Experimental research on the performance of a Novel Geo-filament anchor for an Earthen architectural site

At present, most studies of anchorage techniques for earthen architecture ruins are conducted so as to improve the anchor force, however, the damage caused to the ruins by anchor reinforcement has not been fully considered in practice and no special anchor technique has been applied to reinforce the small sliding mass. This paper summarizes the application and R&D of anchor rod techniques as applied to the protection of the Gaochang Ruins, Turpan, in China. Based on the reinforcement of the small sliding mass of the earthen historical ramparts, a new type of Geotechnical Filament Anchor (GFA) is designed. By changing six parameters, including anchorage length (L), GF thickness (H), bore diameter (D), slurry strength (S), GFA surface state (R) and inclination Angle (A), the tensile strength, failure mode, load displacement (P-S) relationship and strain (ζ-L) distribution characteristics are studied, and corresponding analysis is performed on the test data and phenomena. (1) A formula for the design value of the anchorage force N is presented. (2) Combining the data on the strain distribution at the GF-slurry interface under the action of N, the shear stress distribution model of the anchorage system is obtained. (3) Taking into account the soil mechanical properties of the above-mentioned ruins, the shear stress diffusion coefficient (α) is conceptualized, the formula for the shear strength of the grouting material is obtained, and the allowable ranges of L, D, H, R, and S determined. A new design is proposed for the application of anchorage techniques to earthen ruins in the context of protection of cultural relics, which promotes the design and calculation method described in this paper.


Introduction
Earthen architectural sites remain some of the most treasured cultural legacies for humankind. Due to the diverse construction materials, vulnerability, structural complexity, and varied classifications of earthen architectural sites, their protection has always been a difficult problem in the field of protection of cultural relics [1].
Currently, support and anchorage are the main measures to solve the problem of structural stability in earthen architectural sites. Anchorage technology has been widely used in the field of geotechnical engineering. Research to use steel bar, steel strand, GFRP(Glass Fiber Reinforced Polymer) and AFRP (Aramid Fiber Reinforced Polymer.) as the bolt material,have been well developed, and their anchorage performance, interface mechanics theory and anchorage parameters [2][3][4][5][6][7][8] are well established. However, the structural reinforcement of earthen architecture ruins is very special, so reinforcement techniques and materials adopted in other fields cannot be simply copied and applied.
Therefore, a new type of anchor rod for the reinforcement on the structure of earthen architecture ruins has been developed and applied [9][10][11], whose rod materials are mainly Phyllostachys pubescens, wood, and rebar [12][13][14][15]. Relevant research primarily focuseson the influence of anchor rod length and grouting material on anchorage force and the stress distribution characteristics of the rod-slurry interface [16][17][18][19][20][21][22][23][24] However, relevant studies have yet to fully considered the requirements of strengthened anchorage performance for the site's soil characteristics and have not taken a unique anchor technique to reinforce the small sliding mass. There is no corresponding design theory and calculation method for the anchorage technology of earthen ruins, and the design basis is mainly based on experience and test data.
This paper summarizes the application and R&D of anchor rod techniques as applied to the protection of the Gaochang Ruins, Turpan, in China. Based on the reinforcement of the small sliding mass of the earthen historical ramparts, a new type of Geotechnical Filament Anchor (GFA) is designed. By changing six parameters, including anchorage length (L), GF thickness (H), bore diameter (D), slurry strength (S), GFA surface state (R) and inclination Angle (A), the tensile strength, failure mode, load displacement (P-S) relationship and strain (ζ-L) distribution characteristics are studied, and corresponding analysis is performed on the test data and phenomena. This study also attempts to summarize the design concepts and computational methods of GFA, and simultaneously discuss and rethink the application and development of the anchor technique for earthen ruins.

