of the site is a plain filling soil with a thickness of 0.8 m, the second layer of soil is silt clay with a thickness of 2.6 m, the third layer is clay with a thickness of 3.5 m, and the fourth layer is silt fine sand with a thickness of 4.6 m. Before the experiment, the physical mechanical indexes of the two soil layers are measured according to the SL237-1999 code [25]. The filling soil does not have stable properties, and its gravity density is . The silt clay was tested by means of a laboratory direct shear test and was found to have the following properties: gravity density , natural water content , , cohesive force = 20.5 kPa, and effective angle of friction = approximately 9°.
Figure 2 shows a field test picture of the proposed inflatable recyclable anchor. Before conducting the pullout test of the anchor, a hole drilling machine was used to form the hole in advance, and concrete bearings were made on both sides of the hole. During the test, the anchor bar was placed vertically into the hole, and the reaction force device, the inflator, and the loading system were installed in sequence. The inflation pressure was provided by an air compressor, and a microcomputer controlled the inflation rate and the regulated time. The loading method was graded loading. After applying every 2 kN load and stabilizing the pressure for 3 minutes, the data of the pullout bearing capacity and displacement were recorded. When the displacement continues to increase and the pullout force of the anchor is stable, the loading was completed. The air pressure was gradually decreased to 0, and the inflation anchor was recovered.
2.2. An Inflatable Retrievable Anchor
An inflatable recyclable anchor consists of a rubber pneumatic bag, a movable steel plate, and a dowel bar, as shown in Figure 3. The rubber pneumatic bag and movable steel plate constitute the anchoring part of the inflatable recyclable anchor. As shown in Figure 4, the anchoring part of the anchor is wrapped by a steel plate on the outside of the rubber balloon, and the steel plate material is a seamless steel tube with an outer diameter of 114 mm and a wall thickness of 5 mm. The seamless steel pipe is first cut into four equal parts and then combined into an integral body by an upper/bottom bracket. The rubber pneumatic bag is a cylindrical bladder made of high-quality butyl rubber material with an outer diameter of 100 mm and a wall thickness of 2 mm. The rubber pneumatic bag is protected by upper and lower end brackets and 4 steel plates. When inflated, the airbag inflates and drives the steel plates into contact with the soil and thus generates an anchorage force. The dowel bar is the free part of the inflatable recyclable anchor, and the dowel bar is an HRB400 threaded steel rod with a diameter of 18 mm. For convenience of recycling, a dowel bar is cut to a length of 1 m per section and is connected through the screw socket. At the top of the anchor hole, a special changeover screw socket is used to convert the four dowel bars into one pull rod.
2.3. Test Sequence
(i)Use the anchor drill to make holes with a diameter of 120 mm.(ii)Assemble the inflation anchorage section. The cylindrical airbag is first surrounded by four steel plates. The upper end bracket is fixed by a metal cuff. The lower end bracket is tightened by a movable puller wire, and the airbag inflation pipeline is led out.(iii)Assemble the free section of the inflation anchor. Connect the transmission reinforcement bar to the upper end bracket of the anchoring end, then use a threaded sleeve to lengthen the rod according to the hole depth, and then connect the four transmission rod members and one tension reinforcement bar through the conversion sleeve.(iv)Place the assembled inflatable anchor into the anchorage hole.(v)Install the metal reaction beam, pullout instrument, lifting jack, data acquisition instrument, and so on.(vi)Inflate the rubber pneumatic bag according to the preset pressure value.(vii)Conduct the pullout test, and collect the data of the pullout bearing capacity and the displacement of the anchor.(viii)Recycle the inflation anchor. First, discharge the gas in the airbag, then gradually dismantle the data acquisition system, loading system, and counterforce system, and then pull out the whole gas-filled anchor rod from the anchor hole, and pull the pulling rod and the force transmission rod out in order.
3. Experimental Results
Through a series of field tests, the anti-pull bearing performance of the inflatable retractable anchor rod was studied and the possibility of its engineering application was also discussed. Table 1 shows changes in the relevant parameters of the test.
