1.1                  ALLOWABLE BEARING PRESSURE, BOND OR FRICTION OF GROUND

The allowable bearing pressure, bond or friction of soils and rocks should be determined by one of the methods given in clauses 2.2.1 to 2.2.5.

1.1.1                            RATIONAL DESIGN METHOD

Rational design method for calculating the ultimate capacity should be based on sound engineering approach and should include:

• the reasonable interpretation of the results of site investigation;
  • the assessment of test results obtained in situ or from samples in the laboratory; and
  • an analysis based on the laws of physics and recognized engineering principles taking into account the ground conditions and foundation geometry, or an established empirical method proven with adequate correlation.

Normally, the allowable capacity is estimated by applying a factor of safety of 3 to the calculated ultimate bearing capacity. However, other factors of safety may be adopted having regard to the nature of the soil or rock, its variability over the site and the reliability of the design method.

1.1.2                            PRESUMED VALUES

• General

In lieu of a rational design method, the allowable capacity for soils and rocks may also be taken as those presumed values derived from empirical correlation and as stipulated below provided that the following conditions are complied with:

• the planning, conducting and supervision of the ground investigation and the interpretation of the results are carried out in accordance with the recommendations given in Chapter 3; and
  • the structures are not unduly sensitive to settlement or other displacement or movement that may be required to mobilize the allowable capacity.

The presumed values for rock are based on the assumption that slip of the rock will not occur. Therefore, where the rock profile is inclined at such an angle that the bearing capacity of the rock mass may be affected, the rock joints should be checked to ensure that there is no unfavourable joint orientation that  could permit slip of the rock to occur.

(2)    Allowable Vertical Bearing Values

The allowable vertical bearing pressure for foundations on horizontal ground may be estimated from Table 2.1 on the basis of the material description.

(3)    Allowable Lateral Bearing Pressure for Rock

The allowable lateral bearing pressure for rock may be taken as one third of the allowable vertical bearing pressure provided that no adverse rock joints exist.

(4)    Allowable Bond or Friction between Rock and Concrete

The allowable bond or friction between rock and concrete for piles may be estimated from Table 2.2.

(5)    Footings of Minor Temporary Structures

A presumed allowable vertical bearing pressure of 100 kPa (if dry) or 50 kPa (if submerged) may be used for the design of footings on horizontal ground of minor temporary structures such as fencing and hoarding.