7.4 Design methods and design considerations
7.4.1 Design methods
(1)P The design shall be based on one of the following approaches:
- the results of static load tests, which have been demonstrated, by means of calculations or otherwise, to be consistent with other relevant experience;
- empirical or analytical calculation methods whose validity has been demonstrated by static load tests in comparable situations;
- the results of dynamic load tests whose validity has been demonstrated by static load tests in comparable situations;
- the observed performance of a comparable pile foundation, provided that this approach is supported by the results of site investigation and ground testing.
(2) Design values for parameters used in the calculations should be in general accordance with Section 3, but the results of load tests may also be taken into account in selecting parameter values.
(3) Static load tests may be carried out on trial piles, installed for test purposes only, before the design is finalised, or on working piles, which form part of the foundation.
7.4.2 Design considerations
(1)P The behaviour of individual piles and pile groups and the stiffness and strength of the structure connecting the piles shall be considered.
(2)P In selecting calculation methods and parameter values and in using load test results, the duration and variation in time of the loading shall be considered.
(3)P Planned future placement or removal of overburden or potential changes in the ground-water regime shall be considered, both in calculations and in the interpretation of load test results.
(4)P The choice of type of pile, including the quality of the pile material and the method of installation, shall take into account:
- the ground and ground-water conditions on the site, including the presence or possibility of obstructions in the ground;
- the stresses generated in the pile during installation;
- the possibility of preserving and checking the integrity of the pile being installed;
- the effect of the method and sequence of pile installation on piles, which have already been installed and on adjacent structures or services;
- the tolerances within, which the pile can be installed reliably;
- the deleterious effects of chemicals in the ground;
- the possibility of connecting different ground-water regimes;
- the handling and transportation of piles;
- the effects of pile construction on neighbouring buildings.
(5) In considering the aspects listed above, the following items should receive attention:
- the spacing of the piles in pile groups;
- displacement or vibration of adjacent structures due to pile installation;
- the type of hammer or vibrator used;
- the dynamic stresses in the pile during driving;
- for those types of bored pile where a fluid is used inside the borehole, the need to keep the pressure of the fluid at a level to ensure that the borehole will not collapse and that hydraulic failure of the base will not occur;
- cleaning of the base and sometimes the shaft of the borehole, especially under bentonite, to remove remoulded materials;
- local instability of a shaft during concreting, which may cause a soil inclusion within the pile;
- ingress of soil or water into the section of a cast-in-situ pile and possible disturbance of wet concrete by the flow of water through it;
- the effect of unsaturated sand layers around a pile extracting water from the concrete;
- the retarding influence of chemicals in the soil;
- soil compaction due to the driving of displacement piles;
- soil disturbance due to the boring of a pile shaft.