Ch1 Basic - University of...
Transcript of Ch1 Basic - University of...
1-1
1. BASIC SOIL AND ROCK CHARACTERISTICS
1.1. THE PHASE DIAGRAM
Soils are normally composed of three constituents - solid soil particles, air and
water. The air and water occupy the void spaces or pores between the solid particles. In
the case of saturated soils the pore fluid is made up entirely of water.
To facilitate calculations involving various amounts of the three constituents of a
soil it is convenient to represent these constituents by means of a diagram, often known as
a phase diagram , as illustrated in Fig.1.1. The symbols for the masses and volumes of
the constituents are also shown on this figure. The mass of the air occupying the voids is
ignored since it is negligible by comparison with the masses of water and solid.
A number of definitions may now be given in terms of the symbols in Fig.1.1.
Bulk Density ρ = MW + MS
VA + VW + Vs (1.1)
(Sometimes referred to as total density ρt)
Dry Density ρd = Ms
VA + VW + Vs (1.2)
Density of Water ρw = MW
VW (1.3)
ρw is commonly taken as 1000 kg/m3 for convenience
Volume of Voids Vv = VA + VW (1.4)
Void Ratio e = VV
VS (1.5)
Porosity n = VV
VV + VS =
e
1 + e , (1.6)
often expressed as a percentage.
Degree of Saturation S or Sr = VW
VA + VW. (1.7)
usually expressed as a percentage.
1-2
Fig.1.1 Phase diagram for a partly Saturated soil
Fig.1.2 Phase diagram for a Saturated soil
Fig.1.3
Volume
water
solid
air
Mass
MW
MS
VA
VW
VS
Volume
water
solid
Mass
MW
MS
VW
VS
Volume
water
solid
air
Mass
MW
MS
VA
VW
VS
0.0
25
m3
45
.0 k
g
1-3
Density of Solid Particles or Soil Particle Density
ρs = Ms
Vs (1.8)
Specific Gravity of Solid Particles
G = ρs
ρw (1.9)
Water Content or Moisture Content
w = MW
MS , (1.10)
usually expressed as a percentage.
Air Voids Va = VA
VA + VW + VS , (1.11)
usually expressed as a percentage.
With a saturated soil the volume of air becomes zero as illustrated in Fig.1.2.
Referring to the figure a further definition can be given.
Saturated Density ρsat = MW + MS
VW + VS (1.12)
Example
A soil sample having a total volume of 0.025mm3 and total mass of 45.0 kg. has
been removed from the ground. If the water content and specific gravity of the soil are
20.0% and 2.68 respectively calculate:
a) the dry density of the sample,
b) the degree of saturation,
c) the porosity.
Referring to the phase diagram in Fig.1.3 the unknown terms MS, MW, VA, VW,
VS can be calculated as follows:
MW = w x MS = 0.20MS
but MW + MS = 45.0 kg
∴ 1.20MS = 45.0
∴ MS = 37.5 kg
∴ MW = 45.0 - 37.5 = 7.5 kg
1-4
The volumes can now be calculated
VW = MW
ρw =
7.5
1000 = .0075 m3
VS = MS
Gρw =
37.5
2.68 x 1000 = 0.014 m3
∴ VA = .025 - .014 - .0075 = .0035 m3
With these known quantitites the three items required can now be determined:
a) dry density ρd = MS
VA + VW + VS (1.2)
= 37.5
.025 = 1500 kg/m3
b) degree of saturation S = VW
VA + VW (1.7)
= .0075
.0035 + .0075 = 0.682 or 68.2%
c) porosity n = VA + VW
VA + VW + VS
= .0035 + .0075
.025 = 0.440 or 44.0%
1.2 IDENTIFICATION OF SOILS
In addition to the techniques described in Geomechanics 2 for identification of
the mineralogical components of soils, a number of relatively simple laboratory tests
which are useful in identifying various soil types, has been developed. The presentation
here will concentrate on grain size and plasticity characteristics, but reference should be
made to books on soil testing for details of the testing procedures. (Bowles, 1970,
Lambe, 1951, Kezdi, 1980). More sophisticated laboratory tests, which may be used for
soil identification are used on occasions but these will not be discussed in this
introductory presentation.
1.2.1 GRAIN SIZE DISTRIBUTION
Soils are traditionally described by one or more of the names gravel, sand, silt or
clay which indicate sizes of the soil particles. A number of slightly different
1-5
classification systems are in use relating size ranges to these four names but probably the
most widely used is the M.I.T. system as follows:
Gravel - grain size greater than 2mm
Sand - 0.06 mm to 2 mm
Silt - 0.002 mm to 0.06 mm
Clay - grain size less than 0.002 mm
Soils often consist of mixtures of these four ranges resulting in names such as silty
sand, sandy clay, etc. The distribution of grain sizes in the gravel and sand ranges is
found by sieving. A sample of dry soil is passed through a nest of sieves with the
coarsest sieve at the top and the finest sieve at the bottom. The mass of soil retained on
each sieve is measured as shown in the sample calculation in Table 1.1. From this
information a histogram may be constructed as in Fig.1.4. Because of the large range of
grain sizes encountered in soils a log scale is normally used. It has been found more
convenient in soil engineering practice to integrate the histogram and to present the data
as a cumulative distribution curve as illustrated by curve A in Fig.1.5.
Table 1.1
Sieve Analysis of a Sand Soil
_____________________________________________________________________
Sieve Mass Retained Percent Cumulative Percent
Aperture gm Retained Percent Retained Finer
_____________________________________________________________________
2.36 mm 2.5 2.6 2.6 97.4
1.18 mm 9.3 9.8 12.4 87.6
600 µm 25.2 26.5 38.9 61.1
300 µm 28.7 30.2 69.1 30.9
150 µm 18.1 19.0 88.1 11.9
75 µm 6.4 6.7 94.8 5.2
Pan 5.0 5.2 100.0
95.2 100.0
For soil classification purposes two parameters which can be determined from
the grain-size distribution curve are often quoted. These are:
Effective Size which is the grain size corresponding to the 10 percent finer point on the
curve. This can be referred to as D10.
