2.5 Design by prescriptive measures
(1) In design situations where calculation models are not available or not necessary, exceeding limit states may be avoided by the use of prescriptive measures. These involve conventional and generally conservative rules in the design, and attention to specification and control of materials, workmanship, protection and maintenance procedures.
NOTE Reference to such conventional and generally conservative rules may be given in the National annex.
(2) Design by prescriptive measures may be used where comparable experience, as defined in 220.127.116.11, makes design calculations unnecessary. It may also be used to ensure durability against frost action and chemical or biological attack, for which direct calculations are not generally appropriate.
2.6 Load tests and tests on experimental models
(1)P When the results of load tests or tests on large or small scale models are used to justify a design,or in order to complement one of the other alternatives mentioned in 2.1 (4), the following features shall be considered and allowed for:
- differences in the ground conditions between the test and the actual construction;
- time effects, especially if the duration of the test is much less than the duration of loading of the actual construction;
- scale effects, especially if small models are used. The effects of stress levels shall be considered, together with the effects of particle size.
(2) Tests may be carried out on a sample of the actual construction or on full scale or smaller scale models.
2.7 Observational method
(1) When prediction of geotechnical behaviour is difficult, it can be appropriate to apply the approach known as "the observational method", in which the design is reviewed during construction.
(2)P The following requirements shall be met before construction is started:
- acceptable limits of behaviour shall be established;
- the range of possible behaviour shall be assessed and it shall be shown that there is an acceptable probability that the actual behaviour will be within the acceptable limits;
- a plan of monitoring shall be devised, which will reveal whether the actual behaviour lies within the acceptable limits. The monitoring shall make this clear at a sufficiently early stage, and with sufficiently short intervals to allow contingency actions to be undertaken successfuly;
- the response time of the instruments and the procedures for analysing the results shall be sufficiently rapid in relation to the possible evolution of the system;
- a plan of contingency actions shall be devised, which may be adopted if the monitoring reveals behaviour outside acceptable limits.
(3)P During construction, the monitoring shall be carried out as planned.
(4)P The results of the monitoring shall be assessed at appropriate stages and the planned contingency actions shall be put into operation if the limits of behaviour are exceeded.
(5)P Monitoring equipment shall either be replaced or extended if it fails to supply reliable data of appropriate type or in sufficient quantity.