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Several Issues in Geotechnical Anchor Bolts and Anchorage Structure Design

Release date:

2022-01-04

Rock-soil anchors and anchoring systems are an innovative, lightweight engineering technology that actively harnesses and fully utilizes the inherent strength and self-stabilizing capabilities of rock and soil. Over the past half-century, these systems have experienced unprecedentedly rapid and widespread adoption across civil, hydraulic, architectural, transportation, and mining sectors. They have demonstrated unparalleled advantages over traditional passive support structures—particularly in enhancing both the stability and cost-effectiveness of engineering projects. In recent years, China has achieved remarkable success in advancing new theories and developing cutting-edge technologies for rock-soil anchoring. As the country continues to ramp up infrastructure investments, especially in critical areas like water resources, transportation, energy, and urban development, rock-soil anchors and anchoring systems are poised to unlock even greater potential for growth and innovation.

However, with the widespread application of geotechnical anchoring technology, several engineering accidents have occurred during the design or construction process—often due to insufficient understanding of anchoring mechanisms or a lack of design experience—resulting in significant economic losses. To address the engineering challenges arising from the promotion and application of geotechnical anchoring techniques, we have compiled Professor Cheng Liangkui’s articles, including "Several Mechanical Conceptual Issues in Geotechnical Anchoring Projects" and "Research and Recent Advances in Geotechnical Anchoring," into a comprehensive guide titled "Key Issues in the Design of Geotechnical Anchor Bolts and Anchored Structures." This resource is intended as a practical reference for engineers and technicians involved in geotechnical engineering design and practice.

Several Issues in Geotechnical Anchor Bolts and Anchorage Structure Design

Cheng Liangkui

Geotechnical anchors and anchoring structures represent an emerging engineering discipline that fully unlocks and leverages the potential of soil and rock materials. As China's geotechnical anchoring theories continue to evolve, innovative technologies and methodologies are constantly being developed. Today, geotechnical anchoring techniques have been widely applied in various construction projects, including slope stabilization, excavation support, tunneling, underground caverns, mining operations, dam construction, waterway projects, reservoirs, airports, port facilities, as well as in structures designed to resist overturning and buoyancy forces. Geotechnical anchors and anchoring systems offer significant advantages over traditional passive retaining and gravity-based structures, providing enhanced stability and cost-effectiveness for engineering projects. Their unparalleled performance has injected fresh vitality into the construction industry, positioning them with tremendous growth potential and a promising future ahead.

However, with the widespread application of geotechnical anchoring technology, several engineering accidents have occurred during design or construction due to inadequate understanding of anchoring mechanisms or insufficient design experience, resulting in significant economic losses. The following sections will discuss some of the technical issues that have emerged in these projects.

The design of the reinforcement section, free length, and anchorage body of the anchor rod, as well as the tensile bearing capacity of the anchor rod.

1.1 Design of the Cross-Sectional Area for Prestressing Tendons

Prestressed anchor rods are a tensile system designed to transfer tensile forces into stable or suitable soil and rock masses, typically consisting of an anchor head, a free-length section of the rod, and an anchored section of the rod. Their structure is illustrated in Figure 1. Regarding the design of the cross-sectional area of the prestressing tendons, JGJ120—2012 "Technical Code for Retaining Structures of Building Excavations" specifies that it should be calculated according to Equation (1):

(1)

Where: N For the design value of the anchor rod's axial tensile force, f _py For the design value of the tensile strength of prestressing steel, A _p For the cross-sectional area of the prestressing steel.

