Understanding Ground Anchors

What are Ground Anchors?

Ground anchoring is an invaluable geotechnical technique that involves installing tensioned tendons into the ground to reinforce structures and improve slope stability. It allows economical support compared to large gravity retaining structures.

A ground anchor assembly consists of a prestressing tendon, commonly a high strength steel strand or bar, and a bond length where the tendon is grouted into the ground. The free end of the tendon is secured to the structure using an anchor head.

By tensioning the tendon using hydraulic jacks, an active prestress force is induced which gets transferred to the soil/rock via friction along the bond length. This provides reinforcing action, increasing the resisting forces available to stabilize the structure.

Ground anchors can be installed in a wide range of geological conditions like rock, clay, sand, gravel, etc. Installation involves specialized drilling and grouting techniques. Corrosion protection is critical to ensure long-term performance.

Anchors are designed based on the ground conditions and load requirements. Load testing of every installed anchor is essential to validate the design through proof tests. Monitoring using load cells is also increasingly adopted.

Ground anchoring finds extensive applications in retaining structures, excavations, slope stabilization projects and foundation uplift resistance. It is an economical and adaptable technique. But it requires careful design, construction and testing to be effective. Proper specifications, workmanship and quality control are critical.

Ground Anchors - Explain Like I'm 5 - ELI5

Imagine you want to hold a tree in place so it doesn't fall over. What could you do? You could tie a rope from the tree to a stake in the ground. The rope is the "anchor" providing a pulling force to keep the tree upright.

Ground anchors work the same way! They are like really big strong ropes used to provide a pulling force holding up man-made structures like walls or slopes.

A ground anchor has two main parts - the tendon and the bond length. The tendon is a super strong steel cable or bar. One end of the tendon is attached to the structure we want to hold up. The other end of the tendon is grouted deep into the ground.

The part embedded in the ground is called the bond length. It is fixed firmly into the soil or rock using a high-strength grout, like cement. This creates a very strong bond.

To activate the anchor, the tendon is first tensioned, or pulled tight. Hydraulic jacks are used to stretch the steel cable and apply a pre-tension force. This force gets transferred through the bond length into the ground.

The ground resists the pulling force, creating an equal opposite reaction force holding up the structure. This technology allows us to reinforce and strengthen the ground supporting important structures like buildings, bridges and slopes.

So next time you see a big retaining wall or deep excavation held up securely, there's a good chance some super strong "underground ropes" called ground anchors are doing the tough job of providing the needed strength!

Design features of a typical Ground Anchor

Here are the key design features of Ground Anchors:

Tendon - High strength steel bar or multi-strand tendon to provide tensile resistance.
Bond length - Length of the tendon that is bonded to the ground to transfer the tensile loads.
Corrosion protection - Tendon is protected against corrosion using greases, grouts, plastic sheathing etc.
Anchor head - Head where the tendon is anchored and tensioned. Transfers the load to the structure.
Free stressing length - Ungrouted portion of the tendon that can elongate during stressing.

Key advantages of using Ground Anchors

Here are the main advantages of using Ground Anchors:

Cost effective - More economical compared to large gravity retaining structures.
Adaptability - Can be installed in difficult ground conditions and geometry.
Monitoring - Loads can be monitored to assess ground movements.
Minimal disturbance - Does not require extensive excavation or fill works.
Prestress - Provides active reinforcement to the soil/rock mass.
Reusability - Anchors can be re-stressed and loads adjusted.

Ground Anchors as part of Retaining Wall Systems

Ground anchors are commonly used with retaining walls to provide lateral support and improve overall stability.

Here are some ways ground anchors can be combined with other systems in retaining walls:

Anchored retaining walls - Ground anchors installed through the wall face provide direct lateral support. Typical in top-down basement construction.

Hybrid Anchored Walls - Combination of ground anchors with passive supports like rakers or buttresses. Provides optimized design.

Anchored Sheet Pile Walls - Ground anchors installed through sheetpiles maximize wall height and stability. No excavation needed.

Soldier Pile walls - Ground anchors used with soldier piles provide lateral bracing and increase wall resistance.

Soil Nailing - Shallow soil nails provide local face stability while ground anchors provide overall stability.

Contiguous Bored Piles - Ground anchors between secant/tangent bored piles enhance overall stability.