Research background
Gaochang Ruins, one of the important ruins in the Silk Road, was built in the first century BC (middle years of the Western Han Dynasty) and abandoned in the late thirteenth century due to war, its usage period is more than a thousand three hundred years, and it has been more than two thousand years since it was built.
Most existing rampart ruins were reconstructed and built based on Gaochang City in Tang Dynasty. The whole city is an irregular square (as shown in Fig. 1) with a floor area of about 198 ha. It is composed of three parts: outer city, inner city, and imperial city. The rampart is 6-11 m high and about 10 m wide (bottom), which is rammed and built with raw soil, with a thickness of the tamping layer of 8-10 cm. Due to the natural and artificial destruction over one hundred years, the rampart ruins are damaged to varying degrees (as shown in Fig. 2).
In this paper, on the research background of reinforcement protection for the rampart ruins of Gaochang Ruins, five large-scale protection projects have been performed to protect the earthen ruins of Gaochang Ruins since 2005. Grouting, adobe support, anchor rod reinforcement, and other technical measures are mainly taken for the reinforcement of the structure of rampart ruins.
The reinforced anchor rod made of bamboo or wood can provide greater anchorage force and effectively control displacement. They can give full play to their a Alnert Gruenwedel's hand drawing (1902).
b Vertical photograph.     advantages when reinforcing the potential large-scale sliding masses (as shown in Fig. 3) with certain opening depth and diameter requirements. The problem of excessive intervention will occur when they are used to reinforce small sliding masses (as shown in Fig. 4). In view of the above problems, a new GFA is designed to reinforce small sliding masses of rampart ruins of Gaochang Ruins.

Experiment scheme
Geo-filament is a high-fiber synthetic material that can form a stiffed belt after extrusion molding, as shown in Fig. 5. The tensile strength of geo-filament is very high, and all its physical and mechanical indexes are shown in Table 1.

Experiment design and specimen preparation
To research the influence of changes in anchor rod length (L), thickness (H), bore diameter (D), grouting material (S), ribbing spacing (R), and arrangement angle (A) (the angle between the opening axis and the horizontal direction) on the anchorage performance, it is designed with six independent test batches, including 19 groups of tests in total, of which 3 test pieces are processed in each group of test. The single-factor test method is adopted, where the effect of a single factor on the anchorage force at a given time is evaluated under similar environmental conditions, keeping the other five factors constant.
The six independent test batches are, respectively: anchor length L (5 groups); thickness H (3 groups); bore diameter D (3 groups); grouting strength S (3 groups); ribbing spacing R (3 groups); arrangement angle A (3 groups). The test grouping design parameters are shown in Table 2.
The geo-filament is cut according to the test's needs, with a 400 mm free section reserved for the anchorage of the drawing instruments. The outside of the anchor rod is evenly ribbed with hemp rope, which the test scheme determines the spacing. The end of the anchor rod is wound with three rounds of hemp, then the surface of the hemp is evenly coated with epoxy resin. It is necessary to ensure that the hemp rope is thoroughly soaked with epoxy during the coating process so it can be boned tightly to the geo-filament.
To test the geo-filament-slurry interface stress distribution, the researcher divided the filament anchor rod into four equally spaced segments, with strain gauges arranged at the ends and the demarcation points, and the compensation plate is placed in the middle of the anchor rod. The schematic diagram of anchor production is shown in Fig. 6. The diagram of the cross-section of the anchor is shown in Fig. 7. The strain gauge is sealed with epoxy resin and waterproof tape, and the lead and shielded wires are numbered successively after welding. The field production of the test piece is shown in Fig. 8.

Material characteristics of the experiment
Mechanical properties of the rammed earth (1) The rampart relics of Gaochang are mainly built with rammed earth. To reduce the test deviation, the researcher samples the drill core in the in-situ test area, and measures the parameters of rammed earth materials, as shown in Table 3.
(2) Mechanical properties of the grouting body Three kinds of slurry are prepared according to the actual engineering experience (S1, S2, S3), and the slurry proportion is calculated according to the weight proportion.
The water cement ratio of three kinds of slurry is 30% (weight ratio). Make slurry test blocks with 7.07 ×7.07 mm mortar test mold, and measure its compressive mechanical property 30 days after curing, as shown in Table 4.

Test piece installation
To avoid interaction among anchor rods, the drilling spacing in vertical and horizontal directions shall be more than 1 m. The bore diameter is first cleared with a brush and hair dryer. To ensure that the slurry is closely knit, the pore end is blocked with modified loess mud.