Figure 1: Schematic of Prestressed Anchorage

Equation (1) is suitable for non-prestressed anchors with conventional reinforcing bars as the tendon material, but it is inappropriate for prestressed anchors that typically use multi-strand steel cables as reinforcement, as this could lead to insufficient tendon cross-sectional capacity. Prestressed anchors used in geotechnical applications are a classic example of post-tensioned structures, and they must meet the requirements for tension control stress. Since these prestressed anchors incorporate multi-strand steel cables embedded within the ground under challenging conditions, the 4.5 mm diameter wires are particularly vulnerable to corrosion when exposed to groundwater or moist environments. Moreover, after the tendon is tensioned, the tensile stresses among individual strands and wires become uneven, with variations potentially reaching as high as 10% to 20%. Operating under such high-stress conditions, the wires are prone to developing micro-cracks, significantly increasing the risk of stress degradation over time. When designing an anchor system, the applied tension stress on the tendon should not exceed 60% of the steel’s ultimate tensile strength to ensure safety. International standards specify minimum tensile safety factors for anchor tendons—ratios of the tendon’s ultimate tensile capacity to the design tensile force—for different regions. For instance, in the U.S., the safety factor is set at 1.67; in Japan, it’s 1.54 for temporary applications and 1.67 for permanent ones; in China, it’s 1.6 for temporary projects and 1.8 for permanent ones; and in the UK, it’s 1.6 for temporary installations. Additionally, the UK’s anchor standards mandate that, for projects involving high risks of ground corrosion or severe consequences in case of failure, the tendon’s tensile safety factor must be no less than 2.0. However, calculations based on Equation (1 reveal that, when using commonly employed 1860-grade steel cables, the resulting tendon cross-section yields a tensile safety factor of only 1.4. This falls short of the required safety margin for geotechnical anchoring projects, posing potential risks to structural integrity.

1.2 Design of the Free Length of Rock Bolts

The free length of a prestressed anchor rod refers to the portion of the rod extending between the anchor head and the anchorage segment. Its primary function is to ensure that tensile forces are fully transferred to the anchored body and the surrounding ground layers. In anchor design, it is crucial to provide a free length that meets specific requirements: 1) The free segment of the anchor must extend beyond the critical failure plane by at least 1.5 meters (as shown in Figure 2). This ensures that the anchorage segment remains sufficiently distant from any potential failure surface, allowing the anchor to effectively resist shear forces while maintaining an adequate zone of compressive stress between the excavation face and the sliding surface. 2) A longer free segment facilitates positioning the anchorage segment within ground layers that exhibit higher shear strength, enhancing the overall effectiveness of the anchoring system. 3) It helps maintain the structural integrity and stability of the entire system, ensuring that the anchor rods work seamlessly with the supporting structure. 4) An adequately long free length also mitigates the risk of significant fluctuations in initial prestress caused by displacement changes. Specifically, this prevents both excessive reductions in load transfer—resulting from insufficient clamping between steel strands and anchor devices or from load losses in the force-transmitting systems like bearing plates—and dramatic increases in load transfer due to amplified ground movements. For these reasons, geotechnical anchor rod standards across various countries mandate that the free length of an anchor rod should not be less than 4.5 to 5.0 meters.

Figure 2: Positioning of the Anchorage Section of the Rock Bolt

1.3 Design of the Anchorage Section of Rock Bolts

The function of the prestressed anchor rod's anchorage section is to transfer the tensile force acting on the anchor rod’s body to the surrounding ground layers, using either grout or mechanical devices. As for the pull-out bearing capacity of bonded-type anchor rods: R It is determined by the length of the anchoring element. L _A • Anchor bolt diameter D • Bond strength between the grouting material in the anchorage section and the surrounding strata f _mg The decision previously relied on Equation (2) for calculation:

(2)

Equation (2) indicates that the uplift resistance of an anchor bolt increases proportionally with the length of its anchorage segment. As a result, some commercial software programs still rely on traditional calculation methods to determine anchor bolt lengths, leading to a widespread issue in China: excessively long anchorage segments are commonly used in foundation pit anchor-supported pile walls. However, extensive experimental and theoretical analyses reveal that, under tensile loading, the bond stress between the grout within the anchor’s anchorage zone and the surrounding ground is unevenly distributed along the length of the segment—typically following the pattern illustrated in Figure 3. Specifically, when the applied tension is low, the bond stress is concentrated over a relatively short section. As the tensile force increases, the peak bond stress gradually shifts toward the anchor’s root, while the bond stress near the near end of the anchorage segment drops sharply. Eventually, when the peak bond stress reaches the root, the residual bond stress at the near end of the anchorage segment plummets to very low levels—or even triggers debonding between the grout and the surrounding soil/rock. In essence, the shorter the anchor’s anchorage segment, the higher its average bond strength tends to be. Importantly, the effective length of the anchorage segment, which can truly leverage the shear strength of the surrounding soil or rock, is inherently limited.

