6.8 Resin grout
Resins and resin mortars may be used in ground anchor construction as an alternative to cement grout if their applicability has been proven by a system test appropriate to their application.
Laboratory and field tests should be undertaken to verify mixture, mixing efficiency, setting times and performance.
6.9 Corrosion protection of steel tendon and stressed steel components
There is no certain way of identifying corrosive circumstance with sufficient precision to predict corrosion rates of steel in the ground. All steel components which are stressed shall be protected against corrosion for their design life. Corrosion protection elements shall be capable of transmitting tendon loads, where required.
The standard of corrosion protection is classified by the anchor design life:
- temporary ground anchors are defined as those which are required to be in service no longer than two years;
- permanent ground anchors are defined as those which are to have a design life of more than two years.
|1. Tendon bond length
All installed tendons shall be provided with a minimum 10 mm cement grout cover to the borehole wall. Where aggressive ground conditions are known to exist, it may be appropriate to enhance the protection for example by the use of a single corrugated duct around the tendon(s).
|2. Tendon free length
The protection system shall have low frictional properties and allow movement of the tendon within the borehole. This may be achieved by the provision of one of the following:
a) plastic sheath surrounding each individual tendon, end sealed against ingress of water;
b) plastic sheath surrounding each individual tendon, completely filled with corrosion protection compound;
c) plastic or steel sheath or duct common to all tendons, end sealed against ingress of water;
d) plastic or steel sheath or duct common to all tendons completely filled with corrosion protection compound;
b) or d) is appropriate for extended temporary use or in aggressive conditions.
|3. Transition between anchor head and free length (inner anchor head)
The free length sheath or duct may be sealed to the bearing plate/anchor head, or a metal sleeve or plastic duct may be sealed or welded to the bearing plate. It shall overlap the free length sheath or duct and for extended temporary use be filled with corrosion protection compound, cement or resin which is contained at the lower end.
|4. Anchor head
Where the anchor head is accessible for inspection and possible re-coating the following protection is acceptable:
a) a coating of non-fluid corrosion protection compound; or
b) a combination of corrosion protection compound and tape which is impregnated with corrosion protection compound.
Where the anchor head is not accessible a metal or plastic cap shall be fitted and filled with corrosion protection compound for extended use.
Where aggressive conditions are known to exist, a metal or plastic cap shall be filled with corrosion protection compound.
6.9.2 Temporary ground anchor
The steel components of a temporary ground anchor shall be provided with protection which will inhibit or prevent corrosion over a minimum design life of two years.
If there is a possibility that the design life of a temporary ground anchor is to be extended on a temporary basis or if the anchor is installed in ground conditions known to be corrosive then measures, approved by the Client's Technical Representative, shall be taken to protect all parts of the anchor from corrosion.
Examples of corrosion protection which may be considered to satisfy the above principles of protection to temporary anchors are described in Table 2.
6.9.3 Permanent ground anchor
The minimum corrosion protection surrounding the tendon(s) of the anchor shall be a single continuous layer of corrosion preventive material which does not degrade during the design life of the anchor.
The tendon(s) of a permanent ground anchor shall be provided with either:
- a) two protective barriers to corrosion such that if one barrier is damaged during installation or anchor loading, the second barrier remains intact; or
- b) a single protective barrier to corrosion, the integrity of which shall be proven by testing each anchor insitu (see annex A);
- c) a corrosion protection system provided by a steel duct tube-a-manchette type anchor (see 6.10.4 and 6.10.9);
- d) a corrosion protection system provided by a corrugated plastic duct tube-a-manchette type anchor (see 6.10.4 and 6.10.9);
- e) a corrosion protection system provided by a steel duct compression tube type anchor (see 6.10.4 and 6.10.6).
Examples of corrosion protection which may be considered to satisfy the above principles of protection to permanent anchors are described in Table 3.