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Jaxon Perez
Jaxon Perez

BS 8081-1989: A Comprehensive Guide to Ground Anchorages


What is BS 8081-1989 and why is it important?




BS 8081-1989 is a British standard that provides recommendations for soil and rock anchorage systems of the grouted or mechanical type. It was first published in 1989 and superseded by BS 8081:2015 in 2015. However, it is still widely used and referenced by engineers and contractors who deal with ground anchorages.




BS 8081-1989, Code of practice for ground anchorages.pdf



Ground anchorages are devices that transfer tensile forces from a structure to the ground. They are used for various purposes, such as stabilizing slopes, retaining walls, dams, bridges, tunnels, foundations, and other structures that are subject to uplift or lateral forces. Ground anchorages can also be used to resist seismic loads, wind loads, or buoyancy forces.


BS 8081-1989 covers the basis for geotechnical design and structural design of ground anchorages. It also provides guidance on the materials, installation, testing, monitoring, and maintenance of ground anchorages. It aims to ensure that ground anchorages are designed and constructed safely, efficiently, and durably.


What are ground anchorages and how do they work?




A ground anchorage consists of three main components: an anchorage head, a tendon, and a bond length. The anchorage head is the part that connects the tendon to the structure that needs to be anchored. The tendon is the part that transmits the tensile force from the anchorage head to the bond length. The bond length is the part that transfers the tensile force from the tendon to the surrounding soil or rock.


The principle of a ground anchorage is to create a frictional bond between the tendon and the soil or rock along the bond length. This bond resists the pull-out force that acts on the tendon when it is tensioned. The bond strength depends on several factors, such as the type and condition of the soil or rock, the diameter and shape of the tendon, the grouting method and material, and the installation quality.


The performance of a ground anchorage depends on its load-displacement behavior. This behavior describes how much the anchorage displaces (or elongates) when it is subjected to a certain load (or tension). Ideally, a ground anchorage should have a high load capacity and a low displacement. However, in reality, there is always some degree of displacement due to elastic deformation of the tendon and creep of the soil or rock.


Types of ground anchorages




There are two main types of ground anchorages: grouted anchorages and mechanical anchorages. Each type has its own advantages and disadvantages depending on the site conditions and design requirements.


Grouted anchorages




A grouted anchorage is a type of ground anchorage that uses cementitious grout to fill the annular space between the tendon and the borehole wall along the bond length. The grout acts as a bonding agent that creates friction between the tendon and the soil or rock. Grouted anchorages can be further classified into two subtypes: post-tensioned grouted anchorages and pre-tensioned grouted anchorages.


A post-tensioned grouted anchorage is a grouted anchorage that is tensioned after the grout has hardened. This type of anchorage allows for a better control of the tendon stress and displacement. However, it requires a longer curing time and a more complex installation process.


A pre-tensioned grouted anchorage is a grouted anchorage that is tensioned before the grout has hardened. This type of anchorage allows for a shorter curing time and a simpler installation process. However, it may result in a lower bond strength and a higher displacement.


Mechanical anchorages




A mechanical anchorage is a type of ground anchorage that uses mechanical devices to lock the tendon to the soil or rock along the bond length. The mechanical devices can be wedges, cones, plates, or other shapes that create an interlocking effect between the tendon and the borehole wall. Mechanical anchorages can be further classified into two subtypes: expansion shell anchorages and rock bolt anchorages.


An expansion shell anchorage is a mechanical anchorage that uses an expansion shell device to grip the tendon to the borehole wall. The expansion shell device consists of a hollow metal cylinder with slits and wedges inside. When the tendon is inserted into the cylinder and tensioned, the wedges are forced outward and press against the borehole wall. This creates a radial force that locks the tendon in place.


A rock bolt anchorage is a mechanical anchorage that uses a rock bolt device to attach the tendon to the borehole wall. The rock bolt device consists of a metal rod with threads or deformations along its length. When the rod is inserted into the borehole and rotated, it cuts into the soil or rock and forms a bond. This creates an axial force that holds the tendon in place.


Design considerations for ground anchorages




The design of ground anchorages involves two aspects: geotechnical design and structural design. Geotechnical design deals with the interaction between the ground anchorages and the soil or rock. Structural design deals with the interaction between the ground anchorages and the structure.


Geotechnical design




The geotechnical design of ground anchorages aims to ensure that the ground anchorages can resist the design load without exceeding the allowable displacement or causing failure of the soil or rock. The geotechnical design involves the following steps:


  • Ground assessment: This step involves collecting and analyzing data on the soil or rock properties, such as density, strength, stiffness, permeability, and groundwater level. The data can be obtained from field investigations, laboratory tests, or empirical correlations.



  • Load analysis: This step involves determining the magnitude and direction of the load that acts on the ground anchorages. The load can be derived from the structural analysis or from empirical formulas.



  • Bond length determination: This step involves calculating the required bond length for each ground anchorage based on the load analysis and the bond strength of the soil or rock. The bond length should be sufficient to mobilize enough friction along the bond length to balance the pull-out force.



  • Displacement analysis: This step involves estimating the displacement of each ground anchorage under the design load based on the load-displacement behavior of the soil or rock and the tendon. The displacement should be within the allowable limit for serviceability and stability.



  • Safety analysis: This step involves checking the safety factor of each ground anchorage against failure modes such as bond failure, tendon rupture, or soil or rock failure. The safety factor should be greater than or equal to a specified value for reliability.



Structural design




The structural design of ground anchorages aims to ensure that the ground anchorages can transfer the tensile force from the structure to the soil or rock without exceeding the allowable stress or causing damage to the structure. The structural design involves the following steps:


  • Tendon selection: This step involves choosing the type, size, shape, material, and coating of the tendon for each ground anchorage based on the load analysis and the corrosion resistance. The tendon should have enough strength, stiffness, and durability to withstand the tensile force and the environmental conditions.



