Anchorages

This clause on anchorages is quite short and for British use should be supplemented by reference to BS 8081. Anchorages are to be designed for actions based on 2.4.2 and the earlier clauses ofSection 8.

The approach taken to geotechnical design of ground anchorages is based entirely on load testing. Calculations of appropriate sizes, shapes, grout pressures etc. are not mentioned and are seen merely as tools used in the process of selecting an anchor for testing. For structural design of the anchorages, the user is directed to Eurocode 3 (EC7,8.8.2(6)P).

Construction of ground anchorages is the topic of prEN 1537 'Ground anchors'. This also gives advice on the preliminary design of anchorages. In the terms of EC7, such preliminary design is the sizing of an anchor to be constructed for test purposes.

C8.8.1 General

The code says that this subclause refers to any type of anchorage, including soil nails, dead-man anchors, as well as pre-stressed anchors. However, most of the text is only relevant to pre-stressed anchorages. Dead-man anchors can be designed according to Section 8 as a whole, considering Clause 8.8 only where relevant.

C8.8.2 Anchorage design

Typically, calculations for a retaining wall or other type of structure will have been carried out at ultimate limit state, for Cases B and C, and A if relevant. From each of these, a required minimum capacity will have been calculated for the anchors. It is necessary to derive from these the target values for assessment and acceptance tests, together with an appropriate lock-off force for the anchors and design forces for structural elements such as tendons and walings. EC7 does not state clearly how these values are to be derived.

Table C8.2 illustrates the process involved in anchor design, from the analysis of the retaining wall to design of structural members. Approximate relative values of the forces and actions are indicated by a ♦ symbol.

It is recommended in this commentary that the minimum required anchor capacity, derived from wall or other calculations, should be treated as the minimum ULS design resistance of the anchors. This design resistance must equal the characteristic resistance divided by the factor ym (Equation 8.4). This leads to a minimum value for the characteristic resistance.

Calculations may also be performed for the serviceability limit state, yielding another minimum required resistance; the characteristic resistance of the anchors should also be not less than this value.

This approach to anchorage design differs from pile design in Section 7, in which piles are designed for Case B (factored loads, unfactored soil or unfactored load tests - ym = 1) and for Case C (unfactored (permanent) loads, factored soil or factored load tests). However, it is proposed here that anchors are designed for Case B and/or C, which effectively give factored anchor loads, together with factored load tests. The anchor loads derived from Case B or C are treated as required design anchor capacities.

It is considered that this approach generally leads to anchor designs similar to conventional practice, though possibly more conservative in some cases. An alternative might be to consider the required capacities derived from ULS wall calculations to be the required characteristic resistances of the anchors. This would be similar to the approach taken to pile design, but would be unconservative compared with conventional anchor design.

Assessment tests on anchors will normally prove the capacity of the grout/ground interface but will not fail the tendon/grout interface or the tendons themselves. Hence separate partial factors are quoted for the tendon/grout interface and the tendons themselves.

A worked example is presented in E16.

C8.8.3 Construction considerations

This subclause gives only a very bare outline of the specification needed for corrosion protection. Both BS 8081 and prEN 1537, Section 6 give guidance on corrosion protection for temporary and permanent anchors.

C8.8.4 Anchorage testing

C8.8.5 Assessment tests C8.8.5(4)P

In prEN 1537:1996 'Ground Anchors', the creep limit is defined as the maximum creep displacement rate permitted at a specific load level. The creep displacement rate (ks) is defined as:

ks = (s2 - Sj) / log10 (t2 / tj) (note that ks has units of displacement) where s; = displacement at time t;.

For ULS the creep limit load is defined as the load at which ks is equal to 2 mm. prEN 1537 states that the limit of 2 mm will be used as one of the ULS failure criteria in an assessment (on-site suitability) test. The creep limit rate should not exceed 1 mm at proof load where investigation tests have been carried out (or 0.8 mm where no investigation tests have been carried out).

The measurement of the creep displacement rate may be carried out using a maintained load test, as defined in prEN 1537 which recommends time intervals for measurement of anchor head displacement of 1,2,3, 5,10,15,20, 30,45,60 minutes. For fine grained soils, longer time intervals may be needed.

C8.8.6 Acceptance tests

C8.8.7 Supervision of construction and monitoring

Comment

A subclause should be added requiring that records be kept of ground conditions encountered.

0 0

Post a comment