Grouting Services is the recognised leader in providing rock and soil anchoring solutions to the civil engineering and construction markets.
Ground anchors are effectively restraining devices used in many different types of structures including retaining walls, dams, wharves, bridge abutments and foundations for buildings. Ground anchors are stressed (active anchorage) to prevent structural movement and they typically transfer their load over a fixed length.
These are commonly referred to as tension anchors and are suited for strong rock conditions. For anchors founded in soil or weak rock, load distributive compressive (and tension) anchors are used as they rely on the succession of small successive bond lengths rather than one unique longer bond length.
Classification of Ground Anchors
Ground anchors are classified according to their service life, purpose, installation procedures and method of load transfer from the anchor to the ground. An anchor with a service life greater than 24 months is generally considered permanent. Permanent anchors shall always have some type of corrosion protection system based on the service life of the structure, the known aggressivity of the environment and corrosive properties of the soil and consequences of tendon failure.
Also, anchors can be classified into frictional type anchors that are supported by the friction of the grout and the ground, ground pressure type anchors that acquire anchoring force with the passive resistance of the ground using ground pressure boards or piles, and complex type anchors that are a combination of the above two types, based on the supporting method of the fixation ground. Frictional type anchors can also be classified into tensile type anchors and compressive type anchors based on the load application method to the grout. Lastly, compressive type anchors can be classified into load concentrative type anchors and load distributive type anchors depending on the distribution of the load.
We design and construct both permanent and temporary anchors using either multi-strand or stress bar in accordance with internationally recognised codes that have been adopted in New Zealand which include; British Standard Code of practice for Ground Anchorages BS8081:1989, Execution of special geotechnical work – Ground Anchors BS EN 1537:2000, FIP Design and construction of prestressed ground anchorages April 1996 and US Federal Highway Administration Geotechnical Engineering Circular No. 4 Ground Anchors and Anchored Systems June 1999.
We have an extensive database of ground anchor testing and geotechnical ultimate bond ruptures in a wide range of materials nationwide. However, in any ground anchoring project, site specific geotechnical investigations for the construction of the ground anchors are critical to minimise the risk profile for all parties. Access to this information means we are better able to qualify potential drilling difficulties and any potential for loss of grout from the drill hole during the anchor installation.
For multi-strand anchors, the strands are run through our specialist greasing and sheathing machine. This machine parts the individual wires of the strands followed by immersion into a grease bath before completely encapsulating the strand in the outer sheathing to ensure no voids are present.
We also provide mini-piles (drilled and grouted micro-piles up to 300mm diameter) which are structural supporting members that improve the stability and load bearing capacity of structures. These are constructed by drilling small diameter holes and constructing piles with high tensile steel and high compressive strength grouts to achieve tremendous load bearing capacities. Strengthening works commonly incorporate micro-piles to provide additional tension and or compression capacity to existing structures.
We utilise the technique of high-pressure (1000psi) post-grouting in weak compressible soils to significantly improve the bond capacity, and where required, fabric socks are used to ensure containment of grout within the drill hole thus ensuring full bond potential is realised.
Grouting Services, through its partnership with SAMWOO of Korea, offers world-leading ground anchor technology that includes:
The load distributive compression (and tension) type removable anchors provide significant advantages to building owners as once the anchors are removed at the end of the project construction, there are no obstructions left in the ground that will conflict with any future developments.
Load Concentrative Tension Type Anchor
When stress is applied to tension type anchor, load transfer occurs to bond length through adhesion of steel strand and grout. Due to load concentration, the parts of tension type anchor attached with steel strand and grout become unzipped and this leads to crack and load reduction.
In addition, tension type anchor has the weakness of progressive debonding and time-dependent load reduction (creep) occurrence when friction of load concentration zone exceeds the extreme skin friction of the target ground. As shown by (Fig. A), tension at the earlier phase displays the state as of ①.
Then, as the parts attached with steel strand and grout become unzipped, it changes into the state as of ②. The relatively concentrated skin friction of anchor becomes higher than the allowed value between ground and grout body to progress into the state as of ③. Accordingly, load reduction takes place. The drawbacks of tensile type anchors are that a progressive destruction occurs due to the jacking crack in the grout and creeps due to load concentration, greatly reducing the load.
Therefore, as in the vicinity friction distribution graph(Figure A), the load transference distribution is as shown in curve 1) at the initial point when the load is applied, which changes into curve 3) due to the above mentioned reasons with the progress of time, reducing the load.
Load Concentrative Compression Type Anchor
Compression type anchors consist of an unbonded polyethylene (PE)-coated steel strand which transfers the jacking force / load directly to a structural element located at the distal end of the anchor. Unlike the tension type anchors, the grout body for compression type anchors is loaded in compression which is capable of securing much higher loads. However, due to the concentrative design of these anchors, the use of high-strength grout is frequently required to secure the jacking forces at the distal end. Also, it is often difficult to secure concentrative anchorage force in weak soils. Similar to the tension type anchors, compression type anchors are subject to the occurrence of progressive debonding and time-dependent load reduction (creep) as displayed in state ① as shown in (Fig. B). In this case the friction required to secure the concentrated load exceeds that of the skin friction for that zone. This effect causes grout debonding and loss of soil confinement pressure resulting in load reduction as displayed in states ②and③.
Load Distributive Tension / Compression Type Anchor
As discussed, high stresses from tension and conventional compression type anchors transfer concentrated loads to the soil and grout body which can become overstressed resulting in failure. Therefore, load distributive compression type anchors have been developed and are being used, which uniformly distribute the anchor load to the grout body and soil along the theoretical length of the bond zone. In addition the grout strength requirements are reduced as well as applied eccentricity. As a result high loads can be achieved even in normal soil condition. Recently, load distributive tension type anchors have been developed which are capable of securing stable loads in even relatively weak soils such as clay and silts. These anchors do not require high strength grout and have low eccentricity as well. The use of load distributive anchors results in a more uniform distribution of the anchor force to the soil as illustrated in figure C below. Therefore, load reduction and creep are minimized, enabling the anchor to maintain initial design load.