Bearing Capacity and Shallow Foundations - Elements of Soil Mechanics - page 284

Table 9.2 Presumed safe bearing capacity, q s , values (based on BS 8004: 1986 ).

q s (kPa)

Rocks
(Values based on assumption that foundation is carried down to unweathered rock)
Hard igneous and gneissic 10000

Hard sandstones and limestones 4000

Schists and slates 3000

Hard shale and mudstones, soft sandstone 2000

Soft shales and mudstones 1000-600

Hard chalk, soft limestone 600

Cohesionless soils
(Values to be halved if soil submerged)
Compact gravel, sand and gravel > 600

Medium dense gravel, or sand and gravel 600-200

Loose gravel, or sand and gravel < 200

Compact sand > 300

Medium dense sand 300-100

Loose sand < 100

Cohesive soils
(Susceptible to long-term consolidation settlement)
Very stiff boulder clays and hard clays 600-300
Stiff clays 300-150
Firm clays 150-75
Soft clays and silts < 75
Very soft clays and silts Not applicable

9.7.2 Design by prescriptive method

As mentioned in Chapter 5, Eurocode 7 Part 1 states that a prescriptive method may be used to check
a limit state on occasions where calculation of the soil properties is not possible or necessary, provided
that generally conservative rules of design are used. British Standard BS 8004: 1986 provided a list of safe
bearing capacity values (reproduced in Table 9.2) and this list forms the basis of such a prescriptive
approach for the case of the checking of the bearing resistance limit state. The values are based on the
following assumptions:

(i) The site and adjoining sites are reasonably level.
(ii) The ground strata are reasonably level.
(iii) There is no softer layer below the foundation stratum.
(iv) The site is protected from deterioration.

Foundations designed to these values will normally have adequate protection to satisfy the requirements
of the bearing resistance limit state, provided that they are not subjected to inclined loading. It should
be remembered however that settlement effects would also have to be considered, and thus a serviceabil-
ity limit state check (see Example 11.8) should be undertaken.
For cohesive soils the consistency is related to the undrained strength, c u . Such a relationship is sug-
gested in BS 5930 and is reproduced in Table 9.3.

9.8 Non-homogeneous soil conditions

The bearing capacity equations (6)-(10) are based on the assumption that the foundation soil is homoge-
neous and isotropic.