Test method
The measurement range of anchor rod tension meter is 0-100 kN, the pull stroke is 150 mm, and the model is ZY-10. The strain acquisition instrument shall be a TDS-303 data acquisition instrument produced by Japan, with a measurement range from − 20,000 to 20,000. The KFG series general foil strain gauge is used, and the dimension of the sensitive grid is 3 ×10 mm (with a resistance value of 120.8 ± 0.1 and the sensitivity of 2.14 ± 1%). To cooperate with the anchor rod tension meter, make a counterforce frame with a thick steel plate. The middle of the counterforce frame and the ruins are padded with a wood block, and the displacement sensor is installed on the steel counterforce frame. Additional file 1. The schematic diagram of the test device is shown in Fig. 9.
The test was performed in the primary soil of non-cultural relics near the eastern section of the northern ramparts of Gaochang Old City in Turpan, Xinjiang, in May. Considering location and climate factors, the researcher performed an in-situ pull-out test after the anchor rod was implanted and the slurry was consolidated entirely (about 40 days).      To eliminate the stress clearance among devices, the researcher pre-applied 200 N pull before data acquisition and adopted the electric servo hydraulic pressure method for loading. The loading rate is 50 N/s until the anchorage system is damaged.

Analysis of the anchorage performance
Through the observation of the experimental phenomenon and data analysis, the corresponding relationship between the failure stages and the anchorage forces during the pull-out process of GFA in each test group is obtained, as shown in Tables 5, 6 There are four main types of destruction modes of the GFA during the pull-out process in the experiment, which is listed as follows: geo-filament pullout (Fig. 10a); slurry damage (Fig. 10b); geo-filament fracture (Fig. 10c); soil-part loosening and grouting material pull-out (Fig. 10d).
By summarizing the destruction phenomena and the properties of each destruction stage, the following conclusions can be drawn: a. The destruction mode of the GFA is mainly GF pullout (Fig. 10a), and the performance of the anchoring system has not been fully exploited. When the strength of the slurry is sufficiently strong, the destruction mode of soil-part loosening and grouting material pull-out (Fig. 10d) will occur. By this time, the performance of the anchoring system has been fully exploited. Hence, the soil-part loosening and grouting material pullout appear to be an ideal   The curve of L-P-S mode of destruction. However, this results in secondary damage to the earthen ruins, so the destruction mode appears non-conducive to heritage preservation. b. When the anchorage length reaches about 3000 mm, the ultimate anchorage force is close to the breaking force of geo-filament, and the destruction mode of GF fracture (Fig. 10c) occurs. c. Changing the anchorage length (L), grouting material strength (S), and surface state (R) of GFA can significantly improve the anchorage performance. Chang-ing the anchor thickness (H), bore diameter (D), and anchorage angle (A) of GFA cannot significantly improve the anchorage performance

Load-displacement (P-S) relationship
The measured data of each group is sorted out, the bad values are eliminated and the available data is averaged. The load-displacement curves (P-S) of L, H, D, R, S, and A are obtained, as shown in Figs. 11, 12, 13, 14, 15, 16. According to the division of each destruction stage in Table 5, the load-displacement curves are distinguished by different colors.
a. The green area of the load-displacement curve represents the geo-filament deformation stage. The maximum load at this stage is expressed as N i . b. The yellow area of the load-displacement curve represents the stage where the grouting material crack or the pullout appears. The maximum load at this stage is expressed as N j . (1) Influence of anchorage length L on the load-displacement curve.
The L-P-S curve is shown in Fig. 11. With the increase of anchor length, the anchorage force is significantly improved, and the anchor displacement is effectively controlled under the same pulling force.
The anchorage force increases with the increase of anchor length. If the anchor length is in the range of 800 mm-1500 mm, the growth rate of ultimate anchorage force increases step by step. If the anchor length is in the range of 1500 mm-3000 mm, the growth rate of ultimate anchorage force decreases; If the anchor length is about 3000 mm, the ultimate anchorage force is close to the breaking force of geo-filament.

