Application of a Hot Melt Recoverable Anchor Cable in Foundation Pit Support

Abstract

With the continuous development of the social economy, the depth of foundation pit excavation of proposed construction projects in various places gradually increases, which is closer to the building red line, and the surrounding environment is relatively complex; therefore, there are few supporting schemes for foundation pit selection. Based on the Vanke B-6 plot of a real estate development project, the project has close neighboring buildings. If we use a normal anchor cable, underground cut line problems are possible, and so we choose the hot melt-type recoverable anchor; with three element dispersion pressure, the anchor cable can minimize broken sections of brush slope excavation and has advantages of a simplified construction process and good seismic performance. Except for the cable barrel, the other parts and all steel strands can be recycled, and recycled parts can be reused. The recyclable bolt (cable) overcomes the disadvantages of the conventional bolt (cable) as a temporary support, such as pollution of the underground environment, encroachment on the underground space of adjacent buildings, and becoming an underground obstacle in follow-up projects. It accords with the characteristics of the times of environmental protection and sustainable development, and has achieved good economic and social benefits.

1. Introduction

At present, the bolt–cable support system is more and more widely used in deep foundation pit support. The use of anchoring technology in geotechnical engineering can make reasonable use of the strength and self-stability of the soil itself, simplify the structural system, improve the stability of the structure, ensure construction safety, save engineering materials, shorten the construction period, and reduce the cost; so, all countries in the world are developing geotechnical anchoring technology. Especially in the United Kingdom, Germany, France, Switzerland, Austria, Japan and other countries, the development of anchoring technology is quite rapid, the application range is very wide, and the construction machinery is also very advanced [1,2,3,4,5]. In the 1960s, China began to use ordinary mortar bolt and shotcrete support in mine roadway, railway tunnel and slope engineering, and it has developed rapidly so far, with the gradual use of some new anchor cables [6,7,8].

With the development of society, the demand for green construction has gradually increased, and recyclable anchor cables have begun to be applied in projects [9,10,11,12]. In foreign countries and Hong Kong, there are provisions prohibiting the violation of the “red line” overseas space, and Shanghai, Beijing, and other places in China also have clear restrictions. Recycling anchor cables and reducing underground construction waste is a very important environmental protection project, and a good consensus has been formed in the whole society. In recent years, domestic and foreign scholars have conducted relevant studies on the mechanical mechanism of recoverable anchor cables [7,13,14,15,16]. Based on the Kelvin solution, Lu et al. [17] deduced the distribution equations of elastic bonding stress and normal stress in the anchorage section of the pressure-type anchor bolt, and combined with the field measured data, further verified the feasibility of the theoretical calculation formula. Zhang et al. [18] established a hyperbolic function model of bolt load transfer, further deduced the distribution law of frictional resistance and shear displacement of the bolt along the anchoring length, and analyzed the load transfer characteristics of the bolt in viscous soil combined with field experiments, and the measured values were in good agreement with the calculation results of the model. Pang et al. [19] analyzed and obtained the effective length of recoverable anchor solid and rock mass in a field test. A PVC casing can be used to isolate the anchor and anchor solid so as to avoid corrosion of the anchor. This method can be applied to temporary or short-term projects. Zhang et al. [20] made a comprehensive summary and study of recyclable bolts at home and abroad, gave the design method and anchoring mechanism of recyclable bolts, summarized the relevant factors affecting the interface bond stress, and prospected the future development of recyclable bolts. Li et al. [21] summarized the recent development of new types of anchor bolts at home and abroad, mainly including recyclable anchor bolts for urban foundation pit reinforcement and automatic cut-off recyclable anchor bolts, etc., and put forward some problems and construction difficulties in the current development of recyclable anchor bolts.

Facing this new anchor cable, it is necessary to deeply understand and master the key points of the support construction technology of the anchor cable frame, and realize the harmonious development of human and natural ecology under the premise of ensuring the quality of support construction [22,23,24]. In addition, the use of this construction technology for construction is more simple and can save a lot of money, so as to be widely used in engineering. Based on the real estate development project of Nanbeikang B-6 block of Vanke, this paper introduces the use of a hot melt recoverable anchor cable, a three-unit pressure dispersive anchor cable, which can minimize the excavation and brush slope in the broken section, and has the advantages of a simplified construction process and good seismic performance. At the same time, except for the anchor cable cylinder, the rest of the components and all the steel strands can be recycled, and the recycled parts can be reused. The use of this cable not only obtains a good construction effect but also obtains an excellent economic benefit. The main methodological processes used in this paper are summarized in the flowchart shown in Figure 1.