3.1. Effect of Inflation Pressure (P)
Figure 5 shows the effect of varying the inflation pressure on the pullout resistance of the inflatable retrievable anchor. For this series of tests, the embedment depth (H) was 2000 mm, the rubber pneumatic bag length (L) was 1000 m, and the inflation pressure (P) varied from 100 kPa to 400 kPa. It can be seen from Figure 5 that the pullout resistance of the anchor gradually increases with the inflation pressure, and the load-displacement curve shows changes from the elastic phase to the plastic phase and then to the ductile phase. When the inflation anchor reaches the ultimate pullout bearing capacity, the displacement of the inflation anchor continues to increase, but the residual pullout bearing capacity of the inflation anchor is basically stable at 0.85 to 0.95 times the limit value. When the inflation pressure increases from 100 kPa to 400 kPa, the ultimate pullout bearing capacity of the anchor increases from 10.82 kN to 40.98 kN, an increase of approximately 3.8 times. The residual pullout capacity increases from 9.32 kN to 36.01 kN, an increase of approximately 3.9 times. When reaching the ultimate pullout capacity, the corresponding displacement value increases from 7.187 mm to 26.587 mm, an increase of approximately 3.7 times.
Further analysis of the relationship between anchor pullout bearing capacity and inflation pressure is conducted. As shown in Figure 6, both the ultimate pullout capacity and the residual pullout capacity vary linearly with the inflation pressure.
3.2. Effect of Embedment Depth (H)
Figure 7 shows the relationship curves of the pullout bearing capacity and displacement of the inflatable recyclable anchor for various depths. For this series of tests, the inflation pressure (P) was 300 kPa, the rubber pneumatic bag length (L) was 1000 m, and the embedment depth (H) varied from 1.5 m to 3.0 m. As shown in Figure 7, as the embedment depth of the inflation anchor increases, the pullout bearing capacity gradually increases. When the embedment depth of the inflation anchor is 1.5 m, the ultimate and residual pullout bearing capacity of the anchor bar are 20.20 kN and 17.31 kN, respectively. When the embedment depth of the inflation anchor increases to 2.0 m and to 3.0 m, the ultimate pullout bearing capacities of the anchor bar increase to 29.46 kN and 38.01 kN, corresponding to increases of 46% and 29%, respectively. However, the residual pullout bearing capacities of the anchor bar increase to 27.50 kN and 31.52 kN, corresponding to increases of 59% and 15%, respectively.
It can be seen that, with each incremental increase of the embedment depth, the relative increase in bearing capacity of the inflation anchor is gradually reduced. This may be explained given the following two aspects. On the one hand, it is a consequence of critical burial depth. When the embedment depth of the inflation anchor is 1.5 m, the soil layer over the anchorage section has a thickness of only 0.3 m. When the soil around the anchor hole expands and is compressed, shear failure of the soil body occurs easily. On the other hand, it may be related to the soil property of the anchorage section. When the depth is 1.5 m, the upper soil of the anchorage section is in the plain fill layer with a thickness of 0.5 m, and the lower section with a thickness of 0.7 m is in the silty clay layer. While at the depth of 2.0 m and 3.0 m, the anchorage sections are all located in the silty clay layer.
3.3. Rubber Pneumatic Bag Length (L)
Figure 8 shows the relationship curves of the pullout bearing capacity and displacement of the inflatable recyclable anchor for various lengths. For this series of tests, the embedment depth (H) was 2000 mm, the inflation pressure (P) was 300 kPa, and the rubber pneumatic bag length (L) varied from 250 mm to 1000 mm. When the length of airbag is 250 mm, the ultimate and residual pullout bearing capacities of the anchor bar are 8.64 kN and 7.39 kN, respectively. While the length of airbag increases to 500 mm and 1000 mm, the ultimate pullout bearing capacities of the anchor bar increase to 15.34 kN, and 29.46 kN, respectively; the residual pullout bearing capacities of the anchor bar increase to 13.88 kN and 27.50 kN. The displacements corresponding to the ultimate pullout bearing capacity are 14.438 mm, 15.843 mm, and 17.011 mm, respectively. Therefore, it can be observed that when the length of airbag increases by 4 times from 250 mm to 1000 mm, the ultimate and residual pullout bearing capacities of the anchor bar increase by approximately 3.4 and 3.7 times, but the corresponding displacement increases by only 2.5 mm.