1-6
Fig. 1.4 Histogram from a Sieve Analysis
Fig. 1.5 Grain size distribution curves
1-7
Uniformity Coefficient (Cu) which is a measure of the uniformity of grain size in the
soil and is defined as the ratio of the 60% finer size (D60) to D10.
that is Cu = D60
D10 (1.13)
For curve A in Fig.1.5 the uniformity coefficient is:
Cu = 0.57
0.14 = 4.1
which indicates a relatively uniform soil (sometimes referred to as poorly graded).
A grain size distribution curve for a soil with a uniformity coefficient larger than
that for soil A in Fig.1.5 is illustrated by curve B (well graded soil) in Fig.1.5. For the
silty clay soil represented by curve C in Fig.1.5 it is not possible to determine the
uniformity coefficient since the effective size is unknown.
Coefficient of Cuvature (Cc) is a value that can be used to identify a poorly graded soil.
6010
2
30
.
)(
DD
DCc = (1.14)
A well graded soil has Cc between 1 and 3 as long as Cu is also greater than 4 for gravels
and 6 for sands.
The finer portions of the grain size curves B and C cannot be determined by
sieving since a sieve with an aperture of about 75 µm is normally the finest sieve used in
this type of test. For silt and clay size soils the grain size distributions are found by
means of a sedimentation procedure in which a sample of the soil is allowed to settle in
water. This procedure utilizes Stokes Law which relates the size of a sphere to its fall
velocity in a fluid (usually water) by means of the expression:
D2 = 18000 η v
g(Gs - Gw) (1.15)
where D is the sphere diameter in mm
η is the dynamic viscosity of water in N sec/m2
v is the fall velocity of the sphere in cm/sec.
g is the gravitational acceleration in cm/sec2.
Gs is the specific gravity of the sphere solid
Gw is the specific gravity of the water.
1-8
The concentration of solids in the water at a particular time after the
commencement of sedimentation is found by measuring the specific gravity of the
suspension with an hydrometer. Alternatively the concentration may be found by taking a
small volume of suspension from a particular depth by means of a pipette. The mass of
solids is determined by drying off the water.
During preparation of the suspension for a sedimentation (or hydrometer) test a
deflocculating agent such as sodium hexametaphosphate or sodium silicate is
customarily added to prevent the formation of soil flocs. Some clay soils behave in such
a way that a variety of grain size distribution curves may be obtained depending upon the
type and concentration of deflocculating agent that is used. Discussion of the
recommended procedure for determining the grain size distribution of soils is given in the
S.A.A. Standard AS1289.
1.2.2 SOIL PLASTICITY
Changes in soil water content can produce significant changes in soil behaviour.
It is not surprising therefore to find that two widely used identification tests involve
observations of soil behaviour at two different water contents. In these tests water
contents, known as the Atterberg Limits, at which particular soil characteristics develop
are measured.
The larger water content, known as the Liquid Limit (wl) is the water content at
which the soil flows in a specially made cup when subjected to a series of small blows.
The liquid limit device permits the cup containing the soil in which a small groove has
been cut, to be lifted and dropped a small distance. The liquid limit is the water content
at which the groove closes when the soil has been subjected to 25 blows. The test is
performed by counting the number of blows to close the groove at various water contents.
The results are then plotted on a diagram such as Fig.1.6, from which the liquid limit may
be interpolated. The liquid limit may be estimated from the results of a test at a single
value of water content (w). If the number of blows for this test is n then the following
expression can be used to provide an estimate of wl.
wl = w(n
25 )0.121 (1.16)
For many Australian soils the following expression has been found to provide a
better estimate of wl.
wl = w(n
25 )0.091 (1.17)
1-9
Small laboratory cone penetrometers are increasingly being used for the
measurement of liquid limit. The British (BS 1377-1975) device for example is 35mm
long cone and has a 30o tip and a mass of 80 g. The liquid limit is taken to be the water
content of the soil when the penetration of this cone is 20 mm.
The smaller water content, known as the Plastic Limit (wp) is the water content at which
small threads of the soil crumble when rolled to a diameter of 3mm.
Fig. 1.6 Flow curve for Liquid Limit determination
These two tests are conducted on clayey and silty soils. These tests cannot be
conducted on granular soils such as sands and gravels. (See S.A.A. Standard AS1289).
The typical liquid and plastic limits for some clay soils are illustrated in Table 1.2 which
demonstrates the magnitude of the influence of the adsorbed cation as well as the type of
clay mineral.
1-10
TABLE 1.2
Typical Atterberg Limits for some Clay Soils
___________________________________________________________
Soil Liquid Limit (wl) Plastic Limit (wp)
(%) (%)
_____________________________________________________________________
Sodium Kaolinite 50 30
Calcium Kaolinite 40 28
Sodium montmorillonite 700 50
Calcium montmorillonite 500 80
Sodium Illite 120 50
Calcium Illite 100 45
_____________________________________________________________________
Some frequently used terms which involve the Atterberg Limits are:
Plasticity Index IP = wl - wp (1.18)
and gives a measure of the range of water content over which the soil is in a plastic state.
Liquidity Index = w - wp
wl - wp (1.19)
Consistency Index = wl - w
wl - wp (1.20)
The liquididty and consistency indices are measures of the natural water content
(w) of a soil in relation to the liquid and plastic indices.
Activity = Plasticity Index
percent of soil finer than 2 µm (1.21)
High activity is associated with high water retention capability, high
compressibility, low strength, high swelling and shrinking by comparison with low
activity soils. Soil with an activity within the range of 0.75 to 1.25 is considered normal.
Inactive soils have values below 0.75 while active soils have values above 1.25. Some
typical values of activity are:
Sodium montmorillonite 6
Calcium montmorillonite 1.5
Illite 0.9
Kaolinite 0.4
A graphical plot of plasticity index against liquid limit (called a plasticity chart) is
frequently used to classify fine grained soils (silts and clays) as illustrated in Fig.1.7.
