imesa - institute of municipal engineering of southern africa
TRANSCRIPT
anatysis is often considers ttlernsnt of road smban P
asis is therefore given to
SIS
erp van padopvullings word
ontledlng.
kument handel oor die roeline of eenvoudi rd. Vir meer ingewikkeMe gevalie m e t daar van spesialis geategniese ku
gter nie dat geotegnisse kund ord nie. Dit word benad Die gebruik van toep
beveel dat sekere for
nvenience the investigation a B
carried out in stages. These ra
sophisticated invest
the earlier stages in some detail
ith the more complex prob
not identified and not because th
ation and design
process described, although se
reassd geometric standards for modern roads havs led to much h
eased capital costs and utilisation have demande
these factors Rave led to the necess
votvgd in embankment most obvious and
atbn are necessarily c
B A S I C STEPS IN SYS'$EM P--
A . AIM OF INVE
M = METHODS OF INFORMATION
$ : EVALUATION C R l T E W l A
Q 2 DECISION
STAGES IN S Y S T E M p-.
S I T E l ! \ cVESTIGATIQfy --p----
L licl . d
I s ? S T A G E
~---.1.1
2 nd S T A G E
seen as a ration
nerally of increasi
er each of the sta
ective, of the invest
s construded ecommically.
reas of the country in
isture contents on slope stabiliB
Iso directly related to embankment heights. Hence, in
re probable than for low embankments. Cogni
e relative probabilities of distress. It is, never-tta
nce to the embankment height to the exten% that
mbartkments ar ere are for example, numerous embankments below 5m height which have
th excessive settlement a instability and, conversely,
ssibility of distress.
the process of evaluating the res
aluation is no more than straightf
because it is to
ms. In other cases, it may be
ses, the evaluation phase may be the
hasized that evalu
re,
separate formal pr
nvestigatbn, ie pits, auger holes, reholes, etc. shouM th
reholes so that maimurn information is obta
embankments cross alluvial ds its there will probably be a
ation for the structur
e boreholes to provide informatbn for both the ernbankme
t the structure will have to be piled and information
he rock quality. These aspects are of little relevanc
ch information on the overtying soil is required. The type of the
then be defined by the requirements of the embankment,
to h piled, information on the soil cover is still required in order to choose the
sed embankments are fairly I e, information from conve
icient for the embankment de . If further boreholes or pro
sitbned with care. The topography and the arr photos, and even th
on as to where further investigation should be located. Onl
es at regular chainages and at standard offsets from the c
sitioning of boreholes grobabty reflects the most pessimist
o account to avoid excessive consewatism.
slope, for exampia, W
ith small crossfalls, ie
es of failure, are essentially similar to flat
ntinuities, such as fau&s or
to assess their
n and construction,
ossfallis, there are not likely to e large depths of
tx>t necessarily require such conditions.
use instability. Colluvium djacent to the main
of the investigation a appear to have a sat
e after construction, bue to a
arameters based on the initia
se sites are similar to those encountered at
ter course exists. It is likely that there
to the crossfali, and it is im
ent construction.
particularly aimed at ex
investigation. In partimlar, v
he same contrac? It is
ugh to the other; it coul
ught that further investigation ma
wide just sufficient information so fh
sy to achieve and all too often the invesli
01%; frequently, th re expensive, at? vious, mistake is
by excessive laboratory testing, the results from
, nor used in the design
f that stratum then
t appropriate investig
example, a series
tes, a l thugh some borehol
rns are classified under stab~lrty and/or settlement. The relevant soil charaaeristics
th of the subsoil is required to estimate the stability of slopes. This can
of the material, or from tn-situ tests, such as the CPT, SPT and vane shear
evaluation of the tests and S equent analyses are
sign. if these indicate stability problem, re sophisticated test
ial testing. The shear strength of clays is
h, c, or the drained stre
ely the situations of a rapid applrcation of load,
licalion of bad, which
tests. The emphasis S
of the distribution
blern, then carefutly c
never be justified.
The analysis of th els by calculations of fact rs sf satev for stab~P~ty,
input parameters have been selected
ever, in defining accepta dsrs of safety an
erne level required tor the c
stabiffly factors of safet
ts, @g factor of safety not less than 1.5 or sett1
rticular circurnstanc
in the realms sf sp
f safety, it is likely to be the st
fety for road embankme
ntually back to the same
figure, is cakulated for
nt will be necessary.