2. Project Overview

The proposed B-6 block project is located in the west of Provincial Road S103, Shizhong District, Jinan City, south of the planned B-5 block and east of the B-13 block. The proposed buildings include 57F and 34F super high-rise office buildings, 16F and 22F office towers, 12F apartment buildings and 3~4F shopping centers, with five underground garages as a whole. The foundation pit is generally distributed in a parallelogram, the width from east to west is about 187.0 m, the length from north to south is about 196.6 m, and the total length of the supporting section is about 787.2 m. The excavation depth of the foundation pit is large, the distance from the red line is relatively close, the surrounding environment is complex, the deformation requirements are strict, the safety of the foundation pit support is comprehensively determined to be level, and the design service life of the foundation pit is 22 months. The site is located on the west side of Xinglong Mountain, which is a sloping plain geomorphic unit in front of the mountain, with a higher terrain and a lower terrain in the east and west. The site is partially filled with construction waste and muck, and the depth of the quaternary system is up to 27.2 m. In the scope of the investigation depth, the inner layer can be divided into 5 large layers and their sublayers according to their deposition age and engineering properties. The stratigraphic situation of the proposed site is shown in

Table 1. Site soil layer.

Stratigraphic Sequence Number	Landlord Stratum	Site Sublevel
①	Filling (Q4ml)	Mixed fill, plain fill
②	Loess crushed stone (Q4al+pl)	Loess, gravel
③	Silty clay crushed stone, cemented crushed stone (Q3al+pl)	Silty clay, crushed stone, cemented crushed stone, silty clay mixed crushed stone
④	Silty clay with clay and gravel (Q3al+pl)	Silty clay, clay, crushed stone, silty clay mixed crushed stone, cemented crushed stone
⑤	Limestone (O1m)	More intact weathered limestone, more broken weathered limestone, broken weathered limestone, clay, gravel, clay mixed gravel

. The groundwater level of the site is low, and all of them are below the basement; so, the influence of groundwater on the project can be ignored.

3. Hot Melt Retrievable Anchor Cable

At present, the recyclable anchor cable types widely used at home and abroad mainly include the DYWIDAG recyclable anchor cable from Langenfeld, Germany, SBMA recyclable anchor cable developed by Anthony, et al., KTB load-distributed recyclable anchor cable developed by KTB Association in Tokyo, Japan, and GCE recyclable anchor cable developed by Japan National Disaster Prevention Co., Ltd. (Tsukuba, Japan).

As a structural member driven into the soil, the compressive stress dispersible recoverable anchor cable can transfer the tensile load to the anchored soil layer, strengthen the enclosure structure, control the displacement, and reduce the deformation of the foundation pit. The main components of the recovered bolt are the anchor, free section and anchoring section [25], as shown in Figure 2.

The anchor is composed of an anchor head and a pressure plate. The function of the pressure plate is to transfer the prestress from the steel strand to the ground or the support system. The length of the free section is the elastic part of the prestressed steel strand, which mainly transfers the pulling force of the anchoring section to the outside structure. The length of the anchoring section is the length of the cement slurry contained in the bearing plate of the bolt with dispersed compressive stress.

3.1. Test Scheme

Through the test, the corresponding P-s curve of the bolt can be determined, the difference between the ultimate bearing capacity of the pressure-dispersed bolt and the tension-type bolt under the same conditions in the soft rock medium can be compared, and the approximate increase in its bearing capacity can be determined. At the same time, the deformation and failure characteristics of the pressure-dispersed bolt can be observed. The influence of carrier spacing and the quality of rock and soil layers on the bearing capacity of pressure distributed bolt is analyzed to provide some useful suggestions and references for the engineering application of pressure distributed bolt in soft rock medium.