Figure 3: Bond Stress Distribution Along the Entire Length of the Load-Intensive Anchor Bolt in the Anchorage Zone

The GB 50086—2015 "Technical Code for Geotechnical Anchors and Shotcrete Support Engineering" correctly specifies that the anchorage length for load-concentrated anchors should ideally be 3–8 meters in rock and 6–12 meters in soil. However, excessively long anchorage lengths are neither necessary nor cost-effective—they can actually reduce anchor installation efficiency, increase project costs, and delay the optimal timing for anchoring operations.

There are numerous uncertainties involved in anchor design, which is why it’s crucial to strictly adhere to code requirements by incorporating safety factors. Depending on the extent of potential public safety risks following failure in an anchoring project, the pull-out safety factor for the permanent anchor’s embedded section should be no less than 1.8 to 2.2, while the tensile safety factor for the anchor rod itself should be at least 1.6 (for reinforcing bars) and 1.8 (for steel strands). Additionally, assuming that the bond stress along the anchorage length is uniformly distributed may lead to an inaccurate estimation of the anchor’s load-carrying capacity. In reality, the bond strength decreases as the anchorage length increases. Therefore, when calculating the anchor’s ultimate load capacity, it’s essential to account for the influence coefficient ψ, which reflects how anchorage length affects bond strength. Specifically: - For anchorage lengths greater than 6.0 meters (for rock anchors) or 10.0 meters (for soil anchors), the influence coefficient ψ can be set within the range of 0.6 to 1.0. - For shorter anchorage lengths—less than 6.0 meters (rock anchors) or 10.0 meters (soil anchors)—the coefficient ψ should be higher, typically ranging from 1.0 to 1.6.

1.4 Selection of Anchor Types in Anchorage Design

For geotechnical projects that explicitly require anchor forces to resist structural overturning, vertical displacement, sliding along the base or shear surfaces, and potential large-scale instability or collapse, prestressed anchors should always be employed. For permanent rock slopes and the support of large-scale rock caverns, it is advisable to use an anchoring system that combines long, high- or moderately prestressed anchors (cables) with short, low-prestress anchors, depending on the specific stability needs.

Rock tunnel support should actively incorporate low-prestress steel rock bolts equipped with mechanical anchors at the headings, or those secured using high-strength grouting materials such as resins or cement-based anchoring agents. For mine roadway projects where the surrounding rock has a short self-stabilization time, exhibits significant creep behavior, or is susceptible to blasting-induced vibrations, friction-type low-prestress anchors—such as split-sleeve or hydraulically expanded designs—are particularly well-suited.

For slope or excavation projects in soft rock and soil, it is advisable to use load-dispersing anchoring systems. Permanent anchoring projects should utilize pressure-dispersive anchor rods, while temporary anchoring projects can rely on tension-dispersive anchor rods. For structural uplift resistance applications, pressure-type, pressure-dispersive anchor rods, or enlarged-diameter anchor rods are recommended.

Non-prestressed rock bolts can be used for supporting tunnel caverns with smaller spans (＜10m) located in Class II and III surrounding rock conditions, helping to control small-scale rock sliding or soil deformation between prestressed anchor bolts in slope stabilization projects, as well as reinforcing the slopes of excavations with shallow digging depths.

Non-prestressed, fully bonded anchor rods (soil nails) used to reinforce excavated soil typically suffer from a significant reduction in soil strength due to the presence of groundwater, rainwater, leaks from underground pipelines, and localized water sources. This, in turn, diminishes the frictional resistance between the soil and the soil nails, often leading to the collapse of soil-nail walls in water-saturated areas. Conversely, when soil-nail support is applied above the groundwater table—or if artificial methods are employed to lower the groundwater level—and where surface water is effectively managed through robust drainage systems, the reinforcing effect of the soil nails and the overall stability of the soil-nail wall are markedly enhanced.

Several Effective Methods to Enhance the Pullout Bearing Capacity of Rock Bolts

1) The Single-Hole Composite Anchoring Method (Figure 4a) involves installing two or more individual anchor units within a single borehole. Each unit features a separate free-length segment and an anchorage segment along its rod. When tensile forces are applied individually to each unit, the method ensures a uniform distribution of bond stress between the grout and the surrounding ground, significantly reducing stress peaks. This approach maximizes the utilization of the shear strength of the ground surrounding the anchorage segments. Additionally, the pull-out resistance of the anchor increases proportionally with both the number of individual anchoring units and the total length of the anchored body.