1-11
The plot is divided into four regions by the two lines as shown. The group symbols in
these regions are interpreted as follows:
C - clay
M - silt
O - organic soil
H - high plasticity
L - low plasticity
As an example the symbol CH means inorganic clays of high plasticity. The
montmorillonites in Table 1.2 are CH soils.
The relationship between the Atterberg limits and the engineering properties of
soils by means of the plasticity chart was first observed by Casagrande (1932).
Fig. 1.7 Plasticity chart for classification of fine grained soils.
1-12
1.3 SOIL CLASSIFICATION
Several soil classificaton systems are in common use, most of them being based
upon grain size distributions of soils and some based upon a combination of grain size
and plasticity characteristics. One widely used system is the Unified Soil Classification
System which is detailed in Tables 1.3 and 1.4 and in which soils are initially sub-divided
into coarse grained or fine grained on the basis of grain size. Further sub-divisions are
made into various groups depending upon grain size and plasticity characteristics. The
above tables which are metricated and are taken from AS1726-1975, SAA Site
Investigtion Code, follow the original Unified Classification System(USBR Earth
Manual) and ASTM D2487-69 except that they adopt the particle size limits given in
AS1289 and other standards, viz:
Gravel 2-50mm
Sand 0.06-2mm
Silt and Clay < 0.06mm
The system excludes the boulder and cobble fractions of the soil and classifies only the
material less than 60mm in size. In the original Unified Classification System the grain
sizes used corresponded to the No. 200 (74µm) and No. 4 (4.7mm) sieves, whereas in this
metricated system the grain sizes (in Tables 1.3 and 1.4) are 0.06mm and 2.0mm
respectively. As 60mm, 2mm and 0.06mm sieve sizes are not normally used, the
percentages passing these sizes can be obtained from a particle size distribution curve
determined from a laboratory test. Alternatively, the percentages passing may be
estimated in the field.
The plasticity chart (Fig.1.7) is used to classify the fine grained soils and the fines
(fraction smaller than 0.06mm) that may be present in the coarse grained soils. The
meanings of the letters used for the group symbols are given partly in Section 1.2.2, the
remainder being given below:
G - gravel
S - sand
W - well graded
P - poorly graded.
Some typical engineering characteristics of the soil groups in Table 1.3 are listed
in Table 1.5. The Unified Soil Classification System has been described in more detail by
the U.S. Corps of Engineers (1953).
1-13
Example
Classify the following soils according to the Unified Soil Classification System
and comment briefly on their suitability for the impervious zone of an earth dam.
Soil A B C D
% finer than 0.06mm 4 58 25 18
% finer than 2.0mm 40 85 70 62
Liquid Limit (%) - 55 40 35
Plastic Limit (%) - 15 20 27
Soil A is a gravel since more than half is larger than 0.06mm and more than half is larger
than 2.0mm. It is a clean gravel since there are less than 5% fines (finer than 0.06mm).
The grain size curve for this gravel has been estimated from the two known points in
Fig.1.8. Although the uniformity coefficient Cu is not known it is certainly greater than 4
and the value of Cc is probably around unity - consequently the soil may be classified as
GW, a well graded gravel.
Because this soil is highly permeable it would be unsuitable for the
impervious zone of an earth dam.
Soil B is a fine grained soil since more than half is finer than 0.06mm. This soil plots in
the CH region of the plasticity chart Fig.1.7 based on the Atterberg limits. The soil is
therefore CH, a highly plastic clay.
Because this soil is very impermeable it could be suitable for the impervious
core of an earth dam but only if a thin core is used becuase CH soils are low in strength
by comparison with other more suitable impervious soils.
Soils C and D are sands since more than half of the material is larger than 0.06mm and
more than half of the coarse fraction is smaller than 2.0mm. Because both soils contain
more than 12% fines, the soils must classify as either SC or SM. From the plasticity chart
soil C plots above the A line whereas soil D plots below the A line. Therefore
soil C is SC, a clayey sand
and soil D is SM, a silty sand.
Both types of soil would be suitable for the impervious core of an earth dam.
1-14
TABLE 1.3
UNIFIED SOIL CLASSIFICATION SYSTEM
(from Add. No. 1 (Feb. 1978) to AS1726 - 1975)
TABLE 1.3
UNIFIED SOIL CLASSIFICATION SYSTEM
(from Add. No. 1 (Feb. 1978) to AS1726 - 1975
1-15
TABLE 1.4
UNIFIED SOIL CLASSIFICATION SYSTEM (from Add. No. 1 (Feb. 1978) to AS1726 - 1975)
Giv
ety
pica
lnam
es:i
ndic
ate
ap-
prox
imat
epe
rcen
tage
sof
sand
and
grav
el:m
axim
umsi
ze:
angu
lari
ty,s
urfa
ceco
nditio
n,an
dha
rdne
ssof
the
coar
segr
ains
:loc
alor
geol
ogic
alna
me
and
othe
rpe
rtin
entd
escr
iptive
info
rmat
ion
and
sym
boli
npa
rent
hese
s.
For
undi
stur
bed
soils
add
info
r-m
atio
non
stra
tifica
tion
,deg
ree
ofco
mpa
ctne
ss,c
emen
tation
,m
oist
ure
cond
ition
san
ddr
ain-
age
char
acte
rist
ics.
