Designing a seismically resilient foundation is a complex and specialist undertaking that is well beyond the scope of this guidance. However, a few of the basic concepts are briefly introduced here. In each case, the nuances, complexities and engineering understanding that make these foundations work in practice have been omitted. In any real-world situation, it is important to seek proper advice from a professional engineer before any work is undertaken.
The design of a seismically resilient foundation begins by considering three factors:
- Site characteristics – the composition and quality of the soil beneath the foundation and the seismicity of the region.
- Design loads – all the loads imposed on the foundation by the completed structure, including lateral seismic forces.
- Degree of resilience – the desired behaviour of the foundation and the structure it supports during an earthquake.
In preparing its technical guidance Repairing and rebuilding houses affected by the Canterbury earthquakes, MBIE developed a prescriptive criterion for foundation strength and stiffness to account for the high variability of liquefaction effects on Canterbury TC2 and TC3 sites.
The guidance states:
- Design Type C house foundations for the potential for differential settlement of the supporting ground that will allow a maximum unsupported length for the ground floor of 4 m beneath sections of the floor and 2 m at the extremes of the floor (ie, ends and outer corners).
- Design to ensure that the floor does not hog or sag more than:
- 1 in 400 (ie, 5 mm hog or sag at the centre of a 4 m length) for the case of no support over 4 m, and
- no more than 1 in 200 for the case of no support of a 2 m cantilever at the extremes of the floor.
- Appropriate provision should be made for ‘flexible’ services entry to the dwelling to accommodate the potential differential settlement of the foundation as indicated in the geotechnical report.
- Designs should accommodate total settlements of up to 50 mm for an SLS event and up to 100 mm for a ULS event [from Table 5.3]
A geotechnical investigation , as carried out by or under the supervision of a suitably qualified geotechnical engineer, will establish several important ground characteristics, such as geological history, soil composition, soil compressibility, bearing strength and stratification, which can be used to build a model of the site.
The investigation should extend through soil strata considered able to affect the behaviour of the site and the building foundations. It should then continue to a sufficient additional depth into the bearing stratum to ensure all potential problem soils have been identified given the type of foundation to be used. If deep pile foundations are being considered, the investigation also needs to study at least 10 pile diameters into the bearing stratum.
The objective of the investigation is to develop a subsurface geological model for the site so as to reliably predict the future behaviour of the ground at the site so that the designer can create a suitable foundation and understand the constructability and risk issues. The investigation should be sufficiently detailed so that the designer has confidence that the foundation will perform satisfactorily given the building’s performance specifications.