In this test, when the steel strand is operated, the material is cut according to the actual length plus 80 cm to ensure that there is enough length of the steel strand outside the hole after the anchor rod is installed for tension operation. The force transfer mechanism of the pressure dispersed bolt requires that the steel strand of the bolt be detached from the mortar in the full length to form a free section without bond so that the tension load can be transferred directly to the carrier. Generally, it can be used to install an ordinary steel strand with a sleeve or directly use an unbonded steel strand finished product to ensure that the steel strand of pressure dispersion-type anchor bolt is in a free state and unbonded. For the pressure dispersion test bolt, the parts within the full length of the tunnel are treated without bonding.

3.2. Test Parameter

The anchor cable used in this project adopts a pressure-dispersive hot melt decore (recovery) grouting anchor cable (relevant parameters are shown in

Table 2. Parameters of the hot melt detachable (reclaimed) anchor cable.

Serial Number	Bolt Type	Bearing Plate Size	Suitable Cable Type	Steel Strand Specifications	Remark
1	Full casing anchor cable	Φ100	1~5	Φ12.7/15.2	Suitable for Φ133 casing
2	Full casing anchor cable	Φ112	1~5	Φ12.7/15.2	Suitable for Φ146 casing
3	Full casing anchor cable	Φ135	2~10	Φ12.7/15.2	Suitable for Φ168 casing

), and the final load is 1.4 times of the standard value of the design internal force according to the specification [26].

4. Anchor Cable Support Scheme

According to the depth of the foundation pit and the complex surrounding environmental conditions, the total thickness of the soil layer within the excavation depth range is about 20.0 m, the thickness of the medium weathered limestone above the foundation pit on the east side is about 3.0~6.0 m, and the thickness of the soil layer below the foundation pit on the west side is about 6.0~10.0 m. In order to ensure the safety of foundation pit engineering, the construction method of one pile in the bottom is adopted, and the embedment depth is 3.0 m (rock layer)~8.0 m (soil layer). The construction technology of the pressure dispersion-type recycled anchor rod is adopted on the east side to reduce the influence on the later subway construction.

The foundation pit is mainly divided into 12 supporting units, and the supporting form of “supporting pile + prestressed anchor cable” combined with local slope release is adopted. This paper selects 1-1~3-3 supporting units for introduction.

4.1. The 1-1 Section of the Supporting Unit

The depth of the foundation pit is about 20.1 m, and the support form of “cast-in pile + prestressed anchor cable” combined with slope excavation is adopted. Shotcrete surface layer is used on the whole slope. A6.5@200×200 steel mesh is built into the excavation section at the top of slope, and A2.0@50×50 finished steel wire mesh is built into the vertical excavation section. The thickness of the surface layer is 80 mm and the strength is C20.

The supporting pile diameter is 1000 mm, the spacing is 1500 mm, the pile length is 24.6 m, the strength is C30, the embedded section is 6.0 m, and 6 prestressed anchor cables are set. Support forms are shown in Figure 3.

4.2. The 2-2 Section of the Supporting Unit

The depth of the foundation pit is about 21.6 m, and the support form of “cast-in pile + prestressed anchor cable” combined with slope excavation is adopted. Shotcrete surface layer is used on the whole slope. A6.5@200×200 steel mesh is built into the excavation section at the top of slope, and A2.0@50×50 finished steel wire mesh is built into the vertical excavation section. The thickness of the surface layer is 80 mm and the strength is C20.

The diameter of the supporting pile is 1000 mm, the spacing is 1500 mm, the length of the supporting pile is 24.1 m, the strength is C30, the embedded section is 4.0 m, and 6 prestressed anchor cables are set. Support forms are shown in Figure 4.

4.3. The 3-3 Section of the Supporting Unit

The depth of the foundation pit is about 21.6 m, and the support form of “cast-in pile + prestressed anchor cable” combined with slope excavation is adopted. Shotcrete surface layer is used on the whole slope. A6.5@200×200 steel mesh is built into the excavation section at the top of slope, and A2.0@50×50 finished steel wire mesh is built into the vertical excavation section. The thickness of the surface layer is 80 mm and the strength is C20.