Exa
mpl
e:
Wel
lgra
ded
grav
els,
grav
el-
sand
mix
ture
s,littl
eor
nofine
s
Poor
lygr
aded
grav
els,
grav
el-
sand
mix
ture
s,littl
eor
nofine
s
Silty
grav
els,
poor
lygr
aded
grav
el-s
and-
silt
mix
ture
s
Cla
yey
grav
els,
poor
lygr
aded
grav
el-s
and-
clay
mix
ture
s
Wel
lgra
ded
sand
s,gr
avel
lysa
nds,
little
orno
fine
s
Poor
lygr
aded
sand
s,gr
avel
lysa
nds,
little
orno
fine
s
Silty
sand
s,po
orly
grad
edsa
nd-s
iltm
ixtu
res
Cla
yey
sand
s,po
orly
grad
edsa
nd-c
lay
mix
ture
s
GW
GP
GM
GC
SW SP SM SC
Wid
era
nge
ofgr
ain
size
and
subs
tant
ial
amou
nts
ofal
lint
erm
edia
tepa
rtic
lesi
zes
Pred
omin
antly
one
size
ora
rang
eof
size
sw
ith
som
ein
term
edia
tesi
zes
mis
sing
Non
-pla
stic
fine
s(f
orid
entif
icat
ion
proc
edur
esse
eM
Lbe
low
)
Plas
tic
fine
s(f
orid
entif
icat
ion
pro-
cedu
res
see
CL
belo
w)
Wid
era
nge
ingr
ain
size
san
dsu
b-st
antia
lam
ount
sof
alli
nter
med
iate
partic
lesi
zes
Pred
omin
ante
lyon
esi
zeor
ara
nge
ofsi
zes
with
som
ein
term
edia
tesi
zesm
issi
ng
Non
-pla
stic
fine
s(f
orid
entif
icat
ion
pro-
cedu
res,
see
ML
belo
w)
Plas
tic
fine
s(f
orid
entif
icat
ion
pro-
cedu
res,
see
CL
belo
w)
ML
CL,C
I
OL
MH
CH
OH
Pt
Dry
stre
ngth
crus
hing
char
acte
r-is
tics
Non
eto
slig
ht
Med
ium
tohi
gh
Slig
htto
med
ium
Slig
htto
med
ium
Hig
hto
very
high
Med
ium
tohi
gh
Rea
dily
iden
tified
byco
lour
,odo
ursp
ongy
feel
and
freq
uent
lyby
fibr
ous
text
ure
Dila
tenc
y(r
eact
ion
tosh
akin
g)
Qui
ckto
slow
Non
eto
very
slow
Slow
Slow
tono
ne
Non
e
Non
eto
very
high
Toug
hnes
s(c
onsi
sten
cyne
arpl
astic
limit)
Non
e
Med
ium
Slig
ht
Slig
htto
med
ium
Hig
h
Slig
htto
med
ium
Inor
gani
csi
lts
and
very
fine
sand
s,ro
ckfl
our,
silty
orcl
ayey
fine
sand
sw
ithsl
ight
plas
tici
tyIn
orga
nic
clay
sof
low
tom
ediu
mpl
astic
ity,
grav
elly
clay
s,sa
ndy
clay
s,si
ltycl
ays,
lean
clay
s
Org
anic
silts
and
orga
nic
silt-
clay
sof
low
plas
tici
ty
inor
gani
csi
lts,
mic
aceo
usor
dict
omac
eous
fine
sand
yor
silty
soils
,ela
stic
silts
Inor
gani
ccl
ays
ofhi
ghpl
astic
ity,
fatc
lays
Org
anic
clay
sof
med
ium
tohi
ghpl
astici
ty
Peat
and
othe
rhi
ghly
orga
nic
soils
Giv
ety
pica
lnam
e;in
dica
tede
gree
and
char
acte
rof
plas
tici
ty,
amou
ntan
dm
axim
umsi
zeof
coar
segr
ains
:col
ourin
wet
con-
dition
,odo
urif
any,
loca
lor
geol
ogic
alna
me,
and
othe
rpe
rt-
inen
tdes
crip
tive
info
rmat
ion,
and
sym
boli
npa
rent
hese
s
For
undi
stur
bed
soils
add
info
r-m
atio
non
stru
ctur
e,st
ratif-
icat
ion,
cons
iste
ncy
and
undi
s-tu
rbed
and
rem
ould
edst
ates
,m
oist
ure
and
drai
nage
cond
ition
s
Exa
mpl
eC
laye
ysi
lt,br
own:
slig
htly
plas
tic:
smal
lper
cent
age
offi
nesa
nd:
num
erou
sve
rtic
alro
otho
les:
firm
and
dry
inpl
aces
;loe
ss;(
ML)
Fie
ldid
entif
icat
ion
proc
edur
es(E
xclu
ding
partic
les
larg
erth
an75
mm
and
basi
ngfr
actio
nson
estim
ated
wei
ghts
)
Gro
upsy
mbo
ls1
Typi
caln
ames
Info
rmat
ion
requ
ired
for
desc
ribi
ngso
ils
Labo
rato
rycl
assi
fica
tion
criter
ia
C=
Gre
ater
than
4D D----
60 10U
C=
Bet
wee
n1
and
3(D
)
Dx
D----------
----------
--30
10c
2 60
Not
mee
ting
allg
rada
tion
requ
irem
ents
forG
W
Atter
berg
limits
belo
w"A
"lin
eor
PIle
ssth
an4
Atter
berg
limits
abov
e"A
"lin
ew
ith
PIgr
eate
rth
an7
Abo
ve"A
"line
with
PIbe
twee
n4
and
7ar
ebo
rder
line
case
sre
quirin
gus
eof
dual
sym
bols
Not
mee
ting
allg
rada
tion
requ
irem
ents
forSW
C=
Gre
ater
than
6D D----
60 10U
C=
Bet
wee
n1
and
3(D
)
Dx
D-----
----------
-------
30
10c
2 60
Atter
berg
limits
belo
w"A
"lin
eor
PIle
ssth
an4
Atter
berg
limits
abov
e"A
"lin
ew
ith
PIgr
eate
rth
an7
Abo
ve"A
"line
with
PIbe
twee
n4
and
7ar
ebo
rder
line
case
sre
quirin
gus
eof
dual
sym
bols
Determinepercentagesofgravelandsandfromgrainsizecurve
Usegrainsizecurveinidentifyingthefractionsasgivenunderfieldidentification
Dependingonpercentagesoffines(fractionsmallerthan.075mmsievesize)coarsegrainedsoilsareclassifiedasfollowsLessthan5%Morethan12%5%to12%
GW,GP,SW,SPGM,GC,SM,SCBordelinecaserequiringuseofdualsymbols
The.075mmsievesizeisaboutthesmallestparticlevisibletothenakedeye
FinegrainedsoilsMorethanhalfofmaterialissmallerthan
.075mmsievesize
CoarsegrainedsoilsMorethanhalfofmaterialislargerthan
.075mmsievesize
Siltsandclaysliquidlimit
greaterthan50
Siltsandclaysliquidlimitlessthan50
SandsMorethanhalfofcoarsefractionissmallerthan
2.36mm
GravelsMorethanhalfofcoarse
fractionislargerthan2.36mm
Sandswithfines
(appreciableamountoffines)
Cleansands(littleorno
fines)
Gravelswithfines
(apreciableamountoffines)
Cleangravels(littleorno
fines)
Iden
tific
atio
npr
oced
ure
onfr
action
smal
ler
than
.425
mm
siev
esi
ze
Hig
hly
orga
nic
soils
Uni
fied
soil
clas
sifi
cation
(inc
ludi
ngid
entif
icat
ion
and
desc
ript
ion)
Silty
sand
