Grouted anchors

Grouted anchors are used as anchoring elements, typically as prestressed anchors, when breaks in the terrain need to be secured for construction pits or traffic routes. The permissible anchor forces and the length of the load-bearing elements depend on geological and site-specific boundary conditions. They are calculated individually for each construction scheme.

Various drilling methods are available for making the holes for the load-bearing elements. The wide variety of available design variants allows us to adapt to the most diverse subsoil conditions and construction site scenarios in order to achieve the desired load-bearing capacities.

We have our own drilling equipment to help us implement your requirements. This allows us to drill in extremely small spaces with limited working height as well as at great heights.

Grouted anchors are used to anchor construction pit walls such as diaphragm walls or bored pile walls. Underpinning elements can also be anchored back using grouted anchors. If anchoring is not possible for any reason, bracing can be used as an alternative.

Common uses for grouted anchors include:

• Temporary construction pit systems
• Embankment reinforcements
• Anti-buoyancy control
• Dam rehabilitation
• Rope bracing
   

The wide range of available design variants means that optimum solutions can be found for every construction site and every subsoil.

• Flexible use due to individual adaptation to a wide range of subsoil conditions
• No obstructive bracing in the construction pit
• Very high anchor loads are possible
• Production is vibration-free and low-noise
• Can be designed as removable anchors
• Can also be used at low working heights
• They are produced in a tried and tested high-precision method carried out with the General Type Approval of the German Institute for Building Technology (Deutsches Institut für Bautechnik, DIBt).
• Safety during execution is monitored by applying high quality assurance standards.
• Economical implementation and high daily output with the flush drilling method
• Pressure-sensitive drilling using the flush drilling method means that grouted anchors can also be used if the site has not been declared free of explosive ordnance (permission in accordance with section 7 of the German Explosives Act (Sprengstoffgesetz, SprengG)).
• The grouted piles are produced in our own facilities.
   

Standards and regulations

Grouted anchors are dimensioned in accordance with:

• DIN EN 1997-1: Eurocode 7 – Geotechnical design – Part 1: General rules
• German version of EN 1997-1:2004 + AC:2009 + A1:2013 with DIN 1054: Subsoil – Safety analyses in earthworks and foundation engineering – Supplementary regulations to DIN EN 1997-1
• Recommendations of the DGGT Construction Pits Working Group )
   

The following apply to the production of grouted anchors on the construction site:

• DIN EN 1537 Execution of work in specialist civil engineering - Grouted anchors
• German version of EN 1537:2013 in conjunction with DIN SPEC 18537 Title (German): Supplementary specifications for DIN EN 1537:2014-07, Execution of work in foundation engineering – Grouted anchors
   

Grouted anchors are tested in accordance with:

• DIN EN 22477-5: Geotechnical investigation and testing – Testing of geotechnical structures and components – Part 5: Testing of grouted anchors
• German version of EN ISO 22477-5:2018
   

How do grouted anchors work?

Grouted anchors transfer an applied tensile force (prestressing force) to a load-bearing layer of subsoil. Each anchor consists of an anchor head, a tendon free length and a fixed anchor length. This is often also referred to as a grout body.

There are two types of anchors: composite anchors and pressure pipe anchors.

• In the case of composite anchors, the prestressing force is dissipated into the subsoil from the beginning of the fixed anchor length and the grouted anchor is tension loaded.
• In the case of pressure pipe anchors, the force is applied from the end of the fixed anchor length via the pressure pipe, so that the grout body is compression loaded.
   

Dimensioning

Dimensioning grouted anchors involves defining both the necessary anchor force and the length of the grouted anchor. Care must be taken to ensure that the fixed anchor length comes to rest in a uniform soil layer. An estimate is also made of the length of the grouted body. Only low anchor forces can be applied in non-cohesive soil types such as clay and silt than in as gravel/sand or rock.

Manufacturing steps

The production of grouted anchors on the construction site comprises four basic work stages. Each phase has a special significance for the load-bearing capacity and durability of the grouted anchor.

1. Drilling the anchor borehole
   The selected drilling method can have a strong influence on the load-bearing capacity of the grouted anchor. This is why it is important to choose the right drilling method. Our many years of experience have equipped us to work with a wide variety of soil types, so that we can always use the best possible method.
    
2. Installation and grouting
   Care must be taken during storage, transport and installation of the load-bearing element in the borehole to ensure that the protective anti-corrosion casing is not damaged. The construction teams are specially trained for this work stage. After the anchor has been installed, the grout body is formed by applying grouting pressure. It is important to choose the right type of cement and the correct amount of grouting pressure. Targeted follow-up injections, which can be carried out several times if necessary, can significantly increase the load-bearing capacity in non-cohesive soil types.
    
3. Testing and prestressing
   The testing and prestressing stage, which usually takes place about a week after the installation of the grouted anchors, must be carried out carefully and adapted to the soil type. The acceptance tests verify that each anchor is fully functional.
    
