Archistruct: Learning Architecture from Structure: PPP Notes

SOILS DEFINITIONS: TERMS AND CLASSIFICATIONS

(These notes are compiled from AGS)

CLAY

SILT

Silt consists of inorganic particles between .003 in. and .0002 in. in diameter.
Fine-grained particles are similar in composition to the rocks from which they are derived, and are not plastic in nature.
Organic silt is found on the bottom of lakes and river deltas.

SAND

Classifications of sand vary from fine to coarse.
Rock sizes range from .003 in. to .079 in. in diameter.
Adequately compacted, sand makes an ideal bearing material.
The coarser the sand, the higher the allowable bearing pressures.
Fine sands are susceptible to becoming quick when subjected to unbalanced hydrostatic pressures, and may liquefy when they are loose, saturated, or subjected to seismic forces.
Settlement is usually immediate, with little long-term settlement.

GRAVEL

Classifications of gravel vary from fine to coarse, and these unconsolidated rock fragments range from .75 in. to about 3 in.
Except for gravels composed of shale, this material makes a good foundation material.
Depending on the compactness and underlying material, very high bearing pressures are allowed by some building codes.

COBBLES

Ranging in size from about 3 in. to about 10 in., these rock fragments can make reliable foundation-bearing materials.
Is, however, difficult to properly compact when used for fill.
Cobble-sized materials can interfere with pile driving and drilled-pier construction causing significant problems.

BOULDERS

Typically classified as rock fragments greater than 10 in.
Boulders are used as part of a fill mass if the voids between the boulders are filled with finer-grained sands and silts.
These materials are generally not considered suitable for direct foundation support because of their size and shape, and the difficulty in excavating the material to desired shapes.
As with cobbles, boulders can cause significant problems during construction.

BEDROCK

Unbroken hard rock that is not over any other materials is considered bedrock.
Depending upon its composition, it can be capable of withstanding extremely high bearing pressure.
Desirable for foundations supporting high loads.
If the rock has been weathered or is cracked, its bearing capacity may be compromised.
Settlement of buildings on bedrock is primarily limited to the elastic settlement of the foundation.

RESIDUUM

Residuum consists of soil derived from the in-place decomposition of bedrock materials.
In general, these soils are more weathered near the surface, and gradually transition to a more rocklike material with depth.
Where residual soils reveal evidence of the stratification and structure of the parent rock, they are known as saprolitic materials.

ALLUVIAL SOILS

Because materials are eroded, transported, and deposited through the action of flowing water, these soils are typically loose and saturated, hence often are unsuitable for support of structures or pavements.

COLLUVIAL SOILS

Because materials are transported by gravity, typically associated with landslides, these soils are generally irregular in composition and loose.
They require improvement prior to being used to support buildings and pavements.

AEOLIAN SOILS

These soils are transported and deposited by the wind.
They consist of silt or sand-sized soils.
Loess, one of the more common types of Aeolian soils, is composed of fine-cemented silt.
While this material is competent in place, it loses much of its strength when disturbed or recompacted.

TILL

Till is a mixture of clay, silt, sand, gravel, and boulders deposited by glaciers.
Consolidated tills that are well graded (indicated by a uniform distribution of particle size) are exceptionally strong and make excellent foundation strata.
Loose tills can cause differential settlements if used as a bearing material.

LOAM

This organic material, made up of hummus and sand, silt or clay, provides excellent material for agriculture but should not be used for foundations.
Organic materials will settle a great deal over time, and even lightly loaded slabs on grade will settle if bearing on loam.

COHESIONLESS SOILS

These types of soils consist of cobbles, gravels, sands, and nonplastic silts.
Generally formed from the mechanical weathering of bedrock brought about by water, ice, heat, and cold.
Typically composed of the same minerals as the parent rock.
Strength of cohesionless materials is derived primarily from interparticle friction.

COHESIVE SOILS

These types of soils contain clay minerals with an unbalanced chemical charge.
Tend to attract water and bond together.
Strength of cohesive materials is derived from a combination of these chemical bonds and from interparticle friction.

CONSOLIDATION

When soils are subjected to loads, water within the void spaces initially supports the change in stress through an increase in pressure.
Excess pressures gradually dissipate in proportion to the permeability of the soil.
Coarse-grained materials drain rapidly, while finer-grained silts and clays drain more slowly.
As the excess pore pressures dissipate, the void spaces compress and transfer the loads to the soil grains.
The resulting reduction in volume over time is known as consolidations.

UNDERCONSOLIDATED SOILS

Soils that have built up in river deltas and other water bodies are deposited in a very loose state.
These soils are often underconsolidated, in that they have never experienced stresses equal to or greater than current overburden stresses.
These materials tend to consolidate under their owner weight over time, until all excess pore pressures have been dissipated and the soils become “normally consolidated.”
Foundations bearing on underconsolidated soils can typically expect large short- and long- term settlement.

OVERCONSOLIDATED SOILS

Unlike many other types of materials, soils are not elastic.
When stresses are applied to soils, they compress.
However, when the same stress is removed, they do not rebound to the same height.
When reloaded, the soils “remember” previously loaded conditions and compress to their historical level of stress.
Soils that have previously been loaded to stresses above those created by the current soil overburden are considered to be overconsolidated.
Foundations bearing on overconsolidated soils can typically expect less short- and long- term settlements.

DESICCATION

All soils typically contain some moisture within the voids between soil particles.
When soils are dried, capillary tension tends to pull the soil grains together, causing the soil to shrink and lose volume.
This action can cause the soil to become overconsolidated, as the capillary tension results in stress.

Archistruct: Learning Architecture from Structure