Figure 4: Several Methods to Enhance Anchorage Strength

2) Post-High-Pressure Grouting Anchoring Method (Fig. 4b). After inserting the anchor rod, equipped with specialized devices such as sleeve-valve tubes and sealing bags, into the borehole, gravity grouting is first applied to the anchorage section, forming a cylindrical grout body. Once the grout reaches a strength of 5.0 MPa, high-pressure grouting fluid—applied at a pressure no less than 2.5 MPa—is used to split the initial grout body. This process allows the grout to penetrate, spread, and exert lateral pressure into the surrounding ground around the anchorage zone, significantly enhancing the bond strength between the grout and the formation. As a result, the ultimate pull-out resistance of the anchor rod is dramatically increased, often by several times its original capacity.

3) The enlarged-head anchoring method (Fig. 4c). This technique leverages the bearing resistance of the soil at the varying cross-section of the anchorage segment, significantly enhancing the ultimate pullout capacity of the anchor rod.

Load-distributing (single-hole composite) anchoring system's load-transfer mechanism

Due to the concentrated load, the bond stress distribution between the grout in the anchorage section of the anchor rod and the surrounding ground is uneven. The shorter the anchorage segment, the higher the average bond stress becomes, allowing for more effective utilization of the shear strength of the ground surrounding the anchor rod's anchorage area. To address the drawbacks of the load-concentrated transfer mechanism in conventional anchor rods, a pressure-dispersive (removable-core) anchor technology—developed independently by Cheng Liangkui and his team (as shown in Figure 5)—has been introduced. Typically, this type of anchor consists of 2 to 4 individual unit anchors, each with a relatively short anchorage length, usually ranging from 2 to 4 meters. As a result, the load borne by each unit anchor is only about 1/2 to 1/4 of that carried by a load-concentrated anchor. Compared to load-concentrated anchor rods, pressure-dispersive anchors exhibit significantly superior mechanical properties (as illustrated in Figure 6).

Figure 5: Schematic Diagram of the Pressure-Distributed Anchor Bolt Configuration

Figure 6: Internal Force Distribution of Pressure-Type Anchors

1) It can significantly reduce the peak axial force in the grouting body of the anchor bolt and the shear (adhesive) stress between the grouting material and the surrounding ground, markedly improving the uniformity of both the axial force distribution within the grout and the shear (adhesive) stress distribution.

2) As the number of unit rock bolts increases and the total anchored length of the bolts grows, the pullout resistance capacity of the bolts can be improved proportionally.

3) When the anchor rod is loaded, the bond stress distribution between the grout in the anchorage zone and the surrounding ground tends to become more uniform, significantly reducing stress concentrations. This helps minimize creep in the anchored ground and limits shear displacement between the ground and the grout, thereby effectively controlling the initial loss of anchor prestress and enhancing the long-term performance of the anchor rod.

4) The rod of the stress-dispersing anchor bolt consists of bare steel strands coated with grease and encased in a protective PE layer. When the bolt is subjected to tension, the grout surrounding the anchor remains predominantly under compression, making it highly resistant to cracking. This design significantly enhances the corrosion protection of the bolt’s steel core, thereby improving the overall durability of the anchor bolt.

Anchoring Timing

After excavation of slopes, tunnels, and caverns, failure to promptly install rock bolts to provide adequate anchoring resistance can lead to deformation of the surrounding rock and soil due to unloading effects caused by excavation. Over time, this deformation tends to worsen. Additionally, as the excavation area expands, factors such as vibrations, rainfall, weathering, and temperature fluctuations further reduce the shear strength of both the rock-soil mass and its structural planes. Meanwhile, weak fill materials within rock joints and fractures may gradually wash away, accelerating the decline in the rock mass's inherent load-bearing capacity.