3.4. Grouted Anchors
Pullout tests of conventional grouted anchor rods were carried out at the same site. The grouting anchor rod is a rebar steel with a diameter of 18 mm, and the preforming hole has a diameter of 120 mm and a depth of 2 m. The anchoring end has a length of 1.2 m and is located in a silty clay layer. The grouting material is selected as cement mortar with a water-cement ratio of 1 : 0.5, and the pullout tests are performed after 28 days. The pullout bearing capacity-displacement curves of the three grouted anchor rods are shown in Figure 9. Taking the average of the pullout resistance of the three grouted anchor rods, the ultimate pullout bearing capacity is 19.73 kN, and the corresponding displacement is 20.33 mm.
The bearing performance of the inflation anchor and grouted anchor were compared and analysed. When the lengths of the anchorage end, anchor, the embedment depths, and drill hole diameters of the two anchors are the same, the ultimate pullout capacity (F = 19.73 kN) of the grouted anchor bar is comparable to the ultimate pullout capacity (F = 19.83 kN) of the inflatable anchor with an inflation pressure of 200 kPa, but the displacement (S = 20.33 mm) of the grouted anchor corresponding to the ultimate pullout capacity is significantly greater than that of the inflatable anchor (S = 10.6 mm). For the residual pullout capacity, the residual pullout bearing capacity of the inflatable anchor rod is 17.45 kN, 88% of the ultimate pullout bearing capacity. However, the residual pullout bearing capacity of the grouted anchor sharply reduces after reaching its ultimate state. The displacement continues to increase, and no stable values of residual pullout bearing capacity were measured.
4. Discussion
4.1. Advantages of Inflatable Recyclable Anchors
The above tests show that increasing the inflation pressure, deepening the anchor embedment depth, or increasing the length of the airbag can increase the pullout bearing capacity of the inflatable recyclable anchor. Among the three major influencing factors, the most significant influencing factors are the inflation pressure and the rubber pneumatic bag length. When the inflation pressure (P) or the airbag length (L) is multiplied, the ultimate pullout capacity and the residual pullout capacity of the anchor also increase with a nearly equal multiplying factor. However, when the depth of the anchor increases, the increase of the pullout bearing capacity is limited.
Compared with the anchors proposed by Newson et al. [18], Cao et al. [22], and others, the inflatable anchor not only has good residual pullout bearing capacity but also its ultimate pullout bearing capacity has been greatly improved. When the inflation pressure is 100 kPa, the ultimate pullout bearing capacity of the anchor proposed by Newson et al. [18] and Cao et al. [22] is approximately 0.3 kN with a corresponding displacement of approximately 75 mm. While the inflatable recyclable anchor developed in this study has an ultimate pullout capacity of 10.82 kN, an increase of approximately 36 times, and a corresponding displacement of just 7.187 mm, a reduction ratio of approximately 90%. It can be seen that the pullout bearing performance of the inflatable anchor has been greatly improved.
Compared with grouted anchors commonly used in engineering, the inflatable recyclable anchor has a comparable or even higher ultimate pullout bearing capacity and has a large residual pullout bearing capacity and a small anchor rod displacement. In addition, the inflatable anchor has the following obvious advantages. It does not occupy all the underground resources and can be reused to reduce costs, and the anchoring force can be formed quickly to shorten the construction period. Therefore, the inflatable recyclable anchor is a green, low-carbon, economic, and environmental-friendly anchoring technology with good engineering application potential.
4.2. Field Tests
To further discuss the engineering application prospect of the proposed anchor, we use the international house built by our university as the engineering example. The excavation depth of the building’s underground garage is 5.6 m, and it is close to the surrounding existing buildings. The engineering geological conditions of the site are shown in Table 2. The mechanical properties of the soil are poor. Therefore, to ensure safety, when the underground garage is excavated, temporary supports for the foundation pit slope must be used.
