34 / Chapter 2 Foundations and Sitework

them. The ability of these soils to sup-

port building loads without shifting

depends primarily on friction between

the particles to keep the particles from

sliding past one another. This resis-

tance to internal sliding, called shear strength, varies with the degree of inter-

locking between particles and the con-

fining force of the surrounding soil.

Where coarse-grained soils are densely

packed with little space between par-

ticles and securely confined by sur-

rounding soils, it is relatively difficult

for particles to move past one another.

Soils such as these exhibit relatively

high strength and can support greater

loads. Where coarse-grained soils are

loosely packed or poorly confined,

particles can more easily slide past one

another, and less load can be safely

supported. Soils that rely primarily

on internal friction for strength are

termed frictional or cohesionless. Smaller-grained soils may be sub-

ject to a wider array of interparticle

forces. As particle size decreases, sur-

face area increases in relation to weight

and size, and the spaces between the

particles, called soil pores, get smaller.

In essence, the particles become

lighter and more easily pushed and

pulled by electrostatic forces, chemi-

cal interactions, and forces related to

the presence of water in the soil.

For example, whereas gravels are

generally little affected by moisture in

the soil, the properties of sand can vary

noticeably with moisture content. As

any beachgoer knows, wet sand makes

a stronger sand castle than dry sand, as

capillary forces acting between parti-

cles help to hold the particles in place.

And wet sand responds more firmly

to the pressure of our feet as we walk

on the beach than does dry sand, as

the hydrostatic pressure of the water

helps to distribute the load exerted

on the soil. A dramatic example of the

effects of moisture on smaller-grained

soils is a phenomenon called soil liq- uefaction. Water-saturated sands or silts

may lose virtually all of their strength

and behave as a liquid when subjected

to sudden, large changes in load, such

as may occur during an earthquake.

Figure 2.3 The Unified Soil Classification System, from ASTM D2487. The Group Symbols are a universal set of abbreviations for soil types, as seen, for example, in Figure 2.6.

Group Symbol Descriptive names of soil within this group

C oa

rs e-

G ra

in ed

S oi

ls

G ra

ve ls

C le

an

G ra

ve ls

GW Well-graded gravel or well-graded gravel with sand, little or no fines

GP Poorly graded gravel or poorly graded gravel with sand, little or no fines

G ra

ve ls

w

it h

Fi ne

s GM Silty gravel, silty gravel with sand

GC Clayey gravel, clayey gravel with sand

Sa nd

s

C le

an

Sa nd

s

SW Well-graded sand or well-graded sand with gravel, little or no fines

SP Poorly graded sand or poorly graded sand with gravel, little or no fines

Sa nd

s w

it h

Fi ne

s

SM Silty sand, silty sand with gravel

SC Clayey sand, clayey sand with gravel

ML Silt or silt-sand-gravel mixtures, low plasticity

Fi ne

-G ra

in ed

S oi

ls

Si lt

s an

d C

la ys L iq

ui d

L im

it <

5 0

CL Lean clay or clay-sand-gravel mixtures, low plasticity

OL Organic clay or silt (clay or silt with significant organic content), or organic clay- or silt-sand-gravel mixtures, low plasticity

L iq

ui d

L im

it ≥

5 0

MH Elastic silt, silt-sand-gravel mixtures

CH Fat clay or clay-sand-gravel mixtures, high plasticity

OH Organic clay or silt (clay or silt with significant organic content), or organic clay- or silt-sand-gravel mixtures, high plasticity

H ig

hl y

O rg

an ic

So

ils

PT Peat, muck, and other highly organic soils

Allen, Edward. Fundamentals of Building Construction : Materials and Methods, John Wiley & Sons, Incorporated, 2013. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/washington/detail.action?docID=7103638. Created from washington on 2023-04-10 04:35:52.

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