,gra
velly;
abou
t20%
hard
angu
largr
avel
part
icle
s12
.5m
mm
axim
umsi
ze;r
ound
edan
dsu
bang
ular
sand
grai
nsco
arse
tofine
,abo
ut15
%no
n-pl
astic
lines
with
low
dry
stre
ngth
;wel
lcom
pact
edan
dm
oist
inpl
aces
;alluv
ials
and;
(SM
)
010
2030
4050
6070
8090
100
Liqu
idlim
it
0102030405060
Plasticityindex
CH
OH or MH
OL
ML
or
CL
"A"lin
e
Com
pari
ngso
ilsat
equa
lliq
uid
limit
Toug
hnes
san
ddr
yst
reng
thin
crea
se
wit
hin
crea
sing
plas
ticity
inde
x
Plas
ticity
char
tfo
rla
bora
tory
clas
sifi
catio
nof
fine
grai
ned
soils
CI
CL
-ML
CL
-ML
TABLE 1.5
ENGINEERING CHARACTERISTICS OF MAJOR SOIL TYPES
_______________________________________________________________________________________________________________________________________________________
MAJOR DIVISIONS LETTER COMPRESSIBILITY DRAINAGE VALUE FOR EMBANKMENTS PERMEABILITY COMPACTION CHARACTERISTICS
& EXPANSION CHARACTERISTICS cm/sec
________________________________________________________________________________________________________________________________________________________
GW Almost Excellent Very stable, pervious shells k > 10-2 Good, tractor, rubber-tyred,
None of dikes and dams steel wheeled roller
_______________________________________________________________________________________________________________________________________
GRAVEL Almost Excellent Reasonably stable, pervious k > 10-2 Good, tractor, rubber-tyred,
and GP None shells of dikes and dams steel-wheeled roller.
GRAVELLY
SOILS GM Very Fair to Poor Reasonably stable, not k = 10-3 Good, with close control
slight to to practically paticularly suited to shells, to 10-6 rubber-tyred, steel wheeled
COARSE slight impervious but may be used for roller.
impervious cores or blankets.
GRAINED
SOILS GC Slight Poor to Fairly stable, may be used k = 10-6 to Fair rubber-tyred, sheeps
practically for impervious core. 10-8 foot roller.
impervious
____________________________________________________________________________________________________________________________________________
SW Almost Excellent Very stable, pervious k > 10-3 Good, tractor.
None sections slope protection
required.
_______________________________________________________________________________________________________________________________
SAND SP Almost Excellent Reasonably stable, may be k > 10-3 Good, tractor.
and None used in dike section with
SANDY flat slopes.
SOILS
Very Fair to poor Fairly stable, not k = 10-3 Good with close control
SM slight to to practically particularly suited to shells, to 10-6 rubber-tyred, sheeps foot
slight to impervious but may be used for roller.
medium impervious cores or dikes.
______________________________________________________________________________________________________________________________
SC Slight to Poor to Fairly stable, use for k = 10-6 Fair, sheeps foot roller
medium practically impervious core for flood to 10-8 Rubber-tyred.
impervious control structures.
___________________________________________________________________________________________________________________________________________________
1-17
TABLE 1.5 (cont.)
ML Slight to Fair to Poor Poor stability, may be used k = 10-3 Good to poor, close control
SILTS medium for embankments with proper to 10-6 essential, rubber-tyred roller
control.
____________________________________________________________________________________________________________________________
and
CLAYS CL Medium Practically Stable, impervious cores and k = 10-6 Fair to good, sheeps foot roller,
impervious blankets to 10-8 rubber tyred
_____________________________________________________________________________________________________________________________
FINE wl < 50 OL Medium to Poor Not suitable for embankments k = 10-4 Fair to poor,
High to 10-6 sheeps foot roller.
___________________________________________________________________________________________________________________________________________
GRAINED MH High Fair to poor Poor stability, core of hydraulic k = 10-4 Poor to very poor, sheeps
SOILS fill dam, not desirable in to 10-6 foot roller.
rolled fill construction.
____________________________________________________________________________________________________________________________
SILTS CH High Practically Fair stability with flat slopes k = 10-6 Fair to poor, sheeps foot roller
and impervious thin cores, blankets and dike to 10-8
CLAYS sections.
_____________________________________________________________________________________________________________________________
wl > 50 OH High Practically Not suitable for embankments k = 10-6 Poor to very poor, sheeps foot
impervious to 10-8 roller.