  • Anchorage head design: This step involves designing the anchorage head for each ground anchorage based on the tendon selection and the connection method. The anchorage head should have enough strength, stiffness, and ductility to transfer the tensile force from the tendon to the structure without causing stress concentration, slippage, or fatigue.



  • Corrosion protection: This step involves applying corrosion protection measures for each ground anchorage based on environment and the service life of the ground anchorages. The corrosion protection measures can include coating, wrapping, sleeving, or cathodic protection of the tendon and the anchorage head.



Installation and testing of ground anchorages




The installation and testing of ground anchorages aims to ensure that the ground anchorages are constructed in accordance with the design specifications and achieve the required performance and quality. The installation and testing involves the following steps:


  • Drilling and grouting: This step involves drilling the borehole for each ground anchorage using appropriate drilling equipment and methods. The borehole diameter, depth, inclination, and alignment should match the design values. The borehole should be cleaned and flushed before grouting. The grouting process should follow the specified grout mix, pressure, volume, and injection method.



  • Tensioning and locking: This step involves tensioning each ground anchorage using a hydraulic jack and a load cell. The tensioning force should be applied gradually and maintained for a certain time until the elongation stabilizes. The tensioning force should be within the design range and verified by a calibrated gauge. The anchorage head should be locked by a nut or a plate to prevent loss of tension.



  • Monitoring and maintenance: This step involves monitoring the performance and condition of each ground anchorage during and after installation. The monitoring methods can include load tests, displacement measurements, strain gauges, corrosion sensors, or visual inspections. The monitoring frequency and duration should be based on the design requirements and site conditions. The maintenance actions can include adjusting the tension, repairing the corrosion protection, or replacing the damaged components.



Benefits and limitations of ground anchorages




Ground anchorages have many benefits and limitations that should be considered before choosing them as a solution for a geotechnical or structural problem. Some of the benefits and limitations are summarized below.


Advantages of ground anchorages




  • They can provide a high load capacity and a low displacement compared to other types of soil or rock reinforcement.



  • They can reduce the excavation volume and the retaining wall height compared to conventional methods.



  • They can accommodate complex geometries and variable ground conditions.



  • They can improve the stability and durability of structures that are subject to uplift or lateral forces.



  • They can be installed quickly and easily with minimal disturbance to the surroundings.



  • They can be adjusted, monitored, and maintained during their service life.



Challenges and risks of ground anchorages




  • They require a thorough ground assessment and a careful design to ensure their safety and reliability.



  • They are susceptible to corrosion and deterioration due to environmental factors or installation defects.



  • They may cause damage to existing utilities or structures during drilling or grouting.



  • They may induce stress changes or deformations in the soil or rock that affect their performance or cause adverse effects on adjacent structures.



  • They may face legal or contractual issues regarding ownership, responsibility, or liability of the ground anchorages.



Conclusion




Ground anchorages are an effective and versatile technique for transferring tensile forces from a structure to the ground. They can be used for various applications in geotechnical and structural engineering, such as stabilizing slopes, retaining walls, dams, bridges, tunnels, foundations, and other structures that are subject to uplift or lateral forces. Ground anchorages can also be used to resist seismic loads, wind loads, or buoyancy forces.


However, ground anchorages also pose many challenges and risks that require a comprehensive understanding of their principles, design, installation, testing, monitoring, maintenance, and corrosion protection. BS 8081-1989 is a British standard that provides recommendations for soil and rock anchorage systems of the grouted or mechanical type. It covers the basis for geotechnical design and structural design of ground anchorages. It also provides guidance on the materials, installation, testing, monitoring, and maintenance of ground anchorages. It aims to ensure that ground anchorages are designed and constructed safely, efficiently, and durably.


FAQs




  • Q: What is the difference between a ground anchorage and a soil nail? A: A ground anchorage is a device that transfers tensile forces from a structure to the ground, while a soil nail is a device that transfers shear forces from the ground to a structure. A ground anchorage has a free length and a bond length, while a soil nail has only a bond length. A ground anchorage is tensioned after installation, while a soil nail is not.



  • Q: What are the main factors that affect the bond strength of a ground anchorage? A: The main factors that affect the bond strength of a ground anchorage are the type and condition of the soil or rock, the diameter and shape of the tendon, the grouting method and material, and the installation quality.



  • Q: How can corrosion be prevented or reduced in ground anchorages? A: Corrosion can be prevented or reduced in ground anchorages by applying corrosion protection measures, such as coating, wrapping, sleeving, or cathodic protection of the tendon and the anchorage head. The corrosion protection measures should be selected based on the service life of the ground anchorages, the aggressivity of the environment, installation methods and consequences of failure.



  • Q: How can the performance and condition of ground anchorages be monitored and maintained? A: The performance and condition of ground anchorages can be monitored and maintained by using monitoring methods, such as load tests, displacement measurements, strain gauges, corrosion sensors, or visual inspections. The monitoring frequency and duration should be based on the design requirements and site conditions. The maintenance actions can include adjusting the tension, repairing the corrosion protection, or replacing the damaged components.



  • Q: What are some of the benefits and limitations of ground anchorages? A: Some of the benefits of ground anchorages are that they can provide a high load capacity and a low displacement, reduce the excavation volume and the retaining wall height, accommodate complex geometries and variable ground conditions, improve the stability and durability of structures, and be installed quickly and easily. Some of the limitations of ground anchorages are that they require a thorough ground assessment and a careful design, are susceptible to corrosion and deterioration, may cause damage to existing utilities or structures, may induce stress changes or deformations in the soil or rock, and may face legal or contractual issues.



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