Soil foundations - Geotechnical Investigation and Design Tables - page 266

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Treatment by compaction was covered previously.

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Relative order of cost depends on the site specifics and proposed development.
Time and land constraints often govern rather than the direct costs.

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Further discussions on specialist ground treatments are not covered.

21.2 Types of foundations
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The foundations are classified according to their depth.

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Typically when the embedded length > 5 × Bearing surface dimension, then the

foundation is considered deep.

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Deep foundations are more expensive but are required where the surface layer
is not competent enough to support the loads in terms of bearing strength or
acceptable movement.

Table 21.2 Foundation types.

Classification Foundation type Typically use

Shallow Strip Edge beams for lightly loaded buildings

Pad To support internal columns of buildings

Raft To keep movements to a tolerable amount

Deep Driven piles Significant depth to competent layer

Bored piles Large capacity required

- Combinations and variations of the above occur, ie piles under some edge
beams, or pad foundations connected by ground beams.

21.3 Strength parameters from soil description
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The bearing value is often assessed from the soil description in the borelog. The
presumed bearing value is typically given in the geotechnical engineering assess-
ment report based on the site conditions, but often without the benefit of specifics

Table 21.3 Preliminary estimate of bearing capacity.

Material Description Strength Presumed bearing value (kPa)

Clay V. Soft 0-12 kPa 25 <

Soft 12-25 kPa 25-50

Firm 25-50 kPa 50-100

Stiff 50-100 kPa 100-200

V. Stiff 100-200 kPa 200-400

Hard 200 kPa 400 > >

Sands* V. Loose D r 15% 0 ◦ 50 < φ < <

Loose D r 15-35% 30-35 ◦ 50-100 = φ =

Med dense D r

=

35-65%

φ =

35-40 ◦ 100-300

Dense D r 65-85% 40-45 ◦ 300-500

= φ =

V. dense D r 85% 45 ◦ 500

> φ > >