Micropiles - Ground anchors used with micropile walls optimize stability and construction sequence.

Reinforced Slopes - Ground anchors used with granular fill slopes and reinforcements for steep slopes.

Rock Bolting - Anchoring unstable rocks behind wall using rock bolts integrates with wall stability.

Monitoring - Load cells used with anchors allow monitoring of lateral loads and wall movements.

Reinforced Earth - Integration of ground anchors with reinforced earth structures like MSE walls.

Proper combination of ground anchoring with other systems and techniques results in optimised and cost-effective retaining structures. Ground anchors also allow flexibility in construction staging.

Tools and Equipment for Ground Anchors

Here is some typical equipment and tools required for ground anchor construction:

Drilling Equipment
Drill rigs - Rotary, percussive or vibratory rigs for drilling anchor boreholes.
Drill bits - Roller cone bits, drag bits, hammers for different ground conditions.
Casing - Temporary casings used to stabilize weak boreholes.
Drilling fluid systems - For wet drilling, slurry removal and borehole cleaning.

Installation Equipment
Hydraulic jacks - For inserting and pushing anchor tendon in borehole.
Centralizers - To center the tendon in the borehole.
Grout plant - For mixing and pressurizing cement grout.
Grout pumps - Positive displacement pumps for grout injection.

Stressing Equipment
Hydraulic jack - For pulling and stressing the tendon to the design load.
Pressure gauge - To measure the hydraulic pressure on tendon.
Load cell - To directly measure the load on the tendon.
Elongation measurement - To measure extension of tendon during stressing.

Testing Equipment
Hydraulic jack - For proof testing loaded anchors.
Load cell - To measure test load applied accurately.
Dial gauges - To measure movement and elongation during testing.
Packers - For isolating sections during water loss testing.

Safety Equipment
Personal protective equipment - Hard hats, gloves, safety shoes, harnesses.
Shoring frames - For securing access shafts and boreholes.
Safety barriers - For safe access control around construction area.

Proper equipment selection, inspection and maintenance is key to efficient and safe ground anchor construction.

Key steps in installing Ground Anchors

Here are the general steps for installing ground anchors:

Drilling anchor borehole - Drilling is done using suitable drilling methods like rotary, percussive, wet drilling etc. based on the ground conditions.
Borehole cleaning - The borehole is cleaned by circulating drilling fluid and flushing repeatedly to remove all drilling cuttings.
Water testing (optional) - Water loss test done to check for natural voids by measuring water loss rate along bond length.
Tendon preparation - Corrosion protection and centralizers are installed on the tendon as per design.
Tendon insertion - The tendon assembly is carefully inserted into the borehole using a hydraulic jack.
Grouting - Cement grout is injected into the borehole at low and high pressure along the bond length in stages.
Stressing - After the grout has cured, the tendon is stressed incrementally to the lock-off load using a hydraulic jack.
Lock-off - The anchor head is permanently secured after stressing to maintain the design prestress in the tendon.
Testing - The installed anchor undergoes proof testing for acceptance as per standards. Load, movement are measured.
Anchor head protection - Corrosion protection cap or cover is installed to protect the anchor head.
Records - Borehole data, grouting details, prestress records, and test results are documented for each anchor.

Proper site supervision, quality control testing, health and safety procedures are implemented in each step to ensure performance.

Temporary Ground Anchors vs Permanent Ground Anchors

The key differences between temporary and permanent ground anchors are:

Design life - Temporary anchors are designed for short-term use during construction, usually less than 2 years. Permanent anchors are designed for the project design life.
Load capacity - Temporary anchors typically designed for lower loads than permanent anchors. Safety factors can also be reduced.
Corrosion protection - Temporary anchors have minimal corrosion protection to last the construction duration. Permanent anchors have robust long-term corrosion protection.
Testing - Testing loads and acceptance criteria can be reduced for temporary anchors. Permanent anchors follow full testing requirements.
Monitoring - Temporary anchors require minimal monitoring or maintenance. Permanent anchors require long-term monitoring and maintenance plans.
Cost - Temporary anchors aim to optimize cost for the construction phase. Permanent anchors are designed for maximum long-term economy.

Why are temporary anchors used in construction design?

Here are some of the main reasons temporary ground anchors are used in construction design:

Cost savings - Temporary anchors are cheaper to install and test compared to permanent anchors. This reduces construction costs.

