Environmental Engineering Reference
In-Depth Information
TABLE 7.6
Typical Residual Soil Profile and Engineering Characteristics of Gneiss in Humid Climate
a
Zone
Description
Engineering Characteristics
b
Upper (A and
Surficial humus zone (A horizon) grades to mature
A horizon is usually porous since it is the zone of
B horizons)
residual soil, usually reddish- to yellowish-brown,
maximum leaching. Strengths are low to medium. B
heterogeneous mixtures of silt, sand, and clay
horizon: If desiccated or cemented, strengths will be high,
(ML or CL-ML groups) with kaolin clays
compressibility low (see
Figure 7.4b)
.
Relatively impervious.
(
Figure 7.3)
.
Desiccation or cementation
If uncemented and not desiccated, as often exists when deposit
following leaching can result in a stiff
is permanently below GWL; low permeability, but can be
crust from 1 to 2 m, and occasionally 3 m
compressible with moderate strengths.
c
SPT values often range
thick. Luterite gravel can be found on the
from 5 to 10: void ratios, 1.0 to 1.5 and higher
surface and at the contact with the
intermediate zone in alternating wet and
dry climates
Intermediate
Red, brown, yellow stiff to hard clays or
Can be strong and relatively incompressible with allowable
heterogeneous sand-clay mixtures, often
bearing of about 10 tsf
d
with SPT ranging from 30 to 50: or it
micaceous with quartz veins
can be relatively weak and compressible (SPT 5-9) as shown on
the boring log from Atlanta
(Figure 7.5d)
Saprolite
Relict rock: has the fabric and structure of
Permeability can be relatively high. SPT values are high and
parent rock but the consistency of soil:
void ratios low. Partially saturated specimens can have
with quartz and mica sand particles
strengths 20% higher than saturated specimens. Strength is
and kaolin clays (see
Figure 6.84).
higher perpendicular to the foliations than parallel:
Colors vary from while to gray to
compressibility is higher perpendicular to foliations Compaction
brown or reddish. Grades erratically
characteristics:
In situ
particles are angular, but soft and readily
to moderately fresh rock
crushable by compaction equipment in the field, particularly the
micas
a
For general profile see
Figure 6.82.
b
At a given site, characteristics can be extremely variable and differential settlements require careful consideration. See Tabie 7.5 for typical engineering properties.
c
In one case, a large structure imposing contact pressures of about 3 kg/cm
2
suffered differential settlements of 10 cm within the first 3 years, and the rate thereafter con-
