Geotechnical Engineering

DHARM

BEARING CAPACITY 579

*14.10 BEARING CAPACITY FROM PENETRATION TESTS

Penetration tests are those in which the resistance to penetration of a soil for a standard value

of penetration is determined in a standard or specified manner. Devices known as

‘penetrometers’ are used for this purpose. A wide variety of these tests has become available,

but the more important are the ‘Standard Penetration Test’ and the ‘Dutch Cone Penetration

Test’. These are more commonly employed for cohesionless soils. More detailed information on

these devices and test will be provided in Chapter 18 on ‘‘Soil Exploration’’.

At this juncture, it is sufficient to know that the standard penetration test results are

commonly in the form of ‘Penetration Number’, N, which indicates the number of blows re-

quired to cause 300 mm penetration of a split-spoon sampler into the soil under test by means

of a 65 kg hammer falling through 750 mm.

This value has been correlated to Terzaghi’s bearing capacity factors, density index and

angle of shearing resistance, φ (Peck, Hansen and Thornburn, 1953). Terzaghi and Peck have

prepared charts for allowable bearing pressure, based on a standard allowable settlement, for

footings of known widths on sand, whose N-values are known. (Terzaghi and Peck, 1948).

These correlations and charts will be presented in Sec. 14.13.

*14.11 BEARING CAPACITY FROM MODEL TESTS—HOUSEL’S

APPROACH

Housel (1929) has suggested, based on extensive experimental investigations, a practical method

of determining the bearing capacity of a prototype foundation in a foundation soil which is

reasonably homogeneous in depth by means of two or more small-scale model tests. It is as-

sumed that the load-carrying capacity of a foundation for a predetermined allowable settle-

ment consists of two distinct components—one which is carried by the soil column directly

beneath the foundation, and the other which is carried by the soil around the perimeter of the

foundation. The first component is a function of the area and the second, a function of the

perimeter of the foundation.

This concept is expressed by the formula

W = qs. A = σ. A + mP ...(Eq. 14.113)

where W = total ultimate load which the foundation can carry (kN),

qs = bearing capacity of the foundation (kN/m^2 ) for a specified settlement,

σ = contact pressure developed under the bearing area of the foundation (or an experi-

mental constant) (kN/m^2 ),

m = perimeter shear (or an empirical experimental constant) (kN/m)

A = bearing area of the foundation (m^2 ), and

P = perimeter of the foundation (m).

Equation 14.109 may be modified as

qs = σ + m.

P

A ...(Eq. 14.114)

or qs = mx + σ ...(Eq. 14.115)