Soil type affects selection of types of foundations, foundation depth and foundation sizes.

Following are the considerations for foundation based on soil types:

Soil Type

Foundation

Trouble areas

Types

Reason for use

Sand

Footings

Easy to construct and economical

Bearing capacity may be a problem but in most cases it is sufficient

Excessive settlement in wet and loose deposits.

Confining pressure is usually low.

Retaining Structures

Must be used since sand cannot support themselves

Deep foundations (Piles)

Uses friction resistance but low in bearing capacity

Clay

Footings

Economic but may have problem with bearing capacity in saturated clays

Low bearing capacity.

Generally low shear strength when wet.

High consolidation in soft clays
Swelling is possible.

Over-consolidated clays may contain cracks and fissures.

Retaining Structures

Clays are self-supportive up to a certain height (critical). Must be used if height increases beyond the critical.

Deep foundations (Piles)

If bearing capacity is low, piles may be driven to rock. May change formation of clay.

  foundation-on-sand

Following table shows solutions to some problems in foundations based on soil types:

Soil Type

Nature of problem

Possible solution

Sand

Settlement

Loose sands must be compacted

Lowering water table may result in sand densification

Bearing Capacity

Compaction increases cohesion and friction thus bearing capacity increases

Use of deep foundation

Clay

Consolidation

Lowering water table

Pre-loading

Drive pile to rock

Bearing capacity

Compaction

Use of deep foundations

Expansion or swelling

Treat or stabilize soil

Maintain constant water table

Alter soil nature (similar to stabilization)

Include swell pressure in design