Shorter design life - They only need to perform during the construction phase, not the full design life of the permanent works.

Excavation support - Used to support excavations and cut slopes that are required temporarily during construction.

Temporary structures - To support temporary structures like construction platforms, formwork or access facilities.

Staged construction - Used in early stages of projects until permanent structural supports are established.

Removing later - Temporary anchors can be removed later on once they are no longer needed. Permanent anchors are left in-place.

Load adjustments - Loads can be adjusted on temporary anchors as construction sequencing changes.

Optimized spacing - Temporary anchors can use closer spacing as they will be removed later.

Simplified corrosion protection - Only need corrosion protection to last through construction duration.

Construction loading - Used to resist temporary construction loads which exceed permanent design loads.

Monitoring - Can allow greater deformation monitoring compared to permanent anchors.

The ability to remove them or leave some in-place provides flexibility in the construction sequence and potential cost savings in building the permanent works.

How ground conditions affect Ground Anchor designs

The type of ground conditions can significantly affect the design of ground anchors in the following ways:

Rock - Good rock mass provides excellent bond zone for anchors. Corrosion protection may be reduced. High capacity anchors can be designed.

Strong intact rock - May require special high-capacity anchors bars or cables instead of strands. Permanent applications are best.

Weathered/fractured rock - Lower bond strengths and anchor capacities. Corrosion protection important. May limit applications.

Stiff overconsolidated clay - Good anchor bond but high grout pressures needed. Corrosion protection critical.

Soft normally consolidated clay - Poor anchor bond due to remoulding. Limited applications possible.

Sand - Good bond in dense sands, but capacity varies with density. Pregrouting may help. High water table challenging.

Gravel - Excellent bond but bleeding of grout into gravel needs prevention. Water loss testing important.

Fill - Poor grip, short bond lengths. Corrosion likely. Limited capacity anchors, close spacing.

Variable ground - Need good investigation. Zoned design with variable bond lengths, spacings.

Aggressive ground - Robust corrosion protection essential. May affect service life, reliability.

Seismic areas - Need ductile components. Corrosion protection for reliability. Testing important.

Proper ground investigation, characterization tests and field trials are required to optimize the anchor design for the site's ground conditions.

How are Ground Anchors tested?

Testing of ground anchors is critical to ensure the installed anchors meet the required performance criteria. Here are some key steps in testing ground anchors:

Investigation Testing - Initial testing on sacrificial test anchors to determine the anchor bond capacity and creep behavior in the site's soil/rock.
Suitability Testing - Testing of initial production anchors to confirm they achieve the required bond capacity and creep performance.
Acceptance Testing - Testing of every anchor using the following steps:

Lifting off - Any existing lock-off load is removed to test the anchor from an unloaded state.
Alignment Load - A small alignment load is applied to take up any slack in the tendon.
Lock-off Load - The design lock-off load is applied and held for 1 minute to simulate service conditions.
Proof Load - The load is increased to a specified proof load (commonly 120-150% of design load) and held for 10 mins.
Creep Test - The load is held constant for a prolonged time (up to 60 mins) while monitoring movement.
Unloading - The anchor is unloaded in steps back to the lock-off load.
Lock-off - The anchor is locked off permanently at the design load.

Movements are recorded at each step to check immediate elastic movements and creep deformations remain within specified limits. Testing ensures every anchor has the required short-term and long-term load capacity before accepting it.

What are the factors that influence the cost of Ground Anchors?

Some key factors that can influence the costs of ground anchor installation are:

Site conditions - Costs are higher in difficult ground conditions like rocks, high water tables etc. that require more complex drilling and grouting.

Anchor capacity - Higher capacity anchors require larger drill rigs, stronger tendons, more materials.

Anchor type - Multi-strand anchors are generally more expensive than single bar anchors.

Degree of corrosion protection - More corrosion protection for permanent anchors increases material costs.

Testing requirements - More elaborate investigation and proof testing requirements increase testing costs.

Monitoring - Providing monitoring devices like load cells adds to costs.

Access constraints - Tight access leads to specialized equipment needs which add costs.

Drilling method - Wet drilling in tight access may require desanding plants. Rotary can be costlier than percussion drilling.

Bond length - Longer bond lengths require longer drilling, more grout, larger tendons.