__________________________________________________________________________________________________________________________________________________
HIGHLY Pt Not used for construction Compaction not practical
ORGANIC
SOILS
FIG
1.8
1-19
1.4 ROCK CLASSIFICATION SYSTEMS
In discussing rock classification systems a distinction needs to be made between
rock mass and rock substance. A rock mass consists of an aggregate of blocks of rock
substance separated by discontinuities : structural features such as bedding planes,
cleavage planes, joint planes, fissures and solution cavities. Locally, planes of structural
weakness may be open and air-filled, water-filled or infilled with alteration products or
materials of a nature different from that of the rock blocks. Alternatively, the rock mass
may be traversed by fracture planes on either side of which the rock blocks abut tightly.
The rock mass will conform to the geological structure of the area and may be affected by
folding and faulting.
The blocks of rock substance lying between discontinuities or joint planes are
composed of aggregates of mineral particles together with voids which may be isolated or
interconnected and air- or water-filled. Additionally the rock substance may contain
closed or incipient joints which are not always visible to the naked eye.
Rock classification systems, many of which were developed to assist in assessing
rock mass behaviour in tunnelling, are used to:
(a) Divide a particular rock mass into zones of similar behaviour;
(b) Provide a basis for understanding the characteristics of each zone;
(c) Yield quantitative or semi-quantitative data for engineering design;
(d) Provide a common basis for communication.
Examples of widely used classification systems, a few of which are described in
more detail on the following pages, are:
1. Rock loads acting on tunnel supports (Terzaghi, U.S.A., 1946)
2. Stand-up time of unsupported tunnels (Lauffer, Austria, 1958)
3. Degrees of weathering of rock substance (Moye, Australia, 1958)
4. Rock Quality Designation, RQD (Deere, U.S.A., 1964)
5. Description of Rock Properties for Foundation Purposes (ASCE, 1972)
6. Rock Structure Rating, RSR (Wickham, U.S.A., 1972)
7. Geomechanics Classification, or Rock Mass Rating, RMR (Bieniawski, South
Africa, 1973)
8. Rock Mass Quality, Q (Barton, Norway, 1974)
9. Uniaxial compressive strength (International Society for Rock Mechanics, 1979)
10. Basic Geotechnical Description (I.S.R.M., 1981).
1-20
1.4.1 Rock Substance Weathering
The Moye classification was originally developed for the granitic rocks of the
Snowy Mountains area. In spite of some deficiencies it is now generally applied to most
rocks.
FR Fresh: no visible sign of weathering.
FRST Fresh, with Limonite Stained Joints : weathering limited to the surfaces of
major discontinuities.
SW Slightly Weathered: penetrative weathering developed on open discontinuity
surfaces, but only slight weathering of rock substance.
MW Moderately Weathered: weathering extends throughout the rock mass, but
the rock substance is not friable.
HW Highly Weathered: weathering extends throughout the rock mass, and the
rock substance is partly friable.
CW Completely Weathered: the rock is wholly decomposed and in a friable
condition, but the rock texture and structure are preserved.
RS Residual Soil: a soil material with the original texture, structure and
mineralogy of the rock completely destroyed.
A slightly different classification system (MDB) was developed by McMahon,
Douglas and Burgess. In contrast to the Moye system, the MDB system does not assume
that a progressive loss of strength always occurs as an effect of increased weathering.
1.4.2 Rock Quality Designation (RQD)
This is an index based on modified core recovery, from diamond drilling with
double tube core barrels, of at least NX size (54 mm diameter). Only the sound pieces of
core, 100mm or more in length, are considered.
RQD is expressed as the percentage of the total length drilled that is recovered in
lengths of at least 100mm.
RQD 91 - 100 Excellent
76 - 90 Good
51 - 75 Fair
25 - 50 Poor
0 - 24 Very Poor
1-21
1.4.3 The Geomechanics Classification (RMR System)
This engineering classification of rock masses uses the following parameters,
which can be obtained from bore cores, or measured in the field:
Uniaxial Compressive Strength of Rock Substance
Rock Quality Designation (RQD)
Spacing of Discontinuities
Condition of Discontinuities
Groundwater Conditions
Orientation of discontinuities
From the RMR value which is obtained by adding the five ratings in Table 1.6 and
adjusting the total in accordance with Table 1.7, the probable stand-up time for a given
diameter tunnel in the described rock mass can be estimated and the method of
excavation can be recommended. The effective deformation modulus (EM) of foundation
rock can also be deduced from its RMR:
EM = 2(RMR) - 100 GPa (1.22)
for RMR greater than 50, and
EM = 10X GPa (1.23)
where X = (RMR - 10)/40
for RMR less than 50
1-22
TABLE 1.6 - RMR - Classification Parameters and their Ratings
Parameter Ranges of Values
Point-load
strength
index
>10 MPa 4-10 MPa 2-4 MPa 1-2 MPa
For this low range-
uniaxial comp test is
preferred
Strength
of intact
rock
material Uniaxial
compressive
strength
>250 MPa 100-250 MPa 50-100 MPa 25-50 MPa 5-25
MPa
1-5
MPa
<1
MPa
1
Rating 15 12 7 4 2 1 0
Drill core quality RQD 90-100% 75-90% 50-75% 25-50% < 25% 2
Rating 20 17 13 8 3
Spacing of
discontinuities >2 m 0.6-2 m 200-600mm 60-200mm <60mm
3
Rating 20 15 10 8 5
Condition of
discontinuities
Very rough
surface.Not
continuous.N
o
separation.Un
weathered
rock
Lightly rough
surfaces.
Seperation<1
mm. Slightly
weathered
walls
Lightly
rough
surfaces.
Seperation<
1mm.