Maximizing the reduction of the time interval between excavation of the rock and soil mass and the development of anchor bolt resistance is crucial. This approach minimizes both the duration and the area of the excavation face exposed without any anchoring support, which is the primary condition and fundamental principle for fully leveraging the self-supporting capacity of the excavated materials and enhancing the stability of slopes and tunnels. For large-span, high sidewall caverns and tunnel projects located in IV- and V-grade surrounding rock conditions, systematic anchor bolt support systems should comprehensively adopt low-prestress anchors such as expansion-shell mechanical anchors, resin-cased anchors, and fast-setting cement-cased anchors—replacing the conventional passive full-length bonded anchors. These low-prestress anchors provide active support forces, effectively controlling early-stage deformation of the rock mass during excavation. By rapidly mobilizing the natural self-supporting capacity of the surrounding rock, they ensure that fractured rock blocks within the anchored zone are tightly interlocked and firmly engaged, forming a compressive rock-bearing ring (arch) that significantly improves the overall stability of underground structures.

Anchorage Design for Large-Span High-Height Excavations with High-Stress, Low-Strength Rock Mass

1) The areas surrounding the cavern (including the crown and sidewalls) should be promptly and comprehensively reinforced with low-prestressed (tensioned) rock bolts and steel-fiber-reinforced shotcrete immediately after excavation. Carefully selected support parameters will enable the rapid formation of a robust, high-strength "bolt-spray–rock" load-bearing ring around the cavern perimeter—characterized by adequate thickness, exceptional rigidity, and a stable three-dimensional compression state—to effectively control the severe deformations triggered by initial rock stress release during excavation.

2) By using prefabricated elements as the load-transfer structure, the interval between the low-stress anchor bolts of the installation system and the prestressed long anchors is minimized to the greatest extent. The tensioning and locking of the prestressed long anchors should ideally be completed within 20 to 30 days after rock excavation. This ensures that the anchored rock ring surrounding the cavern is securely connected to the stable bedrock layers deep below, effectively restraining any subsequent deformation of the cavern’s surrounding rock mass.

3) Reduce the spacing of the prestressed long anchor rods to increase the initial prestress value applied per unit area of the tunnel wall, ensuring that this value is no less than 120 kN/m. ²。

4) The initial prestress (locking load) of the prestressed anchor bolts should ideally be set at 50% of the design tensile capacity of the bolts. This allows for the possibility that, as rock stresses continue to release and surrounding rock deformation increases, the tensile force in the bolts may rise accordingly. In turn, the increased resistance of the bolts will help counteract further development of surrounding rock deformation, ultimately enabling the surrounding rock and the support system (stiffness) to achieve stable, coordinated growth over time.

5) Strengthen displacement monitoring and information feedback for cavern engineering. If sustained, rate-like increases in surrounding rock displacement occur—indicating an imbalance in the interaction between the surrounding rock and support forces—immediate reinforcement with additional prestressed anchor bolts is required.

Slope Anchorage Effect

6.1 Stability Calculation for Anchored Slopes

Currently, when applying the rigid-body limit equilibrium method to assess the anti-sliding stability of anchored slopes in China, it is often observed that even with a high number of prestressed anchors—each capable of supporting substantial loads—their calculated contribution to the overall slope stability remains surprisingly small or severely limited. This discrepancy stands in stark contrast to the actual stability conditions seen in real-world anchored slopes. The root cause of this issue lies in the fact that current calculations fail to account for how anchor prestress can significantly boost both the cohesion (c) at the potential failure surface and the rock’s elastic modulus (E). Moreover, these analyses tend to underestimate the tangential resistance provided by anchor forces acting directly on the failure plane. When sufficient data or empirical evidence is available, a practical approach is to conservatively increase the cohesion value (c) as a safety margin. Additionally, in conventional methods used to calculate the safety factor of anchored slopes, the tangential component of the anchor force is typically included in the numerator of the formula, inadvertently leading to an underestimation of the anchors' true stabilizing effect. A more accurate approach would involve placing this tangential force in the denominator of the equation, treating it as a reduction in the driving下滑力 (downward sliding force).

6.2 Anchor Bolt Layout on Rock Slopes

When designing rock slope anchoring projects, it is essential to clearly understand the slope's lithology, rock mass structure, and the interplay among structural planes, as well as the relationship between these structural planes and the slope face. Additionally, geological factors such as the distribution of weak zones within the rock and groundwater conditions must be thoroughly assessed. Based on this analysis, the type or failure mode of the slope should be identified. Finally, the anchor layout scheme can be selected according to the most likely failure pattern.