4. Applying anti-corrosion protection to the anchor head
   Anti-corrosion protection measures must be implemented comprehensively and to a high standard of quality if the anchor is to be durable. This is especially the case for permanent anchors. This is only achieved if the seals are in good working order and the anchor head area is completely filled with certified anti-corrosion protection. For this reason, special attention is paid to this point and only specially trained tensioning professionals are assigned to this task.

Long-term monitoring

Specific subsoil conditions may make it necessary to equip the structure with anchor load cells or deformation measuring devices and monitor them over the long term.

We ensure high execution quality with:

• Individually coordinated instructions
• Our own DIBt general type approval for grouted anchors
• Compliance with the requirements of execution standard DIN EN 1537
• In-house production of the grouted anchors in Germany
• In-house and third-party production monitoring
• Clear processes via our quality management system in accordance with DIN EN 9001:2015
• Suitability and acceptance tests in accordance with DIN EN 1537
   

We supply grouted anchors in the form of strand anchors and bar anchors. A further distinction is made between:

Temporary anchors / short-term anchors

Service life up to two years

Temporary anchors with a service life of up to two years according to the DIN EN 1537 standard are indispensable for low-deformation construction pit production. Anti-corrosion protection is governed by the standard. Type approval is therefore no longer required for these systems, only for the method used to tension the anchors. Both single-rod and strand anchors are used as temporary anchors.

Extending the period of use in exceptional cases

In some cases, the two-year period of use may be exceeded. Special solutions must be defined in these cases. Our specialist engineers can provide solutions tailored to the individual problem. For example, anti-corrosion protection can be specifically improved at the anchor head and greased strands can be used for strand anchors.

Permanent anchors

Lasting security

Permanent anchors secure slopes and breaks in the terrain in the long term. This means that particularly high demands are placed on the durability of the load-bearing element and the implementation. In addition to the protection of the load-bearing element during transportation and installation, special attention is paid to the careful execution of anti-corrosion protection measures on the anchor head. These processes are defined and taught in detail at Stump-Franki. Construction site personnel and tensioning specialists are especially familiarised with and trained in the expert production of permanent anchors.

High anchor forces

We have a wealth of experience in the production of permanent anchors with high anchor forces up to characteristic forces of 7,500kN. Series production in our own facilities and consistent quality control during execution ensure a high level of security.

Flush drilling

Economical and flexible

The external flushing system enables us to extract drilling cuttings as efficiently as possible. The grouting section is produced cleanly with an uneven surface (root effect). This helps us to achieve rapid construction progress and thus work at a low cost, thanks to the high daily output. Transportation, assembly and dismantling can all be carried out quickly thanks to the compact equipment and minimal site facilities. In this way, even small projects can be implemented economically and flexibly throughout the country.

Pressure sensitive drilling

For sites that have not been proven to be free of explosive ordnance

Sites that have not been proved to be free of explosive ordnance pose a particular challenge. In this case, we use our drilling rig for pressure-sensitive drilling. Sample drillings are conducted across the suspected areas in the company of an expert in accordance with section 20 SprengG in order to obtain a drilling permit. Stump-Franki has a special permit for this in accordance with section 7 SprengG. This means that we can implement your project – even in areas that are difficult to investigate – safely, straightforwardly, and in compliance with the regulations of the respective federal state regarding the handling of explosive ordnance.

Staggered anchors

Solutions for subsoils with poor load-bearing capacity

If only minor forces can be transferred to the ground (e.g. in clay) via grouted anchors, construction pits are often designed to include bracing. Staggered anchors, however, offer a safe and economical alternative.

How does a staggered anchor work?

Staggered anchors have the fixed anchor lengths of the individual strands arranged at different depths in the borehole, but these come together in a single anchor head. All the strands are tested in a specially designed acceptance test and the anchor is then prestressed and secured. The arrangement of the fixed anchor lengths of the strands at different depths ensures that the subsoil is optimally utilised and not overloaded. This allows higher anchor forces to be achieved in the same borehole.

Removable anchors

Removal design and patented process for economic dismantling

In addition to existing or planned underground transportation and utility facilities, construction schemes in inner-city areas increasingly involve orders to remove grouted anchors, in part or in full. Different systems are available for this purpose. The most suitable system must be selected according to the surrounding ground and the length of the grouted anchors. Our specialist engineers will be happy to assist you with the selection process as early as the planning phase to ensure safe removal during the execution phase.

Stump-Franki employees have optimised the removal processes and developed a patented procedure from this.

Drilling start point below the groundwater level

Experience and expertise for safe execution

High groundwater levels in cities and the construction of deep and dense trough construction pits in coastal areas call for an individual adaptation of the design: In this case, grouted anchors are produced with attachment points that are significantly below the groundwater level. The design and manufacture of these anchors must be carried out with particular care. Our teams have many years of experience and maintain a constant exchange of knowledge, so that even boreholes with several bars of counterpressure from the surrounding groundwater can be carried out safely.

Producing anchors below groundwater level by enclosing the construction pit requires preparatory measures as early as during the production of the construction pit wall as a sheet pile or diaphragm wall. Our expert teams will be happy to assist you early on to ensure safe implementation.