Highlyweath
ered walls
Slickensided
Or Gorge<5
mm thick Or
Seperation
1-5 mm
continuous
Soft gorge >5 mm
thick Or Seperation >
5 mm continuous
4
Rating 30 25 20 10 0
Inflow per 10m
tunnel length None
< 10
liters/min
10-25
liters/min
25-125
liters/min >125 liters/min
Ratio (Joint
water
pressure/major
principal stress)
0 0.0-0.1 0.0-0.2 0.2-0.5 > 0.5
Ground
water
General
conditions
Completely
dry Damp Wet Dripping Flowing
5
Rating 15 10 7 4 0
TABLE 1.7 RMR - RATING ADJUSTMENT FOR DISCONTINUITY
ORIENTATIONS
_____________________________________________________________________
Strike and dip Very Favourable Fair Unfavourable Very
orientations of joints favourable unfavourable
Tunnels 0 -2 -5 -10 -12
Ratings Foundations 0 -2 -7 -15 -25
Slopes 0 -5 -25 -50 -60
1-23
TABLE 1.8 RMR - ROCK MASS CLASSES DETERMINED FROM TOTAL
RATINGS
_____________________________________________________________________
Rating 100<−−81 80<−−61 60<−−41 40<−−21 < 20
Class No I II III IV V
Description Very good rock Good rock Fair rock Poor rock Very poor
rock
TABLE 1.9 RMR - MEANING OF ROCK MASS CLASSES
Fig. 1.9 Use of RMR to estimate Stand-up Time
1-24
1.4.4 Rock Mass Quality (Q)
The six parameters chosen to describe the rock mass quality Q are as follows:
RQD = rock quality designation
Jn = joint set number
Jr = joint roughness number
Ja = joint alteration number
Jw = joint water reduction factor
SRF = stress reduction factor.
These parameters are combined in pairs and are found to be crude measures of:
1. relative block size (RQD/Jn)
2. inter-block shear strength (Jr/Ja) (≅ tan φ)
3. active stress (Jw/SRF)
The overall quality Q is equal to the product of the three pairs:
Q = (RQD/Jn) . (Jr/Ja) . (Jw/SRF) (1.24)
Thus, the following rock mass would be most favourable for tunnel stability: high
RQD-value, small number of joint sets, appreciable joint roughness, minimal joint
alteration of filling, minimal water inflow, and favourable stress levels. From a large
number of case histories an approximate relationship has been developed between Q and
RMR.
RMR = 9 ln(Q) + 44 (1.25)
Ratings for the six parameters are given in Tables 1.10 - 1.15.
TABLE 1.10 ROCK QUALITY DESIGNATION (RQD)
See section 1.4.2. Where RQD is reported or measured as ≤ 10, (including 0) a
nominal value of 10 is used to evaluate Q in equation (2.23). RQD intervals of 5, i.e.
100, 95, 90 etc. are sufficiently accurate.
1-25
TABLE 1.11 JOINT SET NUMBER (Jn)
_____________________________________________
A. Massive, no or few joints 0.5-1.0
B. One joint set 2
C. One joint set plus random 3
D. Two joint sets 4
E. Two joint sets plus random 6
F. Three joint sets 9
G. Three joint sets plus random 12
H. Four or more joint sets, random,
heavily jointed, "sugar-cube" etc. 15
J. Crushed rock, earthlike 20
Note: (i) For intersections use (3.0 x Jn)
(ii) For portals use (2.0 x Jn)
TABLE 1.12 JOINT ROUGHNESS NUMBER (Jr)
________________________________________________
(a) Rock wall contact and
(b) Rock wall contact before 10 cm shear
A. Discontinuous joints 4
B. Rough or irregular, undulating 3
C. Smooth, undulating 2
D. Slickensided, undulating 1.5
E. Rough or irregular, planar 1.5
F. Smooth, planar 1.0
G. Slickensided, planar 0.5
Note: (i) Descriptions refer to small scale features and intermediate scale features,
in that order.
(c) No rock wall contact when sheared
H. Zone containing clay minerals thick enough
to prevent rock wall contact 1.0
J. Sandy, gravelly or crushed zone thick enough
to prevent rock wall contact 1.0
Note: (ii) Add 1.0 if the mean spacing of the relevant joint set is greater than 3m.
(iii) Jr = 0.5 can be used for planar slickensided joints having lineations,
provided the lineations are orientated for minimum strength.
1-26
TABLE 1.13 JOINT ALTERATION NUMBER (Ja)
_________________________________________________________
(Ja) (φr)
(a) Rock wall contact (approx)
A. Tightly healed, hard, non-softening,
impermeable filling i.e. quartz or epidote 0.75 (-)
B. Unaltered joint walls, surface staining only 1.0 (25-35o) C. Slightly altered joint walls. Non-softening mineral coatings, sandy particles, clay-free disintegrated rock etc. 2.0 (25-30o) D. Silty-, or sandy-clay coatings, small clay fraction (non-soft) 3.0 (20-25o) E. Softening or low friction clay mineral coatings, i.e. kaolinite or mica. Also chlorite, talc, gypsum, graphite etc., and small quantities of swelling clays. 4.0 ( 8-16o) (b) Rock wall contact before 10 cm shear F. Sandy particles, clay-free disintegrated rock etc. 4.0 (25-30o) G. Strongly over-consolidated non- softening clay mineral fillings (continuous, but <5mm thickness) 6.0 (16-24o) H. Medium or low over-consolidation, softening clay, mineral fillings (continuous but <5mm thickness) 8.0 (12-16o) J. Swelling -clay fillings, i.e. montmorillonite (continuous, but <5mm thickness). Value of Ja depends on percent of swelling clay-size particles, and access to water etc. 8-12 ( 6-12o) (c) No rock wall contact when sheared K, Zones or bands, of disintegrated L, or crushed rock and clay (see M. G,H,J for description of 6,8 clay condition) or 8-12 ( 6-24o) N. Zones or bands of silty- or sandy- clay, small clay fraction (non- softening) 5.0 ( - ) O, Thick, continuous zones or P, bands of clay (see G,H,J for 10,13, R. description of clay condition) or 13-20 ( 6-24o)
1-27
TABLE 1.14 JOINT WATER REDUCTION FACTOR (Jw)
____________________________________________________
(Jw) Approx.
water pres. (kg/cm2) A. Dry excavations or minor inflow i.e. <5 1/min. locally 1.0 < 1 B. Medium inflow or pressure, occasional outwash of joint fillings 0.66 1-2.5 C. Large inflow or high pressure in competent rock with unfilled joints 0.5 2.5-10 D. Large inflow or high pressure, considerable outwash of joint fillings 0.33 2.5-10 E. Except ionally high inflow or water pressure at blasting, decaying with time 0.2-0.1 >10 F. Exceptionally high inflow or water pressure continuing without noticeable decay 0.1-0.05 >10 Note: (i) Factors C to F are crude estimates. Increase Jw if drainage measures are installed. (ii) Special problems caused by ice formation are not considered.