(2) Influence of geo-filament thickness H on the loaddisplacement curve.
The H-P-S curve is shown in Fig. 12. The when thickness of H2 is twice as much as H1, the ultimate anchorage force will increase 17.5% but the increase in the thickness of the geo-filament does not effectively control the displacement of anchorage system. Changing H can not obviouly improving anchorage performance.
Because the geometry of the geo-filament is flaky, the strength of the soil in the anchorage system is higher than that of the grouting material, and the increase of the thickness (H) has a certain weakening effect on the integrity of the slurry. From the test phenomena, it can be concluded that the increasing the thickness of the geo-filament has little effect on improving the anchorage performance, on the contrary, it will accelerate the slurry crash.

(3) Influence of bore diameter D on the load displacement curve
The D-P-S curve is shown in Fig. 13. The increase of pore diameter (D) leads to the increase of slurry proportion and radial dry shrinkage of the anchorage system. As a result, the radial pressure of the slurry is reduced, the bond strength between the geo-filament and the slurry interface is weakened. Increasing the pore diameter will cause the anchorage system to fail prematurely. In practical engineering, increasing the pore diameter would cause greater damage to the heritage.

(4) Influence of grouting strength S on the load displacement curve.
The S-P-S curve is shown in Fig. 14. The pull-out bearing capacity of the anchor increases by strength grade of the grouting material. Due to the increased strength of grouting material, the bond strength between the slurryrod interface get improved. At the same tensile force, the displacement of the S1 and S2 anchorage systems is about twice as much as S3's anchorage system, which indicating that increasing the strength of the grouting material can effectively control the displacement of the anchorage systems.

(5) Influence of anchor rod surface state R on the load displacement curve.
The R-P-S curve is shown in Fig. 15. The maximum anchorage force is 4.33kN for R0 without rib and 7.53 kN and 11.06 kN for R100 and R30 with rib. The ultimate anchorage force is 73.90% and 155.42% higher than the rib-free anchor rod, respectively. When the tensile force reaches 3 kN, the displacement of ribbed anchorage system is significantly less displaced than the rib-free anchorage system. This suggests that changing the surface state of geo-filament can not only effectively improve the ultimate anchorage force, but also effectively control the displacement.

(6) Influence of anchorage arrangement angle A on the load displacement curve.
The A-P-S curve is shown in Fig. 16. When the arrangement angle is 0°, the ultimate pull-out force of the anchor is the maximal. As the the arrangement angle increases, the ultimate pull-out force of the anchor decreases. Because the applied load is horizontal, the larger the arrangement angle is, the larger vertical component force bears, and the anchor rod cannot give full play to its anchorage performance. However, given the the requirements of the construction process, it is recommended that the arrangement angle of the anchor rod should be 10°.