The foundation pit supporting method adopted by the design company is detailed as follows. The upper 1.5 m soil layer is unloaded with a slope ratio of 1 : 1, leaving a 2.0 m wide platform, and the lower 4.1 m vertical slope is reinforced with 3 rows of cement-soil mixing piles with a diameter of 500 mm, a spacing of 350 mm, and a length of 10 m. In the vertical slope, two rows of soil grouting anchors are placed at 1.0 m and 3.0 m from the top of the pile. The inflation anchor is chosen as threaded steel with a diameter of 22 mm, and the preforming hole has a diameter of 50 mm and a depth of 9.0 m. The anchor is inclined at an angle of 15° with the horizontal direction. It is located in a silty clay layer. The cement mortar with a water to cement ratio of 1 : 0.5 is used as the grouting material.
The test field of the inflatable recyclable anchor is chosen as the north side of the foundation pit, and the north side support section is divided into east and west sections. The east section adopts grouted anchors according to the design blueprint, and the west section replaces the grouted anchor with an inflatable recyclable anchor. The hole for the inflatable recyclable anchor is preformed by using a drilling machine and has an aperture of 120 mm and a hole depth of 6.0 m. The rubber pneumatic bag of the anchorage end has a length of 1000 mm and an inclination angle of 15° (Figure 10). To ensure the safety of the foundation pit, the supporting effect of the new anchor is examined. The deep horizontal displacement monitor of the soil and the observation points of the building on top of the slope are set in both the east and west supporting sections.
The excavation operation lasted 105 days from the excavation operation to backfill. As the excavation depth increases, the gas pressure of the inflation anchor gradually increases and finally stabilizes at 360 kPa. The horizontal displacement of the soil and the settlement of the building monitored in real time in the east and west supporting sections are shown in Figures 11 and 12. Figure 11 shows the total horizontal displacement of the soil with varying depths of the two supporting sections. The displacement of the soil below the bottom of the foundation pit is relatively small, but the horizontal displacement of the soil increases significantly in the excavation depth of the foundation pit, and near the top of the pit, the horizontal displacement of the soil reaches its maximum value. The maximum displacement of the inflation anchor section is 25.233 mm, and the grouting anchor section is 34.361 mm, both of which are within the design tolerance of 40 mm. The settlement monitoring results of the buildings around the foundation pit are shown in Figure 12. It is found that, with the excavation of the foundation pit, the settlement of the buildings gradually increases. After the excavation to the bottom of foundation, the settlement tends to be stable. The settlement values of the grouting anchor supporting section are generally larger than those of the inflation anchor section. The maximum settlement of the inflation anchor section is approximately 21.311 mm, and the maximum settlement value of the grouting anchor section is 26.927 mm, both of which are less than the design allowable value of 32 mm.
Before the foundation pit is backfilled, three anchors are selected for pullout tests in each of the supporting sections. The pullout capacity-displacement curves of the two types of anchors are shown in Figure 13.
According to the pullout test results for the two types of anchors, the average ultimate pullout capacity of inflatable recyclable anchors is 38.733 kN with a displacement of 23.745 mm, and the residual pullout bearing capacity is 33.310 kN. The average ultimate pullout resistance of the grouted anchors is 39.470 kN with a displacement of 30.335 mm. It can be seen from Figure 13 that, before reaching their ultimate pullout capacities, the displacements of the two types of the anchor increases with the increasing pullout bearing capacity. The displacement-capacity curve of the inflatable anchor shows a nonlinear relationship, while that of the grouted anchor resembles an ideal elastoplastic deformation stage. The ultimate pullout capacities of the two types of anchors are almost the same, but the corresponding displacement of the inflatable anchor is clearly smaller than that of the grouted anchor. After the ultimate pullout capacity is reached, the pullout bearing capacity of the grouted anchor is drastically reduced and the displacement is increased, which means that the anchor basically fails to function. However, the inflation anchor stabilizes at a certain value after experiencing a temporary decrease in the pullout strength. The residual pullout bearing capacity is relatively large, approximately 85.91% of the ultimate pullout capacity.
Before the foundation pit was backfilled, a recovery test for the inflation anchor was performed (Figure 14). According to the recovery design, the inflation pressure of the anchor is first reduced, and then, a professional recovery device is used to recover the pulling bars one by one. Finally, the anchorage end of the expansion anchor is recovered, and the anchor can be reused after being recycled for cleaning and maintenance.
From field tests of inflatable recyclable anchors, the new anchors have the same pullout capacity as conventional grouted anchors, and their supporting performance is superior to traditional grouted anchors. They are easy to recycle and can be reused. They can be used as a supporting measure for temporary foundation pit slope engineering.