Productivity rates - Lower production rates due to site conditions or interruptions increase costs.

Operator experience - Inexperienced teams require more supervision and cause construction delays.

Health and safety - Tight safety measures in risky projects lead to slower production.

Specifications - Stringent project specifications requiring higher safety factors or QA/QC testing.

Proper site characterization through investigations, efficient construction planning and techniques, balanced specifications and experienced crews help optimize ground anchor costs.

What are design changes that can help reduce the cost of a Ground Anchor installations?

Here are some design approaches that can help reduce costs of ground anchor installations:

Optimized spacing - Use closer spacing of smaller capacity anchors rather than fewer large anchors.

Single bar anchors - Specify single bar anchors instead of multi-strand anchors where possible.

Higher strength steel - Use higher grade steel bars and strands to reduce cross-sectional area.

Reduced free length - Minimize free stressing length based on structural design requirements.

Shorter bond length - Optimize fixed anchor length based on ground strength.

Temporary anchors - Use temporary anchors where possible instead of permanent ones.

Reduced test loads - Review proof test load requirements to avoid overly conservative values.

Corrosion protection - Specify galvanization for temporary anchors instead of greases if man power for greasing is costly.

Monitoring - Review need for monitoring devices like load cells for all anchors.

Construction loads - Design anchors for actual construction loads instead of factored loads

Load factors - Review reliability requirements to avoid highly conservative load factors.

Specifications - Check for any "over-specification" that increases costs without benefit.

Value engineering - Review designs/specifications along with experienced contractors.

The goal should be to develop the most economical design that still meets the reliability requirements for the project. balanced with long-term maintenance needs.

Ground Anchors Frequently Asked Questions

What types of structures or sites are ground anchors suitable for?
Retaining walls, excavations, slopes, dams, foundation uplift resistance etc. where economic lateral support is needed.

When is it more economical to use ground anchors versus other retaining systems?
For heights above 6-10 meters, ground anchors become more economical than pure gravity walls. They require less materials, space and foundations.

How do ground anchors compare to methods like soil nailing, rock bolting, etc?
Ground anchors provide active reinforcement and can carry higher loads over longer lengths. But installation requires more specialized equipment and skills.

What factors determine the anchor capacity needed - height, loading, soil strength?
Height determines the lateral earth pressure load. Surcharges, water pressures add load. Soil/rock shear strength determines how much load can be transferred.

How does anchor spacing, capacity, and layout get determined?
Based on loads, soil strength, structural geometry using limit equilibrium stability and finite element analyses. Verified by field pullout tests.

How much space is needed on site for anchor installation equipment?
About 25-50 sqm working platform for a typical anchor drilling rig. More space needed for wet drilling supporting plants.

How disruptive is the installation process to nearby structures or utilities?
Noise and vibration can be disruptive but methods are available to minimize impact. Care taken near utilities.

How long does it take to install anchors for a typical project?
Production rates vary from 10-30 meters a day for individual anchors depending on ground conditions and capacity.

What considerations are important for temporary versus permanent anchors?
Temporary anchors focus on construction phase - cost, rapid installation, minimal corrosion protection. Permanent anchors focus on long-term performance - capacity, reliability, durability.

How reliable are ground anchors in different ground conditions?
Well designed and tested anchors are generally reliable across ground conditions. But unexpected ground conditions always pose a risk.

How is corrosion protection ensured for long-term permanent applications?
Multilevel protection is provided - sacrificial anodes, plastic sheathing, corrosion inhibitors, grout protection. Regular maintenance also critical.

What kind of inspection and maintenance is required after installation?
Visual, corrosion protection checks, load monitoring through gauges or load cells. Frequency from 1 to 5 years.

What building codes or specifications govern the design and testing of anchors?
British Standards, FHWA guidelines, Eurocode 7, AASHTO specifications among others. Varying requirements globally.

Who are qualified specialists that can design, install, and test ground anchors?
Geotechnical engineering consultants, specialist foundation contractors, anchor testing agencies with proven expertise.

What risks should be watched out for with ground anchors?
Inadequate ground investigation, improper installation, ineffective corrosion protection, poor testing.

How can the project be designed to optimize use of ground anchors?
Early involvement of specialists, integrated design optimizing anchor layouts, capacities and construction staging.