Strain distribution ( ξ − L ) characteristics of GF-grouting interface
According to the destruction results in the test, the failure of the geo-filament anchorage system is caused by the pull-out of the anchor, due to bond-slip between the geo-filament and the slurry interface. The bond-slip section is in the red area of the P-S curve (Figs. 11,12,13,14,15,16). The failure of this section is similar to the brittle failure. There is no obvious deformation of the anchorage system, and the anchor is suddenly pulled out.
The choice of the anchor force design value N needs to take into account two aspects, that is, the reinforcement and protection of the ruins cultural relics and the effective performance of the anchorage system.
According to the load displacement curve, the value N is discussed by combination of the test phenomenon and the characteristics of strain distribution ( ξ − L ) at geofilament-grouting interface. Considering the safety stock of anchorage system, the calculation formula of N value is shown in Formulas (1) or (2) Here N i represents the extreme value of the deformation stage of the geo-filament, and the anchorage system is in the elastic stage;N j represents the extreme value when the grouting is cracked or pulled out and the anchorage system is in the plastic stage; N k is the extreme value of the geo-filament pull-out stage, and the anchorage system is in the bond-slip phase. (1) The specific values of N i , N j and N k are shown in Tables 5, 6 , 7, 8, 9, 10; N 1 and N 2 values of different test batches (L, H, D, S, R and A) can be obtained from Formulas (1), (2). The anchor enters the bond stage from plastic that is an essential manifestation of the anchorage system being about to fail; Therefore, the extreme values N j when the grouting is cracked or pulled out are compared with N 1 and N 2 . The sorting out of values are shown in Table 11.   Table 11, in most cases, N1 < Nj. If the anchorage force design value is N1, the anchorage performance cannot be fully exploited. Therefore, most of the N1 values are treated as invalid values during data processing. The data marked with ''*'' in Table 11 is invalid. The strain distribution curve of geo-filament-grouting interface corresponding to the valid data in Table 11 are extracted respectively (L、H、D、R、S、A), as shown in Figs. 17, 18, 19, 20, 21, 22. (1) Design value N of anchorage force based on the anchor length L According to the analysis from Figs. 17,18,19,20,21,22, with the increase of anchorage length (L), grouting material strength S, and geo-filament surface roughness R, the anchorage performance of geo-filament improves markedly; in which the anchorage length (L) has the most prominent impact on the performance of anchorage system, and the change of anchor length (L) is the most common in practical application.
According to Fig. 17, the maximum transmission depth of the anchorage force of the geo-filament anchorage system is about 2.2 m, and the stress in the middle section of the anchor varies considerably.
The longer the anchor length is, the more uniform the stsress distribution is and the smaller the change of adjacent strain is. The stress distribution of the anchor in the 800-1200 mm section is significantly different from that in the 1500-2000 mm section. Based on the analysis of the anchor failure stage in Table 5if the length of the anchor is greater than 2 m, the grouting pull-out destruction mode occurs. Considering the possibility of subsequent failure of the reinforced earthen ruins, the mode of destruction by filament pull-out is more favorable for the complete protection of the ruins, on the premise of providing the required anchorage force. Therefore, the design value of the anchorage depth of the geo-filament anchor should be at most 2 m, and the optimal length of the anchorage should be between 1.2 m and 1.5 m.
By comparing and analyzing the strain distribution curves of each group (L、H、D、R、S、A), it is reasonable to take the anchor length L as the discriminant index to determine the calculation formula of anchorage force. Considering the safety stock of the anchorage system, the researcher gives the formula for calculating the value of N in Eq. 3.

(2) Grouting material strength design value
When the destruction of the anchorage system occurs between the slurry and the ruins, the anchorage performance of the anchor can be fully exploited, which belongs to the ideal destruction location. However, such kind of destruction will cause damage to the earthen ruins and is not conducive to the protection of earthen cultural relics. The load of the geo-filament anchor is mainly transmitted through the bond stress (shear stress) between the anchor and the slurry, and the failure form of the slurry is a shear failure.
By comparing the stress distribution at the geofilament-grouting interface in Figs. 17,18,19,20,21,22 and comparing and analyzing the obtained data, it can be concluded that the L/5 section at the anchor end bears about 45% of the anchorage force under the action of anchorage force design value N. The anchorage system's damage begins with the end grouting's crack. After a certain simplification, it is possible to obtain a plot of the stress distribution of the anchorage system of the geo-filament anchor under the action of anchorage force N, as shown in Fig. 23.
In Fig. 23, α = π D 2d represents the bond stress between the L/5 section of the geo-filament end and the grouting interface, and τ 1 represents the bond stress between the L/5 section grouting of the geo-filament end and the ruins soil interface. According to the balance of internal and external force τ 1 , τ 2 can be obtained as: where d is the width of the geo-filament, and D depicts the bore diameter of grouting (Fig. 7). τ 1 = π D 2d τ 2 can be obtained from Eqs. (4), (5), α = π D 2d is defined as the shear stress diffusion coefficient. The relationship of τ 1 and τ 2 can be written as: where we need to satisfy τ 2 ≤ τ max 2 n , and n is the safety factor. Therefore, the relation between τ 1 and τ 2 can be further written as: Through comprehensive analysis, it can be concluded that the shear strength of the grouting material should not be more than α n times that of the soil for the design of the geo-filament bolt.

Conclusion and reflection
This paper is a discussion, reflection, and summary of the experience of anchoring techniques in reinforcing of the earthen architectural sites. That is more than just an introduction to implementing a new type of bolt mechanical performance test. It is essential to summarize the experience of the engineering field test to reflect on the problems in the application and study of the anchoring techniques for the earthen ruins.
The test in this paper was carried out on the Gaochang Old City site protection project site. There are no ideal indoor test conditions. The processing, drilling, grouting, and other procedures of the anchor rod are not simply designed to meet the testing effect but also combine the convenience and operability of construction. Therefore, the obtained test data can reflect the actual situation relatively completely.