1-28
TABLE 1.15 STRESS REDUCTION FACTOR (SRF) _____________________________________________________ (a) Weakness zones intersecting excavation, which may cause loosening of rock mass when tunnel is excavated A. Multiple occurrences of weakness zones containing clay or chemically disintegrated rock, very loose surrounding rock (any depth) 10 B. Single weakness zones containing clay or chemically disintegrated rock (depth of excavation ≤ 50m) 5 C. Single weakness zones containing clay or chemically disintegrated rock (depth of excavation >50m) 2.5 D. Multiple shear zones in competent rock (clay-free), loose surrounding rock (any depth) 7.5 E. Single shear zones in competent rock (clay-free) (depth of excavation ≤ 50m) 5.0 F. Single shear zones in competent rock (clay-free) (depth of excavation > 50m) 2.5 G. Loose open joints, heavily jointed or "sugar cube" etc. (any depth) 5.0 Note: (i) Reduce these values of SRF by 25-50% if the relevant shear zones only influence but do not intersect the excavation. (b) Competent rock, rock stress problems σc/σ1 σt/σ1 (SRF) H. Low stress, near surface >200 >13 2.5 J. Medium stress 200-10 13-0.66 1.0 K. High stress, very tight structure (usually favourable to stability, may be unfavourable for wall stability) 10-5 0.66-.33 0.5-2 L. Mild rock burst (massive rock) 5-2.5 0.33-.16 5-10 M. Heavy rock burst (massive rock) < 2.5 <0.16 10-20 Note: (ii) For strongly anisotropic virgin stress field (if measured): when 5 ≤ σ1/σ3 ≤ 10, reduce σc and σt to 0.8 σc and 0.8 σt. When σ1/σ3 > 10, reduce σc and σt to 0.6 σc and 0.6 σt, where: σc = unconfined compression strength, and σt = tensile strength (point load) and σ1 and σ3 are the major and minor principal stresses. (iii) Few case records available where depth of crown below surface is less than span width. Suggest SRF increase from 2.5 to 5 for such cases (see H). (c) Squeezing rock plastic flow of incompetent rock under the influence of high rock pressure N. Mild squeezing rock pressure 5-10 O. Heavy squeezing rock pressure 10-20 (d) Swelling rock chemical swelling activity depending on presence of water P. Mild swelling rock pressure 5-10 R. Heavy swelling rock pressure 10-15
1.29
1-29
Additional notes relating to use of Tables 1.10-1.15:
1. When borecore is unavailable, RQD can be estimated from the number of joints
per unit volume, in which the number of joints per metre for each joint set are added. A
simple relation can be used to convert this number to RQD for the case of clay-free rock
masses:
RQD = 115 - 3.3 Jv (approx.)
where
Jv = total number of joints per m3
(RQD = 100 for Jv < 4.5)
2. The parameter Jn representing the number of joint sets will often be affected by
foliation, schistosity, slatey cleavage or bedding etc. If strongly developed these parallel
"joints" should obviously be counted as a complete joint set. However, if there are few
"joints" visible, or only occasional breaks in bore core due to these features, then it will
be more appropriate to count them as "random joints" when evaluating Jn in Table 1.11.
3. The parameters Jr and Ja (representing shear strength) should be relevant to the
weakest significant joint set or clay filled discontinuity in the given zone. However, if
the joint set or discontinuity with the minimum value of (Jr/Ja) is favourably oriented for
stability, then a second, less favourably orientated joint set or discontinuity may
sometimes be of more significance, and its higher value of (Jr/Ja ) should be used when
evaluating Q from equation (1.24). The value of (Jr/Ja) should in fact relate to the
surface most likely to allow failure to initiate.
REFERENCES
Bieniawski, Z.T. - "Engineering classification of jointed rock masses". Trans. S. Afr.
Inst. Civ. Engrs, Vol. 15, No. 12, 1973, pp 335-344.
Bowles, J.E. - "Engineering Properties of Soils and their Measurement". McGraw-Hill
Book Company, 187 p., 1970.
Casagrande, A. - "Research on the Atterberg Limits of Soils", Public Roads, 13, pp 121-
136, 1932.
Deere, D.V. - "Technical description of rock cores for engineering purposes. Rock
Mechanics and Engineering Geology, Vol. 1, No. 1, 1964, pp 17-22.
I.S.R.M. Commission on Classification of Rocks and Rock Masses - "Basic
Geotechnical Description of Rock Masses". Int. J. Rock Mech. Min.Sci, Vol. 18, 1981,
pp 85-110.
Kezdi, A. - "Handbook of Soil Mechanics", Vol. 2, Soil Testing, Elsevier Scientific
Publishing Company, 258 p., 1980.
Lambe, T.W. - "Soil Testing for Engineers", John Wiley & Sons, 165 pp. 1951.
1-30
Moye, D.G. - "Engineering geology for the Snowy Mountains Scheme". J.I.E. Aust., Vol.
27, 1955, pp 281-299.
Moye, D.G. - "Engineering Geology Manual". Snowy Mountains Hydroelectric
Authority, 1958.
Standards Association of Australia - "Method of Testing Soils for Engineering Purposes",
Australian Standard AS1289.
U.S. Corps of Engineers, Waterways Experiment Station - "Unified Soil Classification
System", Tech. Memo. 3-357, 1953.
Wickham, G.E, Tiedemann, H.R. & Skinner, E.H. - "Support determinations based on
geologic predictions", Rapid Excavation & Tunneling Conference, Chicago 1972,
pp 43-64.
1-31