For sedimentary or metamorphic rock bodies presenting gently dipping discontinuities β < α ), slopes prone to planar failure typically employ prestressed anchors uniformly arranged along the slope surface (Figure 7a); for steeply inclined, discontinuous-rock slopes β > α ), it may be considered to avoid installing pre-stressed anchor rods in the system, opting instead for protective measures directly on the slope surface. However, for rock slopes featuring horizontal jointed rock masses (such as thin-layered sedimentary rocks) where circular sliding is likely or where weathered, weak layers occur at the slope toe, pre-stressed anchor rods can be strategically placed at stress-concentration zones near the slope toe (Figures 7b and 7c). For slopes prone to toppling failure, it is advisable to install pre-stressed anchor rods in the upper to middle sections of the slope, with the rods oriented at an upward angle (Figure 7d). This arrangement helps enhance the normal force acting on discontinuous surfaces, effectively resisting both toppling and flexural-buckling failures of the rock mass.

Figure 7: Rock Rock Slope Structure and Anchor Bar Layout

Project Showcase

China Bank Headquarters Foundation Excavation Anchored Wall Support Project

Anchored and Shotcrete Support Project for the Underground Powerhouse of the Three Gorges Hydropower Station

Shijiazhuang Concrete Gravity Dam Anchorage Project

Jinping First Power Station Left Dam Abutment 530m High Slope Anchoring Project

About the Author

Cheng Liangkui, male, born in November 1935, is from Liyang, Jiangsu Province. He formerly served as the Deputy Chief Engineer at the General Institute of Building Research under the Ministry of Metallurgy and currently holds the title of a senior engineer at the professor level. He is now or has previously held positions such as Executive Director of the China Society for Rock Mechanics and Engineering, Chairman of the Technical Advisory Committee of the same society, member of the Chinese Group within the International Society for Rock Mechanics, President of the China Association for Rock-Soil Anchorage Engineering (for its first, second, and third sessions), and Vice Chairman of the Underground Rock Engineering Professional Committee under the China Society for Rock Mechanics and Engineering. Additionally, he serves as a part-time professor at Dalian University of Technology, Beijing University of Science and Technology, and China University of Mining and Technology, and is an editorial board member for journals including *Chinese Journal of Civil Engineering* and *Journal of Rock Mechanics and Engineering*.

Cheng Liangkui has long been dedicated to the research and application of geotechnical anchoring, shotcrete technology, and the stability of tunnels, slopes, and deep excavations. He was among the first in China to lead successful research on shotcrete and its supporting techniques, developing shotcrete-anchoring systems tailored to diverse working conditions and varying geological settings of surrounding rock. Cheng also pioneered theories on reinforcing rock arches and controlling rock mass creep through shotcrete-anchoring methods, significantly advancing the understanding of the interaction between rock and anchors. Recognized as a pioneer and leader in China’s shotcrete structures and geotechnical anchoring systems, Cheng Liangkui, as the primary contributor, has received 18 national and provincial-level awards for scientific and technological progress, including first-, second-, and third-place honors, as well as the National Science and Technology Conference Award. Notably, he was awarded two National Science and Technology Progress Prizes. Additionally, he has played a key role in drafting or revising seven national and industry standards. Cheng has authored or co-authored nine specialized books and published over 200 papers both domestically and internationally. His groundbreaking work has been instrumental in establishing and advancing China’s expertise in geotechnical anchorages and anchoring structures, driving fundamental transformations and leaps forward in the country’s engineering practices—particularly in areas such as tunnel construction, large-scale underground chambers, slope stabilization, deep excavation support, and innovative solutions for structural uplift and tensile foundations. In recognition of his outstanding contributions, Cheng was honored with a special government allowance from the State Council in 1991. In 2012, he was featured in the *Biographical Dictionary of Chinese Science and Technology Experts*, compiled by the China Association for Science and Technology, specifically in the Engineering Technology section of the Metallurgy volume. Most recently, in 2019, he was included in the *Tribute: Celebrating the Spirit of Scientists* series, part of the China Rock Mechanics and Engineering Society’s collection honoring distinguished scientists.

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