Test conclusions
In reinforcing small sliders for earthen architectural sites, a GFA has its advantages, including easiness in installation, due to its relatively small perforation and effectiveness in deformation control. A GFA employs a full-length bonding anchoring mechanism, of which an underapplied-force simulation reveals a failure process in three phases, namely filament deformation (elastic deformation), slurry cracking (plastic deformation), and Geofilament dislodging (adhesive detachment). Its maximum anchoring force is primarily determined by the anchorage length (L). However, its force transmission mechanism and destruction mode could be improved by adjusting its grouting material strength (S) and surface roughness (R).). The design value of the anchorage force (N) is determined by the extreme value of pulling force in each phase of its destruction process and L. Approximate 45% of the N is supported by the segment, 1/5 of L, near the end of the bolt. The stress distribution at the soil-anchor interface could thus be simplified as a uniform distribution on the (6)  On the premise of providing the required anchorage force, the ''flaky'' rod can change the destruction mode of the traditional anchor. Considering the possibility of failure in the life cycle of the reinforced earthen ruins, the anchorage system using a ''flaky'' rod mostly fails in the form of filament pullout, which is more conducive to the complete protection of ruins, the anchorage depth of GFA should not be greater than 2 m, the optimal anchorage length should be between 1.2 m and 1.5 m, the number of filaments should not be greater than one filament, and the bore diameter should be between 40 and 50 mm.

Test defects
The test design scheme in this paper also suffers from some drawbacks. Adding concrete to the grout material does not comply with the concept of earthen ruins protection, as the salt in the concrete will subsequently promote salt precipitate on the ruins. In the comparative analysis of the test data, it was found that there are certain defects in the strain gauge arrangement in the length-affected test group. In the L12, L15, L20, and L30 test groups, the test gauge shall be added; that is, the strain gauge shall be placed at 800 mm in L12; the strain gauge shall be placed at 800 mm and 1200 mm in L15, and so forth to facilitate the comparison and analysis of the data results.
Therefore, the characteristics of cultural relics shall be fully considered in the testing and theoretical analysis of new reinforcement materials and reinforcement technologies. We must not rush to apply new materials or technologies to cultural relics protection projects by simply setting the quality of reinforced materials and technical measures as the standard.

Technology reflection
The original purpose of the test is to test the anchorage performance of an anchorage techniques used to reinforce small sliding masses in earthen ruins. With the development of the test, a new understanding of the preservation techniques of earthen ruins has been generated through the analysis of test phenomena and data.
Past studies routinely adopted, from geotechnical engineering, anchoring theories and testing methods which were ill-suited to solving the peculiar problems encountered in earthen site protection. To challenge this trend, this paper puts forward a new design calculation method and design concept for the anchor bolt of the earthen ruins through the corresponding analysis of the test phenomenon and test data, combined with the protection characteristics of the earthen ruins and the concept of cultural relic protection.
This paper raises the question of small and mediumsized slip bodies protecting earthen sites. The author holds that all the stability problems to be solved in the protection of the earthen ruins should belong to solving the stability of small-scale slip bodies.
Structural reinforcement of earthen ruins requires first understanding the mechanisms of their structural instability to develop a scientific reinforcement plan, rather than the anchor rod design work currently done only for reinforcement. Due to the complex factors affecting the instability mechanism of earthen sites, it is necessary to study the instability mechanism from multiple perspectives and adopt more comprehensive analysis methods to achieve scientific assessment and analysis [25][26][27].
The protection of earthen sites has its particularity, which is not that the stronger the reinforcement measures are, the better the effect is. The mechanical characteristics of cultural relics shall be considered to predict the possibility of damage in the life cycle of cultural relics. The anchorage calculation theory shall not copy the existing theories in the field of geotechnical engineering. It shall start from the characteristics of earthen ruins protection to form its design concept and calculation method.