The diameter of the supporting piles is 1000 mm, the spacing is 1500 mm, the length of the piles is 24.1 m, the strength is C30, the embedded section is 4.0 m, and 7 prestressed anchor cables are set. Support forms are shown in Figure 5.

5. Anchor Cable Construction Scheme and Effect

5.1. Construction Scheme

Taking into account the characteristics of the recoverable anchor cable, it is necessary to have good ground adaptability, provide different bearing capacities and meet different anchoring depths for different strata, and its construction technology should be simple and easy to recover. The main construction process of the compressive stress dispersing hot melt recoverable anchor cable is as follows:

Determine the hole position → drill hole position → adjust the angle → drill hole → clean water hole → cement slurry washing hole → lower anchor cable → grouting in casing → take out casing → secondary grouting → construction anchor cable waist beam → tensioning → anchor head locking → remove excess steel hinge line of the anchor head to protect the anchor head → anchor cable core removal and recovery. The construction site is shown in Figure 6.

The hot melt recoverable anchor cable used is a three-unit four-cable pressure distribution cable, and the tensioning method is equal load tensioning. The concrete steps of cable tensioning are as follows: First, the cable is pre-tensioned according to 0.2 times the design tension value. Then, the anchor clip of the tension tool is installed on the steel strand of the anchor rod of the first unit body and stretched to P3 (41 kN). Then, install the tool anchor clip on the steel strand of the second unit anchor rod and continue to stretch to P4 (100 kN). Then, install the tool anchor clip on the steel strand of the third unit body anchor rod and continue to stretch to 0.5 times the design value (210 kN). Finally, each unit bolt combination is stretched to 1.1 times the design force value and locked.

Among them, there are several key issues in the construction process for the recoverable bolt. First of all, the demountable anchor cable is a pressure cable, and the strength quality of its anchoring section is the key point of the project; so, the quality of hole washing is the main factor that directly determines the bearing capacity of the anchor cable. Second, the protective cover of PE wire with an exposed length of anchor cable should be protected to avoid wire damage in the process of binding and excavation of reinforcement bars in the construction of concrete crown beam, which will affect the later core removal work of the anchor cable. Third, the locking end anchor must be in the same straight line with the anchor to avoid an angle between the anchor and the anchor cable that results in prestress damage.

5.2. Test Results

For the test, a comparative analysis was carried out on the available bolt, and the basic test results of the bolt are as follows: under the condition of the same stratum and the same specification bolt (φ180, the same anchoring length), the average value of the ultimate tensile strength of the hot melt type is 703.7 kN, which is greater than 630.0 kN of the design requirement. The average mechanical ultimate pulling capacity is 509.2 kN, which does not meet the requirement of the design. Under the condition of non-ultimate failure of the bolt body, both kinds of recoverable bolts can achieve 100% recovery. The ultimate friction resistance of the rock mass and anchor solid measured in the basic test meets the design requirements. The test results are shown in Figure 7 and

Table 3. Anchor bolt (φ180) basic test design parameters and test results.

Bolt Type	Bolt Numbering	Loading Form	Design Bolt Diameter (mm)	Design of the Ultimate Pulling Force of the Bolt (kN)	Maximum Load Value (kN)	Termination Loading Condition	Measured Ultimate Bearing Capacity of the Bolt (kN)
Mechanically recoverable bolt	S-1-1 J	Multiple cycle loading test	4 roots φ15.2	630	655	The bolt body is pulled out	582
	S-1-2 J	Single cycle loading test	4 roots φ15.2	630	509	Anchor head displacement does not converge	436
	S-1-3 J	Single cycle loading test	4 roots φ15.2	630	582	Anchor head displacement does not converge	509
	S-2-3 J	Single cycle loading test	4 roots φ15.2	630	655	Anchor head displacement does not converge	582
	S-2-1 J	Single cycle loading test	4 roots φ15.2	630	509	Anchor head displacement does not converge	436
Hot melt recoverable bolt	S-3-1 R	Single cycle loading test	4 roots φ15.2	630	728	Load to maximum load	728
	S-3-2 R	Single cycle loading test	4 roots φ15.2	630	728	Shaft failure	655
	S-3-3 R	Multiple cycle loading test	4 roots φ15.2	630	728	Load to maximum load	728
Ordinary bolt	S-4-1 P	Single cycle loading test	5 roots φ15.2	630	728	Load to maximum load	728
	S-4-2 P	Single cycle loading test	5 roots φ15.2	630	728	Load to maximum load	728
	S-4-3 P	Multiple cycle loading test	5 roots φ15.2	630	728	Load to maximum load	728

.