4.3. Anchoring Force of the Inflatable Retrievable Anchor
According to the failure characteristics of the inflatable retrievable anchor, its anchoring force is expressed in four forms, namely, support anchoring force, tension anchoring force, expansion anchoring force, and tangent anchoring force, as shown in Figure 15 [26].
The support anchoring force of the inflatable retrievable anchor is the normal supporting force imposed on the surface of the surrounding soil by the pallets. And it can be calculated by the following equation:where is the support anchoring force, is the outer radius of the pallet, the inner radius of the pallet, [] is the tensile strength of the pallet, t is the thickness of the pallet, and is the coefficient.
The tangent anchoring force is the restraint force which limits the soil to slide or open along the weak plane. So when the bolt passes through a single weak plane, the tangent anchoring force can be calculated by the following equation:
The expansion anchoring force is the normal stress applied on inner surface of the hole, when the inflatable anchor is fully expanded. Its value can be calculated according to the elastic mechanical model as shown in Figure 16. The model assumes that the inflatable anchor is in infinite body under the plain strain status. The ground stress on the outer surface of the surrounding soil is p. The displacement of the inner surface of the surrounding rock is , as shown in the following equation:where is the final radius of the inflatable anchor when it is fully expanded and is the radius of the drill hole.
This elastic mechanical model is axisymmetric. The normal stress of any point in the surrounding soil can be calculated as follows:where is the normal stress of any point, is the radius of any point, and A and C are the undetermined constants.
The corresponding displacement of any point can be calculated as follows:where is the displacement of any point, is the elastic modulus of surrounding soil, and is the Poisson ratio of surrounding soil.
According to the boundary conditions of the elastic mechanical model, which are and , the normal stress imposed on the inner surface of the surrounding soil can be calculated as follows:
Then, the expansion anchoring force can be calculated as follows:where is the expansion anchoring force of the inflatable anchor and is the length of the rubber pneumatic bag.
The tension anchoring force of the inflatable anchor is the frictional force between the bolt and the surrounding soil, whose normal force is the expansion anchoring force. So the tension anchoring force can be calculated by the following equation:where [] is the shearing strength between the bolt and the surrounding soil and is the frictional factor between the bolt and the surrounding soil.
5. Conclusions
In this paper, through a series of field tests, the pullout bearing performance of a novel inflatable anchor is studied, and the proposed anchor is further used in a temporary foundation pit supporting project to explore its pullout bearing capacity, supporting effect, and recovery performance. The main conclusions obtained are as follows:(i)Inflatable recyclable anchor rods consisting of a rubber pneumatic bag, movable steel plates, and dowel bar.(ii)The pullout performance of the inflatable recyclable anchor is related to the inflation pressure, the length of the rubber pneumatic bag, and the embedment depth. Among the three major influencing factors, the inflation pressure and the length of the rubber pneumatic bag have a significant influence on the pullout bearing performance of the proposed anchor.(iii)Inflatable recyclable anchors have good pullout bearing properties. Their ultimate pullout bearing capacity is comparable or even higher than that of the traditional grouted anchors, and they are superior to grouted anchors with lower displacement and higher residual pullout performance.(iv)Inflatable recyclable anchors have good engineering application potential. When applied to temporary supporting projects such as slope foundation pits, they not only have the advantages of rapid formation of anchorage force and the ability to adjust the size according to needs but also can be recycled for reuse, making them low-carbon, economical, and environmental friendly.(v)Inflatable recyclable anchors are a new type of anchorage technology. The related research is still in its infancy, and there are still many problems to be solved. Further experimental and theoretical research is needed. For example, the impact of fatigue of the airbag and steel plates on the pullout bearing characteristics, the applicability in different rock and soil conditions, the formation mechanism of anchorage force, the stability of engineering applications, and effective monitoring methods are all future research topics of interest.
Data Availability
The data used to support the findings of this study are available from the corresponding author upon request.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Acknowledgments
The work described in this paper was supported by funds from the National Natural Science Foundation of China (no. 51678066). The authors also wish to thank the technical staff of Civil Engineering at Yangtze University for all their help.