1. ACCEPTABLE SETTLEMENT AND ROTATION

• General

Acceptance of estimated settlement and rotation of foundations should be considered on a case-by-case basis, as different structures will have different tolerance in accommodating movements of their foundations. The acceptable settlement and rotation for foundations should therefore be determined for each individual case with respect to integrity, stability and functionality of the supported structure.

Where differential settlement is anticipated, it should be assessed accurately or conservatively, and its effect on the supported structure should be checked to ensure that it is acceptable in respect of strength and serviceability.

(2)                   Reference Criteria

For buildings or structures not particularly sensitive to movement, the following movement criteria, evaluated at the base of a shallow foundation or

in case of a deep foundation, the base of pile cap, may be used as a reference for developing case specific criteria:

• The maximum total settlement should not exceed 30 mm;
  • The differential settlement between columns / vertical elements should be limited to 1:500; and
  • The maximum angular rotation should not exceed 1:500 due to wind or other transient loads.

The above criteria should be assessed based on working loads. For criteria (a) and (b), the full dead loads should be considered, and the imposed loads may be reduced in accordance with the Code of Practice for Dead and Imposed Loads.

In general, criterion (a) could be deemed to be satisfied if the foundation rests directly on categories 1(a), 1(b), 1(c), 1(d) and 2 rock or if the foundation elements are driven to sound bearing strata with SPT N values ≥ 200.

Differential settlement should be considered in situations where its evaluation is considered necessary, for example, mixed foundation systems, piles with significant difference in lengths, substantial variation in the properties or depths of compressible strata under the foundations. The differential settlements should be properly controlled or appropriately catered for in the design of superstructure.

(3)                   Individual Case

Where the anticipated movement of the foundation is in excess of the reference criteria specified in (2) above, an assessment should be carried out to demonstrate that its effect:

• will not cause or induce any overstress in the building or structures supported by the foundations, or in any nearby buildings, structures, or surrounding ground; and
  • will not cause any strength or serviceability problem either in the connections of services or utilities, or in the connections with the surrounding structures, pavements, streets or roads.