ts of construction muM be ntrolled il necessary, the am
struction situatio
en be not less th
the check analyse
snt variable, ie it re -water level, it is
of safety. Such embankment
S. The acceptable factors
,O for the initial asse
re conditions. From
actor of safety shouM
Is to express these variable the form of simple critsri
il is necessary to use ju
ankment, in terms of th amount and time. Thes
roximate limits, from the initial or sscond-stage assessmen
, embankment settlement predictions mu
tcs five years. Ali such estimates
s are acceptable; in so
, so that time is
rary placing of a higher fill than
nt in the time available than
surcharge is primarily dependent on th
eneral situatbn only and each c
uires fairly sophisticated investigation a
urn surcharge height will be in
worth the additional pr
torn the stntdurs
ankments over soft
e slopes and gulleys. Such em
y firm, the large pressures caus
e the cross-fall, and thus heigh
cceptable settlement shuM be
from the preliminary a
riod, it is necessary to procee
ns of the paramete
r that the Izictclr of safety is extremely dependerlt an %he
cohesion T h e former, however, has a fairly smalf ran
The ar~alysis 1s Wherefore most sensitive to variations In the pore pressure ratm
to assess for ail but the most s~rnple slo stablllty analyses. This
of reailistcally nmdelling the geology and rhe
PhaP can occur qtdrte f ~ e uently m scsuttlern
the usual sensiPivrPy analyses, 1s "rat of large c
n-sr'tu subsorls These may be stiff at the time af investig
eaker under an embankment in these cases
lgEhs of t h e wefled sods. {and &his 1s d[f%tcuM becabrse ass
difticultf or to analyre ?he em ankmenl an the basi of %he effective stress
B are already familiar with the a plication of prcsba
nl constn~ctron, so the advantages of these rnetho
l LIT
,
ents settle, but very fs
ility of many embankments.
stages, of increasing levels of so
+case of most road embankments
of the fill can be
f appropriate soil
35
le of the Pibt a
th of the day layer is assumed to = 1, then the factor of s
sensitivity analysis can very e
oukl then de~rease to 2,10.
to check the influenc
t fits of using berms.
rri
I a factor of safety les S
spts.
solidated clays, c' is t ' varies within a r
endent variable (See sect
e uses the same
st be emphasized that this is a very crude $9
ssessrnent by an experienced person.
&hough this test was originally devised for th
een cone pressures, Q, and
se can be expressed by the folio
= Cone Factor and varies from 10 to 30
y =3 density of the subsoil
D = depth of subsoil
Far recent alluvial y be taken as 14 fort
is usually small co
rehinary analyses, t
mare sophisticate
menil assessments,
ssessment of the future
M
n of embankment settle
s order of maagnitu sefltlement, viz B0 m
words, in the majorsy of cases
tests.
The methods of estimating settlements are subsections in the order in which
firs! action rs ta define the
late the requtred reit
nt s n the rnagr~~tuds of the
the other considerations
ramme wi%! asst~lt in the pavemen! layers $:eing built a year or
B1 there are no stwc h $h@ particular embarakment,
ss is to consider t
er of magnitude of t
le standard materi
Rcaw@ver, there is ask settlement equation may be use
E
where 6 -- settlement
o = increase in stress due to embankment
H = thickness of compressible stratum
E = modulus = I f q ,
= coefficient of cornpressibility
e , tf the serbsaii is deseri ed as medium dense sand and is I Q m thick undertyi
out 20 MPa, U is about 200 kPa since the embankment dens
20 k ~ / d , therefore:
then be necessaq to valuate the consequences of this estimated se
ankment, and it can be constmcte
fy satisfactory, teadin
x cutvert passed throu
te of the coefficient of
er)/ f~ssured, with frequent
10 rn2iyeai in this case the time
be smfficsent, say 2 years
pie 1, therefore is 0.5 years, and only 50 rnrn seillemenl will occur aller this perbd
further invsstigation wiiB !hen be re
onstralg initial estimates based on n more than t@!Ea56e
al descriptions sl th
I f , as a result of the init1 1 assessment of settlemen
reqwred, Bhrs wsll generally take 181
and hence seR-tlements e
( a ) SPT The tables m t
T values given a
t a refers ia the mean increase rn stress ue to the embankment !@ad at the centre
l-
erlying clomprcssible strata
ate thicknesses of corn
the thickness 01 subsoil is lar
ation:
se in veflical eflective stress at centre of stratum
ressure at centre af stratum
ives continuous readings of cone pressure, it is comrraon to su
gnitude of cone pressures, as well
nr is then carried out in a tabular form and rna
rnative method is the use of a set?lernent chart (Figure 1 1 )
uisman-de Beer equation given a
th sunicient accuracy f
e average cone press
f these cases m r e W histicated in-situ t , such as the piezo
ring piessuremeter (S
er cases, undisfu&e
ocedrnent to describe either this testin
recl in the geotechnical literature on the subject.
here more sophisticated testi methods are required, it
h suggested in this document, it is likely that such expertise wouM have
e of the investigation, since the planning is itself a fu
ation is organized, it should never result in the g
le logs and consolidation test resuhs, and asked to
lies such as OM infilled chan
ne limitation of the seismic techn us is that the relatkrely small energy in
dudes reliable investigation at depths in excess of
extent, be overcome y using more sophisticated equipment or by using
recommended that the work shou only be carried out by personnel with
training.
sistivity methods may also be used. These re ecialist input, sinc
e field operatbn techniques appear to be si
very useful for detecti anomalies such as intrus
of marked changes in materials and for detecting water.
(W in many cases sa
rcient information for the first-stags inve
on of the subsoil in a relatively ondistu
in art excavation dee
is used primarily on satur
re many variations in the rnetho
e more usual methods.
chniques can be divided into so
number of different mac
necessary to obtain a
De from SPTs or CPTs. This is unusu
ion of the embankmen
ignificant pmblem i
as by fieM tests, such
that the more clayey samples are