Hollow Bar Micropiles/ Self Drilling Anchor Systems For Micropiles
A micropile is a small diameter, less than 300 mm, drilled and grouted pile. Micropiles can carry considerable axial compression and tension loads due to the large grout/- ground shear resistance, which facilitates load transfer by friction. Micropiles have been used for retrofitting existing foundations, and to support new construction, especially when the use of larger diameter piles is not feasible. There has been a rapid growth in using micropiles over the last few decades.
In recent years hollow bar micropiles (HBMP), also known as Self Drilling Anchor (SDA) Systems have been popular due to their fast installation and efficient load transfer mechanism.
The tubular grouted micropiles can be used for permanent applications with a design life exceeding 100 years.
The hollow bar simulteneously acts, as the drilling rod, injection tube and reinforcement for the micropile. The Self Drilling Anchor System can be used in any type of soil, including unstable soils. Injecting a cement grout through the hollow steel tendon ensures that the borehole is filled from the bottom to the top. Additional operations such as inserting and extracting casings and complex multi-stage grouting are unecessary.
The SDA system is fast, simple and flexible. The applications are as below
Dynamic/cyclic load application
The advantages are
No casings required, self drilling anchors/hollow bars can be drilled into loose soils without needing casings to support the boreholes.
Both rope threads and trapezoid threads are robust and ideal for rotary-percussive drilling and ensuring a high level of bond with the borehole gout.
Fast drilling and installation, as drilling, installation and grouting are in a single operation.
The hollow core not only serves for flushing during drilling, but also for grouting after drilling.
Continuous threads ensure the bars can be cut and coupled at any point, or extended.
Different anchor bits are available for different ground conditions.
Micropiling / Underpinning
The principles for the application and execution of the micropiling foundations are specified in micropile standard BS EN 14199 defining the execution of micropiling works. A micropile is small diameter pile which is used in various applications with Ground engineering. Generally, pile foundations serves to transfer structural loads to the underlying formations of construction ground and/or to limit deformations according to the principles of the execution of geotechnical and civil works. The micropile may thereby only be subjected to axial tensile, compressive or alternating loads. Self-drilling anchor (SDA)/ Hollow Bar micropiles are featured products for micropiling and preferred method of installation in these applications is simultaneous drilling and grouting.
Micropiles are devided in two groups regarding the load bearing type:
End Bearing Piles:
In this type the micropile is installed into the bed rock/hard stratum and most of bearing capacity is from the base. This type of micropiles are used when the bed rock or a hard stratum is available in shallow depths
Skin Friction Piles:
Where there is no dense underlying stratum available in shallow depths, the skin friction ones is often used and the bearing capacity depends on the frictions forces that are mobilised along the length of the pile.
Soil Nailing / Slope Stabilization
Soil/Rock Nailing is a construction method used to maintain or enhance the stability of the soil or rock mass by installing reinforcing elements (nails) in accordance to the principles for the execution of the geotechnical works. Nailing (sometimes combined with other elements like shotcrete, drainage system, Retaining walls, etc.) forms a retaining structure of reinforced Soil/Rock mass which nails are predominantly subjected to tensile stress. In addition, they might be also subjected to bending the shear stresses.
The area of applications for the above nail types in accordance with BS EN 14490 are:
Slope stabilization for vertical and inclined use
Underground primary support for tunnel construction, radial ground reinforcement and face stabilization.
Self-drilling anchor (SDA)/ Hollow Bar micropiles are part of the excavation process and serve as primary soil/rock support for:
Radial consolidation of the tunnel circumference supports the rock, forming a load bearing ring thus enabling the rock supporting itself (together with shotcrete and other facing elements).
Umbrellas and Fore piling as pre-support in poor rock formation, shallow overburden and critical stress conditions.
Face stabilisation to sustain rock pressure for excavation with large cross sections.
Root piles to divert stress.
The conventional method of installation involves drilling the Self-drilling anchor (SDA)/ Hollow Bar micropiles with water flush to the design layout and depth and in a second phase, independent of drilling, a grout adapter is added and a dense cement based grout mix is injected through the hollow core to exit through the flush holes at the drill bit filling the borehole until the grout mix appears at the borehole mouth, once cured, enabling the transfer of stress from the rock/soil formation.
Our Services We offer a broad range of foundation related services, including geotechnical design, foundation project planning, professional engineering services, basement construction, jet grouting and more, based on our years of experience.
Slope Stabilization with Self Drilling Anchor (SDA)/ Hollow Bar Micropiles
1. Soil Nailing
Construction proceeds from the top to bottom, and head plates are installed on each nail. Shotcrete or concrete is typically applied on the excavation face to provide continuity when a soil nail wall is constructed.
2. Retaining Wall Structures by Self Drilling Anchor (SDA)/ Hollow Bar Drilling
Ground anchors are used to secure retaining walls during excavation works to prevent the wall from moving. Ground anchor is usually designed to prevent deformation of the ground or retained structure.
3. Embarkment Stabilisation by Self Drilling Anchor (SDA)/ Holow Bar Drilling
Embankment stabilization is to stabilize an unstable or inadequately stable slope. The purpose of slope stabilization techniques is to increase the Factor of Safety of a slope to a level that is considered adequate.
Foundation with Self-drilling Anchor (SDA)/Holow Bar Micropiles
A Micropiling is a small diameter pile which is used in various applications within Ground engineering. Underpinning is the method of strengthening the foundation of an existing building or other structure in confined or low-overhead spaces, difficult to access jobsites, basements and small tunnels.
2. Micropiling for Tensioned Foundations
Self-drilling Anchor (SDA) / Hollow bar drilling tensioned piles, also known as uplift piles or anchor piles, are a type of pile foundation that is used to resist uplift forces that might otherwise cause it to be extracted from the ground.
3. Micropiling to Prevent Buoyancy
Self Drilling Anchor (SDA) / Hollow bar Micropiling to prevent buoyancy is constructed to resist the uplift forces can be called as Tension piles. Uplift forces can develop because of hydrostatic pressure, seismic activity or overturning moments.
Tunnelling with Self-drilling Anchor (SDA)
1. Radial Systematic Anchoring by Self-drilling Anchor (SDA)/Hollow Bar Drilling
Radial consolidation of the tunnel circumference supports the rock, forming a load bearing ring thus enable the rock supporting itself together with shotcrete and Self-drilling Anchor (SDA)/Hollow Bar Micropiles.
2. Stabilization of Tunnel Portals
Self-drilling Anchor (SDA)/Hollow Bar Micropiling is a ground stabilisation technique that can be used on Tunnel Portals either natural or excavated slopes. It involves drilling holes, grouting, and anchoring in a single operation.
Self-drilling Anchor (SDA)/Hollow Bar micropiles was introduced for temporary tunnel surface stabilization and long- term reinforcement around the excavation surface area.
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