5.3. Comparative Analysis

Based on the test, it can be found that the reasonable setting of carrier spacing is an important condition to give full play to the advantages of stress performance of the pressure-distributed bolt. If the carrier spacing of the pressure-distributed bolt is too small, the pressure load transferred by the carrier to the mortar body cannot be completely balanced by the friction resistance between the mortar body and the rock and soil layer in the anchoring section of the unit, and the excess load will be transferred to the next anchoring section of the unit, thus further increasing the load in the next anchoring section. Layer upon layer superposition makes the stress state of the pressure dispersion-type bolt deteriorate continuously, which is very unfavorable to the anchoring performance of the bolt. On the other hand, if the carrier spacing of the pressure-dispersed bolt is too large, the utilization rate of the anchoring section of the bolt will be low, and there will be too much safety reserve in the unit anchoring section after absorbing the pressure load borne by the section, which is not economical to apply in the project.

In practical application, the pressure distribution-type bolt has many incomparable advantages over the tension-type bolt, such as its bearing capacity can be increased proportionally with an increase in the anchoring section length, the deformation resistance of the bolt is stronger, the bolt can be recycled without affecting the development of surrounding strata, the corrosion resistance of the bolt is stronger, the prestress loss of the bolt is smaller, the use of the bolt is more economical, etc. The results show that the pressure dispersion-type bolt has a very good engineering application. Through the basic test of the bolt, it can be found that the bearing capacity of the bolt with pressure dispersion in soft rock strata is greatly improved compared with that of the traditional tension bolt. Based on the P-s curve and other data obtained from the test, there are significant differences between the stress and deformation of the pressure-dispersed bolt and that of the traditional tension-type bolt, which also provides a certain reference for the future engineering application of the pressure-dispersed bolt in soft rock formation medium in mountainous areas.

The test results show that the hot melt recoverable bolt has a significant advantage among the three kinds of bolts, as its F-s curve changes more evenly, and the obtained rock and soil body and anchor solid limit friction meet the relevant technical requirements. At the same time, the medium pressure stress-dispersing hot melt recoverable anchor cable is quick and convenient to make, and the construction technology is simple and easy to recover. Only the working anchor piece of the anchor cable is relaxed during recovery, and then the steel strands can be extracted one by one using a jack or a hoist. Under the condition of the deep foundation pit, the basic construction requirements can be guaranteed by using this kind of anchor cable. At the same time, after the completion of the support project, the anchor cable was recovered and great economic benefits were obtained.

6. Conclusions

(1)

The hot melt recoverable anchor cable has excellent engineering characteristics. In the project, the grouting body in the anchoring section of the anchor cable and the anchor rod are under pressure, which can exert high compressive strength of the cement material. Meanwhile, the anchor cable is jointly carried by each unit anchoring section, and the formation strength is used to achieve a high anchoring force.

(2)

The hot melt recyclable anchor cable has a strong anti-corrosion ability. There will be no tension cracks in the anchoring section of the anchor cable. At the same time, a non-bonded steel strand is used to wrap the PE plastic pipe, which can be used for permanent engineering reinforcement.

(3)

The hot melt recoverable anchor cable has good economic benefits. The cable is quick and convenient to make, simple to construct, and easy to recover. Recovery simply relaxes the working anchor pieces and pull them out one by one using a jack or winch.

(4)

The hot melt recoverable anchor cable has broad application prospects. In temporary projects, recycling can be easily achieved after the completion of use, will not cause pollution to the underground space near the project and subsequent development obstacles, and will have broad application prospects in urban construction.

(5)

At present, there are still some problems in the application of recyclable anchor cables, requiring further clarification of the use of recycling bolts and cables, establishing a certain incentive mechanism, encouraging the enthusiasm for recycling bolt production and recycling manufacturers, forming an industrial development chain, and promoting the use of recycling bolts and cables.