Geology Reference
In-Depth Information
To convert from
To
Multiply by
UNIT WEIGHT
tons (metric)/cubic metre
grams/cubic centimetre
1.00
megagrams/cubic metre
1.00
pounds/cubic foot
62.42
pounds/cubic foot
tons(metric)/cubic metre
0.01
1.57 × 10
−
1
kilonewtons/cubic metre
kilonewtons/cubic metre
tons (metric)/cubic metre
0.10
pounds/cubic foot
6.36
A cubic metre of water weighs about 10,000N= 10 kilonewtons = 10 kN. By comparison, a cubic metre
of granite weighs about 27 kNbecause the SPECIFICGRAVITY of quartz, one of the main components
of granite, is 2.69, i.e. it weighs 2.69 times the weight of the same volume of water. The UNITWEIGHT
is WEIGHT/UNIT VOLUME. The UNIT WEIGHT,
γ
R
, of granite is 27 kN/m
3
and for water
γ
W
=10
kN/m
3
. The unit weight of soil
γ
S
is lower than that of granite and typically between 12 and 18 kN/m
3
.
This is because, whilst the minerals in soil usually have similar speci
c gravities to the minerals in
granite, soil has many voids
filled either with air or water.
we often use the term DENSITY, which is MASS/VOLUME and usually expressed
in Mg/m
3
. DRY DENSITY
Instead of
'
weight
'
is an easy index test to
measure in the laboratory and often correlates quite well with degree of weathering or consolidation of a
rock or soil. BULK DENSITY is the mass of natural soil (with contained moisture) per unit volume.
RELATIVE DENSITY is a term used to describe the relative compaction of soil compared to its densest
condition achievable naturally or by compaction. The terms used are:
-
the mass of the dry sample divided by volume
-
Relative density (%)
Term
0
-
15
Very loose
15
-
35
Loose
35
-
65
Medium
65
-
85
Dense
85
-
100
Very dense
For natural granular materials (mostly sands), the same terms are used, correlated to SPT
'
N
'
values, as
described in
Chapter 4.
